Monolithic Porous Polymer Architectures for Visible-to-UV Upconversion-Driven Photochemical Reactions

Triplet-triplet annihilation-based photon upconversion (TTA-UC) expands the accessible spectral window for photochemical transformations by generating high-energy photons from low-energy excitation. Although TTA-UC has been widely demonstrated in solution and in various solid-state formats, examples of polymer materials capable of efficient visible-to-ultraviolet (vis-to-UV) upconversion under low excitation intensities remain scarce. Achieving vis-to-UV TTA-UC in a porous solid is particularly challenging because it requires balancing chromophore proximity for TTA with the suppression of aggregation-induced quenching, while simultaneously preserving substrate accessibility. Here, we introduce porous polymer monoliths that enable solid-state TTA-UC specifically designed for heterogeneous photochemistry. By embedding sensitizer-annihilator dye pairs into polymer matrices with controlled co-continuous porosity, we create monolithic architectures that display upconverted UV emission from blue light at excitation intensities below a few mW cm^{- 2}, with the operational wavelength further extended to green light excitation. The porous architecture enhances energy transfer to reaction substrates, providing a versatile platform for heterogeneous photochemical processes. Owing to their robustness and tunability, these monoliths are promising candidates as heterogeneous photoreactors, opening new opportunities for scalable and sustainable photochemistry.