Triplet-Mediator Ligand-Protected Metal Nanocluster Sensitizers for Photon Upconversion

Triplet-triplet annihilation photon upconversion (TTA-UC) is attracting a great deal of attention as a viable approach to exploit unutilized wavelengths of light in solar-driven devices. Recently, ligand-protected metal nanoclusters have emerged as a compelling platform for serving as triplet sensitizers for TTA-UC. In this study, we developed an atomically precise, triplet-mediator ligand (TL)-protected metal nanocluster, Au₂Cu₆(S-Adm)₆[P(DPA)₃]₂ (AuCuDPA; S-Adm = 1-adamanthanethiolate, DPA = 9,10-diphenylanthracene). In AuCuDPA, the excitation of the Au₂Cu₆ core rapidly generates a metal-to-ligand charge transfer state, followed by the formation of the long-lived triplet state (approximately 150 μs) at a DPA site in the TL. By combining AuCuDPA with a DPA annihilator, we achieved a red-to-blue upconversion quantum yield (Φ_UCg) of 20.7 ± 0.4% (50% max.) with a low threshold excitation intensity of 36 mW cm^-2 at 640 nm. This quantum yield almost reaches the maximum limit achievable using a DPA annihilator and establishes a record-setting value, outperforming previously reported nanocrystal and nanocluster sensitizers. Furthermore, strong upconversion emission based on a pseudo-first-order TTA process was observed under 1 sun illumination, indicating that the AuCuDPA sensitizer holds promise for applications in solar-energy-based systems.