Time-resolved Raman study on the photoinduced intermolecular electron–transfer reaction of biphenyl in alcohols

Abstract

Time-resolved Raman spectroscopy has uncovered a new type of photoinduced intermolecular electron–transfer reaction of biphenyl in alcohols. The Raman bands of the first excited singlet (S1) state, the cation radical and the anion radical of biphenyl were simultaneously observed in the time range 0–200 ns after photoexcitation. The two ion radicals exhibit distinct rise and decay behaviors from each other. A kinetic analysis shows that the electron–transfer reaction consists of the following three elementary processes; (i) monophotonic formation of a biphenyl cation radical and a solvated electron via the S1 state, (ii) migration of the electron in the solvent medium, and (iii) anion radical formation resulting from the capture of the electron by a biphenyl molecule in the ground state. The formation of the cation radical from the S1 state is a slow process with a rate not greater than 10−7 s−1.