Optimized Photochromic Performance of Spiropyran through Incorporation into Hydrogen-Bonded Organic Frameworks and Applications in Anticounterfeiting and Information Encryption

Photostimulus-responsive fluorescent materials are promising for anticounterfeiting and UV printing due to rapid response and simple preparation. In this paper, we propose a novel strategy to prepare photostimulus-responsive materials SP@HOF-olefin by integrating the photochromic molecule spiropyran (SP) with postsynthetic modified hydrogen-bonded organic frameworks (HOF-olefin). Compared to SP@HOF, the composites SP@HOF-olefin exhibit enhanced photochromic properties, such as a fast response speed, pronounced color contrast, and exceptional fatigue resistance. The improvements can be attributed to the reduction of residual carboxyl groups in the framework, which subsequently decreases the polarity of the framework. Besides, the SP loading content in SP@HOF-olefin was significantly enhanced. Furthermore, HOF-olefin can be transformed into HOF films via photopolymerization. These films demonstrated excellent flexibility, which can be folded and twisted at any angle ranging from 0 to 180°. By utilizing the photomask method, letters such as "Z", "T", "S", and "U", along with other patterns were successfully imprinted on the film. Besides, we investigated the utilization of these films in advanced anticounterfeiting applications. This work serves as a representative case demonstrating how functionalized HOFs improved the photochromic properties of SP, showcasing potential applications in anticounterfeiting and information encryption.