Dr. H. Nishihara has been leading a new research area on highly integrated metal-containing π-conjugated and/or redox-active systems constructed by "coordination programming". He created new classes of multi-functional materials and their combination with interface affording one-dimensional (1D) and two-dimensional (2D) molecular networks and applied them to construct chemical devices. His representative research achievements are summarized as follows.
1. Interfacial synthesis of π-conjugated redox complex oligomer/polymer wires on electrodes and analysis of their electron conduction mechanism and kinetics
An ultimate goal of molecular electronics is to control electron conduction in molecular wires and networks at will by combining appropriate molecular units. Dr. Nishihara has developed a facile and convenient interfacial bottom-up method to fabricate linear and branched molecular wires with the desired number of redox bis(terpyridine)metal (M(tpy)2, M = Fe, Co) complex units combined with π-conjugated linkers and with the designed hetero-metal sequence at metal (Au) or semiconductor (Si) surface. The superior long-range electron conduction abilities of internal molecular wires as well as strong dependencies of the resistivity at the electrode-molecular wire junction and the terminal hetero-redox molecular connection on the chemical structures of molecular segments were quantitatively analyzed. This enables evaluation of the total performance of the molecular wires based on the combination of molecular units, contributing significantly to the research field of molecular electronics and devices.
2. Creation of coordination nanosheets
Two-dimensional (2D) materials such as graphene and molybdenum disulfide have attracted plenteous recent attention because of their unique physical and chemical properties useful for a variety of applications. Dr. Nishihara discovered a facile bottom-up method to synthesize coordination nanosheet (CONASH) materials with variable thickness from submicrometer (multilayer) to subnanometer (monolayer) using coordination reaction of π-conjugated multiple-way bridging ligands with metal ions at the liquid-liquid or gas-liquid interface in ambient conditions. His first report was on nickelladithiolene CONASH prepared by a reaction of nickel ion with benzenehexathiol. This CONASH exhibits reversible redox activity and thus modulation of its oxidation state and electronic conductivity is controllable using redox reactions. Metallic electron conducting nature was found in the oxidized form where the bis(dithiolato)nickel unit is in the neutral form. After the discovery of this nickelladithiolene CONASH, it was theoretically predicted to be an organic 2D topological insulator applicable to new-concept molecular devices.
Dr. Nishihara further succeeded in the synthesis of electro-conductive bis(dithiolato)palladium CONASH, electrochromic bis(terpyridine)metal CONASH (metal = iron and cobalt), and photo-functional bis(dipyrrinato)zinc CONASH. His pioneering work on functional CONASH demonstrates the high potentiality of CONASH as a new class of innovative materials with a variety of interests in basic and applied researches.
3. Development of bio-photosensor using combination of cyanobacterial photosystem I and molecular wire
Highly efficient photoelectron conversion is an important target of research on molecular device, for which it is necessary to control electron conduction in a molecular circuit precisely by combining appropriate molecular-based components. Photosystem I (PSI) is an excellent candidate of the components for molecular devices, since this bio-component for photosynthesis undergoes most efficient and optimized photoelectric conversion performance seen in nature with a quantum yield of nearly 100%. Together with his colleagues in biology and electronics fields, Dr. Nishihara developed a new type of photosensor utilizing cyanobacterial PSI, in which vitamin K1, a redox component in PSI, was reconstituted with an artificial molecular wire containing a 1-methylnaphtoquinone derivative to connect to electrode or transistor gate. Higher sensitivity was achieved by employing single-electron transfer nature of a gold nanoparticle (AuNP) in hydrophobic environment, downsizing the electrode, and tuning of the chemical structure of the artificial molecular wire.
4. Synthesis of expanded multinuclear metalladithiolene families and clarification of their electronic properties
Dr. Nishihara created new expanded metalladitholene families. One of them is triangular π-conjugated trinuclear metalladitholenes, which exhibit unique electronic and magnetic properties and redox behaviors due to the strong internulcear interaction. This study led to the discovery of CONASH, a new class of 2D materials as noted above. He also found the metal-metal bond formation reaction of metalladithiolenes with carbonyl complexes to afford hetero-metal cluster complexes for the first time. This reaction is further combined with the π-conjugated metalladithiolene dimer and trimer, affording more integrated hetero-metal cluster complexes.
5．Creation of metal complexes and nanoparticles responsive to external field
Dr. Nishihara focused on the further functionalization of π-conjugated metal complexes and created several intelligent metal complex systems. One example is the family of photochromic metal complexes in which organic photochromic ligands are coordinated to transition metals. This new class compounds display various unique characteristics such as redox-combined reversible isomerization with a single light source, redox-combined switching of memory depth, multi-stable isomeric states changeable by different wavelength light irradiation, redox-assisted isomerization, and photo-induced alteration of magnetic properties. Another example is a family of pyrylium-containing fused ring aromatic compounds showing multi-step valence tautomerism prepared by the protonation-induced cyclization of ethynylanthraquinones (acting as electron acceptor) with electron donor moieties such as ferrocene or triaylamine. He also presented unique molecular systems such as radical ligand containing complexes showing SOMO-HOMO level conversion and single molecular rotors synchronized with redox and luminescence properties. Coordination chemistry of metal nanoparticles (MNP) is also the research subject of Dr. Nishihara. He presented new synthetic methods of MNP, assembly of MNPs on the solid surface, electrochemical observation of single electron transfer of AuNP in aqueous solution, hydrogen absorption in isonitrile-stabilized PdNP, and construction of porous MNP network.
In conclusion, research achievements of Dr. Nishihara are concerning various types of π-conjugated and/or redox transition metal complex systems and their relating new materials, which could be carried out based on his unique perspective and original ideas. His remarkable accomplishments based on coordination chemistry, electrochemistry, and photochemistry have been spreading to the wide fields such as materials science, solid-state science, nanoscience, the nanotechnology to as above. The Chemical Society of Japan has recognized that his achievements are worthy of the society's award.