The Pivotal Role of Ligands Determining the Properties of Atomically Precise Gold Nanoclusters

The thesis work I am about to present explores the fascinating world of atomically precise nanoclusters (NCs), investigating their properties and potential applications. NCs have gained immense scientific interest due to their promise for various applications. However, research into these intriguing systems is still active, with fundamental studies ongoing. Our research group (Professor Maran) has played a key role in unraveling the principles governing NCs and their connection to their structure. We have particularly emphasized the crucial role of ligands in shaping the nanoclusters' properties. Ligands have long been underestimated, often considered superficially or even incorrectly. Over the years, our research has highlighted their profound impact on the overall molecular properties of NCs. Among all NCs, Au25 (protected by thiolates) is the most widely studied, thanks to its practical advantages (high-yield synthesis, stability in three oxidation states, and early discovery) and unique properties. From an electrochemical perspective, NCs are remarkable in that they consist of thiolates completely surrounding the metal core, akin to a 3D analog of 2D self-assembled monolayers on extended gold surfaces. However, unlike the well-shielded monolayers on extended surfaces, the 3D monolayer on a small cluster allows exogenous molecules to penetrate and experience the properties of a unique nanoenvironment. Our group's extensive research has led to a series of publications, contributing significantly to the recognition within the scientific community that ligands are not merely ancillary components, but rather fundamental tools with profound implications. This recognition has driven my doctoral thesis. In this thesis work, after introducing the concept of NCs (Chapter 1) and the methods and characterization techniques used (Chapter 2), I present a series of studies focused on manipulating NC ligand chemistry. These studies demonstrate that carefully tailoring the ligands on the nanocluster surface allows us to modify (Chapter 4, 5, 9) and even precisely control (Chapter 3) properties such as electron transfer in solution (Chapter 5), conductivity of nanocluster films and their associated electron transfer in the solid state (Chapters 3, 5), and overall redox properties (Chapter 9). Additionally, I explore the possibility of enabling the formation of novel species with unique properties through chemical bonding between distinct NC units, after ad-hoc surface modification, via Ligand Exchange Reaction (LER) (Chapters 6, 8). Finally, Chapters 4 and 7 showcase instances where nanocluster surface modification has facilitated their application-oriented utilization. Chapters 5 and 8 highlight two particularly groundbreaking achievements. Chapter 5 demonstrates the profound impact of the isotopic effect on the solid-state packing properties of NCs, as evidenced by the first-ever crystal structure determination of a fully deuterated cluster. Chapter 8, on the other hand, presents the synthesis of an Au25 dimer linked by a molecular bridge comprising a metal atom complex, which was found to be a multicomponent redox-active molecule. This achievement was made possible through a novel LER strategy that utilizes electrostatic driving forces followed by a change in product solubility, ensuring the complete absence of byproducts arising from successive exchanges. This novel synthesis approach is a significant breakthrough that could lead to the development of new complex systems with a wide range of applications. In conclusion, we believe this thesis work makes an important contribution to scientific research on atomically precise nanoclusters and confirms the importance of ligands in determining the properties of these molecules. We believe this work can stimulate further research on modifying the properties of NCs by surface modification.