We report the discovery of an unprecedentedly large, 2.2 nm diameter, thiolate protected gold nanocrystal characterized by single crystal X-ray crystallography (sc-XRD), Au(SPh-tBu) named Faradaurate-279 (F-279) in honor of Michael Faraday's (1857) pioneering work on nanoparticles. F-279 nanocrystal has a core-shell structure containing a truncated octahedral core with bulk face-centered cubic-like arrangement, yet a nanomolecule with a precise number of metal atoms and thiolate ligands. The AuS geometry was established from a low-temperature 120 K sc-XRD study at 0.90 Å resolution. The atom counts in core-shell structure of Au follows the mathematical formula for magic number shells: Au@Au@Au@Au@Au, which is further protected by a final shell of Au. Au core is protected by three types of staple motifs, namely: 30 bridging, 18 monomeric, and 6 dimeric staple motifs. Despite the presence of such diverse staple motifs, AuS structure has a chiral pseudo-D symmetry. The core-shell structure can be viewed as nested, concentric polyhedra, containing a total of five forms of Archimedean solids. A comparison between the Au and Au cuboctahedral superatom model in shell-wise growth is illustrated. F-279 can be synthesized and isolated in high purity in milligram quantities using size exclusion chromatography, as evidenced by mass spectrometry. Electrospray ionization-mass spectrometry independently verifies the X-ray diffraction study based heavy atoms formula, AuS, and establishes the molecular formula with the complete ligands, namely, Au(SPh-tBu). It is also the smallest gold nanocrystal to exhibit metallic behavior, with a surface plasmon resonance band around 510 nm.

Thiolate-protected gold nanoparticles (AuNPs) are a special class of nanomaterials that form atomically precise NPs with distinct numbers of Au atoms ( n) and thiolate (-SR, R = hydrocarbon tail) ligands ( m) with molecular formula [Au (SR)]. These are generally termed Au nanomolecules (AuNMs), nanoclusters, and nanocrystals. AuNMs offer atomic precision in size, which is desired to underpin the rules governing the nanoscale regime and factors affecting the unique properties conferred by quantum confinement. Research since the 1990s has established the molecular nature of these compounds and investigated their unique size-dependent optical and electrochemical properties. Pioneering work in X-ray crystallography of Au(SCHCOOH) and Au(SCHPh) revolutionized the field by providing significant insight into the structural assembly of AuNMs and surface protection modes. Recent discoveries involving bulky and rigid ligands to favor crystal growth as a solution to the nanostructure problem have led to crystal structure determinations of several AuNMs ( n = 18 to 279). However, there are several open questions, such as the following: How does the structure evolve with size? Does the atomic structure determine the properties? What determines the atomic structure? What factors govern the stability: geometry or electronic properties or ligands? Where does the molecule-to-metal transition occur? Answering these questions requires the elucidation of governing rules in the nanoscale regime. In this Account, we discuss patterns and trends observed in structures, growth, and surface protection modes of 4- tert-butylbenzenethiolate (TBBT)-protected AuNMs and others to answer some of the important open questions. The TBBT series of AuNMs comprises Au(SR), Au(SR), Au(SR), Au(SR), Au(SR), Au(SR), and Au(SR), where Au to Au are molecule-like with discrete electronic structures and Au exhibits metal-like properties with a surface plasmon resonance (SPR) at 510 nm. The TBBT series of AuNMs have dihedral symmetry, except for Au(SR), which has no symmetry. We synthesize the scaling law and the rules of surface assembly, one-, two-, and three-dimensional growth patterns, the structural evolution trend, and an overarching trend for diverse types of thiolate-protected AuNMs. This Account sheds light on a new perspective in structural evolution for the TBBT series based on observations, namely, face-centered cubic (FCC) to decahedral to icosahedral to FCC, which contrasts with the contemporary understanding of the structural evolution of naked metal clusters (NMCs) from icosahedral to decahedral to FCC. We also hope that this Account will be of pedagogical value and spur further experimental and computational studies on this wide range of structures to delineate the underlying stability factors in the magic series.

We report a detailed study on the optical properties of Au(SR) using steady-state and transient absorption measurements to probe its metallic nature, time-dependent density functional theory (TDDFT) studies to correlate the optical spectra, and density of states (DOS) to reveal the factors governing the origin of the collective surface plasmon resonance (SPR) oscillation. Au is the smallest identified gold nanocrystal to exhibit SPR. Its optical absorption exhibits SPR at 510 nm. Power-dependent bleach recovery kinetics of Au suggests that electron dynamics dominates its relaxation and it can support plasmon oscillations. Interestingly, TDDFT and DOS studies with different tail group residues (-CH and -Ph) revealed the important role played by the tail groups of ligands in collective oscillation. Also, steady-state and time-resolved absorption for Au, Au, and Au were studied to reveal the molecule-to-metal evolution of aromatic AuNMs. The optical gap and transient decay lifetimes decrease as the size increases.

Understanding the evolution of the structure and properties in metals from molecule-like to bulk-like has been a long sought fundamental question in science, since Faraday's 1857 work. We report the discovery of a Janus nanomolecule, Au 191 (SPh-tBu) 66 having both molecular and metallic characteristics, explored crystallographically and optically and modeled theoretically. Au 191 has an anisotropic, singly twinned structure with an Au 155 core protected by a ligand shell made of 24 monomeric [−S−Au−S−] and 6 dimeric [−S−Au−S−Au−S−] staples. The Au 155 core is composed of an 89-atom inner core and 66 surface atoms, arranged as [Au 3 @Au 23 @Au 63 ]@Au 66 concentric shells of atoms. The inner core has a monotwinned/stackingfaulted face-centered-cubic (fcc) structure. Structural evolution in metal nanoparticles has been known to progress from multiply twinned, icosahedral, structures in smaller molecular sizes to untwinned bulk-like fcc monocrystalline nanostructures in larger nanoparticles. The monotwinned inner core structure of the ligand capped Au 191 nanomolecule provides the critical missing link, and bridges the size-evolution gap between the molecular multipletwinning regime and the bulk-metal-like particles with untwinned fcc structure. The Janus nature of the nanoparticle is demonstrated by its optical and electronic properties, with metal-like electron−phonon relaxation and molecule-like long-lived excited states. Firstprinciples theoretical explorations of the electronic structure uncovered electronic stabilization through the opening of a shell-closing gap at the top of the occupied manifold of the delocalized electronic superatom spectrum of the inner core. The electronic stabilization together with the inner core geometric stability and the optimally stapled ligand-capping anchor and secure the stability of the entire nanomolecule.

The nature of the ligands dictates the composition, molecular formulae, atomic structure and the physical properties of thiolate protected gold nanomolecules, Aun(SR)m. In this review, we describe the ligand effect for three classes of thiols namely, aliphatic, AL or aliphatic-like, aromatic, AR, or bulky, BU thiol ligands. The ligand effect is demonstrated using three experimental setups namely: (1) The nanomolecule series obtained by direct synthesis using AL, AR, and BU ligands; (2) Molecular conversion and interconversion between Au38(S-AL)24, Au36(S-AR)24, and Au30(S-BU)18 nanomolecules; and (3) Synthesis of Au38, Au36, and Au30 nanomolecules from one precursor Aun(S-glutathione)m upon reacting with AL, AR, and BU ligands. These nanomolecules possess unique geometric core structure, metal-ligand staple interface, optical and electrochemical properties. The results unequivocally demonstrate that the ligand structure determines the nanomolecules' atomic structure, metal-ligand interface and properties. The direct synthesis approach reveals that AL, AR, and BU ligands form nanomolecules with unique atomic structure and composition. Similarly, the nature of the ligand plays a pivotal role and has a significant impact on the passivated systems such as metal nanoparticles, quantum dots, magnetic nanoparticles and self-assembled monolayers (SAMs). Computational analysis demonstrates and predicts the thermodynamic stability of gold nanomolecules and the importance of ligand-ligand interactions that clearly stands out as a determining factor, especially for species with AL ligands such as Au38(S-AL)24.