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 [AgxAu25−x(PPh3)10(SR)5Cl2]2+ (abbreviated as AgxAu25−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 AgxAu25−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 AgxAu25−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 AgxAu25−x NCs can sensitize molecular triplets and efficiently induce red‐to‐blue photon upconversion via triplet–triplet annihilation.

Structurally Flexible Au–Cu Alloy Nanoclusters Enabling Efficient Triplet Sensitization and Photon Upconversion

“…(c) Time-resolved emission decay of Au 4 Cu 4 (10 μM) in deaerated THF as a function of perylene concentration upon 478 nm excitation (upper panel) and its Stern–Volmer plot (lower panel). The time constants determined from analysis using the Runge–Kutta method are also shown. The Stern–Volmer constant ( K SV ) and coefficient of determination ( R 2 ) were obtained from the least-squares fit (solid line).…”

“…In addition, NCs comprise plural heavy-metal elements (Au, Ag, Cu, etc.) with large spin–orbit coupling (SOC) constants, which allow rapid spin-flip and inevitably generate excited triplet states with high efficiency. − Owing to such an intrinsic triplet nature, NCs frequently exhibit relatively long excited-state lifetimes of several microseconds. Such photophysical properties render them highly desirable as photosensitizers that effectively promote photocatalysis, − singlet oxygen generation, − and photochemical upconversion (UC). − , …”

“…Additionally, they often contain toxic elements, such as Pb and Cd, which are problematic because they hinder the fabrication of ecofriendly materials. Recently, our group showed that NCs such as [PtAg 24 (DMBT) 18 ] 2– (DMBT = 2,4-dimethylbenzenethiolate); [Au 25 (PET) 5 (TPP) 10 Cl 2 ] 2+ (PET = 2-phenylethanethiolate; TPP = triphenylphosphine); [Ag x Au 25– x (PET) 5 (TPP) 10 Cl 2 ] 2+ ( x = 12, 13); [Au 13 (DPPE) 5 Cl 2 ] 3+ (Au 13 ; DPPE = 1,2-bis­(diphenylphosphino)­ethane); and Au 2 Cu 6 (S-Adm) 6 (TPP) 2 ( Au 2 Cu 6 ; S-Adm = 1-adamantanethiolate) can be utilized as new platforms for triplet sensitizers for TTA-UC. In addition to low toxicity, NCs possess advantages similar to those of semiconductor nanocrystals, such as the facile tunability of optoelectronic properties, surface functionalization by ligands, and a ubiquitous triplet nature.…”

Alloying and dealloying of Au18Cu32nanoclusters at precise locationsviacontrolling the electronegativity of substituent groups on thiol ligands

“…Compared with homo-metal nanoclusters, alloy nanoclusters have been extensively studied owing to their ability to manipulate the geometric and electronic structure by changing the type, quantity, and position of the doped metal. [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] For instance, a substantial improvement in fluorescence has been observed by doping a single Ag atom into the center of [Ag 12 Ag 13 (PPh 3 ) 10 (SC 2 H 4 Ph) 5 Cl 2 ] 2+ . 23 Ag 29 (SSR) 12 (PPh 3 ) 4 and Ag 25 (SR) 18 exhibit a similar phenomenon.…”

“…23 Ag 29 (SSR) 12 (PPh 3 ) 4 and Ag 25 (SR) 18 exhibit a similar phenomenon. [24][25][26][27][28][29][30][31][32][33] Furthermore, compared with the undoped parent Au 38 (SR) 24 , the centerdoped Pt 1 Au 37 (SR) 24 and Pt 2 Au 36 (SR) 24 nanoclusters exhibit distinctly different catalytic activities. 34,35 nanocluster can precisely tailor its molecular orbitals and thus determine the optical properties of the metal nanocluster and is therefore of great research interest.…”