Effect of salts on the quantum yield and the decay of lons produced by photoinduced electron transfer from zinc tetraphenylporphyrin to 1,4-benzoquinone in 4-methylpentan-2-one

Abstract

The effects of salts on the electron transfer from the excited triplet state of zinc tetraphenylporphyrin (ZnTPP) to 1,4-benzoquinone (BQ) in 4-methylpentan-2-one have been studied by using laser flash photolysis. Addition of neutral salts, such as LiClO₄, NaClO₄ or tetrabutylammonium perchlorate (Bu₄NClO₄), enhances the quantum yield of the porphyrin radical cations (ZnTPP^˙+), separated from the radical ion pairs, [ZnTPP^˙+⋯ BQ^˙–], and prolongs the lifetime of ZnTPP^˙+ markedly. The enhancement of the quantum yield depends on the nature of the counter-cation of the added salts and is in the order of Bu₄N⁺ < Na⁺ < Li⁺. The decay of ZnTPP^˙+ in the presence of LiClO₄ follows second-order kinetics as in the case of salt-free solutions, whereas the decay in the presence of NaClO₄ or Bu₄NClO₄ has been found to be composed of an earlier second-order decaying component and a later first-order decaying component. The decay rate constant for the second-order decaying component was determined by separation of the first-order component from the composition. The second-order decay rate is decreased markedly by addition of each salt and is independent of the salt concentration in the range 5 × 10^–4–1.5 × 10^–1 mol dm^–3. The effect of salts on the cation decay depends on the nature of the counter-cation and is as large as the enhancing effect for the quantum yield. These results are well interpreted by Coulombic stabilization of the radical ion pairs due to specific interaction with the added salt.