Synthesis, properties, and theoretical characterization of largely π-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 electronic properties have been characterized by both. experimental techniques and quantum-chemical calculations. Cyclic voltammetry measurements show a two-electron oxidation wave to form the dication, which is mainly located on the dithiole rings. The second irreversible oxidation wave to form the trication-radical corresponds to the oxidation of the polyacenic backbone. The temperature dependence of the reduction peak corresponding to the donor(2+) --> donor(0) process is explained in terms of the low stability of the cation and the high aromaticity of the dication. Charge-transfer complexes are formed with the strong acceptor 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 semiempirical, ab initio, and density functional theory levels. The steric hindrance introduced by lateral benzoannulation determines the loss of planarity of the neutral molecules, which adopt butterfly shaped structures. 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 polyacenic moiety and two orthogonal, singly charged dithiole rings. The destabilization of the cations and the high aromaticity of the dications explain the redox properties observed experimentally. Theoretical calculations also help to rationalize the UV-vis data since they predict the appearance of a low-energy charge-transfer absorption band for the neutral compounds where the laterally fused polyacenic units act as accepters.