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

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