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 four 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(−αδ r ) ( α = 9.0 Å -1 ) as a function of the tunneling distance δ 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.