Photoinduced Intermolecular Electron Transfer in Liquid Solutions

Abstract

Intermolecular photoinduced electron transfer between Rhodamine 3B cation (R3B+), and dimethylanaline (DMA) is studied in a variety of solvents using pump−probe spectroscopy from ultrashort times (∼100 fs) to long times (∼10 ns). Excitation of R3B+ results in the transfer of an electron from DMA and the production of the neutral radical R3B and the DMA+ radical cation. Using a very broadband continuum probe, the generation of the R3B neutral radical is observed (430 nm) as well as the ground state bleach (550 nm), an excited state absorption (445 nm), and stimulated emission (620 nm). A good spectrum of the R3B radical is obtained by removing the overlapping excited state absorption. The forward electron transfer is examined by monitoring the time dependence of the stimulated emission. The data are analyzed with a previously presented detailed theory of through-solvent electron transfer for diffusing donors and acceptors, which includes the influences solvent structure and the hydrodynamic effect. Previous studies have shown that the theory works well for times >100 ps. It is found that in a non-hydrogen-bonding solvent (acetonitrile) and in mixtures of hydrogen-bonding solvents, the theory works well down to a few hundred femtoseconds with only one adjustable parameter, the contact electronic coupling matrix element. However, in pure hydrogen-bonding solvents, it is necessary to increase the solvent hard sphere radius used in the radial distribution to theoretically describe the data, which suggest a larger solvent structural unit than a single solvent molecule.