Synthesis, Crystal Structures, and Electrical Properties of Anion Radical Salts of Novel Electron Acceptors, 2,6-Dicyanomethylene-4-oxo-2,6-dihydrocyclopentadithiophene (CPDT-TCNQ) and Its Diselenophene Analogue (CPDS-TCNQ), Having Three Electron-Withdrawing Groups

Abstract

Abstract Novel heterophene analogues of tetracyanodiphenoquinodimethane (TCNDQ) having three electron-withdrawing groups in one molecule, 2,6-dicyanomethylene-4-oxo-2,6-dihydrocyclopentadithiophene (CPDT-TCNQ: 1) and 2,6-dicyanomethylene-4-oxo-2,6-dihydrocyclopentadiselenophene (CPDS-TCNQ: 2), have been synthesized. CPDT-TCNQ and CPDS-TCNQ have a good coplanar conformation and have a fairly high electron-accepting ability due to the existence of the central carbonyl group in addition to the two terminal dicyanomethylene groups. The anion radical salts, Me4X(CPDT-TCNQ)2 (X = N, P, and As), Et4N(CPDT-TCNQ)2, and Me4X(CPDS-TCNQ)2 (X = P and As), showed a metallic conducting behavior (σrt = 260 to 42 S cm−1) down to 130–255 K. In the crystal structures of Me4X(CPDT-TCNQ)2 (X = N, P, and As), which are isostructural with each other, the acceptor molecules form rigid and tight two-dimensional networks consisting of strong S···N and O···H inter-column contacts in the side-by-side direction. However, these salts have an extremely one-dimensional electronic structure along the stacking direction. This fact indicates that the chalcogen atoms in the acceptor molecules do not increase the dimensionality of the electronic structures of the anion radical salts; that property is significantly different from the role of chalcogen atoms of TTF or BEDT-TTF type donor molecules. The existence of the central carbonyl group in CPDT-TCNQ and in CPDS-TCNQ plays a very important role to give metallic anion radical salts due to the rigid conformation. The phase transition at 130 K on Me4N(CPDT-TCNQ)2 is regarded as 2kF CDW instability. The origins of the phase transitions at 165 K for Me4P(CPDT-TCNQ)2 and 185 K for Me4As(CPDT-TCNQ)2 are regarded as 4kF CDW. Thus the phase transition mechanisms of these salts are significantly different from each other, although the crystal structures are isostructural.