Showing papers in "Accounts of Chemical Research in 1999"

Chemistry and Physics in One Dimension: Synthesis and Properties of Nanowires and Nanotubes

Abstract: Dimensionality plays a critical role in determining the properties of materials due to, for example, the different ways that electrons interact in three-dimensional, twodimensional (2D), and one-dimensional (1D) structures.1-5 The study of dimensionality has a long history in chemistry and physics, although this has been primarily with the prefix “quasi” added to the description of materials; that is, quasi-1D solids, including square-planar platinum chain and metal trichalcogenide compounds,2,6 and quasi2D layered solids, such as metal dichalcogenides and copper oxide superconductors.3-5,7,8 The anisotropy inherent in quasi-1D and -2D systems is central to the unique properties and phases that these materials exhibit, although the small but finite interactions between 1D chains or 2D layers in bulk materials have made it difficult to address the interesting properties expected for the pure low-dimensional systems. Are pure low-dimensional systems interesting and worth pursuing? We believe that the answer to this question is an unqualified yes from the standpoints of both fundamental science and technology. One needs to look no further than past studies of the 2D electron gas in semiconductor heterostructures, which have produced remarkably rich and often unexpected results,9,10 and electron tunneling through 0D quantum dots, which have led to the concepts of the artificial atom and the creation of single electron transistors.11-15 In these cases, lowdimensional systems were realized by creating discrete 2D and 0D nanostructures. 1D nanostructures, such as nanowires and nanotubes, are expected to be at least as interesting and important as 2D and 0D systems.16,17 1D systems are the smallest dimension structures that can be used for efficient transport of electrons and optical excitations, and are thus expected to be critical to the function and integration of nanoscale devices. However, little is known about the nature of, for example, localization that could preclude transport through 1D systems. In addition, 1D systems should exhibit density of states singularities, can have energetically discrete molecularlike states extending over large linear distances, and may show more exotic phenomena, such as the spin-charge separation predicted for a Luttinger liquid.1,2 There are also many applications where 1D nanostructures could be exploited, including nanoelectronics, superstrong and tough composites, functional nanostructured materials, and novel probe microscopy tips.16-29 To address these fascinating fundamental scientific issues and potential applications requires answers to two questions at the heart of condensed matter chemistry and physics research: (1) How can atoms or other building blocks be rationally assembled into structures with nanometer-sized diameters but much longer lengths? (2) What are the intrinsic properties of these quantum wires and how do these properties depend, for example, on diameter and structure? Below we describe investigations from our laboratory directed toward these two general questions. The organization of this Account is as follows. In section II, we discuss the development of a general approach to the rational synthesis of crystalline nanowires of arbitrary composition. In section III, we outline key challenges to probing the intrinsic properties of 1D systems and illustrate solutions to these challenges with measurements of the atomic structure and electronic properties of carbon nanotubes. Last, we discuss future directions and challenges in section IV.

Looking for Stable Carbenes: The Difficulty in Starting Anew

Abstract: A decade ago we initiated research, the goal of which was isolation of a stable carbene. Our success has helped to catalyze a resurgence of interest in readily available and easily handled carbenes. Research on stable (nucleophilic) carbenes is again a popular theme worldwide. Efforts in the general area of stable carbenes now focus not only on chemistry of the carbenes themselves but also on their applications to other chemical systems, where their chemical properties create technical opportunities that are unavailable with other functional groups. The quest for isolable carbenes is a long saga whose origin can be traced back to the first half of the 1800s. A recently published history of this quest provides an important backdrop for the research described in this Account.1 It is the intent of this Account to delineate the events and environment that led to the report from DuPont laboratories of the first isolation of a stable crystalline carbene. Getting Started

Luminescence Photophysics in Semiconductor Nanocrystals

Abstract: Introduction Sp3-hybridized semiconductors (including InP, GaAs, CdSe, and Si) are remarkable from the perspective of physical chemistry. A single electron created by HOMOLUMO promotion moves rapidly in response to an applied electric field, because there is little lattice distortion (i.e., small Franck-Condon factors) accompanying its creation. According to Marcus-Hush electron transfer theory, electron motion is resonant in the limit of vanishing reorganization energy. Franck-Condon factors are also small around an electron-hole pair, which is an electronically excited state. As a consequence, radiationless internal conversion (unimolecular decay converting electronic energy into heat) is extremely slow. Excited states decay radiatively in a defect-free, direct gap semiconductor such as CdSe. These simple spectroscopic facts have important practical consequences. Semiconductor lightemitting diodes have narrow emission bands and can show near 30% efficiency in converting electrical power into light. Semiconductor lasers and diodes also show excellent long-term stability against photochemical and current-induced degradation, when compared with many organic materials. All these properties reflect the strong chemical bonding, and the extremely delocalized nature of the electronic wave functions. Semiconductor nanocrystals lie between the traditional regimes of chemistry and solid-state physics.1 Nanocrystal research was initially motivated by an effort to understand the evolution of bulk structural and electronic properties from the molecular scale.2 Presently, technological interest in nanocrystals stems from the prospect of creating novel materials with distinct physical properties. Nanocrystals act like molecules as they interact with light via their electronic transition dipoles. Yet, their delocalized solid-state parentage causes them to display unusual photophysics relative to molecules. In many molecules vibronic interaction in the excited state is strong as the wave function is localized on just one or a few bonds. The molecular excited state has a different structure which promotes fast nonradiative deactivation into the ground state. Emission quantum yields can be low, and often sensitive to quenching by the local environment. The situation is different in nanocrystals. In a 23 A diameter nanocrystal, for example, the wave function is delocalized over ∼100 unit cells with little probability density at the surface. This suggests that, in the absence of defects, internal or surface, a nanocrystal should exhibit near unity fluorescence quantum yield, and partial protection from quenching. The emission spectrum should be sharp as the Franck-Condon factors are small. At room temperature nanocrystals can be better photoemitters than bulk semiconductors because in nanocrystals the electron and hole remain superimposed due to quantum confinement. Nanocrystals have the potential to serve as ideal chromophores if their surface chemistry can be understood and controlled. In this Account we describe nanocrystal photophysics and make comparisons between inorganic solid-state materials and organic dye molecules.

Glycosidase Mechanisms: Anatomy of a Finely Tuned Catalyst

Abstract: In order to accelerate the hydrolysis of glycosidic bonds by factors approaching 1017-fold, glycosidases have evolved finely tuned active sites optimally configured for transition-state stabilization. Structural analyses of various enzyme complexes representing stable intermediates along the reaction coordinate, in conjunction with detailed mechanistic studies on wild-type and mutant enzymes, have delineated the contributions of nucleophilic and general acid/base catalysis, as well as the roles of noncovalent interactions, to these impressive rate enhancements.

Electrical rectification by a molecule : the advent of unimolecular electronic devices

Abstract: In 1959, the late Richard P. Feynman proposed, in his usual witty way, that there was “plenty of room at the bottom”, i.e., that atomic and molecular dimensions had not yet been exploited in information storage.1 In electronic technology, what was initially called “microminiaturization” did provide fantastic economies of scale, cost, and speed: the integrated circuits (IC) introduced by Noyce and Kilby were the beginning of this trend. It was observed that the scale of ICs or “computer chips” has halved, at first every 2 years, then every 18 months;2 this brought a concomitant increase in computing speed (“VAX on a chip”, then “Cray on a chip”) and an astonishing decrease in unit cost. However, there is trouble ahead. Circuit designers talk about “design rules”, the closest distance between adjacent electronic components in the IC. These design rules define the clock cycle, which is the time required to travel between the furthest components on the chip: shorter cycles mean faster computing. These design rules have now crept down to about 180 nm commercially. If photolithography is used, the design rules are limited, by Rayleigh’s criterion,3 to about one-half the wavelength of light used. Capacitative coupling between components and heat dissipation are perennial headaches. Three-dimensional integration (rather than planar integration) has remained an elusive goal. To achieve better performance, i.e., going to design rules of 100 nm or below, requires abandoning UV radiation and resorting to X-ray or electron beam lithography, with much higher error rates. At 50 nm, an even more drastic limit sets in: one can no longer “dope” Si uniformly. Present projections are that this 50 nm “silicon wall” will be reached by the year 2005.4 The idea of using molecules as electronic devices has gained attention and respectability in the past quarter century. By chemical insertion of electron-donating or electron-withdrawing groups, molecules can become oneelectron donors (D) or one-electron acceptors (A). To work properly, the oxidation or reduction of these molecules must be chemically reversible. In group IV chemistry (today, group 14: Si, Ge), one dopes a crystal of Ge or Si with dilute concentrations of interstitial or substitutional electron-rich elements (group V, or 15: N, P, As, etc.) to achieve an “n-doped” material. To make a “p-doped” crystal, one dopes with group III (or 13: Al, Ga, In, etc.). Thus, “D” corresponds to “n”, and “A” corresponds to “p”. By accosting a micrometer-thick film of organic D molecules to a micrometer-thick film of an organic A molecules, one gets a microscopic DA rectifier (one-way conductor) of electrical current, equivalent to an inorganic pn rectifier.5 In the 1960s, and particularly in the early 1970s, organic charge-transfer crystals and conducting polymers yielded organic equivalents of inorganic electronic systems: semiconductors, metals, superconductors, batteries, etc.6 But this wave of “me-too-ism” did not create a new technology: the organic systems did not perform better, or less expensively, than their inorganic counterparts. The two niche areas that survived are liquid crystal displays and (maybe) light-emitting diodes based on conducting polymers. In the early 1980s, sparked by three scientific conferences organized by the late Forrest L. Carter, the idea of “molecular electronics”, that is, electronic devices consisting solely of molecules, gained large-scale interest.7-9 Aficionados of biological processes started talking about “biomolecular electronics”. The term “molecular electronics” was extended to all electronic properties of polymers, crystals, etc.swhat we might call “large-scale molecular electronics”. This field, as outlined above, has not fared well in the marketplace. A persistent view has been that unimolecular, or “oligomolecular”,10,11 or “molecular-scale” 12 electronics have a very bright future, just as the new millennium begins. Molecules, with their 1-3 nm sizes, should step in where inorganic chemistry finally fails. Thus, unimolecular electronics will come to the rescue: they will finally find a central role in electronic technology.

Architectonic Quantum Dot Solids

Abstract: Three things largely determine the electronic properties of a crystal: the energy levels of the atoms or lattice sites, the coupling between adjacent sites, and the symmetry of the solid. Imagine being able to control each of these properties separately, and therefore being able to “design” a solid with a prescribed set of electronic properties. For an atomic solid, this would amount to being able to tune the electronegativity of an atom, to control the strength of the covalent interactions within the lattice, and to choose the crystal structure. While the chemistry of the periodic table does not permit such control, the chemistry of “artificial atoms”,1 or quantum dots (QDs), does. In this Account, we will discuss one example of “designing” a unique electronic property into a QD solid. This property, which is the ability of the solid to reversibly pass through a metal−insulator (MI) transition under ambient conditions, is something that has not been previously demonstrated for a more traditional solid. A two-dimensional (2D) hexagonal superlattice of organically passivated silver QDs was fabricated as a Langmuir monolayer. Selecting a particular size of QD controlled the (super)lattice site energies. The coupling between adjacent QDs was coarsely controlled by selection of the organic surface passivant, and precisely controlled by compressing the superlattice using the Langmuir technique. By using the title “Architectonic2 Quantum Dot Solids”, we emphasize that many of the collective properties of the superlattice were rationally designed into the material.

Transitions, Transition States, Transition State Analogues: Zinc Pyrazolylborate Chemistry Related to Zinc Enzymes

Abstract: Zinc is known to be functional in almost 300 enzymes representing all subgroups of the enzyme classification.1 Although it is present in organisms only about half as much as its famous congener iron, it competes with the latter in terms of versatility. In comparison, the divalent metals magnesium and calcium, being 1-2 orders of magnitude more abundant, rarely share the biochemical abilities and rarely interfere with the biological functions of zinc, and the homologues of zinc, cadmium and mercury, just manage to exist as highly poisonous impurities. The question “Why?” suggests itself. What makes zinc, the “boring element without properties”, so unique? Like all “Why?”, questions this one is difficult to answer. Definitely the redox inertness and the hard-soft properties of zinc play an important role. Unlike all other firstrow transition metals, zinc cannot get caught in a redox trap, i.e., change its properties by changing its oxidation state. Unlike calcium and magnesium, zinc likes to be coordinated by the soft donor functions in a protein environment, i.e., cysteine thiolate and histidine imidazole. Unlike cadmium and mercury, it does not hold on irreversibly to these donors. The intermediate nature of zinc in this respect, not really hard and not really soft, certainly makes it suitable to be bound to and to act in all available donor environments in living organisms. The two aforementioned properties illustrate the availability of zinc for biological purposes. When it comes to usefulness, other properties must be named. The first of these is a combination of high (kinetic) lability and low (thermodynamic) stability, which characterizes the complexes of zinc more than those of any other transition metal. Unfortunately this combination, which is the essence of catalysis, is an unpleasant symptom for a preparative chemist trying to build enzyme models. In our opinion two other properties are the most important ones in zinc enzymes, specifically for the largest part of them effecting group transfer. They are the high nucleophilicity of “relevant” anions (OH, OR, SR, OPO2X) when bound to tetrahedral zinc and the high coordinative flexibility in the ligand sphere of the metal. In an enzyme environment, the Zn-OH function which exists at neutral pH is an equivalent of free OH-, and all zinc-catalyzed group-transfer reactions involve a rapid change of coordination numbers and coordination geometries at the zinc center.1 This Account is meant to provide some information from zinc coordination chemistry related to an understanding of these latter two phenemena. Both involve atomic interactions and atomic motions in the immediate environment of the metal ion, i.e., the inorganic part of zinc-catalyzed organic or biochemical reactions. We are well aware that the properties of “biological zinc” are a consequence of its “biological environment” and that the catalytic processes can only be understood by a full analysis of all environmental effects, i.e., the interactions of the substrate with the catalyst, solvent, enzyme interior, or cofactors. But the catalytic bond breaking and bond making take place when the reagents are bound to zinc. With this in mind we put up for discussion some observations from coordination chemistry and some generalizations drawn thereof. On the basis of the fact that most zinc enzymes contain the catalytic zinc tetrahedrally coordinated by three protein side chains and the reagents (water, hydroxide, or substrate) as depicted below, we developed the “Freiburg enzyme model”, which reliably reproduces this bonding situation.2 This enzyme model mimics the protein environment by attachment of the metal to three heterocyclic nitrogen donors of a tris(pyrazolyl)borate ligand and by encapsulation of the metal by the three substituents R attached at the 3-positions of the pyrazole rings. Throughout this Account, the graphical symbol annotated as Tp*Zn will be used to represent the “enzyme” part of this model. The Tp*Zn-X complexes have been found versatile in many respects. They allow a wide variation of substituents X. They are suitable for studying the properties of single Zn-X units in a tetrahedral and hydrophobic environment, in this context the nucleophilicity of X. Due to their reasonably inert nature, they offer themselves for mechanistic studies, and the large amount of structural data gathered for them provides a basis for an evaluation of mechanistic pathways. Our contributions to pyrazolylborate-zinc chemistry started with invaluable help from the inventor of the ligands, Jerry Trofimenko. They grew complementary, in competition and in cooperation with the contributions by our colleagues G. Parkin, W. Klaui, W. Tolman, and the late N. Kitajima. Heinrich Vahrenkamp received his Dr. degree in 1967 at the University of Munchen, working under the direction of H. Noth in boron chemistry. After picking up cluster chemistry and X-ray crystallography from L. F. Dahl at the University of Wisconsin, he worked at the Universities of Marburg and Munchen on organometallic Lewis bases, becoming Privatdozent in 1972. In 1973 he was called to the chair of inorganic chemistry at the University of Freiburg where he has worked until today. After more than 20 years of research on polynuclear organometallic compounds, the main interest of his research group has now shifted to the coordination chemistry of zinc related to an understanding of its biological functions. Acc. Chem. Res. 1999, 32, 589-596

Ground State and Transition State Contributions to the Rates of Intramolecular and Enzymatic Reactions

Abstract: reactions, rate enhancements of 108 M have been observed (Table 1).2 Intramolecular reactions where severe ground state strain is relieved upon formation of the transition state are known to be as large as 1016 M.3 Kirby4 has compiled a compendium of intramolecular reactions and has pointed out the relationship of rate to exothermicity. The importance of ground state conformations and the lack of translational entropy in intramolecular and enzymatic reactions have drawn attention from Menger5 and ourselves, while Jencks and Page6 have offered an explanation based on entropic driving forces stemming from the freezing out of motions and the dampening of vibrational frequencies in the transition state. Houk,7 in a scholarly study, has provided a correlation between rate constants for certain lactonization reactions and transition state stabilization. We provide here an account of our recent computational results8-10 dealing with the driving forces for enzymatic and intramolecular reactions. We introduce8 the term near attack conformation (NAC) to define the required conformation for juxtaposed reactants to enter a transition state (TS). The greater the mole fraction of reactant conformations that are present as NACs, the greater the rate constant. Rate constants for bond making and breaking in enzymatic reactions depend on, (i) the fraction of E‚S present as NACs, (ii) the change in solvation of reactant species within the NAC, as compared to water, and (iii) electrostatic forces11 which can stabilize the TS. The latter may include hydrogen bonds and metal ligation. Covalency in metal ligation and hydrogen bonding (low barrier hydrogen bonding12) most probably are introduced in the ground state. These features are best appreciated when ground state conformations and TS structures can be examined separately.

Enantioselective Free Radical Reactions

Abstract: Organic free radicals have historically been regarded as intermediates poorly suited for selective reactions because of their high reactivity. In the past 25 years it has become apparent that radicals can react in a chemoand regioselective manner. Furthermore, in the past decade, stereoselective transformations of free radicals have been achieved.1 During this time, a variety of useful synthetic strategies have been developed for radical-based C-C bond-forming reactions, and an understanding of the determining parameters for diastereoselective transformations has emerged.1 On the other hand, enantioselective transformations of organic radicals remain uncommon. As recently as 1995, the prevailing view, stated in a review of the field,1a was that “Acyclic diastereocontrol has emerged only recently, but progress has been rapid. Enantioselective reactions of radicals are rare at present, but we believe that their development is now inevitable.” Since this statement was published, inevitability has become reality, and there are now several examples in the literature of enantioselective radical transformations. Early enantioselective radical transformations relied on precedents from successful strategies utilized for other reaction types, as was the case in the development of diastereoselective radical reactions. Thus, chiral auxiliaries used in diastereoselective radical transformations were developed based upon precedents elaborated for enolate alkylations,2 and the enantioselective variants have benefited from strategies used in catalytic DielsAlder3 and other processes. In this Account, we present developments in enantioselective radical transformations that have occurred in the past three years in the context of previous efforts to control configuration in diastereoselective radical reactions. We focus principally on advances made in our own laboratories but mention also important developments from other research groups.