Molecular Engineering of Stabilized Silicon-Rosindolizine Shortwave Infrared Fluorophores.

Fluorescence-based biological imaging in the shortwave infrared (SWIR, 1000-1700 nm) is an attractive replacement for modern in vivo imaging techniques currently employed in both medical and research settings. Xanthene-based fluorophores containing heterocycle donors have recently emerged as a way to access deep SWIR emitting fluorophores. A concern for xanthene-based SWIR fluorophores though is chemical stability toward ambient nucleophiles due to the high electrophilicity of the cationic fluorophore core. Herein, a series of SWIR emitting silicon-rosindolizine (SiRos) fluorophores with emission maxima >1300 nm (up to 1550 nm) are synthesized. The SiRos fluorophore photophysical properties and chemical stability toward nucleophiles are examined through systematic derivatization of the silicon-core alkyl groups, indolizine donor substitution, and the use of o-tolyl or o-xylyl groups appended to the fluorophore core. The dyes are studied via absorption spectroscopy, steady-state emission spectroscopy, solution-based cyclic voltammetry, time-dependent density functional theory (TD-DFT) computational analysis, X-ray diffraction crystallography, and relative chemical stability over time. Optimal chemical stability is observed via the incorporation of the 2-ethylhexyl silicon substituent and the o-xylyl group to protect the core of the fluorophore.

Design and Synthesis of RhodIndolizine Dyes with Improved Stability and Shortwave Infrared Emission up to 1250 nm.

The design of shortwave infrared (SWIR) emissive small molecules with good stability in water remains an important challenge for fluorescence biological imaging applications. A series of four SWIR emissive rhodindolizine (RI) dyes were rationally designed and synthesized to probe the effects of nonconjugated substituents, conjugated donor groups, and nanoencapsulation in a water-soluble polymer on the stability and optical properties of the dyes. Steric protecting groups were added at the site of a significant LUMO presence to probe the effects on stability. Indolizine donor groups with added dimethylaniline groups were added to reduce the electrophilicity of the dyes toward nucleophiles such as water. All of the dyes were found to absorb (920-1096 nm peak values) and emit (1082-1256 nm peak values) within the SWIR region. Among xanthene-based emissive dyes, emission values >1200 nm are exceptional with 1256 nm peak emission being a longer emission than the recent record setting VIX-4 xanthene-based dye. Half-lives were improved from ∼5 to >24 h through the incorporation of either steric-based core protection groups or donors with increased donation strength. Importantly, the nanoencapsulation of the dyes in a water-soluble surfactant (Triton-X) allows for the use of these dyes in biological imaging applications.

Size Exclusion Chromatography: An Indispensable Tool for the Isolation of Monodisperse Gold Nanomolecules.

Highly monodisperse and pure samples of atomically precise gold nanomolecules (AuNMs) are essential to understand their properties and to develop applications using them. Unfortunately, the synthetic protocols that yield a single-sized nanomolecule in a single-step reaction are unavailable. Instead, we observe a polydisperse product with a mixture of core sizes. This product requires post-synthetic reactions and separation techniques to isolate pure nanomolecules. Solvent fractionation based on the varying solubility of different sizes serves well to a certain extent in isolating pure compounds. It becomes tedious and offers less control while separating AuNMs that are very similar in size. Here, we report the versatile and the indispensable nature of using size exclusion chromatography (SEC) as a tool for separating nanomolecules and nanoparticles. We have demonstrated the following: (1) the ease of separation offered by SEC over solvent fractionation; (2) the separation of a wider size range (∼5-200 kDa or ∼1-3 nm) and larger-scale separation (20-100 mg per load); (3) the separation of closely sized AuNMs, demonstrated by purifying Au137(SR)56 from a mixture of Au329(SR)84, Au144(SR)60, Au137(SR)56, and Au130(SR)50, which could not be achieved using solvent fractionation; (4) the separation of AuNMs protected by different thiolate ligands (aliphatic, aromatic, and bulky); and (5) the separation can be improved by increasing the column length. Mass spectrometry was used for analyzing the SEC fractions.

The Missing Link: Au191(SPh-tBu)66 Janus Nanoparticle with Molecular and Bulk-Metal-like Properties.

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, Au191(SPh-tBu)66 having both molecular and metallic characteristics, explored crystallographically and optically and modeled theoretically. Au191 has an anisotropic, singly twinned structure with an Au155 core protected by a ligand shell made of 24 monomeric [-S-Au-S-] and 6 dimeric [-S-Au-S-Au-S-] staples. The Au155 core is composed of an 89-atom inner core and 66 surface atoms, arranged as [Au3@Au23@Au63]@Au66 concentric shells of atoms. The inner core has a monotwinned/stacking-faulted 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 Au191 nanomolecule provides the critical missing link, and bridges the size-evolution gap between the molecular multiple-twinning 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. First-principles 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.

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

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 n(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- tert-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 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.

Au36(SePh)24 nanomolecules: synthesis, optical spectroscopy and theoretical analysis.

Here, we report the synthesis of selenophenol (HSePh) protected Au36(SePh)24 nanomolecules via a ligand-exchange reaction of 4-tert-butylbenzenethiol (HSPh-tBu) protected Au36(SPh-tBu)24 with selenophenol, and its spectroscopic and theoretical analysis. Matrix assisted laser desorption ionization (MALDI) mass spectrometry, electrospray ionization (ESI) mass spectrometry and optical characterization confirm that the composition of the as synthesized product is predominantly Au36(SePh)24 nanomolecules. Size exclusion chromatography (SEC) was employed to isolate the Au36(SePh)24 and temperature dependent optical absorption studies and theoretical analysis were performed. Theoretically, an Independent Component Maps of Oscillator Strength (ICM-OS) analysis of simulated spectra shows that the enhancement in absorption intensity in Au36(SePh)24 with respect to Au36(SPh)24 can be ascribed to the absence of interference and/or increased long-range coupling between interband metal core and ligand excitations. This work demonstrates and helps to understand the effect of Au-Se bridging on the properties of gold nanomolecules.

Crystal Structure of Faradaurate-279: Au279(SPh-tBu)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-tBu)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-D3 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-tBu)84. It is also the smallest gold nanocrystal to exhibit metallic behavior, with a surface plasmon resonance band around 510 nm.

Indolizine-Squaraines: NIR Fluorescent Materials with Molecularly Engineered Stokes Shifts.

The development of deep red and near infrared emissive materials with high quantum yields is an important challenge. Several classes of squaraine dyes have demonstrated high quantum yields, but require significantly red-shifted absorptions to access the NIR window. Additionally, squaraine dyes have typically shown narrow Stokes shifts, which limits their use in living biological imaging applications due to dye emission interference with the light source. Through the incorporation of indolizine heterocycles we have synthesized novel indolizine squaraine dyes with increased Stokes shifts (up to >0.119 eV, >50 nm increase) and absorptions substantially further into the NIR region than an indoline squaraine benchmark (726 nm versus 659 nm absorption maxima). These materials have shown significantly enhanced water solubility, which is unique for squaraine dyes without water-solubilizing substituents. Absorption, electrochemical, computational, and fluorescence studies were undertaken and exceptional fluorescence quantum yields of up 12 % were observed with emission curves extending beyond 850 nm.

Synthesis of Aromatic Thiolate-Protected Gold Nanomolecules by Core Conversion: The Case of Au36(SPh-tBu)24.

Ultrasmall nanomolecules (<2 nm) such as Au25(SCH2CH2Ph)18, Au38(SCH2CH2Ph)24, and Au144(SCH2CH2Ph)60 are well studied and can be prepared using established synthetic procedures. No such synthetic protocols that result in high yield products from commercially available starting materials exist for Au36(SPh-X)24. Here, we report a synthetic procedure for the large-scale synthesis of highly stable Au36(SPh-X)24 with a yield of ∼42%. Au36(SPh-X)24 was conveniently synthesized by using tert-butylbenzenethiol (HSPh-tBu, TBBT) as the ligand, giving a more stable product with better shelf life and higher yield than previously reported for making Au36(SPh)24 from thiophenol (PhSH). The choice of thiol, solvent, and reaction conditions were modified for the optimization of the synthetic procedure. The purposes of this work are to (1) optimize the existing procedure to obtain stable product with better yield, (2) develop a scalable synthetic procedure, (3) demonstrate the superior stability of Au36(SPh-tBu)24 when compared to Au36(SPh)24, and (4) demonstrate the reproducibility and robustness of the optimized synthetic procedure.

Synthesis of Au38(SCH2CH2Ph)24, Au36(SPh-tBu)24, and Au30(S-tBu)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-tBu)24, and Au30(S-tBu)18, exclusively, starting from a common Aun(glutathione)m (where n and m are number of gold atoms and glutathiolate ligands) starting material upon reaction with HSCH2CH2Ph, HSPh-tBu, and HStBu, respectively. The systematic synthetic approach involves two steps: (i) synthesis of kinetically controlled Aun(glutathione)m 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-tBu)24, and Au30(S-tBu)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), para-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.

A Computational and Experimental Study of Thieno[3,4-b]thiophene as a Proaromatic π-Bridge in Dye-Sensitized Solar Cells.

Four D-π-A dyes (D=donor, A=accpetor) based on a 3,4-thienothiophene π-bridge were synthesized for use in dye-sensitized solar cells (DSCs). The proaromatic building block 3,4-thienothiophene is incorporated to stabilize dye excited-state oxidation potentials. This lowering of the excited-state energy levels allows for deeper absorption into the NIR region with relatively low molecular weight dyes. The influence of proaromatic functionality is probed through a computational analysis of optimized bond lengths and nucleus independent chemical shifts (NICS) for both the ground- and excited- states. To avoid a necessary lowering of the TiO2 semiconductor conduction band (CB) to promote efficient dye-TiO2 electron injection, strong donor functionalities based on triaryl- and diarylamines are employed in the dye designs to raise both the ground- and excited-state oxidation potentials of the dyes. Solubility, aggregation, and TiO2 surface protection are addressed by examining an ethylhexyl alkyl chain in comparison to a simple ethyl chain on the 3,4-thienothiophene bridge. Power conversion efficiencies of up to 7.8 % are observed.

Au133(SPh-tBu)52 nanomolecules: X-ray crystallography, optical, electrochemical, and theoretical analysis.

Crystal structure determination has revolutionized modern science in biology, chemistry, and physics. However, the difficulty in obtaining periodic crystal lattices which are needed for X-ray crystal analysis has hindered the determination of atomic structure in nanomaterials, known as the "nanostructure problem". Here, by using rigid and bulky ligands, we have overcome this limitation and successfully solved the X-ray crystallographic structure of the largest reported thiolated gold nanomolecule, Au133S52. The total composition, Au133(SPh-tBu)52, was verified using high resolution electrospray ionization mass spectrometry (ESI-MS). The experimental and simulated optical spectra show an emergent surface plasmon resonance that is more pronounced than in the slightly larger Au144(SCH2CH2Ph)60. Theoretical analysis indicates that the presence of rigid and bulky ligands is the key to the successful crystal formation.

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 m/z (∼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)n-Ph (where n = 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.

Single crystal XRD structure and theoretical analysis of the chiral Au30S(S-t-Bu)18 cluster.

Au30S(S-t-Bu)18 cluster, related closely to the recently isolated "green gold" compound Au30(S-t-Bu)18, has been structurally solved via single-crystal XRD and analyzed by density functional theory calculations. The molecular protecting layer shows a combination of monomeric (RS-Au-SR) and trimeric (RS-Au-SR-Au-SR-Au-SR) gold-thiolate units, bridging thiolates, and a single sulfur (sulfide) in a novel µ3-coordinating position. The chiral gold core has a geometrical component that is identical to the core of the recently reported Au28(SPh-t-Bu)20. Both enantiomers of Au30S(S-t-Bu)18 are found in the crystal unit cell. The calculated CD spectrum bears a close resemblance to that of Au28(SPh-t-Bu)20. This is the first time when two structurally characterized thiol-stabilized gold clusters are found to have such closely related metal core structures and the results may increase understanding of the formation of gold clusters when stabilized by bulky thiolates.

Au24(SAdm)16 nanomolecules: X-ray crystal structure, theoretical analysis, adaptability of adamantane ligands to form Au23(SAdm)16 and Au25(SAdm)16, and its relation to Au25(SR)18.

Here we present the crystal structure, experimental and theoretical characterization of a Au24(SAdm)16 nanomolecule. The composition was verified by X-ray crystallography and mass spectrometry, and its optical and electronic properties were investigated via experiments and first-principles calculations. Most importantly, the focus of this work is to demonstrate how the use of bulky thiolate ligands, such as adamantanethiol, versus the commonly studied phenylethanethiolate ligands leads to a great structural flexibility, where the metal core changes its shape from five-fold to crystalline-like motifs and can adapt to the formation of Au(24±1)(SAdm)16, namely, Au23(SAdm)16, Au24(SAdm)16, and Au25(SAdm)16. The basis for the construction of a thermodynamic phase diagram of Au nanomolecules in terms of ligands and solvent features is also outlined.

Neat and complete: thiolate-ligand exchange on a silver molecular nanoparticle.

Atomically precise thiolate-protected noble metal molecular nanoparticles are a promising class of model nanomaterials for catalysis, optoelectronics, and the bottom-up assembly of true molecular crystals. However, these applications have not fully materialized due to a lack of ligand exchange strategies that add functionality, but preserve the properties of these remarkable particles. Here we present a method for the rapid (<30 s) and complete thiolate-for-thiolate exchange of the highly sought after silver molecular nanoparticle [Ag44(SR)30](-4). Only by using this method were we able to preserve the precise nature of the particles and simultaneously replace the native ligands with ligands containing a variety of functional groups. Crucially, as a result of our method we were able to process the particles into smooth thin films, paving the way for their integration into solution-processed devices.

Super-stable, highly monodisperse plasmonic Faradaurate-500 nanocrystals with 500 gold atoms: Au(~500)(SR)(~120).

Determining the composition of plasmonic nanoparticles is challenging due to a lack of tools to accurately quantify the number of atoms within the particle. Mass spectrometry plays a significant role in determining the nanoparticle composition at the atomic level. Significant progress has been made in understanding ultrasmall gold nanoparticles such as Au25(SR)18 and Au38(SR)24, which have Au core diameters of 0.97 and 1.3 nm, respectively. However, progress in 2-5 nm-diameter small plasmonic nanoparticles is currently impeded, partially because of the challenges in synthesizing monodisperse nanoparticles. Here, we report a plasmonic nanocrystal that is highly monodisperse, with unprecedentedly small size variability. The composition of the superstable plasmonic nanocrystals at 115 kDa was determined as Au(500±10)SR(120±3). The Au(~500) system, named Faradaurate-500, is the largest system to be characterized using high resolution electrospray (ESI) mass spectrometry. Atomic pair distribution function (PDF) data indicate that the local atomic structure is consistent with a face-centered cubic (fcc) or Marks decahedral arrangement. High resolution scanning transmission electron microscopy (STEM) images show that the diameter is 2.4 ± 0.1 nm. The size and the shape of the molecular envelope measured by small-angle X-ray scattering (SAXS) confirms the STEM and PDF analysis.

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.

Enhanced single molecule mass spectrometry via charged metallic clusters.

Nanopore sensing is a label-free method for characterizing water-soluble molecules. The ability to accurately identify and characterize an analyte depends on the residence time of the molecule within the pore. It is shown here that when a Au25(SG)18 metallic cluster is bound to an α-hemolysin (αHL) nanopore, the mean residence time of polyethylene glycol (PEG) within the pore is increased by over 1 order of magnitude. This leads to an increase in the range of detectable PEG sizes and improves the peak resolution within the PEG-induced current blockade distribution. A model describing the relationship between the analyte residence time and the width of the peaks in the current blockade distribution is included. Finally, evidence is presented that shows the Coulombic interaction between the charged analyte and cluster plays an important role in the residence time enhancement, which suggests the cluster-based approach could be used to increase the residence time of a wide variety of charged analyte molecules.

Core size conversion: route for exclusive synthesis of Au38 or Au40 nanomolecules.

Gold nanomolecules with a precise number of gold atoms and ligands have promise for catalytic, optical, and biomedical applications. For practical applications, it is essential to develop synthetic protocols to prepare monodisperse gold nanomolecules. A typical synthesis yields a number of nanomolecules with discrete numbers of core atoms. Thermochemical treatment in the presence of excess thiol, etching, is known to narrow down the number of discrete nanomolecules, by selective degradation of sizes with lower stability. Au38(SR)24 and Au40(SR)24 are abundantly formed in these etching reactions due to their extraordinary stability to chemical etching. These nanomolecules are of high interest due in part to its stability, X-ray crystallographic structure availability (Au38), and intrinsic chirality arising from the arrangement of the Au-SR interface. However, the synthetic routes typically yield a mixture of Au38 and Au40, demanding extensive separation protocols. Here, we present a synthetic route to prepare either Au38 or Au40 exclusively in the product of etching. This was made possible by conducting a comprehensive mechanistic study starting from single-sized reactant. Au67 on etching yields Au40 exclusively. Au(103-105)(SR)(45-46) on etching also yields Au40 exclusively. Clusters of various sizes smaller than Au67 on etching yield Au38 exclusively. This is the first direct evidence for the exclusive formation of Au38 and Au40 nanomolecules by core size conversion. Mass spectrometry was used to study the core size conversion reactions to understand the mechanism. Au38 and Au40 nanomolecules form via different intermediates, as observed in the mass spectrometry data.