トピックス

ナノ粒子合成のためのカスタムメイド樹状フェニルアゾメチン

2022.03.18

特定の原子数を持つ金属クラスターを準備規模で合成して高度な特性を研究することは依然として課題である。デンドリマーテンプレート法は、サイズや原子数を制御したナノ粒子を合成するための強力な方法であるが、従来のデンドリマーではすべての原子数にアクセスすることはできない。この問題に対処するために、限られた数の配位サイト（n = 16）と非配位性の大きなポリフェニレンシェルを持つ新しいカスタムメイドのフェニルアゾメチンデンドリマー（DPA）が設計された。非対称デンドロンとアダマンタンコアの四置換デンドリマー（PPDPA16）は、成功裏に合成された。配位挙動により、PPDPA16に16個の金属ルイス酸（RhCl3、RuCl3、およびSnBr2）が蓄積されることが確認された。複合体の還元後、制御されたサイズの低原子価金属ナノ粒子が得られた。このカスタムメイドのデンドリマーは、望ましい原子性を持つさまざまな金属クラスターを合成するための有望なアプローチである。

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.

多核金属錯体分子の金属原子をラベルとした立体構造解析

2021.08.06

二価/四価の金属を貯蔵する原子模倣デンドリマーの重合

2016.12.02

K. Albrecht, Y. Hirabayashi, M. Otake, S. Mendori, Y. Tobari, Y. Azuma, Y. Majima, K. Yamamoto
Science Adv. 2016, 2, e1601414. フェニルアゾメチンデンドリマー（DPA）は、ボーア原子のアナログである層ごとの電子密度勾配を持ち、原子模倣を実現している。この電子対模倣と組み合わせることで、原子模倣デンドリマーの重合が達成された。模倣する原子の原子価はデンドリマーの化学構造を変えることで制御された。二価原子を模倣することで一次元（1D）ポリマーが得られ、平面四価原子を模倣することで二次元（2D）ポリマーが得られた。これらのポリ（デンドリマー）ポリマーは、未占有軌道にルイス酸（SnCl2）を貯蔵できるため、これらのポリ（デンドリマー）ポリマーが一連のナノコンテナで構成されていることを示している。Polymerization of a divalent/tetravalent metal-storing atom-mimicking dendrimer 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. By mimicking 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.

Bismuth Complexes in Phenylazomethine Dendrimers: Controllable Luminescence and Emission in the Solid State

2016.09.22

T. Kambe, A. Watanabe, T. Imaoka, K. Yamamoto
Angew. Chem. Int. Ed. 2016, 55, 13151-13154.
Dendritic phosphors were obtained by the stepwise integration of BiCl3 in phenylazomethine dendrimers. The bismuth-coordinated phenylazomethines displayed photoluminescence at 500–800 nm, and the intensity could be tuned by changing the stoichiometry of BiCl3 and the dendrimer. This phosphor did not show serious luminescence quenching even though the local concentration of BiCl3 in the dendrimer was as high as 20 M, and luminescence was also observed in the solid state. The absorption and emission properties could be reversibly switched by addition of a Lewis base or under electrochemical redox control, which induced the reversible complexation of BiCl3 in the dendrimer.

Carbazole Dendrimers as Solution-Processable Thermally Activated Delayed-Fluorescence Materials

2015.03.05

K. Albrecht, K. Matsuoka, K. Fujita, K. Yamamoto
Angew. Chem. Int. Ed. 2015, 54, 5677-5682.
溶液塗布可能な遅延蛍光材料としてのカルバゾールデンドリマー
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).

Macromolecular semi-rigid nanocavities for cooperative recognition of specific large molecular shapes

2013.05.27

T. Imaoka, Y. Kawana, T. Kurokawa, K. Yamamoto
Nature Commun. 2013, 4, 2581.
【協働的に大型分子の形状を認識する少しだけ硬い高分子精密ナノ空間】
Molecular shape recognition for larger guest molecules (typically over 1 nm) is a difficult task because it requires cooperativity within a wide three-dimensional nanospace coincidentally probing every molecular aspect (size, outline shape, flexibility and specific groups). Although the intelligent functions of proteins have fascinated many researchers, the reproduction by artificial molecules remains a significant challenge. Here we report the construction of large, well-defined cavities in macromolecular hosts. Through the use of semi-rigid dendritic phenylazomethine backbones, even subtle differences in the shapes of large guest molecules (up to ~2 nm) may be discriminated by the cooperative mechanism. A conformationally fixed complex with the best-fitting guest is supported by a three-dimensional model based on a molecular simulation. Interestingly, the simulated cavity structure also predicts catalytic selectivity by a ruthenium porphyrin centre, demonstrating the high shape persistence and wide applicability of the cavity.

Enhancing the Photoelectric Effect with a Potential-Programmed Molecular Rectifier

2012.05.08

T. Imaoka, H. Ueda, K. Yamamoto
J. Am. Chem. Soc. 2012, 134, 8412-8415.
Enhancing the Photoelectric Effect with a Potential-Programmed Molecular Rectifier
Dendrimer-based electron rectifiers were applied to photoconducting devices. A remarkable enhancement of the photocurrent response was observed when a zinc porphyrin as the photosensitizer was embedded in the dendritic phenylazomethine (DPA) architecture. The dendrimer-based sensitizer exhibited a 20-fold higher current response than the non-dendritic zinc porphyrin. In sharp contrast, a similar application of the dendrimer with poly(vinylcarbazole) as the electron donor resulted in a decreased response. This is consistent with the idea that the DPA facilitates electron transfer from the core to its periphery along a potential gradient, as predicted by density functional theory calculations.

Controlled Storage of Ferrocene Derivatives as Redox-Active Molecules in Dendrimers

2010.03.19

Y. Ochi, M. Suzuki, T. Imaoka, M. Murata, H. Nishihara, Y. Einaga, K. Yamamoto
J. Am. Chem. Soc. 2010, 132, 5061-5069. Controlled Storage of Ferrocene Derivatives as Redox-Active Molecules in Dendrimers Dendritic polyphenylazomethines (DPA) could encapsulate ferroceniums by complexation of the electron-donating skeleton of the DPA imines. Upon addition of ferroceniums to a series of dendritic polyphenylazomethines (DPAGX, where X is the generation number, X = 1−4), the UV−vis spectra showed changes in a manner similar to that observed for the complexation of metal ions with DPAGX. Stepwise shifts in the isosbestic point were consistently observed with the number of imine groups in the first and second layers of the generation-4 dendrimer (DPAG4). DPAG2 and DPAG3 were also found to trap 6 equiv of ferroceniums. To investigate the complexation, UV−vis spectroscopy, 57Fe Mössbauer spectroscopy, electrospray ionization-mass spectroscopy (ESI-MS), cyclic voltammetry (CV), and fluorescence spectroscopy were performed. We confirmed that neutral ferrocenes cannot complex with the imine group while ferroceniums can. Utilizing the redox property of ferrocenes, we were able to electrochemically control the encapsulation and release of ferrocenes into the DPA in a manner similar to redox-responsive proteins such as ferritin. In addition to ferrocenes, oligoferrocenes could also be trapped in the DPA. The biferrocene cation(1+) was particularly suitable for electrochemical switching due to its stable mixed valence condition. The terferrocene dication(2+) encapsulated into DPAG4 could be fabricated into a thin film, which exhibited the near-infrared absorption of an intervalence charge-transfer (IV-CT) band, pointing the way toward the use of such systems in material science.

Dendritic Structure Having a Potential Gradient: New Synthesis and Properties of Carbazole Dendrimers

2009.01.28

K. Albrecht, K. Yamamoto
J. Am. Chem. Soc. 2009, 131, 2244-2251. Dendritic Structure Having a Potential Gradient: New Synthesis and Properties of Carbazole Dendrimers 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.

Additive-Free Synthesis of Poly(phenylene oxide):  Aerobic Oxidative Polymerization in a Base-Condensed Dendrimer Capsule

2007.07.11

K. Yamamoto, Y. Kawana, M. Tsuji, M. Hayashi, T. Imaoka
J. Am. Chem. Soc. 2007, 129, 9248-9532. Additive-Free Synthesis of Poly(phenylene oxide):  Aerobic Oxidative Polymerization in a Base-Condensed Dendrimer Capsule The additive-free synthesis of poly(phenylene oxides) by aerobic oxidation was achieved using a copper complex in a dendritic phenylazomethine structure. The outer shell composed of Schiff base units assists the catalytic function of the inner Cu complex moiety. It is important that the hybrid structure should be designed with a rigid architecture not to deactivate the catalytic center. Due to the rigid π-conjugating structure of the phenylazomethine dendrimer, the metal center can retain its fine conformation. This catalyst drastically decreased the total waste required to produce the PPO derivatives and provided a new approach to green polymerization.