Raman spectroscopy of benzenethiolates on nanometer-scale gold clusters

Near-infrared (1064-nm) irradiation of neat solid samples of benzenethiolate monolayer-protected gold clusters (MFCs) yields strong, well-resolved Raman spectra of the thiolate groups, comparable to those obtained for the same groups adsorbed at roughened gold electrodes. These clusters are formulated as TOA_Z[Au_N(SPh)_M]^Z-, N ≫ M and Z = 3-6, with core diameters of 1.7 and 1.5 nm, and were characterized previously by X-ray scattering, mass spectrometry, infrared spectroscopy, optical spectroscopy, nuclear magnetic resonance, and elemental analysis [Price, R. C.; Whetten, R. L. J. Am. Chem. Soc. 2005]. Numerous previous attempts to obtain spectra on various MFCs yielded only diffuse luminescence bands, as did benzenethiolate MFC samples of TOA ₂[Au₄₄(SPh)₂₈]^2-, with 1.1-nm core diameters. The clusters are free of excess tetraoctylammonium bromide (TOABr) from the synthetic procedure, containing only the necessary TOA⁺ to maintain charge balance. In situ thermometry, using the anti-Stokes/Stokes intensity ratios, indicated the sample temperature remained below the onset of thermal decomposition. The Raman spectra of the clusters bear a strong resemblance to those obtained for nonmetallic (Au^(I)SPh)_x polymer samples that are not in resonant absorption at the laser wavelength. The smaller of the two cores, nominally TOA₆[Au₁₁₀(SPh) ₆₂] shows clearly a band at 505 cm^-1 assigned to a S-S stretch, suggestive of a moiety resembling diphenyl disulfide on the cluster surface. These results are interpreted with reference to recent reports suggesting a substantial "reconstruction" of the outermost gold layer upon thiolate adsorption (SAM formation).