Modulating the Photochromic Kinetics of Spiropyran/Polyacrylate Coatings via N-Substituent Chain Length for Anticounterfeiting Applications

Spiropyran has three steady-state colors that are difficult to replicate, which show excellent application prospects in anticounterfeiting. To achieve dynamic anticounterfeiting with time-dependent characteristics, the key is to control the solid-state color-changing rate. In this study, three spiropyrans with varying N-bromoalkyl chain lengths were synthesized and mixed with poly(n-butyl methacrylate) (PnBMA) or poly(ethyl methacrylate) (PEMA) to prepare composite coatings. Kinetic studies revealed that spiropyran/PnBMA composites had faster discoloration rates than spiropyran/PEMA, and longer N-substituted chains accelerated the discoloration of spiropyran in the PnBMA matrix. This is probably because longer N-substituted chains lower the Tg content and increase the free volume. Two composite coatings with different discoloration rates were patterned on paper to demonstrate the anticounterfeiting applications.