Methacrylate-based copolymers as tunable hosts for triplet-triplet annihilation upconversion

The ability to convert light to higher energies through triplet-triplet annihilation upconversion (TTA-UC) is attractive for a range of applications including solar energy harvesting, bioimaging and anti-counterfeiting. Practical applications require integration of the TTA-UC chromophores within a suitable host, which leads to a compromise between the high upconversion efficiencies achievable in liquids and the durability of solids. Herein, we present a series of methacrylate copolymers as TTA-UC hosts, in which the glass transition temperature (T _g), and hence upconversion efficiency can be tuned by varying the co-monomer ratios (n-hexyl methacrylate (HMA) and 2,2,2-trifluoroethyl methacrylate (TFEMA)). Using the model sensitiser/emitter pair of palladium(ii) octaethylporphyrin (PdOEP) and diphenylanthracene (DPA), the upconversion quantum yield was found to increase with decreasing glass transition temperature, reaching a maximum of 1.6 ± 0.2% in air at room temperature. Kinetic analysis of the upconversion and phosphorescence decays reveal that increased PdOEP aggregation in the glassy polymers leads to a competitive non-radiative relaxation pathway that quenches the triplet state. Notably, the threshold intensity is highly sensitive to the glass transition temperature, ranging from 1250 mW cm^-2 for PHMA₉₀TFEMA₁₀ (T _g = -9.4 °C) to ∼200 mW cm^-2 for more 'glassy' hosts, e.g. PHMA₃₃TFEMA₆₇ (T _g = 20.1 °C), suggesting the TTA-UC mechanism switches from diffusion-based collisions to triplet exciton migration at localised sensitiser-emitter pairs.