Efficient Broad-Power-Range Nonlinear Ultraviolet Emission via Endothermic Triplet-Triplet Annihilation Upconversion

Molecules and materials capable of nonlinear ultraviolet (UV) emission have garnered considerable research interest for applications in high-end manufacturing, information technology, and biomedicine. Herein, we demonstrate nonlinear UV emission sustained over an exceptionally broad excitation power range through the construction of an endothermic triplet-triplet annihilation upconversion (TTA-UC) system. In this endothermic TTA-UC molecular pair, a Schiff base platinum complex featuring intense broadband visible absorption spanning 500-600 nm serves as the sensitizer, whereas a naphthalene derivative with a triplet energy (T1) slightly higher than that of the sensitizer functions as the annihilator. Under 532 nm excitation, the resulting nonlinear UV emission achieves an upconversion quantum yield of up to 5.4% (ΦUC, upper limit of 50%), which significantly outperforms conventional nonlinear UV emission approaches, including lanthanide-doped inorganic upconversion nanoparticles and frequency conversion based on nonlinear optical crystals. This study establishes a new pathway toward efficient nonlinear UV emission operable across a wide excitation power window, providing fresh opportunities to advance the development and practical implementation of nonlinear UV photonics.