Introduction to π-Electron Materials
There has been a long history of research on artificial electrical conductors based on π electrons aimed at developing new technologies as in the rapid development of silicon technology. Just after the end of the Second World War, the concept of “organic semiconductors” was proposed by Professor Inokuchi’s group at the University of Tokyo. The concept originated from their study on electrical conductivity of various aromatic hydrocarbons extracted from raw carbon chars, and charge transfer complexes with high electrical conductivity were synthesized as a result . From the latter half of the 1950s, photoconductivity of highly-purified single crystals of insulating polyaromatic hydrocarbons, such as anthracene, naphthalene and tetracene, was studied extensively and the mechanisms of generation and transport of charge carriers were fully discussed  . These efforts have become profitable very recently as an enormous industry of organic photoconductors used in copying machines and laser-beam printers. The discovery of superconductivity in graphite intercalation compounds was made in the mid 1960s  but there have been few studies succeeding this pioneering work, probably because the critical temperature was extremely low (0.13–0.55 K) . The next generation of research on organic semiconductors was on ion-radical salts based on 7,7,8,8-tetracyanoquinodimethane (TCNQ) , where highly conductive or metalic phenomena of various TCNQ complexes drew extensive attention.
KeywordsCritical Temperature Organic Semiconductor Charge Transfer Complex Charge Density Wave Organic Conductor
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