Femtosecond dynamics on 2-(2′-hydroxy-4′-diethylaminophenyl) benzothiazole

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 × 1013 s-1), HABT undergoes a relatively slow rate (≈ 5.4 ± 0.5 × 1011 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 salvation equilibrium in the normal excited state (Neq*), ESIPT takes place with an appreciable barrier. The results also show Neq*(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 CH2Cl2. The observed spectroscopy and dynamics are rationalized by a significant difference in dipole moment between Neq* and Teq*, while the dipolar vector for the enol form in the ground state (N) is in between that of Neq* and Teq*. Upon N→N* Franck-Condon excitation, ESIPT is energetically favorable, and its rate is competitive with the salvation relaxation process. Upon reaching equilibrium configurations Neq* and Teq*, forward and/or backward ESIPT takes place with an appreciable solvent polarity induced barrier due to differences in polarization equilibrium between Neq* and Teq*.