The mean size of the gold (Au) core in the synthesis of dodecanethiolate-stabilized Au cluster compounds can be finely adjusted by choice of the Au:dodecanethiolate ratio and the temperature and rate at which the reduction is conducted. The Au clusters have been examined with a large number of independent analytical tools, producing a remarkably consistent picture of these materials. Average cluster and core dimensions, as ascertained by 1H NMR line broadening, high-resolution transmission electron microscopy, small-angle X-ray scattering, and thermogravimetric analysis, vary between diameters of 1.5 and 5.2 nm (∼110−4800 Au atoms/core). The electronic properties of the Au core were examined by UV/vis and X-ray photoelectron spectroscopy; the core appears to remain largely metallic in nature even at the smallest core sizes examined. The alkanethiolate monolayer stabilizing the Au core ranges with core size from ∼53 to nearly 520 ligands/core, and was probed by Fourier transform infrared spectroscopy, differential scanning calorimetry, contact-angle measurements, and thermal desorption mass spectrometry. The dodecanethiolate monolayer on small and large core clusters exhibits discernable differences; the line dividing “3-dimensional” monolayers and those resembling self-assembled monolayers on flat Au (2-dimensional monolayers) occurs at clusters with ∼4.4 nm core diameters.

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Electronic conductivity, σEL, in solid-state films of alkanethiolate monolayer protected Au clusters (Au MPCs) occurs by a bimolecular, electron self-exchange reaction, whose rate constant is controlled by (a) the core-to-core tunneling of electronic charge along alkanethiolate chains and (b) the mixed valency of the MPC cores (e.g., a mixture of cores with different electronic charges). The tunneling mechanism is demonstrated by an exponential relation between the electronic conductivity of Au309(C n )92 MPCs (average composition) and n, the alkanethiolate chainlength, which varies from 4 to 16. The electron tunneling coefficient β n = 1.2/CH2 or, after accounting for alkanethiolate chain interdigitation, βdis = 0.8 Å-1. Quantized electrochemical double layer charging of low polydispersity Au140(C6)53 MPCs was used to prepare solutions containing well-defined mixtures of MPC core electronic charges (such as MPC0 mixed with MPC1+). Electronic conductivities of mixed-valent, solid-state Au140(C6)53 MPC films cast from such solutions are proportional to the concentration product [MPC0][MPC1+], and give a MPC0/1+ electron self-exchange rate constant of ca. 1010 M-1 s-1.

Rotated disk electrode voltammetry is described for CH2Cl2 solutions of cluster molecules with nanometer-sized gold cores and stabilizing ligand shells consisting of mixed monolayers of octanethiolate and ω-ferrocenyloctanethiolate ligands in molar ratios ranging from 2:1 to 24:1. Voltammograms for the cluster molecules exhibit a ferrocene oxidation wave with a limiting current that is under hydrodynamic mass transport control. The current−potential curves preceding (“prewave”) and following (“postwave”) the ferrocene wave, which are ideally flat, are decidedly sloped. The Δi/ΔE slopes are proportional to the square root of electrode rotation rate, i.e., are also under hydrodynamic control. The Δi/ΔE slopes are due to the charging of the electrical double layers of the cluster molecules, showing them to act as diffusing, molecule-sized “nanoelectrodes”. A theoretical analysis is presented of the transport control of the double layer charging. Possible reasons that the values of the cluster molecule capacitance (per unit surface area of cluster molecule, which entails use of models for the shape of the Au core of the cluster) are somewhat larger than the literature expectation for octanethiolate monolayers on flat gold surfaces are discussed. The tiny capacitances of the cluster molecules means that changing their charges by small potential increments can require an average of less than a single electron per cluster molecule.