Photophysical properties of 5-methoxyflavone and its protonated species influenced by intermolecular hydrogen bonding and environmental rigidity

The excited-state intramolecular proton transfer (ESPT intra ) process in flavonoids has attracted considerable attention due to its fundamental role in their photophysics. This study systematically investigates the photophysical properties of 5-methoxyflavone (5MF), a flavonoid derivative incapable of ESPT intra . It was demonstrated that intermolecular hydrogen bonding with protic solvents effectively suppress the non-radiative ππ∗-nπ∗ internal conversion in 5MF, leading to significant fluorescence enhancement. In highly acidic aqueous solutions (pH < 3.3), an excited-state intermolecular proton transfer (ESPT inter ) process takes place, generating protonated 5MF (5MFH + ) and resulting in fluorescence quenching. Notably, when incorporated in acidic poly(vinyl alcohol) (PVA) films, the excited-state protonated species 5MFH + exhibits intense fluorescence with a long fluorescence lifetime of 8.4 ns, owing to the suppression of non-radiative decay pathways by the rigid polymer network. Additionally, the dense hydroxyl groups in the PVA matrix also enhance the emission of neutral 5MF. These results highlight the potential of both 5MF and 5MFH + as efficient luminescent chromophores for the design of optical materials based on hydrogen-bonding polymer systems.