Ultrafast excited-state electron transfer at an organic liquid/aqueous interface.

Ultrafast excited-state electron transfer has been monitored at the liquid/liquid interface for the first time. Second harmonic generation (SHG) pump/probe measurements monitored the electron transfer (ET) occurring between photoexcited coumarin 314 (C314) acceptor and dimethylaniline (DMA) donor molecules. In the treatment of this problem, translational diffusion of solute molecules can be neglected since the donor DMA is one of the liquid phases of the interface. The dynamics of excited-state C314 at early times are characterized by two components with exponential time constants of 362 +/- 60 fs and 14 +/- 2 ps. The 362 fs decay is attributed to the solvation of the excited-state C314, and the 14 ps to the ET from donor to acceptor. We are able to provide conclusive evidence that the 14 ps component is the ET step by monitoring the formation of the radical DMA cation. The formation time is 16 ps in agreement with the 14 ps decay of C314*. The recombination dynamics of DMA+ plus C314- was determined to be 163 ps from the observation of the DMA+ SHG signal.