Language：English   Publisher：Journal of the American Chemical Society  

Luminescent organic radicals often exhibit a red-shifted, broad, and structureless emission band when assembled in a host matrix. This emission has been attributed to pairs of neighboring photoexcited and ground-state radicals forming an excimer. However, the electronic character of this “excimer-like” (EX) state has not been definitively clarified through experiments. Here we present an experimental characterization of this EX state using a spatially confined luminescent diradical as a model for radical dimers. Comprehensive spectroscopic investigations revealed that the EX state of the diradical is a zwitterionic (ZI) state with a closed-shell singlet electronic configuration generated by intramolecular symmetry-breaking radical-to-radical charge transfer. In combination with previous magnetoluminescence studies, we experimentally show that the formation of the ZI state is specific to the singlet spin state. Our results enable a detailed understanding of the photophysics and photochemistry of radical multichromophoric systems for future spin photonics applications.

DOI： 10.1021/jacs.5c17112

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Language：English   Publisher：Chemistry A European Journal  

μ-Oxo dimers of octaaryl-substituted silicon porphyrazines (SiPzs) were synthesized for the first time. The steric congestion due to the peripheral aryl groups attenuated the rotation of the SiPz units around the Si–O–Si axis, resulting in the fixed D<inf>4</inf> symmetric conformations with the torsion angle of ∼23° at room temperature or lower, which were unambiguously elucidated by the single-crystal X-ray diffraction analysis and variable-temperature NMR studies. The interlayer molecular orbital interactions between the stacked SiPz units perturbed UV/vis absorption spectra and electrochemical properties of the dimers. In addition, the hetero-type dimer, bearing electron-donating and electron-withdrawing aryl-substituents on each SiPz unit, showed a large two-photon absorption cross-section based on the reverse saturable absorption mechanism mediated by the intramolecular charge transfer interactions.

DOI： 10.1002/chem.202502913

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Language：English   Publisher：Chemical Science  

Liquid is the most flexible state of condensed matter and shows promise as a functional soft material. However, these same characteristics make it challenging to achieve efficient room-temperature phosphorescence (RTP) from metal-free organic molecular liquids. Herein, we report efficient RTP from liquefied thienyl diketones bearing one or two dimethyloctylsilyl (DMOS) substituents. These solvent-free liquids exhibit high RTP quantum yields up to 5.6% in air and 25.6% under Ar due to their large RTP rate constant exceeding 5000 s⁻¹. Both liquids undergo excited-state conformational changes and afford monomer RTP, exhibiting essentially the same narrowband spectra as in solution. Moreover, introducing two DMOS substituents sufficiently suppresses aggregation-caused quenching of the molecularly emissive phosphors, illustrating a design principle for RTP-active liquid materials.

DOI： 10.1039/d5sc03768a

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Language：English   Publisher：Journal of Physical Chemistry Letters  

Boron nitride nanotubes (BNNTs) are used for the self-assembly of tetracene (Tc) molecules without chemical modification in their one-dimensional nanocavities. The Tc aggregated state can be changed depending on the encapsulated Tc amount that is controlled by the sublimation conditions of Tc molecules for the complexation (Tc@BNNTs). As a result, the photophysical processes, including singlet fission in the Tc assemblies, are varied, which enables the emission mode switching of the aggregation-based fluorescence and the excimer emission for Tc@BNNTs. Consequently, the host–guest complexation using BNNTs allows not only the fabrication and evaluation of optically functionalized molecular assemblies in the nanocavity but also the modulation of their photophysical processes, by which the emission colors of Tc@BNNTs are also tuned.

DOI： 10.1021/acs.jpclett.5c01618

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Publisher：Chemphotochem  

Development of near-infrared (NIR)-light-driven photocatalytic hydrogen (H<inf>2</inf>) production is indispensable for an efficient use of solar energy in a sustainable society. Herein, a strategy is reported for achieving higher quantum yields of NIR-light-driven photocatalytic H<inf>2</inf> production using salts of diprotonated dodecaphenylporphyrin (H<inf>4</inf>DPP²⁺(X⁻)<inf>2</inf>), showing large saddle distortion as organic photosensitizers and platinum nanoparticles as catalysts. When a perchlorate ion (ClO<inf>4</inf>⁻) is employed as a counter ion (X⁻) of H<inf>4</inf>DPP²⁺(X⁻)<inf>2</inf>, the quantum yield of the photocatalytic H<inf>2</inf> production reaches to 36% by excitation at 750 nm, which is the highest value among those of NIR-light-driven H<inf>2</inf> production reported so far. Such a drastic enhancement of the quantum yield is achieved by selection of X⁻ as an appropriate counter anion of H<inf>4</inf>DPP²⁺, judging from second-order rate constants of photoinduced electron transfer (ET) from electron donors to ³(H<inf>4</inf>DPP²⁺(X⁻)<inf>2</inf>)* and the reorganization energy of ET for H<inf>4</inf>DPP²⁺(X⁻)<inf>2</inf>.

DOI： 10.1002/cptc.202500009

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Language：English   Publisher：Journal of Chemical Physics  

Introducing heavy atoms, or replacing atoms with heavier ones, is a routine approach for accelerating spin-flipping photophysical processes. However, predicting its impact on phosphorescence efficiency is not straightforward. Herein, we report an unexpected consequence of bromine-to-iodine substitution in a bromothienyl diketone derivative, TIPS-BrTn, that exhibits outstanding room-temperature phosphorescence (RTP) in cyclohexane solution. Contrary to our expectation, the iodo-congener TIPS-ITn exhibited feeble photoluminescence, which we confirmed as RTP by ultrafast spectroscopy. Further experimental and theoretical studies revealed that, in the T<inf>1</inf> state, an excited-state symmetry breaking occurred on TIPS-ITn while TIPS-BrTn preserved the centrosymmetric geometry. We identified the driving force for the symmetry breaking as an intramolecular two-center three-electron bonding interaction between iodine and carbonyl oxygen in the (n,π*) excited state. Consequently, while the direct T<inf>1</inf>-S<inf>0</inf> spin-orbit coupling (SOC) in TIPS-BrTn is symmetry-forbidden and zero, that of TIPS-ITn is non-zero due to the loss of centrosymmetry, thereby accelerating nonradiative T<inf>1</inf>-S<inf>0</inf> decay to diminish the RTP. Importantly, the phosphorescence rate constant is not solely dictated by the direct T<inf>1</inf>-S<inf>0</inf> SOC; instead, it can be rationalized by the intensity borrowing from higher singlet states. Thus, our work highlights the importance of controlling molecular symmetry, which could suppress the direct T<inf>1</inf>-S<inf>0</inf> SOC and lead to a preferential acceleration of radiative decay over nonradiative decay for achieving efficient RTP.

DOI： 10.1063/5.0255535

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Language：English   Publisher：Angewandte Chemie International Edition  

Although a widely used and important industrial chemical, carbon disulfide (CS<inf>2</inf>) poses a number of hazards due to its volatility and toxicity. As such, the development of multifunctional materials for the selective capture and easy recognition of CS<inf>2</inf> is one of the crucial issues. Herein, we demonstrate completely selective CS<inf>2</inf> adsorption among trials involving H<inf>2</inf>O, alcohols, volatile organic compounds (including thiol derivatives), N<inf>2</inf>, H<inf>2</inf>, O<inf>2</inf>, CH<inf>4</inf>, CO, NO, and CO<inf>2</inf>. We also showcase its fine detection using remarkable luminescent response in a gold(I)-based metal−organic framework (MOF) of {Zn^II(pz)[Au^I(CN)<inf>2</inf>]<inf>2</inf>} (pz=pyrazine; 1) with a two-fold interpenetration network. Ex situ single crystal X-ray diffraction for 1 and CS<inf>2</inf>-accommodated 1 suggested that the Au ⋅⋅⋅ Au atoms are not only luminescent centers but also act as interaction sites for CS<inf>2</inf> modulating the Au ⋅⋅⋅ Au contacts. These experiments revealed the specificity of CS<inf>2</inf> and how changes in the CS<inf>2</inf>-induced structure. Based on the obtained structural transformation, 1 exhibited a sensitive detecting ability for CS<inf>2</inf> with an ultrafast response time of less than 10 s. Moreover, ex situ time-resolved photoluminescence analyses developed in this work implied that CS<inf>2</inf> varied the energetic relaxation at the excited states related to the luminescent efficiency of the resultant MOF system.

DOI： 10.1002/anie.202413830

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Language：English   Publisher：Chemical Communications  

Peripherally unsubstituted dioxadiaza[8]circulene, as the first example of structurally identified pristine hetero[8]circulene, was synthesized by the substituent detachment reactions. The solid-state structures and photophysical properties were analysed to elucidate intermolecular interactions. Herzberg–Teller type emission was considered to explain the optical behavior.

DOI： 10.1039/d4cc05539j

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Publisher：Advanced Functional Materials  

Photomechanical crystals have applications in light-fueled actuators and soft robots. Herein, light-responsive, versatile, anthraquinone dye crystal oscillators actuated via natural vibrations that are resonated by a photothermal effect are described. A black needle-shaped crystal cantilever oscillates at 70 Hz in the first mode of natural vibration upon irradiation by broad-wavelength light ranging from the ultraviolet through the visible to the near-infrared, and also under continuous-wavelength light (400–2000 nm). The second and third natural vibration modes are induced at higher frequencies (530 and 1350 Hz) and evidence complex flagellum-like motions. The frequency can be readily tuned by moving the support of a crystal piece; this is analogous to playing a guitar. The crystal exhibits high durability (more than 10 000 cycles): the high elasticity prevents deterioration. Oscillatory motions can be designed via simulations using finite element analysis. This work will facilitate the use of photomechanical crystals in light-fueled soft robots.

DOI： 10.1002/adfm.202410671

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Language：English   Publisher：Inorganic Chemistry  

We have explored the photophysical properties and one-electron reduction process in the dyad photocatalyst for CO<inf>2</inf> photoreduction, ZnP-phen=Re, in which the catalyst of fac-[Re(1,10-phenanthoroline)(CO)<inf>3</inf>Br] is directly connected with the photosensitizer of zinc(II) porphyrin (ZnP), using time-resolved infrared spectroscopy, transient absorption spectroscopy, and quantum chemical calculations. We revealed the following photophysical properties: (1) the intersystem crossing occurs with a time constant of ∼20 ps, which is much faster than that of a ZnP single unit, and (2) the charge density in the excited singlet and triplet states is mainly localized on ZnP, which means that the excited state is assignable to the π-π* transition in ZnP. The one-electron reduction by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole occurs via the triplet excited state with the time constant of ∼100 ns and directly from the ground state with the time constant of ∼3 μs. The charge in the one-electron reduction species spans ZnP and the phenanthroline ligand, and the dihedral angle between ZnP and the phenanthroline ligand is rotated by ∼24° with respect to that in the ground state, which presumably offers an advantage for proceeding to the next CO<inf>2</inf> reduction process. These insights could guide the new design of dyad photocatalysts with porphyrin photosensitizers.

DOI： 10.1021/acs.inorgchem.4c02271

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Language：English   Publisher：Chemical Science  

We report metal-free organic 1,2-diketones that exhibit fast and highly efficient room-temperature phosphorescence (RTP) with high colour purity under various conditions, including solutions. RTP quantum yields reached 38.2% in solution under Ar, 54% in a polymer matrix in air, and 50% in crystalline solids in air. Moreover, the narrowband RTP consistently dominated the steady-state emission, regardless of the molecular environment. Detailed mechanistic studies using ultrafast spectroscopy, single-crystal X-ray structure analysis, and theoretical calculations revealed picosecond intersystem crossing (ISC) followed by RTP from a planar conformation. Notably, the phosphorescence rate constant k<inf>p</inf> was unambiguously established as ∼5000 s⁻¹, which is comparable to that of platinum porphyrins (representative heavy-metal phosphor). This inherently large k<inf>p</inf> enabled the high-efficiency RTP across diverse molecular environments, thus complementing the streamlined persistent RTP approach. The mechanism behind the photofunction has been elucidated as follows: (1) the large k<inf>p</inf> is due to efficient intensity borrowing of the T<inf>1</inf> state from the bright S<inf>3</inf> state, (2) the rapid ISC occurs from the S<inf>1</inf> to the T<inf>3</inf> state because these states are nearly isoenergetic and have a considerable spin-orbit coupling, and (3) the narrowband emission results from the minimal geometry change between the T<inf>1</inf> and S<inf>0</inf> states. Such mechanistic understanding based on molecular orbitals, as well as the structure-RTP property relationship study, highlighted design principles embodied by the diketone planar conformer. The fast RTP strategy enables development of organic phosphors with emissions independent of environmental conditions, thereby offering alternatives to precious-metal based phosphors.

DOI： 10.1039/d4sc02841d

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Language：English   Publisher：Angewandte Chemie International Edition  

We synthesized a (1-propylpyridinium)<inf>2</inf>[ReN(CN)<inf>4</inf>]-type organic–inorganic hybrid exhibiting water-vapor-induced drastic structural changes of the [ReN(CN)<inf>4</inf>]²⁻ assemblies. Specifically, upon exposure to water vapor, dehydrated nitrido-bridged chains were converted to hydrated cyanido-bridged tetranuclear clusters via rearrangements of large molecular building units in the crystals. These switchable assembly forms display substantially different photo-physical properties, although in both cases the emission is caused by a metal-centered d–d transition. The nitrido-bridged chain exhibited a near-infrared (749 nm) emission, which blue-shifted as the temperature increased, while a visible (561 nm) emission and its red shift was demonstrated by the cyanido-bridged cluster.

DOI： 10.1002/anie.202306853

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DOI： 10.1002/anie.202307093

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Language：English   Publisher：Angewandte Chemie International Edition  

This study reports the facile syntheses of tetra-boron difluoride (tetra-BF<inf>2</inf>) complexes, flag-hinge-like molecules that exhibit intense green-to-orange luminescence in solution and yellow-to-red emission in the solid states. Single-crystal structure analysis and density functional theory calculations suggested a bent structure of this series of compounds. The complexes also exhibited excellent optical properties, with quantum yields reaching 100 % and a large Stokes shift. These properties were attributed to the altered bending angle of the molecule in the S<inf>1</inf> excited state. As the rotational motion was suppressed around the 2,2′-bipyrrole axis, atropisomers with axial chirality were formed, which are optically resolvable into (R) and (S)-enantiomers through a chiral column. The atropisomers thus function as circularly polarized luminescent (CPL) materials, in which the color (green, green-yellow, and yellow) can be varied by controlling the aggregation state. This rational design of multi-BF<inf>2</inf> complexes can potentially realize novel photofunctional materials.

DOI： 10.1002/anie.202204358

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Publisher：Bulletin of the Chemical Society of Japan  

We investigated the correlation between the photophysical properties and detailed excited-state characteristics of a multiple-resonance-type thermally activated delayed fluorescence (TADF) molecule, DABNA-1, using time-resolved infrared vibrational spectroscopy. By comparing the distinctive vibrational spectra in the fingerprint region (10001700 cm¹¹) to the simulated spectra, we found the optimal calculation conditions for density functional theory calculations to retrieve the vibrational spectra. Based on the calculations, the excited-state geometries and molecular orbitals in the lowest excited singlet (S<inf>1</inf>) and triplet (T<inf>1</inf>) states, as well as the ground state (S<inf>0</inf>), were determined. Consequently, we revealed that the similarity between the potential surfaces of T<inf>1</inf> and S<inf>0</inf> suppressed non-radiative decay and improved the high fluorescence quantum yield via TADF. Furthermore, we calculated the spin-orbit coupling matrix elements (SOCMEs) considering the experimentally confirmed geometries, and revealed that twisting of the main skeleton increases the SOCMEs.

DOI： 10.1246/bcsj.20210403

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