Heteroatom substitution effect on electronic structures, photophysical properties, and excited-state intramolecular proton transfer processes of 3-hydroxyflavone and its analogues: A TD-DFT study

Abstract

The effects of the electron-donating capacity altered by heteroatom substituents on the electronic structures, photophysical properties, and excited-state intramolecular proton transfer (ESIPT) processes of 3HX analogues (3HF, 3HQ, 3HTF, and 3HSO where X = O, NH, S, and SO2, respectively) have been investigated by both static calculations and dynamic simulations using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods at B3LYP/TZVP level for ground state (S0) and excited-state (S1), respectively. The static results indicate that the intramolecular hydrogen bonds of all molecules are strengthened in the S1 state, confirmed by the red-shift of IR vibrational spectra and the topology analysis. Heteroatom substitutions cause the red-shift on enol absorption and keto emission spectra of 3HX with relatively larger Stoke shift corresponding to their HOMO−LUMO gaps compared with that of 3HF. Frontier molecular orbitals (MOs) show that upon the photoexcitation, the charge redistribution between the proton donor and proton acceptor groups have induced the ESIPT process. Moreover, the potential energy curves (PECs) of proton transfer (PT) processes of all molecules reveal that the PT processes of all molecules are most likely to proceed in the S1 state because of low barrier and exothermic reaction. The chance of ESIPT for all molecules is in this order: 3HSO > 3HTF > 3HF > 3HQ. The results of dynamic simulations confirm that the ESIPT processes of all molecules easily occur with the ultrafast time scale (48, 55, 60, 70 fs for 3HSO, 3HTF, 3HF, and 3HQ, respectively). Furthermore, the PT time is anti-correlated with the electronegativity of heteroatoms in 3HX, supported by Mulliken analysis. The ESIPT process of 3HSO is the fastest among 3HX in accordance with its highest intramolecular hydrogen bond strength, lowest PT barrier, and highest exothermic reaction. Nevertheless, after the ESIPT is complete, the twisted structure of 3HSO has initiated the conical intersection, leading to no keto emission observed in the experiment.