Aromatic Thiolate-Protected Series of Gold Nanomolecules and a Contrary Structural Trend in Size Evolution

ConspectusThiolate-protected gold nanoparticles (AuNPs) are a special class of nanomaterials that form atomically precise NPs with distinct numbers of Au atoms (<i>n</i>) and thiolate (−SR, R = hydrocarbon tail) ligands (<i>m</i>) with molecular formula [Au_<i>n</i>(SR)_<i>m</i>]. 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 Au₁₀₂(SC₆H₄COOH)₄₄ and Au₂₅(SC₂H₄Ph)₁₈^– 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 (<i>n</i> = 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-<i>tert</i>-butylbenzenethiolate (TBBT)-protected AuNMs and others to answer some of the important open questions. The TBBT series of AuNMs comprises Au₂₈(SR)₂₀, Au₃₆(SR)₂₄, Au₄₄(SR)₂₈, Au₅₂(SR)₃₂, Au₉₂(SR)₄₄, Au₁₃₃(SR)₅₂, and Au₂₇₉(SR)₈₄, where Au₂₈ to Au₁₃₃ are molecule-like with discrete electronic structures and Au₂₇₉ exhibits metal-like properties with a surface plasmon resonance (SPR) at 510 nm. The TBBT series of AuNMs have dihedral symmetry, except for Au₁₃₃(SR)₅₂, 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 <i>contrasts</i> 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

Crystal Structure of Faradaurate-279: Au₂₇₉(SPh‑<i>t</i>Bu)₈₄ Plasmonic Nanocrystal Molecules

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), Au₂₇₉(SPh-<i>t</i>Bu)₈₄ 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 Au₂₇₉S₈₄ 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 Au₂₇₉ follows the mathematical formula for magic number shells: Au@Au₁₂@Au₄₂@Au₉₂@Au₅₄, which is further protected by a final shell of Au₄₈. Au₂₄₉ 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, Au₂₇₉S₈₄ structure has a chiral pseudo-<i>D</i>₃ 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 Au₂₇₉ and Au₃₀₉ 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, Au₂₇₉S₈₄, and establishes the molecular formula with the complete ligands, namely, Au₂₇₉(SPh-<i>t</i>Bu)₈₄. It is also the smallest gold nanocrystal to exhibit metallic behavior, with a surface plasmon resonance band around 510 nm

Crystal Structure of Faradaurate-279: Au₂₇₉(SPh‑<i>t</i>Bu)₈₄ Plasmonic Nanocrystal Molecules

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), Au₂₇₉(SPh-<i>t</i>Bu)₈₄ 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 Au₂₇₉S₈₄ 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 Au₂₇₉ follows the mathematical formula for magic number shells: Au@Au₁₂@Au₄₂@Au₉₂@Au₅₄, which is further protected by a final shell of Au₄₈. Au₂₄₉ 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, Au₂₇₉S₈₄ structure has a chiral pseudo-<i>D</i>₃ 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 Au₂₇₉ and Au₃₀₉ 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, Au₂₇₉S₈₄, and establishes the molecular formula with the complete ligands, namely, Au₂₇₉(SPh-<i>t</i>Bu)₈₄. It is also the smallest gold nanocrystal to exhibit metallic behavior, with a surface plasmon resonance band around 510 nm

Au₂₇₉(SR)₈₄: The Smallest Gold Thiolate Nanocrystal That Is Metallic and the Birth of Plasmon

We report a detailed study on the optical properties of Au₂₇₉(SR)₈₄ 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. Au₂₇₉ is the smallest identified gold nanocrystal to exhibit SPR. Its optical absorption exhibits SPR at 510 nm. Power-dependent bleach recovery kinetics of Au₂₇₉ suggests that electron dynamics dominates its relaxation and it can support plasmon oscillations. Interestingly, TDDFT and DOS studies with different tail group residues (−CH₃ and −Ph) revealed the important role played by the tail groups of ligands in collective oscillation. Also, steady-state and time-resolved absorption for Au₃₆, Au₄₄, and Au₁₃₃ were studied to reveal the <i>molecule-to-metal</i> evolution of aromatic AuNMs. The optical gap and transient decay lifetimes decrease as the size increases

Data_Sheet_1_Ligand Structure Determines Nanoparticles' Atomic Structure, Metal-Ligand Interface and Properties.PDF

<p>The nature of the ligands dictates the composition, molecular formulae, atomic structure and the physical properties of thiolate protected gold nanomolecules, Au_n(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 Au_n(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.</p