Initial Process of Proton Transfer in Salicylideneaniline Studied by Time-Resolved Photoelectron Spectroscopy

Abstract

Excited state intramolecular proton transfer (ESIPT) in salicylideneaniline (SA) molecules expanded in a supersonic gas jet has been investigated by femtosecond time-resolved photoelectron spectroscopy. Although ESIPT in SA was predicted to take place in a planar structure, the fattening process of a molecule from a twisted Franck–Condon state has never been resolved. Here, we identified the twisting motion of the anilino ring during the fattening process in the decay dynamics of the photoelectron yield, taking account of the energy surface of the S1(π, π ^∗) state of the enol form and the potential surface of ESIPT calculated by a time-dependent density functional theory (TDDFT). The twisting motion was found to be slower in the bromide and methylated SAs, while that in the nitrated SA did not change significantly. These substitution effects are explained by the modification of the potential barriers by the substituents, also predicted by the TDDFT calculation, and support the assignment of the decay dynamics to the twisting motion of the anilino ring prior ESIPT.