Aminal‐linked Covalent Organic Frameworks for Light Energy Upconversion

ABSTRACT The conversion of two low‐energy photons into a single higher‐energy photon at low irradiance is highly desirable for bioimaging and solar energy harvesting. Yet translating established solution‐phase triplet–triplet annihilation upconversion (TTA‐UC) systems into robust solid‐state platforms remains a challenge. Here, we report the first covalent organic frameworks (COFs) capable of sensitized TTA‐UC. Two aminal‐linked frameworks integrating anthracene chromophores, Ant‐COF‐H and Ant‐COF‐OH , were synthesized and structurally characterized, revealing high crystallinity and strong photoluminescence ( Φ F ≈ 40%). When sensitized with a palladium porphyrin complex, both COFs display upconverted emission with quantum yields up to 1.8%, surpassing the performance of the conventional all‐in‐solution reference system. Notably, the onset of saturation occurs at excitation power densities as low as 100 mW cm −2 . Time‐resolved emission spectroscopy reveals fast energy‐transfer consistent with intra‐framework triplet migration rather than diffusion. Finally, we correlate framework structural features with energy‐loss pathways, providing design guidelines for further improvement. This work establishes a foundation for practical, low‐power light management in crystalline polymers by demonstrating that aminal‐linked COFs can be engineered to support efficient energy transfer and function as solid‐state upconverters.