Thermally activated delayed fluorescence Au-Ag-oxo nanoclusters: From photoluminescence to radioluminescence

Thermally activated delayed fluorescence (TADF) materials have numerous applications in energy conversion and luminescent imaging. However, they are typically achieved as metal-organic complexes or pure organic molecules. Herein, we report the largest Au-Ag-oxo nanoclusters to date, Au₁₈Ag₂₆(R₁COO)₁₂(R₂C≡C)₂₄(μ₄-O)₂(μ₃-O)₂ (Au₁₈Ag₂₆, where R₁ = CH₃-, Ph-, CHOPh- or CF₃Ph-; R₂ = Ph- or FPh-). These nanoclusters exhibit exceptional TADF properties, including a small S₁-T₁ energy gap of 55.5 meV, a high absolute photoluminescence quantum yield of 86.7%, and a microseconds TADF decay time of 1.6 μs at ambient temperature. Meanwhile, Au₁₈Ag₂₆ shows outstanding stability against oxygen quenching and ambient conditions. Atomic level analysis reveals the strong π⋯π and C-H⋯π interactions from the aromatic alkynyl ligands and the enhancement of metal-oxygen-metal interactions by centrally coordinated O²⁻. Modeling of the electronic structure shows spatially separated highest occupied molecular orbital and lowest unoccupied molecular orbital, which promote charge transfer from the ligand shell, predominantly carboxylate ligands, to O²⁻-embedded metal core. Furthermore, TADF Au-Ag-oxo nanoclusters exhibit promising radioluminescence properties, which we demonstrate for X-ray imaging. Our work paves the way for the design of TADF materials based on large metal nanoclusters for light-emission and radioluminescence applications.