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

摘要

Numerous reports hitherto show that the photoluminescence (PL) propertiesof metal nanoclusters (NCs) can be enhanced by alloying the metalcores. In particular, biicosahedral [Ag_xAu_(25?x)(PPh_3)_(10)(SR)_5Cl_2]~(2+) (abbreviatedas Ag_xAu_(25?x) hereafter; with PPh_3 = triphenylphosphine; SR = thiolate ligand)NCs attract significant attention because their PL quantum yield is improvedby 200 times when 13 Au atoms are replaced with Ag atoms (x = 13). In thiscontribution, the origin of the PL in the Ag_xAu_(25?x) system and its remarkableenhancement are investigated on the basis of spectroscopic investigationsof 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 Ag_(13)Au_(12) butalso Ag_(12)Au_(13) NCs contribute to the PL; and (ⅲ) replacing the central vertexatom of the biicosahedron with an Ag atom causes a blue shift of the tripletstates, which suppresses the T_1–S_0 intersystem crossing and enhancesthe phosphorescence emission. Additionally, the results of single-particlePL spectroscopy and defocused imaging with rotation of linearly polarizedexcitation light reveal that the phosphorescence transition dipole moment ofAg_(13)Au_(12) exists in the long axis direction of the biicosahedron. Furthermore,the Ag_xAu_(25?x) NCs can sensitize molecular triplets and efficiently induce redto-blue photon upconversion via triplet–triplet annihilation.