Nanostructured Polymers Enable Stable and Efficient Low‐Power Photon Upconversion

Photon upconversion based on sensitized triplet–triplet annihilation ( s TTA‐UC) is a wavelength‐shifting technique with potential use in actuators, sensing, and solar technologies. In s TTA‐UC, the upconverted photons are the result of radiative recombination of high‐energy singlets, which are created through the fusion of metastable triplets of two annihilator/emitter molecules. The emitter triplets are populated via energy transfer (ET) from a low‐energy absorbing light‐harvester/sensitizer. The process is highly efficient at low powers in solution but becomes relatively ineffective in solid matrices since the limited molecular mobility precludes bimolecular interactions. The realization of efficient solid‐state upconverters that exhibit long‐term stability and are compatible with industrial fabrication processes is an open challenge. Here, nanophase‐separated polymer systems synthesized under ambient conditions that contain the upconverting dyes in liquid nanodomains is reported. The nanostructured polymers show an excellent optical quality, an outstanding upconversion efficiency of up to ≈23%, and excellent stability in air, with only negligible performance losses over a period of three months. Moreover, the dyes’ confinement in nanosized domains <50 nm results in an increased effective local density of chromophores that enables hopping‐assisted ET and TTA and confers to the upconversion process peculiar kinetics that enhances the material performance at low powers.