Ultrafast proton transfer dynamics of 2-(2'-hydroxyphenyl)benzoxazole dye in different solvents

The excited-state intramolecular proton transfer of 2-(2'-hydroxyphenyl)benzoxazole dye in different solvents is investigated using ultrafast femtosecond transient absorption spectroscopy combined with quantum chemical calculations. Conformational conversion from the syn-enol configuration to the keto configuration is proposed as the mechanism of excited-state intramolecular proton transfer. The duration of excited-state intramolecular proton transfer is measured to range from 50 fs to 200 fs in different solvents. This time is strongly dependent on the calculated energy gap between the N-S₀ and T-S₁ structures in the S₁ state. Along the proton transfer reaction coordinate, the vibrational relaxation process on the S₁ state potential surface is observed. The duration of the vibrational relaxation process is determined to be from 8.7 ps to 35 ps dependent on the excess vibrational energy.