Photoswitchable polymer adhesives enable strong bonding and on-demand debonding with minimal damage to adherends due to the high spatiotemporal resolution and therefore promise to contribute to material recycling as complex architectures consisting of dissimilar materials are being developed in various fields. In a variety of photosystems, the photoisomerization between noncharged spiropyran (SP) and zwitterionic merocyanine (MC) with large polarity changes is a primary candidate because molecular polarity is a dominant factor to generate adhesion force. However, SP has not been chemically incorporated into polymer adhesives. In this study, we developed photoswitchable adhesives of SP homopolymers with different molecular mobility of the SPs, which were located in the side chains and linked to the main chains through alkyl spacers with different lengths. Photoswitching of the adhesion strength by the polarity changes between SP and MC was demonstrated on both hydrophilic and hydrophobic substrates.

Molecular photoswitches have been widely used as molecular machines in various fields due to the small structures and simple motions generated in reversible isomerization. However, common photoswitches, as represented by azobenzene (AB), cannot combine both large motions and high thermal stability, which are critically important for some practical applications in addition to high photoisomerization yields. Here, we focus on a promising photoswitch, stiff stilbene (SS), and its derivative, sterically hindered SS (HSS). The detailed investigation of their performance with a comparison to AB demonstrated that HSS is an outstanding photoswitch offering larger motions than AB and SS, ca. 90% photoisomerization in both E-to-Z and Z-to-E directions, and significantly high thermal stability with a half-life of ca. 1000 years at room temperature. The superior performance of HSS promises its use in various applications, even where previous photoswitches have troubles and are unavailable.