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

Abstract

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 S 1 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 S 0(1 A g ) → S 1(1 B u ) transition of the enol form at 4178.1 Å, is attributed to vibronic coupling with the nonplanar excited keto form due to H tunneling.