Nanoconfinement‐Mediated Concentration Decoupling Enabled Efficient Triplet–Triplet Annihilation Upconversion for Time‐Resolved Temperature Sensing

Organic molecule‐based triplet–triplet annihilation upconversion (TTA‐UC) has already shown enormous potential for applications in bioimaging, disease diagnostics, and additive manufacturing with its ultrahigh upconversion efficiency and low excitation intensity. However, TTA‐UC relies on intermolecular Dexter energy transfer, rendering its upconversion strongly concentration‐dependent. Here, a strategy is presented to decouple the concentration dependence of TTA‐UC in small, uniform nanoparticles (NPs), addressing a critical limitation in its practical applications. By encapsulating ultralow‐concentration TTA‐UC pairs (sensitizer/annihilator: 244 n m /6.5 µ m ) in solid micellar NPs, an exceptional upconversion quantum efficiency of 15.9% (100% normalized) is achieved in aqueous media, which surpasses the reported results of NIR upconversion nanomaterials by two orders of magnitude. The TTA‐UC NPs present ultra‐low critical micellar concentration which suppresses micelle dissociation. Through rational annihilator T 1 energy level engineering, efficient, lifetime‐tunable upconversion emission (96.0–47.8 µs range) is further demonstrated within water‐dispersible NPs while maintaining multicolor output under single‐wavelength excitation. Further, unprecedented TTA‐UC‐based time‐resolved temperature sensing with a thermal sensitivity of 4.1% K −1 is constructed. This study establishes a versatile platform for developing high‐performance TTA‐UC materials, opening new avenues for their implementation in nanophotonics and background‐free sensing.