Picosecond measurements of phenol excited-state proton transfer in clusters. I. Solvent basicity and cluster size effects

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Abstract
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≥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≂3). Proton transfer was not observed in the less-basic solvent clusters (CH3OH)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.