Transmitter-Free Interfacial Sensitization in ZnSe/ZnS Quantum Dots Enables Efficient Upconversion

Triplet-triplet annihilation upconversion (TTA-UC) from the visible to ultraviolet (UV) holds promise for UV-driven photochemistry, yet the design principles governing interfacial triplet energy transfer (TET) and overall efficiency remain debated. Here, we compare two sensitization strategies for TTA-UC using ZnSe/ZnS core/shell quantum dots, including transmitter-mediated (with 4-biphenylcarboxylic acid, BCA) and transmitter-free direct assembly. Two high-triplet-energy annihilators, 2,6-di-tert-butylnaphthalene (DTBN) and p-terphenyl (TP), are investigated. Time-resolved photoluminescence reveals that while DTBN exhibits faster interfacial TET (k_TET = 0.030 ns^-1 vs. 0.022 ns^-1 for TP in direct assemblies), its low fluorescence quantum yield (4.77% vs. 54.59% for TP) severely limits UC efficiency. Consequently, the direct ZnSe/ZnS-TP assembly achieves a maximum UC efficiency of 7.9%, outperforming both its BCA-mediated counterpart (6.9%) and DTBN-based systems (≤1.5%). These findings establish that simplifying the donor-acceptor interface while prioritizing the annihilator emissivity can yield superior performance, providing a key design principle for high-efficiency visible-to-UV upconversion materials.