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.

No abstract

Electron hopping in films of arenethiolate (benzylthiolate, phenylethylthiolate, phenylbutanethiolate, and cresolthiolate) monolayer-protected cluster molecules (MPCs) is investigated through measurements of solid-state electronic conductivity. Electron donor−acceptor hopping rates between the Au cores of arenethiolate MPCs exceed those of previously studied solid-state alkanethiolate MPC films, but the electronic coupling term, β = 0.8 Å-1, is nearly the same. Rate constants range from 108 to 1011 s-1 across the series of arenethiolate MPCs; for the case of cresolthiolate, the rate corresponds to a single molecule resistance of ∼107 Ω/cresolthiolate ligand. The 4−8 kJ/mol activation-barrier energies for electron hopping are generally in line with the Marcus theory prediction. The low barrier energies and large rate constants arise from a combination of the low dielectric medium environment of the reactants (the MPC cores) and the partly aromatic tunneling bridges. The sharp increase in film conductivity upon substituting arenethiolate ligands for >50% of the hexanethiolates on a hexanethiolate-protected MPC suggests a percolation effect.

NMR spectroscopy and computer modeling were used to characterize tiopronin monolayer-protected gold clusters (MPCs). These MPCs contain gold cores with a distribution of radii ranging from 0.4 to 2.6 nm. NOESY and HMQC spectra yielded assignments for all NMR sensitive nuclei in the tiopronin ligands. DOSY and T 2 experiments provided information about the particle size distribution as a function of proton frequency shift. Further information was obtained from hole-burning and amide-exchange experiments. The spectroscopic data reveal two classes of ligands, a network of hydrogen bonds, and considerable inhomogeneous and homogeneous line broadening. The methyl and methine protons clearly exhibit two components with separations that decrease strongly with the number of bonds separating the proton from the gold core. Spin−echo experiments clearly show that a range of T 2 values is associated with each resonance frequency in both the upfield and downfield components for each type of proton but that the most probable value is larger for the upfield component. Various models that may be consistent with the NMR data and the properties of reported crystal structures were considered. It is suggested that bimodal frequency distributions result from chemical shifts that are associated with a mixture of primarily two gold cluster structure types that differ in the mode of core packing. It is suggested that the Knight shift contributes to the large downfield shift observed for the methine protons in the larger particles.

Monolayer-protected clusters (MPCs) are used to prepare solid, continuous metal films containing a single white metal or an alloy thereof. MPCs consist of nanoscopic metal cores coated with monolayers of thiolate ligands. In one method, multilayer films of carboxylate-functionalized alkanethiolate MPCs are assembled using Cu2+ coordinative bridges. The MPC film can be thermally decomposed at moderate temperature (<350 °C) to produce films of the core metal; the thiolate ligands escape as volatile disulfides. In another method, solutions of MPCs with alkanethiolate monolayers are cast or painted onto substrates, followed again by thermolysis to produce films of core metals. Films prepared from MPCs having metal alloy cores tend to exhibit metal segregation. A third method uses electrochemical generation of iodide at a Pt electrode to destabilize the MPC monolayer, an action that effectively coats the electrode with Au. The metal films are analyzed by stylus profilometry, atomic force microscopy, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and electrochemistry.