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Showing papers in "Accounts of Chemical Research in 2002"


Journal ArticleDOI
TL;DR: This Account summarizes techniques for fabrication and applications in biomedicine of microfluidic devices fabricated in poly(dimethylsiloxane) (PDMS).
Abstract: This Account summarizes techniques for fabrication and applications in biomedicine of microfluidic devices fabricated in poly(dimethylsiloxane) (PDMS). The methods and applications described focus on the exploitation of the physical and chemical properties of PDMS in the fabrication or actuation of the devices. Fabrication of channels in PDMS is simple, and it can be used to incorporate other materials and structures through encapsulation or sealing (both reversible and irreversible).

2,490 citations


Journal ArticleDOI
TL;DR: In this article, the authors present the development of crystal-engineering strategies toward the synthesis of non-centrosymmetric infinite coordination networks for use as second-order nonlinear optical (NLO) materials.
Abstract: Crystal engineering, the ability to predict and control the packing of molecular building units in the solid state, has attracted much attention over the past three decades owing to its potential exploitation for the synthesis of technologically important materials. We present here the development of crystal-engineering strategies toward the synthesis of noncentrosymmetric infinite coordination networks for use as second-order nonlinear optical (NLO) materials. Work performed mainly in our laboratory has demonstrated that noncentrosymmetric solids based on infinite networks can be rationally synthesized by combining unsymmetrical bridging ligands and metal centers with well-defined coordination geometries. Specifically, coordination networks based on 3D diamondoid and 2D grid structures can be successfully engineered with a high degree of probability and predictability to crystallize in noncentrosymmetric space groups. We have also included noncentrosymmetric solids based on 1D chains and related helical structures for comparison.

2,344 citations


Journal ArticleDOI
TL;DR: The aim of this article is to highlight some features common to all hydrogen bonds and further to suggest that the term hydrogen bridge is perhaps a better descriptor for them.
Abstract: A hydrogen bond, X-H...A, is an interaction wherein a hydrogen atom is attracted to two atoms, X and A, rather than just one and so acts like a bridge between them. This attraction always increases with increasing electronegativity of X and A, and in the classical view all hydrogen bonds are highly electrostatic and sometimes even partly covalent. Gradually, the concept of a hydrogen bond became more relaxed to include weaker interactions, provided some electrostatic character remains. In the limit, these weak hydrogen bonds have considerable dispersive-repulsive character and merge into van der Waals interactions. A great variety of hydrogen bonds are observed in the solid state and the aim of this article is to highlight some features common to all these bonds and further to suggest that the term hydrogen bridge is perhaps a better descriptor for them. Such a description recognizes an interaction without borders and one that admits of much variation in its relative covalent, electrostatic, and van der Waals content.

1,850 citations


Journal ArticleDOI
TL;DR: The origins and basis of green chemistry chart a course for achieving environmental and economic prosperity inherent in a sustainable world.
Abstract: Over the course of the past decade, green chemistry has demonstrated how fundamental scientific methodologies can protect human health and the environment in an economically beneficial manner. Significant progress is being made in several key research areas, such as catalysis, the design of safer chemicals and environmentally benign solvents, and the development of renewable feedstocks. Current and future chemists are being trained to design products and processes with an increased awareness for environmental impact. Outreach activities within the green chemistry community highlight the potential for chemistry to solve many of the global environmental challenges we now face. The origins and basis of green chemistry chart a course for achieving environmental and economic prosperity inherent in a sustainable world.

1,830 citations


Journal ArticleDOI
Hongjie Dai1
TL;DR: In this paper, the van der Waals self-assembly forces and applied electric fields are used to control the growth direction of carbon nanotubes in a patterned growth approach.
Abstract: Synthesis of carbon nanotubes by chemical vapor deposition over patterned catalyst arrays leads to nanotubes grown from specific sites on surfaces. The growth directions of the nanotubes can be controlled by van der Waals self-assembly forces and applied electric fields. The patterned growth approach is feasible with discrete catalytic nanoparticles and scalable on large wafers for massive arrays of novel nanowires. Controlled synthesis of nanotubes opens up exciting opportunities in nanoscience and nanotechnology, including electrical, mechanical, and electromechanical properties and devices, chemical functionalization, surface chemistry and photochemistry, molecular sensors, and interfacing with soft biological systems.

1,732 citations


Journal ArticleDOI
TL;DR: Using the directional-bonding methodology, this work has synthesized high-symmetry ensembles resembling the Platonic solids and the Archimedean solids, as well as other cages, like trigonal bipyramids, adamantanoids, and trigonal prisms.
Abstract: We provide a summary of our results in three-dimensional, coordination-driven self-assembly based on the directional-bonding methodology, in which the stoichiometric mixing of complementary building blocks, with appropriate, predefined geometries, leads to targeted, nanoscopic cages. Using this motif, we have synthesized high-symmetry ensembles resembling the Platonic solids, such as dodecahedra, and the Archimedean solids, such as truncated tetrahedra and cuboctahedra, as well as other cages, like trigonal bipyramids, adamantanoids, and trigonal prisms. The synthesis and characterization of these compounds is discussed, as is some host-guest chemistry.

1,553 citations


Journal ArticleDOI
TL;DR: Representative results concerning the solubility, dispersion, defunctionalization, and optical properties of the functionalized carbon nanotubes are presented.
Abstract: Carbon nanotubes can be functionalized via amidation and esterification of the nanotube-bound carboxylic acids. The solubility of these functionalized carbon nanotubes makes it possible to characterize and study the properties of carbon nanotubes using solution-based techniques. Representative results concerning the solubility, dispersion, defunctionalization, and optical properties of the functionalized carbon nanotubes are presented. Several examples for the use of functionalized carbon nanotubes in the fabrication of polymeric carbon nanocomposites, the probing of nanotube-molecule interactions, and the conjugation with biological species are highlighted and discussed.

1,542 citations


Journal ArticleDOI
TL;DR: It is shown that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution-based chemical transformations, chromatography, and spectroscopy.
Abstract: In this Account we highlight the experimental evidence in favor of our view that carbon nanotubes should be considered as a new macromolecular form of carbon with unique properties and with great potential for practical applications. We show that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution-based chemical transformations, chromatography, and spectroscopy. It is already clear that the nascent field of nanotube chemistry will rival that of the fullerenes.

1,411 citations


Journal ArticleDOI
TL;DR: Using mechanistic reasoning, over 20 new processes of varying complexity have been designed and implemented that involved C-C bond-forming reactions and the prospect for such developments is probed in the context of ruthenium-catalyzed reactions.
Abstract: An important first step in making organic reactions more environmentally benign by design requires processes that are, to a first approximation, simple additions with anything else needed only catalytically. Since so few of the existing reactions are additions, synthesis of complex molecules requires the development of new atom-economic methodology. The prospect for such developments is probed in the context of ruthenium-catalyzed reactions. Using mechanistic reasoning, over 20 new processes of varying complexity have been designed and implemented. While some involved oxidation−reduction processes, most involved C−C bond-forming reactions.

1,130 citations


Journal ArticleDOI
TL;DR: Three methods are described, in the context of the guiding principles of green chemistry, for the catalytic oxidation of alcohols, using a recyclable oligomeric TEMPO catalyst and sodium hypochlorite as the oxidant in a bromide-free and chlorinated hydrocarbon solvent-free system.
Abstract: Three methods are described, in the context of the guiding principles of green chemistry, for the catalytic oxidation of alcohols. The first employs a recyclable oligomeric TEMPO catalyst (PIPO) and sodium hypochlorite as the oxidant in a bromide-free and chlorinated hydrocarbon solvent-free system. The second involves a ruthenium/TEMPO catalyst and oxygen as the oxidant. The third consists of a recyclable water-soluble palladium-diamine complex in conjunction with air as the oxidant in an aqueous biphasic system. The mechanisms of the ruthenium/TEMPO- and palladium-catalyzed oxidations are discussed, and the mechanism of the former is compared with that of the analogous copper/TEMPO catalyst.

1,004 citations


Journal ArticleDOI
Phaedon Avouris1
TL;DR: In this article, the authors review the electrical properties of carbon nanotubes, and then focus on carbon Nanotube field effect transistors (CNTFETs) and compare their electrical properties with those of Si MOSFETs.
Abstract: Carbon nanotubes have unique properties that make them a most promising system on which to base molecular electronics. We briefly review the electrical characteristics of carbon nanotubes, and then focus on carbon nanotube field-effect transistors (CNTFETs). Procedures by which hole-transport, electron-transport and ambipolar CNTFETs can be fabricated are presented, and their electrical characteristics are discussed and compared with those of Si MOSFETs. Ways to fabricate arrays of CNTFETs are also demonstrated, and electron and hole CNTFETs are integrated to form complementary logic circuits.

Journal ArticleDOI
TL;DR: DMC possesses properties of nontoxicity and biodegradability which makes it a true green reagent to use in syntheses that prevent pollution at the source and avoiding the formation of undesirable inorganic salts as byproducts.
Abstract: Dimethyl carbonate (DMC) is a versatile compound that represents an attractive eco-friendly alternative to both methyl halides (or dimethyl sulfate) and phosgene for methylation and carbonylation processes, respectively. In fact, the reactivity of DMC is tunable: at T = 90 °C, methoxycarbonylations take place, whereas at higher reaction temperatures, methylation reactions are observed with a variety of nucleophiles. In the particular case of substrates susceptible to multiple alkylations (e.g., CH2-active compounds and primary amines), DMC allows unprecedented selectivity toward mono-C- and mono-N-methylation reactions. Nowadays produced by a clean process, DMC possesses properties of nontoxicity and biodegradability which makes it a true green reagent to use in syntheses that prevent pollution at the source. Moreover, DMC-mediated methylations are catalytic reactions that use safe solids (alkaline carbonates or zeolites), thereby avoiding the formation of undesirable inorganic salts as byproducts. The r...

Journal ArticleDOI
TL;DR: In this paper, the cleavage and addition of ortho C−H bonds in various aromatic compounds such as ketones, esters, imines, imidates, nitrile, and aldehydes to olefins and acetlylenes can be achieved with the aid of ruthenium catalysts.
Abstract: The cleavage and addition of ortho C−H bonds in various aromatic compounds such as ketones, esters, imines, imidates, nitrile, and aldehydes to olefins and acetlylenes can be achieved catalytically with the aid of ruthenium catalysts. The reaction is generally highly efficient and useful in synthetic methods. The coordination to the metal center by a heteroatom in directing groups such as carbonyl and imino groups is the key. The reductive elimination to form a C−C bond is the rate-determining step.

Journal ArticleDOI
TL;DR: This Account describes studies aimed at understanding the mechanisms of electronic communication in porphyrin-based architectures that undergo excited-state energy migration and ground-state hole/electron hopping.
Abstract: Understanding electronic communication among the constituents in multicomponent macromolecular architectures is essential for the rational design of molecular devices for photonic, electronic, or optoelectronic applications. This Account describes studies aimed at understanding the mechanisms of electronic communication in porphyrin-based architectures that undergo excited-state energy migration and ground-state hole/electron hopping. Porphyrins are ideal building blocks for such constructs owing to their attractive and versatile physical properties and amenability to synthetic control. These properties have permitted the creation of covalently linked multiporphyrin arrays wherein the rates of excited-state energy migration and ground-state hole/electron hopping can be tuned over a wide range.

Journal ArticleDOI
TL;DR: N-tert-Butanesulfinyl aldimines 3 and ketimines 4 are exceedingly versatile intermediates for the asymmetric synthesis of amines and provide a new family of ligands for asymmetric catalysis.
Abstract: N-tert-Butanesulfinyl aldimines 3 and ketimines 4 are exceedingly versatile intermediates for the asymmetric synthesis of amines. The N-tert-butanesulfinyl imines are prepared in high yields by condensing enantiomerically pure tert-butanesulfinamide 1, which is readily available in either configuration, with a wide range of aldehydes and ketones. The tert-butanesulfinyl group activates the imines for the addition of many different classes of nucleophiles, serves as a powerful chiral directing group, and after nucleophilic addition is readily cleaved by treatment of the product with acid. A wide range of highly enantioenriched amines, including α-branched and α,α-dibranched amines, α- and β-amino acids, 1,2- and 1,3-amino alcohols, and α-trifluoromethyl amines, are efficiently synthesized using this methodology. In addition, N-tert-butanesulfinyl imine derivatives provide a new family of ligands for asymmetric catalysis.

Journal ArticleDOI
TL;DR: This work has used knowledge of the electronic structure of excited states of acids to design molecules that exhibit enhanced excited-state acidity and are the strongest reversible photoacids known.
Abstract: We have used knowledge of the electronic structure of excited states of acids to design molecules that exhibit enhanced excited-state acidity. Such “super” photoacids are the strongest reversible photoacids known and allow the time evolution of proton transfer to be examined in a wide array of organic solvents. This includes breaking/formation of the hydrogen bonds in hundreds of femtoseconds, solvent reorientation and relaxation in picoseconds, proton dissociation, and, finally, diffusion and geminate recombination of the dissociated proton, observed in nanoseconds.

Journal ArticleDOI
TL;DR: New types of Lewis acids as water-compatible catalysts have been developed and catalytic asymmetric aldol reactions in such media have been attained, which will contribute to reducing the use of harmful organic solvents and to develop efficient catalytic systems which cannot be realized in conventional organicsolvents.
Abstract: New types of Lewis acids as water-compatible catalysts have been developed. Various metal salts were found to work as Lewis acid catalysts in aqueous media, and catalytic asymmetric aldol reactions in such media have been attained. Furthermore, Lewis acid-surfactant combined catalysts, which can be used for reactions in water without using any organic cosolvents, have been also developed. These investigations will contribute to reducing the use of harmful organic solvents and to develop efficient catalytic systems which cannot be realized in conventional organic solvents.

Journal ArticleDOI
TL;DR: This specific account mainly summarizes the own experiences in developing rapid, robust, and selective microwave-assisted transition metal-catalyzed homogeneous reactions, which will be of importance in the search for green laboratory-scale synthesis.
Abstract: The efficiency of microwave flash heating in accelerating organic transformations (reaction times reduced from days and hours to minutes and seconds) has recently been proven in several different fields of organic chemistry. This specific account mainly summarizes our own experiences in developing rapid, robust, and selective microwave-assisted transition metal-catalyzed homogeneous reactions. Applications include selective Heck couplings, cross-couplings, and asymmetric substitutions. The science of green chemistry was developed to meet the increasing demand for environmentally benign chemical processes. We believe the combination of metal catalysis and microwave heating will be of importance in the search for green laboratory-scale synthesis.

Journal ArticleDOI
TL;DR: The aim of the present Acount is to investigate theoretically the effects of different types of defects on the electronic properties of carbon nanotubes, and to propose new potential applications in nanoelectronics.
Abstract: Carbon nanotubes are quasi one-dimensional nanostructures with unique eletrical prroperties that make them prime candidates for molecular electronics, which is certainly a most promising direction in nanotechnology. Early theoretical works predicted that the electronic properties of "ideal" carbon nanotubes depend on their diameter and chirality. However, carbon nanotubes are probably not as perfect as they were once thought to be. Defects such as pentagons, heptagons, vacancies, or dopant are found to modify drastically the electronic properties of these nanosystems. Irradiation processes can lead to interesting, highly defective nanostructures and also to the coalescence of nanotubes within a rope. The introduction of defects in the carbon network is thus an interesting way to tailor its intrinsic properties, to create new potential nanodevices. The aim of the present Acount is to investigate theoretically the effects of different types of defects on the electronic properties of carbon nanotubes, and to propose new potential applications in nanoelectronics.

Journal ArticleDOI
TL;DR: Single-stranded DNA and RNA molecules in solution can be driven through a nanoscopic pore by an applied electric field and information about length, composition, structure, and dynamic motion of the molecule can be deduced from modulations of the current blockade.
Abstract: Single-stranded DNA and RNA molecules in solution can be driven through a nanoscopic pore by an applied electric field. As each molecule occupies the pore, a characteristic blockade of ionic current is produced. Information about length, composition, structure, and dynamic motion of the molecule can be deduced from modulations of the current blockade.

Journal ArticleDOI
TL;DR: A simple method for purifying and healing structural defects in the nanotubes is described and the effects of reactor temperature, reaction time, and carbon partial pressure on the yield, purity, and size of the MWNTs produced are reported.
Abstract: Chemical vapor deposition (CVD) is the most promising synthesis route for economically producing large quantities of carbon nanotubes. We have developed a low-cost CVD process for the continuous production of aligned multiwall carbon nanotubes (MWNTs). Here we report the effects of reactor temperature, reaction time, and carbon partial pressure on the yield, purity, and size of the MWNTs produced. A simple method for purifying and healing structural defects in the nanotubes is described. The dispersion of nanotubes in polymer matrices has been investigated as a means of deriving new and advanced engineering materials. These composite materials have been formed into fibers and thin films and their mechanical and electrical properties determined.

Journal ArticleDOI
TL;DR: Solid acids and especially those based on micelle-templated silicas and other mesoporous high surface area support materials are beginning to play a significant role in the greening of fine and specialty chemicals manufacturing processes.
Abstract: Solid acids and especially those based on micelle-templated silicas and other mesoporous high surface area support materials are beginning to play a significant role in the greening of fine and speciality chemicals manufacturing processes. A wide range of important organic reactions can be efficiently catalyzed by these materials, which can be designed to provide different types of acidity as well as high degrees of reaction selectivity. The solid acids generally have high turnover numbers and can be easily separated from the organic components. The combination of this chemistry with innovative reaction engineering offers exciting opportunities for innovative green chemical manufacturing in the future.

Journal ArticleDOI
TL;DR: Comparison between the 3D structures of catechol oxidase and hemocyanins reveals the structural reasons for the divergence in function.
Abstract: The crystal structure of catechol oxidase reveals new insight into the functional properties of the type-3 copper proteins. This class of proteins includes the closely related and better-known tyrosinase as well as hemocyanin, an oxygen transport protein. All these proteins have a dinuclear copper center, have similar spectroscopic behaviors, and show close evolutionary and functional relationships. Comparison between the 3D structures of catechol oxidase and hemocyanins reveals the structural reasons for the divergence in function.

Journal ArticleDOI
TL;DR: This report analyzes the impact of scCO(2) on green organometallic catalysis on the basis of the experience in laboratories and concludes that it is likely that the material will have an important role in the development of catalytic synthetic methods.
Abstract: Carbon dioxide in its liquid or supercritical state (scCO(2)) has a prodigious potential as an environmentally benign reaction medium for sustainable chemical synthesis Since the mid-1990s, rapidly increasing research efforts have shown that scCO(2) can replace conventional and potentially hazardous solvents in a wide range of processes There is also increasing evidence that the application of scCO(2) can broaden the scope of catalytic synthetic methodologies On the basis of the experience in our laboratories, this report analyzes the impact of scCO(2) on green organometallic catalysis

Journal ArticleDOI
TL;DR: Recent scanning tunneling microscopy studies of the intrinsic electronic properties of single-walled carbon nanotubes (SWNTs) are overviewed and the implications for understanding fundamental one-dimensional physics and future nanotube device applications are discussed.
Abstract: Recent scanning tunneling microscopy studies of the intrinsic electronic properties of single-walled carbon nanotubes (SWNTs) are overviewed in this Account. A brief theoretical treatment of the electronic properties of SWNTs is developed, and then the effects of finite curvature and broken symmetry on electronic properties, the unique one-dimensional energy dispersion in nanotubes, the interaction between local spins and carriers in metallic nanotubes systems, and the atomic structure and electronic properties of intramolecular junctions are described. The implications of these studies for understanding fundamental one-dimensional physics and future nanotube device applications are also discussed.

Journal ArticleDOI
TL;DR: The design and successful isolation of a series of soluble di- and polynuclear alkynyl complexes of selected metals with d(8) and d(10) electronic configurations are described, found to exhibit rich photophysical and photochemical properties that are unique to the presence of the alkyny ligand.
Abstract: In this Account, the design and successful isolation of a series of soluble di- and polynuclear alkynyl complexes of selected metals with d(8) and d(10) electronic configurations are described. These organometallic complexes are found to exhibit rich photophysical and photochemical properties that are unique to the presence of the alkynyl ligand. By a systematic variation of the nature of the ligands, the metal centers, and certain structural features, the photophysical properties could be readily varied and their spectroscopic origins elucidated. Some of these complexes have been shown to be ideal building blocks for the design of luminescent organometallic oligomers and metal-based functional materials.

Journal ArticleDOI
TL;DR: The results obtained prove that the addition of fluorine drastically enhances the reactivity of the nanotube side walls, and the use of this strategy as a versatile tool for preparation and manipulation of SWNTs with variable side-wall functionalities has been demonstrated.
Abstract: This Account focuses on the most recent and systematic efforts in the area of functionalization chemistry of the single-wall carbon nanotubes (SWNTs) which utilizes direct fluorination for the preparation of "fluoronanonotubes" and their subsequent derivatization. The results obtained prove that the addition of fluorine drastically enhances the reactivity of the nanotube side walls. The use of this strategy as a versatile tool for preparation and manipulation of SWNTs with variable side-wall functionalities has been demonstrated. The functionalized SWNTs have shown an improved solubility in selected solvents and significantly altered electrical, mechanical, and optical properties. An overview of new synthetic methods for preparation and a discussion of characterization data for the functionalized SWNTs are provided.

Journal ArticleDOI
TL;DR: This Account chronicles the conceptual development, proof of principle, exploration of scope, and mechanistic investigations of a newly developed class of palladium-catalyzed cross-coupling reactions of silicon derivatives.
Abstract: This Account chronicles the conceptual development, proof of principle, exploration of scope, and mechanistic investigations of a newly developed class of palladium-catalyzed cross-coupling reactions of silicon derivatives. Under the influence of fluoride activation a myriad of oxygen-containing silicon moieties undergo mild and stereospecific cross-coupling. The diversity of methods for introduction of silicon groupings into organic molecules and the range of organic electrophiles that can be used are outlined.

Journal ArticleDOI
TL;DR: The physical chemistry of solution silicate species and surfactants in the synthesis of mesoporous silicas is reviewed and controls in surfactant packing and liquid crystalline phase transformation can lead to various tailored synthesis strategies.
Abstract: Micelle-templated mesoporous silica materials are rapidly becoming important in many fields of chemistry for hosting reactants or catalysts in confined space. Fine control of the pore size, wall structure, surface functionalization, defects, and morphology is needed for fine-tuning the pores as nanoreactors. We review the physical chemistry of solution silicate species and surfactants in the synthesis of mesoporous silicas. Controls in surfactant packing and liquid crystalline phase transformation can lead to various tailored synthesis strategies. Postsynthesis treatments further make more stable mesoporous materials.

Journal ArticleDOI
TL;DR: A broad structural variety of products emerges that have not only diverse chemical functionalities but also hydrophilic "residual sugar" portions that render them water-soluble and readily biodegradable.
Abstract: With reference to the impending transition of chemical industry from depleting fossil raw materials to renewable feedstocksthe end of cheap oil is predicted for 2040 at the latestthis Account gives an overview on chemically transforming carbohydrates, by far the major part of the annually regrowing biomass, into the following unsaturated O- and N-heterocycles with versatile industrial application profiles: furans, pyrans, dihydropyranones, pyrroles, pyrazoles, imidazoles, pyridinols, pyrazines, and quinoxalines. Although the emphasis was laid on conversions that can be effected in practical one-pot procedures or in a few large-scale-adaptable steps, a broad structural variety of products emerges that have not only diverse chemical functionalities but also hydrophilic “residual sugar” portions that render them water-soluble and readily biodegradable.