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 (ΦF ≈ 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.
Brzeziński et al. (Mon,) studied this question.