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理論化学研究に立脚した量子干渉現象の分子エレクトロニクスへの展開

光合成細菌の反応中心複合体を用いた高効率な太陽電池の開発

Language：English   Publishing type：Research paper (scientific journal)  

The lithium-richest phase in the binary Li-Tt system (Tt = Si, Ge, Sn, and Pb) has a stoichiometry of Li₁₇Tt₄. In the beginning of this paper, the structural complexity of Li₁₇Tt₄ is gradually stripped away using the concept of the M26 cluster found in γ-brass structures and a Tt-centered polyhedral representation. By means of the first-principles electronic structure calculations, which are followed by the analyses of the electron localization function (ELF), Bader charges, and spin density, we observe non-nuclear maxima of the ELF, electron density, and spin density. Since the electron densities off the atoms are confined in crystalline voids, separated from each other, and behaving as an anion, Li₁₇Tt₄ can be identified as a potential zero-dimensional electride. This finding agrees with a simple Zintl picture, which suggests a valence electron count of [(Li⁺)₁₇(Tt⁴¹)₄¢e¹]. Detailed analyses on the band structures, the projected density of states, and crystal orbitals at the ¥ point in the reciprocal space hint at the potential of forming a bond between the non-nuclear electron density and the neighboring atoms. Signatures of bonding and anti-bonding orbital interactions can be witnessed.

DOI： 10.1246/bcsj.20190040

Language：English   Publishing type：Research paper (scientific journal)  

In this paper, we explore quantum interference (QI) in molecular conductance from the point of view of graph theory and walks on lattices. By virtue of the Cayley-Hamilton theorem for characteristic polynomials and the Coulson-Rushbrooke pairing theorem for alternant hydrocarbons, it is possible to derive a finite series expansion of the Green's function for electron transmission in terms of the odd powers of the vertex adjacency matrix or Hückel matrix. This means that only odd-length walks on a molecular graph contribute to the conductivity through a molecule. Thus, if there are only even-length walks between two atoms, quantum interference is expected to occur in the electron transport between them. However, even if there are only odd-length walks between two atoms, a situation may come about where the contributions to the QI of some odd-length walks are canceled by others, leading to another class of quantum interference. For nonalternant hydrocarbons, the finite Green's function expansion may include both even and odd powers. Nevertheless, QI can in some circumstances come about for nonalternants from cancellation of odd- and even-length walk terms. We report some progress, but not a complete resolution, of the problem of understanding the coefficients in the expansion of the Green's function in a power series of the adjacency matrix, these coefficients being behind the cancellations that we have mentioned. Furthermore, we introduce a perturbation theory for transmission as well as some potentially useful infinite power series expansions of the Green's function.

DOI： 10.1021/acs.chemrev.7b00733

Language：English   Publishing type：Research paper (scientific journal)  

In pursuit of new lithium-rich phases and potential electrides within the Li-N phase diagram, we explore theoretically the ground-state structures and electronic properties of Li₄N at P = 1 atm. Crystal structure exploration methods based on particle swarm optimization and evolutionary algorithms led to 25 distinct structures, including 23 dynamically stable structures, all quite close to each other in energy, but not in detailed structure. Several additional phases were obtained by following the imaginary phonon modes found in low-energy structures, as well as structures constructed to simulate segregation into Li and Li₃N. The candidate Li₄N structures all contain NLi_n polyhedra, with n = 6-9. They may be classified into three types, depending on their structural dimensionality: NLi_n extended polyhedral slabs joined by an elemental Li layer (type a), similar structures, but without the Li layer (type b), and three-dimensionally interconnected NLi_n polyhedra without any layering (type c). We investigate the electride nature of these structures using the electron localization function and partial charge density around the Fermi level. All of the structures can be characterized as electrides, but they differ in electronic dimensionality. Type-a and type-b structures may be classified as two-dimensional (2-D) electrides, while type-c structures emerge quite varied, as 0-D, 2-D, or 3-D. The calculated structural variety (as well as detailed models for amorphous and liquid Li₄N) points to potential amorphous character and likely ionic conductivity in the material.

DOI： 10.1021/jacs.6b09067

Language：English   Publishing type：Research paper (scientific journal)  

An empirical observation of a relationship between a striking feature of electronic transmission through a π-system, destructive quantum interference (QI), on one hand, and the stability of diradicals on the other, leads to the proof of a general theorem that relates the two. Subject to a number of simplifying assumptions, in a π-electron system, QI occurs when electrodes are attached to those positions of an N-carbon atom N-electron closed-shell hydrocarbon where the matrix elements of the Green's function vanish. These zeros come in two types, which are called easy and hard. Suppose an N+2 atom, N+2 electron hydrocarbon is formed by substituting 2 CH₂ groups at two atoms, where the electrodes were. Then, if a QI feature is associated with electrode attachment to the two atoms of the original N atom system, the resulting augmented N+2 molecule will be a diradical. If there is no QI feature, i.e., transmission of current is normal if electrodes are attached to the two atoms, the resulting hydrocarbon will not be a diradical but will have a classical closed-shell electronic structure. Moreover, where a diradical exists, the easy zero is associated with a nondisjoint diradical, and the hard zero is associated with a disjoint one. A related theorem is proven for deletion of two sites from a hydrocarbon.

DOI： 10.1073/pnas.1518206113

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An exponential falloff with separation of electron transfer and transport through molecular wires is observed and has attracted theoretical attention. In this study, the attenuation of transmission in linear and cyclic polyenes is related to bond alternation. The explicit form of the zeroth Green's function in a Hückel model for bond-alternated polyenes leads to an analytical expression of the conductance decay factor β. The β values calculated from our model (β_CN values, per repeat unit of double and single bond) range from 0.28 to 0.37, based on carotenoid crystal structures. These theoretical β values are slightly smaller than experimental values. The difference can be assigned to the effect of anchoring groups, which are not included in our model. A local transmission analysis for cyclic polyenes, and for [14]annulene in particular, shows that bond alternation affects dramatically not only the falloff behavior but also the choice of a transmission pathway by electrons. Transmission follows a well-demarcated system of π bonds, even when there is a shorter-distance path with roughly the same kind of electronic matter intervening.

DOI： 10.1021/acsnano.5b04615

Language：English   Publishing type：Research paper (scientific journal)  

For transmission of electrons through a π system, when the Landauer theory of molecular conductance is viewed from a molecular orbital (MO) perspective, there obtains a simple perturbation theoretic dependence, due to Yoshizawa and Tada, on a) the product of the orbital coefficients at the sites of electrode attachment, and b) the MO energies. The frontier orbitals consistently and simply indicate high or low transmission, even if other orbitals may contribute. This formalism, with its consequent reinforcement and/or interference of conductance, accounts for the (previously explained) difference in direct vs. cross conjugated transmission across an ethylene, as well as the comparative ON/OFF ratios in the experimentally investigated dimethyldihydropyrene and dithienylethene-type single-molecule switches. A strong dependence of the conductance on the site of attachment of the electrodes in a π system is an immediate extrapolation; the theory then predicts that for some specified sites the switching behavior will be inverted; i.e. the "open" molecular form of the switch will be more conductive. The phase and amplitude of the frontier molecular orbitals at the sites that are connected to electrodes play an essential role in determining transmission of electrons through a π system. When applied to two diarylethene switches, theory then predicts that for some specified sites the switching behavior will be inverted; that is, the "open" molecular form of the switch will be more conductive.

DOI： 10.1002/anie.201311134

Language：English   Publishing type：Research paper (scientific journal)  

Site-specific electron transport phenomena through benzene and benzenedithiol derivatives are discussed on the basis of a qualitative Hückel molecular orbital analysis for better understanding of the effect of anchoring sulfur atoms. A recent work for the orbital control of electron transport through aromatic hydrocarbons provided an important concept for the design of high-conductance connections of a molecule with anchoring atoms. In this work the origin of the frontier orbitals of benzenedithiol derivatives, the effect of the sulfur atoms on the orbitals and on the electron transport properties, and the applicability of the theoretical concept on aromatic hydrocarbons with the anchoring units are studied. The results demonstrate that the orbital view predictions are applicable to molecules perturbed by the anchoring units. The electron transport properties of benzene are found to be qualitatively consistent with those of benzenedithiol with respect to the site dependence. To verify the result of the Hückel molecular orbital calculations, fragment molecular orbital analyses with the extended Hückel molecular orbital theory and electron transport calculations with density functional theory are performed. Calculated results are in good agreement with the orbital interaction analysis. The phase, amplitude, and spatial distribution of the frontier orbitals play an essential role in the design of the electron transport properties through aromatic hydrocarbons.

DOI： 10.1021/ja111021e

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACS Applied Materials and Interfaces  

Various catalytic reactions such as water oxidation to O2 and CO2 reduction have been achieved using the surfaces of inorganic materials as reaction sites, as exemplified by oxygen-defect-induced reactions in metal oxides. In recent years, anion-controlled post-transition-metal oxide materials have attracted attention. These materials have been shown to exhibit unique reactivity that cannot be achieved with oxides because of the effect of paired anion species coexisting with oxide anions. They have also been shown to enable highly difficult reactions because of the surface and reaction properties resulting from the molecular nature of the anion species. This Perspective reports on recent developments in small-molecule conversion reactions catalyzed by anion-controlled inorganic materials.

DOI： 10.1021/acsami.5c00825

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Publishing type：Research paper (scientific journal)   Publisher：Journal of Materials Chemistry A  

The growing need for cost-effective and efficient energy conversion technologies drives the development of advanced catalysts for the oxygen evolution reaction (OER). Our research focuses on high-entropy spinel oxides (HESOs) as efficient OER electrocatalysts. Using the molten salt synthesis (MSS) method, we prepared HESO nanoparticles from Fe, Ni, Co, Mn, and Zn. By adjusting the precursor ratios, we obtained equimolar (Ni0.2Fe0.2Co0.2Mn0.2Zn0.2)3O4, CoMn-rich, and NiFe-rich samples to examine compositional effects. Among these, the CoMn-rich HESO sample exhibited superior catalytic performance in 1 M KOH solution, with an overpotential of 330.1 mV at 10 mA cm−2 and a Tafel slope of 53.5 mV dec−1. Its promising long-term stability and enhanced reaction kinetics are significant. The synergistic effect of Co and Mn with high valence states and enhanced oxygen adsorption on the CoMn-rich HESO lower the energy barrier and accelerate electron transfer, improving the reaction kinetics. Density functional theory (DFT) calculations further reveal the relationship between orbital hybridization and catalytic performance, emphasizing the contribution of high valence metal active centers in improving performance. The density of states (DOS) analysis further demonstrates the stronger covalency between the 3d orbitals of the metal active site and the O 2p orbitals on the surface of CoMn-rich samples, which favors the absorption of oxygen species and thus improves the electrochemical performance. This work presents an effective method for HESO synthesis and opens new avenues for energy conversion research.

DOI： 10.1039/d4ta08485c

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Publishing type：Research paper (scientific journal)   Publisher：Journal of Materials Chemistry C  

Organic-inorganic hybrid metal halides (OIMHs) are emerging functional materials with diverse applications. However, the exploration of mechanically soft OIMHs remains limited. This study introduces tin(iv) OIMH crystals (1 and 1w), incorporating a naphthalenediimide (NDI)-based organic cation (3pmNDI), which exhibit elastic flexibility, mechanochromism, and photothermal conversion. The unique one-dimensional (1D) slip-stacked assembly of 3pmNDI cations, influenced by the presence or absence of lattice water (1wvs.1), dictates their mechanical properties and chromic behavior. Compound 1 exhibits an exceptionally low elastic modulus (Er = 1.84 ± 0.21 GPa), as measured by nanoindentation, and a photothermal conversion efficiency of 63%. These findings showcase the potential of OIMHs as multifunctional flexible crystals.

DOI： 10.1039/d5tc00891c

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Publishing type：Research paper (scientific journal)   Publisher：Small Science  

Ethanol (EtOH) is a ubiquitous compound with critical applications across various industries, necessitating accurate and reliable sensing for sanitation, quality control, and environmental monitoring. Chromism-based sensors, known for their simplicity, portability, and real-time detection capabilities, have faced limitations in EtOH sensing due to insufficient sensitivity, low selectivity, irreversibility, and low color perception. Herein, a groundbreaking solvato/vapochromism-based EtOH sensor utilizing a Cu-based metal–organic framework (MOF) thin film, Cu-MOF-74, is reported. The conversion of Cu-based ceramics to Cu-MOF-74 facilitates the fabrication of solvato/vapochromic MOF thin films with low light scattering, enabling effective colorimetric analysis. The Cu-MOF-74 thin films demonstrate rapid and reversible solvato/vapochromism upon the adsorption of guest molecules, including water and EtOH. This unique behavior allows for the precise and reliable detection of EtOH across the entire concentration range. Furthermore, a smartphone application is developed to detect EtOH concentrations, enabling rapid and convenient evaluation of EtOH levels. The findings represent a significant advancement in EtOH sensing technology, overcoming the limitations of traditional methods. The Cu-MOF-74-based sensor offers a versatile and reliable solution for various applications, including environmental monitoring, process control, and healthcare.

DOI： 10.1002/smsc.202400634

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Inorganic Chemistry  

Ferroelectric ion conductors composed of noncentrosymmetric host structures and guest water molecules have recently garnered attention. These systems exhibit colossal polarization driven by long ion displacement facilitated by water molecules; however, the manner in which water molecules are perturbed by the polar backbone remains unclear. In this study, we investigated water migration behavior within the noncentrosymmetric channels of the ferroelectric proton conductor K2MnN(CN)4·H2O using various first-principles computational methods, including climbing image nudged elastic band (CI-NEB) calculations, potential energy surface (PES) scans, and ab initio molecular dynamics (AIMD) simulations. The energetic and dynamic characteristics governing water migration, obtained through CI-NEB and PES scans, revealed a significant directional preference for migration. Specifically, a lower activation barrier for migration in the negative c-axis direction compared to the positive one suggested rectification characteristics. These direction-dependent transition state energies were attributed to anisotropic arrangements of CN ligands, whose π orbitals interact with the highest occupied molecular orbital of the water molecule. In addition, AIMD simulations demonstrated that water molecules exhibit dynamically biased fluctuations around their equilibrium positions, corroborating the role of the polar framework as an internal electric field that directs water flow.

DOI： 10.1021/acs.inorgchem.4c05053

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DOI： 10.21203/rs.3.rs-5717389/v1

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Language：English   Publisher：Science and Technology of Advanced Materials  

Discoveries and technological innovations over the past decade are transforming our understanding of the properties of ceramics, such as ‘hard’, ‘brittle’, and ‘homogeneous’. For example, inorganic crystals containing molecular anions exhibit excellent secondary battery characteristics, and the fusion of inorganic solids and molecules results in innovative catalytic functions and physical properties. Different from the conventional ceramics such as metal oxides that are formed by monatomic cations and anions, unique properties and functions can be expected in molecular-incorporated inorganic solids, due to the asymmetric and dynamic properties brought about by the constituent molecular units. We name the molecular-incorporated inorganic materials that produce innovative properties and functions as supra-ceramics. In this article, we describe various kinds of supra-ceramics from the viewpoint of synthesis, analysis and physical properties/functions for a wide range of applications.

DOI： 10.1080/14686996.2024.2416384

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Inorganic Chemistry  

Two-dimensional (2D) materials are known for their potential to exhibit anisotropic transport properties due to their layered structures. However, the anisotropic ion conduction of 2D metal-organic frameworks (MOFs) has been rarely explored. In this study, we investigated the anisotropic proton conduction along the in-plane and stacking directions of two analogs of undulating 2D MOFs: [Mn(salen)]2[Pt(CN)4]·H2O (MnPt) and [Mn(salen)]2[PtI2(CN)4]·H2O (MnPtI). This investigation was conducted using both experimental methods, involving single crystals, and theoretical calculations. Compared to the relatively isotropic proton conduction of MnPt at 85 °C and 95% relative humidity (RH), with a stacking direction conductivity (σstacking) of 1.8 × 10-5 S/cm, which is approximately 2.9 times the in-plane conductivity (σin-plane), MnPtI exhibited highly anisotropic proton conduction. The σstacking of MnPtI under the same conditions (85 °C, 95% RH) was 1.5 × 10-4 S/cm, which is 83 times higher than its σin-plane. Additionally, the activation energy for proton conduction in MnPtI ranged from 0.65 to 0.73 eV, which is higher than the 0.48 eV observed for MnPt. Theoretical calculations confirmed that slight differences in local structures, including node distortions between MnPt and MnPtI, significantly influenced the activation energies for water migration. This was attributed to the formation of hydrogen bonds between layers and water molecules.

DOI： 10.1021/acs.inorgchem.4c03816

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nature Communications  

Catalysis-based electrical sensing of volatile organic compounds on metal oxide surfaces is a powerful method for molecular discrimination. However, catalyst deactivation caused by the poisoning of catalytic sites by analytes and/or catalyzed products remains a challenge. This study highlights the underestimated role of van der Waals interactions between hydrophobic aliphatic alkyl chains and hydrophilic ZnO surfaces in mitigating catalyst deactivation during aliphatic aldehyde sensing. By immobilizing octadecylphosphonic acid (ODPA) on ZnO nanowire sensors, recovery times for nonanal detection are significantly reduced without compromising sensitivity. Temperature-programmed measurements demonstrate a reduction in desorption temperature of carboxylates on ODPA-modified ZnO to below 150 °C, whereas carboxylates on bare ZnO remain above 300 °C, indicating a significant decrease in catalyst deactivation. Density functional theory calculations reveal that accumulated van der Waals interactions between alkyl chains and ZnO surfaces significantly contributed to adsorption molecular kinetics. IR spectroscopy using deuterated self-assembled monolayers (SAMs) reveals conformational changes of alkyl chains within the SAMs caused by aldehyde adsorption, supporting the suggested adsorption kinetics. A model is proposed based on the dynamic surface-covering by alkyl chains destabilizes catalytically oxidized carboxylic acids.

DOI： 10.1038/s41467-024-53577-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Chemistry Chemical Physics  

CeO2 supported with Ni nanoparticles has emerged as a promising catalyst for enhancing the efficiency of dry reforming of methane (DRM) reaction. Methane dissociation (CH4 → CH3 + H) was reported as one of the rate-determining steps in the DRM reaction. We elucidated the reaction mechanism and explored methods for reducing the activation energy using density functional theory (DFT) calculations, where the activation energy of methane dissociation was determined at multiple Ni4 cluster sites on CeO2. In parallel, we experimentally evaluated methane dissociation based on the methane consumption rate in the DRM reaction using newly developed flower-like Ni-supported CeO2 catalyst (Ce(F)). The experimental activation energy was determined to be 0.69 eV (15.91 kcal mol−1), closely matching the DFT-calculated value of 0.80 eV (18.45 kcal mol−1) for the Ni4 cluster model, validating our theoretical predictions. Additionally, we discovered that positively charging the Ni4 can lower the activation energy of methane dissociation. These findings contribute to a deeper understanding of how to control the activation energy of the methane dissociation reaction in DRM.

DOI： 10.1039/d4cp01324g

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of the American Chemical Society  

Dividing orbital interactions into through-space (TS) and through-bond (TB) modes is valuable for understanding various molecular properties. In this paper, we elucidate how the quantum interference phenomenon known as σ interference in electron transport through σ systems arises from TS and TB interactions. We performed electron transport calculations using a combination of density functional theory and nonequilibrium Green’s function methods, focusing on ethylenediamine, a classical molecule that effectively highlights the contrast between TS and TB interactions. Our results confirm that destructive σ interference occurs in the syn and gauche conformers of this molecule. To further investigate both TS and TB interactions, we employed two analytical methods: the fragment molecular orbital (FMO) method, which captures the effects of both TS and TB interactions, and the chemical graph theory method, which specializes in TB interactions. The FMO analysis demonstrated that TB interactions lead to the characteristic distribution and energy level alignment of the frontier orbitals. Additionally, it was clarified that a change in TS interaction, due to a variation in the dihedral angle of the molecule, alters the energy gap between these orbitals, resulting in the manifestation of σ interference in the syn and gauche conformers, but not in the trans conformer. The chemical graph theory analysis based on the ladder C model, aimed at exploring the topological origin of σ interference from the network of TB interactions, revealed that σ interference is caused by the cancellation between the walk associated with geminal interactions (σ-conjugation) and the one related to vicinal interaction (σ-hyperconjugation). Notably, it was found that the vicinal interaction, which changes sign with the dihedral angle, has a decisive influence on whether this cancellation occurs. These findings clarify that σ interference arises from the interplay between TS and TB interactions. This insight will be valuable for designing molecular systems that utilize σ interference.

DOI： 10.1021/jacs.4c09771

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

The molecular mechanism of adhesion of two epoxy resins based on diglycidylether of bisphenol A (DGEBA) cured with 4,4′-diaminodiphenyl sulfone (DDS) and 4,4′-diaminodiphenylmethane (DDM) to the carbon fiber (CF) surface is investigated by employing density functional theory (DFT) calculations. The CF surface was modeled by the armchair-edge structure of graphite functionalized with carboxyl (COOH) groups. Two adhesion interfaces were constructed using the CF surface: one with the DGEBA-DDS molecule (CF/DGEBA-DDS interface) and the other with the DGEBA-DDM molecule (CF/DGEBA-DDM interface). The interfacial properties were analyzed by calculating the maximum adhesion stress (Smax) at the interface. The adhesion stress-displacement curve revealed that Smax is 1160.37 MPa, higher for the CF/DGEBA-DDS interface compared to the CF/DGEBA-DDM interface, which is 1060.48 MPa. The energy decomposition analysis showed a similar DFT contribution to adhesion stress for both interfaces, but the dispersion contribution is more significant at the CF/DGEBA-DDS interface. The crystal orbital Hamilton population (COHP) analysis revealed distinct interfacial interactions despite similar DFT contributions. Hydrogen bonding (H-bonding) between the functional groups at both interfaces including feeble OH−π interactions between the benzene rings of epoxy resins and COOH groups on the CF surface were observed. The orbital interaction energies calculated from integrated COHP, i.e., IpCOHP, revealed that the CF/DGEBA-DDS interface has six H-bonding interactions with large absolute IpCOHP values (>1 eV), whereas the CF/DGEBA-DDM interface has five. The interaction between the amine group of the DGEBA-DDM molecule and the CF surface has a large IpCOHP value among all interactions. The sulfone group being at the center of the DDS molecule and its strong surface interaction positioned the DGEBA-DDS molecule closer to the CF surface than the DGEBA-DDM molecule, enhancing dispersion interaction at the CF/DGEBA-DDS interface. Hence, the CF surface exhibits a stronger affinity toward the DGEBA-DDS molecule than the DGEBA-DDM molecule through dispersion interaction.

DOI： 10.1021/acs.langmuir.4c02473

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

Distorted honeycomb coordination layers are extremely flexible and exhibit giant anisotropic thermal expansion.

DOI： 10.1039/d4cc01232a

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Langmuir  

Epoxy resins are widely used adhesives in industrial fields. To use epoxy resin as an adhesive, it is necessary to mix the epoxy resin with a hardener. Hardeners have various functional groups and skeletons, and the properties of epoxy resins vary depending on the hardener. Although the adhesion of epoxy resins has been extensively studied using density functional theory (DFT) calculations, few studies have evaluated the effect of hardener molecules. Therefore, in this study, DFT calculations of adhesion energies and bonding structures on Cu (111) and Cu2O (111) surfaces are performed for model molecules of adducts of epoxy resin with hardeners having various functional groups and skeletons to evaluate the influence of the hardeners on the adhesion of epoxy resin to the metal surface. The adhesion energy to the Cu (111) surface is governed by the energy due to dispersion forces. Hardeners of the thiol type, which contain relatively heavy sulfur atoms, and hardeners with aromatic rings, displaying high planarity, enable the entire molecule to approach the metal surface, resulting in a relatively high adhesion strength. The calculations for the Cu2O (111) surface show the adhesion strength is more strongly influenced by interactions such as hydrogen bonds between the surface and adhesive molecules than by dispersion forces. Therefore, in adhesion to Cu2O (111), the benzylamine-epoxy adduct with hydrogen bonding and OH-π interactions with the surface, in addition to having a relatively flexible framework, shows a high adhesion strength.

DOI： 10.1021/acs.langmuir.4c01093

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Langmuir  

The adhesion of epoxy adhesives to aluminum materials is an important issue in assembling parts for lightweight mobility. Aluminum surfaces typically possess an oxide layer, which readily adsorbs water. In this study, the aggregation states of water and its effect on the curing reaction were examined by placing a water layer between an amorphous alumina surface and a mixture of epoxy and amine components. This study used molecular dynamics simulations and density functional theory calculations. Before the reaction, water molecules strongly adsorbed onto the alumina surface, aggregating excess water. Some water diffused into the epoxy/amine mixture, accelerating the diffusion of unreacted substances. This led to faster reaction kinetics, particularly in proximity to the alumina surface. The adsorption of water molecules onto the alumina surface and the aggregation of excess water were similarly observed even after the curing process. Subsequently, the interaction between the alumina surface and various functional groups of the epoxy/amine mixture was evaluated before and after the reaction. Epoxy monomers had little interaction with the alumina surface before the reaction, whereas hydroxy groups formed by the ring-opening reaction of epoxy groups exhibited notable interaction. Conversely, sulfonyl and amino groups in amine compounds formed hydrogen bonds with OH groups on the alumina surface before the reaction. However, after the reaction, amino groups weakened their interaction with the alumina OH groups as they transformed from primary to tertiary during the curing reaction. Both epoxy and amine monomers/fragments similarly interacted with water molecules, both before and after the reaction. The insights gained from this study are expected to contribute to a better understanding of the impact of moisture absorption on the application of epoxy resins.

DOI： 10.1021/acs.langmuir.4c01081

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：ACS Applied Energy Materials  

In this study, a comprehensive theoretical analysis was undertaken to elucidate the remarkably efficient conversion of CO2 into HCOO- employing a coordination polymer featuring Pb-S bonds, namely [Pb(tadt)]n (where tadt stands for 1,3,4-thiadiazole-2,5-dithiolate), referred to as KGF-9. The catalytic activity of this visible-light responsive solid photocatalyst has been carefully compared with that of PbS, a typical compound that also contains the Pb-S bond. The former shows a very high catalytic activity, while the latter shows almost no activity. The photoreduction process of CO2 on the KGF-9 surface was analyzed in detail using periodic density functional theory calculations. The reduced catalyst surface was modeled as a hydrogenated surface. The reaction at the active center of a formate dehydrogenase provides an interesting contrast, suggesting that the S-H group plays an important role in the conversion of CO2 to HCOO-. However, the S-H group on the reduced PbS surface does not facilitate the conversion to the same extent as KGF-9. This is because the electrons supplied to CO2 on the PbS surface come from deep within the solid, whereas on KGF-9, they come from the top surface. This difference is due to differences in the electronic structure of the S-H bond, band gap, and valence band maximum position between the two surfaces, accounting for the marked difference in their catalytic activity. These insights are consistent with experimental and computational results on the thermodynamic and kinetic characteristics of the CO2 reduction reaction of KGF-9 and PbS, and provide guidance for the design of CO2 photoreduction catalysts.

DOI： 10.1021/acsaem.4c00408

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Langmuir  

A better understanding of the aggregation states of adhesive molecules in the interfacial region with an adherend is crucial for controlling the adhesion strength and is of great inherent academic interest. The adhesion mechanism has been described through four theories: adsorption, mechanical, diffusion, and electronic. While interfacial characterization techniques have been developed to validate the aforementioned theories, that related to the electronic theory has not yet been thoroughly studied. We here directly detected the electronic interaction between a commonly used thermosetting adhesive, cured epoxy of diglycidyl ether of bisphenol A (DGEBA) and 4,4′-diaminodiphenylmethane (DDM), and copper (Cu). This study used a combination of density functional theory (DFT) calculations and femtosecond transient absorption spectroscopic (TAS) measurements as this epoxy adhesive-Cu pairing is extensively used in electronic device packaging. The DFT calculations predicted that π electrons in a DDM molecule adsorbed onto the Cu surface flowed out onto the Cu surface, resulting in a positive charge on the DDM. TAS measurements for the Cu/epoxy multilayer film, a model sample containing many metal/adhesive interfaces, revealed that the electronic states of excited DDM moieties at the Cu interface were different from those in the bulk region. These results were in good accordance with the prediction by DFT calculations. Thus, it can be concluded that TAS is applicable to characterize the electronic interaction of adhesives with metal adherends in a nondestructive manner.

DOI： 10.1021/acs.langmuir.4c00711

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Langmuir  

In the development of adhesives, an understanding of the fracture behavior of the bonded joints is inevitable. Two typical failure modes are known: adhesive failure and cohesive failure. However, a molecular understanding of the cohesive failure process is not as advanced as that of the adhesive failure process. In this study, research was developed to establish a molecular understanding of cohesive failure using the example of a system in which epoxy resin is bonded to a hydroxyl-terminated self-assembled monolayer (SAM) surface. Adhesive failure was modeled as a process in which an epoxy molecule is pulled away from the SAM surface. Cohesive failure, on the other hand, was modeled as the process of an epoxy molecule separating from another epoxy molecule on the SAM surface or breaking of a covalent bond within the epoxy resin. The results of the simulations based on the models described above showed that the results of the calculations using the model of cohesive failure based on the breakdown of intermolecular interactions agreed well with the experimental results in the literature. Therefore, it was suggested that the cohesive failure of epoxy resin adhesives is most likely due to the breakdown of intermolecular interactions between adhesive molecules. We further analyzed the interactions at the adhesive failure and cohesive failure interfaces and found that the interactions at the cohesive failure interface are mainly accounted for by dispersion forces, whereas the interactions at the adhesive failure interface involve not only dispersion forces but also various chemical interactions, including hydrogen bonds. The selectivity between adhesive failure and cohesive failure was explained by the fact that varying the functional group density affected the chemical interactions but not the dispersion forces.

DOI： 10.1021/acs.langmuir.3c04015

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Graphene and other two-dimensional (2D) materials can be used to create electronic and optoelectronic devices. However, their development has been limited by the lack of effective large-area transfer processes. Here we report a transfer method that uses functional tapes with adhesive forces controlled by ultraviolet light. The adhesion of the tape is optimized for the transfer of monolayer graphene, providing a yield of over 99%. Once detached from the growth substrate, the graphene/tape stack enables easy transfer of graphene to the desired target substrate. The method can be used to transfer other 2D materials, including bilayer graphene, transition metal dichalcogenides, hexagonal boron nitride and stacked heterostructures. The solvent-free nature of the final release step facilitates transfer to various target substrates including flexible polymers, paper and three-dimensional surfaces. The tape/2D material stacks can also be cut into desired sizes and shapes, allowing site-selective device fabrication with reduced loss of 2D materials.

DOI： 10.1038/s41928-024-01121-3

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Other Link： https://www.nature.com/articles/s41928-024-01121-3

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Langmuir  

The adhesion mechanism of epoxy resin to the γ-alumina (110) surface was investigated using first-principles density functional theory (DFT). Aluminum materials are lightweight and are used in a wide range of industrial fields. Its surface is oxidized to alumina, and the stable surface is known as the γ-alumina (110) surface. The coverage of hydroxy groups by chemisorbed water molecules on this surface varied depending on the pretreatment temperature. In this study, we investigated the adhesive interactions of epoxy resin on four alumina surfaces with different densities of surface hydroxy groups (0, 3, 6, and 9 OH/nm2) and have discussed their effects. At each interface, the energy curves of the vertically displaced epoxy resin were calculated and the adhesive forces were estimated by differentiating these curves. As the coverage of the surface hydroxy groups increased from 0 to 6 OH/nm2, the adhesive strength gradually decreased. However, the adhesive strength at 9 OH/nm2 was relatively large and almost equal to that at 3 OH/nm2. This inverse volcano-type behavior was analyzed via the decomposition of adhesive forces and the crystal orbital Hamilton population (COHP). The decomposition of adhesive forces into DFT and dispersion components revealed that the inverse volcano-type behavior is derived from the DFT component, and the interfacial interactions owing to the DFT component are accompanied by charge transfer. These were investigated using a COHP analysis, which revealed that this behavior was caused by changes in the activity of the aluminum atoms on the surface and surface reconstruction by chemisorbed water molecules. It is noteworthy that the adhesive strength for 9 OH/nm2 was only 6.9% lower than that for 0 OH/nm2 wherein the chemisorbed water molecules were completely removed from the surface. These results are expected to provide a guideline for the adhesion of epoxy resin to aluminum materials.

DOI： 10.1021/acs.langmuir.3c02883

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Journal of Organic Chemistry  

Face-to-face stacking of aromatic compounds leads to stacked antiaromaticity, while that of antiaromatic compounds leads to stacked aromaticity. This is a prediction with a long history; in the late 2000s, the prediction was confirmed by high-precision quantum chemical calculations, and finally, in 2016, a π-conjugated system with stacked aromaticity was synthesized. Several variations have since been reported, but essentially, they are all the same molecule. To realize stacked aromaticity in a completely new and different molecular system and to trigger an extension of the concept of stacked aromaticity, it is important to understand the origin of stacked aromaticity. The Hückel method, which has been successful in giving qualitatively correct results for π-conjugated systems despite its bold assumptions, is well suited for the analysis of stacked aromaticity. We use this method to model the face-to-face stacking systems of benzene and cyclobutadiene molecules and discuss their stacked antiaromaticity and stacked aromaticity on the basis of their π-electron energies. By further developing the discussion, we search for clues to realize stacked aromaticity in synthesizable molecular systems.

DOI： 10.1021/acs.joc.3c01167

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Repository Public URL： https://hdl.handle.net/2324/7236797

Language：English   Publisher：American Chemical Society (ACS)  

Face-to-face stacking of aromatic compounds leads to stacked antiaromaticity, while that of antiaromatic compounds leads to stacked aromaticity. This is a prediction with a long history; in the late 2000s, the prediction was confirmed by high-precision quantum chemical calculations, and finally, in 2016, a π-conjugated system with stacked aromaticity was synthesized. Several variations have since been reported, but essentially, they are all the same molecule. To realize stacked aromaticity in a completely new and different molecular system and to trigger an extension of the concept of stacked aromaticity, it is important to understand the origin of stacked aromaticity. The Hückel method, which has been successful in giving qualitatively correct results for π-conjugated systems despite its bold assumptions, is well suited for the analysis of stacked aromaticity. We use this method to model the face-to-face stacking systems of benzene and cyclobutadiene molecules and discuss their stacked antiaromaticity and stacked aromaticity on the basis of their π-electron energies. By further developing the discussion, we search for clues to realize stacked aromaticity in synthesizable molecular systems.

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

This paper details the use of computational and informatics methods to design metal nanocluster catalysts for efficient ammonia synthesis. Three main problems are tackled: defining a measure of catalytic activity, choosing the best candidate from a large number of possibilities, and identifying the thermodynamically stable cluster catalyst structure. First-principles calculations, Bayesian optimization, and particle swarm optimization are used to obtain a Ti8 nanocluster as a catalyst candidate. The N2 adsorption structure on Ti8 indicates substantial activation of the N2 molecule, while the NH3 adsorption structure suggests that NH3 is likely to undergo easy desorption. The study also reveals several cluster catalyst candidates that break the general trade-off that surfaces that strongly adsorb reactants also strongly adsorb products.

DOI： 10.1021/acsomega.3c03456

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physical Chemistry A  

Recent studies have theoretically and experimentally demonstrated that antiaromatic molecules with 4n πelectrons exhibit stacked aromaticity according to π-πstacking when arranged in a face-to-face manner. However, the mechanism of its occurrence has not been clearly studied. In this study, we investigated the mechanism of stacked aromaticity using cyclobutadiene. When the antiaromatic molecules are stacked in a face-to-face manner, the orbital interactions between the degenerate singly occupied molecular orbitals (SOMOs) of the monomer unit cause a larger energy gap between the degenerate highest-occupied molecular orbitals (HOMOs) and the lowest-unoccupied molecular orbitals (LUMOs) of the dimer. However, the antiaromatic molecules are more stable in less symmetric conformations, mainly because of pseudo-Jahn-Teller distortions. In the case of cyclobutadiene, the two SOMOs of the monomer unit split into HOMO and LUMO because of the bond alternation. When the molecules are stacked in a face-to-face manner, the HOMO-LUMO gap of the dimer is smaller than that of the monomer due to the interactions between the HOMOs and LUMOs of the two monomer units. When the monomer units are within a specific distance of each other, the HOMO and LUMO of the dimer, which correspond to antibonding and bonding between the units, respectively, are interchanged. This alternation of molecular orbitals may result in an increase in the bond strength between the monomer units, exhibiting stacked aromaticity. We demonstrated that it is possible to control the distance exhibited by stacked aromaticity by engineering the HOMO-LUMO gap of the monomer units.

DOI： 10.1021/acs.jpca.3c00360

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：ChemCatChem  

The nonoxidative coupling of methane has attracted much attention because it yields two useful products: ethane and hydrogen. However, low conversion at low temperatures and carbon deposition at high temperatures are considered problematic. In this Concept article, a solution to these problems is presented. On the basis of the reaction enthalpy for the initial C−H bond cleavage of methane and the energy difference between C1 species (CH3 and CH) on the catalyst surface, a catalyst search guideline is provided to suppress carbon deposition while keeping the conversion rate as high as possible. Alloys are considered catalyst candidates. Few materials satisfy both of these requirements simultaneously; however, several alloys, including MgPt, are shown to be promising. The results of validation experiments for the catalytic performance of MgPt are also discussed.

DOI： 10.1002/cctc.202201488

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

Catalytic reactions of mixed substrates sometimes behave differently from those of individual substrates. For example, the hydrogenation of propylbenzene over Rh/SiO2 proceeds 120% faster in the presence of toluene. Such an acceleration effect does not agree with the well-accepted Langmuir-Hinshelwood reaction model. In this paper, we examined its mechanism experimentally and computationally. The hydrogenation experiment of vaporised aromatics confirmed that the acceleration was specific to the liquid phase with the isopropanol solvent. Direct adsorption measurements revealed that toluene adsorption synchronises with propylbenzene adsorption. Density functional theory calculations confirmed the associates of toluene and propylbenzene on the catalyst surface in the polar environment. The formation of associates increased the adsorption energy of toluene and decreased that of propylbenzene. Lowered adsorption energy reduces the activation barrier for catalytic reaction and intensifies the reaction rate beyond the Langmuir-Hinshelwood model prediction.

DOI： 10.1039/d3re00032j

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Language：Japanese   Publisher：The Japan Society of Vacuum and Surface Science  

<p>Molecules can be regarded as a kind of graph with atoms as vertices and bonds as edges. Therefore, it is possible to discuss the physical properties and reactivity of various molecules by applying the ideas of graph theory and network theory. Such research has flourished in the field of chemical graph theory. In this manuscript, it is shown that the lowest occupied molecular orbital (LOMO) can be interpreted graph theoretically as eigenvector centrality. This is one measure of centrality that characterizes the importance or influence of a node on a network. As such, LOMO coefficient can be regarded as a manifestation of centrality in an aggregate of atoms, indicating which atom plays the most important role in that aggregate or has the greatest influence on the atom network. Using such properties of LOMO, an analysis of the network of metal atoms in metal clusters is performed. The predictability of the binding energies of the constituent atoms of the metal clusters using LOMO coefficients is discussed.</p>

DOI： 10.1380/vss.66.158

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Small  

To increase chemical reaction rates, general solutions include increasing the concentration/temperature and introducing catalysts. In this study, the rate constant of an electrophilic metal coordination reaction is accelerated 23-fold on the surface of layered aluminosilicate (LAS), where the reaction substrate (ligand molecule) induces dielectric polarization owing to the polar and anionic surface. According to the Arrhenius plot, the frequency factor (A) is increased by almost three orders of magnitude on the surface. This leads to the conclusion that the collision efficiency between the ligands and metal ions is enhanced on the surface due to the dielectric polarization. This is surprising because one side of the ligand is obscured by the surface, so the collision efficiency is expected to be decreased. This unique method to accelerate the chemical reaction is expected to expand the range of utilization of LASs, which are chemically inert, abundant, and environmentally friendly. The concept is also applicable to other metal oxides which have polar surfaces, which will be useful for various chemical reactions in the future.

DOI： 10.1002/smll.202205857

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Hole-collecting monolayers have drawn attention in perovskite solar cell research due to their ease of processing, high performance, and good durability. Since molecules in the hole-collecting monolayer are typically composed of functionalized π-conjugated structures, hole extraction is expected to be more efficient when the π-cores are oriented face-on with respect to the adjacent surfaces. However, strategies for reliably controlling the molecular orientation in monolayers remain elusive. In this work, multiple phosphonic acid anchoring groups were used to control the molecular orientation of a series of triazatruxene derivatives chemisorbed on a transparent conducting oxide electrode surface. Using infrared reflection absorption spectroscopy and metastable atom electron spectroscopy, we found that multipodal derivatives align face-on to the electrode surface, while the monopodal counterpart adopts a more tilted configuration. The face-on orientation was found to facilitate hole extraction, leading to inverted perovskite solar cells with enhanced stability and high-power conversion efficiencies up to 23.0%.

DOI： 10.1021/jacs.3c00805

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Chemical Communications  

The low-temperature activation of methane is highly important as a reaction that can dissociate the strongest C-H bond and convert it into useful compounds. This study demonstrated that supported platinum oxide was found to activate methane near room temperature and selectively afford methanol in the presence of oxygen.

DOI： 10.1039/d2cc05351a

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Journal of the American Chemical Society  

A coordination-induced trigger for catalytic activity is proposed on an N-heterocyclic carbene (NHC)-decorated ceria catalyst incorporating Cr and Rh (ICy-r-Cr0.19Rh0.06CeOz). ICy-r-Cr0.19Rh0.06CeOz was prepared by grafting 1,3-dicyclohexylimidazol-2-ylidene (ICy) onto H2-reduced Cr0.19Rh0.06CeOz (r-Cr0.19Rh0.06CeOz) surfaces, which went on to exhibit substantial catalytic activity for the 1,4-arylation of cyclohexenone with phenylboronic acid, whereas r-Cr0.19Rh0.06CeOz without ICy was inactive. FT-IR, Rh K-edge XAFS, XPS, and photoluminescence spectroscopy showed that the ICy carbene-coordinated Rh nanoclusters were the key active species. The coordination-induced trigger for catalytic activity on the ICy-bearing Rh nanoclusters could not be attributed to electronic donation from ICy to the Rh nanoclusters. DFT calculations suggested that ICy controlled the adsorption sites of the phenyl group on the Rh nanocluster to promote the C-C bond formation of the phenyl group and cyclohexenone.

DOI： 10.1021/jacs.2c07290

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Numerous biomimetic molecular catalysts inspired by methane monooxygenases (MMOs) that utilize iron or copper-oxo species as key intermediates have been developed. However, the catalytic methane oxidation activities of biomimetic molecule-based catalysts are still much lower than those of MMOs. Herein, we report that the close stacking of a μ-nitrido-bridged iron phthalocyanine dimer onto a graphite surface is effective in achieving high catalytic methane oxidation activity. The activity is almost 50 times higher than that of other potent molecule-based methane oxidation catalysts and comparable to those of certain MMOs, in an aqueous solution containing H2O2. It was demonstrated that the graphite-supported μ-nitrido-bridged iron phthalocyanine dimer oxidized methane, even at room temperature. Electrochemical investigation and density functional theory calculations suggested that the stacking of the catalyst onto graphite induced partial charge transfer from the reactive oxo species of the μ-nitrido-bridged iron phthalocyanine dimer and significantly lowered the singly occupied molecular orbital level, thereby facilitating electron transfer from methane to the catalyst in the proton-coupled electron-transfer process. The cofacially stacked structure is advantageous for stable adhesion of the catalyst molecule on the graphite surface in the oxidative reaction condition and for preventing decreases in the oxo-basicity and generation rate of the terminal iron-oxo species. We also demonstrated that the graphite-supported catalyst exhibited appreciably enhanced activity under photoirradiation owing to the photothermal effect.

DOI： 10.1021/jacsau.2c00618

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DOI： 10.26434/CHEMRXIV-2023-JM387

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Language：Japanese   Publisher：Japan Society for Molecular Science  

<p>The desire to understand chemical reactions is probably a sentiment shared by researchers involved in molecular science. In this account, methods for qualitatively discussing chemical reactions using Walsh diagrams, orbital correlation diagrams, and orbital interaction diagrams are described. These methods are suitable for the analysis of reactions of organic molecules and organometallic complexes. These methods have been further applied by the author to the analysis of surface reactions. This account illustrates how the Walsh diagrams, orbital correlation diagrams, and orbital interaction diagrams can be applied to surface reaction analysis, using as examples the C-H bond activation of methane on the surface of IrO₂, H-H and C-C bond formation processes on the surface of a typical Pt catalyst. Based on the findings obtained by applying these methods to surface systems, hints for designing new catalysts are also given.</p>

DOI： 10.3175/molsci.17.a0124

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Binary alloy catalysts have the potential to exhibit higher activity than monometallic catalysts in nitrogen activation reactions. However, owing to the multiple possible combinations of metal elements constituting binary alloys, an exhaustive search for the optimal combination is difficult. In this study, we searched for the optimal binary alloy catalyst for nitrogen activation reactions using a combination of Bayesian optimization and density functional theory calculations. The optimal alloy catalyst proposed by Bayesian optimization had a surface energy of ∼0.2 eV/Å2and resulted in a low reaction heat for the dissociation of the NN bond. We demonstrated that the search for such binary alloy catalysts using Bayesian optimization is more efficient than random search.

DOI： 10.1021/acsomega.2c05988

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Physical Chemistry Chemical Physics  

Shear adhesive strength between epoxy resin and copper surfaces: a density functional theory study

DOI： 10.1039/D2CP03354B

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The adsorption sites of the top and hollow on the close-packed surfaces of transition metals are well known. In this paper, which site is more preferred for the adsorption of atoms and molecular fragments on the metal surfaces is discussed based on the topology of the adsorption geometry. For this purpose, the method of moments for the electronic density of states is applied to the surface. Adsorption at the hollow site generates a triangular topology, leading to a more negative value of the third moment (μ_3) than that at the top site, which generates no triangular topology. When the difference in energy between the two adsorption sites is plotted against the band filling of the metal surface, a characteristic node at around the intermediate band filling can be found. This is a signature that the energy difference curve is controlled by μ_3. Roughly speaking, the hollow-site adsorption, which has a more negative μ_3 value, takes precedence at low band fillings, while the top site adsorption, which has a less negative μ_3 value, takes precedence at high band fillings. One can conclude that an adsorption structure with more three-membered rings on a surface is more stable at low electron counts whereas that with less three-membered rings is more stable at high electron counts. However, if the strength of the metal–adsorbate bond is significantly greater than that of the metal–metal bond, the effect of the second moment (μ_2) on the energy difference curve cannot be neglected. The hollow-site adsorption leads to a larger value of μ_2 due to the topological feature of a larger coordination number around the adsorbate atom. As a result, the hollow-site adsorption is preferred over the top site at any band filling.

DOI： 10.1021/acs.jpcc.2c04656

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Repository Public URL： https://hdl.handle.net/2324/6795486

Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

21 types of modifiers are screened for palladium catalysed semi-hydrogenation of alkynes with varying catalyst type, reaction time, and target substrate using an automated flow reactor system.

DOI： 10.1039/d2re00147k

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Adhesive strength is known to change significantly depending on the direction of the force applied. In this study, the peel and tensile adhesive forces between the hydroxylated silica (001) surface and epoxy resin are estimated based on quantum chemical calculations. Here, density functional theory (DFT) with dispersion correction is used. In the peel process, the epoxy resin is pulled off from the terminal part, while in the tensile process, the entire epoxy resin is pulled off vertically. As a result of these calculations, the maximum adhesive force in the peel process is decreased to be about 40% of that in the tensile process. The adhesion force−displacement curve for the peeling process shows two characteristic peaks corresponding to the process where the adhesive molecule horizontally oriented to the surface shifts to a vertical orientation to the surface and the process where the vertical adhesive molecule is dissociated from the surface. Force decomposition analysis is performed to further understand the peel adhesion force; the contribution of the dispersion force is found to be slightly larger than that of the DFT force. This feature is common to the tensile process as well. Each force in the peel process is about 40% smaller than the corresponding force in the tensile process.

DOI： 10.1021/acsomega.2c01544

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Journal of Chemical Physics  

In this study, we employ the Sachs graph theory to formulate the conduction properties of a single-molecular junction consisting of a molecule in which one carbon atom of an alternant hydrocarbon is replaced with a heteroatom. The derived formula includes odd and even powers of the adjacency matrix, unlike the graph of the parental structure. These powers correspond to odd- and even-length walks. Furthermore, because the heteroatom is represented as a self-loop of unit length in the graph, an odd number of passes of the self-loop will change the parity of the length of the walk. To confirm the aforementioned effects of heteroatoms on conduction in an actual sample, the conduction behavior of meta-connected molecular junctions consisting of a heterocyclic six-membered ring, whose conductive properties have already been experimentally determined, was analyzed based on the enumerated number of walks.

DOI： 10.1063/5.0083486

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

The catalytic function of metal nanoclusters has attracted much attention because of their specific activity and selectivity. The structures of metal clusters are very diverse, especially when adsorbates are adsorbed on them. This is an obstacle when approaching metal nanocluster catalysts with computational chemistry. In this manuscript, a prescription for this problem is presented. With metal nanoclusters catalyzing reactions involving hydrogen in mind, a comprehensive, systematic, and efficient search for stable structures of metal nanoclusters with an adsorbed hydrogen atom is presented. This can be achieved through a good use of a supercomputer while using the particle swarm optimization algorithm and density functional theory together. In this attempt, three metallic elements, Fe, Ni, and Cu, are selected. When clustered, what kind of structure these elements form and how their affinity for hydrogen changes are detailed. Eventually, a path is presented to explore clusters that are actually useful as catalysts, using surface calculations as a reference.

DOI： 10.1007/s11244-021-01512-2

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Language：Japanese   Publisher：Society of Computer Chemistry, Japan  

<p>π-stacked single-molecule junctions stacked with π-conjugated molecules have the potential to be used as building blocks for single-molecule scale three-dimensional integrated circuits. In this study, we investigate the relationship between π-stacking distance and conductance in face-to-face π-stacked single-molecule junctions with benzene as the monomer unit using the non-equilibrium Green's function (NEGF), which combines the Hückel molecular orbital (HMO) and density functional theory (DFT) methods. As the π-stack distance between two benzene molecules decreases, the pseudo-<i>para</i> junction, which is insulating, turns conductive. Furthermore, it was found that the cause of this change can be explained by orbital interactions.</p>

DOI： 10.2477/jccj.2023-0002

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The presence of adsorbed water on hydrophilic solid surfaces should be taken into account, especially in humid environments. It significantly reduces the adhesive strength between the epoxy resin and the adherend surface. Here, the adhesion structure of interfacial water sandwiched between bisphenol A epoxy resin and a hydroxylated silica (001) surface is investigated with microsecond molecular dynamics simulations. Specifically, interfacial water layers with initial thicknesses of 7.5, 10, and 20 Å are modeled. The density curves of water and the diglycidyl ether of bisphenol A show that at room temperature, the surface of the silica with hydroxyl groups is completely covered with a thick layer of water. For water layers thinner than 10 Å, the density of epoxy resin on the silica surface increases when the system is heated and does not return to the original density when the system is cooled. Furthermore, calculation of the interaction energy revealed that the exclusion of water from the hydroxylated surface by epoxy resin during heating can contribute to the increase in the adhesive interaction between the epoxy resin and the silica surface with hydroxyl groups.

DOI： 10.1021/acs.langmuir.1c02653

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Clarification of adhesive interactions in semiconductor packages can improve reliability of power electronics. In this study, the adhesion interfaces between the epoxy molding compound and Cu-based lead frames were analyzed using the density functional theory. A resin fragment was prepared based on the polymer framework formed in the curing reaction of epoxy cresol novolac (ECN) and phenol novolac (PN), which are typical molding materials. The resin fragment was optimized on the surfaces of Cu and Cu2O. We calculated the charge density differences for adhesion structures and discussed the origin of adhesive interactions. The ECN-PN fragment's adhesion to the Cu surface relied mainly on dispersion forces, whereas in the case of Cu2O, the resin bonded chemically to the surface via (1) σ-bonds formed between the ECN-PN's OH group oxygen and coordinatively unsaturated copper (CuCUS) and (2) hydrogen bonds between resin's OH groups and coordinatively unsaturated oxygen (OCUS) located close to to CuCUS, resulting in a stable adhesive structure. The energy required to detach the resin fragment from the optimized structure was determined using the nudged elastic band method in each model of the adhesive interface. Morse potential curve was used to approximate the obtained energy, and the energy differentiation by detachment distance yielded the theoretical adhesive force. The maximum adhesive stress was 1.6 and 2.2 GPa for the Cu and Cu2O surfaces, respectively. The extent to which the ECN-PN fragment bonded to the Cu2O surface stabilized was 0.5 eV higher than in the case of the Cu surface.

DOI： 10.1021/acsomega.1c05914

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<title>Abstract</title>Understanding chemical effects on the plasmonic properties of a metal nanomaterial due to the surface molecules on that metal is of great importance in the field of plasmonics and these effects have yet to be completely elucidated. Here, we report mechanisms of the chemically induced change in the electronic state at the metal-ligand interface of silver nanoparticles due to the ligand molecules, and the effect of this change on the plasmonic properties of those nanoparticles. It was found that changes in the electron density of states at the metal-ligand interface cause alterations in the induced and permanent dipole moments, and eventually to the permittivity at the interface, when the wave function near the Fermi level is localized at the interface. These alterations play a key role in determining the plasmonic properties of silver nanoparticles. The present findings provide a more precise understanding of the interconnection between the electronic states at the metal-organic interface and the plasmonic properties of the metal.

DOI： 10.1038/s43246-021-00159-6

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Hexagonal boron nitride (h-BN) is a material with excellent thermal conductivity and electrical insulation, used as an additive to various matrices. To increase the affinity of h-BN to them, hydrogen bonds should be formed at the interface. In reality, however, they are not formed; the N atoms are not capable of accepting hydrogen bonds due to the delocalization of their lone pair electrons over the B-N π bonds. To make it form hydrogen bonds, one may need to break the planarity of h-BN so that the orbital overlap in the B-N π bonds can be reduced. This idea is verified with first-principles calculations on the adsorption of a water molecule on hypothetical h-BN surfaces, the planarity of which is broken. One can do it in silico but not in vitro. BN nanotubes (BNNTs) are considered as a more realistic BN surface with nonplanarity. The hydrogen bond is shown to become stronger as the curvature of the tube increases. On the contrary, the strength of the dispersion force acting at the interface becomes weaker. In water adsorption, these two interactions are in competition with each other. However, in epoxy adhesion, the interaction due to dispersion forces is overwhelmingly stronger than that due to hydrogen bonding. The smaller the curvature of the surface, the smaller the distance between more atoms at the interface; thus, the interaction due to dispersion forces maximized.

DOI： 10.1021/acs.langmuir.1c01935

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There is a need for a catalyst that can directly convert methane into useful substances. The use of Ni as a catalyst for the steam reforming of methane has led us to look at Ni nanoclusters as potential candidates for the direct conversion of methane. Fe, Co, Cu, and Zn nanoclusters are also focused on. How the type of C1fragments (CH4, CH3, CH2, CH, and C) stabilized by the metal nanoclusters as adsorbed species varies with metal species is theoretically investigated. The particle swarm optimization algorithm, which is based on swarm intelligence, as well as density functional theory, is used for this calculation. The Ni nanoclusters are found to preferentially adsorb C as a stable species, the Fe and the Co nanoclusters both CH and CH3, and the Cu nanoclusters CH3; the Zn nanoclusters are found not to chemisorb any C1fragment. The methane activation capacity can be ranked in the order of Ni > Fe > Co > Cu > Zn. The highest methane activation capacity of Ni is due to the strongest covalent nature of the interaction between Ni and the adsorbed species. The ionicity of the bond between Fe and the adsorbed species is higher than that between Co and the adsorbed species, while the covalent nature of the bonds is comparable for both. The weak methane activation ability of Cu compared to Fe, Co, and Ni is found to be due to the fact that both the covalent and ionic bond strengths between Cu and the adsorbed species are weak. Zn and the adsorbed species form neither ionic nor covalent bonds. These results indicate that the Fe and the Co nanoclusters as well as the Ni may lead to the over-oxidation of methane, whereas the Zn nanoclusters cannot activate methane in the first place; therefore, their application to direct methane conversion catalysts is unlikely. Since the Cu nanoclusters do not adsorb C and CH as stable species, but CH3stably, the Cu nanoclusters are expected to work as a catalyst for the direct conversion of methane.

DOI： 10.1039/d1cp00345c

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<div><div><div><p>Herein, we report that the close-stacking of a double-decker-type dinuclear iron phthalocyanine complex on a graphite surface is effective for achieving high methane oxidation activity, comparable to those of certain MMOs, in an aqueous solution. </p></div></div></div>

DOI： 10.26434/chemrxiv.14728860.v1

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Although the C-H bond of methane is very strong, it can be easily dissociated on the (110) surface of β-PtO2. This is because a very stable Pt-C bond is formed between the coordinatively unsaturated Pt atom and CH3 on the surface. Owing to the stable nature of the Pt-C bond, CH3 is strongly bound to the surface. When it comes to methanol synthesis from methane, the Pt-C bond has to be cleaved to form a C-O bond during the reaction process. However, this is unlikely to occur on the β-PtO2 surface: The activation energy of the process is calculated to be so large as 47.9 kcal/mol. If the surface can be modified in such a way that the ability for the C-H bond activation is maintained but the Pt-C bond is weakened, a catalyst combining the functions of C-H bond cleavage and C-O bond formation can be created. For this purpose, analyzing the orbital interactions on the surface is found to be very useful, resulting in a prediction that the Pt-C bond can be weakened by replacing the O atom trans to the C atom with a N atom. This would be a sort of process to make β-PtO2 a mixed anion compound. Density functional theory simulations of catalytic reactions on the β-PtO2 surface show that the activation energy of the rate-limiting step of methanol synthesis can be reduced to 27.7 kcal/mol by doping the surface with N.

DOI： 10.1021/acsomega.1c01476

Language：Others   Publishing type：Research paper (scientific journal)  

In this study, the adhesive interaction between gold and epoxy resin is theoretically investigated. These materials make up crucial components of a wide range of electronic devices. The objectives of the study are (1) to elucidate the bonding mechanism between epoxy resin and a realistic gold surface, and (2) to obtain a device-design guideline for superior adhesion, thus reducing the bonding breakage that may potentially cause device failure. Die pad surfaces used in chip attachment methods for microelectronics are usually fabricated using an electrolytic plating technique. This technique involves ionic gold solutions like K[Au(CN)2]. The combined theoretical and experimental studies previously carried out by the authors have revealed that the CN- counteranion of the gold cation has a high affinity for gold and is likely to remain on the realistic gold surface generated by plating. However, the cyano group content on the surface of the plated gold is still unknown. Therefore, gold surfaces embedded with cyano groups with various coverages are constructed. The effect of the varying coverage of the cyano groups on the adhesion strength is inspected using first-principles density functional theory calculations. As the number of cyano groups on the surface increases, the direct interaction between the gold surface and the epoxy resin is hindered, but the hydroxy and amino groups in the epoxy resin and hardener form more hydrogen bonds with the cyano groups adsorbed on the surface. It is found that the surface with intermediate cyano coverage (about 33%) yields the highest adhesive strength.

DOI： 10.1021/acs.langmuir.1c00285

Language：Others   Publishing type：Research paper (scientific journal)  

Transition metals with a higher-lying Fermi level (a smaller work function) such as Ti can easily activate nitrogen and hydrogen reductively to form nitrides and hydrides on its surface, but ammonia synthesis does not occur. H atoms adsorbed on the surface of Ti have a negative charge, identified as H-. In order for an N-H bond to form on the Ti surface, the hydrogen atom must dump its extra charge into empty levels above the Fermi level of the surface. The high Fermi level of Ti impedes this process. Although the Fermi level of TiH2 is as high as that of Ti, TiH2 has been reported to act as an ammonia synthesis catalyst under Haber-Bosch conditions, characterized by its robust catalytic activity. To clarify the electronic aspects of the catalytic activity of TiH2, a theoretical study on the surface of TiH2 using density functional theory calculations is carried out. The negative charge of the H atom about to bond to the N atom in ammonia synthesis on the TiH2 surface is as large as that of the H atom on the Ti surface. However, since surface hydrides of TiH2 present nearby interact with the H atom in an antibonding manner and the electronic state of the H atom is destabilized, the energy required for dumping the extra charge upon the formation of the N-H bond is substantially reduced. Nitrides in the surface of TiH2, the formation of which during the reaction has been suggested experimentally, make the amount of the charge of the H atom smaller by oxidizing the Ti atoms nearby. This further reduces the activation barrier associated with the N-H bond formation, leading to a good agreement between theory and experiment.

DOI： 10.1021/acs.jpcc.0c10907

Language：Others   Publishing type：Research paper (scientific journal)  

In this paper, the nature of the lowest-energy electrons is detailed. The orbital occupied by such electrons can be termed the lowest occupied molecular orbital (LOMO). There is a good correspondence between the Hückel method in chemistry and graph theory in mathematics; the molecular orbital, which chemists view as the distribution of an electron with a specific energy, is to mathematicians an algebraic entity, an eigenvector. The mathematical counterpart of LOMO is known as eigenvector centrality, a centrality measure characterizing nodes in networks. It may be instrumental in solving some problems in chemistry, and also it has implications for the challenge facing humanity today. This paper starts with a demonstration of the transmission of infectious disease in social networks, although it is unusual for a chemistry paper but may be a suitable example for understanding what the centrality (LOMO) is all about. The converged distribution of infected patients on the network coincides with the distribution of the LOMO of a molecule that shares the same network structure or topology. This is because the mathematical structures behind graph theory and quantum mechanics are common. Furthermore, the LOMO coefficient can be regarded as a manifestation of the centrality of atoms in an atomic assembly, indicating which atom plays the most important role in the assembly or which one has the greatest influence on the network of these atoms. Therefore, it is proposed that one can predict the binding energy of a metal atom to its cluster based on its LOMO coefficient. A possible improvement of the descriptor using a more sophisticated centrality measure is also discussed.

DOI： 10.1021/acsomega.0c04913

Language：Others   Publishing type：Research paper (scientific journal)  

Epoxy resin adhesives are widely used for joining metal alloys in various industrial fields. To elucidate the adhesion mechanism microscopically, we investigated the interfacial interactions of epoxy resin with hydroxylated silica (0 0 1) and γ-alumina (0 0 1) surfaces using periodic density functional theory calculations as well as density of states (DOS) and crystal orbital Hamilton population (COHP) analyses. To better understand the interfacial interactions, we employed and analyzed water and benzene molecules as hydrophilic and hydrophobic adsorbates, respectively. Structural features and calculated adhesion energies reveal that these small adsorbates have a higher affinity for the γ-alumina surface than that for the silica surface, while a fragmentary model for the epoxy resin exhibits a strong interaction with the silica surface. This discrepancy suggests that the structural features of the hydroxylated silica surface dictate its affinity to a specific species. Partial DOS and COHP curves provide evidence for the presence of OH-πinteractions between the OH groups on the surfaces and the benzene rings of the epoxy resin fragments. The orbital interaction energies of the H-bonding and OH-πinteractions evaluated from the integrated COHP indicate that the OH-πinteraction is a nonnegligible origin of the adhesion interaction, even when polymers with hydrophobic benzene rings are adsorbed on hydroxylated surfaces.

DOI： 10.1021/acsomega.0c03798

Language：Others   Publishing type：Research paper (scientific journal)  

This is a longish, theoretical paper dealing with the molecular conductance of H2 and π-stacked ethylene. At different levels of theory ranging from single-determinant to multireference, from open boundary to periodic boundary, from semiempirical to ab initio, from Green's function theory to graph theory, and from localized atomic orbitals to plane waves, the molecular junctions of H2 and ethylene were calculated and analyzed. It was found based on simplistic models as well as sophisticated, higher-level simulations that moderately stretching the H-H bond or compressing the ethylene π-stack increases not only the diradical character of these systems but also their conductance in the range where these two parameters show a positive correlation. Negative correlation is also observed under extreme stretching of the H-H bond or extreme compression of the ethylene πstack. Challenges in experimental realization of the proposed molecular junctions and verification of the theoretical predictions were discussed. Digressions seen here and there in this paper may be informative and taken as a demonstration of theoreticians' way of applying insight gained from a simplistic model to a realistic system.

DOI： 10.1021/acs.jpcc.0c06198

Language：Others   Publishing type：Research paper (scientific journal)  

In this paper, an orbital correlation diagram is proposed for the purpose of understanding and predicting a surface reaction. To deal with the electronic structure of a surface, one may need to rely on a cluster model or a surface slab model. Band (crystal) orbitals calculated at the Σ point in the reciprocal space for the unit cell of the slab model with periodicity are found helpful for the construction of the correlation diagram. By using the diagram thus established, the C-H bond activation reaction of methane on an IrO2 surface is investigated. The energy level of the dz2 orbital of a coordinatively unsaturated Ir atom in the surface is found to be important for determining the activation barrier of the reaction. The activation energy can be reduced by lowering the energy level of the dz2 orbital. Conversely, a rise in the energy of the dz2 orbital leads to an increase in the activation barrier. To make the dz2 orbital energy change, the concept of mixed-anion compounds is adopted. The replacement of an oxide with a different anion allows one to tune the crystal field splitting of metal oxides. IrO2 doped with F as an axial ligand yields a lower-lying dz2 orbital level, while the orbital goes up in energy when IrO2 is doped with N. This trend is consistent with what is expected from the electronegativity of each dopant. A perfect inverse linear correlation is found between the activation energy of the reaction and the electronegativity. By changing the dopant, one may have control over the reactivity of IrO2.

DOI： 10.1021/acs.jpcc.0c04541

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Adhesion  

A die bonding interface consisting of a plated gold surface and conductive silver paste was investigated using two approaches: One was a computational method for the ideal interaction, while the other was an experimental technique to get some ideas about the actual bonding. A density functional theory (DFT) calculation was employed to clarify the adhesive interface between the Au (1 1 1) surface and bisphenol-A type epoxy resin which included dicyandiamide as a curing agent. The computational results showed that the cyano group involved in the hardener strongly binds to a gold atom on the ideal surface. This interaction is likely to be assigned to sigma-donation and pi-back donation. The predicted adhesive strength was 1.15 GPa. To evaluate the real adhesion strength, small-sized single lap-shear joints were fabricated and tensile shear tests were performed. The measured adhesive stress was about 27 MPa. The results of surface analytical methods indicated that the actual die pad surface was not smooth. Furthermore, an altered layer terminated with Au(CN)(x)was found on the top of the die pad. It was inferred that the mechanism of the actual die bonding interaction would be changed to the formation of hydrogen bonds. This could be a reason why reduced adhesive strength was observed.

DOI： 10.1080/00218464.2020.1807958

Web of Science

Scopus

Language：Others   Publishing type：Research paper (scientific journal)  

Work function is one of the most fundamental and important physical quantities in surface science. Materials with either lower work function or higher work function would find various applications, such as electronic devices and high-performance catalysts. However, it would be challenging to find a material with the optimal work function exploiting the all-search or random approach, whether it is based on an experimental or theoretical method. In this paper, we use the Bayesian optimization (BO) approach, which is one of the most powerful machine-learning tools for optimization, in order to effectively explore a candidate material with a higher or lower work function value out of hundreds of thousands of materials registered in a material database. We introduce a quick measure of the work function based on the depth of the Fermi level calculated from the first-principles computation for the crystalline bulk structure of a material. We call this the approximate work function, treating it as the objective function of our BO scheme. Since we do not need any time-consuming surface calculation with the slab model for the evaluation of the approximate work function, a quick search of a material with the highest or the lowest work function is achieved. As input variables for our BO implementation, we employ some bulk-specific properties of materials, which can be fetched from the database. The demonstration of our BO-based exploration of the database shows that materials with both low and high limits of the approximate work function can be discovered more efficiently in BO than a random exploration. The top 10 lowest work function materials thus found are in line with our chemical intuition in that all of them include either alkali or alkaline earth metal. On the other hand, we found the top 10 highest work function materials with amazement because they also include either alkali or alkaline earth metal and a lanthanide element.

DOI： 10.1021/acs.jpcc.0c01106

Language：English   Publishing type：Research paper (scientific journal)  

Work function is one of the most fundamental and important physical quantities in surface science. Materials with either lower work function or higher work function would find various applications, such as electronic devices and high-performance catalysts. However, it would be challenging to find a material with the optimal work function exploiting the all-search or random approach, whether it is based on an experimental or theoretical method. In this paper, we use the Bayesian optimization (BO) approach, which is one of the most powerful machine-learning tools for optimization, in order to effectively explore a candidate material with a higher or lower work function value out of hundreds of thousands of materials registered in a material database. We introduce a quick measure of the work function based on the depth of the Fermi level calculated from the first-principles computation for the crystalline bulk structure of a material. We call this the approximate work function, treating it as the objective function of our BO scheme. Since we do not need any time-consuming surface calculation with the slab model for the evaluation of the approximate work function, a quick search of a material with the highest or the lowest work function is achieved. As input variables for our BO implementation, we employ some bulk-specific properties of materials, which can be fetched from the database. The demonstration of our BO-based exploration of the database shows that materials with both low and high limits of the approximate work function can be discovered more efficiently in BO than a random exploration. The top 10 lowest work function materials thus found are in line with our chemical intuition in that all of them include either alkali or alkaline earth metal. On the other hand, we found the top 10 highest work function materials with amazement because they also include either alkali or alkaline earth metal and a lanthanide element.

DOI： 10.1021/acs.jpcc.0c01106

Language：Japanese   Publishing type：Research paper (scientific journal)  

Theoretical Study on the Relation between the Frontier Orbital and the Conductance in Aromatic Single-Molecular Parallel Circuits

DOI： 10.2477/jccj.2019-0038

Language：English   Publishing type：Research paper (scientific journal)  

In electronic devices, as the number of paths connecting source and drain electrodes increases, the conductance of the device will also increase. However, this is not always the case on the nanoscale. According to the current superposition law at work in the macroscopic electrical circuits, doubling the number of paths should double the conductance, but when such paths are examined on the basis of the frontier orbital theory for nanoscale electrical circuits, more complex scenarios arise. When the number of paths in a molecule is doubled, the conductance may get more than doubled, remain unchanged, or even be reduced. We propose a classification of conducting systems falling into each of these scenarios with the help of aromaticity. The present work involves a theoretical study using the nonequilibrium Green's function that shows that these varying outcomes are closely related to the presence or absence of aromatic rings. This work serves to characterize molecular conductance characteristics based on frontier orbital theory, orbital interactions, and a local transmission concept. Some discrete mathematical aspects of the relationship between atom connectivity and electron conductivity are also described.

DOI： 10.1021/acs.jpcc.9b08595

Language：Others   Publishing type：Research paper (scientific journal)  

In electronic devices, as the number of paths connecting source and drain electrodes increases, the conductance of the device will also increase. However, this is not always the case on the nanoscale. According to the current superposition law at work in the macroscopic electrical circuits, doubling the number of paths should double the conductance, but when such paths are examined on the basis of the frontier orbital theory for nanoscale electrical circuits, more complex scenarios arise. When the number of paths in a molecule is doubled, the conductance may get more than doubled, remain unchanged, or even be reduced. We propose a classification of conducting systems falling into each of these scenarios with the help of aromaticity. The present work involves a theoretical study using the nonequilibrium Green's function that shows that these varying outcomes are closely related to the presence or absence of aromatic rings. This work serves to characterize molecular conductance characteristics based on frontier orbital theory, orbital interactions, and a local transmission concept. Some discrete mathematical aspects of the relationship between atom connectivity and electron conductivity are also described.

DOI： 10.1021/acs.jpcc.9b08595

Language：Others   Publishing type：Research paper (scientific journal)  

Theoretical Study on the Electronic Structure of Heavy Alkali-Metal Suboxides
On the metal-rich side of the phase diagrams of the Rb-O, Cs-O, and Rb-Cs-O systems, one can find a variety of stoichiometries: for example, Rb O , Rb O, Cs O, Cs O, Cs O , RbCs O , and Rb Cs O . They may be termed heavy alkali-metal suboxides. The application of the standard electron-counting scheme to these compounds suggests the presence of surplus electrons. This motivated us to carry out a theoretical study using the first-principles density functional theory (DFT) method. The structures of these compounds are based on either a formally cationic Rb O or Cs O cluster. The analyses of the partial charge density just below the Fermi level and the electron localization function (ELF) have revealed that there exist surplus electrons in interstitial regions of all the investigated suboxides so that the excess positive charge of the cluster can be compensated. Density of states (DOS) calculations suggest that all of the compounds are metallic. Therefore, the suboxides listed above may be regarded as a new family of metallic electrides, where coreless electrons reside in interstitial spaces and provide a conduction channel. Except for the phases of Rb O and Cs O , the suboxide structures include both the cationic clusters and alkali-metal matrix. Several charge analyses indicate that the interstitial surplus-electron density can be assigned to the alkali-metal atoms in the metal matrix, leading to the possibility of the presence of negatively charged alkali-metal atoms, namely Rb (rubidide) and Cs (caeside) ions, a.k.a. alkalides. In Rb O, Rb , Rb , and Rb are found to coexist in the same crystal structure. Similarly, in Cs O, one can find the three types of Cs atoms. However, in Cs O, no Cs state is identified. In the Rb-Cs-O ternary suboxides, Rb takes a negatively charged anion state or neutral state, while all of the Cs atoms are found to be cationic because they get involved in the Cs O cluster and all the Rb atoms exist in interstitial sites. Orbital interactions between the clusters are analyzed to understand how the condensation of the clusters into the solid happens and how the electride nature ensues. These clusters are found to have some superatomic character. 9 2 6 4 7 11 3 11 3 7 11 3 9 2 11 3 9 2 11 3 6 7 4 11 3 - - - 0 + 0

DOI： 10.1021/acs.inorgchem.9b03046

Language：English   Publishing type：Research paper (scientific journal)  

When certain pairs of atoms in a π-conjugated molecule are connected with nanometer-scale source and drain electrodes, the remarkable quantum interference (QI) effect may arise. In this case, the electron transmission probability is significantly suppressed due to the QI effect. Tight-binding approaches, such as the Hückel molecular orbital (HMO) model, have revealed important features of this quantum phenomenon. However, important deviations from experiments and from more sophisticated calculations are known for a variety of cases. Here, we propose an extension of the HMO method to include non-nearest-neighbor interactions. Such long-range interactions (LRIs) are implemented in the HMO model in the form of a damping function that decays as the topological distance - the number of bonds separating two atoms - gets larger. The proposed model is further developed so that a geometric modification, i.e., the rotation around a single bond, can be taken into account. Our results show that LRI affects both the location of the antiresonance peak due to QI and the intensity of QI, even suppressing it in some cases. These results agree well with what was observed in a Density Functional based Tight-Binding (DFTB) study reported in the literature. These properties can be interpreted on the basis of a graph-theoretic path-counting model as well as the molecular orbital theory. In addition, the geometric LRI model is shown to reproduce the change of transmission as a function of rotation around the single bond separating two benzene rings in biphenyl, in agreement with what was observed in both experiment and DFTB calculation.

DOI： 10.1063/1.5097330

Language：Others   Publishing type：Research paper (scientific journal)  

Influence of long-range interactions on quantum interference in molecular conduction. A tight-binding (Hückel) approach
When certain pairs of atoms in a π-conjugated molecule are connected with nanometer-scale source and drain electrodes, the remarkable quantum interference (QI) effect may arise. In this case, the electron transmission probability is significantly suppressed due to the QI effect. Tight-binding approaches, such as the Hückel molecular orbital (HMO) model, have revealed important features of this quantum phenomenon. However, important deviations from experiments and from more sophisticated calculations are known for a variety of cases. Here, we propose an extension of the HMO method to include non-nearest-neighbor interactions. Such long-range interactions (LRIs) are implemented in the HMO model in the form of a damping function that decays as the topological distance - the number of bonds separating two atoms - gets larger. The proposed model is further developed so that a geometric modification, i.e., the rotation around a single bond, can be taken into account. Our results show that LRI affects both the location of the antiresonance peak due to QI and the intensity of QI, even suppressing it in some cases. These results agree well with what was observed in a Density Functional based Tight-Binding (DFTB) study reported in the literature. These properties can be interpreted on the basis of a graph-theoretic path-counting model as well as the molecular orbital theory. In addition, the geometric LRI model is shown to reproduce the change of transmission as a function of rotation around the single bond separating two benzene rings in biphenyl, in agreement with what was observed in both experiment and DFTB calculation.

DOI： 10.1063/1.5097330

Language：Others   Publishing type：Research paper (scientific journal)  

Methane activation is usually assumed to take place on top of metal surfaces or metal clusters. It can also occur at the metal-support interface in metal-supported catalysts with reducible oxides, such as CeO . In the present work, we exploit density functional theory with an additional Hubbard-like parameter (DFT + U) to calculate the activation of methane at an O site interfacing a Ni metal cluster on a support, CeO (111) surface. Two reaction routes, namely, radical and nonradical routes, are taken into account. We show that the nonradical route is favored with an apparent activation energy of 18.1 kcal/mol, which is lower than that for the radical route by 15.0 kcal/mol. In the nonradical route, the formation of a four-centered transition-state structure is observed while a C-H bond of methane is being cleaved to form an OH moiety and a CH fragment that is being bound to the interfacial Ni atom. It is also found that the interfacial O atoms are out of the CeO surface plane with Ce-O bond distances being much longer than those in the crystalline bulk CeO , which allows them to be easily reduced, and hence, the interfacial O atoms become more reactive toward methane, as compared to the surface O atoms. The interactions between Ni cluster and the CeO (111) surface result in the reduction of two Ce ions to Ce , improving the reducibility of the interfacial O atoms. This should be an important key to the facile methane activation. 2 4 2 3 2 2 4 2 4+ 3+

DOI： 10.1021/acs.jpcc.8b11973

Language：Others   Publishing type：Research paper (scientific journal)  

This study examined the nature of the electronic structure of representative cross-conjugated polyenes from a valence bond (VB) perspective. Our VBSCF calculations on a prototypical dendralene model reveal a remarkable inhibition of the delocalization compared to linear polyenes. Especially along the C-C backbone, the delocalization is virtually quenched so that these compounds can essentially be considered as sets of isolated butadiene units. In direct contrast to the dendralene chains, quinodimethane compounds exhibit an enhancement in their delocalization compared to linear polyenes. We demonstrate that this quenching/enhancement of the delocalization is inherently connected to the relative weights of specific types of long-bond VB structures. From our ab initio treatment, many localization/delocalization-related concepts and phenomena, central to both organic chemistry and single-molecule electronics, emerge. Not only do we find direct insight into the relation between topology and the occurrence of quantum interference (QI), but we also find a phenomenological justification of the recently proposed diradical character-based rule for the estimation of the magnitude of molecular conductance. Generally, our results can be conceptualized using the "arrow-pushing" concept, originating from resonance theory.

DOI： 10.1021/jacs.9b01420

Language：English   Publishing type：Research paper (scientific journal)  

Methane activation is usually assumed to take place on top of metal surfaces or metal clusters. It can also occur at the metal-support interface in metal-supported catalysts with reducible oxides, such as CeO
₂
. In the present work, we exploit density functional theory with an additional Hubbard-like parameter (DFT + U) to calculate the activation of methane at an O site interfacing a Ni
₄
metal cluster on a support, CeO
₂
(111) surface. Two reaction routes, namely, radical and nonradical routes, are taken into account. We show that the nonradical route is favored with an apparent activation energy of 18.1 kcal/mol, which is lower than that for the radical route by 15.0 kcal/mol. In the nonradical route, the formation of a four-centered transition-state structure is observed while a C-H bond of methane is being cleaved to form an OH moiety and a CH
₃
fragment that is being bound to the interfacial Ni atom. It is also found that the interfacial O atoms are out of the CeO
₂
surface plane with Ce-O bond distances being much longer than those in the crystalline bulk CeO
₂
, which allows them to be easily reduced, and hence, the interfacial O atoms become more reactive toward methane, as compared to the surface O atoms. The interactions between Ni
₄
cluster and the CeO
₂
(111) surface result in the reduction of two Ce
⁴⁺
ions to Ce
³⁺
, improving the reducibility of the interfacial O atoms. This should be an important key to the facile methane activation.

DOI： 10.1021/acs.jpcc.8b11973

Language：English   Publishing type：Research paper (scientific journal)  

This study examined the nature of the electronic structure of representative cross-conjugated polyenes from a valence bond (VB) perspective. Our VBSCF calculations on a prototypical dendralene model reveal a remarkable inhibition of the delocalization compared to linear polyenes. Especially along the C-C backbone, the delocalization is virtually quenched so that these compounds can essentially be considered as sets of isolated butadiene units. In direct contrast to the dendralene chains, quinodimethane compounds exhibit an enhancement in their delocalization compared to linear polyenes. We demonstrate that this quenching/enhancement of the delocalization is inherently connected to the relative weights of specific types of long-bond VB structures. From our ab initio treatment, many localization/delocalization-related concepts and phenomena, central to both organic chemistry and single-molecule electronics, emerge. Not only do we find direct insight into the relation between topology and the occurrence of quantum interference (QI), but we also find a phenomenological justification of the recently proposed diradical character-based rule for the estimation of the magnitude of molecular conductance. Generally, our results can be conceptualized using the "arrow-pushing" concept, originating from resonance theory.

DOI： 10.1021/jacs.9b01420

Language：English   Publishing type：Research paper (scientific journal)  

This study examined the nature of the electronic structure of representative cross-conjugated polyenes from a valence bond (VB) perspective. Our VBSCF calculations on a prototypical dendralene model reveal a remarkable inhibition of the delocalization compared to linear polyenes. Especially along the C-C backbone, the delocalization is virtually quenched so that these compounds can essentially be considered as sets of isolated butadiene units. In direct contrast to the dendralene chains, quinodimethane compounds exhibit an enhancement in their delocalization compared to linear polyenes. We demonstrate that this quenching/enhancement of the delocalization is inherently connected to the relative weights of specific types of long-bond VB structures. From our ab initio treatment, many localization/delocalization-related concepts and phenomena, central to both organic chemistry and single-molecule electronics, emerge. Not only do we find direct insight into the relation between topology and the occurrence of quantum interference (QI), but we also find a phenomenological justification of the recently proposed diradical character-based rule for the estimation of the magnitude of molecular conductance. Generally, our results can be conceptualized using the "arrow-pushing" concept, originating from resonance theory.

DOI： 10.1021/jacs.9b01420

Language：English   Publishing type：Research paper (scientific journal)  

Adhesion interaction of epoxy resin with the basal surfaces of h-BN and graphite is investigated with the first-principles density functional theory calculations in conjunction with the dispersion correction. The h-BN/epoxy and graphite/epoxy interfaces play an important role in producing nanocomposite materials with excellent thermal dissipation properties. The epoxy resin structure is simulated by using four kinds of fragmentary models. Their structures are optimized on the h-BN and graphite surfaces after an annealing simulation. The distance between the epoxy fragment and the surface is about 3 Å. At the interface between h-BN and epoxy resin, no H-bonding formation is observed, though one could expect that the active functional groups of epoxy resin, such as hydroxyl (OH) group, would be involved in a hydrogen-bonding interaction with nitrogen atoms of the h-BN surface. The adhesion energies for the two interfaces are calculated, showing that these two interfaces are characterized by almost the same strength of adhesion interaction. To obtain the adhesion force-separation curve for the two interfaces, the potential energy surface associated with the detachment of the epoxy fragment from the surface is calculated with the help of the nudged elastic band method and then the adhesion force is obtained by using either the Morse-potential approximation or the Hellmann-Feynman force calculation. The results from both methods agree with each other. The maximum adhesion force for the h-BN/epoxy interface is as high as that for the graphite/epoxy interface. To better understand this result, a force-decomposition analysis is carried out, and it has been disclosed that the adhesion forces working at both interfaces mainly come from the dispersion force. The trend of increase in the C
₆
parameters used for the dispersion correction for the atoms included in the h-BN or graphite surface is in the order: N < C < B, which reasonably explains why the strengths of the dispersion forces operating at the two interfaces are similar. Also, the electron localization function analysis can explain why the h-BN surface cannot form an H bond with the hydroxyl group in epoxy resin.

DOI： 10.1021/acsomega.9b00129

Language：Others   Publishing type：Research paper (scientific journal)  

Adhesion interaction of epoxy resin with the basal surfaces of h-BN and graphite is investigated with the first-principles density functional theory calculations in conjunction with the dispersion correction. The h-BN/epoxy and graphite/epoxy interfaces play an important role in producing nanocomposite materials with excellent thermal dissipation properties. The epoxy resin structure is simulated by using four kinds of fragmentary models. Their structures are optimized on the h-BN and graphite surfaces after an annealing simulation. The distance between the epoxy fragment and the surface is about 3 Å. At the interface between h-BN and epoxy resin, no H-bonding formation is observed, though one could expect that the active functional groups of epoxy resin, such as hydroxyl (OH) group, would be involved in a hydrogen-bonding interaction with nitrogen atoms of the h-BN surface. The adhesion energies for the two interfaces are calculated, showing that these two interfaces are characterized by almost the same strength of adhesion interaction. To obtain the adhesion force-separation curve for the two interfaces, the potential energy surface associated with the detachment of the epoxy fragment from the surface is calculated with the help of the nudged elastic band method and then the adhesion force is obtained by using either the Morse-potential approximation or the Hellmann-Feynman force calculation. The results from both methods agree with each other. The maximum adhesion force for the h-BN/epoxy interface is as high as that for the graphite/epoxy interface. To better understand this result, a force-decomposition analysis is carried out, and it has been disclosed that the adhesion forces working at both interfaces mainly come from the dispersion force. The trend of increase in the C parameters used for the dispersion correction for the atoms included in the h-BN or graphite surface is in the order: N < C < B, which reasonably explains why the strengths of the dispersion forces operating at the two interfaces are similar. Also, the electron localization function analysis can explain why the h-BN surface cannot form an H bond with the hydroxyl group in epoxy resin. 6

DOI： 10.1021/acsomega.9b00129

Language：English   Publishing type：Research paper (other academic)  

In the electronic packages, gold surfaces are useful for stable connections. In this paper, adhesion interface for die bonding structure with conductive silver paste is investigated. The adhesive interface between gold surface and epoxy resin is investigated in two approaches. The first-principles calculations are employed to optimize the structure and calculate the theoretical bonding strength. The fragment model is constructed for bisphenol-A type epoxy resin and a curing agent which includes dicyandiamide. The results show that a cyano group included in the hardener greatly interacts with ideal gold surface, whereas a hydroxyl group seems not to interact with them regardless of high polarity. This interaction is likely to come from the π-back donation. The predicted bonding strength is as high as 1.15GPa. Next, the actual die pad surfaces prepared with plating gold are characterized to presume the substantial bonding structure by analytical method. Besides, measurements of adhesive strength have been performed by using small size lap shear tests under various temperatures. These results show that there are altered layers at the top of plating gold, which seems to affect the interaction mechanisms to hydrogen bond and make the adhesive strength lower.

DOI： 10.1109/ICSJ.2018.8602784

Language：Others  

In the electronic packages, gold surfaces are useful for stable connections. In this paper, adhesion interface for die bonding structure with conductive silver paste is investigated. The adhesive interface between gold surface and epoxy resin is investigated in two approaches. The first-principles calculations are employed to optimize the structure and calculate the theoretical bonding strength. The fragment model is constructed for bisphenol-A type epoxy resin and a curing agent which includes dicyandiamide. The results show that a cyano group included in the hardener greatly interacts with ideal gold surface, whereas a hydroxyl group seems not to interact with them regardless of high polarity. This interaction is likely to come from the π-back donation. The predicted bonding strength is as high as 1.15GPa. Next, the actual die pad surfaces prepared with plating gold are characterized to presume the substantial bonding structure by analytical method. Besides, measurements of adhesive strength have been performed by using small size lap shear tests under various temperatures. These results show that there are altered layers at the top of plating gold, which seems to affect the interaction mechanisms to hydrogen bond and make the adhesive strength lower.

DOI： 10.1109/ICSJ.2018.8602784

Language：English   Publishing type：Research paper (scientific journal)  

In recent years, a considerable interest has grown in the design of molecular nanowires with an increasing conductance with length. The development of such nanowires is highly desirable because they could play an important role in future molecular-scale circuitry. Whereas the first experimental observation of this nonclassical behavior still has to be realized, a growing number of candidate wires have been proposed theoretically. In this Letter, we point out that all the wires with an anti-Ohmic increasing conductance with length proposed so far share a common characteristic: their diradical character increases with length. The conceptual connection between diradical character and conductance enables a systematic design of such anti-Ohmic wires and explains the difficulty in their syntheses. A strategy is proposed to balance the stability and conductance so that this nonclassical phenomenon can be observed.

DOI： 10.1021/acs.nanolett.8b03503

Language：Others   Publishing type：Research paper (scientific journal)  

In recent years, a considerable interest has grown in the design of molecular nanowires with an increasing conductance with length. The development of such nanowires is highly desirable because they could play an important role in future molecular-scale circuitry. Whereas the first experimental observation of this nonclassical behavior still has to be realized, a growing number of candidate wires have been proposed theoretically. In this Letter, we point out that all the wires with an anti-Ohmic increasing conductance with length proposed so far share a common characteristic: their diradical character increases with length. The conceptual connection between diradical character and conductance enables a systematic design of such anti-Ohmic wires and explains the difficulty in their syntheses. A strategy is proposed to balance the stability and conductance so that this nonclassical phenomenon can be observed.

DOI： 10.1021/acs.nanolett.8b03503

Language：English   Publishing type：Research paper (scientific journal)  

It has been well accepted that when quantum interference (QI) occurs in a single molecular junction comprised of a π-conjugated molecule, the elastic π-electron transmission is blocked, while the elastic σ-electron transmission remains unchanged. When it comes to inelastic transport, in which passing electrons across the molecule trapped in between two metallic electrodes lose their energy through electron-phonon coupling, it is not necessarily obvious whether vibration affects the QI feature or not. In this paper, on the basis of a Hückel/tight-binding model, we address the inelastic transport through linear and cyclic polyenes which are conditioned to show QI. The zeroth-order Green's function approximated by the negative inverse of the adjacency matrix of a molecular graph is used in conjunction with the lowest order expansion of the self-consistent Born approximation. Owing to the simplification of the model, it just finds the limited applicability for the π-to-π scattering. Only topological aspects of dephasing are included. In such a theoretical construct, the alternant nature of the π-conjugated molecule is found helpful for classifying the dephasing patterns based on the parity of atomic sites. A rule is proposed, and it says that when both starred or both unstarred atoms are connected with the electrodes, QI always occurs, and atoms which belong to a different partite set from that of the atoms connected with the electrodes contribute to the inelastic π-to-π scattering. If QI occurs when a starred atom and an unstarred atom are connected with the electrodes, the contribution of the inelastic π-to-π scattering to the transport is expected to be unimportant.

DOI： 10.1063/1.5048955

Language：Others   Publishing type：Research paper (scientific journal)  

It has been well accepted that when quantum interference (QI) occurs in a single molecular junction comprised of a π-conjugated molecule, the elastic π-electron transmission is blocked, while the elastic σ-electron transmission remains unchanged. When it comes to inelastic transport, in which passing electrons across the molecule trapped in between two metallic electrodes lose their energy through electron-phonon coupling, it is not necessarily obvious whether vibration affects the QI feature or not. In this paper, on the basis of a Hückel/tight-binding model, we address the inelastic transport through linear and cyclic polyenes which are conditioned to show QI. The zeroth-order Green's function approximated by the negative inverse of the adjacency matrix of a molecular graph is used in conjunction with the lowest order expansion of the self-consistent Born approximation. Owing to the simplification of the model, it just finds the limited applicability for the π-to-π scattering. Only topological aspects of dephasing are included. In such a theoretical construct, the alternant nature of the π-conjugated molecule is found helpful for classifying the dephasing patterns based on the parity of atomic sites. A rule is proposed, and it says that when both starred or both unstarred atoms are connected with the electrodes, QI always occurs, and atoms which belong to a different partite set from that of the atoms connected with the electrodes contribute to the inelastic π-to-π scattering. If QI occurs when a starred atom and an unstarred atom are connected with the electrodes, the contribution of the inelastic π-to-π scattering to the transport is expected to be unimportant.

DOI： 10.1063/1.5048955

Language：English   Publishing type：Research paper (scientific journal)  

Methane strongly adsorbs on the (110) surface of IrO
₂
, a rutile-type metal dioxide. Its C-H bond is facilely dissociated even below room temperature, as predicted in a few theoretical works and actually observed in a recent experimental study. Thence, three questions are posed and answered in this paper: First, why does methane adsorb on the IrO
₂
surface so strongly? Second, why is the surface so active for the C-H bond breaking reaction? Third, is there any other rutile-type metal dioxide that is more active than IrO
₂
? A second-order perturbation theoretic approach is successfully applied to the analysis of the electronic structure of methane, which is found to be significantly distorted on the surface. Regarding the first point, it is clarified that an attractive orbital interaction between the surface Ir 5d
_z2
orbital and the distorted methane's highest occupied molecular orbital leads to the strong adsorption. As for the second point, the bond strength between the surface metal atom and the CH
₃
fragment generated after the C-H bond scission of methane is correlated well with the activation barrier. A substantial bonding interaction between CH
₃
's nonbonding orbital and the dz
²
orbital hints at the strong Ir-CH
₃
bond and hence high catalytic activity ensues. Last but not least, β-PtO
₂
, a distorted rutile-type dioxide, is identified as a more active catalyst than IrO
₂
. Here again, a perturbation theoretic line of explanation is found to be of tremendous help. This paper is at the intersection of theoretical, catalytic, inorganic, and physical chemistry. Also, it should serve as a model for the design and study of metal-oxide catalysts for the C-H bond activation of methane.

DOI： 10.1021/acs.jpcc.8b03184

Language：Others   Publishing type：Research paper (scientific journal)  

Methane strongly adsorbs on the (110) surface of IrO , a rutile-type metal dioxide. Its C-H bond is facilely dissociated even below room temperature, as predicted in a few theoretical works and actually observed in a recent experimental study. Thence, three questions are posed and answered in this paper: First, why does methane adsorb on the IrO surface so strongly? Second, why is the surface so active for the C-H bond breaking reaction? Third, is there any other rutile-type metal dioxide that is more active than IrO ? A second-order perturbation theoretic approach is successfully applied to the analysis of the electronic structure of methane, which is found to be significantly distorted on the surface. Regarding the first point, it is clarified that an attractive orbital interaction between the surface Ir 5d orbital and the distorted methane's highest occupied molecular orbital leads to the strong adsorption. As for the second point, the bond strength between the surface metal atom and the CH fragment generated after the C-H bond scission of methane is correlated well with the activation barrier. A substantial bonding interaction between CH 's nonbonding orbital and the dz orbital hints at the strong Ir-CH bond and hence high catalytic activity ensues. Last but not least, β-PtO , a distorted rutile-type dioxide, is identified as a more active catalyst than IrO . Here again, a perturbation theoretic line of explanation is found to be of tremendous help. This paper is at the intersection of theoretical, catalytic, inorganic, and physical chemistry. Also, it should serve as a model for the design and study of metal-oxide catalysts for the C-H bond activation of methane. 2 2 2 z2 3 3 3 2 2 2

DOI： 10.1021/acs.jpcc.8b03184

Language：English   Publishing type：Research paper (scientific journal)  

In this paper, we explore quantum interference (QI) in molecular conductance from the point of view of graph theory and walks on lattices. By virtue of the Cayley-Hamilton theorem for characteristic polynomials and the Coulson-Rushbrooke pairing theorem for alternant hydrocarbons, it is possible to derive a finite series expansion of the Green's function for electron transmission in terms of the odd powers of the vertex adjacency matrix or Hückel matrix. This means that only odd-length walks on a molecular graph contribute to the conductivity through a molecule. Thus, if there are only even-length walks between two atoms, quantum interference is expected to occur in the electron transport between them. However, even if there are only odd-length walks between two atoms, a situation may come about where the contributions to the QI of some odd-length walks are canceled by others, leading to another class of quantum interference. For nonalternant hydrocarbons, the finite Green's function expansion may include both even and odd powers. Nevertheless, QI can in some circumstances come about for nonalternants from cancellation of odd- and even-length walk terms. We report some progress, but not a complete resolution, of the problem of understanding the coefficients in the expansion of the Green's function in a power series of the adjacency matrix, these coefficients being behind the cancellations that we have mentioned. Furthermore, we introduce a perturbation theory for transmission as well as some potentially useful infinite power series expansions of the Green's function.

DOI： 10.1021/acs.chemrev.7b00733

Language：English   Publishing type：Research paper (scientific journal)  

We explore a new strategy to tune the conductivity of molecular electronic devices: captodative substitution. We demonstrate that a careful design of such substitution schemes on a benzene parental structure can enhance the conductivity by almost an order of magnitude under small bias. Once this new strategy has been established, we apply it to molecular wires and demonstrate that it enables the unprecedented anti-Ohmic design of wires whose conductivity increases with the length. Overall, the captodative substitution approach provides a very promising pathway toward full chemical control of the conductivity of molecules which opens up the possibility to design molecular switches with an improved on/off ratio among others.

DOI： 10.1021/acs.jpcc.7b10877

Language：English   Publishing type：Research paper (scientific journal)  

We explore a new strategy to tune the conductivity of molecular electronic devices: captodative substitution. We demonstrate that a careful design of such substitution schemes on a benzene parental structure can enhance the conductivity by almost an order of magnitude under small bias. Once this new strategy has been established, we apply it to molecular wires and demonstrate that it enables the unprecedented anti-Ohmic design of wires whose conductivity increases with the length. Overall, the captodative substitution approach provides a very promising pathway toward full chemical control of the conductivity of molecules which opens up the possibility to design molecular switches with an improved on/off ratio among others.

DOI： 10.1021/acs.jpcc.7b10877

Language：Japanese   Publishing type：Research paper (scientific journal)  

Three Years at Cornell

DOI： 10.1380/vss.61.91

Language：English   Publishing type：Research paper (scientific journal)  

How heteroatomic substitutions affect electron transport through π-conjugated hydrocarbons has been the subject of some debate. In this paper we investigate the effect of heteroatomic linkers in a molecular junction on the electron-transmission spectrum, focusing on the occurrence of quantum interference (QI) close to the Fermi level, where conductivity can be significantly suppressed. We find that the substitution or addition of heteroatoms to a carbon skeleton at the contact positions does not change the main feature of QI due to the underlying carbon skeleton. QI in the overall system thus remains a robust feature. This empirical observation leads us to derive, in two mathematical ways, that these findings can be generalized. We note that addition or substitution of a carbon atom by a heteroatom at the contact positions will increase or decrease the number of electrons in the π-system, which will lead to a change in the alignment of the molecular orbitals of the isolated system relative to the electrode Fermi level. Both Hückel and density functional theory calculations on model systems probe the effect of this Fermi level change and confirm qualitatively the implications of the underlying mathematical proofs.

DOI： 10.1021/acs.jpcc.7b03493

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 American Chemical Society. The nature of the electronic-structure of polyenes, their delocalization features, and potential diradicaloid characters constitute a fundamental problem in chemistry. To address this problem, we used valence bond self-consistent field (VBSCF) calculations and modeling of polyenes, C2nH2n+2 (n = 2-10). The theoretical treatment shows that starting with n = 5, the polyene's wave function is mainly a shifting 1,4-diradicaloid, a character that increases as the chain length increases, while the contribution of the fundamental Lewis structure with alternating double and single bonds (1) decays quite fast and becomes minor relative to the diradicaloid pack. We show how, nevertheless, it is this wave function that predicts that polyenes will still exhibit alternating short/long CC bonds like the fundamental structure 1. Furthermore, despite the decay of the VB contribution of 1, it remains the single structure with the largest weight among all the individual structures. The mixing of all the 1,4-diradicaloid structures into 1 follows perturbation theory rules, with the result that the delocalization energy due to this mixing is additive and behaves as a linear function of the number of the double bonds, ΔEdel = -6.9 × n (kcal mol-1). The VB modeling shows that while the conjugation stabilizes structure 1, this stabilization energy is energetically overridden by the Pauli repulsion between two adjacent double bonds. Nevertheless, unsubstituted polyenes remain planar; this observation is addressed. Potential manifestations of the diradicaloid nature of polyenes are discussed, and it is concluded that the diradicaloid character is clearly not a well-defined physical property as in real diradicals. Thus, we went full circle to realize that our philosophical question may not be strictly resolved. The localized/delocalized properties of polyenes seem to define a "chemical duality principle". This duality of molecular wave functions is a ubiquitous beguiling phenomenon.

DOI： 10.1021/jacs.7b04410

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 American Chemical Society. How heteroatomic substitutions affect electron transport through π-conjugated hydrocarbons has been the subject of some debate. In this paper we investigate the effect of heteroatomic linkers in a molecular junction on the electron-transmission spectrum, focusing on the occurrence of quantum interference (QI) close to the Fermi level, where conductivity can be significantly suppressed. We find that the substitution or addition of heteroatoms to a carbon skeleton at the contact positions does not change the main feature of QI due to the underlying carbon skeleton. QI in the overall system thus remains a robust feature. This empirical observation leads us to derive, in two mathematical ways, that these findings can be generalized. We note that addition or substitution of a carbon atom by a heteroatom at the contact positions will increase or decrease the number of electrons in the π-system, which will lead to a change in the alignment of the molecular orbitals of the isolated system relative to the electrode Fermi level. Both Hückel and density functional theory calculations on model systems probe the effect of this Fermi level change and confirm qualitatively the implications of the underlying mathematical proofs.

DOI： 10.1021/acs.jpcc.7b03493

Language：English   Publishing type：Research paper (scientific journal)  

The nature of the electronic-structure of polyenes, their delocalization features, and potential diradicaloid characters constitute a fundamental problem in chemistry. To address this problem, we used valence bond self-consistent field (VBSCF) calculations and modeling of polyenes, C_2nH_2n+2 (n = 2-10). The theoretical treatment shows that starting with n = 5, the polyene's wave function is mainly a shifting 1,4-diradicaloid, a character that increases as the chain length increases, while the contribution of the fundamental Lewis structure with alternating double and single bonds (1) decays quite fast and becomes minor relative to the diradicaloid pack. We show how, nevertheless, it is this wave function that predicts that polyenes will still exhibit alternating short/long CC bonds like the fundamental structure 1. Furthermore, despite the decay of the VB contribution of 1, it remains the single structure with the largest weight among all the individual structures. The mixing of all the 1,4-diradicaloid structures into 1 follows perturbation theory rules, with the result that the delocalization energy due to this mixing is additive and behaves as a linear function of the number of the double bonds, ΔE_del = -6.9 × n (kcal mol^-1). The VB modeling shows that while the conjugation stabilizes structure 1, this stabilization energy is energetically overridden by the Pauli repulsion between two adjacent double bonds. Nevertheless, unsubstituted polyenes remain planar; this observation is addressed. Potential manifestations of the diradicaloid nature of polyenes are discussed, and it is concluded that the diradicaloid character is clearly not a well-defined physical property as in real diradicals. Thus, we went full circle to realize that our philosophical question may not be strictly resolved. The localized/delocalized properties of polyenes seem to define a "chemical duality principle". This duality of molecular wave functions is a ubiquitous beguiling phenomenon.

DOI： 10.1021/jacs.7b04410

Language：English   Publishing type：Research paper (scientific journal)  

The wave-particle duality of electrons gives rise to quantum interference (QI) in single molecular devices. A significant challenge to be addressed in molecular electronics is to further develop chemical intuition to understand and predict QI features. In this study, an orbital rule is markedly ameliorated so that it can capture the manifestation of QI not only in alternant hydrocarbons but also in nonalternant ones. The orbital-based prediction about the occurrence of QI in a nonalternant hydrocarbon shows good agreement with experimental results. A simple perturbation theoretic line of reasoning suggests that frontier orbital phase and splitting play a pivotal role in QI phenomena.

DOI： 10.1021/acs.jpcc.7b02274

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 American Chemical Society. The wave-particle duality of electrons gives rise to quantum interference (QI) in single molecular devices. A significant challenge to be addressed in molecular electronics is to further develop chemical intuition to understand and predict QI features. In this study, an orbital rule is markedly ameliorated so that it can capture the manifestation of QI not only in alternant hydrocarbons but also in nonalternant ones. The orbital-based prediction about the occurrence of QI in a nonalternant hydrocarbon shows good agreement with experimental results. A simple perturbation theoretic line of reasoning suggests that frontier orbital phase and splitting play a pivotal role in QI phenomena.

DOI： 10.1021/acs.jpcc.7b02274

Language：English   Publishing type：Research paper (scientific journal)  

We show in this work that conjugated π-electron molecular chains can, in quite specific and understood circumstances, become more conductive the longer they get, in contradiction to what would be expected intuitively. The analysis, done in the framework of the source and sink potential method, and supported by detailed transmission calculations, begins by defining "relative transmission," an inherent measure of molecular conduction. This, in turn, for conjugated hydrocarbons, is related to a simple molecular orbital expression - the ratio of secular determinants of a molecule and one where the electrode contacts are deleted - and a valence bond idea, since these secular determinants can alternatively be expressed in terms of Kekulé structures. A plausible argument is given for relating the relative transmission to the weight of the diradical resonance structures in the resonance hybrid for a molecule. Chemical intuition can then be used to tune the conductivity of molecules by "pushing" them towards more or less diradical character. The relationship between relative transmission (which can rise indefinitely) and molecular transmission is carefully analyzed - there is a sweet spot here for engineering molecular devices. These new insights enable the rationalization of a wide variety of experimental and theoretical results for π-conjugated alternant hydrocarbons, especially the striking difference between extended oligophenylenes and related quinoid chains. In this context, oligo-p-phenylene macrocycles emerge as a potential molecular switch.

DOI： 10.1063/1.4972992

Language：English   Publishing type：Research paper (scientific journal)  

Applications of the Houckel (tight binding) model are ubiquitous in quantum chemistry and solid state physics. The matrix representation of this model is isomorphic to an unoriented vertex adjacency matrix of a bipartite graph, which is also the Laplacian matrix plus twice the identity. In this paper, we analytically calculate the determinant and, when it exists, the inverse of this matrix in connection with the Green's function, G, of the N x N Houckel matrix. A corollary is a closed form expression for a Harmonic sum (Eq. (12)). We then extend the results to d-dimensional lattices, whose linear size is N. The existence of the inverse becomes a question of number theory. We prove a new theorem in number theory pertaining to vanishing sums of cosines and use it to prove that the inverse exists if and only if N + 1 and d are odd and d is smaller than the smallest divisor of N + 1. We corroborate our results by demonstrating the entry patterns of the Green's function and discuss applications related to transport and conductivity. Published by AIP Publishing.

DOI： 10.1063/1.4977080

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 Author(s) We show in this work that conjugated π-electron molecular chains can, in quite specific and understood circumstances, become more conductive the longer they get, in contradiction to what would be expected intuitively. The analysis, done in the framework of the source and sink potential method, and supported by detailed transmission calculations, begins by defining "relative transmission," an inherent measure of molecular conduction. This, in turn, for conjugated hydrocarbons, is related to a simple molecular orbital expression - the ratio of secular determinants of a molecule and one where the electrode contacts are deleted - and a valence bond idea, since these secular determinants can alternatively be expressed in terms of Kekulé structures. A plausible argument is given for relating the relative transmission to the weight of the diradical resonance structures in the resonance hybrid for a molecule. Chemical intuition can then be used to tune the conductivity of molecules by "pushing" them towards more or less diradical character. The relationship between relative transmission (which can rise indefinitely) and molecular transmission is carefully analyzed - there is a sweet spot here for engineering molecular devices. These new insights enable the rationalization of a wide variety of experimental and theoretical results for π-conjugated alternant hydrocarbons, especially the striking difference between extended oligophenylenes and related quinoid chains. In this context, oligo-p-phenylene macrocycles emerge as a potential molecular switch.

DOI： 10.1063/1.4972992

Language：English   Publishing type：Research paper (scientific journal)  

Applications of the Huckel (tight binding) model are ubiquitous in quantum chemistry and solid state physics. The matrix representation of this model is isomorphic to an unoriented vertex adjacency matrix of a bipartite graph, which is also the Laplacian matrix plus twice the identity. In this paper, we analytically calculate the determinant and, when it exists, the inverse of this matrix in connection with the Green's function, G, of the N × N Huckel matrix. A corollary is a closed form expression for a Harmonic sum (Eq. (12)).We then extend the results to d-dimensional lattices, whose linear size is N. The existence of the inverse becomes a question of number theory. We prove a new theorem in number theory pertaining to vanishing sums of cosines and use it to prove that the inverse exists if and only if N + 1 and d are odd and d is smaller than the smallest divisor of N + 1. We corroborate our results by demonstrating the entry patterns of the Green's function and discuss applications related to transport and conductivity.

DOI： 10.1063/1.4977080

Language：Japanese   Publishing type：Research paper (scientific journal)  

Fundamental and Applied Science, Academia and Industry, a Creative Tension in Today's Chemistry

Language：Japanese   Publishing type：Research paper (scientific journal)  

Catalyst Informatics on Methane Activation on Various Metal Alloys

DOI： 10.2477/jccj.2017-0058

Language：English   Publishing type：Research paper (scientific journal)  

In pursuit of new lithium-rich phases and potential electrides within the Li N phase diagram, we explore theoretically the ground-state structures and electronic properties of Li4N at P = 1 atm. Crystal structure exploration methods based on particle swarm optimization and evolutionary algorithms led to 25 distinct structures, including 23 dynamically stable structures, all quite close to each other in energy, but not in detailed structure. Several additional phases were obtained by following the imaginary phonon modes found in low-energy structures, as well as structures constructed to simulate segregation into Li and Li3N. The candidate Li4N structures all contain NLin polyhedra, with n = 6-9. They may be dassified into three types, depending on their structural dimensionality: NLin extended polyhedral slabs joined by an elemental Li layer (type a), similar structures, but without the Li layer (type b), and three-dimensionally interconnected NLin polyhedra without any layering (type c). We investigate the electride nature of these structures using the electron localization function and partial charge density around the Fermi level. All of the structures can be characterized as electrides, but they differ in electronic dimensionality. Type-a and type-b structures may be classified as two-dimensional (2-D) electrides, while type-c structures emerge quite varied, as 0-D, 2-D, or 3-D. The calculated structural variety (as well as detailed models for amorphous and liquid Li4N) points to potential amorphous character and likely ionic conductivity in the material.

DOI： 10.1021/jacs.6b09067

Language：English   Publishing type：Research paper (scientific journal)  

There is a consensus that long-range electron transfer/transport occurs by a through-bond rather than through-space mechanism. In helical all-Z, all-s-cis oligoenes, one can set up an interesting competition in the medium-separation regime between a closer (in distance) through-space path and a more distant through-bond one. Although such oligoene conformations/isomers are unstable (by around 4 kcal mol^-1 per double bond relative to all-E, all-s-trans isomers), recent synthetic efforts on truncated helicenes and oligothiophenes have provided related molecules. On the way to transmission calculations with electrodes attached to the termini of helical oligoenes, we uncover an interesting conformational ambiguity in all-Z, all-s-cis oligoenes, the existence of a broad conformational minimum for helical compression, with hints of end-to-end frontier-orbital-caused stabilization. There is relationship between helical oligoene structures and the corresponding substructure of a helicene, but there are also significant differences in the number of olefin subunits per helix turn. In Hückel transport calculations, the role of TB or TS mechanisms is obscured to an extent by variations in bond alternation and dihedral angle along the oligomer chain. However, the operation of a dominant through bond mechanism emerges clearly in local transmission plots. In moving the electrodes to carbon position related by quantum interference, it is possible to uncover a through space mechanism.

DOI： 10.1002/chem.201600042

Language：English   Publishing type：Research paper (scientific journal)  

There is a consensus that long-range electron transfer/transport occurs by a through-bond rather than through-space mechanism. In helical all-Z, all-s-cis oligoenes, one can set up an interesting competition in the medium-separation regime between a closer (in distance) through-space path and a more distant through-bond one. Although such oligoene conformations/isomers are unstable (by around 4kcalmol(-1) per double bond relative to all-E, all-s-trans isomers), recent synthetic efforts on truncated helicenes and oligothiophenes have provided related molecules. On the way to transmission calculations with electrodes attached to the termini of helical oligoenes, we uncover an interesting conformational ambiguity in all-Z, all-s-cis oligoenes, the existence of a broad conformational minimum for helical compression, with hints of end-to-end frontier-orbital-caused stabilization. There is relationship between helical oligoene structures and the corresponding substructure of a helicene, but there are also significant differences in the number of olefin subunits per helix turn. In Huckel transport calculations, the role of TB or TS mechanisms is obscured to an extent by variations in bond alternation and dihedral angle along the oligomer chain. However, the operation of a dominant through bond mechanism emerges clearly in local transmission plots. In moving the electrodes to carbon position related by quantum interference, it is possible to uncover a through space mechanism.

DOI： 10.1002/chem.201600042

Language：English   Publishing type：Research paper (scientific journal)  

We return to an old puzzle - the short metal-metal separation and electrical conductivity of the apparently unoxidized one-dimensionally stacked structure of a d⁸ Ir(I) complex, Ir(CO)₃Cl. One would expect neither a short Ir-Ir distance of 2.84 Å, nor metallicity in an unoxidized stacked square-planar d⁸ array. We build up dimer, trimer, one-dimensional polymer and model 3-dimensional structures, in both molecular and extended structure plane wave calculations. The short Ir-Ir separation in the polymer, with a substantial contribution of 6_pz-5d^z2 bonding to it, is obtained without any oxidation. There is computational evidence for an important level crossing in the polymer. The metallicity remains unexplained, but likely arises from partial oxidation. And that remains an outstanding experimental issue.

DOI： 10.1016/j.poly.2015.09.050

Language：English   Publishing type：Research paper (scientific journal)  

We return to an old puzzle - the short metal-metal separation and electrical conductivity of the apparently unoxidized one-dimensionally stacked structure of a d(8) Ir(I) complex, Ir(CO)(3)Cl. One would expect neither a short Ir-Ir distance of 2.84 angstrom, nor metallicity in an unoxidized stacked square-planar d(8) array. We build up dimer, trimer, one-dimensional polymer and model 3-dimensional structures, in both molecular and extended structure plane wave calculations. The short Ir-Ir separation in the polymer, with a substantial contribution of 6p(z)-5d(z)2 bonding to it, is obtained without any oxidation. There is computational evidence for an important level crossing, in the polymer. The metallicity remains unexplained, but likely arises from partial oxidation. And that remains an outstanding experimental issue. (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.poly.2015.09.050

Language：English   Publishing type：Research paper (scientific journal)  

An empirical observation of a relationship between a striking feature of electronic transmission through a pi-system, destructive quantum interference (QI), on one hand, and the stability of diradicals on the other, leads to the proof of a general theorem that relates the two. Subject to a number of simplifying assumptions, in a pi-electron system, QI occurs when electrodes are attached to those positions of an N-carbon atom N-electron closed-shell hydrocarbon where the matrix elements of the Green's function vanish. These zeros come in two types, which are called easy and hard. Suppose an N+2 atom, N+2 electron hydrocarbon is formed by substituting 2 CH2 groups at two atoms, where the electrodes were. Then, if a QI feature is associated with electrode attachment to the two atoms of the original N atom system, the resulting augmented N+2 molecule will be a diradical. If there is no QI feature, i.e., transmission of current is normal if electrodes are attached to the two atoms, the resulting hydrocarbon will not be a diradical but will have a classical closed-shell electronic structure. Moreover, where a diradical exists, the easy zero is associated with a nondisjoint diradical, and the hard zero is associated with a disjoint one. A related theorem is proven for deletion of two sites from a hydrocarbon.

DOI： 10.1073/pnas.1518206113

Language：English   Publishing type：Research paper (scientific journal)  

An exponential falloff with separation of electron transfer and transport through molecular wires is observed and has attracted theoretical attention. In this study, the attenuation of transmission in linear and cyclic polyenes is related to bond alternation. The explicit form of the zeroth Green's function in a Huckel model for bond-alternated polyenes leads to an analytical expression of the conductance decay factor beta. The beta values calculated from our model (beta(CN) values, per repeat unit of double and single bond) range from 0.28 to 0.37, based on carotenoid crystal structures. These theoretical beta values are slightly smaller than experimental values. The difference can be assigned to the effect of anchoring groups, which are not included in our model. A local transmission analysis for cyclic polyenes, and for [14]annulene in particular, shows that bond alternation affects dramatically not only the falloff behavior but also the choice of a transmission pathway by electrons. Transmission follows a well-demarcated system of pi bonds, even when there is a shorter-distance path with roughly the same kind of "electronic matter" intervening.

DOI： 10.1021/acsnano.5b04615

Language：Others  

Diradicals, lurking

Language：English   Publishing type：Research paper (scientific journal)  

The explicit form of the zeroth Green's function in the Hückel model, approximated by the negative of the inverse of the Hückel matrix, has direct quantum interference consequences for molecular conductance. We derive a set of rules for transmission between two electrodes attached to a polyene, when the molecule is extended by an even number of carbons at either end (transmission unchanged) or by an odd number of carbons at both ends (transmission turned on or annihilated). These prescriptions for the occurrence of quantum interference lead to an unexpected consequence for switches which realize such extension through electrocyclic reactions: for some specific attachment modes the chemically closed ring will be the ON position of the switch. Normally the signs of the entries of the Green's function matrix are assumed to have no physical significance; however, we show that the signs may have observable consequences. In particular, in the case of multiple probe attachments - if coherence in probe connections can be arranged - in some cases new destructive interference results, while in others one may have constructive interference. One such case may already exist in the literature.

DOI： 10.1063/1.4903043

Language：English   Publishing type：Research paper (scientific journal)  

The explicit form of the zeroth Green's function in the Huckel model, approximated by the negative of the inverse of the Huckel matrix, has direct quantum interference consequences for molecular conductance. We derive a set of rules for transmission between two electrodes attached to a polyene, when the molecule is extended by an even number of carbons at either end (transmission unchanged) or by an odd number of carbons at both ends (transmission turned on or annihilated). These prescriptions for the occurrence of quantum interference lead to an unexpected consequence for switches which realize such extension through electrocyclic reactions: for some specific attachment modes the chemically closed ring will be the ON position of the switch. Normally the signs of the entries of the Green's function matrix are assumed to have no physical significance; however, we show that the signs may have observable consequences. In particular, in the case of multiple probe attachments - if coherence in probe connections can be arranged - in some cases new destructive interference results, while in others one may have constructive interference. One such case may already exist in the literature. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4903043

Language：English   Publishing type：Research paper (scientific journal)  

For transmission of electrons through a system, when the Landauer theory of molecular conductance is viewed from a molecular orbital (MO) perspective, there obtains a simple perturbation theoretic dependence, due to Yoshizawa and Tada, on a)the product of the orbital coefficients at the sites of electrode attachment, and b)the MO energies. The frontier orbitals consistently and simply indicate high or low transmission, even if other orbitals may contribute. This formalism, with its consequent reinforcement and/or interference of conductance, accounts for the (previously explained) difference in direct vs. cross conjugated transmission across an ethylene, as well as the comparative ON/OFF ratios in the experimentally investigated dimethyldihydropyrene and dithienylethene-type single-molecule switches. A strong dependence of the conductance on the site of attachment of the electrodes in a system is an immediate extrapolation; the theory then predicts that for some specified sites the switching behavior will be inverted; i.e. the open molecular form of the switch will be more conductive.

DOI： 10.1002/anie.201311134

Language：English   Publishing type：Research paper (scientific journal)  

A mechanism of the adhesion between carbon fiber and epoxy resin is studied by using density functional theory (DFT) calculations. Surface structures of carbon fiber were modeled by the armchair-edge structure of graphite functionalized with OH and COOH groups. DFT calculations were performed to construct two realistic models of adhesion interface consisting of the functionalized carbon surface and a fragment of epoxy resin. Adhesive properties of the model interfaces were evaluated based on the binding energy (E _b) between the carbon surface and the resin as well as the maximum adhesive force (F_max) acting at the interface. Calculated values of E_b are 13.8 kcal/mol for the OH-functionalized surface and 19.1 kcal/mol for the COOH-functionalized surface. The binding energy per hydrogen bond is calculated to be 6.9 kcal/mol (OH model; two H-bonds) and 6.3 kcal/mol (COOH model; three H-bonds), both of which are virtually similar and reasonable for the bond energy of a typical OH···O hydrogen bond. Analysis of adhesive force-displacement curves derived from energy-displacement plots revealed that F_max is 0.52 nN for the OH model and 0.70 nN for the COOH model. Calculated adhesive properties are in good agreement with those previously reported for the interface between an aluminum oxide surface and an epoxy resin [J. Phys. Chem. C 2011, 115, 11701], strongly suggesting that hydrogen bonds between the oxygen-containing functional groups play a crucial role in the adhesive interaction in the carbon fiber/epoxy resin system.

DOI： 10.1021/jp407835d

Language：English   Publishing type：Research paper (scientific journal)  

A mechanism of the adhesion between carbon fiber and epoxy resin is studied by using density functional theory (DFT) calculations. Surface structures of carbon fiber were modeled by the armchair-edge structure of graphite functionalized with OH and COOH groups. DFT calculations were performed to construct two realistic models of adhesion interface consisting of the functionalized carbon surface and a fragment of epoxy resin. Adhesive properties of the model interfaces were evaluated based on the binding energy (E-b) between the carbon surface and the resin as well as the maximum adhesive force (F-max) acting at the interface. Calculated values of E-b are 13.8 kcal/mol for the OH-functionalized surface and 19.1 kcal/mol for the COOH-functionalized surface. The binding energy per hydrogen bond is calculated to be 6.9 kcal/mol (OH model; two H-bonds) and 6.3 kcal/mol (COOH model; three H-bonds), both of which are virtually similar and reasonable for the bond energy of a typical OH center dot center dot center dot O hydrogen bond. Analysis of adhesive force-displacement curves derived from energy-displacement plots revealed that F-max is 0.52 nN for the OH model and 0.70 nN for the COOH model. Calculated adhesive properties are in good agreement with those previously reported for the interface between an aluminum oxide surface and an epoxy resin [J. Phys. Chem. C 2011, 115, 11701], strongly suggesting that hydrogen bonds between the oxygen-containing functional groups play a crucial role in the adhesive interaction in the carbon fiber/epoxy resin system.

DOI： 10.1021/jp407835d

Language：English   Publishing type：Research paper (scientific journal)  

The conductance through single 7,7,8,8-tetracyanoquinodimethane (TCNQ) connected to gold electrodes is studied with the nonequilibrium Green's function method combined with density functional theory. The aim of the study is to derive the effect of a dicyano anchor group, =C(CN)₂, on energy level alignment between the electrode Fermi level and a molecular energy level. The strong electron-withdrawing nature of the dicyano anchor group lowers the LUMO level of TCNQ, resulting in an extremely small energy barrier for electron injection. At zero bias, electron transfer from electrodes easily occurs and, as a consequence, the anion radical state of TCNQ with a magnetic moment is formed. The unpaired electron in the TCNQ anion radical causes an exchange splitting between the spin-α and spin-β transmission spectra, allowing the single TCNQ junction to act as a spin-filtering device.

DOI： 10.1002/cphc.201300136

Language：English   Publishing type：Research paper (scientific journal)  

Diarylethenes are photosensitive pi-conjugated molecules, being of great promise in potential applications to various molecular devices. Although the switching properties of diarylethenes have been widely investigated experimentally and theoretically, little is known about their rectifying diode-like behavior. In this study, electron-transport properties of asymmetric diarylethenes incorporating two different heterocyclic five-membered rings with opposite electronic demands are investigated with the nonequilibrium Green function combined with density functional theory. The aim of this study is to derive the effect of the heteroatomic defects on not only switching but also rectifying characteristics of the asymmetric diarylethenes. Obtained results show that a silicon atom involved in the diarylethenes plays an important role in the current rectifying as well as switching performance. It is found that maximum rectification ratios of the asymmetric diarylethenes increase linearly with an increase in electronegativity difference between the asymmetrically arranged heteroatoms. The silicon- and oxygen-containing asymmetric diarylethene is suggested to be a good potential candidate for a novel molecular electronic device combining a switch and a diode.

DOI： 10.1246/bcsj.20130089

Language：English   Publishing type：Research paper (scientific journal)  

The conductance through single 7,7,8,8-tetracyanoquinodimethane (TCNQ) connected to gold electrodes is studied with the nonequilibrium Green's function method combined with density functional theory. The aim of the study is to derive the effect of a dicyano anchor group, =C(CN)2, on energy level alignment between the electrode Fermi level and a molecular energy level. The strong electron-withdrawing nature of the dicyano anchor group lowers the LUMO level of TCNQ, resulting in an extremely small energy barrier for electron injection. At zero bias, electron transfer from electrodes easily occurs and, as a consequence, the anion radical state of TCNQ with a magnetic moment is formed. The unpaired electron in the TCNQ anion radical causes an exchange splitting between the spin-α and spin-β transmission spectra, allowing the single TCNQ junction to act as a spin-filtering device. © 2013 WILEY-VCH Verlag GmbH &amp
Co. KGaA, Weinheim.

DOI： 10.1002/cphc.201300136

Language：English   Publishing type：Research paper (scientific journal)  

Diarylethenes are photosensitive φ-conjugated molecules, being of great promise in potential applications to various molecular devices. Although the switching properties of diarylethenes have been widely investigated experimentally and theoretically, little is known about their rectifying diode-like behavior. In this study, electron-transport properties of asymmetric diarylethenes incorporating two different heterocyclic five-membered rings with opposite electronic demands are investigated with the nonequilibrium Green function combined with density functional theory. The aim of this study is to derive the effect of the heteroatomic defects on not only switching but also rectifying characteristics of the asymmetric diarylethenes. Obtained results show that a silicon atom involved in the diarylethenes plays an important role in the current rectifying as well as switching performance. It is found that maximum rectification ratios of the asymmetric diarylethenes increase linearly with an increase in electronegativity difference between the asymmetrically arranged heteroatoms. The silicon- and oxygen-containing asymmetric diarylethene is suggested to be a good potential candidate for a novel molecular electronic device combining a switch and a diode.

DOI： 10.1246/bcsj.20130089

Language：English   Publishing type：Research paper (scientific journal)  

Extended π-stacked molecules have attracted much attention since they play an essential role in both electronic devices and biological systems. In this article electron transport properties of a series of multilayered cyclophanes with the hydroquinone donor and quinone acceptor units in the external positions are theoretically studied with applications to molecular rectifiers in mind. Calculations of electron transport through the π-π stacked structures in the multilayered cyclophanes are performed by using nonequilibrium Green's function method combined with density functional theory. Calculated transmission spectra show that the conductance decreases exponentially with the length of the molecule with a decay factor of 0.75 Å^-1, which lies for the values between π-conjugated molecules and σ-bonded molecules. Applied bias calculations provide current-voltage curves, which exhibit good rectifying behavior. The rectification mechanism in the coherent transport regime is qualitatively explained by the response of the frontier orbital energy levels, especially LUMO levels, to the applied bias, where the rectifying direction is expected to be opposite to the Aviram-Ratner model. The maximum value of rectification ratio increases with an increase in the number of stacking layers due to the effective separation of the donor and acceptor parts, where effects from the opposite electrodes to the donor and acceptor are negligible. Multilayered donor-acceptor cyclophanes are suitable materials for investigating the relationship among electron transport properties, rectification properties, and molecular length (separation between the donor and acceptor parts).

DOI： 10.1021/jp308849t

Language：English   Publishing type：Research paper (scientific journal)  

Extended pi-stacked molecules have attracted much attention since they play an essential role in both electronic devices and biological systems. In this article electron transport properties of a series of multilayered cyclophanes with the hydroquinone donor and quinone acceptor units in the external positions are theoretically studied with applications to molecular rectifiers in mind. Calculations of electron transport through the pi-pi stacked structures in the multilayered cyclophanes are performed by using nonequilibrium Green's function method combined with density functional theory. Calculated transmission spectra show that the conductance decreases exponentially with the length of the molecule with a decay factor of 0.75 angstrom(-1), which lies for the values between pi-conjugated molecules and sigma-bonded molecules. Applied bias calculations provide current-voltage curves, which exhibit good rectifying behavior. The rectification mechanism in the coherent transport regime is qualitatively explained by the response of the frontier orbital energy levels, especially LUMO levels, to the applied bias, where the rectifying direction is expected to be opposite to the Aviram-Ratner model. The maximum value of rectification ratio increases with an increase in the number of stacking layers due to the effective separation of the donor and acceptor parts, where effects from the opposite electrodes to the donor and acceptor are negligible. Multilayered donor-acceptor cyclophanes are suitable materials for investigating the relationship among electron transport properties, rectification properties, and molecular length (separation between the donor and acceptor parts).

DOI： 10.1021/jp308849t

Language：English   Publishing type：Research paper (scientific journal)  

Electron transport properties through benzene molecules disubstituted with π-accepting cyanide and isocyanide anchor groups at their para and meta positions are investigated on the basis of a qualitative orbital analysis at the Hückel molecular orbital level of theory. The applicability of a previously derived orbital symmetry rule for electron transport is extended to the systems perturbed by the π-accepting anchor groups, where the HOMO-LUMO symmetry in the molecular orbital energies relative to the Fermi level is removed. The conservation of the HOMO-LUMO symmetry in the spatial distribution of the molecular orbitals between the unperturbed benzene molecule and the perturbed molecules with the anchor groups rationalizes symmetry-allowed electron transport through the para isomers. On the other hand, destructive interferences between the nearly 2-fold degenerate frontier orbitals constructed from the 2-fold degenerate orbitals of the unperturbed benzene molecule and the anchor groups lead to symmetry-forbidden electron transport through the meta isomers. The qualitative orbital thinking is supported by more quantitative density functional theory (DFT) calculations combined with the nonequilibrium Green's function (NEGF) method. The orbital analysis is a powerful tool for the understanding and rational design of molecular devices composed of π-conjugated hydrocarbons and those perturbed by the π-accepting anchor groups.

DOI： 10.1021/jp3068156

Language：English   Publishing type：Research paper (scientific journal)  

Electron transport properties through benzene molecules disubstituted with pi-accepting cyanide and isocyanide anchor groups at their para and meta positions are investigated on the basis of a qualitative orbital analysis at the Huckel molecular orbital level of theory. The applicability of a previously derived orbital symmetry rule for electron transport is extended to the systems perturbed by the pi-accepting anchor groups, where the HOMO-LUMO symmetry in the molecular orbital energies relative to the Fermi level is removed. The conservation of the HOMO-LUMO symmetry in the spatial distribution of the molecular orbitals between the unperturbed benzene molecule and the perturbed molecules with the anchor groups rationalizes symmetry. allowed electron transport through the para isomers. On the other hand, destructive interferences between the nearly 2-fold degenerate frontier orbitals constructed from the 2-fold degenerate orbitals of the unperturbed benzene molecule and the anchor groups lead to symmetry-forbidden electron transport through the meta isomers. The qualitative orbital thinking is supported by more quantitative density functional theory (DFT) calculations combined with the nonequilibrium Green's function (NEGF) method. The orbital analysis is a powerful tool for the understanding and rational design of molecular devices composed of pi-conjugated hydrocarbons and those perturbed by the pi-accepting anchor groups.

DOI： 10.1021/jp3068156

Language：English   Publishing type：Research paper (scientific journal)  

Electron transport properties of boron- and nitrogen-doped polycyclic aromatic hydrocarbons and cyclophanes are investigated with the nonequilibrium Greens function method and compared to transport properties of the unsubstituted species. The aim of the study is to derive the effect of the heteroatomic defects on the conductance of nanographenes and to propose new effective ways for current control and design of carbon devices. Of special interest are the electrical current rectifying properties of asymmetrically doped nanographenes and cyclophanes, as well as the rectification mechanism. The mechanisms of donor-π bridge-acceptor and donor-σ bridge-acceptor rectification are used to explain the diode-like properties of asymmetrically doped nanographenes and cyclophanes. The electron-rich nitrogen and electron-poor boron heteroatoms introduce conductance channels within the highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of the hydrocarbons and cyclophanes and significantly enhance the conductance. The combination of nitrogen and boron impurities in one polycyclic aromatic hydrocarbon leads to asymmetrical I/V curves. The rectification is further enhanced in the cyclophanes where the boron impurities are located in one of the layers and the nitrogen impurities in the other. Owing to the efficient separation of the donor and acceptor parts, a higher rectification ratio is estimated. The rectifying properties of the asymmetrically doped carbon materials are derived from the nonequilibrium Greens function theory. The main reason for the rectification is found to be the interaction of the external electric field induced between the electrodes with the molecular orbitals of the asymmetrically doped hydrocarbons.

DOI： 10.1021/jp303843k

Language：English   Publishing type：Research paper (scientific journal)  

Spintronic properties of cyclobutadiene (CBD) systems are investigated based on a qualitative frontier orbital analysis. CBD undergoes a Jahn-Teller distortion from the square triplet state to the rectangular singlet state. According to the qualitative Huckel molecular orbital analysis, the electron transport through the square triplet state is symmetry allowed, whereas that through the rectangular singlet state is symmetry forbidden. The magnetic triplet state is a possible coexisting system of conductivity and magnetism. Sophisticated first-principles quantum chemical calculations are performed by using a realistic molecular junction model. Obtained results are in good agreement with the prediction based on the qualitative orbital analysis. Interesting spin filtering properties are found in the square-shaped CBD system. The high- and low-spin states of the square-shaped CBD system produce the spin-alpha and spin-beta polarized conductance, respectively. The qualitative orbital analysis is useful as a guiding principle for designing molecular spintronics.

DOI： 10.1021/jp305448q

Language：English   Publishing type：Research paper (scientific journal)  

Electron transport properties of boron- and nitrogen-doped polycyclic aromatic hydrocarbons and cyclophanes are investigated with the nonequilibrium Green's function method and compared to transport properties of the unsubstituted species. The aim of the study is to derive the effect of the heteroatomic defects on the conductance of nanographenes and to propose new effective ways for current control and design of carbon devices. Of special interest are the electrical current rectifying properties of asymmetrically doped nanographenes and cyclophanes, as well as the rectification mechanism. The mechanisms of donor-pi bridge-acceptor and donor-sigma bridge-acceptor rectification are used to explain the diode-like properties of asymmetrically doped nanographenes and cyclophanes. The electron-rich nitrogen and electron-poor boron heteroatoms introduce conductance channels within the highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of the hydrocarbons and cyclophanes and significantly enhance the conductance. The combination of nitrogen and boron impurities in one polycyclic aromatic hydrocarbon leads to asymmetrical I/V curves. The rectification is further enhanced in the cyclophanes where the boron impurities are located in one of the layers and the nitrogen impurities in the other. Owing to the efficient separation of the donor and acceptor parts, a higher rectification ratio is estimated. The rectifying properties of the asymmetrically doped carbon materials are derived from the nonequilibrium Green's function theory. The main reason for the rectification is found to be the interaction of the external electric field induced between the electrodes with the molecular orbitals of the asymmetrically doped hydrocarbons.

DOI： 10.1021/jp303843k

Language：English   Publishing type：Research paper (scientific journal)  

Spintronic properties of cyclobutadiene (CBD) systems are investigated based on a qualitative frontier orbital analysis. CBD undergoes a Jahn-Teller distortion from the square triplet state to the rectangular singlet state. According to the qualitative Hückel molecular orbital analysis, the electron transport through the square triplet state is symmetry allowed, whereas that through the rectangular singlet state is symmetry forbidden. The magnetic triplet state is a possible coexisting system of conductivity and magnetism. Sophisticated first-principles quantum chemical calculations are performed by using a realistic molecular junction model. Obtained results are in good agreement with the prediction based on the qualitative orbital analysis. Interesting spin filtering properties are found in the square-shaped CBD system. The high- and low-spin states of the square-shaped CBD system produce the spin-α and spin-β polarized conductance, respectively. The qualitative orbital analysis is useful as a guiding principle for designing molecular spintronics.

DOI： 10.1021/jp305448q

Language：English   Publishing type：Research paper (scientific journal)  

A mechanism of the adhesion interaction between an aluminum oxide surface and an epoxy resin is investigated by using density-functional-theory (DFT) calculations. To understand effects of adsorbed water molecules on the adhesion interaction, hydroxylated aluminum oxide surfaces with adsorbed water molecules are prepared. Geometry optimization is performed for a model of adhesiveadherend complex, which is comprised of a fragment of epoxy resin and a wateradsorbed aluminum oxide surface. DFT calculations demonstrate that hydroxy groups and ether groups of epoxy resin can interact with the adherend surface via a hydrogen-bond network of adsorbed water molecules, which leads to a critical factor in the adhesion interaction. Plots of energy versus vertical distance of the resin from the surface are nicely approximated by the Morse potential. The force required for detachment of the resin from the surface can be estimated from the maximum value of the forcedistance curve, which is obtained from the derivative of the potential energy curve. Obtained results demonstrate that the hydrogen-bond network via adsorbed water molecules significantly affects the adhesion mechanism. The adsorbed water molecules provide a long-distance adhesion interaction but exert little influence over the maximum value of the adhesion force.

DOI： 10.1246/bcsj.20120028

Language：English   Publishing type：Research paper (scientific journal)  

A mechanism of the adhesion interaction between an aluminum oxide surface and an epoxy resin is investigated by using density-functional-theory (DFT) calculations. To understand effects of adsorbed water molecules on the adhesion interaction, hydroxylated aluminum oxide surfaces with adsorbed water molecules are prepared. Geometry optimization is performed for a model of adhesive adherend complex, which is comprised of a fragment of epoxy resin and a water-adsorbed aluminum oxide surface. DFT calculations demonstrate that hydroxy groups and ether groups of epoxy resin can interact with the adherend surface via a hydrogen-bond network of adsorbed water molecules, which leads to a critical factor in the adhesion interaction. Plots of energy versus vertical distance of the resin from the surface are nicely approximated by the Morse potential. The force required for detachment of the resin from the surface can be estimated from the maximum value of the force-distance curve, which is obtained from the derivative of the potential energy curve. Obtained results demonstrate that the hydrogen-bond network via adsorbed water molecules significantly affects the adhesion mechanism. The adsorbed water molecules provide a long-distance adhesion interaction but exert little influence over the maximum value of the adhesion force.

DOI： 10.1246/bcsj.20120028

Language：Japanese   Publishing type：Research paper (scientific journal)  

Molecular Theory of the Adhesion between Metal and Resin

Language：English   Publishing type：Research paper (scientific journal)  

Electron transport through π-π stacked materials has been studied theoretically and experimentally so far with versatile applications in mind. In this paper a novel π-π stacked molecular rectifier is proposed. Electron transport properties through cyclophane-type quinhydrone are investigated by using nonequilibrium Green's function method combined with density functional theory. The investigated molecule has a quinhydrone structure comprised of π-π stacked donor (hydroquinone) and acceptor (benzoquinone) pair due to the in-phase orbital interaction between the HOMO of hydroquinone and the LUMO of benzoquinone. A computed current-voltage curve shows rectifying behavior in the direction perpendicular to the ring plane. The maximum value of rectification ratio of 2.37 is obtained at 0.8 V. In this system the LUMO level plays a key role, and asymmetrical evolution of the LUMO level for positive and negative biases leads to the rectifying behavior. The present study is a basic step for further functionalization of a molecular rectifier based on transannular electron transport. The understanding of insight into the electron transport through a π-π stacked system will provide motivation for design of future molecular devices.

DOI： 10.1021/jp209547a

Language：English   Publishing type：Research paper (scientific journal)  

Electron transport through pi-pi stacked materials has been studied theoretically and experimentally so far with versatile applications in mind. In this paper a novel pi-pi stacked molecular rectifier is proposed. Electron transport properties through cyclophane-type quinhydrone are investigated by using nonequilibrium Green's function method combined with density functional theory. The investigated molecule has a quinhydrone structure comprised of pi-pi stacked donor (hydroquinone) and acceptor (benzoquinone) pair due to the in-phase orbital interaction between the HOMO of hydroquinone and the LUMO of benzoquinone. A computed current-voltage curve shows rectifying behavior in the direction perpendicular to the ring plane. The maximum value of rectification ratio of 2.37 is obtained at 0.8 V. In this system the LUMO level plays a key role, and asymmetrical evolution of the LUMO level for positive and negative biases leads to the rectifying behavior. The present study is a basic step for further functionalization of a molecular rectifier based on transannular electron transport. The understanding of insight into the electron transport through a pi-pi stacked system will provide motivation for design of future molecular devices.

DOI： 10.1021/jp209547a

Language：Japanese   Publishing type：Research paper (scientific journal)  

Molecular Theory of the Adhesion between Metal and Resin

Language：English   Publishing type：Research paper (scientific journal)  

The symmetry of a molecule junction has been shown to play a significant role in determining the conductance of the molecule, but the details of how conductance changes with symmetry have heretofore been unknown. Herein, we investigate a naphthalenedithiol single-molecule system in which sulfur atoms from the molecule are anchored to two facing gold electrodes. In the studied system, the highest single-molecule conductance, for a molecule junction of 1,4-symmetry, is 110 times larger than the lowest single-molecule conductance, for a molecule junction of 2,7-symmetry. We demonstrate clearly that the measured dependence of molecule junction symmetry for single-molecule junctions agrees with theoretical predictions.

DOI： 10.1021/ja2033926

Language：English   Publishing type：Research paper (scientific journal)  

The symmetry of a molecule junction has been shown to play a significant role in determining the conductance of the molecule, but the details of how conductance changes with symmetry have heretofore been unknown. Herein, we investigate a naphthalenedithiol single-molecule system in which sulfur atoms from the molecule are anchored to two facing gold electrodes. In the studied system, the highest single-molecule conductance, for a molecule junction of 1,4-symmetry, is 110 times larger than the lowest single-molecule conductance, for a molecule junction of 2,7-symmetry. We demonstrate clearly that the measured dependence of molecule junction symmetry for single-molecule junctions agrees with theoretical predictions.

DOI： 10.1021/ja2033926

Language：English   Publishing type：Research paper (scientific journal)  

A mechanism of the adhesion between an aluminum oxide surface and an epoxy resin is investigated by using density functional theory (DFT) calculations. Force field simulations are carried out for a better understanding of the dynamic behavior of the resin on the surface and for constructing models for DFT calculations. Stable structures of a resinsurface complex, adhesion energies, and details about interaction sites are obtained from geometry optimizations for some models based on DFT calculations with a plane-wave basis set and periodic boundary conditions. DFT calculations reveal that hydroxyl groups of the epoxy resin interact with the surface of aluminum oxide to form hydrogen bonds, which work as a main force for the adhesion. Plots of energy versus vertical distance of the resin from the surface are nicely approximated by the Morse potential. The force required for detachment of the resin from the surface can be estimated from the maximum value of the force-distance curve, which is obtained from the derivative of the potential energy curve. Obtained results demonstrate that hydrogen bonds play a central role for the adhesion between an aluminum oxide surface and an epoxy resin.

DOI： 10.1021/jp202785b

Language：English   Publishing type：Research paper (scientific journal)  

A mechanism of the adhesion between an aluminum oxide surface and an epoxy resin is investigated by using density functional theory (DFT) calculations. Force field simulations are carried out for a better understanding of the dynamic behavior of the resin on the surface and for constructing models for DFT calculations. Stable structures of a resin-surface complex, adhesion energies, and details about interaction sites are obtained from geometry optimizations for some models based on DFT calculations with a plane-wave basis set and periodic boundary conditions. DFT calculations reveal that hydroxyl groups of the epoxy resin interact with the surface of aluminum oxide to form hydrogen bonds, which work as a main force for the adhesion. Plots of energy versus vertical distance of the resin from the surface are nicely approximated by the Morse potential. The force required for detachment of the resin from the surface can be estimated from the maximum value of the force-distance curve, which is obtained from the derivative of the potential energy curve. Obtained results demonstrate that hydrogen bonds play a central role for the adhesion between an aluminum oxide surface and an epoxy resin.

DOI： 10.1021/jp202785b

Language：English   Publishing type：Research paper (scientific journal)  

Site-specific electron transport phenomena through benzene and benzenedithiol derivatives are discussed on the basis of a qualitative Huckel molecular orbital analysis for better understanding of the effect of anchoring sulfur atoms. A recent work for the orbital control of electron transport through aromatic hydrocarbons provided an important concept for the design of high-conductance connections of a molecule with anchoring atoms. In this work the origin of the frontier orbitals of benzenedithiol derivatives, the effect of the sulfur atoms on the orbitals and on the electron transport properties, and the applicability of the theoretical concept on aromatic hydrocarbons with the anchoring units are studied. The results demonstrate that the orbital view predictions are applicable to molecules perturbed by the anchoring units. The electron transport properties of benzene are found to be qualitatively consistent with those of benzenedithiol with respect to the site dependence. To verify the result of the Huckel molecular orbital calculations, fragment molecular orbital analyses with the extended Huckel molecular orbital theory and electron transport calculations with density functional theory are performed. Calculated results are in good agreement with the orbital interaction analysis. The phase, amplitude, and spatial distribution of the frontier orbitals play an essential role in the design of the electron transport properties through aromatic hydrocarbons.

DOI： 10.1021/ja111021e

Language：English   Publishing type：Research paper (scientific journal)  

The conductance through short DNA molecules connected to gold electrodes is studied with density functional theory and nonequilibrium Green's function method combined with density functional theory. The anchoring of the molecules to the electrodes is investigated, and in addition to the covalent S-Au bond, weak interactions between the aromatic heterocyclic bases and the electrodes are found. These weak interactions are important for the electron transport through DNA molecules. A tunneling mechanism is suggested, and the conductive properties of the nucleotides in a metal-molecule-metal junction are compared. Different four-nucleotide DNA sequences are investigated. A significant value for the current, 20 pA, is calculated for 1.5 V applied bias for a DNA sequence consisting of guanine and cytosine nucleotides. It is shown that adenine-thymine nucleotide pairs introduce potential barriers for the electron transport and therefore significantly decline the conductance. The obtained results are compared with recent experimental observations (Nanotechnology2009, 20, 115502) and confirm the possibility for electron transport through DNA molecules as well as provide an explanation for the reduced conductance through DNA sequences, which contain adenine-thymine nucleotide pairs. The results are compared with a previous theoretical study, performed with the extended Hückel method (ChemPhysChem2003, 4, 1256), which reports low conductance for DNA molecules. The difference in the conclusions is due to the applied bias self-consistent field calculations used in the recent study, which take into account the changes of the transmission probabilities with the bias.

DOI： 10.1021/jp110803a

Language：English   Publishing type：Research paper (scientific journal)  

The conductance through short DNA molecules connected to gold electrodes is studied with density functional theory and nonequilibrium Green's function method combined with density functional theory. The anchoring of the molecules to the electrodes is investigated, and in addition to the covalent S-Au bond, weak interactions between the aromatic heterocyclic bases and the electrodes are found. These weak interactions are important for the electron transport through DNA molecules. A tunneling mechanism is suggested, and the conductive properties of the nucleotides in a metal-molecule-metal junction are compared. Different four-nudeotide DNA sequences are investigated. A significant value for the current, 20 pA, is calculated for 1.5 V applied bias for a DNA sequence consisting of guanine and cytosine nucleotides. It is shown that adenine-thymine nucleotide pairs introduce potential barriers for the electron transport and therefore significantly decline the conductance. The obtained results are compared with recent experimental observations (Nanotechnology 2009, 20, 115502) and confirm the possibility for electron transport through DNA molecules as well as provide an explanation for the reduced conductance through DNA sequences, which contain adenine-thymine nucleotide pairs. The results are compared with a previous theoretical study, performed with the extended Huckel method (ChemPhysChern 2003, 4, 1256), which reports low conductance for DNA molecules. The difference in the conclusions is due to the applied bias self-consistent field calculations used in the recent study, which take into account the changes of the transmission probabilities with the bias.

DOI： 10.1021/jp110803a

Language：English   Publishing type：Research paper (scientific journal)  

Phenylene and biphenyl compounds with dibenzophosphole sulfide (DBPS) as an anchoring group for single molecule junctions were synthesized. The conductance measurements revealed that the phosphine sulfide indeed acts as an anchoring group for Au electrodes. Theoretical calculations including metal electrodes demonstrated that the LUMO level of the DBPS-terminated biphenyl is close to the Au Fermi level, leading to the electron conduction of the Au-molecule-Au junction based on the resonance-tunneling mechanism.

DOI： 10.1246/cl.2011.174

Language：English   Publishing type：Research paper (scientific journal)  

Phenylene and biphenyl compounds with dibenzophosphole sulfide (DBPS) as an anchoring group for single molecule junctions were synthesized. The conductance measurements revealed that the phosphine sulfide indeed acts as an anchoring group for Au electrodes. Theoretical calculations including metal electrodes demonstrated that the LUMO level of the DBPS-terminated biphenyl is close to the Au Fermi level, leading to the electron conduction of the AumoleculeAu junction based on the resonance-tunneling mechanism.

DOI： 10.1246/cl.2011.174

Language：English   Publishing type：Research paper (scientific journal)  

Diarylethenes ire photosensitive pi-conjugated molecules whose application to various molecular devices is expected. The molecular and electronic structures of diarylethenes are switchable upon photoirradiation with their reversible structural isomerization. Site-specific electron transport phenomena through a diarylethene molecule, 1,2-di(2-methyl-1-naphthyl) perfluorocyclopentene, are studied by using the nonequilibrium Green's function method combined with the Huckel molecular orbital method (NEGF-HMO) and density functional theory (NEGF-DFT). Oil the basis of the orbital symmetry rule, the conductance of the diarylethene is predicted to be efficiently switchable when the C3 and C10 atoms are appropriately connected with electrodes. Transmission spectra, spatial distribution of the MPSH (molecular projected self-consistent Hamiltonian) states, and I-V curves are obtained from DFT Calculations. These results obtained from the higher-level DFT calculations are consistent with the prediction based on the qualitative frontier orbital analysis at the HMO level of theory. The Computed Current for the closed-ring form of the 3-10 connection is about 3 orders of magnitude high compared with those for other connections. The phase, amplitude, and spatial distribution of the frontier orbitals play an essential role in designing the electron transport properties through the photoswitching system.

DOI： 10.1021/jp905663r

Language：English   Publishing type：Research paper (scientific journal)  

Diarylethenes are a class of photochromic molecules whose conductance switches with their photoisomerization. We have investigated the conductance of diarylethenes using non-equilibrium Green's function method combined with Hückel method (NEGF-HMO) and density functional theory (NEGF-DFT). In this study we have found that the qualitative predictions based on frontier orbital analysis are consistent with the DFT calculations and can be used for prediction of the electron transport properties of molecular devices.

DOI： 10.1016/j.tsf.2009.07.037

Language：English   Publishing type：Research paper (scientific journal)  

Diarylethenes are a class of photochromic molecules whose conductance switches with their photoisomerization. We have investigated the conductance of diarylethenes using non-equilibrium Green's function method combined with Huckel method (NEGF-HMO) and density functional theory (NEGF-DFT). In this study we have found that the qualitative predictions based on frontier orbital analysis are consistent with the DFT calculations and can be used for prediction of the electron transport properties of molecular devices. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2009.07.037

Language：English   Publishing type：Research paper (scientific journal)  

Diarylethenes are photosensitive π-conjugated molecules whose application to various molecular devices is expected. The molecular and electronic structures of diarylethenes are switchable upon photoirradiation with their reversible structural isomerization. Site-specific electron transport phenomena through a diarylethene molecule, 1,2-di(2-methyl-1-naphthyl) perfluorocyclopentene, are studied by using the nonequilibrium Green's function method combined with the Hückel molecular orbital method (NEGF-HMO) and density functional theory (NEGF-DFT). On the basis of the orbital symmetry rule, the conductance of the diarylethene is predicted to be efficiently switchable when the C3 and C10 atoms are appropriately connected with electrodes. Transmission spectra, spatial distribution of the MPSH (molecular projected self-consistent Hamiltonian) states, and I-V curves are obtained from DFT calculations. These results obtained from the higher-level DFT calculations are consistent with the prediction based on the qualitative frontier orbital analysis at the HMO level of theory. The computed current for the closed-ring form of the 3-10 connection is about 3 orders of magnitude high compared with those for other connections. The phase, amplitude, and spatial distribution of the frontier orbitals play an essential role in designing the electron transport properties through the photoswitching system.

DOI： 10.1021/jp905663r

Language：Japanese   Publishing type：Research paper (scientific journal)  

Phosphine Sulfide As an Anchor Unit for Metal-Molecule Junction

DOI： 10.11494/kisoyuki.2009.0.221.0

Event date： 2018.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：愛知, 名古屋大学   Country：Japan  

14族のリチウム化合物はリチウムテトレライドと呼ばれる。その中でも、もっともリチウムの割合が高いLi17Tt4 (Tt = Si, Ge, Sn, Pb)は余剰電子を有していることが密度汎関数計算から明らかとなった。その余剰電子は核を離れて格子間に存在しているため、電子自身がアニオンとして振る舞う電子化物（エレクトライド）の一種であると考えられる。状態密度や電子密度の解析により、余剰電子の波動関数はそれを取り囲むイオンの波動関数と軌道相互作用をしており、新奇な結合様式が見いだされた。

Event date： 2018.3

Language：English   Presentation type：Oral presentation (general)  

Venue：カンボジア   Country：Japan  

The wave-particle duality of electrons gives rise to quantum interference (QI) in single
molecular devices. A significant challenge to be addressed in molecular electronics is to
further develop chemical intuition to understand and predict QI features. In this study, an
orbital rule is markedly ameliorated so that it can capture the manifestation of QI not only in
alternant hydrocarbons but also in nonalternant ones. The orbital-based prediction about the
occurrence of QI in a nonalternant hydrocarbon shows good agreement with experimental
results. A simple perturbation theoretic line of reasoning suggests that frontier orbital phase
and splitting play a pivotal role in QI phenomena.

Language：Japanese  

Venue：福岡、九州大学筑紫キャンパス   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Language：English   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：神奈川大学みなとみらいキャンパス   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京理科大学野田キャンパス   Country：Japan  

Language：English   Presentation type：Oral presentation (general)  

Venue：九州大学伊都キャンパス   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：新化学技術推進協会   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：大阪大学   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：島根大学   Country：Japan  

Language：English   Presentation type：Oral presentation (general)  

Venue：理研   Country：Japan  

Language：English   Presentation type：Oral presentation (general)  

Venue：東京工業大学   Country：Japan  

Language：English   Presentation type：Oral presentation (general)  

Venue：国立台湾大学   Country：Taiwan, Province of China  

Language：Japanese  

Venue：オンライン   Country：Japan  

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Language：Japanese  

Venue：東京理科大学野田キャンパス   Country：Japan  

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Venue：神奈川大学みなとみらいキャンパス   Country：Japan  

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Venue：オンライン   Country：Japan  

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Venue：福岡、九州大学筑紫キャンパス   Country：Japan  

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Venue：九州大学伊都キャンパス   Country：Japan  

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Venue：オンライン   Country：Japan  

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Language：Japanese  

Venue：新化学技術推進協会   Country：Japan  

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Event date： 2022.5

Language：Japanese  

Venue：オンライン   Country：Japan  

Event date： 2022.5

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Venue：オンライン   Country：Japan  

Event date： 2022.5

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Venue：オンライン   Country：Japan  

Event date： 2022.5

Language：Japanese  

Venue：オンライン   Country：Japan  

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Event date： 2022.5

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Venue：オンライン   Country：Japan  

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Event date： 2022.5

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Venue：オンライン   Country：Japan  

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Event date： 2022.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.3

Language：Japanese  

Venue：オンライン   Country：Japan  

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Event date： 2021.12

Language：English  

Venue：オンライン   Country：Japan  

Event date： 2021.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2021.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2021.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2021.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2021.5

Language：Japanese  

Venue：オンライン   Country：Japan  

Event date： 2021.5

Language：Japanese  

Venue：オンライン   Country：Japan  

Event date： 2021.3

Language：Japanese  

Venue：オンライン   Country：Japan  

Event date： 2021.3

Language：Japanese  

Venue：オンライン   Country：Japan  

Event date： 2021.3

Language：Japanese  

Venue：オンライン   Country：Japan  

Event date： 2021.3

Language：Japanese  

Venue：オンライン   Country：Japan  

Event date： 2021.3

Language：English  

Venue：オンライン   Country：Japan  

Event date： 2020.11

Language：Japanese  

Venue：オンライン   Country：Japan  

Event date： 2020.9

Language：Japanese  

Venue：オンライン   Country：Japan  

Event date： 2019.12

Language：English  

Venue：横浜シンポジア   Country：Japan  

Event date： 2019.12

Language：English   Presentation type：Oral presentation (general)  

Venue：横浜シンポジア   Country：Japan  

Event date： 2019.3

Language：English   Presentation type：Oral presentation (general)  

Venue：甲南大学　岡本キャンパス   Country：Japan  

Event date： 2019.1

Language：English  

Venue：京都大学　化学研究所（宇治市五ヶ庄） おうばくプラザ   Country：Japan  

Event date： 2018.11

Language：Japanese  

Venue：東京大学本郷キャンパス小柴ホール   Country：Japan  

Event date： 2018.11

Language：Japanese  

Venue：北海道大学 創成科学研究棟   Country：Japan  

Event date： 2018.10

Language：Japanese  

Venue：筑紫キャンパス　総合研究棟（C-CUBE）   Country：Japan  

Event date： 2018.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：広島、JMSアステールプラザ   Country：Japan  

Event date： 2018.10

Language：English  

Venue：京都大学　福井謙一記念研究センター   Country：Japan  

Event date： 2018.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：福岡国際会議場   Country：Japan  

Event date： 2018.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：福岡国際会議場   Country：Japan  

Event date： 2018.7

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京大学（本郷キャンパス）   Country：Japan  

Event date： 2018.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海度、国民休暇村　支笏湖   Country：Japan  

Event date： 2018.5

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：愛知、岡崎   Country：Japan  

近年、IrO2 (110)面上においてメタンのCH結合が容易に解離するということが実験的に報告されている。我々はIrO2表面がCH結合を活性化するプロセスについてバンド計算及び分子軌道計算の両面から詳細に調べた。その結果、メタンの構造変形に伴う、HOMOのエネルギー準位および軌道分布の変化が重要であるということが明らかとなった。

Event date： 2018.3

Language：English   Presentation type：Oral presentation (general)  

Venue：千葉、日本大学   Country：Japan  

The wave-particle duality of electrons gives rise to quantum interference (QI) in single
molecular devices. A significant challenge to be addressed in molecular electronics is to
further develop chemical intuition to understand and predict QI features. In this study, an
orbital rule is markedly ameliorated so that it can capture the manifestation of QI not only in
alternant hydrocarbons but also in nonalternant ones. The orbital-based prediction about the
occurrence of QI in a nonalternant hydrocarbon shows good agreement with experimental
results. A simple perturbation theoretic line of reasoning suggests that frontier orbital phase
and splitting play a pivotal role in QI phenomena.

Event date： 2018.1

Language：English  

Venue：九州大学   Country：Japan  

The wave-particle duality of electrons gives rise to quantum interference (QI) in single
molecular devices. A significant challenge to be addressed in molecular electronics is to
further develop chemical intuition to understand and predict QI features. In this study, an
orbital rule is markedly ameliorated so that it can capture the manifestation of QI not only in
alternant hydrocarbons but also in nonalternant ones. The orbital-based prediction about the
occurrence of QI in a nonalternant hydrocarbon shows good agreement with experimental
results. A simple perturbation theoretic line of reasoning suggests that frontier orbital phase
and splitting play a pivotal role in QI phenomena.

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Language：Japanese  

Venue：福岡、北九州国際会議場   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：横浜、慶應大矢上キャンパス   Country：Japan  

Language：Japanese  

Venue：札幌、北海道大学   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：横浜、慶応大矢上キャンパス   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：長野、信州大学工学部   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：千葉、東京理科大   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：慶應義塾大学 矢上キャンパス   Country：Japan  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：神奈川大学みなとみらいキャンパス   Country：Japan  

Language：English  

Venue：東京理科大学　野田キャンパス   Country：Japan  

Language：Japanese  

Venue：札幌、北海道大学   Country：Japan  

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Venue：オンライン   Country：Japan  

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Language：Japanese  

Language：Japanese  

Language：Japanese  

Venue：横浜、慶應大矢上キャンパス   Country：Japan  

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Language：Japanese  

Venue：横浜、慶応大矢上キャンパス   Country：Japan  

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Venue：福岡、北九州国際会議場   Country：Japan  

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Language：Japanese  

Venue：慶應義塾大学 矢上キャンパス   Country：Japan  

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Language：Japanese  

Venue：長野、信州大学工学部   Country：Japan  

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Venue：神奈川大学みなとみらいキャンパス   Country：Japan  

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Language：Japanese  

Language：Japanese  

Venue：千葉、東京理科大   Country：Japan  

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Language：Japanese  

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

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Language：Others   Publisher：Comprehensive Inorganic Chemistry III, Third Edition  

The activation and formation of bonds on inorganic surfaces are important elementary processes in heterogeneous catalysis. The purpose of this chapter is to provide one way to clarify these processes in terms of the electronic structure of solid surfaces. In particular, the reader will learn how to rationalize the origin of the activation energy for bond breaking and formation on surfaces, focusing on the orbital interactions between the surface and the bond. There are very useful theoretical chemistry tools such as crystal orbital overlap population and crystal orbital Hamilton population that can be used to effectively retrieve information on bonds that are obscured in a bunch of surface bands. Local information on bonding on the surface is obtained. Information on the occupancy of electrons in the bonding and antibonding orbitals of the bond will prove to be very useful. Using such tools, one prescription is offered for the question of how to understand why the activation energy of bond breaking and formation on inorganic surfaces is high or low. What is covered in this chapter begins with simple metal surfaces and ends with more complex oxide and hydride surfaces. The interaction of various inorganic surfaces with bonds will be overviewed using examples from important topics in catalytic chemistry such as CO activation, Fischer-Tropsch synthesis, methane activation, and ammonia synthesis.

DOI： 10.1016/B978-0-12-823144-9.00132-1

Scopus

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Language：Japanese   Publisher：日本塗装技術協会  

CiNii Books

CiNii Research

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Language：Japanese  

Language：Japanese   Publisher：技術情報協会  

CiNii Books

CiNii Research

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： https://doi.org/10.2477/jccj.2021-0013

Language：Japanese  

Language：English  

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Language：Japanese  

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： https://doi.org/10.2477/jccj.2019-0038

Language：Japanese  

Language：Japanese  

本稿では，アメリカのニューヨーク州イサカ市にある
コーネル大学にポスドクとして留学した 3 年間の経験を
研究と生活の面から書かせていただこうと思う

DOI： https://doi.org/10.1380/vss.61.91

Language：Japanese  

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：17

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：10

Type：Competition, symposium, etc. 

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：12

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Number of participants：30

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：10

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：31

This study examined the nature of the electronic structure of representative cross-conjugated polyenes from a Valence Bond (VB) perspective. Our VBSCF calculations on a prototypical dendralene model reveal a remarkable inhibition of the delocalization compared to linear polyenes. Especially along the C-C backbone, the delocalization is virtually quenched, so that these compounds can essentially be considered as sets of isolated butadiene units.

In recent years, a considerable interest has grown in the design of molecular nanowires with an increasing conductance with length. The development of such nanowires is highly desirable because they could play an important role in future molecular-scale circuitry. Whereas the first experimental observation of this nonclassical behavior still has to be realized, a growing number of candidate wires have been proposed theoretically. In this Letter, we point out that all the wires with an anti-Ohmic increasing conductance with length proposed so far share a common characteristic: their diradical character increases with length.

In this paper, we explore quantum interference (QI) in molecular conductance from the point of view of graph theory and walks on lattices. By virtue of the Cayley–Hamilton theorem for characteristic polynomials and the Coulson–Rushbrooke pairing theorem for alternant hydrocarbons, it is possible to derive a finite series expansion of the Green’s function for electron transmission in terms of the odd powers of the vertex adjacency matrix or Hückel matrix. This means that only odd-length walks on a molecular graph contribute to the conductivity through a molecule. Thus, if there are only even-length walks between two atoms, quantum interference is expected to occur in the electron transport between them.

Site-specific electron transport phenomena through benzene and benzenedithiol derivatives are discussed on the basis of a qualitative Hückel molecular orbital analysis for better understanding of the effect of anchoring sulfur atoms. A recent work for the orbital control of electron transport through aromatic hydrocarbons provided an important concept for the design of high-conductance connections of a molecule with anchoring atoms. In this work the origin of the frontier orbitals of benzenedithiol derivatives, the effect of the sulfur atoms on the orbitals and on the electron transport properties, and the applicability of the theoretical concept on aromatic hydrocarbons with the anchoring units are studied. The results demonstrate that the orbital view predictions are applicable to molecules perturbed by the anchoring units. The electron transport properties of benzene are found to be qualitatively consistent with those of benzenedithiol with respect to the site dependence. To verify the result of the Hückel molecular orbital calculations, fragment molecular orbital analyses with the extended Hückel molecular orbital theory and electron transport calculations with density functional theory are performed. Calculated results are in good agreement with the orbital interaction analysis. The phase, amplitude, and spatial distribution of the frontier orbitals play an essential role in the design of the electron transport properties through aromatic hydrocarbons.

An empirical observation of a relationship between a striking feature of electronic transmission through a π-system, destructive quantum interference (QI), on one hand, and the stability of diradicals on the other, leads to the proof of a general theorem that relates the two. Subject to a number of simplifying assumptions, in a π-electron system, QI occurs when electrodes are attached to those positions of an N-carbon atom N-electron closed-shell hydrocarbon where the matrix elements of the Green’s function vanish. These zeros come in two types, which are called easy and hard. Suppose an N+2 atom, N+2 electron hydrocarbon is formed by substituting 2 CH2 groups at two atoms, where the electrodes were. Then, if a QI feature is associated with electrode attachment to the two atoms of the original N atom system, the resulting augmented N+2 molecule will be a diradical. If there is no QI feature, i.e., transmission of current is normal if electrodes are attached to the two atoms, the resulting hydrocarbon will not be a diradical but will have a classical closed-shell electronic structure. Moreover, where a diradical exists, the easy zero is associated with a nondisjoint diradical, and the hard zero is associated with a disjoint one. A related theorem is proven for deletion of two sites from a hydrocarbon.