Electronic jet spectroscopy of 2,5-bis-(5-t-butyl-2-benzoxazolyl)phenol: deuterium isotope effect

We have examined intramolecular proton transfer in 2,5-bis-(5-t-butyl-2-benzoxazolyl)phenol (BBP). In n-hexane BBP exhibits dual fluorescence: the ‘blue’ from the excited enol form and the ‘red’ emission from the keto tautomer. The fluorescence excitation spectra and fluorescence lifetimes monitored on the ‘red’ and ‘blue’ wavelengths are equal. In a supersonic free jet the fluorescence occurs entirely from the keto form. Vibronic bands in the laser induced fluorescence (LIF) excitation spectrum are broad. The OH/OD substitution induces a line narrowing. This effect suggests the existence of low energy barrier for proton translocation. The fluorescence disappears at above 400 cm−1 energy excess; this marks the onset of a very effective nonradiative channel for energy dissipation.