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Colloidal Gold Nanoparticles as Catalyst for Carbon-Carbon Bond Formation: Application to Aerobic Homocoupling of Phenylboronic Acid in Water
H. Tsunoyama, H. Sakurai, N. Ichikuni,Y. Negishi,
and T. Tsukuda*
Langmuir (Letters), 20, 11293-11296 (2004).
Gold nanoparticles (<2 nm) stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP NPs) were prepared by reduction of AuCl4− with NaBH4 in the presence of PVP and characterized via array of methods including optical absorption spectroscopy, TEM, XRD, XANES, EXAFS, and XPS. It is found for the first time that the Au:PVP NPs act as catalyst toward homocoupling reactions of phenylboronic acid in water under aerobic conditions. Suppression of the biphenyl formation under anaerobic condition indicates that molecular oxygen dissolved in water is intimately involved in the coupling reactions.
Magic-Numbered Aun Clusters Protected by Glutathione Monolayers (n=18, 21, 25, 28, 32, 39): Isolation and Spectroscopic Characterization
Y. Negishi, Y. Takasugi, S. Sato, H. Yao, K. Kimura, and T. Tsukuda*
J. Am. Chem. Soc. (communications), 126, 6518-6519 (2004).
Small gold clusters (≤1 nm) protected by glutathione (GSH) monolayer were fractionated into six components by polyacrylamide gel electrophoresis (PAGE) and their chemical compositions were investigated by electrospray ionization (ESI) mass spectroscopy. The results demonstrate isolation of a series of magic-numbered gold clusters, Au18(SG)11, Au21(SG)12, Au25(SG)14, Au28(SG)16, Au32(SG)18, and Au39(SG)23. Their optical absorption spectra are highly structured with clear absorption onsets, which shift toward higher energies with reduction of the core size. These molecular-like gold clusters exhibit visible photoluminescence. The results reported herein provide helpful guidelines or starting points for further experimental and theoretical studies on structures, stabilities and optical properties of small gold MPCs.
Structures and Stabilities of Alkanethiolate Monolayers on Palladium Clusters as Studied by Gel Permeation Chromatography
H. Murayama,T. Narushima,Y. Negishi, and T. Tsukuda*
J. Phys. Chem. B, 108, 3496-3503 (2004).
Palladium clusters protected by a series of n -alkanethiolates, Pd:SCn (SCn=n-CnH2n+1S, n=10, 12, 14, 16, and 18), were prepared by a ligand exchange approach: Pd clusters protected by poly (N-vinyl-2-pyrrolidone) (PVP) were transferred from aqueous phase to the toluene phase containing the thiols. The structures and stabilities of the thiolate monolayers of the Pd:SCn clusters were investigated by gel permeation chromatography (GPC) together with TEM, XPS, and FT-IR. The thicknesses of the thiolate layers formed on the Pd clusters were evaluated from the differences between the hydrodynamic diameters and core diameters of the Pd:SCn clusters, determined by GPC and TEM, respectively. The thicknesses thus obtained are in good agreement with the lengths of the corresponding thiolates in the all-trans conformations, illustrating that the alkanethiolates in nearly straight configurations are aligned almost perpendicularly to the core surfaces. Fractionation of the Pd:SC18 clusters by GPC yielded a series of the purified samples: the clusters in each fraction are different in their core sizes. The GPC measurements on the Pd:SCn clusters with small n revealed the decomposition of the monolayers through spontaneous etching and their reconstruction by heat treatment in the presence of the free thiols. The mechanism of these processes is discussed. The present study demonstrates that the GPC provides an elemental and versatile means to characterize and purify the monolayer-protected clusters.
Visible Photoluminescence from Nearly Monodispersed Au12 Clusters Protected by meso-2,3-Dimercaptosuccinic Acid
Y. Negishi and T. Tsukuda*
Chem. Phys. Lett., 383, 161-165 (2004).
Nearly monodispersed Au12 clusters protected by monolayers of meso-2,3-dimercaptosuccinic acid (DMSA) exhibited luminescence at 630 nm with a quantum yield of 1x10-6 upon the photoexcitation at 395 nm to the lowest electronic excited state. The visible PL is assigned to phosphorescence originating from a triplet like excited state based on the large Stokes shift (1.2 eV). The PL quantum yield was enhanced up to ≈0.9%, greater by 8 orders of magnitude than that of bulk gold, by thickening the protecting layer and freezing a solvent at 77 K. The emission peak energy is discussed within the context of core-size dependent electronic structures by comparison with those of thiolate-protected gold clusters reported in the literature.
Highly Oxygenated Fullurene Anions C60On− Formed by Corona Discharge Ionization in the Gas Phase
H. Tanaka*, K. Takeuchi, Y. Negishi, and T. Tsukuda
Chem. Phys. Lett., 384, 283-287 (2004).
Oxygenated fullerenes anions were produced by a vaporization source equipped with a corona discharge ionizer in the presence of a trace amount of oxygen. In situ mass analysis revealed that the species formulated as C60On− (n⩽30) were formed in the source and that the degree of oxygenation could be altered by the discharge current. Formation of the epoxide structure in the C60On− was suggested by XPS measurements for thin films prepared by deposition of the C60On− beam. The structures and formation processes of higher analogues (C60)mOn− (m=2, 3) are briefly discussed.