Highly conjugated p-quinonoid π-extended tetrathiafulvalene derivatives:: A class of highly distorted electron donors

A new class of pi-extended TTF-type electron donors (11a-c) has been synthesized by Wittig-Horner olefination of bianthrone (9) with 1,3-dithiole phosphonate esters (10a-c). In cyclic voltammetry experiments, donors 11a-c reveal a single, electrochemically irreversible oxidation-yielding the corresponding dicationic products-at relatively low oxidation potentials (similar to0.7-0.8 V). Theoretical calculations, performed at the DFT level (B3 P86/6-31 G*), predict a highly-folded C-2h structure for 11a. In the ground state, the molecule adopts a double saddle-like conformation to compensate the steric hindrance. The calculations suggest that the intramolecular charge transfer associated with the HOMO-->LUMO transition is responsible for an absorption band observed above 400 nm. While the radical cation 11a(.+) retains the folded C-2h, structure predicted for the neutral molecule as the most stable conformation, the dication 11a(2+) has a fully aromatic D-2 structure, formed by an orthogonal 9,9'-bianthryl central unit to which two singly-charged dithiole rings are attached. The drastic conformational changes that compounds 11 undergo upon oxidation account for their electrochemical properties. By means of pulse radiolysis measurements, radical-induced one-electron oxidation of 11a-c was shown to lead to the radical cation species (11a-c(.+)), which were found to disproportionate with generation of the respective dication species (11a-c(2+)) and the neutral molecules (11a-c).