Theoretical study on the excited-state intramolecular proton-transfer reaction of 10-hydroxybenzo[h]quinoline in methanol and cyclohexane

The dynamics of the excited-state intramolecular proton-transfer (ESIPT) reaction of 10-hydroxybenzoquinoline (HBQ) in different solvents, have been investigated based on the time-dependent density functional theory (TD-DFT) in detail. Upon excitation, the intramolecular hydrogen bond between the hydroxyl and phenanthrene functionality is significantly strengthened in the S1 state, which can be used as a reasonable tendency for facilitating the ESIPT process. In addition, the calculated vertical excitation energies in the S0 state and S1 state reproduce the experimental UV-vis absorbance and fluorescence emission spectra well. Through calculating the fluorescence spectra of the HBQ chromophore, two outcomes for this chromophore were found in the S1 state, which demonstrates that the ESIPT process occurs. The potential energy curves have been calculated to account for the mechanism of the proton-transfer process in the excited-state. As a result, the barrierless ESIPT process can occur in the S1 state with proton transfer from the O atom to the N atom. And maybe the ESIPT process is easier in methanol solvent due to the higher potential energy difference.