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.