It was a quarter century ago when the first organic superconductor, (TMTSF)2PF6, was reported by the group of Profs. Jérome and Bechgaard. The characteristic crystal structure of (TMTSF)2PF6 with compact Se...Se network suggested a crucial hint to design the two-dimensional metal based on the planar π donors with many peripheral chalcogen atoms, which was considered to be essential to avoid the metal instability inherent in the organic conductors developed in 1970s. With the purpose of obtaining the method to evaluate the dimensionality of the molecular conductor including not only organic conductors but also the molecular metals and superconductors based on transition metal complex molecules with dithiolate ligands first developed by Prof. Underhill and Dr. Cassoux, Prof. Kobayashi et al. have introduced the tight-binding band calculations in the studies on the design of molecular conductors. By adopting very simple extended-Hückel approximation, it became possible that both the anisotropy of crystal structure and the anisotropy of the frontier orbitals constructing conduction band are taken in the band structure calculation simultaneously, which made it possible to visualize the dimensionality of the molecular conductor as the shape of Fermi surface. The validity of extended-Hückel tight-binding band calculation was confirmed by the low-temperature oscillatory magnetoresistance experiments performed at the end of 1980s. This very simple band calculation is now regarded as an inevitable tool for the studies on the molecular conductors. Through these studies, Kobayashi et al. have reported various types of new conducting systems including molecular superconductors such as the first "κ-type organic superconductor", κ-(BEDT-TTF)2I3 with almost ideally two-dimensional cylindrical Fermi surfaces and the molecular superconductor without TTF-like π donors, [(CH3)4N][M(dmit)2]2 (M=Ni, Pd). In addition, on the basis of the observation of three-fold structure of the low-temperature insulating phase of (CH3,Cl-DCNQI)2Cu, Kobayashi et al. have pointed out the existence of the organic conductor with π-d mixing metal band, which provided a clue to make clear the intriguing electromagnetic properties of a series of (R1,R2-DCNQI)2Cu (R1, R2= CH3, Cl, Br) systems first developed by Prof. Hünig et al. But the first stage of the development of new organic conductors initiated by the discovery of Bechagaard salt seemed to reach a peak around 1990. In the last decade, various multi-functional molecular conductors exhibiting synergetic actions of conductivity and magnetism have been developed. Kobayashi et al. have discovered the first antiferromagnetic organic superconductor and the first field-induced organic superconductor. Furthermore they have also developed a completely new type of molecular conductor, the "single-component molecular metal".
(1) Magnetic Organic Superconductor
In general, the magnetism and superconductivity are considered to be competitive to each other. Therefore, the magnetic anions were intentionally avoided in the trials of the development of new molecular superconductors in 1980s. In early 1990s, Prof. Kobayashi et al. have developed several organic metals based on π donor molecule, BETS (= bis(ethylenedithio)tetraselenafulvalene) and tetrahalide monoanions (MX4-, M=Ga, Fe, In; X=Cl, Br). Among them, the systems with typical magnetic monoanions such as FeCl4- and FeBr4- were prepared with the aim of obtaining new type of organic conductors where the π metal electrons and localized 3d magnetic moments coexist at low temperature to afford novel electromagnetic properties originated from the interplay of two electron systems. Recently, Kobayashi et al. have intensively studied these BETS conductors and reported many novel electromagnetic properties: (1) the first observation of the field-induced insulator-to-metal transition in λ-(BETS)2FeCl4, which can be regarded as a kind of "colossal magnetoresistance (CMR)" first observed in the organic systems, (2) the unprecedented superconductor-to-insulator transition in λ-(BETS)2FexGa1-xCl4 (x≈0.4), (3) the field-induced superconductivity in organic systems, λ-(BETS)2FeCl4, λ-(BETS)2FexGa1-xCl4 and κ-(BETS)2FeBr4, (4) Magnetic-field insulator↔superconductor↔metal switching in λ-(BETS)2FexGa1-xCl4 (x≈0.4), (5) the first antiferromagnetic organic superconductor, κ-(BETS)2FeX4 (X=Br, Cl) and (6) the competition between metamagnetism and superconductivity in κ-(BETS)2FeBr4. It is well known that Prof. Day et al. reported the organic superconductor with paramagnetic anion layers in 1995 and Prof. Coronado's group reported the first organic metal with ferromagnetic anion layers in 2000. Through these works, new era of magnetic organic conductor has been opened, where novel bi-functional electromagnetic properties unattainable in the traditional organic conductors can be expected. Among the hitherto-reported magnetic organic conductors, the systems discovered by Kobayashi et al. are known to exhibit exceptionally rich electromagnetic properties, which exemplified the large potential of the organic conducting systems.
(2) Single-component molecular metals
It is well-known that in the late 1940s, Prof. Eley examined the conducting properties of the crystals of phtalocyanine and Cu-phtalocyanine for the purpose of obtaining suitable systems to investigate quantitatively the electronic process in the molecular system. He observed semiconducting properties of phthalocyanine crystal in spite of its very low conductivity (ρ(RT)≈10-10 S cm-1) and suggested the possibility of the existence of the electronic band arising by intermolecular overlap of the π orbitals of the phtalocyanine rings. In 1950, Profs. Akamatu and Inokuchi have also reported the condensed aromatic hydrocarbons such as violanthrone and pyranthrone to be intrinsic semiconductors. In spite of these pioneer works performed more than a half century ago, almost no chemist did dare to dream to develop the metallic crystal consisting of the same kind molecules for a long time. Needless to say, the formation of conduction band and the carrier generation by the charge transfer between the molecules forming conduction band and the other chemical species are two essential requirements for the design of the molecular metals. Since the charge transfer is impossible between the same kind molecules, all the molecular conductors developed until quite recently had been composed of more than two chemical species. But Prof. Kobayashi and his coworkers have succeeded to satisfy these two requirements simultaneously by adopting the transition metal complex molecule with extended-TTF type ligands, which have very small HOMO-LUMO gap and the ability to form two-dimensional intermolecular S...S network. In the single-component molecular crystal Ni(tmdt)2, the electron and hole carriers are generated automatically by the self-assembly of the molecules, similar to the case of the typical inorganic metals such as Na and Cu. The existence of the three-dimensional electron and hole Fermi surfaces was confirmed by the observation of de Haas-van Alphen oscillations.
By his distinguished contribution in the field of molecular conductors, it is recognized that Professor Kobayashi deserves the Chemical Society Award of The Chemical Society of Japan.