Ultrafast photoinduced electron transfer from N, N, N′, N′-tetramethyl- p-phenylenediamine and N, N, N′, N′-tetramethylbenzidine to dichloromethane

Abstract

The photoinduced reaction of electron transfer (ET) from N, N, N′, N′-tetramethyl- p-phenylenediamine (TMPD) and N, N, N′, N′-tetramethylbenzidine (TMB) to the dichloromethane solvent has been studied by picosecond transient absorption and time-resolved resonance Raman spectroscopy. In neat CH 2Cl 2, “static” quenching of the lowest excited singlet state (S 1) of these amines is demonstrated. For TMPD, the reaction shows two kinetic contributions ( τ 1<4 ps, τ 2=9.5 ps) that are ascribed to the fractions of excited molecules that react before and after they are vibrationally relaxed, respectively. A faster quenching reaction is observed for TMB ( τ<4 ps) although it is less favourable energetically than for TMPD. This result is ascribed to a stronger acceptor–donor electronic coupling in the case of TMB, principally due a larger delocalization of the S 1 state donor orbital. The observation of a partial back electron transfer (9% yield) is tentatively explained as resulting from an evolution of the reaction product from an initial [TMB + , CH 2Cl 2 − ] ion pair allowing charge recombination to a final form [TMB + , Cl −] inappropriate to back ET.