The Enhanced Proton-Accepting Ability of Bound Water in Poly(vinyl alcohol) Films

Poly(vinyl alcohol) (PVA) films have been widely used as flexible matrixes in advanced optical materials. Most studies concern the rigidification strategy of PVA films, while the physicochemical properties of inside bound water are ignored. In this study, we have employed lumichrome as the fluorescent probe to explore the acid-base property of bound water, which was demonstrated to exhibit an enhanced proton-accepting ability than bulk water, evidenced by the promoted deprotonation of lumichrome in the ground state. Decreasing the water content in a PVA film is demonstrated to further improve the proton-accepting ability. Different from that in bulk solution, a selective prototropism of lumichrome is determined in PVA films, which is induced by the formation of an anchored lumichrome-PVA complex through three hydrogen bonds. This work first points out the enhanced proton-accepting ability of bound water in PVA films, opening a new avenue for the development of flexible optical materials based on proton transfer.