Aromatic Thiolate-Protected Series of Gold Nanomolecules and a

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Aromatic Thiolate-Protected Series of Gold Nanomolecules and a Contrary Structural Trend in Size Evolution Naga Arjun Sakthivel and Amala Dass* Department of Chemistry and Biochemistry, University of Mississippi, Oxford, Mississippi 38677, United States

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S Supporting Information *

CONSPECTUS: 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 [Aun(SR)m]. 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 (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-tertbutylbenzenethiolate (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 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.

1. INTRODUCTION

nanoscale materials at high precision is known as the nanostructure problem.6 One of the widely studied systems in the nanoregime is gold nanoparticles (AuNPs) in various forms. Thiolate-protected AuNPs have become an ideal system to understand and establish the rules governing the nanoregime with atomic precision following the pioneering works of Faraday,7 Brust et al.,8 Whetten et al.,9 Kornberg and co-workers,10 and Murray and coworkers.11 The special class of atomically precise NPs with molecular formula [Aun(SR)m] consisting of distinct numbers of

Structure determines properties of materials on all scales. When the size of a material is reduced, the properties vary, and as it is further reduced to the level where the dimensions are even smaller than the wavelength of light, several unique properties due to the quantum-confinement effect are observed.1−4 Typically, the distinct variations in properties have been attributed arbitrarily on the basis of size and shape. Atomic structure determination is desired in order to be able to understand the properties of matter and manipulate them at the atomic level.5 Structural analyses of nanoscale materials (1−100 nm) and ultrasmall nanomolecules (1−3 nm) have been a challenge. The challenge of structural characterization of © XXXX American Chemical Society

Received: April 3, 2018

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Figure 1. AuNMs formed using the TBBT ligand: (a) 1−100 nm size range, with the 1−3 nm range containing nanomolecules (±0 atoms) and the 3− 100 nm range containing nanoparticles where size dispersity has been achieved but still a variation of ±1000 atoms exists; (b) the TBBT series of AuNMs.

Figure 2. X-ray crystallographic structures of the TBBT series: (a) core-center Au atoms that are not thiolate-protected; (b) core atoms; (c) total structure. Only the heavy atoms Au and S are shown; C and H have been excluded for clarity.

field of thiolate-protected AuNMs.10−12 Whetten et al.13 envisioned the assembly of giant nanocrystals in a crystal lattice as “Crystals of Nanocrystals”, and this came into reality in 2017.14 AuNMs are being intensely pursued because of the feasibility of elucidating their structures by SC-XRD. The

Au atoms (n) and thiolate (−SR, R = hydrocarbon tail) ligands (m) are generally called as gold nanomolecules (AuNMs), nanoclusters, and nanocrystals. Structural determination of Au102(SC6H4COOH)44 and Au25(SC2H4Ph)18− by singlecrystal X-ray crystallography (SC-XRD) revolutionized the B

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2. CRYSTAL STRUCTURES OF THE TBBT SERIES

experimentally determined X-ray crystal structure models serves as a bridge in closing the gap in our limited understanding of the nanoworld by enabling the correlation of experimental and theoretical results and provide insights into the structural assembly and surface protection modes of AuNMs.11,12,15−19 However, until 2015, Au102 was the largest structure reported, and several advances were made for n < 100 Au atoms.18 The recent discovery that bulky and rigid ligands can be employed to favor crystal growth as a solution to the nanostructure problem20 has led to crystal structure determinations of several AuNMs with seven core−shell structures (n > 100 atoms).10,14,20−25 Each ligand forms a series of discrete stable-sized AuNMs because of the difference in stereoelectronic properties of the ligands. The 4-tert-butylbenzenethiolate (TBBT, SPh-tBu)protected AuNM series comprises Au28(SR)20, Au36(SR)24, Au 44 (SR) 28 , Au 52 (SR) 32 , Au 92 (SR) 44 , Au 133 (SR) 52 , and Au279(SR)84 (Figure 1). The TBBT series is the one with the largest number of crystal structures, spanning from complex to molecule to metal, because the bulky and rigid nature of the ligand favors crystal growth.14,20,21,26−33 The crystal structures and packing of AuNMs in the crystal lattice indicate that the ligands have interligand and intermolecular ligand−shell interactions. Therefore, their rigid nature is likely to play a key role in reducing the randomness and favoring orderly packing of AuNMs in three-dimensional (3D) lattices to form single crystals. Various factors governing their crystal growth remain to be investigated. Hereafter, Aun(SR)m is abbreviated as Aun for ease of notation. The atomic structure evolves from facecentered cubic (FCC) kernels (Au28 to Au92) to icosahedral (Ih) (Au133) to a truncated-octahedral (TO) FCC nanocrystal (Au279). The properties of the AuNMs also evolve from molecule to metal, with Au28 to Au133 in the molecule regime and Au279 in the metal regime.3 AuNMs in the metal regime with plasmonic behavior have been termed Faradaurates to honor the seminal work of Michael Faraday (1857) on AuNPs.14,34−36 Despite the availability of several crystal structures, there are several open questions regarding the evolution of the structure with size, the structure−property relationship, factors governing the structure and stability (geometry or electronic properties or ligands), the precise point of the molecule-to-metal transition, etc. Answering these questions require the elucidation of rules governing the nanoregime. In this Account, we discuss patterns and trends observed in the structures, growth, and surface protection modes of TBBTprotected AuNMs and other core−shell AuNMs to answer some of the important open questions. Unlike large AuNMs, the structures of non-core−shell-based AuNMs vary significantly across different series of AuNMs depending on the stereoelectronic properties of ligands. For example, linear aliphatic ligands support Ih and variants, aromatic ligands support FCC, and bulky ligands support body-centered cubic (BCC), hexagonal close-packed (HCP), and FCC. Therefore, to maintain the scope of this Account on the contrary structural evolution trend observed in the TBBT series and to provide an overarching structural evolution trend, we focus on the TBBT series and five other core−shell AuNMs structures that are also protected by aromatic ligands with different para substituents of generic form −SC6H 4R: Au102(SC 6H4 COOH) 44 , Au103S2(SC10H7)41, Au130(SC6H4CH3)50, Au146(SC6H4COOH)57, and Au246(SC6H4CH3)80.10,22−25

The crystal structures of the TBBT series are discussed here and depicted in Figure 2. AuNMs comprise two components: the Au core and the ligand shell. The Au core has core-center Au atoms, which are not directly linked to thiolate ligands, and Au atoms that are protected by the thiolate monolayer. The thiolate monolayer comprises bridging ligands [−Au−S(R)−Au−], monomeric staples [−(R)S−Au−S(R)−] (Au in blue), and dimeric staples [−(R)S−Au−S(R)−Au−S(R)−] (Au in orange; S in yellow). Tables S1 and S2 summarize the structural details and bond length analyses, respectively, for the TBBT series, and Figure S1 shows the poles and bodies of the AuNMs. Section 3 covers the TBBT monolayers in detail. 2.1. Periodic Non-Core−Shell FCC Structures

Au28 has a 20-atom interpenetrating cuboctahedral (i-CO) core reminiscent of a 1D nanorod with two central Au atoms. Au36 has a 28-atom truncated tetrahedral core made of four i-CO units, and the central four atoms form a tetrahedron. Au44 has a 36-atom core made of six i-CO units. Au52 has a 44-atom core made of eight i-CO units. Au28, Au36, Au44, and Au52 are protected by four dimeric staples and 8, 12, 16, and 20 bridging ligands, respectively. Among these periodic FCC-structured AuNMs, Au36 has D2d symmetry while the other three possess chiral D2 symmetry. The periodic trend in the TBBT series is linear from Au28 to Au52 with an empirical formula of Au8n+4(SR)4n+8 where n = 3−6.30 This empirical formula is not obeyed for AuNMs larger than Au52. For example, when n = 11, the formula is Au 92 (SR) 52, whereas Au92(SR)44 is the experimentally determined next size. Au92 has an 84-atom core with 24 core-center Au atoms, which are the centers of 24 i-CO units. The 24-atom core center consists of a 20-atom i-CO unit with four additional Au atoms located on four {111} facets of the 1D nanorod. The core is protected by eight monomeric staples and 28 bridging ligands. 2.2. Core−Shell AuNMs

The TBBT series has two core−shell structures bordering the molecule (Au133) and metal (Au279) regimes. Au133 has an Ih core−shell structure with an Au@Au12@Au42 Ih core. Ih-Au55 is protected by a 52-atom outer shell with three Au atoms on each of 16 {111} facets and one Au atom on each of four {111} facets of the 20-faceted icosahedron. The three Au atoms on each of the 16 {111} facets follow anti-Mackay (a-M) packing (ABCB), while the Au atom on each of the four {111} facets follow Mackay (M) packing (ABCA). The four {111} facets with one Au atom in different packing reduce the Ih symmetry to chiral D2 symmetry and keep the outer shell from being a regular 60-atom rhombicosidodecahedron. The Au107 core is protected by 26 monomeric staples, and the total structure has no symmetry. Au279 has a TO+ 249-atom core ((Au@Au12@Au42@ Au92)CO−Au147@TO−Au54@Au48 truncated cube (TC)) protected by 30 bridging ligands, 18 monomeric staples, and six dimeric staples. The octahedral symmetry is retained in TO201, and there are two adlayers on the poles with antiMackay packing (ABCAC). The hybrid packing of the adlayers on the body and ligand shell reduces the symmetry of Au279 to chiral D3. Au279 is the smallest gold−thiolate nanocrystal to be metallic to date.3 The TBBT series of AuNMs have dihedral symmetry except for Au133. C

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Figure 3. TBBT monolayer assembly on non-core−shell AuNMs: (a) {100} facets; (b) {111} facets.

Figure 4. TBBT monolayer assembly on core−shell AuNMs. (a) Au133 and (b) Au279.

3. TBBT MONOLAYER ASSEMBLY

of numerous AuNMs protected by diverse types of ligands (aliphatic, aromatic, and bulky). The atomic-resolution structures have elucidated the thiolate monolayer assembly on AuNMs with ultimate clarity. However, the trends and pattern in surface assembly of thiolate monolayers with size evolution has not been studied. Does the same ligand possess a similar assembly pattern across distinct sizes? Here we discuss the TBBT monolayer assembly on various facets in AuNMs, as elucidated in Figures 3, 4, and S1.

Self-assembled monolayers (SAMs) at the thiolate−gold interface have been widely studied on various facets ({111}, {100}, {110}) of single-crystalline gold since the 1980s, and a wide range of applications have been developed.37,38 SAMs on AuNPs have also been studied with diverse techniques to understand their modes of protection and overlay structures.37 Thiolate monolayer assembly on AuNPs was not amenable to SC-XRD studies because of the lack of quality single-crystals. It has been proposed since the early 2000s that the type of ligand tail group dictates the atomic structure of AuNMs.39 The advent of AuNMs and the pioneering work in X-ray crystallography over the past decade has led to crystal structure determinations

3.1. Surface Assembly on Non-Core−Shell FCC AuNMs

Au28, Au36, Au44, and Au52 have similar morphologies on the poles, with Au atoms in a 2 × 3 matrix arrangement containing two intrafacet and two interfacet bridging ligands (Figure 3a-1). D

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Figure 5. Structural evolution of (a) NMCs and (b) the TBBT series. The structural evolution of NMCs follows the order Ih to Dh to FCC. In contrast, the TBBT series follows the order FCC to Dh to Ih to FCC. *Detailed studies of Au102 are underway.

arranged in an alternating fashion on its body (Figure 4b).14 The two {111} facets on the poles are protected by six interfacet monomeric staples arranged in a propeller-blade fashion. The six {111} facets on the body are protected by one intrafacet and one interfacet bridging ligand, one intrafacet and one interfacet monomeric staple, and an interfacet dimeric staple. The six {100} facets have three intrafacet bridging ligands, one interfacet bridging ligand, one interfacet monomeric staple, and one dimeric staple.

Au36 has six {100} facets of the same morphology, with two on the poles and four on the body. Two facets on the poles and body (Figure 3a-2) follow same surface protection as in Figure 3a-1. The remaining two facets on the body follow a different mode of protection, with two intrafacet and four interfacet bridging ligands (Figure 3a-3). Au44 has four {100} facets on the body protected by three intrafacet and four interfacet bridging ligands (Figure 3a-4). Au52 has four {100} facets on the body with two modes of protection containing four intrafacet and four interfacet bridging ligands (Figure 3a-5,6). Furthermore, all the {100} facets of Au28−Au52 are protected by two interfacet dimeric staples (Figure 3a-1−6). Au92 has six {100} facets of two types (poles and body) that are longer than any other facets in the TBBT series. The poles and body have six and four intrafacet bridging ligands, respectively. Furthermore, the poles and body are protected by four interfacet bridging ligands and two intrafacet monomeric staples that protect the grooves (Figure 3a-7,8). Au28 has four {111} facets containing one intrafacet and one interfacet bridging ligand and two dimeric staples (Figure 3b-1). Au36−Au52 possess identical {111} facets that are protected by two intrafacet bridging ligands, two interfacet bridging ligands, and one intrafacet dimeric staple (Figure 3b-2). The {111} facets of Au92 have one intrafacet and one interfacet bridging ligand and three interfacet monomeric staples (Figure 3b-3).

4. STABILITY OF THE TBBT SERIES Although the TBBT series has the largest number of crystal structures, not all of the sizes are thermochemically stable. Au36, Au133, and Au279 have been demonstrated to be robust under thermochemical treatment (etching).14,20,28 Au28, Au44, Au52, and Au92 require a kinetically controlled synthetic pathway, and purification by separation techniques makes it challenging to isolate them in >5 mg quantities.29−33 The TBBT series can be arranged in the following decreasing order based on the stability under etching and ease of synthesis as Au36 ≈ Au133 ≈ Au279 > Au44 > Au28 > Au52 > Au92. We have isolated a size in between Au92 and Au133 with an approximate molecular formula of Au102(TBBT)44 as evidenced by mass spectrometry (Figure S2). Detailed characterization of ∼Au102(TBBT)44 is underway, and we predict that this might also possess a Marks-decahedral (mDh) 49 atom core−shell structure like its analogues Au102(SC6H4COOH)44 and Au103S2(SC10H7)41.10,25 Stability of ∼Au102(TBBT)44 is equivalent to that of Au44. Experiments in our laboratory show that high-yielding, robust synthetic protocols are available for remarkably stable sizes14,20,40,41 (Au36, Au133, and Au279) and that synthetic finesse is required to

3.2. Surface Assembly on Core−Shell AuNMs

Au133 has three interfacet monomeric staples and one interfacet monomeric staple protecting 16 {111} and four {111} facets with different surface packings, respectively (Figure 4a). Au279 is a 14-faceted Archimedean polyhedron with six {100} and eight {111} facets. Two {111} facets form the poles of the nanocrystal, and the remaining six {111} facets and the six {100} facets are E

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Figure 6. (a) Overarching structural trend in AuNMs (cyan, blue, green, and magenta indicate core−shell Au). (b) Shell growth pattern and monolayer assembly in AuNMs independent of the ligand type.

5.2. Structural Trends Observed in Other Core−Shell AuNMs

obtain other low-yielding, less stable sizes (Au28, Au44, Au52, Au92, and Au102) in sufficient quantities.

Au102(SC6H4COOH)44 and Au103S2(SC10H7)41 possess mDh49 cores.10,25 Au130(SC6H4CH3)50 has an mDh-75 core.22 They are protected by 15-atom caps on the poles, with three atoms located on each {111} facet in an anti-Mackay packing, leading to mDh-Au79 and mDh-Au105 polyhedral cores, respectively. The Au102 core is protected by 19 monomeric and two dimeric staples. The Au103 core is protected by 16 monomeric, one dimeric, and two trimeric-like staples, where the trimeric-like staples contain the sulfido groups. The Au130 core is protected by 25 monomeric staples. Au146(SC6H4COOH)57 has an Au79 twinned-TO (t-TO) core that is protected by 40 atoms, leading to a t-TO+ 119-atom core.23 The core is protected by seven bridging ligands, 19 monomeric staples, and four dimeric staples. The 79-atom t-TO unit is a 14-faceted polyhedron with eight {111} and six {100} facets, and the mirror plane runs through the equatorial position. The shell growth along the {111} facets takes place in two modes (ABCA and ABCB), with four facets following each mode. Au246(SC6H4CH3)80 has an mDh-146 core with 30-atom caps on the poles containing six atoms in antiMackay packing on each {111} facet. The Au246 core is protected by 10 bridging ligands and 20 monomeric and 10 dimeric staples.24

5. STRUCTURE EVOLUTION 5.1. Contrary Structural Trend

The structural trend observed in the NMCs evolves from Ih to Dh to FCC with increasing size (Figure 5a).42−45 Aliphatic nalkylthiolate-protected AuNMs have been widely studied for over two decades. They also exhibit a similar trend, where Au25 and Au38 exhibit Ih kernels, Au67 is predicted to be Dh, Au144 is predicted to have an Ih or truncated-Dh core, and sizes above Au329 are FCC nanocrystals.4,11,12,35,36,39,46−48 However, in the aromatic TBBT series, a contrary trend in structure evolution with increasing size has been observed across the molecule-tometal regime. The structure evolves from non-core−shell FCC to mDh to Ih and then to FCC nanocrystals (Figure 5b). The non-core−shell FCC structures (Au28−Au52) follow a face-shared 1D growth pattern along {100} facets by crisscross stacking of interpenetrating 1D nanorods. Au92 can be viewed as 2D growth of Au44 or 3D growth of 1D nanorods. It is more of a 2D growth, which follows face- and vertex-shared growth along the {100} and {111} facets, respectively. Then the growth in the TBBT series becomes truly 3D, forming core−shell structures. F

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Accounts of Chemical Research 5.3. Overarching Structural Trend, Core−Shell Growth, and Structure Overlay Patterns Independent of the Ligand Type

AuNMs grow from non-core−shell structures in 1D or 2D fashion and converge to 3D growth, leading to core−shell structures that are twinned. Then the growth converges to untwinned FCC structures (Figure 6). The core−shell AuNMs follow ABAB packing along {100} facets, while {111} facets follow hierarchical growth with FCC (ABC) packing until it forms the convex polyhedra and then follows anti-Mackay packing (ABCB) on the surface. For example, {111} facets on the mDh surface have anti-Mackay-type packing, whereas Ih and t-TO+ structures have mixed packing, which can be partially attributed to relief of the structural strain. The anti-Mackay-type packing on the surface allows maximum adlayer atom packing. The TBBT series has provided significant insight into structural patterns and assembly of the thiolate monolayer. On the basis of the observations, we note that the structural monolayer overlay patterns on {100} facets in AuNMs across all sizes follow c(2 × 2) overlay structural symmetry in FCC and HCP structures but not in the others (Figures 6b and S3).49−52 SAMs on single-crystalline bulk gold {100} facets also exhibit c(2 × 2) overlay symmetry, and {111} facets follow (√3×√3)R30° symmetry.37,38 However, {111} facets in AuNMs identified to date have a high radius of curvature and do not possess longer {111} facets to exhibit the (√3×√3)R30° symmetry observed in bulk gold. Therefore, larger giant nanocrystals might possess such an arrangement. The {111} and {100} facets (in addition to the c(2 × 2) symmetry) in AuNMs possess a mixture of bridging ligands and staples that protect the re-entrant grooves, edges, and vertices on the surface. The staple motifs assemble on the basis of rotational symmetry, whereas bridging ligands on {100} facets follow translational symmetry.33

Figure 7. UV−vis−NIR absorption spectra of the TBBT series: Au28, dark-yellow solid curve; Au36, olive dotted curve; Au44, black solid curve; Au52, orange dashed-dotted curve; Au92, purple dashed curve; Au133, blue solid curve; Au279, red solid curve. The spectra have been normalized for clarity.

The TBBT series exhibit size-dependent optical and electrochemical gaps and electronically excited state lifetimes.3 Au28 has absorption features at 360, 490, and 600 nm with the onset at ∼700 nm (1.77 eV) and an electrochemical gap of ∼1.95 V.29,54 Au36 has absorption features at 375 and 575 nm with the onset at ∼705 nm (1.75 eV) and an electrochemical gap of ∼1.98 V.28,54 Au44 has an absorption feature at 386 nm and a broad shoulder at ∼780 nm with the onset at 820 nm (1.51 eV) and an electrochemical gap of 1.65 V.30,54 Au52 has absorption features at 396 and 800 nm with the onset at 890 nm (1.39 eV).30 Au92 has absorption features at 440, 660, and 850 nm and an electrochemical gap of 0.78 V.32,33 Au133 has a strong absorption at 510 nm and broad features at 430 and 710 nm. Au279 exhibits a SPR at 510 nm. The optical gap for Au92−Au279 cannot be assessed on the basis of the onset as their band gaps are electronic shell closing.

8. CONCLUSIONS AND OPPORTUNITIES The TBBT series follows periodic face-shared 1D growth from Au28 to Au52 and 2D growth in Au92. Then it converges to core− shell structures following 3D growth. The 3D growth is initially twinned and later converges to untwinned FCC nanocrystals. The structural evolution of TBBT series goes from FCC to mDh to Ih to FCC, in contrast to the contemporary understanding based on NMCs and n-alkylthiolate-protected AuNMs. The scaling factor of 0.6 is very close to the Euclidean surface rule of 2/3. The structural patterns studied across various AuNMs have revealed that independent of the ligand employed, the surface overlay pattern follows c(2 × 2) symmetry and the core−shell growth follows ABAB packing along {100} facets and hierarchical growth along {111} facets. The crucial roles of the ligand and geometry in structural evolution and stability have been established. We anticipate that this Account will spur a broad level of interest in experimental and theoretical studies to delineate the factors governing the structural evolution, stability, and structure−property relationship and fill the void in our understanding. Also, future studies of the interplay between geometry, ligand properties, and surface energy in the TBBT series would reveal the major reasons for the varying stability with size. The availability of these experimental structural models would provide a clearer picture and help us write or rewrite the rules governing the nanoscale regime. Nature is very precise, and nothing is random. Likewise, in the nanoscale regime, everything is precise, and every atom counts. AuNMs are the most ideal form of AuNPs, and therefore, the principles learned from this system could be extended to engineer atomically precise conventional larger AuNPs employed in drug delivery. We also hope that this Account will be of pedagogical value.

Figure 8. TBBT series nanoscaling law. The allometric power fit (log− log plot of Aun (N) vs (SR)m (L)) is compared with sphere’s surface area vs volume plot. Slope = 0.60 ± 0.01; intercept = 2.85 ± 0.2; reduced χ2 = 1.49; adjusted R2 = 0.9969.

AuNMs reported up to 2012.55 The decrease in compactness for the TBBT series can be attributed to the more cuboidal and nonplanar structures (Au28−Au92). However, the compactness is still less than that of most compact shape, the sphere, whose compactness value is 4.82.55 Also, the scaling factor of 0.6 is comparable to the 2/3 surface rule. The factors governing the structural evolution and stability of NPs are the geometry, cohesive energy of the element, surface energy, surface stress, and elastic strain.42−45,56,57 The order of stability for the cube, TC, CO, TO, and octahedron based on surface energy might be anticipated to be octahedron, TO, CO, TC, and cube. Interplay between the optimal geometry (surface area/volume) and surface energy results in the following descending order of stability: TO, octahedron, TC, CO, and cube.56 For example, Au279 with the TO core possesses the optimal geometry and surface energy and is most stable, whereas Au92 with a greater number of high-energy {100} facets is the least stable. Surface stress has been shown to greatly influence the structural evolution and molecule-to-metal crossover.57 Depending on the surface stress, the evolution might follow the order Ih to mDh to TO or the order Ih to TO or TO with no twinned particles.57 Also, mDh and Ih are preferred for low and high surface stress, respectively.57 Ligands can be considered as a means to tune the surface stress in AuNMs, which seems to suggest a phenomenology toward dictating the atomic structure and composition. The crucial role of ligands and ligand−ligand interactions in determining the stability of AuNMs has been shown by system comparisons and fragment decomposition analysis.41,49,58 It can be ascribed to the distinct structure and composition of two closely related sizes: mDh-Au130 and IhAu 133 , which have different para substituents on the thiophenolate ligand. Also, the differences in AuNM surface atom packing along the {111} facets for different structures are not anomalous but rather are precise. For example, Au133 follows two types of surface packing, and this can be attributed to relieving the structural strain and surface stress in the form of ligand sterics. Cleveland and Landman note that “in the icosahedron, strain can be relieved by letting the surface facets bow out in a nonplanar and nonuniform fashion at the expense

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.8b00150. Tables summarizing structure, bond length, and electronic property data and figures showing AuNM anatomy, mass spectra, overlay pattern, and standard errors (PDF) H

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Naga Arjun Sakthivel: 0000-0001-8134-905X Amala Dass: 0000-0001-6942-5451 Notes

The authors declare no competing financial interest. Biographies Naga Arjun Sakthivel is currently a doctoral candidate in the Department of Chemistry and Biochemistry at the University of Mississippi. He has a Bachelor’s degree in Chemical and Electrochemical Engineering from CSIR-CECRI in India. His research interest is in atomically precise nanomaterials. Amala Dass is an Associate Professor of Chemistry and Biochemistry at the University of Mississippi. He earned his Ph.D. in Chemistry from Missouri University of Science and Technology in 2005 with Prof. Nicholas Leventis and was a postdoctoral scholar with Prof. Royce Murray at the University of North Carolina at Chapel Hill. His current research interests are in gold and alloy nanomolecules.

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ACKNOWLEDGMENTS NSF-CHE-1255519 supported this work. REFERENCES

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DOI: 10.1021/acs.accounts.8b00150 Acc. Chem. Res. XXXX, XXX, XXX−XXX