Short Hydrogen Bonds Unify the Duality of Confined Water as Both Acid and Base in Poly(vinyl alcohol) Films.

Water confined in poly(vinyl alcohol) (PVA) films exhibits enhanced basicity toward embedded chromophores, yet whether this anomalous reactivity reflects a broader acid-base duality remains unexplored. Here, using 5-methoxyflavone (5MF) as a photobase probe whose C═O stretch is spectrally isolated in FTIR, we uncover that short hydrogen bonds (SHBs) render the same confined water capable of protonating 5MF without external acid. In low-water-content PVA films, the C═O stretch of 5MF red-shifts substantially, confirming SHB formation. Time-resolved fluorescence reveals two protonated emission bands with identical, short lifetimes, distinct from the long-lived free cation in acidic films─pointing to a tightly bound 5MFH+···OH- complex rather than free 5MFH+. DFT calculations suggest that the distinct emissions can be rationalized as two limiting wave function distributions of the same proton undergoing quantum delocalization within the SHB. This unified SHB picture resolves the conflicting acid-base behaviors of confined water and highlights the potential of PVA films as versatile platforms for exploring acid-base phenomena enabled by nanoconfinement.