A Metal–Organic Framework Platform Enabling Visible-to-Ultraviolet Upconversion-Driven Heterogeneous Photochemistry

Triplet–triplet annihilation photon upconversion (TTA-UC) provides an attractive strategy for converting low-energy photons into higher-energy photons under low excitation intensities, enabling diverse applications in photocatalysis, optoelectronics, and solar energy conversion. However, realizing efficient and durable visible-to-ultraviolet (vis-to-UV) TTA-UC in the solid materials remains a major challenge, primarily due to nonemissive annihilator aggregation, phase separation between sensitizers and annihilators, and poor photostability. Herein, we report, for the first time, vis-to-UV TTA-UC realized within a metal–organic framework (MOF) and demonstrate its utility in a heterogeneous photochemical reaction. A hybrid upconversion system is constructed by co-incorporating a p-terphenyl dicarboxylic acid annihilator (TP-(COOH)2) and a tris(2-(5-carboxypyridin-2-yl)-3,5-difluorophenyl)iridium sensitizer (Ir(dFppy-COOH)3) into a Zr-based UiO-68 framework (UiO: University of Oslo). The resulting MOF, composed of π-conjugated organic linkers and highly stable Zr–O clusters, enables triplet exciton migration while maintaining high photo- and chemical-stability. Upon 445 nm excitation, the THF suspension of the hybrid framework exhibits UV emission with an upconversion efficiency of 0.60%, representing respectable performance for MOF-based vis-to-UV TTA-UC materials. Importantly, the rigid and porous MOF architecture effectively suppresses strong chromophore aggregation, affording excellent photostability. Leveraging these features, the MOF-based upconversion system functions as a mercury- and UV-lamp-free UV light source, enabling spatiotemporally controlled uncaging of MNI-caged l-glutamate in a heterogeneous manner. This work establishes a rational, physicochemically stable MOF-based platform for vis-to-UV TTA-UC in suspension and opens new avenues for sustainable photochemistry, photocatalysis, and light-driven functional materials.