Theoretical insights into the excited state intramolecular proton transfer mechanism for a novel 4-methoxy-3-hydroxyflavone system

A novel 3-hydroxyflavone derivative, 4-methoxy-3-hydroxyflavone (4M3HF), is theoretically investigated regarding its excited state intramolecular proton transfer (ESIPT) mechanism based on density functional theory (DFT) and time-dependent DFT (TDDFT) methods. By comparing the solvent polarity, we selected acetonitrile and toluene in this work. First, based on atoms in molecules (AIM) analyses, we verified the formation of intramolecular hydrogen bond of the 4M3HF structure. On investigating the geometrical parameters (i.e., bond lengths and bond angles), we found that the hydrogen bond strength should be enhanced in the S-1 state. By calculating the infrared vibrational spectra, we confirmed the strengthening of the intramolecular hydrogen bond in the S-1 state. We further studied the excitation process using the frontier molecular orbitals method, based on which we concluded that the charge redistribution in solvents facilitates the ESIPT tendency. For exploring the ESIPT behavior, we constructed the potential energy curves along with ESIPT paths in both solvents (acetonitrile and toluene). We found that low potential energy barriers facilitate the ESIPT process in the 4M3HF system, which was also verified by searching the transition state (TS) structure. Comparing these two solvents, we concluded that solvent effects play little role in the ESIPT reaction in the 4M3HF system. And we could successfully explain the previous experimental result