Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Albrecht Ken Last modified date:2023.03.31

Associate Professor / Department of Integrated Materials / Institute for Materials Chemistry and Engineering

1. Sota Yoshida, Masaki Sampei, Naoto Todoroki, Eri Hisamura, Kohei Nakao, Ken Albrecht, Toshimasa Wadayamaa , Surface modification of gold by carbazole dendrimers for improved carbon dioxide electroreduction, Chemical Communications, 10.1039/D3CC00350G, 59, 3459-3462, 2023.02.
2. Kenshiro Matsuda, Xiaotian Rui, Kazuhiro Nakamura, Minori Furukori, Takuya Hosokai, Kosuke Anraku, Kohei Nakao, Ken Albrecht, Photostability of luminescent tris(2,4,6‐trichlorophenyl)methyl radical enhanced by terminal modification of carbazole donor, Chemical Communications, 10.1039/D2CC04481A, 2022.11.
3. Jun WATANABE, Masaki FURUSAWA, Kosuke NAKAMOTO, Yuchao SUN, Masatoshi TASHIMA, Keiko YAMAOKA, Seiko FUJIWARA, Han Seul KIM, Shigeto OKADA, Ken ALBRECHT , 3,6-Diphenyltetrazine as Cathode Active Material for Sodium Ion Batteries, Electrochemistry, 10.5796/electrochemistry.22-00100, 2022.11.
4. Ken Albrecht, Eri Hisamura, Minori Furukori, Yasuo Nakayama, Takuya Hosokai, Kohei Nakao, Hiroki Ikebe, Akira Nakayama, Thermally activated delayed fluorescence of carbazole-benzophenone dendrimers with bulky substituents, POLYMER CHEMISTRY, 10.1039/d2py00255h, 13, 16, 2277-2284, 2022.02.
5. Ken Albrecht, Maki Taguchi, Takamasa Tsukamoto, Tatsuya Moriai, Nozomi Yoshidaa, Kimihisa Yamamoto, Poly-phenylene jacketed tailor-made dendritic phenylazomethine ligand for nanoparticle synthesis, CHEMICAL SCIENCE, 10.1039/d1sc05661a, 13, 20, 5813-5817, 2022.03, Synthesizing metal clusters with a specific number of atoms on a preparative scale for studying advanced properties is still a challenge. The dendrimer templated method is powerful for synthesizing size or atomicity controlled nanoparticles. However, not all atomicity is accessible with conventional dendrimers. A new tailor-made phenylazomethine dendrimer (DPA) with a limited number of coordination sites (n = 16) and a non-coordinating large poly-phenylene shell was designed to tackle this problem. The asymmetric dendron and adamantane core four substituted dendrimer (PPDPA16) were successfully synthesized. The coordination behavior confirmed the accumulation of 16 metal Lewis acids (RhCl3, RuCl3, and SnBr2) to PPDPA16. After the reduction of the complex, low valent metal nanoparticles with controlled size were obtained. The tailor-made dendrimer is a promising approach to synthesize a variety of metal clusters with desired atomicity..
6. Baowei Xie, Ryo Sakamoto, Ayuko Kitajou, Liwei Zhao, Kosuke Nakamoto, Shigeto Okada, Ken Albrecht, Wataru Kobayashi, Masaki Okadae Toshiya Takahara, Enhanced electrochemical performance of Li2.72Na0.31MnPO4CO3 as a cathode material in “water-in-salt” electrolytes, Chemical Communications, 10.1039/D1CC04350A, 57, 12840-12843, 2021.11.
7. Kenji Takada, Mari Morita,Takane Imaoka, Junko Kakinuma, Ken Albrecht, Kimihisa Yamamoto, Metal atom–guided conformational analysis of single polynuclear coordination molecules, Science Advances, 10.1126/sciadv.abd9887, 7, 32, eabd9887, 2021.08.
8. Kohei Iwai, Hiroshi Yamagishi, Colin Herzberger, Yuji Sato, Hayato Tsuji, Ken Albrecht, Kimihisa Yamamoto, Fumio Sasaki, Hiroyasu Sato, Aswin Asaithambi, Axel Lorke, Yohei Yamamoto, Single-Crystalline Optical Microcavities from Luminescent Dendrimers, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 10.1002/anie.202000712, 59, 31, 12674-12679, 2020.07, Microcrystallites are promising minute mirrorless laser sources. A variety of luminescent organic compounds have been exploited along this line, but dendrimers have been inapplicable owing to their fragility and extremely poor crystallinity. Now, a dendrimer family that overcomes these difficulties is presented. First-, second-, and third-generation carbazole (Cz) dendrimers with a carbon-bridged oligo(phenylenevinylene) (COPV2) core (GnCOPV2, n=1-3) assemble to form microcrystals. The COPV2 cores align uni/bidirectionally in the crystals while the Cz units in G2- and G3COPV2 align omnidirectionally. The dendrons work as light-harvesting antennas that absorb non-polarized light and transfer it to the COPV2 core, from which a polarized luminescence radiates. Furthermore, these crystals act as laser resonators, where the lasing thresholds are strongly coupled with the crystal morphology and the orientation of COPV2, which is in contrast with the conventional amorphous dendrimers..
9. Amruth C, Beata Luszczynska, Marek Zdzislaw Szymanski, Jacek Ulanski, Ken Albrecht, Kimihisa Yamamoto, Inkjet printing of thermally activated delayed fluorescence (TADF) dendrimer for OLEDs applications, Organic Electronics, 10.1016/j.orgel.2019.07.011, 74, 218-227, 2019.11, This study presents the inkjet printing of a novel OLED device with fully solution-processed organic layers that employ TADF material as an emitting layer. The ink was formulated using new TADF material, triazine core carbazole dendrimers with tert-butyl group at the periphery (tBuG2TAZ), dissolved in a mixture of two non-chlorinated solvents. The influence of the print resolution and the substrate temperature on morphology of the printed films was studied and optimized in ambient conditions. The optimized TADF dendrimer layer was then incorporated in the OLEDs as the emitting layer. The best-printed OLEDs exhibited a maximum current efficiency of 18 cd/A and maximum luminance of 6900 cd/m2. Such values are comparable to the values obtained in spin coated devices made of the same TADF dendrimer. Further, the mobility of charge carriers extracted from transient electroluminescence measurements of printed OLEDs, when compared to reference OLEDs made by spin coating technique, showed similar values. Finally, we have demonstrated the possibility of patterning of emission the area of complex shapes merely by selectively printing the emission layer. These results demonstrate the potential application of the new dendrimer TADF emitters for the fabrication of efficient OLEDs by an inkjet printing technique..
10. Yusuke Inomata, Ken Albrecht, Naoki Haruta, Kimihisa Yamamoto, Dendrimer-templated synthesis and characterization of tin oxide quantum dots deposited on a silica glass substrate, Chemistry of Materials, 10.1021/acs.chemmater.9b01925, 31, 20, 8373-8382, 2019.10, Tin oxide quantum dots (QDs) have attracted much attention because of their low toxicity and the absence of cadmium and other poisonous elements. In this paper, we report the novel synthetic method for size-controlled tin oxide QDs using dendrimers as a template, and their electronic and structural properties. Hemispherical tin oxide QDs with a size below 2 nm and small size distribution were synthesized on silica glass substrates by the dendrimer-templated synthesis method (Sn12, Sn28, and Sn60 oxide QDs). The structures of the tin oxide QDs were composed not only of Sn(IV) sites, but also Sn(II) sites due to the restriction of the coordination environment to stabilize the structure. Density functional theory calculation showed that a bare tin oxide cluster with a mixed valence state (Sn(II) + Sn(IV)) is more stable than those only with Sn(II) or Sn(IV). The synthesized tin oxide QDs showed the quantum confinement effect caused by the spatial confinement of an electron. The Urbach tail parameter, expressing the disorderliness of the QDs, decreased with the reduced QD size, although the value of each tin oxide QD was higher than that of bulk SnO2. The experimental band gap energy was compared with the effective mass approximation models, which are theoretical models for the quantum confinement effect. We found that the experimental values of Sn28 and Sn60 oxide QDs were consistent with the theoretical values, while Sn12 oxide QDs had a lower value compared to the predicted band gap energy. This could be attributed to the change in the physical parameters of Sn12 oxide QDs, which are not the same as those of Sn28, Sn60 oxide QDs or the bulk SnO2. These results indicate that small tin oxide QDs have a different structure and different electronic properties compared to bulk or conventional nanoparticles and have potential applications in such fields as catalysis and optical and electronic devices..
11. Ken Albrecht, Ken Minagawa, Sae Nakajima, Soh Kushida, Yohei Yamamoto, Akiyoshi Kuzume, Kimihisa Yamamoto, Nanosphere formation of π-conjugated dendrimers by simple precipitation method, Chemistry Letters, 10.1246/cl.190511, 48, 10, 1240-1243, 2019.01, Formation of nanospheres that consist of π-conjugated dendrimers (phenylazomethine and carbazole) is reported. The nanospheres can be prepared by spin-casting a solution of dendrimers in a proper good/poor solvent mixture as kinetically trapped amorphous products. This method could be applied to metallodendrimer and fluorescent dendrimer. By controlling the kinetic process, the size of the spheres could be controlled and the fluorescent dendrimer sphere, with a few ¯m in size, displayed whispering gallery mode (WGM) photoluminescence..
12. Ryo Yamada, Ken Albrecht, Tatsuhiko Ohto, Keigo Minode, Kimihisa Yamamoto, Hirokazu Tada, Single-molecule rectifiers based on voltage-dependent deformation of molecular orbitals in carbazole oligomers, Nanoscale, 10.1039/c8nr06049e, 10, 42, 19818-19824, 2018.11, Current-voltage characteristics of single molecule junctions are governed both by the energy level alignment of molecular orbitals with respect to the Fermi level of the electrodes and by the hybridization of electronic structures at the interface between the molecule and the electrodes. While there have been many studies on tuning the former, only a few works intended to control the latter. In the present study, we demonstrate that molecular junctions based on carbazole oligomers showed a current rectification behavior due to asymmetric-symmetric control of electronic hybridization between the molecule and electrodes at the both terminals. The carbazole oligomers originally showed an asymmetric molecular orbital and, hence, electronic hybridization with the electrodes because of the electric dipole moment. Symmetric electronic hybridization was achieved when the applied electric field between electrodes deformed molecular orbital to be symmetric. This is a novel way to control charge transport in single-molecule junctions..
13. Kenichi Matsuoka, Ken Albrecht, Akira Nakayama, Kimihisa Yamamoto, Katsuhiko Fujita, Highly Efficient Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with Fully Solution-Processed Organic Multilayered Architecture
Impact of Terminal Substitution on Carbazole-Benzophenone Dendrimer and Interfacial Engineering, ACS Applied Materials and Interfaces, 10.1021/acsami.8b09451, 10, 39, 33343-33352, 2018.10, A series of second-generation carbazole-benzophenone dendrimer substituted by several functional groups at terminal positions (subG2B) was investigated toward a thermally activated delayed fluorescence (TADF) emitter for nondoped emissive layer (EML) application in a solution-processed organic light-emitting diode (OLED). Substitution was found to dramatically alter the photophysical properties of the dendritic TADF emitters. The introduction of tert-butyl and phenyl group endows the subG2Bs with aggregation-induced emission enhancement character by suppression of internal conversion in singlet excited states. In the meantime, the introduction of a methoxy group resulted in aggregation-caused quenching character. The device performance of the OLED, where subG2B neat films were incorporated as nondoped EMLs, was found to be highly enhanced by adopting fully solution-processed organic multilayer architecture in comparison to the devices with a vacuum-deposited electron transporting layer (ETL), achieving a maximum external quantum efficiency of 17.0%. Such improvement was attributable to the improved carrier balance via intermixing at solution-processed EML/ETL interfaces. It was also found that the post-thermal annealing of the OLED at appropriate temperatures could be beneficial to enhance OLED performance by promoting the intermixing EML/ETL interface to some extent. Our findings emphasize the potential utility of dendritic TADF emitters in the solution-processed TADF-OLED and increase the importance to manipulate dendrimer/small molecule interfaces..
14. Ken Albrecht, K. Matsuoka, Katsuhiko Fujita, K. Yamamoto, A dendrimer emitter doped in a dendrimer host
Efficient thermally activated delayed fluorescence OLEDs with fully-solution processed organic-layers, Materials Chemistry Frontiers, 10.1039/C7QM00579B, 2, 6, 1097-1103, 2018.01, A thermally activated delayed fluorescence (TADF) carbazole dendrimer (tBuG2TAZ) doped in a carbazole dendrimer host (G3Ph, G4Ph) was employed as an emissive layer (EML) in an OLED device with fully solution processed organic layers. Green (l
= 502 nm) emission with a maximum external efficiency (EQE
) of 16.1% was achieved when tBuG2TAZ was doped in G3Ph. This value was higher than the previously reported OLED device with a neat film of tBuG2TAZ as an EML because of the higher PLQY due to suppressed concentration quenching. The TADF-active carbazole dendrimer doped in a carbazole host film shows excellent miscibility and solvent resistance to ethanol. On the other hand, when a small molecule (rubrene) was doped in the carbazole dendrimer, significant dissolution of rubrene was observed after rinsing the doped film with ethanol. This shows the importance of the solvent resistivity of a dopant emitter when another film is solution-processed onto the doped film. The ''doping dendrimer in dendrimer'' concept is a new concept to allow fabrication of fully solution-processed TADF OLEDs..
15. Sae Nakajima, Ken Albrecht, Soh Kushida, Eiji Nishibori, Takashi Kitao, Takashi Uemura, Kimihisa Yamamoto, Uwe H.F. Bunz, Yohei Yamamoto, A fluorescent microporous crystalline dendrimer discriminates vapour molecules, Chemical Communications, 10.1039/c7cc09342j, 54, 20, 2534-2537, 2018.01, A self-assembled crystalline microporous dendrimer framework (MDF) exhibits novel turn-on and ratiometric fluorescence upon exposure to solvent vapours. The donor-acceptor character, combined with the large surface area (>650 m
), allows the MDF to discriminate vapours of volatile solvents with turn-on and colour change of photoluminescence..
16. Kenichiro Omoto, Nobuhiko Hosono, Mika Gochomori, Ken Albrecht, Kimihisa Yamamoto, Susumu Kitagawa, Anisotropic convergence of dendritic macromolecules facilitated by a heteroleptic metal-organic polyhedron scaffold, Chemical Communications, 10.1039/c8cc02460j, 54, 41, 5209-5212, 2018.01, Anisotropic dendrimers with bipolar shapes were systematically obtained using a heteroleptic metal-organic polyhedron (MOP) as a robust core scaffold. The structure of one of these polyhedral shapes was unambiguously determined by single-crystal X-ray analysis, which revealed that the bulky dendrons converge to both axial positions of the heteroleptic MOP core..
17. Yusuke Inomata, Ken Albrecht, Kimihisa Yamamoto, Size-Dependent Oxidation State and CO Oxidation Activity of Tin Oxide Clusters, ACS Catalysis, 10.1021/acscatal.7b02981, 8, 1, 451-456, 2018.01, The CO oxidation reaction is an industrially important reaction; however, the practical catalysts are limited to noble metals. In this paper, we report a systematic study of the CO oxidation ability on cheap and less noble tin oxide clusters with the aim of quantitatively understanding the active sites. We synthesized size-controlled tin oxide clusters in mesoporous silica using the dendrimer templating method, employing dendritic phenylazomethine with a tetraphenylmethane core (TPMG4) as the template molecule. The clusters had different sizes depending on the added amount of SnCl2 as a precursor to TPMG4. The synthesized tin oxide clusters contained not only stable tetravalent Sn(IV) but also metastable divalent Sn(II) due to the structural stability and had a size-dependent composition. The CO oxidation activity of the tin oxide clusters increased with decreasing cluster size depending on the Sn(II)/Sn(IV) ratio. We also found the correlation between the Sn(II) fraction and the CO oxidation activity, clearly indicating that the partially reduced Sn(II) acted as the active site for the CO oxidation in the tin oxide clusters. This knowledge give a clue on how to design a highly active CO oxidation catalyst with base metal oxides..
18. Tomo Sakanoue, Fumihiro Yonekawa, Ken Albrecht, Kimihisa Yamamoto, Taishi Takenobu, An Ionic Liquid That Dissolves Semiconducting Polymers
A Promising Electrolyte for Bright, Efficient, and Stable Light-Emitting Electrochemical Cells, Chemistry of Materials, 10.1021/acs.chemmater.7b02128, 29, 14, 6122-6129, 2017.07, Light-emitting electrochemical cells (LECs) are composed of blends of semiconducting polymers and electrolytes, in which a unique cooperative action of ions and electrons induces a dynamic p-n junction for efficient emission. One of the crucial issues remaining in LECs is uniformity in blends of polymer and electrolyte; phase separation in between the two components results in poor performance or failure of operation. Here, we overcome this issue by developing an ionic liquid-based electrolyte of alkylphosphonium-phosphate that shows notable compatibility high enough to dissolve even light-emitting polymers. This exceptional compatibility enabled us to prepare uniform film blends with various blue to red emitting polymers, and offered bright and efficient LECs. Especially, a blue-emitting LEC showed excellent performance: the luminance reached ∼20 000 cd m-2 with a high luminance efficiency of ∼5 cd A-1, of which performances significantly exceed a light-emitting diode using the same polymer. The ionic liquid was further applied to the LECs with state-of-the-art light-emitting dendrimers showing thermally activated delayed fluorescence under electrical excitation, giving a high efficiency of 11 cd A-1. These demonstrations remind us of the great importance of the polymer-electrolyte compatibility and the usefulness of ILs for electrolyte of LECs..
19. Keisuke Jono, Akira Suzuki, Munetaka Akita, Ken Albrecht, Kimihisa Yamamoto, Michito Yoshizawa, A Polyaromatic Molecular Clip That Enables the Binding of Planar, Tubular, and Dendritic Compounds, Angewandte Chemie - International Edition, 10.1002/anie.201612489, 56, 13, 3570-3574, 2017.01, By the covalent linkage of two bent bisanthracene amphiphiles with a biphenyl spacer bearing hydrophilic pendants, we synthesized a new molecular clip with a C-shaped conformation. The molecular clip provides an acyclic, open cavity surrounded by four anthracene panels in water. In contrast to previous clip- and tweezers-like compounds as well as cage-shaped compounds, the C-shaped polyaromatic cavity displays unusual wide-ranging capturing abilities toward not only planar perylene-based pigments and cylindrical single-walled carbon nanotubes but also highly branched macromolecules (carbazole dendrimers)..
20. Yasuhisa Naitoh, Ken Albrecht, Qingshuo Wei, Kimihisa Yamamoto, Hisashi Shima, Takao Ishida, Fabrication of sub-1 nm gap electrodes using metal-mask patterning and conductivity measurements of molecules in nanoscale spaces, RSC Advances, 10.1039/c7ra10873g, 7, 84, 53503-53508, 2017.01, We developed a procedure for fabricating sub-1 nm gap Au electrodes using a metal mask for electrode patterning. Self-aligned nanogap formation was achieved using an electromigration method during metal deposition. We also measured the electric conductivities of organic molecules using the new nanogap electrodes. Because the new procedure does not involve wet processing, the ranges of possible electrode and substrate materials for the nanogap electrodes are greatly expanded. Finally, we discussed the molecular orbital energies of bridging and nonbridging 1,4-benzenedithiol molecules between Au electrodes. The new procedure for the fabrication of nanogap electrodes is expected to be useful for measuring the electrical properties of various nanoscale materials..
21. Kenichi Matsuoka, Ken Albrecht, Kimihisa Yamamoto, Katsuhiko Fujita, Mulifunctional dendritic emitter
Aggregation-induced emission enhanced, thermally activated delayed fluorescent material for solution-processed multilayered organic light-emitting diodes, Scientific reports, 10.1038/srep41780, 7, 2017.01, Thermally activated delayed fluorescence (TADF) materials emerged as promising light sources in third generation organic light-emitting diodes (OLED). Much effort has been invested for the development of small molecular TADF materials and vacuum process-based efficient TADF-OLEDs. In contrast, a limited number of solution processable high-molecular weight TADF materials toward low cost, large area, and scalable manufacturing of solution processed TADF-OLEDs have been reported so far. In this context, we report benzophenone-core carbazole dendrimers (GnB, n = generation) showing TADF and aggregation-induced emission enhancement (AIEE) properties along with alcohol resistance enabling further solution-based lamination of organic materials. The dendritic structure was found to play an important role for both TADF and AIEE activities in the neat films. By using these multifunctional dendritic emitters as non-doped emissive layers, OLED devices with fully solution processed organic multilayers were successfully fabricated and achieved maximum external quantum efficiency of 5.7%..
22. Ken Albrecht, Kenichi Matsuoka, Daisuke Yokoyama, Yoshiya Sakai, Akira Nakayama, Katsuhiko Fujita, Kimihisa Yamamoto, Thermally activated delayed fluorescence OLEDs with fully solution processed organic layers exhibiting nearly 10% external quantum efficiency, Chemical Communications, 10.1039/c6cc09275f, 53, 16, 2439-2442, 2017, New solution processable and laminatable terminally modified carbazole-triazine thermally activated delayed fluorescence (TADF) dendrimers are reported. An OLED device with fully solution processed organic layers exhibited an external quantum efficiency of up to 9.4% at 100 cd m−2..
23. Ken Albrecht, Yuki Hirabayashi, Masaya Otake, Shin Mendori, Yuta Tobari, Yasuo Azuma, Yutaka Majima, Kimihisa Yamamoto, Polymerization of a divalent/tetravalent metal-storing atom-mimicking dendrimer, Science Advances, 10.1126/sciadv.1601414, 2, 12, 2016.12, The phenylazomethine dendrimer (DPA) has a layer-by-layer electron density gradient that is an analog of the Bohr atom (atom mimicry). In combination with electron pair mimicry, the polymerization of this atom-mimicking dendrimer was achieved. The valency of the mimicked atom was controlled by changing the chemical structure of the dendrimer. Bymimicking a divalent atom, a one-dimensional (1D) polymer was obtained, and by using a planar tetravalent atom mimic, a 2D polymer was obtained. These poly(dendrimer) polymers could store Lewis acids (SnCl2) in their unoccupied orbitals, thus indicating that these poly(dendrimer) polymers consist of a series of nanocontainers..
24. Kenichi Matsuoka, Ken Albrecht, Kimihisa Yamamoto, Katsuhiko Fujita, Mono-Substituted carbazole dendrimers as solution processable host materials for phosphorescent organic light-emitting diodes, Journal of Photopolymer Science and Technology, 10.2494/photopolymer.29.323, 29, 2, 323-326, 2016.01.
25. Ken Albrecht, Kenichi Matsuoka, Katsuhiko Fujita, Kimihisa Yamamoto, Carbazole dendrimers as solution-processable thermally activated delayed-fluorescence materials, Angewandte Chemie - International Edition, 10.1002/anie.201500203, 54, 19, 5677-5682, 2015.05, Recently, thermally activated delayed fluorescence (TADF) materials have received increasing attention as effective emitters for organic light-emitting diodes (OLEDs). However, most of them are usually employed as dopants in a host material. In this report, carbazole dendrimers with a triphenyl-s-triazine core are reported, which are the first solution-processable, non-doped, high-molecular-weight TADF materials. The dendrimers were obtained by a new and facile synthetic route using the tert-butyldimethylsilyl moiety as a protecting group. All dendrimers showed TADF in toluene. Measurements of the temperature-dependent luminescence lifetime revealed that spin-coated neat films also showed TADF with moderate quantum yields. OLED devices incorporating these dendrimers as spin-coated emitting layers gave external quantum efficiencies of up to a 3.4%, which suggests that this device is harvesting triplet excitons. This result indicates that carbazole dendrimers with attached acceptors are potential TADF materials owing to their polarized electronic structure (with HOMO-LUMO separation). Fluorescent dendrimers: Carbazole dendrimers with a triphenyl-s-triazine core have been developed as the first solution-processable, non-doped, and high-molecular-weight thermally activated delayed fluorescence (TADF) materials. OLED devices with such dendrimers as the spin-coated emitting layer displayed external quantum efficiencies of up to 3.4%, which suggests that this device is harvesting triplet excitons..
26. Ken Albrecht, Noriko Sakane, Yusuke Inomata, Kimihisa Yamamoto, Effect of the Core Structure on the Sequential Coordination of Phenylazomethine Dendrimer, Journal of Inorganic and Organometallic Polymers and Materials, 10.1007/s10904-014-0116-y, 25, 1, 133-139, 2015.01, Phenylazomethine dendrimer (DPA) is a dendritic ligand that coordinates to various Lewis acids in a stepwise radial fashion. Second generation para-substituted and meta-substituted phenylazomethine dendrimers with p-phenylenediamine and m-phenylenediamine core were synthesized and the coordination sequence was investigated by UV–vis titration. Stepwise radial complexation from the outer layer was observed for the m-phenylenediamine core meta-substituted phenylazomethine dendrimer (m-mG2). Other three dendrimers showed stepwise radial complexation from the inner layer. The reason could be explained with the binding constant of the 1st generation dendrimer (model of the 1st layer). This is suggesting that for controlling the coordination sequence of DPA, not only the dendron structure is important, but also the structure of the core is an important factor..
27. Ken Albrecht, Noriko Sakane, Kimihisa Yamamoto, Stepwise radial complexation from the outer layer to the inner layer of a dendritic ligand
A phenylazomethine dendrimer with an inverted coordination sequence, Chemical Communications, 10.1039/c4cc05007j, 50, 81, 12177-12180, 2014.10, A para-substituted phenylazomethine dendrimer (pGnA) coordinates to Lewis acids in a stepwise radial fashion from the inner layer to the outer layer. The inversion of this coordination sequence was achieved for the first time by just changing the substitution position of the phenylazomethine group from the para position to the meta position (mGnA). This journal is.
28. Ken Albrecht, Hideyuki Higashimura, Kimihisa Yamamoto, Synthesis and properties of nitrogen-introduced phenylazomethine dendrimer, Synthetic Communications, 10.1080/00397911.2014.891746, 44, 15, 2239-2247, 2014.08, The pyridine ring-introduced phenylazomethine dendrimer (PyDPA) was synthesized by a dehydration reaction using titanium(IV) chloride. The ultraviolet-visible absorption and the electrochemical study showed that the introduction of the pyridine ring produces a smaller band gap by increasing the highest occupied molecular orbital level and decreasing the lowest unoccupied molecular orbital level. A crystal of a PyDPA-PdCl2 complex was also prepared. Traditional phenylazomethine dendrimers cannot form a complex with palladium, indicating that PyDPA can potentially coordinate with various metal salts such as Co, Ni, Ru, and Mn and can be used for catalytic or electronic applications..
29. Takane Imaoka, Hirokazu Kitazawa, Wang Jae Chun, Saori Omura, Ken Albrecht, Kimihisa Yamamoto, Magic number Pt13 and misshapen Pt12 clusters
Which one is the better catalyst?, Journal of the American Chemical Society, 10.1021/ja405922m, 135, 35, 13089-13095, 2013.09, A relationship between the size of metal particles and their catalytic activity has been established over a nanometer scale (2-10 nm). However, application on a subnanometer scale (0.5-2 nm) is difficult, a possible reason being that the activity no longer relies on the size but rather the geometric structure as a cluster (or superatomic) compound. We now report that the catalytic activity for the oxygen reduction reaction (ORR) significantly increased when only one atom was removed from a magic number cluster composed of 13-platinum atoms (Pt13). The synthesis with an atomic-level precision was successfully achieved by using a dendrimer ligand as the macromolecular template strictly defining the number of metal atoms. It was quite surprising that the Pt12 cluster exhibited more than 2-fold catalytic activity compared with that of the Pt13 cluster. ESI-TOF-mass and EXAFS analyses provided information about the structures. These analyses suggested that the Pt12 has a deformed coordination, while the Pt13 has a well-known icosahedral atomic coordination as part of the stable cluster series. Theoretical analyses based on density functional theory (DFT) also supported this idea. The present results suggest potential activity of the metastable clusters although they have been "missing" species in conventional statistical synthesis..
30. Ken Albrecht, Yuto Kasai, Yasunori Kuramoto, Kimihisa Yamamoto, Dynamic control of dendrimer-fullerene association by axial coordination to the core, Chemical Communications, 10.1039/c3cc43249a, 49, 61, 6861-6863, 2013.08, The effect of axial coordination of pyridine derivatives to the core porphyrin on the fullerene encapsulation of the 4th generation carbazole-phenylazomethine dendrimer (ZnPG2-2) was investigated. The axial coordination of large (bulky) pyridine derivatives affects the cavity in an allosteric manner, and the size-selectivity of the fullerene association could be controlled..
31. Ken Albrecht, Yuto Kasai, Yasunori Kuramoto, Kimihisa Yamamoto, A fourth-generation carbazole-phenylazomethine dendrimer as a size-selective host for fullerenes, Chemical Communications, 10.1039/c2cc36451d, 49, 9, 865-867, 2013.01, A fourth generation carbazole-phenylazomethine dendrimer with a porphyrin core was investigated as a new host for fullerenes (C60, C 70, and C84). This dendrimer has a significantly higher association constant compared to the lower generation dendrimers, and the encapsulation is achieved by the cooperation of the dendron and the core..
32. Hirokazu Kitazawa, Ken Albrecht, Kimihisa Yamamoto, Synthesis of a dendrimer reactor for clusters with a magic number, Chemistry Letters, 10.1246/cl.2012.828, 41, 8, 828-830, 2012.08, A new type of phenylazomethine dendrimer (DPA) that has one more coordination site compared to the tetraphenylmethane core DPA was synthesized for the magic number 13 metal cluster template that has cubic or hexagonal densely packed structures. The stepwise complexation property was revealed, and this property is expected to be the magic number 13 metal cluster template..
33. Ken Albrecht, Roderick Pernites, Mary Jane Felipe, Rigoberto C. Advincula, Kimihisa Yamamoto, Patterning carbazole-phenylazomethine dendrimer films, Macromolecules, 10.1021/ma202485h, 45, 3, 1288-1295, 2012.02, The synthesis of double-layer-type dendrimers with carbazole and phenylazomethine as the dendron with a symmetric tetraphenylmethane core is reported. Structural modeling studies showed that the G3 dendrimer has a rigid and spherical structure. These dendrimers were thermally stable (T d10% over 500 °C) with the TGA-MS study revealing a degradation mechanism occurring first at the inner-layer phenylazomethine group. The metal (Lewis acid) complexation property of these dendrimers was also studied. Electrochemical measurements showed that these dendrimers have the appropriate HOMO level as a hole-transporting material with electropolymerizability on the peripheral carbazole groups. A photo-cross-linking property of the dendrimer film was also observed. Finally, electro-nanopatterning with conducting AFM and photopatterning of the dendrimer film were demonstrated. Thus, the new dendrimer is a potential hole-transporting material that is patternable through oxidation of the peripheral carbazole units by either photochemical or electrochemical methods..
34. Ken Albrecht, Yuto Kasai, Kimihisa Yamamoto, The fluorescence and electroluminescence properties of the carbazole-phenylazomethine double layer-type dendrimer, Journal of Inorganic and Organometallic Polymers and Materials, 10.1007/s10904-008-9239-3, 19, 1, 118-123, 2009.03, The fluorescence properties of double layer-type dendrimers with carbazole as the outer layer and phenylazomethine as the inner layer of the dendron with a zinc porphyrin core were studied. Based on the fluorescence quantum yield (solution), the effect of the bulky dendron to inhibit the interaction of the chromophores was confirmed. Additionally, the efficient light-harvesting property of the double layer-type dendron was revealed by the fluorescence measurement at several excitation wavelengths. These dendrimers were used as an emitting-layer in an OLED device. The electroluminescence of the core porphyrin was observed and it was shown that the double layer-type dendrimer with a luminescent core has the potential to be a hole-transporting/emitting material..
35. Ken Albrecht, Kimihisa Yamamoto, Dendritic structure having a potential gradient
New synthesis and properties of carbazole dendrimers, Journal of the American Chemical Society, 10.1021/ja807312e, 131, 6, 2244-2251, 2009.02, A new synthetic route for carbazole dendrimers was discovered using the copper-catalyzed N-arylation reaction. This synthetic route allowed synthesizing the fourth generation carbazole dendrimer and several derivatives for the first time. The crystal structure, Mark-Houwink-Sakurada plots, and UV-vis and fluorescence studies showed that the dendritic carbazole backbone has a rigid and highly twisted structure. From the measurement of the redox potential of the ferrocene derivatives, the IR spectra of the benzophenone derivatives, and complexation behavior of the phenylazomethine derivatives, the inductive electron-withdrawing effect of the carbazole dendron was revealed. This suggested that the summation of this electron withdrawal from each layer may produce a potential gradient such that the outer layer is electron-rich and the inner layer is electron-poor in the carbazole dendron. By assignment of the 1H and 13C NMR spectra of the dendron, the existence of this kind of potential gradient was proved. Overall, these data show the π-polarization substituent effect of the carbazole unit, and their summation determines the potential gradient in the repeating dendritic structure of the carbazole dendrimer..
36. Ken Albrecht, Atsushi Kimoto, Jun Sang Cho, Yugo Matsuura, Kimihisa Yamamoto, Organic light-emitting diodes with hole-transporting material having metal collecting sites, Journal of Photopolymer Science and Technology, 10.2494/photopolymer.21.181, 21, 2, 181-182, 2008.08.
37. Ken Albrecht, Yuto Kasai, Atsushi Kimoto, Kimihisa Yamamoto, The synthesis and properties of carbazole-phenylazomethine double layer-type dendrimers, Macromolecules, 10.1021/ma800265h, 41, 11, 3793-3800, 2008.06, A new double layer-type dendrimer with carbazole as the outer layer and phenylazomethine as the inner layer of the dendron was synthesized using the Ullmann reaction and dehydration reaction in the presence of titanium tetrachloride. In this dendrimer, the carbazole units act as excellent hole-transporters, the phenylazomethine units act as metal assembling sites, and the combination of both units provides a thermally stable shell for which the 10% weight loss temperature was over 550°C. The dendrimers were used as the hole-transporting layer in an OLED device. The OLED device performance increased when the generation of the carbazole increased, corresponding to the higher HOMO level. Additionally, the enhancement of the hole-transporting property was observed by simple complexation of the metal ions to the imine site. Next, the effect of the generation of phenylazomethine was observed and compared to the asymmetric-type carbazole-phenylazomethine dendrimers. When the generation of phenylazomethine increased in the asymmetric-type dendrimer, the device performance decreased. In contrast, the performance did not change using the double layer-type dendrimer. This indicates that the outer layer carbazole works as a hole-transporting shell, and the double layer-type architecture is an ideal structure..
38. Toshio Nakashima, Norifusa Satoh, Ken Albrecht, Kimihisa Yamamoto, Interface modification on TiO2 electrode using dendrimers in dye-sensitized solar cells, Chemistry of Materials, 10.1021/cm703279u, 20, 7, 2538-2543, 2008.04, The interface modification of dye-sensitized solar cells (DSSCs) in order to remove I3 - on the TiO2 electrodes is important for improving their performance. The suppression of the back electron transfer by a modified interface improves the open-circuit voltage. In this study, we demonstrated that the suppression mechanism using phenylazomethine dendrimers with a triarylamine core (TPA-DPAs) and two other dendrimers, which are a carbazole dendrimer containing a cyclic phenylazomethine of the third generation (CPA-Cz G3) and a half-dendritic phenylazomethine of the fifth generation (Half-DPA G5). Removing I3 - and producing I- on the TiO2 electrode by complexation is critical for the suppression of the back electron transfer and promoting the regeneration of the dye in dendrimers having an inner space. Additionally, the presence of various metal compounds in the dendrimers decreased the resistance of the DSSCs, improving fill factor and energy conversion efficiency. We studied that the dendrimer property is changed by the quantitative metal complexation. The fill factor and energy conversion efficiency of the DSSCs are maximized with 0.5 equiv. of SnCl2 in the TPA-DPAs..
39. Ken Albrecht, Kimihisa Yamamoto, Organic light-emitting diodes with novel carbazole dendrimer as a photocrosslinking hole-transfer layer, Journal of Photopolymer Science and Technology, 10.2494/photopolymer.19.175, 19, 2, 175-176, 2006.08.
40. Norifusa Satoh, Toshio Nakashima, Ken Albrecht, Kimihisa Yamamoto, Dye-sensitized solar cell using π-conjugated dendrimer, Journal of Photopolymer Science and Technology, 10.2494/photopolymer.19.141, 19, 2, 141-142, 2006.08.