Generation of triarylamine radical cations through reaction of triarylamines with Cu(II) in acetonitrile. A kinetic investigation of the electron-transfer reaction

Abstract

Triphenylamine derivatives react with Cu²⁺ in acetonitrile to give radical cations, which subsequently undergo dimerization to provide tetraphenylbenzidine derivatives. Kinetic aspects of radical cation formation were examined by stopped-flow spectrophotometry. A broad range of triphenylamine derivatives were studied, and the driving force for the electron-transfer reaction ranged from +3.67 to -8.56 kcal M^-1 with rate constants varying from 1.09 × 10² to 2.15 × 10⁵ M^-1 s^-1 for these systems. Reorganization energy for the electron-transfer reaction was estimated using experimentally determined activation parameters. Fitting of the rate data to the Marcus equation using different values of the electronic coupling matrix element H_el provided a good fit with H_el = 100 cm^-1 suggesting that electron transfer in the TPA/Cu²⁺ system conforms to the Marcus-type electron transfer. Furthermore, the high reorganization obtained from these studies is consistent with significant bond cleavage in the transition state, and a mechanism consistent with the experimental data is proposed.