Organic Conductors With Narrow Bandwidth Based On 2-(Pyran-4-Ylidene)-1,3-Dithiole

A pi-electron donor incorporating pyran-4-ylidene moiety, 2-(pyran-4-ylidene)-1,3-dithiole (PDT, 1a), and its derivatives (1b-e) have been synthesized. Cyclic voltammetry revealed that the derivatives of 1 exhibited two pairs of redox waves. Comparison of the first redox potentials (E-1) indicated that the donating ability of PDT (E-1 = -0.16 V vs. Fc/Fc(+), in benzonitrile) is stronger than that of TTF (E-1 = -0.09 V), but is weaker than the sulfur analog TPDT (E-1 = -0.19 V). X-ray structure analyses of radical cation salts based on the ethylendithio derivative (1d)(2)X (X = ClO4, ReO4, and GaCl4) revealed that the donors form two-dimensional conducting sheets, in which the donors adopt the so-called 13-type packing with a uniform head-to-tail stacking. Calculation of the overlap integrals of the HOMOs suggest that (1d)(2)X has a small intrastack overlap compared to the TTF-type conductors because of a head-to-tail stacking of the unsymmetrical pi-electron framework. A tight-binding band calculation suggested that all the salts have quasi-one-dimensional Fermi surfaces. They exhibited relatively high conductivity of sigma(rt) = 0.79-20 S cm(-1) on a single crystal and showed weak but metal-like temperature dependence of resistivity.