Fluorescence Enhancement of Nonemissive Monodeprotonated Luteolin in a Poly(vinyl alcohol) Film

Solid polymer matrixes can modulate the electronic states of embedded chromophores and have been widely used in flexible optoelectronic and optical materials. Luteolin is one of the most common natural flavonoids, and its neutral and monodeprotonated forms are nonemissive in aqueous solution induced by ultrafast excited-state proton transfer (ESPT) followed by nonradiative relaxation. In this study, we have incorporated luteolin into poly(vinyl alcohol) (PVA) films and studied their fluorescence behaviors. Neutral and one monodeprotonated luteolin coexist in the PVA film. Weak steady-state fluorescence of neutral luteolin peaking at about 440 nm is observed for the first time. In addition, the monodeprotonated luteolin in PVA film exhibits obvious fluorescence peaking at 500 nm, with a fluorescence quantum yield of as high as 0.4 and a fluorescence lifetime of as long as 2.4 ns. Time-dependent density functional theory calculations have determined that the ESPT of neutral luteolin is barrierless but that of monodeprotonated luteolin needs to surmount a barrier, explaining their distinct emission properties. These results indicate the modulation ability of the PVA film in both ground-state deprotonation and ESPT, broadening the application areas of the solid polymer matrix.