EXCITED-STATE PROTON (OR HYDROGEN-ATOM) TRANSFER IN JET-COOLED 2-(2'-HYDROXYPHENYL)-5-PHENYLOXAZOLE

The fluorescence excitation and dispersed emission spectra of 2-(2'-hydroxyphenyl)-5-phenyloxazole (HPPO) and its OH deuterated derivative (DPPO) have been investigated under gas-phase isolated conditions in a supersonic jet. While the excitation spectrum of HPPO is composed of broad overlapping bands, the deuteration of the OH group induces a drastic narrowing of the vibronic structure. The main vibronic bands of HPPO are well reproduced by Lorentzian line shapes corresponding to a homogeneous width ranging from 24 to 30 cm(-1), while those of DPPO range from 3.6 to 9.7 cm(-1). The dispersed fluorescence of both compounds exhibits an identical large Stokes shift. The results are interpreted in terms of the occurrence of a fast (>4.5 x 10(12) s(-1)) excited-state intramolecular proton-tunneling-transfer reaction in the enol form producing a keto tautomer through an asymmetric potential energy surface with a small energy barrier.