Specific excitation of the solvent coordinate in the S 3→S 1 and S 1→S 0 radiationless decay of the betaines

Abstract

Extensive new ultrafast pump-probe measurements are reported for betaine-30 including excitation of ground state molecules at 800 nm (S 0→S 1) and at 400 nm (S 0→S 3). This paper focuses on the variation of betaine transient pump—probe dynamics with probe wavelength. The observed behavior supports the interpretation that the S 1→S 0 internal conversion is induced by specific energy transfer to the solvent coordinate in polar solvents. In addition, transient spectra resulting from excitation at both 400 and 800 nm are nearly indistinguishable at times longer than ≈ 200 fs. We interpret this as evidence that the S 3→S 1 internal conversion is much more rapid and less solvent sensitive than the S 1→S 0 internal conversion (both are formally intramolecular electron transfer). Comparison of the two charge transfer processes offers insight on electron transfer kinetics in solution.