Theoretical study on the cooperative effects of solvent and π-conjugation extension as well as InterHB on the fluorescent properties and ESIPT behaviors of flavonol fluorophores.

Recently, two flavonol derivatives, 3-hydroxy-2-(p-tolyl)-4H-chromen-4-one (FL1) and 3-hydroxy-2-(p-tolyl)-4H-benzo[g]chromen-4-one (FL2) with excited state intramolecular proton transfer (ESIPT) characteristics were synthesized in the experiment (ACS Sensors, 10 (2025) 2287-2296). However, experimental techniques cannot provide detailed information on the ESIPT mechanisms of FL1 and FL2. There is currently no theoretical study on how different solvents affect their photophysical properties. On this foundation, the photophysical properties and ESIPT behaviors of FL1 and FL2 were systematically investigated, and the influences of nonpolar solvent (cyclohexane, Cy) and protic solvent (methanol, MeOH) as well as the π-conjugation structure were demonstrated via time-dependent density functional theory (TD-DFT) method. Our calculation results indicate that from Cy to MeOH, the fluorescence wavelength redshifts slightly, which is in good accordance with the effect of π-conjugation extension from chromene to benzo[g]chromene on fluorescence performance. Furthermore, the intermolecular hydrogen bond (InterHB) between MeOH and O1 atom strengthens its intramolecular hydrogen bond (IntraHB), thereby decreasing the ESIPT barrier of FL1/FL2, while InterHB between MeOH and O3 atom weakens its IntraHB, thereby increasing the ESIPT barrier of FL1/FL2. The cyclic H-bonded complexes FL1-(MeOH)cyc and FL2-(MeOH)cyc can undergo excited state double proton transfer (ESDPT) process in a concerted but asynchronous protolysis pattern. The energy barrier of ESDPT process is lower than that of ESIPT process.