Excited state isomerization and effect of viscosity- and temperature-dependent torsional relaxation on TICT fluorescence of trans-2-[4-(dimethylamino)styryl]benzothiazole

Effect of viscosity and temperature on twisted intramolecular charge transfer (TICT) fluorescence of trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) have been studied. TICT fluorescence quantum yield in glycerol solution is found to be ∼23-fold greater than that in ethylacetate as a non-viscous solvent. For high-viscosity solvent, the fluorescence quantum yield increases at low temperature due to the decrease in free-volume of the solvent, which favors the decrease in torsional relaxation of the molecule that induces the radiationless decay. However, the free-volume concept is not meaningful at a temperature much above the glass transition temperature when the free-volume is highly abundant. The temperature-dependent phenomenon provides a more accurate description compared to the viscosity-dependent study in a series of solvents of varying viscosity at a given temperature. The stabilization of the TICT state, as a consequence of the restricted motion of the N(CH3)2 group in DMASBT, results in a large Stokes shifted fluorescence band. The TICT fluorescence characteristics of DMASBT are found to be different from that of molecular rotors. In solvents of low polarity, where TICT is practically zero, the molecule exhibits excited state temperature-induced cis–trans isomerization showing fluorescence emissions from both the isomers. However, temperature-induced TICT fluorescence quenching is observed in polar viscous medium without any isomerization. Results indicate that DMASBT can be a potential microsensor for biomimicking as well as real biological systems.