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

TLDR: In this article, 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.

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.