Application of terahertz spectroscopy on monitoring crystallization and isomerization of azobenzene.

Terahertz spectroscopy provides a powerful and informative link between infrared spectroscopy and microwave spectroscopy, and is now beginning to make its transition from initial development to broader use by chemists, materials scientists and biologists. In this study, utilizing terahertz spectroscopy we monitored the crystallization and isomerization of azobenzene. In flash-frozen trans-azobenzene solutions, the processes of crystallization and phase transition were observed. A new phase has been experimentally confirmed to exist stably at low temperatures. The results on gradual-frozen experiment indicate that the formation of the observed new phase is determined by the cooling rate. Besides, based on the distinctive spectral features of the isomers, the thermal- and photo-induced isomerization processes of azobenzene were investigated. This work presents that the terahertz spectroscopy has a great potential to study the phase transitions and crystallization of liquid samples under different freezing conditions.