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.

(NMR) analysis, using the tris [3-(trifluoromethylhydroxymethylene)-(ϩ)camphorato]europium(III) complex. The compound was hydrolyzed to the corresponding acid and then the optical purity was enhanced up to 97% ee by cocrystallization with commercially available R-PEA. The optical purity was checked by NMR and circular dichroism spectra:[␣] D 25 ϭ Ϫ63.2 (C ϭ 1.01 g/liter tetrahydrofuran). 5. The isotherms were measured from A ϭ 100 to 5 Å 2

No abstract

letters to nature 368 NATURE | VOL 389 | 25 SEPTEMBER 1997 membrane lipid/water systems 19 . It has been suggested that two lipid bilayers transform through an inverted micellar intermediate to a cubic mesophase 20 . We propose that the 3-dH film is derived from the parent cubic film by constrained one-dimensional shrinkage during calcination 3 . The cubic → 3-dH transformation can be considered as a constrained one-dimensional distortion of the cubic cell along the [110]-direction. Comparable lattice parameters between the two mesophases allow seamless transitions between ordered domains and cause cubic and 3-dH domains to be essentially indistinguishable in plan-view (Fig. 4c).A surface acoustic wave technique 21 was used to determine pore accessibility of supported films. Figure 5 compares nitrogen sorption isotherms of the incipient 1-dH and cubic 3-dH films. Despite the substantially different degrees of ordering, the isotherms are qualitatively similar. The lack of hysteresis and absence of any appreciable adsorption at relative pressures above 0.3 is consistent with a unimodal porosity with no interparticle meso-or macroporosity 22 . The surface areas calculated for the cubic 3-dH and incipient 1-dH films are 734 and 648 m 2 g −1 , respectively, demonstrating the accessibility of the mesophase porosity. In addition, the trans-film flux of cubic 3-dH films prepared as supported membranes increased by over 1,000× upon calcination, establishing through-thickness pore connectivity (A. Tsai, unpublished results).We have demonstrated a rapid, continuous process, enabling the practical utilization of mesostructures in thin-film form. The uniform three-dimensional pore channel systems of cubic 3-dH films and the absence of granularity suggest applications in molecular separation, catalysis and sensors. The dip-coating procedure combined with the optical probe technique enables us to follow the progressive evolution of the mesostructured films and should provide insight into the synthesis of complex, self-organized organic/inorganic assemblies in general 23 . Ⅺ