Theoretical investigation of excited-state single and double proton transfer mechanisms for 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

Abstract

In this work, excited-state intramolecular single and double proton transfer mechanisms for 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol were investigated using time-dependent density functional theory (TDDFT) method for the first time. Our calculations showed that the intramolecular hydrogen bond N⋯HO is strengthened in the excited state, which provides a driving force to effectively facilitate the proton transfer process. The constructed potential energy surface of the excited state demonstrated that the double proton transfer reaction occurs more readily in both dynamics and thermodynamics aspects, and it is implemented by simultaneous double proton transfer, or successive single transfers. Furthermore, the assignments of multiple fluorescence bands in experiment were confirmed by fluorescence spectral simulation. The calculated emission spectra indicated that the experimental fluorescence maxima at 475 nm should be attributed to normal Stokes shifted emission; the large Stokes shifted fluorescence with peaks at 550 nm originates from the double proton transfer phototautomer emission, and the experimentally observed shoulder peak at ∼493 nm results from the excited state single proton transfer tautomer.