Showing papers in "Topics in Current Chemistry in 1998"

Crystalline Polymorphism of Organic Compounds

Abstract: Crystal polymorphism is encountered in all areas of research involving solid substances. Its occurrence introduces complications during manufacturing processes and adds another dimension to the complexity of designing materials with specific properties. Research on polymorphism is fraught with unique difficulties due to the subtlety of polymorphic transformations and the inadvertent formation of pseudopolymorphs. In this report, a summary of thermodynamic, kinetic and structural considerations of polymorphism is presented. A wide variety of techniques appropriate to the study of organic crystalline polymorphism and pseu-dopolymorphism is then surveyed, ranging from simple crystal density measurement to observation of polymorphic transformations using variable-temperature synchrotron X-ray diffraction methods. Application of newer methodology described in this report is yielding fresh insights into the nature of the crystallization process, holding promise for a deeper understanding of the phenomenon of polymorphism and its practical control.

Supramolecular Synthons and Pattern Recognition

Abstract: The aims of crystal engineering are the understanding of intermolecular interactions and their application in the design of crystal structures with specific architectures and properties. In general, all types of crystal structures may be considered but this article is limited to organic molecular solids. Because of the molecular basis of organic chemistry, the obvious question arises as to whether there are simple connections between the structures of molecules and the crystals that they form. Answers to such questions may be found through a better and more comprehensive understanding of the interactions that control crystal packing. These interactions include strong and weak hydrogen bonds. Patterns of interactions, such as would be useful in a predictive sense, can be obtained by manual inspection or more rigorously with the use of crystallographic databases. Such patterns are termed supramolecular synthons and they depict the various ways in which complementary portions of molecules approach one another. The identification of synthons is then a key step in the design and analysis of crystal structures. Such ideas are also important in the understanding of phenomena such as biological recognition and drug-enzyme binding. Pattern identification also leads to the possibility of comparison of crystal structures. The use of the supramolecular synthon concept facilitates such efforts and in this regard it may be mentioned that synthons combine topological characteristics with chemical information, thereby offering a simplification that is optimal to drawing such comparisons.

Micro Total Analysis Systems: Microfluidic Aspects, Integration Concept and Applications

Abstract: In this contribution three aspects of miniaturized total analysis systems (µTAS) are described and discussed in detail. First, an overview of microfabricated components for fluid handling is given. A description of the importance of sampling- and fluid-handling techniques is followed by details of microvalves, micropumps and micro flowchannels. Secondly, the problems associated with system integration are discussed. As a solution for the realization of microfluidic- and micro analysis systems, the concept of a planar mixed circuit board (MCB) as a platform for the integration of different components is described. In addition, the design, modeling and simulation, and realization of several components in the form of standard modules for integration on a MCB is described. As an illustration of the potential of this approach, the realization of a µTAS demonstrator for the optical detection of the pH change of a pH indicator, is presented. Finally, a number of different applications of µTAS are described, such as on-line process monitoring, environmental monitoring, biomedical and space applications and DNA-analysis.

Microfabrication, Microstructures and Microsystems

Abstract: This review gives a brief introduction to materials and techniques used for microfabrication. Rigid materials have typically been used to fabricate microstructures and systems. Elastomeric materials are becoming attractive, and may have advantages for certain types of applications. Photolithography is the most commonly used technique for the fabrication of structures for microelectronic circuits, microelectromechanical systems, microanalytical devices and micro-optics. Soft lithography represents a set of non-photolithographic techniques: it forms micropatterns of self-assembled monolayers (SAMs) by contact printing and generates microstructures of polymers by contact molding. The aim of this paper is to illustrate how non-traditional materials and methods for fabrication can yield simple, cost-effective routes to microsystems, and now they can expand the capabilities of these systems.

Fluids for Sensor Systems

Abstract: Several techniques for miniaturization of simple chemical and medical analysis systems are described. Miniaturization of total analysis systems realizes a small sample volume, a fast response and reduction of reagents. These features are useful in chemical and medical analysis. During the last decade many micro flow control devices, as well as the micro chemical sensors fabricated by three dimensional microfabrication technologies based on photofabrication, termed micromachining, have been developed. Miniaturized total analysis systems (μTAS) have been studied and some prototypes developed. In microfabricated systems, “microfluidics”, which represent the behavior of fluids in small sized channels, are considered and are very important in the design of micro elements used in μTAS. In this chapter microfluidics applied flow devices, micro flow control devices of active and passive microvalves, mechanical and non-mechanical micropumps and micro flow sensors fabricated by micromachining are reviewed.

Electrochemical and photochemical properties of metal-containing dendrimers

Abstract: Metal complexes are characterized by a precise molecular geometry related to the characteristic coordination number of the metal ion and also, in some cases, to the rigid structure of the ligands. Furthermore, they can exhibit valuable properties such as absorption of visible light, luminescence, and reduction and oxidation levels at accessible potentials. By using metal complexes to construct a dendrimer it is therefore possible to incorporate in the dendritic structure many “pieces of information”. In this paper the available results on the electrochemical and photochemical properties of metal-containing dendrimers are reviewed. It is shown that by a suitable choice of the metal-based building blocks, it is possible to control the number of exchanged electrons at a fixed potential and the pattern of migration of electronic energy. These properties can be exploited for multielectron catalysis and light harvesting.

Hydrogen-Bonded Ribbons, Tapes and Sheets as Motifs for Crystal Engineering

Abstract: The design of new architectures for the purpose of crystal engineering has generated great interest in recent years. In particular, organic compounds have been the focus of many studies due to the presence of functional groups that can form strong and stable intermolecular interactions. Therefore, recognizing geometry and functionality at the molecular level has relevant implications in the design of supramolecular patterns. These patterns in turn can be translated to physical or chemical properties in a solid. This article is a presentation of compounds that have been designed for the purpose of molecular recognition and crystal engineering. In particular our focus will be directed towards organic structures based on the tape, ribbon, and sheet motifs. Several approaches in the design of functional solids will be presented, emphasizing the use of certain complementary intermolecular interactions for this purpose. It is not our attempt to define all concepts used for crystal engineering, but to present recent advances in this field.

Functional Organic Zeolite Analogues

Abstract: There is much current interest in organic solid hosts, whose guest-binding properties are reminiscent of traditional inorganic zeolites. The basic design strategy is to assemble organic and metal-ion building blocks into a network by using directional intermolecular interactions such as hydrogen bonding and coordination. The network structures can be controlled by the geometrical and topological properties of the building blocks. When free from interpenetration, the resulting networks afford cavities or channels (occupying, in some cases, as much as 60–70% of the total volume) capable of selective guest binding. Some coordination and multiply hydrogen-bonded networks are robust enough to withstand the removal of included guests, thus sustaining as large as 10 A guest-free channels. Less robust hydrogen-bonded networks undergo a transition to more dense structures upon guest removal; they are, however, flexible enough to readsorb the guests and restore the single-crystal structures of the host-guest adducts. Guest exchange also occurs, during which crystallinity is retained. Another important consequence of host-guest complexation is activation of trapped guests. Facilitated intracavity reactions, coupled with dynamic guest-binding behaviours (exchange of products and reactants as guests), suggest a potential use of the present type of microporous organic and metal-organic solids as catalysts. This is in fact demonstrated for the Diels-Alder and related ene reactions. The present stage of functional organic zeolite analogues and the problems and prospects associated therewith are discussed from both static and dynamic viewpoints.

Dendrimers with Polymeric Core: Towards Nanocylinders

Abstract: This chapter draws a comprehensive picture of what has been done in the field of dendrimers with polymeric cores putting emphasis first on synthetic issues and then on experiments investigating the aggregation behavior of these intruiging macromolecules both in the solid state and on surfaces. Additionally, experiments will be described which show that some of these dendrimers can be considered cylindrical molecular objects. The macromolecules treated in this chapter may be considered as either dendrimers with polymeric core or alternatively dendronized polymers (or polymers with appendent dendrons) depending on whether one sees them from the vantage point of an organic or macromolecular chemist.

Supramolecular Chemistry within Dendritic Structures

Abstract: This chapter describes reports on supramolecular interactions, i.e., molecular recognition, between dendritic hosts and guest molecules, and non-covalent self assembly of branched monomers. The emphases are associated with the chemistry within dendrimers rather than on the periphery. A brief introduction to the field of supramolecular chemistry is accompanied by a short overview of dendrimers as well as a brief analysis of the syntheses of these macromolecules. Initially, accounts of random molecular inclusion or preliminary non-specific, hydrophilic/hydrophobic-based binding interactions between small guests and dendritic molecules are described, followed by examples of physical encapsulation of guest(s) molecules and their shape selective release. Molecular recognition processes via site-specific interactions including H-bonding host-guest complexations are discussed. Reports dealing with electro- and photo-active dendrimers will be considered. The self-assembly processes involving dendrimers or dendritic building-blocks assisted by forces such as those associated with hydrophilic and lipophilic behavior of amphiphilic species, π-π interactions, and liquid crystalline behavior are reviewed. This is supplemented by a section based on self-assembly processes of dendrons, mediated by intermolecular H-bonding, and mono- and multi-layer aggregation processes of dendritic structures. Lastly the construction and supramolecular chemistry of metallo-dendrimers are considered.

Divergent Approaches to Phosphorus-Containing Dendrimers and their Functionalization

Abstract: Divergent approaches to phosphorus-containing dendrimers are described One of them allows one to prepare the dendrimer of the highest generation known up to now (generation 12, theoretical molecular weight >3 000 000) The reactivity on the surface of these macromolecules is detailed Emphasis is made on a concept of multiplurifunctionalization allowing one to graft at the periphery a large number of sets of two, three or four functional groups Reactivity into the internal voids of phosphorus-containing dendrimers is also reported showing that it is possible to develop a macromolecular chemistry into the cavities of a given dendrimer Lastly the essential contribution of phosphorus in dendrimer chemistry is pointed out

Directional Aspects of Intermolecular Interactions

Abstract: Directional intermolecular interactions can be found by statistical analyses of the surroundings of functional groups in all crystal structures containing them. The most notable such interaction is the hydrogen bond, which, if strong as between OH or NH groups and oxygen or nitrogen acceptors, is approximately linear. This provides an important means of aligning molecules together. Analogous but weaker interactions described here are C-H...O, F...H and H... π, and also those between C-S-C groups and electrophiles or nucleophiles. Some directionality can also be identified in aromatic-aromatic interactions. Metal ion coordination can, in certain instances, also have a directional component, particularly if the coordination geometry is inflexible, as for the octahedral binding of divalent magnesium. The geometries of interactions of metal ions with various functional groups in proteins are described, and in many cases they are more rigid than the analogous interaction involving a hydrogen bond. The emerging use of crystal surfaces as probes of molecular recognition is then discussed. Finally some examples of molecular recognition in biological macromolecules are given; these stress the importance of pattern recognition in hydrogen bonding, together with the significance of weaker interactions.

Biological Application of Microstructures

Abstract: The investigation of living cells, isolated organelles, viruses and macromolecules in microstructures fabricated by semiconductor technology is a new field of research and has found its first biotechnological and medical uses. This chapter contains selected examples of three-dimensional structures to characterise, manipulate and separate suspended and adherently growing cells. It addresses the problem of cell cultivation in strong electric fields. In addition, some theoretical and experimental results dealing with dielectrophoresis, trapping of submicron particles and cryoconservation of cells in microstructures are given. The behaviour of adherently growing animal cells on artificial surfaces is described. Combinations of electrical and optical methodologies are discussed and the perspectives for such semiconductor- or glass-based devices are outlined.

Microreactors for Chemical Synthesis and Biotechnology — Current Developments and Future Applications

Abstract: The modern methods of three dimensional microfabrication have lead to the development of extremely miniaturized chemical and biotechnological systems. These so called microreactors represent novel approaches in respect of production flexibility and chemical reactions not yet applied in chemical processing. This has stimulated world-wide research in this field so that the technical feasibility of such devices has been demonstrated in the laboratory scale.

Integrated Chip-Based Microcolumn Separation Systems

Abstract: The prospects of inexpensive mass production of integrated chemical separation systems with superior analytical performance through modern microfabrication technology has stimulated considerable research activity over the past five years. In addition, these devices hold the promise of being extremely rugged, capable of analysis of very small sample volumes, and suitable for high-throughput analysis through parallel sample processing. The goal of the present article is to review major recent accomplishments in this rapidly advancing field. Since most research efforts up to now have been devoted to electric field driven separation systems, in particular to chip-based capillary electrophoresis, emphasis will be laid on the discussion of the physical-chemical basis of the operation and optimization of these devices.

Iterative Synthesis in Organic Chemistry

Abstract: Iterative synthesis is a useful method in organic chemistry, allowing automation of reaction sequences to synthesize nano-scale molecules. This methodology has recently gained importance as it is the prevailing synthetic strategy to build up dendrimers and other supramolecular systems. Iterative pathways open up an approach to the synthesis of monodisperse oligomers or polymeric precursors. Moreover, linear aliphatic molecules constructed of identical building blocks can be assembled. This contribution gives a brief overview of iterative syntheses with emphasis on organic preparative chemistry. In order to elucidate this principle some biological and biochemical examples are given. The scope and limits of selected iterative applications are discussed and compared to other methods.

Sample Preparation In Microstructured Devices

Abstract: Handling and processing of μl and sub-μl sized samples are challenging, and have a series of attendant problems, such as loss of sample on the walls of transfer devices, loss by evaporation, loss of components of the sample during manipulation and processing, and obtaining a representative sample from an nonhomogeneous specimen. A further difficulty is that the expected concentration of the analyte restricts the scale of miniaturization of the sample. Detection of rare cell types (e.g., cancerous cells, fetal cells in maternal circulation, assessment of minimal residual disease) and microorganisms is problematic, and the expected cell frequency or microbial load will determine a sample size compatible with detection. In such cases a specific selection or a preconcentration step is a more logical approach to ensure the presence of the desired cells or microorganisms. In this review the principles and applications of the current range of microfluidic (e.g., pumps, valves) and microfiltration systems (e.g., silicon microfilters) and electronically controlled microstructured devices relevant to the micro-sample preparation are critically discussed.

Microtechnical Interfaces to Neurons

Abstract: In this chapter, different concepts of microtechnical interfaces to neurons are introduced. A description of hand-crafted neural devices is given which demonstrates the principle applications, limitations and needs of advanced interfaces. The postulation for advanced neural interfaces leading to the discussion on utilizing microelectromechanical systems for neural prostheses. Two on-going research projects are presented highlighting the technical possibilities for the design of neural interfaces. The first example describes a neural prosthesis for an amputee model to control an artificial limb. Secondly, concepts and realization steps are given for a sensory prosthesis which will aid people suffering from a special kind of blindness. A discussion on future concepts follows that outlines possible directions of research and development with the aim of clinical use, implantability of the whole system, wireless signal and energy transmission and closed-loop control of and with biological systems. Possible applications not only include restorative and rehabilitation purposes but also therapeutic applications for pain relief or bio-artificial drug delivery.

The centropolyindanes and related centro-fused polycyclic organic compounds

Abstract: Centropolyindanes constitute a complete family of arylaliphatic polycyclic hydrocarbons containing several indane units. Mutual fusion of the five-membered rings leads to three-dimensional, carbon-rich molecular frameworks bearing a central carbon atom, such as benzo-annelated [3.3.3]propellanes, triquinacenes, and [5.5.5.6]- and [5.5.5.5]fenestranes. In this review, the structural concept of centropolyindanes is contrasted to other fused indane hydrocarbons. Besides the syntheses of the parent centropolyindanes and recently described related indane hydrocarbons, the preparation of a large variety of bridgehead and arene substituted centropolyindanes is presented including strained, heterocyclic, and centrohexacyclic derivatives. In appropriate cases, the particular reactivity and some structural features of these unusual, sterically rigid polycyclic compounds are pointed out.

Modern routes to extended aromatic compounds

Abstract: Aromatic compounds and structures have recently attracted a lot of interest because of their importance in material sciences, the design of molecular devices, metabolism and other biological processes. Although the C-C bond formation is now extensively explored, the accessibility of parent and functionalized aromatics is limited to relatively few examples, especially for high molecular mass polycyclic aromatic hydrocarbons (PAHs). Therefore, this review will concentrate on new developments in the preparation of PAHs in the last decade including pyrolytic approaches to bowl-shaped fullerene fragments and other aromatic compounds, photocyclization and metathesis reactions of stilbenes, [2+2+2] cyclotrimerizations of arynes and alkynes as well as modern Diels-Alder, electrophilic cyclization, and other reactions towards condensed aromatic π-systems. Approaches to symmetrical and unsymmetric (hetero)biaryls as well as other transition metal-catalyzed cyclization reactions will also be discussed briefly. It will be shown that a great variety of synthetic procedures has been developed to prepare PAHs with very interesting molecular structures and/or high efficiency, selectivity, and functionality. The review presents 59 schemes and 318 references.

Polynucleotide Arrays for Genetic Sequence Analysis

Abstract: We describe a new paradigm for genetic analysis based upon high density arrays of polynucleotide probes. Methods for light-directed polynucleotide array synthesis, as well as array packaging, sample preparation, array hybridization, epifluorescence confocal scanning, and data analysis are described. Applications to discovery, genotyping, expression, and resquencing are presented.

Carbon rich cyclophanes with unusual properties : an update

Abstract: Cyclophane chemistry and the research targets in this area have changed over the past two decades. While the unique electronic and structural properties of cyclophanes are still fascinating, more and more attention is now being paid to the function of the molecules, e.g., to the formation of inclusion complexes of large cyclophanes or the use of chiral cyclophanes as auxiliars in asymmetric synthesis. The rigid structures of short bridge cyclophanes has stimulated the synthesis of compounds with extended p-systems. Both their interesting physical properties, e. g., in intramolecular electron transfer processes, and their often beautiful structures of high symmetry are alluring. While synthetic approaches to cyclophanes, their reactivity and inclusion ability have been reviewed extensively, applications of the cyclophane scaffold and its properties are scattered. Therefore the author will focus on the recent developments in the chemistry of cyclophanes that have been used as building blocks for functional devices, as ligands for metal coordination and chiral auxiliaries or chiral ligands in synthesis. It will show that a class of compounds considered esoteric 20 years ago is now even reaching industrial applications. A review with 60 schemes and 122 references.

Design of Novel Aromatics using the Loschmidt Replacement on Graphs

Abstract: Graphs are seen as an excellent place to start the design of a non-natural product for total chemical synthesis. In the design process, the vertices of the graph are replaced by a chemical “radical” to create a molecular graph. Classical examples of this are platonic caged hydrocarbons that arise from CH replacements for the vertices of a platonic polyhedron. In the present context of carbon-rich aromatic structures, we demonstrate how benzene-ring replacement can lead to larger targets as well as a greater variety of theoretically interesting molecules. In honor of the first chemist to think of benzene as a group element, we call this replacement process the Loschmidt replacement.

Unsaturated Oligoquinanes and Related Systems

Abstract: Fully unsaturated oligoquinanes comprise a class of fused five-membered ring compounds which contain extended π-systems with interesting electronic properties. This class of rigid molecules contains planar, bowl- and even ball-shaped structures, and their strain energy increases for each additionally fused five-membered ring. The first three members of this family, fulvene, pentalene and acepentalene have been synthesized or at least generated, whereas all higher members were only approached or studied by computational methods. In this review the class of fully unsaturated oligoquinanes — ranging from fulvene to C20-fullerene — is presented with respect to their syntheses and their properties. Also, related molecules with similar structural features will be discussed, which have not been highlighted in previous volumes of this series [1, 2].