Processing-controlled thickness modulates optical density, vapor response, and visible chromophore retention in anthocyanin–poly(vinylalcohol) films

Transparent chromic polymer films based on natural anthocyanins are promising responsive materials, but their performance is often limited by chromophore instability, deposition-dependent heterogeneity, and poorly controlled transport kinetics. In this work, anthocyanin–poly(vinyl alcohol) (PVA) films were fabricated by spin coating and drop casting to clarify how processing-induced thickness differences affect optical density, vapor-induced chromic response, and light-aging behavior. By varying RedC dye loading from 1 to 2 wt% and changing the deposition method, film thickness was tuned from approximately 10–120 μm. Spin-coated films exhibited rapid chromic activation under acidic vapors, whereas thicker drop-cast films showed slower response, consistent with a longer vapor diffusion path through the polymer layer. Thickness-normalized absorbance analysis indicated that the stronger absorbance of drop-cast films mainly arises from optical path-length effects, although residual differences in attenuation per unit thickness remained detectable. Photostability experiments performed on spin-coated films showed loading-dependent visible chromophore retention, with the 2 wt% formulation retaining more than 90% of its visible absorbance after three months of accelerated irradiation. Overall, the results identify film thickness as a key structural parameter linking processing, optical density, and vapor-response kinetics.