Theoretical study on excited state intramolecular proton transfer process of 2,5-bis(benzoxazol-2-yl) thiophene-3,4-diol

2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (BBT) is a newly synthesized water-soluble fluorophore dye with two intramolecular hydrogen bonds. It was demonstrated that BBT exhibits the excited-state intramolecular single and double proton transfer emissions in recent experiment. In order to explain the phenomenon, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) are used to study the ground states and excited states of BBT and its tautomers. The theoretical results confirm that the hydrogen bonds are strengthened upon photoexcitation. To further reveal the proton transfer mechanism, potential energy surfaces of the S0 and S1 states have been constructed. According to the energy potential barrier of 13.69 kcal/mol, it was indicated that the double proton transfer may occur in stepwise rather than simultaneously in the S1 state unless the excited molecule can overcome the barrier.