Covalent Organic Framework Film with Dynamic C═N─N/C─N═N Conjugated Backbones for Full-Spectrum UV Shielding.

Covalent organic frameworks (COFs) have garnered significant interest for applications in photocatalysis, energy storage, gas storage/separation, and optoelectronics, yet achieving high-quality COF films with high UV-shielding properties still remains a challenge. In this study, transparent COF films are successfully developed and revealed their remarkable UV-shielding capability: effectively blocking 99.9% of UV light while maintaining over 96% visible transmittance. Under UV irradiation, the COF backbone undergoes structural rearrangements between hydrazo (C═N─N) and azo (C─N═N) configurations, leading to electronic redistributions and enhanced conjugation within the framework. This adaptable COF framework efficiently absorbs and dissipates UV energy through non-radiative pathways, providing high UV-blocking properties that dramatically reduce the photooxidation rates of edible oils by up to 16.9-fold compared to uncoated controls. Additionally, the superior photostability of these COF films makes them promising next-generation UV-protective materials for safeguarding sensitive materials and equipment in high-UV environments, such as high-altitude, polar, and extraterrestrial settings.