Theoretical exploration about excited state proton transfer mechanism for a series of phenol-quinoline compounds

In the present work, three novel phenols (10a,11-dihydro-4bH-indeno[1,2-b]quinolin-4-ol (1), 5,6-dihydro-benzo[c]acridin-1-ol (2), and 5,5,7,7a-tetrahydro-4aH-13-aza-benzo[3,4]cyclohepta[1,2-b]naphthalene-1-ol (3)) have been explored theoretically in detail. Using density functional theory (DFT) and time-dependent DFT (TDDFT) methods, we inquire into the intramolecular hydrogen-bonding interactions and the excited-state intramolecular proton transfer (ESIPT) process. Exploring the steady-state absorption and emission spectra under TDDFT/B3LYP/TZVP theoretical level in acetonitrile solvent, our calculated results demonstrate an experimental phenomenon. Based on analysis of the variations of geometrical parameters and infrared (IR) vibrational spectra, we confirm that O-HN should be strengthened in the S-1 state. Investigating the frontier molecular orbitals (MOs) and the charge density difference (CDD) maps, it can be confirmed that the charge redistribution facilitates the tendency of the ESIPT process for 1, 2, and 3 systems. By constructing potential energy curves, we confirm that the proton transfer should occur in the S-1 state. In particular, the ESIPT for 2 and 3 systems are nonbarrier processes in the S-1 state, which confirms that ESIPT should be exothermal spontaneously. This work explains previous experimental results and makes a reasonable assumption about the ESIPT mechanism for 1, 2 and 3 systems. We sincerely hope our work can facilitate understanding and promoting applications about them in future