David N. Reinhoudt

Complete asymmetric chirality in a hydrogen-bonded assembly

Chirality at the supramolecular level involves the non-symmetric arrangement of molecular components in a non-covalent assembly1,2. Supramolecular chirality is abundant in biology, for example in the DNA double helix3, the triple helix of collagen4 and the α-helical coiled coil of myosin5. These structures are stabilized by inter-strand hydrogen bonds, and their handedness is determined by the configuration of chiral centres in the nucleotide or peptide backbone. Synthetic hydrogen-bonded assemblies have been reported that display supramolecular chirality in solution6,7,8 or in the solid state9,10,11,12. Complete asymmetric induction of supramolecular chirality—the formation of assemblies of a single handedness—has been widely studied in polymeric superstructures13,14. It has so far been achieved in inorganic metal-coordinated systems15,16,17, but not in organic hydrogen-bonded assemblies18,19,20. Here we describe the diastereoselective assembly of enantio-pure calix[4]arene dimelamines and 5,5-diethylbarbituric acid (DEB) into chiral hydrogen-bonded structures of one handedness. The system displays complete enantioselective self-resolution: the mixing of homomeric assemblies (composed of homochiral units) with opposite handedness does not lead to the formation of heteromeric assemblies. The non-covalent character of the chiral assemblies, the structural simplicity of the constituent building blocks and the ability to control the assembly process by means of peripheral chiral centres makes this system promising for the development of a wide range of homochiral supramolecular materials or enantioselective catalysts.Chirality at the supramolecular level involves the non-symmetric arrangement of molecular components in a non-covalent assembly,. Supramolecular chirality is abundant in biology, for example in the DNA double helix, the triple helix of collagen and the α-helical coiled coil of myosin. These structures are stabilized by inter-strand hydrogen bonds, and their handedness is determined by the configuration of chiral centres in the nucleotide or peptide backbone. Synthetic hydrogen-bonded assemblies have been reported that display supramolecular chirality in solution or in the solid state. Complete asymmetric induction of supramolecular chirality—the formation of assemblies of a single handedness—has been widely studied in polymeric superstructures,. It has so far been achieved in inorganic metal-coordinated systems, but not in organic hydrogen-bonded assemblies. Here we describe the diastereoselective assembly of enantio-pure calix[4]arene dimelamines and 5,5-diethylbarbituric acid (DEB) into chiral hydrogen-bonded structures of one handedness. The system displays complete enantioselective self-resolution: the mixing of homomeric assemblies (composed of homochiral units) with opposite handedness does not lead to the formation of heteromeric assemblies. The non-covalent character of the chiral assemblies, the structural simplicity of the constituent building blocks and the ability to control the assembly process by means of peripheral chiral centres makes this system promising for the development of a wide range of homochiral supramolecular materials or enantioselective catalysts.

Rare-earth doped polymers for planar optical amplifiers

Optical waveguide amplifiers based on polymer materials offer a low-cost alternative for inorganic waveguide amplifiers. Due to the fact that their refractive index is similar to that of standard optical fibers, they can be easily coupled to existing fibers with low coupling losses. Doping the polymer with rare-earth ions that yield optical gain is not straightforward, as the rare-earth salts are poorly soluble in the polymer matrix. This review article focuses on two different approaches to dope a polymer waveguide with rare-earth ions. The first approach is based on organic cage-like complexes that encapsulate the rare-earth ion and are designed to provide coordination sites to bind the rare-earth ion and to shield it from the surrounding matrix. These complexes also offer the possibility of attaching a highly absorbing antenna group, which increases the pump efficiency significantly. The second approach to fabricate rare-earth doped polymer waveguides is obtained by combining the excellent properties of SiO2 as a host for rare-earth ions with the easy processing of polymers. This is done by doping polymers with Er-doped silica colloidal spheres.

Dinuclear metallo-phosphodiesterase models: application of calix[4]arenes as molecular scaffolds

An important goal in supramolecular chemistry is the synthesis of molecules that exhibit catalytic activity analogous to the activity of enzymes. In this respect, studies toward the biomimetic catalysis of phosphate diester cleavage have received particular attention. In nature this process is catalyzed by enzymes that possess often two or three divalent metal ions in their active sites. In order to mimic the active sites of these metallo-phosphodiesterases chemists generally attempt to connect ligated metal ions by a molecular spacer in such a way that the metal–metal distance matches with the anionic pentacoordinate phosphorus transition state. However, in contrast to enzymes, which bind the transition state by multiple contacts via an induced fit mechanism, many of the low molecular weight model systems exhibit only minor catalytic activity due to lack of catalytic groups and too much rigidity or flexibility. Our approach is to use calix[4]arenes as a molecular scaffold for the dynamic preorganization of multiple catalytic groups. In this review models for dinuclear metallo-phosphodiesterases and the use of calix[4]arenes in such models are described.

Sensor Functionalities in Self-Assembled Monolayers

Sensors based on self-assembled monolayers (SAMs) are the topic of this article. First an overview of the analytical techniques used for the characterization of monolayers is given. This is followed by details of the use of functional SAMs to study interactions with analytes, for example, the complexation of metal ions by crown ether adsorbates.

Nichtkovalente Synthese mit Wasserstoffbrücken

Wasserstoffbrucken ahneln Menschen insofern, als dass sie zur Gruppenbildung neigen. Einzeln sind sie schwach, leicht zu brechen und manchmal schwer zu finden. Handeln sie jedoch gemeinsam, so werden sie viel starker und unterstutzen einander. Bei diesem, unter dem Begriff Kooperativitat bekannten Phanomen ist 1+1 mehr als 2. Unter Nutzung dieses Prinzips haben Chemiker eine grose Vielzahl chemisch stabiler Strukturen entwickelt, deren Bildung auf der reversiblen Knupfung mehrerer Wasserstoffbrucken beruht. Mehr als 20 Jahre des Studiums dieser Phanomene haben zur Entwicklung einer neuen Disziplin innerhalb der organischen Synthese gefuhrt, die man heute als nichtkovalente Synthese bezeichnet. Dieser Aufsatz beschreibt die nichtkovalente Synthese auf der Basis der reversiblen Bildung mehrerer Wasserstoffbrucken. Er beginnt mit einer eingehenden Beschreibung des Wesens der Wasserstoffbrucke, fuhrt dann durch eine Vielzahl von bimolekularen Aggregaten und Assoziaten hoherer Ordnung und erlautert die allgemeinen Prinzipien, die deren Stabilitat zugrunde liegen. Besondere Aufmerksamkeit wird reversibel uber Wasserstoffbrucken gebildeten Kapseln gewidmet, die interessante Einschlussphanomene aufweisen. Weiterhin wird die Rolle von Wasserstoffbrucken in Selbstreplikationsprozessen diskutiert, und abschliesend werden neue Materialien (Nanorohren, Flussigkristalle, Polymere usw.) und Prinzipien (dynamische Bibliotheken) vorgestellt, die in jungster Zeit aus diesem faszinierenden Forschungsgebiet hervorgegangen sind.

An enantiomerically pure hydrogen-bonded assembly

Chiral molecules have asymmetric arrangements of atoms, forming structures that are non-superposable mirror images of each other. Specific mirror images (‘enantiomers’) may be obtained either from enantiomerically pure precursor compounds, through enantioselective synthesis, or by resolution of so-called racemic mixtures of opposite enantiomers, provided that racemization (the spontaneous interconversion of enantiomers) is sufficiently slow. Non-covalent assemblies can similarly adopt chiral supramolecular structures, and if they are held together by relatively strong interactions, such as metal coordination, methods analogous to those used to obtain chiral molecules yield enantiomerically pure non-covalent products. But the resolution of assemblies formed through weak interactions, such as hydrogen-bonding, remains challenging, reflecting their lower stability and significantly higher susceptibility to racemization. Here we report the design of supramolecular structures from achiral calix[4]arene dimelamines and cyanurates, which form multiple cooperative hydrogen bonds that together provide sufficient stability to allow the isolation of enantiomerically pure assemblies. Our design strategy is based on a non-covalent ‘chiral memory’ concept, whereby we first use chiral barbiturates to induce the supramolecular chirality in a hydrogen-bonded assembly, and then substitute them by achiral cyanurates. The stability of the resultant chiral assemblies in benzene, a non-polar solvent not competing for hydrogen bonds, is manifested by a half-life to racemization of more than four days at room temperature.

Optical properties of erbium-doped organic polydentate cage complexes

The optical properties of different erbium (Er)-doped polydentate hemispherand organic cage complexes are studied, for use in polymer-based planar optical amplifiers. Room temperature photoluminescence at 1.54 um is observed, due to an intra-4 f transition in Er31. The Er is directly excited into one of the 4 f manifolds (at 488 nm), or indirectly (at 287 nm) via the aromatic part of the cage. The luminescence spectrum is 70 nm wide (full width at half maximum), the highest known for any Er-doped material, enabling high gain bandwidth for optical amplification. The absorption cross section at 1.54 um is 1.1x10 -20 cm2, higher than in most other Er-doped materials, which allows the attainment of high gain. Measurements were performed on complexes in KBr tablets, in which the complex is present in the form of small crystallites, or dissolved in the organic solvents dimethylformamide and butanol-OD. In KBr the luminescence lifetime at 1.54 um is <0.5 us, possibly due to concentration quenching effects. In butanol-OD solution, the lifetime is 0.8 us, still well below the radiative lifetime of 4 ms estimated from the measured absorption cross sections. Experiments on the selective deuteration of the near-neighbor C–H bonds around the Er3+-ion indicate that these are not the major quenching sites of the Er31 luminescence. Temperature dependent luminescence measurements indicate that temperature quenching is very small. It is therefore concluded that an alternative luminescence quenching mechanism takes place, presumably due to the presence of O–H groups on the Er-doped complex (originating either from the synthesis or from the solution). Finally a calculation is made of the gain performance of a planar polymer waveguide amplifier based on these Er complexes, resulting in a threshold pump power of 1.4 mW and a typical gain of 1.7 dB /cm.

Lanthanide(III)-Doped Nanoparticles That Emit in the Near-Infrared

Redispersible near-infrared emitting lanthanide-doped nanoparticles are reported. The emission and excitation spectra have been examined and the luminescence lifetimes of these materials have been determined. High quantumyields and lifetimes in the millisecond region were found. It is furthermore demonstrated that these nanoparticles can easily be incorporated in polymer materials, thus making them of particular interest as the active material in polymer-based telecommunication components, polymer-based lasers, polymer displays, and polymer light-emitting diodes.

Supramolecular science : where it is and where it is going

Amongst the usual selection of the best from all fields of chemistry, this months issue contains a high proportion of papers on the subject of supramolecular science. We are highlighting this topic in advance of the NATO Advanced Research Workshop celebrating the conclusion of a long-term special programme on Supramolecular Science; Professors Ungaro, Stoddart, and Reinhoudt have contributed an essay looking at the astonishing expansion of research in this field in recent years, and leading researchers have contributed five Concepts articles discussing clearly and sometimes provocatively the current state of research in their own areas. These are listed in the Table of Contents on the next page.

Sugar-integrated gelators of organic solvents - their remarkable diversity in gelation ability and aggregate structure

A drastic change in the solubility and gelation properties of 1-O-methyl-4,6-O-benzylidene derivatives of monosaccharides (see figure) results from the change in the configuration of only one carbon atom.