Enhancement of NIR II Emission of Au₁₂Ag₃₂ Clusters by Tuning the Au Positions

Precise alloying at the preferred location to enhance the optical properties of metal nanoclusters is challenging as often the most stable isomer is produced. To alter the location of Au atoms on the [Au₁₂Ag₃₂(SR)₃₀]^4- cluster, a new approach by changing the reacting Au precursor following an inter-cluster reaction is reported. A Au(I) containing cluster, [Au₁₈Se₈(DPPE)₆]²⁺ as the Au source, while reacting with [Ag₄₄(SR)₃₀]^4-, is used, and the 12 Au atoms occupy the surface position instead of the core. The whole reaction is monitored in-line using high-resolution trapped ion mobility spectrometry (TIMS), and the change in collision cross section (CCS) of the intermediates and the final product reveal that Au atoms can dynamically migrate within the cluster and in the final product is an all Au out isomer of [Au₁₂Ag₃₂(SR)₃₀]^4-. Changing the location of the Au atoms shows an impact on the NIR II emission at ≈1340 nm of parent [Ag₄₄(SR)₃₀]^4-. The emission of the synthesized alloy is enhanced tenfold compared to [Ag₄₄(SR)₃₀]^4- and is ≈20 nm blueshifted. Density functional theory calculations reveal that the Au/Ag atoms on the staple are slightly positively charged, making it convenient for the reacting Au(I)₁₈Se₈ cluster to be exchanged on the surface.