STATE-SELECTIVITY OF EXCITED-STATE INTRAMOLECULAR PROTON-TRANSFER IN A DOUBLE BENZOXAZOLE - JET SPECTROSCOPY AND SEMIEMPIRICAL CALCULATIONS

We examine the fluorescence excitation spectra of free 2,5-bis(2'-benzoxazolyl)-hydroquinone (BBXHQ) cooled in a supersonic jet. By using double-resonance saturation spectroscopy, we show that adjacent vibronic transitions, leading to either blue or red fluorescence, belong to a single molecular species in the electronic ground state. The molecular geometries of the enol and keto forms which are relevant for intramolecular transfer of a single hydrogen atom in the S1 state are obtained by the MNDO/H method. The translocation distance is calculated to be 0.43 angstrom. The complexity of the vibronic spectrum, near the electronic origin for the S0(1A(g)) --> S1 (1B(u)) transition of the enol form at 4178.1 angstrom, is attributed to vibronic coupling with the nonplanar excited keto form due to H tunneling.