Molecular Conductors

A molecular conductor is an electrical conductor based on a molecular component. A chemical modification of the molecule can control the electronic structure and physical properties. We can synthesize an opened-shell molecular conductor by chemical or electrochemical doping of a carrier, although an organic molecule is usually an insulator with a closed-shell structure. The first example of an organic conductor was observed from a bromine-doped perylene.¹⁾ After this observation, a molecular conductor based on tetrathiafulvalene (TTF) was reported in the 1970s,²⁾ and the first case of an organic molecular superconductor was observed from the organic salt of tetraselenafulvalene, (TMTSF)₂X in the 1980s.³⁾ These TTF and TMTSF salts form a one-dimensional or pseudo one-dimensional molecular arrangement. On the other hand, bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) favors forming a two-dimensional molecular arrangement, which is a relatively stable molecular metal toward temperature.^4-6)

Conducting salts of TTF derivatives can be normally obtained by an electrochemical oxidation (electrocrystallization).⁷⁾ These TTF derivatives can function as donor molecules with a hole carrier. Metal dithiolene complexes (M(dmit)₂), 7,7,8,8-Tetracyanoquinodimethane (TCNQ) and fullerene (C₆₀) are acceptor molecules with an electron carrier. An M(dmit)₂ salt produced the first example of an acceptor-based organic superconductor.⁸⁾ Chemical modifications of the M(dmit)₂ salt produced plenty of those organic superconductors by changing the central metal atom and counter cation.⁹⁾

Several alkali-doped nanocarbon and nanographene compounds have shown superconductivity. It has well known that rubidium- and cesium-doped fullerenes have demonstrated superconductivity at more than 30 K.¹⁰⁾ Recently, superconductivity of Cs₃C₆₀ at 38 K was reported.¹¹⁾ Although the molecular conductors synthesized from TTF and M(dmit)₂ exhibit low-dimensional molecular arrangements, these fullerene salts can form three-dimensionality.¹²⁾ Kubozono et al. reported that an alkali-doped picene demonstrated superconductivity at 18 K.¹³⁾ This result indicates that one can observe superconductivity from a planar nanocarbon material as well. In addition to the picene-based superconductor, an alkali-doped phenanthrene (T_c = 5 K),¹⁴⁾ alkali-doped coronene (T_c = 15 K),¹⁵⁾ and alkali-doped 1,2:8,9-dibenzopentacene (T_c = 33 K)¹⁶⁾ have also shown superconductivity.