Synthesis of Au₃₈(SCH₂CH₂Ph)₂₄, Au₃₆(SPh‑<i>t</i>Bu)₂₄, and Au₃₀(S‑<i>t</i>Bu)₁₈ Nanomolecules from a Common Precursor Mixture

Phenylethanethiol protected nanomolecules such as Au₂₅, Au₃₈, and Au₁₄₄ are widely studied by a broad range of scientists in the community, owing primarily to the availability of simple synthetic protocols. However, synthetic methods are not available for other ligands, such as aromatic thiol and bulky ligands, impeding progress. Here we report the facile synthesis of three distinct nanomolecules, Au₃₈(SCH₂CH₂Ph)₂₄, Au₃₆(SPh-<i>t</i>Bu)₂₄, and Au₃₀(S-<i>t</i>Bu)₁₈, exclusively, starting from a common Au_<i>n</i>(glutathione)_<i>m</i> (where <i>n</i> and <i>m</i> are number of gold atoms and glutathiolate ligands) starting material upon reaction with HSCH₂CH₂Ph, HSPh-<i>t</i>Bu, and HS<i>t</i>Bu, respectively. The systematic synthetic approach involves two steps: (i) synthesis of kinetically controlled Au_<i>n</i>(glutathione)_<i>m</i> crude nanocluster mixture with 1:4 gold to thiol molar ratio and (ii) thermochemical treatment of the purified nanocluster mixture with excess thiols to obtain thermodynamically stable nanomolecules. Thermochemical reactions with physicochemically different ligands formed highly monodispersed, exclusively three different core-size nanomolecules, suggesting a ligand induced core-size conversion and structural transformation. The purpose of this work is to make available a facile and simple synthetic method for the preparation of Au₃₈(SCH₂CH₂Ph)₂₄, Au₃₆(SPh-<i>t</i>Bu)₂₄, and Au₃₀(S-<i>t</i>Bu)₁₈, to nonspecialists and the broader scientific community. The central idea of simple synthetic method was demonstrated with other ligand systems such as cyclopentanethiol (HSC₅H₉), cyclohexanethiol­(HSC₆H₁₁), <i>para</i>-methylbenzenethiol­(pMBT), 1-pentanethiol­(HSC₅H₁₁), 1-hexanethiol­(HSC₆H₁₃), where Au₃₆(SC₅H₉)₂₄, Au₃₆(SC₆H₁₁)₂₄, Au₃₆(pMBT)₂₄, Au₃₈(SC₅H₁₁)₂₄, and Au₃₈(SC₆H₁₃)₂₄ were obtained, respectively

Green Gold: Au₃₀(S‑<i>t</i>‑C₄H₉)₁₈ Molecules

Here, we report the synthesis and separation of Au₃₀(<i>tert</i>-thiol)₁₈ (<i>tert</i>-thiol = <i>tert-</i>butanethiol and 1-adamantanethiol) from a mixture of sizes of bulky-ligated nanoparticles. With precisely 30 gold metal atoms and 18 tertiary butyl ligands, this new 30 Au atom molecule is assigned a formula based on results obtained in high-resolution electrospray ionization (ESI) mass spectrometry. UV-vis-NIR spectroscopy shows a distinct electronic transition featured at 620 nm, with a valley in the green wavelength 520–570 nm region, explaining the green appearance. This lack of absorbance in the green region is uncommon, and therefore, metal nanoparticles of green color are extremely rare. Its optical band gap, 1.76 eV is much different than the 1.3 eV reported for Au₂₅­(SR)­₁₈. We report its reproducible direct synthesis (∼10 mg) in a one-pot reaction, with two different ligands. Because of the unique optical properties and altered-discrete sizes, there is a fundamental need for structural analysis of this nanoparticle

Au₃₂₉(SR)₈₄ Nanomolecules: Compositional Assignment of the 76.3 kDa Plasmonic Faradaurates

The purpose of this work is to determine the chemical composition of the previously reported faradaurates, which is a large 76.3 kDa thiolated gold nanomolecule. Electrospray ionization quadrupole-time-of-flight (ESI Q-TOF) mass spectrometry of the title compound using three different thiols yield the 329:84 gold to thiol compositional assignment. The purity of the title compound was checked by matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry. Positive and negative mode ESI-MS spectra show identical peaks denoting that there are no counterions, further reinforcing the accuracy of the assigned composition. We intentionally added Cs⁺ ions to show that the Au₃₂₉(SR)₈₄ is the base molecular ion, with several Cs⁺ adducts. A comprehensive investigation including analysis of the title compound with three ligands, in positive and negative mode and Cs⁺ adduction, leads to a conclusive composition of Au₃₂₉(SR)₈₄. This formula determination will facilitate the fundamental understanding of emergence of surface plasmon resonance in Au₃₂₉(SR)₈₄ with 245 free electrons

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

Au₉₉(SPh)₄₂ Nanomolecules: Aromatic Thiolate Ligand Induced Conversion of Au₁₄₄(SCH₂CH₂Ph)₆₀

A new aromatic thiolate protected gold nanomolecule Au₉₉(SPh)₄₂ has been synthesized by reacting the highly stable Au₁₄₄(SCH₂CH₂Ph)₆₀ with thiophenol, HSPh. The ubiquitous Au₁₄₄(SR)₆₀ is known for its high stability even at elevated temperature and in the presence of excess thiol. This report demonstrates for the first time the reactivity of the Au₁₄₄(SCH₂CH₂Ph)₆₀ with thiophenol to form a different 99-Au atom species. The resulting Au₉₉(SPh)₄₂ compound, however, is unreactive and highly stable in the presence of excess aromatic thiol. The molecular formula of the title compound is determined by high resolution electrospray mass spectrometry (ESI-MS) and confirmed by the preparation of the 99-atom nanomolecule using two ligands, namely, Au₉₉(SPh)₄₂ and Au₉₉(SPh-OMe)₄₂. This mass spectrometry study is an unprecedented advance in nanoparticle reaction monitoring, in studying the 144-atom to 99-atom size evolution at such high <i>m</i>/<i>z</i> (∼12k) and resolution. The optical and electrochemical properties of Au₉₉(SPh)₄₂ are reported. Other substituents on the phenyl group, HS-Ph-X, where X = −F, −CH₃, −OCH₃, also show the Au₁₄₄ to Au₉₉ core size conversion, suggesting minimal electronic effects for these substituents. Control experiments were conducted by reacting Au₁₄₄(SCH₂CH₂Ph)₆₀ with HS-(CH₂)_<i>n</i>-Ph (where <i>n</i> = 1 and 2), bulky ligands like adamantanethiol and cyclohexanethiol. It was observed that conversion of Au₁₄₄ to Au₉₉ occurs only when the phenyl group is directly attached to the thiol, suggesting that the formation of a 99-atom species is largely influenced by aromaticity of the ligand and less so on the bulkiness of the ligand

Photophysical and Redox Properties of Molecule-like CdSe Nanoclusters

Advancing our understanding of the photophysical and electrochemical properties of semiconductor nanoclusters with a molecule-like HOMO–LUMO energy level will help lead to their application in photovoltaic devices and photocatalysts. Here we describe an approach to the synthesis and isolation of molecule-like CdSe nanoclusters, which displayed sharp transitions at 347 nm (3.57 eV) and 362 nm (3.43 eV) in the optical spectrum with a lower energy band extinction coefficient of ∼121 000 M^–1 cm^–1. Mass spectrometry showed a single nanocluster molecular weight of 8502. From this mass and various spectroscopic analyses, the nanoclusters are determined to be of the single molecular composition Cd₃₄Se₂₀(SPh)₂₈, which is a new nonstiochiometric nanocluster. Their reversible electrochemical band gap determined in Bu₄NPF₆/CH₃CN was found to be 4.0 V. There was a 0.57 eV Coulombic interaction energy of the electron–hole pair involved. The scan rate dependent electrochemistry suggested diffusion-limited transport of nanoclusters to the electrode. The nanocluster diffusion coefficient (<i>D</i> = 5.4 × 10 ^–4 cm²/s) in acetonitrile solution was determined from cyclic voltammetry, which suggested Cd₃₄Se₂₀(SPh)₂₈ acts as a multielectron donor or acceptor. We also present a working model of the energy level structure of the newly discovered nanocluster based on its photophysical and redox properties

X‑ray Crystal Structure and Theoretical Analysis of Au_{25–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₁₈^– Alloy

The atomic arrangement of Au and Ag atoms in Au_{25–<i>x</i>}Ag_<i>x</i>(SR)₁₈ was determined by X-ray crystallography. Ag atoms were selectively incorporated in the 12 vertices of the icosahedral core. The central atom and the metal atoms in the six [−SR–Au–SR–Au–SR−] units were exclusively gold, with 100% Au occupancy. The composition of the crystals determined by X-ray crystallography was Au_18.3Ag_6.7(SCH₂CH₂Ph)₁₈. This composition is in reasonable agreement with the composition Au_18.8Ag_6.2(SCH₂CH₂Ph)₁₈ measured by electrospray mass spectrometry. The structure can be described in terms of shells as Au₁@Au_5.3Ag_6.7@6×[−SR–Au–SR–Au–SR−]. Density functional theory calculations show that the electronic structure and optical absorption spectra are sensitive to the silver atom arrangement within the nanocluster

X‑ray Crystal Structure of Au_{38–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₂₄ Alloy Nanomolecules

Herein, we report the X-ray crystallographic structure of a 38-metal atom Au–Ag alloy nanomolecule. The structure of monometallic Au₃₈(SR)₂₄ consists of 2 central Au atoms and 21 Au atoms forming a bi-icosahedral core protected by 6 dimeric and 3 monomeric units. In Au_{38–<i>x</i>}Ag_<i>x</i>(SR)₂₄,where <i>x</i> ranges from 1 to 5, the silver atoms are selectively incorporated into the Au₂₁ bi-icosahedral core. Within the Au₂₁ core, the silver atoms preferentially occupy nine selected locations: (a) the two vertex edges, three atoms on each edge and six atoms total, and (b) the middle face-shared three-atom ring, adding to a total of nine locations. X-ray crystallography yielded a composition of Au_34.04Ag_3.96(SCH₂CH₂Ph)₂₄. The crystal structure of the alloy nanomolecule can be described in terms of shells as Au₂@Au_17.04Ag_3.96@ 6×[−SR–Au–SR–Au–SR] 3×[−SR–Au–SR−]

X‑ray Crystal Structure and Theoretical Analysis of Au_{25–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₁₈^– Alloy

The atomic arrangement of Au and Ag atoms in Au_{25–<i>x</i>}Ag_<i>x</i>(SR)₁₈ was determined by X-ray crystallography. Ag atoms were selectively incorporated in the 12 vertices of the icosahedral core. The central atom and the metal atoms in the six [−SR–Au–SR–Au–SR−] units were exclusively gold, with 100% Au occupancy. The composition of the crystals determined by X-ray crystallography was Au_18.3Ag_6.7(SCH₂CH₂Ph)₁₈. This composition is in reasonable agreement with the composition Au_18.8Ag_6.2(SCH₂CH₂Ph)₁₈ measured by electrospray mass spectrometry. The structure can be described in terms of shells as Au₁@Au_5.3Ag_6.7@6×[−SR–Au–SR–Au–SR−]. Density functional theory calculations show that the electronic structure and optical absorption spectra are sensitive to the silver atom arrangement within the nanocluster

X‑ray Crystal Structure of Au_{38–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₂₄ Alloy Nanomolecules

Herein, we report the X-ray crystallographic structure of a 38-metal atom Au–Ag alloy nanomolecule. The structure of monometallic Au₃₈(SR)₂₄ consists of 2 central Au atoms and 21 Au atoms forming a bi-icosahedral core protected by 6 dimeric and 3 monomeric units. In Au_{38–<i>x</i>}Ag_<i>x</i>(SR)₂₄,where <i>x</i> ranges from 1 to 5, the silver atoms are selectively incorporated into the Au₂₁ bi-icosahedral core. Within the Au₂₁ core, the silver atoms preferentially occupy nine selected locations: (a) the two vertex edges, three atoms on each edge and six atoms total, and (b) the middle face-shared three-atom ring, adding to a total of nine locations. X-ray crystallography yielded a composition of Au_34.04Ag_3.96(SCH₂CH₂Ph)₂₄. The crystal structure of the alloy nanomolecule can be described in terms of shells as Au₂@Au_17.04Ag_3.96@ 6×[−SR–Au–SR–Au–SR] 3×[−SR–Au–SR−]