Photofragmentation Dynamics in Solution Probed by Transient IR Absorption Spectroscopy: πσ*-Mediated Bond Cleavage in p-Methylthiophenol and p-Methylthioanisole.

The 267 nm photodissociation dynamics of p-methylthiophenol (p-MePhSH) and p-methylthioanisole (p-MePhSMe) dissolved in CD3CN have been probed by subpicosecond time-resolved broadband infrared spectroscopy. Prompt (τ < 1 ps) S-H bond fission in p-MePhSH is confirmed by monitoring the time-evolution of the parent (S0) bleach and the transient absorption of the p-MePhS products. Vibrational relaxation of the latter occurs on a ∼8.5 ps time scale, and ∼40% of the total radical population undergoes geminate recombination over a ∼150 ps time scale, yielding (mainly) the p-MePhSH(S0) parent. S-Me bond fission following photoexcitation to the S1 state of p-MePhSMe occurs over a much longer timescale, with a rate that is very dependent on the degree of vibrational excitation within S1. The various findings are compared and contrasted with results from complementary gas-phase photofragmentation studies of both molecules, which are shown to provide a valuable starting point for describing the solution-phase dynamics.