3-Hydroxyflavone-Containing Chitosan/Poly(Vinyl Alcohol) Films: UV-A Response, Water-Related Behavior, and DFT/Multiwfn Descriptors of Isolated Components

Hydrophilic polymer films containing environment-sensitive chromophores are useful model systems for examining optical responses in polar matrices. In this study, chitosan/poly(vinyl alcohol) (CS–PVA) films containing low nominal loadings of 3-hydroxyflavone (3HF; 0.05–0.20 wt.%) were prepared by solution casting and characterized in terms of macroscopic appearance, thickness, ultraviolet–visible (UV–Vis) response before and after ultraviolet-A (UV-A) exposure, Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR), and water-related behavior. All formulations produced continuous and macroscopically translucent films without visible precipitation or phase separation. Film thickness varied substantially among formulations (0.100–0.168 mm), requiring cautious interpretation of raw transmittance data. Before UV-A exposure, the thickness-normalized apparent attenuation coefficient at 365 nm (α_app,365) ranged from 23.84 to 121.74 cm−1 and followed a non-monotonic order: CS–PVA–3HF-0.05 > CS–PVA–3HF-0.20 > CS–PVA–3HF-0.10 > CS–PVA. UV-A exposure for 120 min produced formulation-dependent changes in optical response, with no common concentration-dependent trend. FTIR-ATR spectra showed relatively small variations in the O–H/N–H, carbonyl/amide, and C–O regions; because several band displacements were comparable to the instrumental resolution, these changes were interpreted as variations in the local vibrational environment rather than direct evidence of hydrogen-bond-network restructuring. Water-contact measurements revealed very high apparent water uptake together with post-immersion dry-mass losses above 94%, demonstrating limited aqueous stability. Density functional theory (DFT) calculations combined with Multiwfn wavefunction analysis on isolated 3HF, a three-unit poly(vinyl alcohol) oligomer (PVA3), and a two-unit chitosan oligomer (CS2) described their intrinsic electronic features but did not establish intermolecular complex formation or directly explain the experimental responses. Overall, the results provide a descriptive characterization of the optical and water-related behavior of 3HF-containing CS–PVA films while emphasizing the influence of film geometry and the limitations associated with nominal, rather than analytically verified, chromophore loading and isolated-component computational models.