Femtosecond Fluorescence Study of the Substitution Effect on the Proton Transfer in Thermochromic Salicylideneaniline Crystals

The substitution effect on the dynamics of the proton transfer in thermochromic salicylideneanilines in the crystalline phase was investigated using femtosecond time-resolved fluorescence spectroscopy. From the deuteration effect and low-temperature measurement, it is concluded that the proton tunnels the potential barrier quantum-mechanically in the excited state. The rate of proton transfer was found to decrease with the energy separation between the excited enol and keto forms due to the different potential-barrier heights. The change in the potential-barrier height is qualitatively explained by the electron-donating or -accepting property of the substituent around the hydrogen bond. The thermalization of the vibrational states of the excited enol form was also observed.