The optical absorption spectra of a series of nanocrystal gold moleculesslarger, crystalline Au clusters that are passivated by a compact monolayer of n-alkylthiol(ate)sshave been measured across the electronic range (1.1-4.0 eV) in dilute solution at ordinary temperature. Each of the ∼20 samples, ranging in effective core diameter from 1.4 to 3.2 nm (∼70 to ∼800 Au atoms), has been purified by fractional crystallization and has undergone a separate compositional and structural characterization by mass spectrometry and X-ray diffraction. With decreasing core mass (crystallite size) the spectra uniformly show a systematic evolution, specifically (i) a broadening of the so-called surface-plasmon band until it is essentially unidentifiable for crystallites of less than 2.0 nm effective diameter, (ii) the emergence of a distinct onset for strong absorption near the energy (∼1.7 eV) of the interbandgap (5d f 6sp), and (iii) the appearance in the smallest crystallites of a weak steplike structure above this onset, which is interpreted as arising from a series of transitions from the continuum d-band to the discrete level structure of the conduction band just above the Fermi level. The classical electrodynamic (Mie) theory, based on bulk optical properties, can reproduce this spectral evolutionsand thereby yield a consistent core-sizingsonly by making a strong assumption about the surface chemical interaction. Quantitative agreement with the spectral line shape requires a size-dependent offset of the frequency-dependent dielectric function, which may be explained by a transition in electronic structure just below 2.0 nm (∼200 atoms), as proposed earlier.

An unprecedented small thioaurite cluster compound (with metallic Au0 core) has been isolated in high yield by decomposition of polymeric Au(I)SG compounds, where GSH is the ubiquitous tripeptide glutathione, N-γ-glutamyl-cysteinyl-glycine. The Au:SG clusters appear to share the high stability and robustness of their hydrophobic n-alkyl analogues but are highly water soluble. The most abundant cluster produced by these methods can be easily separated from its homologues by gel electrophoresis. Its total molecular weight is ca. 10.4 kDa, and the mass of its strongly bound inorganic core is 5.6 kDa, suggesting the composition Au28(SG)16. This composition is also consistent with the X-ray diffraction pattern of the crystalline molecular solid. Distinct features in the optical absorption spectroscopy are inherently different from either larger clusters or smaller gold cluster compounds. The compound is optically active, as evidenced by circular dichroism in the near-IR, visible, and near-UV regions. The 13C NMR spectra suggest that the bonding environment of the GS−adsorbate is similar to that of the n-alkyl−adsorbate clusters, and the nonsulfhydryl properties are retained. The cluster is thus envisioned as a large metallic-cluster compound with distinctive optical properties encapsulated by a bioactive peptide monolayer.

Five massive gold-cluster molecules have been isolated in high yield and have undergone separate structural characterization, and their electronic structure has been deduced by optical absorption spectroscopy. These new molecules are distinguished by a crystalline (or quasicrystalline) core of densely packed Au atoms, ranging in size from ∼1.1 nm (∼40 atoms) to ∼1.9 nm (∼200 atoms), surrounded by a compact monolayer of various thio (RS) adsorbates. They are obtained as the thermally and environmentally stable products of the reductive decomposition of nonmetallic (−AuS(R)−) polymer in solution, are separated according to size by fractional crystallization or column chromatography, as monitored by high-mass spectrometry, and are characterized structurally by methods including X-ray diffraction (small and large angle), high-resolution electron microscopy, and scanning tunneling microscopy. The optical absorption spectra of dilute solutions of these molecules show size-dependent steplike structure with an onset near the fcc Au interband edge (Δ = 1.7), indicative of transitions to the discrete lowest unoccupied levels of the conduction band. This structure is evident in the smallest clusters even at room temperature, is enhanced at low temperature, and emerges generally as predicted by Kubo's criterion for quantum size effects. It thus requires no assumption of a transition from the bulk metallic bonding character to a nonmetallic (rehybridized or oxidized) state.

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Atomistic modeling of energetics and structures, coupled with x-ray powder diffraction analyses of size-separated passivated gold nanocrystals in the 1-2 nm size range, shows preferential formation of a stable sequence of three cluster sizes, all with a truncated-decahedral motif. [S0031-9007 (97)03951-3] PACS numbers: 61.46. + w, 36.40.Mr