Femtosecond fluorescence study of proton-transfer process in thermochromic crystalline salicylideneanilines

In order to clarify a proton-transfer mechanism, the dependence of proton-transfer rate on the hydrogen-bond length was investigated. The proton-transfer rate in fc,ur thermochromic N-salicylideneanilines (SAs) in a crystalline phase was estimated by a femtosecond time-resolved fluorescence spectroscopy. The transfer rate is given by k(0) exp(-alpha delta r) (alpha=9.0 Angstrom(-1)) as a function of the tunneling distance delta r, where k(0) is the O-H stretching frequency. The proton-transfer rate is insensitive to a sample temperature and shows a deuteration effect. From these results, it is concluded that the photoinduced proton transfer in the excited state of SAs takes place by the quantum mechanical tunneling.