It remains challenging to achieve efficient and air-stable photon upconversion (UC) in rigid, technologically valuable transparent films. Here, we report the first example of epoxy resins that show an air-stable and efficient triplet-triplet annihilation (TTA)-based UC. Epoxy resins are thermally cross-linked polymers widely used as coating and sealing materials in actual devices. To achieve efficient TTA-UC in rigid epoxy films, it is essential to execute both the triplet sensitization and triplet exciton diffusion processes without relying on molecular diffusion. This requires homogeneously dispersing emitter molecules without aggregation in three-dimensionally cross-linked rigid polymer networks at a high concentration (ca. 1000 mM) such that the inter-emitter distance is less than 1 nm, where dexter energy transfer can occur. This difficult requirement is solved by employing an ionic liquid emitter that consists of 9,10-diphenylanthracene sulfonate and lipophilic phosphonium ions bearing long alkyl chains. The obtained epoxy resins show a high TTA-UC efficiency (ηUC = 3.8%) and low threshold excitation intensity (Ith = 40 mW cm-2) in air. These UC parameters are achieved by virtue of a very high sensitizer-to-emitter triplet energy-transfer efficiency (92.8%) and a significantly long emitter triplet lifetime (17.8 ms) that reflect the high emitter concentration and the rigid chromophore environment, respectively. The bulk transparent upconverting resins can be prepared in air and function in air, which opens a new avenue toward a wide range of real-world applications.
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