State-selectivity of excited-state intramolecular proton transfer in a ‘‘double’’ benzoxazole: Jet spectroscopy and semiempirical calculations

The fluorescence excitation spectra of free 2,5-bis(2′-benzoxazolyl)-hydroquinone (BBXHQ) cooled in a supersonic jet are examined. 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 Å. The complexity of the vibronic spectrum, near the electronic origin for the S0(1Ag) → S1(1Bu) transition of the enol form at 4178.1 Å, is attributed to vibronic coupling with the nonplanar excited keto form due to H tunneling.