Highly Conjugated p-Quinonoid π-Extended Tetrathiafulvalene Derivatives: A Class of Highly Distorted Electron Donors

A new class of π-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 (∼0.7-0.8 V). Theoretical calculations, performed at the DFT level (B3P86/6-31 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 D₂ 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-c²⁺) and the neutral molecules (11a-c).