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 nm/6.5 µm) 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 T₁ 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.