Triplet-Mediator Ligand-Protected Metal Nanocluster Sensitizers for Photon Upconversion
Daichi Arima, Shion Hidaka, So Yokomori, Yoshiki Niihori, Yuichi Negishi, Ryuichi Oyaizu, Takumi Yoshinami, Kenji Kobayashi, Masaaki Mitsui
Abstract
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.