Controlling the conformation changes associated to electron transfer steps through chemical substitution: Intriguing redox behavior of substituted vinylogous TTF

Oxidation of a series of TTF vinylogues has been investigated in acetonitrile and dichloromethane. Depending on the substituent and the solvent, two single-electron transfers or one two-electron process was observed. Density functional modeling calculations and detailed analysis of the electrochemical behavior show that the unusual redox behavior for short-length TTF vinylogues is due to substantial conformation changes concerted with the electron transfers. Flash photolysis and spectroelectrochemistry experiments were also performed to confirm this conclusion. Through fine-tuning of the molecule structure and substituent choice, it is possible to control the relative stabilities of the different redox species. At one end, this leads to situations where the second electron transfer is much easier than the first one. At the ether end, opposite situations are observed with a large increase of the separation between the first and second oxidation potentials by comparison to similar TTF without steric hindrance. The inner sphere reorganization energies remain modest (0.35-0.45 eV), allowing a fast passage between the different conformations during the electron transfers.