Engineering ligand chemistry on Au(25) nanoclusters: from unique ligand addition to precisely controllable ligand exchange

Au(25) nanoclusters (NCs) protected by 18 thiol-ligands (Au(25)SR(18), 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 Au(25)SR(18) NC are scarce. Herein we report the conversion of hydrophobic Au(25)PET(18) (PET = 2-phenylethylthiolate) NCs to Au(25)SR(19) [Au(25)PET(18)(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 Au(25)PET(18) NCs is metal dependent. Ru and Co complexes preferentially lead to the formation of Au(25)SR(19) 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 Au(25)SR(19) NCs. The photophysical properties of Au(25)PET(18)(Ru complex)(1) are distinctly different from Au(25)PET(18). The absorption spectrum is drastically changed upon addition of the extra ligand and the photoluminescence quantum yield of Au(25)PET(18)(Ru complex)(1) is 14 times and 3 times higher than that of pristine Au(25)PET(18) and Au(25)PET(17)(Ru complex)(1), respectively. Interestingly, only one surface ligand (PET) could be substituted by the metal complex when neutral Au(25)PET(18) was used for ligand exchange whereas two ligands could be exchanged when starting with negatively charged Au(25)PET(18). This charge dependence provides a strategy to precisely control the number of exchanged ligands at the surface of NCs.