Water triggers visible-light photochromism in a hydrogen-bonded organic framework

Photochromic materials switched by user-friendly conditions are highly sought after for secure information technologies and adaptive camouflage but remain a formidable challenge. Here, by leveraging dynamic H-bond networks and periodic π-stacked columns in a hydrogen-bonded organic framework (HOF), we report a reversible visible light photochromic system using water as a “trigger” rather than “destroyer”. This behavior arises in a pyrene-based framework, HOF-14, where water adsorption induces a specific lattice contraction via dynamic H-bond network. Such structural change triggers a fundamental redistribution of the framework’s electronic structure, dramatically lowering the exciton binding energy fallen below the ambient thermal energy. This enables spontaneous visible-light-driven electron-hole separation to form pyrene radical cations, which are stabilized by the stacked pyrene columns in HOF. The photochromic transition—a rapid color change from yellow to green within 10 s—is reversible over 50 cycles. We demonstrate high-level information encryption using this HOF ink and its hydrogel formulations, and fabricate adaptive camouflage textiles that dynamically match a foliated environment. These HOF smart textiles are readily recyclable in a closed-loop process, benefiting from the inherent self-repair capability of HOFs—a distinctive feature not found in other classes of porous crystalline materials. Designing photochromic materials which can be switched by user-friendly conditions remains challenging. Here the authors report a reversible visible light photochromic system using water as a trigger rather than destroyer.