Experimental and theoretical investigations of solvent polarity effect on ESIPT mechanism in 4′-N,N-diethylamino-3-hydroxybenzoflavone

Abstract

Excited-state intramolecular proton transfer (ESIPT) is a fundamental photoreaction of significant importance in both chemical and biological systems. This phenomenon typically occurs in chromophores featuring intramolecular hydrogen bonding. Among the molecules undergoing ESIPT, 3-hydroxyflavone derivatives (3-HFs) have garnered significant attention due to their natural origins and environmentally responsive fluorescence properties. A particular 3-HF compound, 4′-N,N-diethylamino-3-hydroxybenzoflavone (D-HBF), distinguished by its extended π-system and red-shifted electronic absorption, has recently been identified as a potent fluorescent probe highly sensitive to changes in environmental polarity. In this study, we systematically explored the ESIPT reaction mechanism of D-HBF in three aprotic solvents: cyclohexane, diethyl ether, and tetrahydrofuran, each possessing varying polarities. Our investigation involved a combination of spectroscopic and theoretical methods. In all three solvents, we observed the characteristic dual emission bands associated with ESIPT, with the intensity ratio of these bands being influenced by the solvent. As solvent polarity increased, we noted a decrease in the rates of both the forward and reverse proton transfer (PT) reactions based on our analysis of fluorescent kinetics. However, the reverse PT was favored. Through density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of bond lengths and bond angles of the intramolecular hydrogen bond in these solvents, we confirmed that the ESIPT reaction in D-HBF is driven by the strengthening of the excited-state hydrogen bond. Notably, upon increasing solvent polarity, the intramolecular hydrogen bonds in the excited N∗ state weakened, as evidenced by the up-shifted IR absorption frequency of the O–H stretching mode in the S₁ state. Electron intensity analysis of frontier orbitals revealed characteristic intramolecular charge transfer (ICT) occurring in D-HBF upon photoexcitation, attributable to the introduction of a strong electron-donating group at the 4′ position (4′-N,N-diethylamino-). Calculations of potential energy curves for the S₀ and S₁ states confirmed that the PT process tends to occur in the S₁ state rather than the S₀ state, and a more polar solvent generates a more significant potential barrier, hindering the corresponding ESIPT reaction. An analysis of the Gibbs free energy of ESIPT further confirmed that increasing solvent polarity favors the equilibrium shifting toward the N∗ state. This research lays the foundation for potential future applications of D-HBF as a biological fluorescent probe sensitive to environmental polarity.