PICOSECOND MEASUREMENTS OF PHENOL EXCITED-STATE PROTON-TRANSFER IN CLUSTERS .1. SOLVENT BASICITY AND CLUSTER SIZE EFFECTS

Excited-state proton transfer (ESPT) rates in molecular clusters were measured as a function of cluster size using picosecond spectroscopy in a molecular beam mass spectrometer. ESPT from the S1 state of phenol to base solvent clusters (NH3)n occurs for a critical solvent cluster size n greater-than-or-equal-to 5, with a rate constant of k = (60 +/- 10 ps)-1 for n = 5-7. ESPT showing critical cluster-size dependencies was also observed in the basic solvent N(CH3)3 (n congruent-to 3). Proton transfer was not observed in the less-basic solvent clusters (CH3 OH)n and (H2O)n. Mixed-solvent studies indicate that the addition of a dissimilar molecule to an otherwise neat solvent cluster impedes ESPT, presumably due to a disruption of the hydrogen bonding network. Evidence is also presented for the direct measure of solvent reorganization following ESPT. For (NH3)n solvation, the solvent reorganization appears as a long-time-scale component (0.3 ns) on the protonated solvent formation traces.