Excited state proton transfer in jet-cooled 2,5-di-(2-benzoxazolyl)phenol

The molecule 2,5-di-(2-benzoxazolyl)phenol (DBP) has been studied by laser spectroscopy in a supersonic free jet. In the jet the fluorescence with a Stokes shift of 7000 cm−1 arises exclusively from the proton-transferred species. The fluorescence excitation spectrum of DBP exhibits the broad fluorescence background underlying some weak, sharp features. The first prominent band has been observed at 27830 cm−1, but from 250 cm−1 above the origin the spectrum becomes congested and broad. OH→OD substitution of DBP produces a blue shift of 103 cm−1. For the deuterated sample (DBP-d1) resolved line structure in the fluorescence excitation spectrum has been observed up to an excess energy of 400 cm−1. To identify spectral features corresponding to deuterated DBP and other species double-resonance depletion spectroscopy has been applied. From analysis of the width of the vibronic bands of DBP-d1, we estimated an upper limit of the excited state deuteron transfer rate constant, kDT=1.7×1012 s−1.