Abstract

Photoinduced intramolecular proton transfer represents a chemical reaction that has mainly been studied in electronically excited molecules. For benzothiazole and -triazole compounds, recent femtosecond experiments gave evidence of excited state proton and deuterium transfer on a time scale of 100 to 200 fs [1,2]. In 2-(2’-hydroxy-5’-methylphenyl)- benzotriazole (trade name: TINUVIN P, TIN), the excited state enol→keto reaction (see structures (I) and (II)) is followed by the radiationless relaxation to the ground state of the keto-type molecules, occurring with a time constant of 150 fs. This species eventually undergoes a proton back-transfer to the ground state of the original enol structure. Until now, there was little information on the ground state reaction of TIN and of other compounds undergoing a closed reaction cycle. In particular, vibrational redistribution processes randomizing the large amount of energy from the initial photoexcitation (≃ 4 eV) have not been observed. In this paper, we discuss the first study of those processes with a time resolution of 100 fs, directly revealing the subpicosecond proton transfer in the ground state of TIN and the strong vibrational heating of the reaction product.

© 1992 The Author(s)