Femtosecond studies of vibrationally hot molecules produced by intramolecular proton transfer in the excited state

The keto-type reaction product of intramolecular proton transfer in the excited state of 2-(2′-hydroxy-5′-methylphenyl) benzotriazole is investigated by femtosecond experiments in a wide spectral range. Keto-type molecules produced on a 100 fs time scale show an emission band between 600 and 800 nm that decays with a time constant of 150 fs. The strong spectral overlap of the emission with the absorption spectrum of the keto-type ground state extending from 400 to 700 nm gives evidence of non-equilibrium vibrational excitations of the product molecules. The keto-type species decays by proton back-transfer to the enol ground state with a time constant of 600 fs. Measurements with different excitation wavelengths demonstrate that the rates of this ultrafast reaction cycle are not affected by the initial amount of vibronic excess energy.