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Showing papers in "Journal of the American Chemical Society in 2001"


Journal ArticleDOI
TL;DR: This paper proves that Cd(CH3)2 can be replaced by CdO and develops a one-pot synthesis which does not require separated preparation of cadmium complex and is reproducible and simple and thus can be readily scaled up for industrial production.
Abstract: High-quality colloidal semiconductor nanocrystals are nanometer-sized, single crystalline fragments of the corresponding bulk crystals, which have well-controlled size and size distribution and are dispersible in desired solvents/media. Recently, semiconductor nanocrystals are of great interest for both fundamental research and technical applications, 1-8 due to their strong size dependent properties and excellent chemical processibility. Synthesis of highquality semiconductor nanocrystals has been playing a critical role in this very active field. 1,9-15 As the most developed system in terms of synthesis, 1,9,10,15high-quality CdSe nanocrystals with nearly monodisperse size and shape are in active industrial development for biological labeling reagents. 5,6 Since Murray et al. 15 reported the synthesis of high quality cadmium chalcogenides nanocrystals using dimethyl cadmium (Cd(CH 3)2) as the cadmium precursor, the synthesis of CdSe nanocrystals using this precursor has been well developed. 1,9,10In comparison, the synthesis of CdTe and CdS15,16are not as advanced. For instance, there is no method to controllably vary the shape of CdTe and CdS nanocrystals. Cd(CH3)2 is extremely toxic, pyrophoric, expensive, unstable at room temperature, and explosive at elevated temperatures by releasing large amount of gas. Due to these reasons, the Cd(CH 3)2related schemes require very restricted equipments and conditions and are not suited for large-scale synthesis. In this paper, we will prove that Cd(CH3)2 can be replaced by CdO. Surprisingly, this new synthetic scheme works significantly better than the Cd(CH3)2-related ones. Without any size-sorting, the quality of quantum-confined dots and rods (quantum dots and quantum rods) of all cadmium chalcognides formed by the new method is comparable to that of the best CdSe nanocrystals reported in the literature. The new scheme is reproducible and simple and thus can be readily scaled up for industrial production. Recently, we identified that Cd(CH 3)2 decomposes in hot trioctylphosphine oxide (TOPO) and generates insoluble metallic precipitate. 9 With a strong ligand, either hexylphosphonic acid (HPA) or tetradecylphosphonic acid (TDPA), Cd(CH 3)2 is immediately converted into cadmium HPA/TDPA complex (Cd HPA/Cd-TDPA) if the cadmium to HPA/TDPA ratio is lower than 1. After the formation of the complex, an injection of Se dissolved in tributylphosphine (TBP) generates high-quality CdSe nanocrystals. This result implies that Cd(CH 3)2 may not be necessary, if we can generate the complex by other means. We first synthesized and purified Cd -HPA from CdCl 2 or Cd(CH3)2. High-quality CdSe nanocrystals were indeed yielded from this complex. This success encouraged us to develop a one-pot synthesis which does not require separated preparation of cadmium complex. We failed to make high-quality CdSe nanocrystals using CdCl2 by the one-pot approach although CdCl 2 can be dissolved in the reaction mixture at elevated temperatures. In contrast, CdO works very well for the one-pot approach. We think this is due to the low stability of CdO relative to phosphonic acids, compared to that of CdCl 2. Experimentally, CdO, TOPO, and HPA/TDPA were loaded in a three-neck flask. At about 300 °C, reddish CdO powder was dissolved and generated a colorless homogeneous solution. Introducing tellurium, selenium, and sulfur stock solutions yields high quality nanocrystals. 17 The samples for all of the measurements shown in this paper are directly from synthesis without any size separation. The growth kinetics of nanocrystals grown by the new approach possesses a pattern similar to that of the best CdSe nanocrystals formed by the Cd(CH3)2 approach (Figure 1). 10 Figure 1 and Figure 2 further reveal that the size of all three kinds of nanocrystals can be close to monodisperse, represented by the sharp absorption peaks if the growth stops in the “focusing of size distribution” regime. 10 Transmission electron microscopy (TEM) measurements indicate that these nanocrystals have very narrow distribution. The relative standard deviation of the size of the nanocrystals shown in Figure 3 (top) is about 10%. The high crystallinity of these wurtzite nanocrystals was confirmed by X-ray powder diffraction. For this CdO approach, the size of relatively monodisperse CdSe nanocrystals can be continuously tuned down to the sizes with the first absorption peak at 440 nm (see the first absorption spectrum in Figure 1). Relatively monodisperse CdSe nanocrystals with the first exciton absorption peak below 480 nm are difficult to synthesize directly with the existing Cd(CH 3)2-related approach. 10,18 (1) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature2000, 404, 59-61. (2) Heath, J. R. (editor). Acc.f Chem. Res. 1999. (3) Alivisatos, A. P.Science1996, 271, 933-937. (4) Huynh, W.; Peng, X.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923927. (5) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (6) Chan, W. C. W.; Nie, S. M. Science1998, 281, 2016-2018. (7) Schlamp, M. C.; Peng, X. G.; Alivisatos, A. P. J. Appl. Phys.1997, 82, 5837-5842. (8) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J. Appl. Phys.1998, 83, 7965-7974. (9) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. , in revision. (10) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344. (11) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J Am. Chem. Soc. 1993, 115, 8706-8715. (12) Nozik, A. J.; Micic, O. I.MRS Bull.1998, 23, 24-30. (13) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019-7029. (14) Dabbousi, B. O.; RodriguezViejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475. (15) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 76657673. (16) Mikulee, F.; Ph.D. Thesis, MIT, Boston, 1998. (17) A typical synthesis for CdTe nanocrystals: 0.0514 g of CdO, 0.2232 g of TDPA and 3.7768 g of TOPO were loaded into a 25 mL flask. The mixture was heated to 300 -320 °C under Ar flow, and CdO was dissolved in TDPA and TOPO. The temperature of the solution was cooled to 270 °C, tellurium stock solution (0.0664 g of tellurium powder dissolved i n 2 g of TOP) was injected. After injection, nanocrystals grew at 250 °C to reach desired size. (18) Bawendi, M. G. Private communication. Figure 1. Temporal evolution of size and size distribution of CdTe, CdSe, and CdS nanocrystals studied by UV -vis. 183 J. Am. Chem. Soc. 2001,123,183-184

2,664 citations


Journal ArticleDOI
TL;DR: The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported, and three of the C(**)N2Ir(acac) complexes were used as dopants for organic light emitting diodes (OLEDs) and show some of the highest efficiencies reported for organic Light Emitting Diodes.
Abstract: The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported. The iridium complexes have two cyclometalated (C∧N) ligands and a single monoanionic, bidentate ancillary ligand (LX), i.e., C∧N2Ir(LX). The C∧N ligands can be any of a wide variety of organometallic ligands. The LX ligands used for this study were all β-diketonates, with the major emphasis placed on acetylacetonate (acac) complexes. The majority of the C∧N2Ir(acac) complexes phosphoresce with high quantum efficiencies (solution quantum yields, 0.1−0.6), and microsecond lifetimes (e.g., 1−14 μs). The strongly allowed phosphorescence in these complexes is the result of significant spin−orbit coupling of the Ir center. The lowest energy (emissive) excited state in these C∧N2Ir(acac) complexes is a mixture of 3MLCT and 3(π−π*) states. By choosing the appropriate C∧N ligand, C∧N2Ir(acac) complexes can be prepared which emit in any color from green to red. Simple, systematic changes in the C∧N ligands, whic...

2,655 citations


Journal ArticleDOI
TL;DR: Single-walled carbon nanotubes are molecular wires that exhibit interesting structural, mechanical, electrical, and electromechanical properties that make for an ideal miniaturized sensor.
Abstract: Single-walled carbon nanotubes (SWNTs) are molecular wires that exhibit interesting structural, mechanical, electrical, and electromechanical properties. 1-3 A SWNT is unique among solidstate materials in that every atom is on the surface. Surface chemistry could therefore be critical to the physical properties of SWNTs and their applications. 3-10 SWNT sidewall functionalization is important to soluble nanotubes, 4-6 self-assembly on surfaces, and chemical sensors. 8-10 For these purposes, it is imperative to functionalize the sidewalls of SWNTs in noncovalent ways to preserve the sp 2 nanotube structure and thus their electronic characteristics. Immobilization of biomolecules on carbon nanotubes has been pursued in the past, motivated by the prospects of using nanotubes as new types of biosensor materials. 11-15 The electronic properties of nanotubes coupled with the specific recognition properties of the immobilized biosystems would indeed make for an ideal miniaturized sensor. A prerequisite for research in this area is the development of chemical methods to immobilize biological molecules onto carbon nanotubes in a reliable manner. Thus far, only limited work has been carried out with multiwalled carbon nanotubes (MWNTs). 11-15 Metallothionein proteins were trapped inside and placed onto the outer surfaces of open-ended MWNTs.11-14 Streptavidin was found to adsorb on MWNTs presumably via hydrophobic interactions between the nanotubes and hydrophobic domains of the proteins. 15 DNA molecules adsorbed on MWNTs via nonspecific interactions were also observed. 12-14

2,516 citations


Journal ArticleDOI
TL;DR: The black dye, when anchored to nanocrystalline TiO(2) films achieves very efficient sensitization over the whole visible range extending into the near-IR region up to 920 nm, yielding over 80% incident photon-to-current efficiencies (IPCE).
Abstract: A new series of panchromatic ruthenium(II) sensitizers derived from carboxylated terpyridyl complexes of tris-thiocyanato Ru(II) have been developed. Black dye containing different degrees of protonation {(C2H5)3NH}[Ru(H3tcterpy)(NCS)3] 1, {(C4H9)4N}2[Ru(H2tcterpy)(NCS)3] 2, {(C4H9)4N}3[Ru(Htcterpy)(NCS)3] 3, and {(C4H9)4N}4[Ru(tcterpy)(NCS)3] 4 (tcterpy = 4,4‘,4‘ ‘-tricarboxy-2,2‘:6‘,2‘ ‘-terpyridine) have been synthesized and fully characterized by UV−vis, emission, IR, Raman, NMR, cyclic voltammetry, and X-ray diffraction studies. The crystal structure of complex 2 confirms the presence of a RuIIN6 central core derived from the terpyridine ligand and three N-bonded thiocyanates. Intermolecular H-bonding between carboxylates on neighboring terpyridines gives rise to 2-D H-bonded arrays. The absorption and emission maxima of the black dye show a bathochromic shift with decreasing pH and exhibit pH-dependent excited-state lifetimes. The red-shift of the emission maxima is due to better π-acceptor properti...

2,477 citations


Journal ArticleDOI
TL;DR: High-temperature (300 degrees C) aging of iron-oleic acid metal complex, which was prepared by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100 degrees C, was found to generate monodisperse iron nanoparticles.
Abstract: The synthesis of highly crystalline and monodisperse γ-Fe2O3 nanocrystallites is reported. High-temperature (300 °C) aging of iron−oleic acid metal complex, which was prepared by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100 °C, was found to generate monodisperse iron nanoparticles. The resulting iron nanoparticles were transformed to monodisperse γ-Fe2O3 nanocrystallites by controlled oxidation by using trimethylamine oxide as a mild oxidant. Particle size can be varied from 4 to 16 nm by controlling the experimental parameters. Transmission electron microscopic images of the particles showed 2-dimensional and 3-dimensional assembly of particles, demonstrating the uniformity of these nanoparticles. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the γ-Fe2O3 structures. Monodisperse γ-Fe2O3 nanocrystallites with a particle size of 13 nm also can be...

1,893 citations


Journal ArticleDOI
TL;DR: A procedure based on density functional theory is used for the calculation of the gas-phase bond dissociation enthalpy (BDE) and ionization potential for molecules belonging to the class of phenolic antioxidants, and it is concluded that in most cases H-atom transfer will be dominant.
Abstract: A procedure based on density functional theory is used for the calculation of the gas-phase bond dissociation enthalpy (BDE) and ionization potential for molecules belonging to the class of phenolic antioxidants. We show that use of locally dense basis sets (LDBS) vs full basis sets gives very similar results for monosubstituted phenols, and that the LDBS procedure gives good agreement with the change in experimental BDE values for highly substituted phenols in benzene solvent. Procedures for estimating the O--H BDE based on group additivity rules are given and tested. Several interesting classes of phenolic antioxidants are studied with these methods, including commercial antioxidants used as food additives, compounds related to Vitamin E, flavonoids in tea, aminophenols, stilbenes related to resveratrol, and sterically hindered phenols. On the basis of these results we are able to interpret relative rates for the reaction of antioxidants with free radicals, including a comparison of both H-atom-transfer and single-electron-transfer mechanisms, and conclude that in most cases H-atom transfer will be dominant.

1,407 citations


Journal ArticleDOI
TL;DR: Nanotubes derivatized with a 4-tert-butylbenzene moiety were found to possess significantly improved solubility in organic solvents and represents the marriage of wire-like nanotubes with molecular electronic devices.
Abstract: Small-diameter (ca. 0.7 nm) single-wall carbon nanotubes are predicted to display enhanced reactivity relative to larger-diameter nanotubes due to increased curvature strain. The derivatization of these small-diameter nanotubes via electrochemical reduction of a variety of aryl diazonium salts is described. The estimated degree of functionalization is as high as one out of every 20 carbons in the nanotubes bearing a functionalized moiety. The functionalizing moieties can be removed by heating in an argon atmosphere. Nanotubes derivatized with a 4-tert-butylbenzene moiety were found to possess significantly improved solubility in organic solvents. Functionalization of the nanotubes with a molecular system that has exhibited switching and memory behavior is shown. This represents the marriage of wire-like nanotubes with molecular electronic devices.

1,390 citations


Journal ArticleDOI
TL;DR: In this article, the shape evolution of CdSe quantum confined nanorods (quantum rods) in nonaqueous solvents with organometallic precursors was studied quantitatively and systematically.
Abstract: The temporal shape evolution of CdSe quantum confined nanorods (quantum rods) in nonaqueous solvents with organometallic precursors was studied quantitatively and systematically. The experimental results revealed three distinguishable stages in the shape evolution. At high monomer concentrations, nanocrystals grow exclusively along the c-axis of the wurtzite structure, making this axis the long axis of the rods. At intermediate concentrations, nanocrystals grow simultaneously in three dimensions. At low monomer concentrations, the aspect ratio decreases in a process controlled by intraparticle diffusion on the surface of the nanocrystals. This intraparticle ripening stage is different from normal Ostwald ripening, which occurs at lower monomer concentrations and is by monomer migration from small to larger ones. Addition of hexylphosphonic acid or tetradecylphosphonic acid, strong cadmium ligands, is important mainly because it enables the high monomer concentrations needed for the growth of quantum rods....

1,249 citations


Journal ArticleDOI
TL;DR: The preparation of two functionalized pentacene derivatives, and the effect of this functionalization on both the solid-state ordering and the electronic properties of the resulting crystals is reported.
Abstract: Molecular order has proven to be a significant factor in the performance of devices based on organic semiconductors. Recent studies involving solubilized versus unsubstituted thiophene oligomers have demonstrated that modifications which increase orbital overlap in the solid state can improve device performance by more than an order of magnitude. 1 Similar studies on pentacene, a compound which has already demonstrated remarkable potential for device applications, 2 have also focused on maximizing orbital overlap by inducing order in films. 3 However, these pentacene studies have thus far relied on substrate modification, rather than on pentacene functionalization, 4 to achieve the desired goals. We report here the preparation of two functionalized pentacene derivatives, and the effect of this functionalization on both the solid-state ordering and the electronic properties of the resulting crystals. Our goal for a functionalized pentacene was two-fold: First, the substituents should impart solubility to the acene, to simplify purification and processing. Second, the substituents should induce some capability for self-assembly of the aromatic moieties into ﷿-stacked arrays to enhance intermolecular orbital overlap. We anticipated that both of these goals could be accomplished by exploiting a rigid spacer to hold the necessarily bulky solubilizing groups well away from the aromatic core, allowing the closest possible contact between the aromatic rings. 5 Our initial targets were the bis(triisopropylsilylethynyl)pentacenes 1 and 2. Both of these compounds are easily prepared in near quantitative yield in a one-pot reaction from 6,13-pentacenequinone and 5,14pentacenequinone, respectively. 6

1,211 citations


Journal ArticleDOI
TL;DR: The substituents on the beta-diiminate ligand exert a significant influence upon the course of the polymerizations, affecting both the degree of stereoselectivity and the rate of polymerization.
Abstract: A series of zinc(II) and magnesium(II) alkoxides based upon a β-diiminate ligand framework has been prepared. [(BDI-1)ZnOiPr]2 [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)...

1,088 citations


Journal ArticleDOI
TL;DR: Experimental results clearly indicated that the initiation stage of the photooxidation of CdSe nanocrystals was not caused by the chemical oxidation of the system kept in air under dark conditions or the hydrolysis of the cadmium-thiol bonds on the surface of the nanocrystal, both of which were magnitudes slower than the photocatalytic oxidization of the surface ligands if they occurred at all.
Abstract: The photochemical instability of CdSe nanocrystals coated by hydrophilic thiols was studied nondestructively and systematically in water. The results revealed that the photochemical instability of the nanocrystals actually included three distinguishable processes, namely the photocatalytic oxidation of the thiol ligands on the surface of nanocrystals, the photooxidation of the nanocrystals, and the precipitation of the nanocrystals. At first, the thiol ligands on the surface of a nanocrystal were gradually photocatalytically oxidized using the CdSe nanocrystal core as the photocatalyst. This photocatalytic oxidation process was observed as a zero-order reaction in terms of the concentration of the free thiols in the solution. The photogenerated holes in a nanocrystal were trapped onto the thiol ligands bound on the surface of the nanocrystal, which initiated the photooxidation of the ligands and protected the nanocrystal from any photooxidation. After nearly all of the thiol ligands on the surface of the ...

Journal ArticleDOI
TL;DR: Analysis of STD NMR experiments performed under competitive conditions proved that the two saccharides studied bind at the same receptor site, thereby ruling out unspecific binding.
Abstract: A protocol based on saturation transfer difference (STD) NMR spectra was developed to characterize the binding interactions at an atom level, termed group epitope mapping (GEM). As an example we chose the well-studied system of galactose binding to the 120-kDa lectin Ricinus communis agglutinin I (RCA120). As ligands we used methyl β-d-galactoside and a biantennary decasaccharide. Analysis of the saturation transfer effects of methyl β-d-galactoside showed that the H2, H3, and H4 protons are saturated to the highest degree, giving evidence of their close proximity to protons of the RCA120 lectin. The direct interaction of the lectin with this region of the galactose is in excellent agreement with results obtained from the analysis of the binding specificities of many chemically modified galactose derivatives (Bhattacharyya, L.; Brewer, C. F. Eur. J. Biochem. 1988, 176, 207−212). This new NMR technique can identify the binding epitope of even complex ligands very quickly, which is a great improvement over ...

Journal ArticleDOI
TL;DR: In this paper, the optical properties of Ag nanoparticles chemically modified with alkanethiol self-assembled monolayers (SAMs) were explored by measuring the localized surface plasmon resonance (LSPR) spectrum using UV−vis extinction spectroscopy.
Abstract: In this paper, we explore the optical properties of Ag nanoparticles chemically modified with alkanethiol self-assembled monolayers (SAMs) by measuring the localized surface plasmon resonance (LSPR) spectrum using UV−vis extinction spectroscopy. For all the experiments presented here, the Ag nanoparticles were fabricated using the technique of nanosphere lithography (NSL) and had in-plane widths of 100 nm and out-of-plane heights of 50 nm. We first demonstrate that unmodified nanoparticles are extremely susceptible to slight changes in 3-dimensional structure when exposed to various solvents. These structural effects can have dramatic effects on the extinction maximum, λmax, of the LSPR shifting it to the blue by over 100 nm. The significant discovery reported here is that λmax for NSL fabricated Ag nanoparticles is extremely sensitive to the SAM properties. We will demonstrate the following new features: (1) λmax of the LSPR linearly shifts to the red 3 nm for every carbon atom in the alkane chain; (2) ...

Journal ArticleDOI
TL;DR: In this paper, a growth mechanism was proposed based on the vapor-liquid-solid (VLS) mechanism which was proposed in the 1960s -1970s for large whisker growth, 17-19 although an oxide assisted growth mechanism has also been proposed.
Abstract: Nanotubes and semiconductor nanowires are of fundamental importance to the study of sizeand dimensionality-dependent chemical and physical phenomena. 1,2 How to rationally synthesize these 1-dimensional nanostructures has been a major challenge, although several strategies have been pursued recently. 3-16 For example, carbon nanotubes have been prepared via condensation of hot carbon plasmas in the presence of certain metals, although the real growth mechanism has been elusive. 3-5 Recently, semiconductor nanowires with different compositions have been successfully synthesized using either vapor 6-12 or solution-based methodologies. 13-16 One key feature of these syntheses is the promotion of anisotropic crystal growth using metal nanoparticles as catalysts. The growth mechanism has been extrapolated from the vapor -liquid-solid (VLS) mechanism which was proposed in the 1960s -1970s for large whisker growth, 17-19 although an oxide-assisted growth mechanism has also been proposed. 2,20


Journal ArticleDOI
TL;DR: Direct asymmetric catalytic aldol reactions have been successfully performed using aldehydes and unmodified ketones together with commercially available chiral cyclic secondary amines as catalysts and the observed stereochemistry of the products is in accordance with the proposed transition state.
Abstract: Direct asymmetric catalytic aldol reactions have been successfully performed using aldehydes and unmodified ketones together with commercially available chiral cyclic secondary amines as catalysts Structure-based catalyst screening identified l-proline and 5,5-dimethyl thiazolidinium-4-carboxylate (DMTC) as the most powerful amino acid catalysts for the reaction of both acyclic and cyclic ketones as aldol donors with aromatic and aliphatic aldehydes to afford the corresponding aldol products with high regio-, diastereo-, and enantioselectivities Reactions employing hydroxyacetone as an aldol donor provide anti-1,2-diols as the major product with ee values up to >99% The reactions are assumed to proceed via a metal-free Zimmerman−Traxler-type transition state and involve an enamine intermediate The observed stereochemistry of the products is in accordance with the proposed transition state Further supporting evidence is provided by the lack of nonlinear effects The reactions tolerate a small amount o

Journal ArticleDOI
TL;DR: The origins of the observed substituent effects as well as the implications of these studies for the design and implementation of new olefin metathesis catalysts and substrates are discussed in detail.
Abstract: This report details the effects of ligand variation on the mechanism and activity of ruthenium-based olefin metathesis catalysts. A series of ruthenium complexes of the general formula L(PR3)(X)2RuCHR1 have been prepared, and the influence of the substituents L, X, R, and R1 on the rates of phosphine dissociation and initiation as well as overall activity for olefin metathesis reactions was examined. In all cases, initiation proceeds by dissociative substitution of a phosphine ligand (PR3) with an olefinic substrate. All of the ligands L, X, R, and R1 have a significant impact on initiation rates and on catalyst activity. The origins of the observed substituent effects as well as the implications of these studies for the design and implementation of new olefin metathesis catalysts and substrates are discussed in detail.

Journal ArticleDOI
TL;DR: Mesoporous semiconducting films consisting of preferentially orientated monoclinic-phase nanocrystals of tungsten trioxide have been prepared using a novel version of the sol-gel method, combining excellent photoresponse to the blue region of the solar spectrum, up to 500 nm, with good transparency at wavelengths larger than 550 nm.
Abstract: Mesoporous semiconducting films consisting of preferentially orientated monoclinic-phase nanocrystals of tungsten trioxide have been prepared using a novel version of the sol−gel method. Transformations undergone by a colloidal solution of tungstic acid, stabilized by an organic additive such as poly(ethylene glycol) (PEG) 300, as a function of the annealing temperature have been followed by means of a confocal Raman microscope. The shape and size of WO3 nanoparticles, the porosity, and the properties of the films depend critically on preparation parameters, such as the tungstic acid/PEG ratio, the PEG chain length, and the annealing conditions. Well-crystallized WO3 films combine excellent photoresponse to the blue region of the solar spectrum, up to 500 nm, with good transparency at wavelengths larger than 550 nm. Particular applications of these nanocrystalline WO3 films include photoelectrochemical and electrochromic devices.

Journal ArticleDOI
TL;DR: A strategy that combines physical templating and capillary forces to assemble monodispersed spherical colloids into uniform aggregates with well-controlled sizes, shapes, and structures that can serve as a useful model system to investigate the hydrodynamic and optical scattering properties of colloidal particles having nonspherical morphologies.
Abstract: This paper describes a strategy that combines physical templating and capillary forces to assemble monodispersed spherical colloids into uniform aggregates with well-controlled sizes, shapes, and structures. When an aqueous dispersion of colloidal particles was allowed to dewet from a solid surface that had been patterned with appropriate relief structures, the particles were trapped by the recessed regions and assembled into aggregates whose structures were determined by the geometric confinement provided by the templates. We have demonstrated the capability and feasibility of this approach by assembling polystyrene beads and silica colloids (> or =150 nm in diameter) into complex aggregates that include polygonal or polyhedral clusters, linear or zigzag chains, and circular rings. We have also been able to generate hybrid aggregates in the shape of HF or H2O molecules that are composed of polymer beads having different diameters, polymer beads labeled with different organic dyes, and a combination of polymeric and inorganic beads. These colloidal aggregates can serve as a useful model system to investigate the hydrodynamic and optical scattering properties of colloidal particles having nonspherical morphologies. They should also find use as the building blocks to generate hierarchically self-assembled systems that may exhibit interesting properties highly valuable to areas ranging from photonics to condensed matter physics.


Journal ArticleDOI
TL;DR: The secondary building unit (SBU) has been identified as a useful tool in the analysis of complex metal-organic frameworks (MOFs) and its applicability to rationalizing MOF crystal structures is illustrated by analysis of nine new MOFs which have been characterized by single-crystal X-ray diffraction.
Abstract: The secondary building unit (SBU) has been identified as a useful tool in the analysis of complex metal-organic frameworks (MOFs). We illustrate its applicability to rationalizing MOF crystal structures by analysis of nine new MOFs which have been characterized by single-crystal X-ray diffraction. Tetrahedral SBUs in Zn(ADC)(2).(HTEA)(2) (MOF-31), Cd(ATC).[Cd(H(2)O)(6)](H2O)(5) (MOF-32), and Zn(2)(ATB)(H2O).(H2O)(3)(DMF)(3) (MOF-33) are linked into diamond networks, while those of Ni(2)(ATC)(H(2)O)(4).(H2O)(4) (MOF-34) have the structure of the Al network in SrAl(2). Frameworks constructed from less symmetric tetrahedral SBUs have the Ga network of CaGa(2)O(4) as illustrated by Zn(2)(ATC).(C(2)H(5)OH)(2)(H2O)(2) (MOF-35) structure. Squares and tetrahedral SBUs in Zn(2)(MTB)(H2O)(2).(DMF)(6)(H2O)(5) (MOF-36) are linked into the PtS network, which is the simplest structure type known for the assembly of these shapes. The octahedral SBUs found in Zn(2)(NDC)(3).[(HTEA)(DEF)(ClBz)](2) (MOF-37) form the most common structure for linking octahedral shapes, namely, the boron network in CaB(6). New structure types for linking triangular and trigonal prismatic SBUs are found in Zn(3)O(BTC)(2).(HTEA)(2) (MOF-38) and Zn(3)O(HBTB)(2)(H2O).(DMF)(0.5)(H2O)(3) (MOF-39). The synthesis, crystal structure, and structure analysis using the SBU approach are presented for each MOF.

Journal ArticleDOI
TL;DR: Both stereochemical and mechanistic considerations suggest that the small organic molecule found at the Fe-only hydrogenase active site and previously modeled as 1,3-propanedithiolate may, in fact, be di-(thiomethyl)-amine.
Abstract: Fe-only hydrogenases, as well as their NiFe counterparts, display unusual intrinsic high-frequency IR bands that have been assigned to CO and CN(-) ligation to iron in their active sites. FTIR experiments performed on the Fe-only hydrogenase from Desulfovibrio desulfuricans indicate that upon reduction of the active oxidized form, there is a major shift of one of these bands that is provoked, most likely, by the change of a CO ligand from a bridging position to a terminal one. Indeed, the crystal structure of the reduced active site of this enzyme shows that the previously bridging CO is now terminally bound to the iron ion that most likely corresponds to the primary hydrogen binding site (Fe2). The CO binding change may result from changes in the coordination sphere of Fe2 or its reduction. Superposition of this reduced active site with the equivalent region of a NiFe hydrogenase shows a remarkable coincidence between the coordination of Fe2 and that of the Fe ion in the NiFe cluster. Both stereochemical and mechanistic considerations suggest that the small organic molecule found at the Fe-only hydrogenase active site and previously modeled as 1,3-propanedithiolate may, in fact, be di-(thiomethyl)-amine.

Journal ArticleDOI
TL;DR: The X-ray crystal structure of a 2:1 Zn(2+):Zinpyr-1 complex has been solved, and is the first structurally characterized example of a complex of fluorescein substituted with metal binding ligands, making them well-suited for intracellular applications.
Abstract: Two new fluorescent sensors for Zn2+ that utilize fluorescein as a reporting group, Zinpyr-1 and Zinpyr-2, have been synthesized and characterized. Zinpyr-1 is prepared in one step via a Mannich reaction, and Zinpyr-2 is obtained in a multistep synthesis that utilizes 4‘,5‘-fluorescein dicarboxaldehyde as a key intermediate. Both Zinpyr sensors have excitation and emission wavelengths in the visible range (∼500 nm), dissociation constants (Kd1) for Zn2+ of <1 nM, quantum yields approaching unity (Φ = ∼0.9), and cell permeability, making them well-suited for intracellular applications. A 3- to 5-fold fluorescent enhancement is observed under simulated physiological conditions corresponding to the binding of the Zn2+ cation to the sensor, which inhibits a photoinduced electron transfer (PET) quenching pathway. The X-ray crystal structure of a 2:1 Zn2+:Zinpyr-1 complex has also been solved, and is the first structurally characterized example of a complex of fluorescein substituted with metal binding ligands.


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TL;DR: A deoxyribonucleotide-based aptamer is adapted in two steps to signal the recognition of cocaine: an instability was engineered in one stem of a three-way junction that forms the cocaine-binding pocket and the resulting short stem was end labeled with a fluorophore and a quencher.
Abstract: We adapted in two steps a deoxyribonucleotide-based aptamer to signal the recognition of cocaine: an instability was engineered in one stem of a three-way junction that forms the cocaine-binding pocket and the resulting short stem was end labeled with a fluorophore and a quencher. In the absence of cocaine, two stems are open, but in its presence they close and the three-way junction forms. This major structural change brings fluorophore and quencher together thereby signaling the presence and concentration of ligand. The sensor is selective for cocaine over its metabolites, can operate in serum, and is useful for the screening of cocaine hydrolases.

Journal ArticleDOI
TL;DR: A new ansatz is presented that combines molecular dynamics simulations with MM-PBSA (Molecular Mechanics Poisson-Boltzmann/surface area) to rank the binding affinities of 12 TIBO-like HIV-1 RT inhibitors, and it is found that this approach can be reliably applied in ranking the ligands from the databases for this important target.
Abstract: In this work, a new ansatz is presented that combines molecular dynamics simulations with MM-PBSA (Molecular Mechanics Poisson-Boltzmann/surface area) to rank the binding affinities of 12 TIBO-like HIV-1 RT inhibitors. Encouraging results have been obtained not only for the relative binding free energies, but also for the absolute ones, which have a root-mean-square deviation of 1.0 kcal/mol (the maximum error is 1.89 kcal/mol). Since the root-mean-square error is rather small, this approach can be reliably applied in ranking the ligands from the databases for this important target. Encouraged by the results, we decided to apply MM-PBSA combined with molecular docking to determine the binding mode of efavirenz SUSTIVA(TM) another promising HIV-1 RT inhibitor for which no ligand-protein crystal structure had been published at the time of this work. To proceed, we define the following ansatz: Five hundred picosecond molecular dynamics simulations were first performed for the five binding modes suggested by DOCK 4.0, and then MM-PBSA was carried out for the collected snapshots. MM-PBSA successfully identified the correct binding mode, which has a binding free energy about 7 kcal/mol more favorable than the second best mode. Moreover, the calculated binding free energy (-13.2 kcal/mol) is in reasonable agreement with experiment (-11.6 kcal/mol). In addition, this procedure was also quite successful in modeling the complex and the structure of the last snapshot was quite close to that of the measured 2,3 A resolution crystal (structure the root-mean-square deviation of the 54 C(alpha) around the binding site and the inhibitor is 1.1 A). We want to point out that this result was achieved without prior knowledge of the structure of the efavirenz/RT complex. Therefore, molecular docking combined with MD simulations followed by MM-PBSA analysis is an attractive approach for modeling protein complexes a priori.


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TL;DR: A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission.
Abstract: A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission. Absorption- and emission spectra and molecular modeling confirm that the bulky dendrimer side chains do not cause extra torsion between the fluorene units. New polyfluorenes with 9,9-diaryl substituents have been prepared to determine the minimum size of substituent necessary for aggregation suppression. An LED using 12 has been demonstrated to produce blue emission with onset voltages below 4 V.

Journal ArticleDOI
TL;DR: This work has shown that field effect transistors (FETs) can be constructed by using individual semiconducting SWNTs, opening up the possibility of building more complex electronic architectures with molecular wire elements.
Abstract: Miniaturization by bottom-up approaches to length scales beyond the current semiconductor processing technology has generated tremendous research efforts in the areas of synthesis, characterization, and assembly of novel nanoscale materials such as quantum dots and wires. 1,2 Single-walled carbon nanotubes (SWNTs) are one-dimensional (1D) molecular wires ideally suited for elucidating basic phenomena in 1D and could play important roles in building miniaturized devices. 3 For instance, it has been shown that field effect transistors (FETs) can be constructed by using individual semiconducting SWNTs, 4 opening up the possibility of building more complex electronic architectures with molecular wire elements. Widely known is that obtaining both pand n-type materials and controlling their charge carrier densities are crucial to the current microelectronics. With SWNTs, an interesting phenomenon has been that tube-FETs under ambient conditions are always p-type with holes as the majority carriers. This has recently been revealed to be due to electron withdrawing by O 2 molecules adsorbed on SWNTs. 5 The ability to tune SWNTs into both nand p-type should be important to their applications in molecular electronics. A classical approach to n-type electron-rich carbon materials is via charge-transfer doping with alkali metals. 6 Indeed, potassium doping has led to n-type SWNT FETs and enabled the derivation of more complex devices such as intra-tube p -n junctions with interesting electronic functions. 3b,7However, alkali dopants suffer from immediate degradation upon exposure to air, making them undesirable for n-doping of SWNTs in practical device applications. Recently, controlled exposure of SWNTs to inorganic (NH 3, NO2) and organic molecules (amines) 7 has revealed the extreme sensitivity of the electrical properties of semiconducting SWNTs to molecular species, and has suggested the possibility of building ultra-sensitive nanosensors. Such sensitivity also hints at a new method of tuning charge carriers in SWNTs by mere adsorption of molecules with suitable functional groups. Rather than a complete ionization of each donor or acceptor as in alkali or halogen doping, partial electron donating or accepting by adsorbed molecular functional groups could be exploited to vary charge carriers in SWNTs. After all, from a theoretical point of view, a doping fraction (number of charges per carbon atom) of ∼10-310-4 is sufficient to shift the Fermi level into the conduction band of a typical semiconducting SWNT, yielding a highly n-doped system. 3b,8