Probing the effect of solvation on photoexcited 2-(2'-hydroxyphenyl)benzothiazole via ultrafast Raman loss spectroscopic studies.

2-(2'-Hydroxyphenyl)benzothiazole (HBT) molecule is known to exhibit efficient excited state intramolecular proton transfer. As a consequence, it shows fluorescence with a large Stokes shift (∼10 000 cm-1) in non-polar solvents. However, fluorescence in polar solvents has a dual-band which corresponds to the emission from both the enol* and the keto* forms. Also, the excited state lifetime significantly varies with the solvent polarity. Recently, Mohammed et al. [J. Phys. Chem. A 115, 7550 (2011)] have shown that the excited state of HBT in acetonitrile (ACN) relaxes back to its ground electronic state through two competitive decay pathways, i.e., intramolecular proton transfer and intramolecular twisting between hydroxyphenyl and benzothiazole units in contrast to its behavior when it is in tetrachloroethene, a non-polar solvent. Here, by following the time-evolution of vibrational features of excited state HBT in ACN through ultrafast Raman loss spectroscopy, we demonstrate a direct evidence for the involvement of torsional motion leading to an ultrashort lifetime of HBT. The time evolution of the C7-N vibrational frequency exhibited a red-shift in its peak position, clearly indicating the evolution of the initially planar cis-keto* form to the more twisted keto* form. Density functional theory calculations also well corroborate the experimental findings. Furthermore, wavepacket analysis of this mode reveals a strong correlation with the torsional motion in ACN.