Au25(SR)18: Metal Doping, Ligand Exchange, and Fusion Reactions

In the past few years, the interest in the study of atomically precise metal nanoclusters has grown very significantly. Main reasons are the refined techniques nowadays available for controlling their structure and composition, their often-intriguing properties, and the possibility to tailor them for specific applications. This Thesis aims at providing new tools to synthesize, control, modify, and characterize thiolate-protected gold nanoclusters. The focus of the research is on the thiolate-protected Au25(SR)18 cluster, which is considered by many the true benchmark system for the study of atomically precise nanoclusters. Due to quantum-confinement effects, these nanoclusters have discrete electron-energy states, and this causes the emergence of molecular properties, such as a HOMO-LUMO gap, distinct optical and redox behavior, and magnetism. Additionally, the composition of the metal part and/or the capping monolayer can be modified to make the cluster exhibit specific, sometimes unexpected properties. The goal of this research is to show that by performing controlled modifications of the cluster core and protecting monolayer, one can indeed introduce new properties, and thereby, explore new frontiers for possible applications of these nanosystems. From the viewpoint of the metal, Au25(SR)18 was modified by introducing one-single foreign metal atom. The synthetic, purification, modification, and characterization procedures were refined to explore new ways for achieving proper understanding of the structure of the doped molecular cluster. Particular emphasis has been put on the NMR characterization of the products, a still unexplored yet very powerful tool to localize the position of the doping metal. It is shown that the actual position of the doping metal changes depending on the element. The effect of Au25 doping is then explored from the viewpoint of the generation and detection of singlet oxygen, which is an area of tremendous interest for the treatment of cancer via photodynamic therapy. Metal nanoclusters exhibit discrete optical transitions and have sufficiently long-lived triplet excited states. This makes them react quite efficiently with triplet ground-state oxygen to form singlet excited-state oxygen. Here we show that by proper tuning of the cluster composition (doping metals and ligands), these nanosystems can be made to exhibit the same singlet-oxygen photosensitization performance of systems currently used in the medical practice. We discovered an intriguing transformation of Au25 core. This can be considered as a fusion reaction that consists in the spontaneous transformation of two Au25(SR)18 clusters to form Au38(SR)24, which is another benchmark gold nanocluster. The radical nature of Au25(SR)180 appears to play an important role in this bimolecular reaction that, importantly, does not require addition of exogenous thiols or other co-reactants. This is indeed a very unexpected result that could modify our view about the relative stability of molecular gold nanoclusters. After exploring core modifications, we also investigated strategies to carry out chemical reactions, namely polymerization, directly on the cluster monolayer. Proper functionalization of the nanocluster, that is, capping the cluster with different thiolates, relies on the possibility of either preparing the cluster directly, starting from a mixture of appropriate thiols, or taking advantage of ligand-place exchange reactions, in which the native thiolates present in preformed clusters are partially exchanged with other thiols. In this Thesis, we have implemented experimental conditions for controlling ligand-place exchange reactions on Au25(SR)18 with the goal of introducing functional groups suitable to react with a specific monomer. After polymerization, a polylysine protected Au25 cluster could be prepared.