Electrosynthesis and redox behavior of vinylogous TTF displaying strong conformational changes associated with electron transfers

Abstract

The redox properties of a series of substituted vinylogous tertrathiafulvalenes (TTF) prepared by oxidative coupling of 1,4-dithiafulvenes (DTF) have been investigated in acetonitrile and dichloromethane. The different steps of the electrodimerization mechanism have been characterized: fast electron transfer, coupling between two cation-radicals and slow deprotonation. Through the substituent choice of DTF, it is possible to control the relative stabilities of the different redox species of the electrogenerated vinylogous TTF. According to the nature and position of the substituent, the structural changes induced by the steric interactions lead to a compression of potential (where the second electron is easier to remove than the first one), or on the contrary to a large increase of the separation between the first and second oxidation potentials (by comparison with similar molecules without steric hindrance). Density functional modeling calculations and detailed analysis of the electrochemical behavior have been used to rationalize the substituent effect. A good agreement with the occurrence of an EE mechanism in which the electron transfer is concerted with the conformation changes is found. The inner reorganization energies are low (0.35–0.45 eV) allowing a fast passage between the different conformations during the electron transfers.