Isomerization of spiropyran photoswitches in microphase-separated block copolymers

Molecular photoswitches are incorporated into materials to impart photoresponsiveness, enabling a wide range of fascinating applications. Although their surrounding environments within materials strongly influence the photoresponsive and thermally reversible behaviors through physical and chemical interactions, these effects remain poorly understood. In this study, we investigate the two-way photoisomerization and thermal back-isomerization of spiropyran (SP) – a representative photoswitch that undergoes large polarity changes upon the reversible isomerization to merocyanine (MC) – chemically integrated into diblock copolymers (dBCPs) exhibiting either ordered or disordered microphase separation. We synthesize a series of dBCPs consisting of a high-glass-transition-temperature (Tg) poly(methyl methacrylate) block and a low-Tg statistical copolymer block of SP acrylate and n-butyl acrylate, with varying compositions, molecular weights, and microphase-separated structures. The results reveal that the rates of both the SP-to-MC and MC-to-SP photoisomerization processes differ substantially between ordered and disordered microphase-separated structures but are comparable among the different ordered morphologies. In contrast, the photoisomerization yields are scarcely affected by the microphase separation. These findings provide valuable insights into the molecular and polymer design of photoresponsive smart materials based on photoswitches and will contribute to their further development and applications.