Engineering ligand chemistry on Au25 nanoclusters: from unique ligand addition to precisely controllable ligand exchange

Abstract

Au₂₅ nanoclusters (NCs) protected by 18 thiol-ligands (Au₂₅SR₁₈, SR is a thiolate ligand) are the prototype of atomically precise thiolate-protected gold NCs. Studies concerning the alteration of the number of surface ligands for a given Au₂₅SR₁₈ NC are scarce. Herein we report the conversion of hydrophobic Au₂₅PET₁₈ (PET = 2-phenylethylthiolate) NCs to Au₂₅SR₁₉ [Au₂₅PET₁₈(metal complex)₁] induced by ligand exchange reactions (LERs) with thiolated terpyridine-metal complexes (metal complex, metal = Ru, Fe, Co, Ni) under mild conditions (room temperature and low amounts of incoming ligands). Interestingly, we found that the ligand addition reaction on Au₂₅PET₁₈ NCs is metal dependent. Ru and Co complexes preferentially lead to the formation of Au₂₅SR₁₉ whereas Fe and Ni complexes favor ligand exchange reactions. High-resolution electrospray ionization mass spectrometry (HRESI-MS) was used to determine the molecular formula of Au₂₅SR₁₉ NCs. The photophysical properties of Au₂₅PET₁₈(Ru complex)₁ are distinctly different from Au₂₅PET₁₈. The absorption spectrum is drastically changed upon addition of the extra ligand and the photoluminescence quantum yield of Au₂₅PET₁₈(Ru complex)₁ is 14 times and 3 times higher than that of pristine Au₂₅PET₁₈ and Au₂₅PET₁₇(Ru complex)₁, respectively. Interestingly, only one surface ligand (PET) could be substituted by the metal complex when neutral Au₂₅PET₁₈ was used for ligand exchange whereas two ligands could be exchanged when starting with negatively charged Au₂₅PET₁₈. This charge dependence provides a strategy to precisely control the number of exchanged ligands at the surface of NCs.