FEMTOSECOND STUDIES OF EXCITED-STATE PROTON AND DEUTERIUM TRANSFER IN BENZOTHIAZOLE COMPOUNDS

Intramolecular proton and deuterium transfer in the excited state of 2-(2'-hydroxyphenyl)benzothiazole (HBT) is investigated on the femtosecond time scale. The reaction steps occurring immediately after photoexcitation are temporally resolved in pump and probe measurements, where the onset of emission of the reaction product is monitored. For HBT in nonpolar solvents, the transfer of the proton occurs with a time constant of 160 +/- 20 fs. The deuterated analogue DBT undergoes a similar reaction with a transfer time of the deuterium of 150 +/- 20 fs. In polar solvents, the hydroxylic proton of HBT is shifted within the molecules on a time scale between 80 and 250 fs. The experimental results point to an essentially barrierless potential energy surface of the reaction in the excited state. The measured time constants correspond to the period of molecular vibrations of low frequency, suggesting proton and deuterium transfer via large-amplitude motions.