TIME-RESOLVED SPECTROSCOPY ON ULTRAFAST PROTON-TRANSFER IN 2-(2'-HYDROXY-5'-METHYLPHENYL)BENZOTRIAZOLE IN LIQUID AND POLYMER ENVIRONMENTS

The intramolecular proton transfer in 2-(2'-hydroxy-5'-methylphenyl)benzotriazole (TIN) is studied in different environments by stationary as well as pico- and femtosecond spectroscopy. Two ground-state conformers with a relative concentration depending on the specific solvent are distinguished from the spectra and the picosecond kinetics. The main species in nonpolar solution and in the polymer shows a weak fluorescence in the red with subpicosecond buildup and decay times. This emission is indicative of excited-state proton transfer resulting in the formation of a planar keto-type tautomer. Transient absorption measured on a femtosecond time scale gives evidence of the ultrafast keto-enol back-reaction occurring in the electronic ground state with time constants between 500 fs and 1.2 ps. In addition, a species of nonplanar geometry in the electronic ground state is detected showing emission in the blue spectral range. The fraction of the latter tautomers lies between less than 10(-3) in nonpolar solvents and up to 60% in polar solution.