Syntheses and properties of derivatives of 2-(thiopyran-4-ylidene)-1,3-dithiole and selenium analogues as novel unsymmetrical electron donors
The systematic replacement of the sulfur atoms of 2-(thiopyran-4-ylidene)-1,3-dithiole (TPDT) by selenium has been studied in order to develop novel unsymmetrical electron donors. In addition, various substitution derivatives of TPDT and selenium analogues bearing methyl, methylthio, and ethylenedithio groups in the five-membered ring and phenyl groups in the five- and/or six-membered rings were prepared. Cyclic voltammetry indicated that TPDT and its derivatives are qualified as strong electron donors, though their donor abilities are more or less perturbed by the molecular modifications, especially in the five-membered ring rather than in the six-membered ring. They thus formed charge-transfer complexes with typical electron acceptors, such as 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,5-dimethyl-7,7,8,8-tetracyanoquinodimethane (DMTCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4), and 3,3′-dichloro-5,5′-dihydro-5,5′-bis(dicyanomethylene)-Δ2,5′-bithiophene (DCBT). Although the resulting complexes showed similar incomplete charge-transfer based on the nitrile vibrational frequencies, their electrical conductivities widely ranged from 10–10 to 10 S cm–1. The high conductivities depended on the appropriate combinations of the donors and the acceptors, and did not seem to be directly correlated with the modifications of the donors by the selenium replacement as well as the methyl, methylthio, and ethylenedithio substitutions. However, most of the complexes with the donors bearing phenyl substituents in the six-membered ring were highly conductive, indicating that the phenyl substitution can contribute to the development of the superior donor character of TPDT owing to considerable extension of the π-conjugation system. In contrast, it was not the case with phenyl substitution in the five-membered ring, which could not sterically make such extension. Almost all of the highly conductive complexes showed a charge-transfer electronic transition in the infrared region, being indicative of a segregated stacking type of crystal structures in a mixed-valence state.