Dynamics of excited-state proton transfer systems via time-resolved photoelectron spectroscopy

We investigate the applicability of time-resolved photoelectron spectroscop y to excited state intramolecular proton transfer (ESIPT) and internal conv ersion dynamics in the model system o-hydroxybenzaldehyde (OHBA) and relate d compounds. Photoelectron spectra of both the excited state enol and keto tautomers were obtained as a function of pump laser wavelength and pump-pro be time delay. The ESIPT was found to occur in less than 50 fs over the who le absorption range of the S-1(pi pi*) state for both OHBA and its monodeut erated analog, suggestive of a small or nonexistent barrier. The subsequent keto internal conversion rate in OHBA varies from 0.63 to 0.17 ps(-1) over the S-1(pi pi*) absorption band and the OD-deuterated analog shows no sign ificant isotope effect. Based upon ab initio calculations and comparison wi th the two-ring analog, 1-hydroxy-2-acetonaphthone (HAN), we suggest that t he internal conversion dynamics in OHBA is influenced by interactions with a close-lying n pi* state. (C) 2001 American Institute of Physics.