SYNTHESIS, PROPERTIES, AND THEORETICAL CHARACTERIZATION OF LARGELY PI-EXTENDED TETRATHIAFULVALENE DERIVATIVES WITH QUINONOID STRUCTURES

A series of highly conjugated tetrathiafulvalene (TTF) analogues with a quinonoid structure has been synthesized, and their structural and e lectronic properties have been characterized by both. experimental tec hniques and quantum-chemical calculations. Cyclic voltammetry measurem ents show a two-electron oxidation wave to form the dication, which is mainly located on the dithiole rings. The second irreversible oxidati on wave to form the trication-radical corresponds to the oxidation of the polyacenic backbone. The temperature dependence of the reduction p eak corresponding to the donor(2+) --> donor(0) process is explained i n terms of the low stability of the cation and the high aromaticity of the dication. Charge-transfer complexes are formed with the strong ac ceptor TCNQF(4) showing a 1:2 (D:A) stoichiometry and a semiconducting behavior. The molecular structures of neutral and oxidized compounds are investigated by performing theoretical calculations at the semiemp irical, ab initio, and density functional theory levels. The steric hi ndrance introduced by lateral benzoannulation determines the loss of p lanarity of the neutral molecules, which adopt butterfly shaped struct ures. The folded structures are retained in the cations, reducing the gain of aromaticity in the first oxidation step. The dications are by contrast predicted to be fully aromatic and are formed by a planar pol yacenic moiety and two orthogonal, singly charged dithiole rings. The destabilization of the cations and the high aromaticity of the dicatio ns explain the redox properties observed experimentally. Theoretical c alculations also help to rationalize the UV-vis data since they predic t the appearance of a low-energy charge-transfer absorption band for t he neutral compounds where the laterally fused polyacenic units act as accepters.