Concerted Mechanisms of Excited-State Proton Intramolecular Transfer for Bis-2,4-(2-benzoxazolyl)-hydroquinone and Its Derivatives

The concerted mechanisms of excited state intramolecular proton transfer (ESIPT) of bis-2,4-(2-benzoxazolyl)-hydroquinone (BBHQ') and its derivatives (BBHQ'(-) and DHBO') have been investigated using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). The calculated absorption and emission spectra of BBHQ' and its derivatives are in good agreement with the experimental results. The calculated bond lengths, bond angles, and IR vibrational spectra linked with hydrogen bond of molecular BBHQ' in the S-0 and S-1 states demonstrate that the hydrogen bond is strengthened in the S-1 state. Compared to BBHQ', BBHQ'(-) has a weak change of hydrogen bond between the S-1 and S-0 states. The calculation results show that there are three stable structures of BBHQ in the S-1 state. We find that the structure corresponding to the 481 nm fluorescence spectrum corresponds to BBHQ'-A rather than BBHQ'-K (Tetrahedron Lett., 2016, 57, 3518). The calculated frontier molecular orbitals (MOs) indicate the nature of the charge distribution and the trend of proton transfer of BBHQ'-A. The constructed potential energy surfaces of BBHQ and DBHO' further elucidate the proposed mechanism that one-proton or two-proton transfer can happen (stepwise or synchronous) in the S-1 states. The proposed ESIPT mechanism can provide a good explanation of the phenomenon of fluorescence quenching of BBHQ and its derivatives. Finally, the weak interaction types are discriminated through the reduced density gradient (RDG) analyses of BBHQ., and BBHQ'(-)