The carbon π-conjugated system is the most essential structural unit that controls the functionality of organic materials. The key to the development in the science and technology of these materials lies in the fundamental study to create π-conjugated compounds with novel structures, which are properly designed for the actualization of desired properties. Dr. Koichi Komatsu has dedicated his efforts to this issue and made outstanding contributions through his synthetic and physical organic studies based on his innovative ideas.
1. Organic Synthesis of Endohedral Fullerenes by Molecular Surgery
Since fullerene C60 was isolated as a real substance, characteristic properties of its exterior surface have been fully elucidated through a great number of studies. In contrast, the development in the science dealing with its hollow interior, i.e., that of the "endohedral fullerenes" has been much retarded due to the technical difficulty in their production, which still relies on classical physical method (like arc-discharge or high-pressure/high-temperature treatment) affording only minute amounts of pure materials.
Komatsu brought about a breakthrough to this situation by establishing an entirely new route to afford a significant amount of a new endohedral fullerene by rationally designed organic synthesis.
(1) Previously, Komatsu had succeeded, for the first time, in highly selective synthesis and X-ray structure analysis of the dumbell-shaped fullerene dimer, C120, based on his unique idea, that is, the application of mechanochemical solid-state reaction to the fullerene chemistry. This success led him to a systematic study on the dependence of the product selectivity on the reaction phase, i.e., the solid state and the liquid phase. He found that the liquid-phase thermal reaction of fullerene with aza-aromatic compounds is particularly suitable to make an opening on the fullerene cage. This was developed into an innovative "molecular surgery" of fullerene, that is, opening a hole on a C60 cage, enlargement of the hole, encapsulation of molecular hydrogen in 100%, and "stitching up" the hole with retention of the encapsulated hydrogen. The open-cage fullerene, synthesized during this transformation, is the first nanometer-sized container of a single molecule of hydrogen. The encapsulated hydrogen is retained under normal conditions and can be released simply by heating at the temperature higher than 160 °C.
(2) In the following step, Komatsu established four-step organic reactions to completely close the hole without serious loss of encapsulated hydrogen, and succeeded in the first synthesis of an entirely new endohedral fullerene incorporating molecular hydrogen, H2@C60, in more than hundred milligrams. This molecule is a unique "hydrocarbon that has no C-H covalent bond." This system can be an ideal research object for physicists and physical chemists to elucidate the intrinsic properties of a single hydrogen molecule in a totally isolated state. This newly developed method is further applicable to encapsulate hydrogen isotopes such as D2 and HD as well as the smallest stable atom, helium, and also to a larger fullerene, C70. This achievement is taken as the first step toward the rational and tailor-made production of a wide variety of endohedral fullerenes in macroscopic amounts in future.
2. Control of the Electronic State of Cyclic π-Conjugated Systems by the Use of Strained σ-Carbon Frameworks
So far, the methodology to control the electronic properties of cyclic π-conjugated systems has been confined to the transformation of the shape of the π-system itself and/or introduction of hetero-atom(s). A totally new concept has been awaited for further investigation of novel electronic structures. Based on his previous discovery on super-stabilization of carbocations by the use of σ-π conjugation, Komatsu succeeded in the creation of π-systems with unique electronic structures utilizing both the electronic and steric effects of bicycloalkene frameworks rigidly fixed around the cyclic systems.
(1) Komatsu chose bicyclo[2.1.1]hexene as a typical strained bicyclic system possessing high p-character and strong Mills-Nixon effect, and achieved an efficient synthesis of a novel cyclooctatetraene (COT) with a completely planar 8-π electron system, together with the corresponding benzene, by cuprate-mediated cross-coupling reaction. This COT is unique in that the antiaromaticity is significantly reduced not only by marked bond-alternation but by a strong σ-π interaction with bicyclic frameworks. The corresponding benzene, obtained also in good yield, was transformed into a unique naphthalene composed of one six-membered ring that is highly aromatic owing to bond-equalization caused by annelation with two bicyclic systems and another one with greater bond alternation.
(2) Although various π-conjugated polymers are known as electric conductors, the mechanism for conduction is not always clear. Komatsu made an inventive molecular design to stabilize cationic species of oligothiophenes, which are models for a polaron and a bipolaron of p-doped states of polythiophenes, by coating the oligothiophene wire with insulating bicyclic σ-frameworks. He succeeded in the synthesis and X-ray structure determination of remarkably stable salts of radical cations as well as dications of these oligomers as single molecules, which do not undergo any π-dimerization, thus elucidating the most essential structural units of p-doped oligothiophenes.
(3) Komatsu also proved that his innovative technique of cation stabilization can be applied to the synthesis and first X-ray crystallography of a series of radical cations of benzenoid condensed aromatics and various sulfur-containing π-systems in addition to the generation and observation of corresponding dications. Furthermore, the first carbon π-conjugated system incorporating silicon, silatropylium ion, was generated and characterized. It is to be noted that the experimental studies on electronic structures of these cationic species of fundamental importance could have never been achieved by any other method known to date. As mentioned above, Komatsu successfully combined his innovative molecular and synthetic δεσιγνσ ωιτη εξπεριμεντσ το χρεατε νοωελ-conjugated systems with a wide range of structures and properties, and made landmark achievements in the field of structural organic chemistry. These accomplishments are widely recognized internationally and domestically.