Excited-State Intramolecular Proton Transfer of Deprotonated Fisetin in Poly(vinyl alcohol) Films

Fisetin is widely employed as a natural fluorescent probe for sensing local environmental dynamics in micelles and membranes, owing to its large Stokes shift emission arising from excited-state intramolecular proton transfer (ESIPT) and the characteristic fluorescence of its anionic form. In contrast to the extensive studies on the ESIPT of neutral fisetin, the ESIPT processes in fisetin anions have received much less attention. In this work, we incorporated 3-hydroxyflavone and fisetin into two poly(vinyl alcohol) (PVA) films that were codoped with oxalic acid and dimethyl sulfoxide, respectively. Anionic 3-hydroxyflavone and fisetin species formed in the films, as evidenced by an increased absorption in the visible spectral region. Dual emission was achieved in the PVA films upon visible-light excitation, originating from the normal and ESIPT tautomer species of divalent fisetin anions. The ESIPT barrier heights for these anions, calculated using time-dependent density functional theory, are comparable to that of neutral fisetin. This study confirms the microbasicity of PVA films and provides fundamental insight into the ESIPT process of deprotonated fisetin, offering a clear basis for its application as a microenvironmental probe in soft matter systems.