Dynamics of excited state electron transfer at a liquid interface using time-resolved sum frequency generation

Femtosecond time resolved vibrational sum frequency generation has been used for the first time to probe a chemical reaction involving interfacial molecules pumped into their excited electronic states. The ultrafast dynamics of electron transfer from ground state N,N-dimethylaniline (DMA) to photoexcited coumarin 314 at a water/DMA monolayer interface was obtained. The forward electron transfer time constant is 16 ± 2 ps, which is faster than electron transfer in bulk DMA. The faster rate is attributed to a lower reorganization free energy, which is a consequence of lower interfacial polarity. The back electron transfer time constant is 174 ± 21 ps.