Femtosecond dynamics on 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole: solvent polarity in the excited-state proton transfer.

Detailed insights into the excited-state enol(N*)-keto(T*) intramolecular proton transfer (ESIPT) reaction in 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole (HABT) have been investigated via steady-state and femtosecond fluorescence upconversion approaches. In cyclohexane, in contrast to the ultrafast rate of ESIPT for the parent 2-(2'-hydroxyphenyl)benzothiazole (>2.9+/-0.3 x 10(13) s(-1)), HABT undergoes a relatively slow rate (approximately 5.4+/-0.5 x 10(11) s(-1)) of ESIPT. In polar aprotic solvents competitive rate of proton transfer and rate of solvent relaxation were resolved in the early dynamics. After reaching the solvation equilibrium in the normal excited state (N(eq)), ESIPT takes place with an appreciable barrier. The results also show N(eq)(enol)<-->T(eq)(keto) equilibrium, which shifts toward N(eq) as the solvent polarity increases. Temperature-dependent relaxation dynamics further resolved a solvent-induced barrier of 2.12 kcal mol(-1) for the forward reaction in CH(2)Cl(2). The observed spectroscopy and dynamics are rationalized by a significant difference in dipole moment between N(eq)* and T(eq), while the dipolar vector for the enol form in the ground state (N) is in between that of N(eq) and T(eq). Upon N-->N Franck-Condon excitation, ESIPT is energetically favorable, and its rate is competitive with the solvation relaxation process. Upon reaching equilibrium configurations N(eq)* and T(eq), forward and/or backward ESIPT takes place with an appreciable solvent polarity induced barrier due to differences in polarization equilibrium between N(eq) and T(eq)*.