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The technological application of sensitized upconversion based on triplet-triplet annihilation (TTA) requires the transition from systems operating in liquid solutions to solid-state materials. Here, we demonstrate that the high upconversion efficiency reported in hyper-cross-linked nanoparticles does not originate from residual mobility of the embedded dyes as it happens in soft hosts. The hyper-reticulation from one side blocks the dyes in fixed positions, but on the other one, it suppresses the nonradiative spontaneous decay of the triplet excitons, reducing intramolecular relaxations. TTA is thus enabled by an unprecedented extension of the triplet lifetime, which grants long excitons diffusion lengths by hopping among the dye framework and gives rise to high upconversion yield without any molecular displacement. This finding paves the way for the design of a new class of upconverting materials, which in principle can operate at excitation intensities even lower than those requested in liquid or in rubber hosts.

Sensitized triplet–triplet annihilation in multicomponent organic systems is already demonstrated to be suitable for obtaining effi cient up-conversion in solution with excitation power densities comparable to solar irradiance, but loses effi ciency in the solid state. Here, it is demonstrated that it is possible to reduce this limitation by incorporating a standard bicomponent system in polymer nanoparticles. The confi nement of all of the involved photophysical processes in a nanometer-scale volume makes each nanoparticle a single and isolated high-effi ciency up-converting unit. As a consequence, these dualdye-loaded nanoparticles can be used to produce drop-cast fi lms, as well as dopants for polymeric matrices, preserving the performances of the starting moieties in solution.