Photophysical characterization and effect of pH on the twisted intramolecular charge transfer fluorescence of trans-2-[4-(dimethylamino)styryl]benzothiazole

Experimental and theoretical photophysical characterization of trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) has been carried out in solutions of different polarities. Steady state fluorescence emission and excitation spectra of DMASBT in the solvents of varying polarities suggest the existence of dual emitting states where locally excited state is responsible for fluorescence in nonpolar solvents; however, in polar solvents fluorescence is from twisted intramolecular charge transfer (TICT) state. Solvatochromic comparison method suggests that the dipolar interactions make the major contribution to the stability of ground as well as excited states with more effect on the latter. Semi-empirical AM1 singly excited configuration interaction (AM1-SCI) calculations support the formation of the TICT state of DMASBT. The twist of the –N(CH3)2 group and the change in its hybridization in the excited state develops a high dipole moment in the S3 state and thereby stabilizing it to give the TICT fluorescence in polar solvents. The TICT fluorescence of DMASBT decreases above the pH value of 3.7 suggesting that hydrogen bonding ability of water molecules becomes efficient above this pH. Results indicate that DMASBT can be a potential sensor based on its remarkably high fluorescence sensitivity towards polarity, viscosity and pH of the medium.