On the Origin of Photoluminescence Enhancement in Biicosahedral Ag<i><sub>x</sub></i>Au<sub>25−</sub><i><sub>x</sub></i> Nanoclusters (<i>x</i> = 0–13) and Their Application to Triplet–Triplet Annihilation Photon Upconversion
Masaaki Mitsui, Daichi Arima, Y. Kobayashi, Eunji Lee, Yoshiki Niihori
Abstract
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 x Au 25− x (PPh 3 ) 10 (SR) 5 Cl 2 ] 2+ (abbreviated as Ag x Au 25− x hereafter; with PPh 3 = 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 x Au 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 x Au 25− x is phosphorescent; (ii) not only Ag 13 Au 12 but also Ag 12 Au 13 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 T 1 –S 0 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 Ag 13 Au 12 exists in the long axis direction of the biicosahedron. Furthermore, the Ag x Au 25− x NCs can sensitize molecular triplets and efficiently induce red‐to‐blue photon upconversion via triplet–triplet annihilation.