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, Au 2 Cu 6 (S-Adm) 6 [P(DPA) 3 ] 2 ( Au 2 Cu 6 DPA ; S-Adm = 1-adamanthanethiolate, DPA = 9,10-diphenylanthracene). In Au 2 Cu 6 DPA , the excitation of the Au 2 Cu 6 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 Au 2 Cu 6 DPA 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 Au 2 Cu 6 DPA sensitizer holds promise for applications in solar-energy-based systems.