On the Origin of Photoluminescence Enhancement in Biicosahedral Ag_xAu_25−x Nanoclusters (x = 0–13) and Their Application to Triplet–Triplet Annihilation Photon Upconversion

Abstract Numerous reports hitherto show that the photoluminescence (PL) properties of metal nanoclusters (NCs) can be enhanced by alloying the metal cores. In particular, biicosahedral [Ag_xAu_25−x(PPh₃)₁₀(SR)₅Cl₂]²⁺ (abbreviated as Ag_xAu_25−x hereafter; with PPh₃ = triphenylphosphine; SR = thiolate ligand) NCs attract significant attention because their PL quantum yield is improved by 200 times when 13 Au atoms are replaced with Ag atoms (x = 13). In this contribution, the origin of the PL in the Ag_xAu_25−x system and its remarkable enhancement are investigated on the basis of spectroscopic investigations of the PL behavior and its quenching by an organic fluorophore, finding that (i) the observed PL of Ag_xAu_25−x is phosphorescent; (ii) not only Ag₁₃Au₁₂ but also Ag₁₂Au₁₃ NCs contribute to the PL; and (iii) replacing the central vertex atom of the biicosahedron with an Ag atom causes a blue shift of the triplet states, which suppresses the T₁–S₀ intersystem crossing and enhances the phosphorescence emission. Additionally, the results of single‐particle PL spectroscopy and defocused imaging with rotation of linearly polarized excitation light reveal that the phosphorescence transition dipole moment of Ag₁₃Au₁₂ exists in the long axis direction of the biicosahedron. Furthermore, the Ag_xAu_25−x NCs can sensitize molecular triplets and efficiently induce red‐to‐blue photon upconversion via triplet–triplet annihilation.