A vinylogue of bis-fused tetrathiafulvalene: novel π-electron framework for two-dimensional organic metals

Abstract

Several derivatives of 2-(1,3-dithiol-2-ylidene)-5-(2-ethanediylidene-1,3-dithiole)-1,3,4,6-tetrathiapentalene (1Aa), DTEDT, which is a vinylogue of bis-fused TTF (BDT–TTP) containing one 2,2′-ethanediylidenebis(1,3-dithiole)(2), have been synthesized. The cyclic voltammograms of the DTEDTs consist of four pairs of single-electron redox waves. Comparison of the first oxidation potentials (E₁) reveals that donating ability of 1Aa is rather weaker than those of 2 and TTF, in spite of the extension of the apparent π-conjugation. Substituent effects on the E₁ values suggest that the first oneelectron oxidation occurs mainly in the vinylogous TTF moiety. The smaller E₂–E₁ value of 1Aa compared with those of BDT–TTP, TTF and 2 suggests a decrease in the on-site coulombic repulsion in the dication. An X-ray structure analysis of the neutral 4,5-dimethyl-4′,5′-bis(methylthio) derivatives (1Be) reveals that the crystal has a so-called ‘κ-type array’ of molecules. An MNDO molecular orbital calculation indicates that the sulfur atoms at the termini of the vinylogous 1,3-dithiole rings are out of phase with the other sulfur atoms in the HOMO. The present donors have produced many molecular complexes exhibiting high conductivity. In particular, all of the radical-cation salts based on 1Aa obtained so far exhibit metallic conducting behaviour down to 1.4–4.2 K, regardless of the counter-anions.