SINGLET EXCITED-STATE INTRAMOLECULAR PROTON-TRANSFER IN 2-(2'-HYDROXYPHENYL)BENZOXAZOLE - SPECTROSCOPY AT LOW-TEMPERATURES, FEMTOSECOND TRANSIENT ABSORPTION, AND MNDO CALCULATIONS

We examine intramolecular proton transfer in 2- (2'-hydroxyphenyl)benzoxazole in its electronic S, state. The experimental methods are fluorescence spectroscopy of the compound isolated in an argon matrix at 11 K and in a supersonically cooled jet, as well as femtosecond pump-and-probe spectroscopy in cyclohexane at 298 K. From the spectroscopic measurements we conclude that the vibronic bands near the electronic origin have a homogeneous width greater-than-or-equal-to 150 cm-1, corresponding to a vibronic lifetime of less than 35 fs. Residual structure reveals a short progression in a vibrational mode of 147 cm-1. The time-resolved measurements indicate a short-lived initial Franck-Condon distribution which evolves, with a time constant of 60 +/- 30 fs, into a distribution of vibrational levels which partly belong to the excited keto form of the molecule. The molecular geometries of the excited enol and keto forms are calculated using MNDO methods. For these geometries, the positions of the mobile hydrogen atom are separated by 0.41 angstrom. We construct a model potential for excited-state intramolecular proton transfer which is consistent with the observations.