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

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 CH2Cl2, “static” quenching of the lowest excited singlet state (S1) of these amines is demonstrated. For TMPD, the reaction shows two kinetic contributions (τ1<4 ps, τ2=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 S1 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+, CH2Cl2−] ion pair allowing charge recombination to a final form [TMB+, Cl−] inappropriate to back ET.