Monolayer protected clusters: synthesis, electrochemistry, ligand exchange kinetics and optical properties

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  • March 21, 2019
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  • Guo, Rui
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Chapter One is an introduction to fundamental properties of Monolayer- Protected Gold Clusters (Gold MPCs) including their synthesis, composition and structure, electrochemistry, ligand exchange mechanism and optical properties. Chapter Two investigates medium effects (supporting electrolyte concentration, type and solvents) on the quantized double layer (QDL) charging capacitance of hexanethiolate coated gold cluster Au140(SC6)53. The dependence of delta V ( e/CMPC ) on the concentration of supporting electrolyte (from 1 to 100 mM), measured using square wave voltammetry, is shown to be caused, primarily, by changes in the diffuse double layer component (CDIFFUSE) of CMPC. A numerical simulation was used to calculate CDIFFUSE successfully. Additionally, significant changes in the magnitude of the compact double-layer component (CCOMPACT ) of CMPC were induced by adding hydrophobic solvent components such as hexane or dodecane or by introducing hydrophobic electrolyte ions (tetrabutyl-, tetrahexyl-, and tetraoctylammonium, perchlorate and tetra-phenylborate). Chapter Three describes the effects of supporting electrolyte concentration, temperature and solvent environment on the capacitance of molecule-like phenylethanethiolate coated gold clusters Au38(SC2Ph)24 at +1 core charge state with square iv wave voltammetry (SWV), differential pulse voltammetry (DPV). The effects are interpreted with both the classical double layer theory treating the two continuous oxidation peaks as quantized double layer (QDL) charging peaks of a monolayer protected gold cluster (MPC) and the concept of "molecular capacitance" treating them as a succession of oxidization peaks of a molecule. Chapter Four compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S-) on the monolayer-protected clusters (MPCs) Au38(SC2Ph)24 and Au140(SC2Ph)53 with rho-substituted arylthiols (rho-X-PhSH), where X = NO2, Br, CH3, OCH3, and OH at 293 K. It was found that second-order rate constants for ligand exchange on Au38(SC2Ph)24 are very close to those of similar exchange reactions on the larger nanoparticle Au140(SC2Ph)53 MPCs indicating vertex site reactivity of these two nanoparticles are ca. the same. However, their ligand exchange extent is different. The reverse exchange reaction was also studied for Au38(rho-X-arylthiolate)24 MPCs (X = NO2, Br, and CH3), where the in-coming ligand is phenylethanethiol. Chapter Five investigates a molecule-like substituent effect on redox formal potentials in the nanoparticle series Au38(SPhX)24. Electron-withdrawing "X" substituents energetically favor reduction and disfavor oxidation, and give formal potentials that correlate with Hammett substituent constants. The ligand monolayer of the nanoparticles is shown thereby to play a strong role in determining electronic energies of the nanoparticle core, and is more than simply a protecting or capping layer. The substituent effect does not, however, detectably change the homo-lumo gap energy. Chapter Six investigates the ligand dependent optical properties of Au38(SC2Ph)24 upon ligand exchange with different in-coming thiols in THF. It was found that the v luminescence of Au38(SC2Ph)24 was enhanced more when more polar thiolate ligands were exchanged. What is more, the luminescence is linearly correlated with the number of incoming ligands exchanged onto the gold core indicating possible existence of localized chemical states of the gold core. Solvent effects on the second order rate constants of ligand exchange reaction were also observed. Chapter Seven describes the synthesis and characterization of ligand exchange product of Au55(PPh3)12Cl6 with pentafluorobenzenethiol. The exchange product was characterized by electrochemistry, TGA, TEM, HPLC, UV- vis, Fluorescence, 1H and 19F NMR spectroscopy.
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  • Murray, Royce W.
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  • October 19, 2010

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