Chiral Supramolecular Assemblies Research Articles

Solvent effect on neutral chiral supramolecular assemblies and their distinct receptor behaviour towards anions.

We describe the distinct receptor behaviour of a neutral chiral Cu(ii) complex in dimethylsulfoxide or methanol towards anions, such as F(-), Cl(-), Br(-), I(-) or OAc(-), where F(-) and OAc(-) show the most colorimetric change, through various spectroscopic techniques. Further insights into this at the molecular level come from the single crystal X-ray structures of both dimethylsulfoxide and methanol solvates which show a solvent effect on their supramolecular network formation. Both chromogenic and fluorogenic sensing of the anions indicate a 2 : 1 receptor-anion formation via anion-π as well as hydrogen bonding interactions.

Maltose-based gelators having azobenzene as light-sensitive unit

Three light-sensitive amphiphiles, based on azobenzene, have been synthesized as supramolecular gelators. A hydrophobic chain with an azobenzene group incorporated at different positions was click coupled to a maltose polar head by a copper(I)-catalysed azide–alkyne [3 + 2] cycloaddition. The liquid crystalline and gel properties of these azo-amphiphilic materials have been studied. Two of these azo-gelators containing maltose give rise to stable gels in water or in a mixture of water and DMSO at room temperature. The chiral supramolecular assemblies of these gelators have been characterised by NMR, electron microscopy and circular dichroism (CD). The light-response of azo-amphiphiles in supramolecular gels has been studied. Also azo-gels, which contained mixtures of the azo-amphiphilic compounds and a similar structural hydrogelator, have been investigated.

ChemInform Abstract: Chiral Hexacoordinated Phosphates: From Pioneering Studies to Modern Uses in Stereochemistry

AbstractReview: 118 refs.

Chiral and Extended π‐Conjugated Bis(2‐pyridyl)phospholes as Assembling N,P,N Pincers for Coordination‐Driven Synthesis of Supramolecular [2,2]Paracyclophane Analogues

Chiral, π-conjugated 3,4-butano-1-phenyl-2,5-bis(2-pyridyl)phosphole derivatives 1a(2,2') and 1a(3) with chiral trans-1,2-diol moieties and fused pinene derivatives, respectively, were prepared from the corresponding chiral diynes by using the Fagan-Nugent method. Their UV/Vis absorption and chiroptical properties (optical rotation and circular dichroism) were studied. Their behavior as N,P,N chelates towards coordination of Cu(I) and formation of chiral supramolecular assemblies with π-conjugated ditopic dicyano ligands was investigated. Chiral C(2)-symmetric rectangles that are [2,2]paracyclophane analogues were obtained, as demonstrated by X-ray crystallography. During the course of this study, the first stable water-soluble phosphole derivative (1a(2)·2 HCl) was prepared. Furthermore, achiral 3,4-butano-1-phenyl-2,5-bis(aza[4]helicene)phosphole 1a(4) was synthesized and displays extended π conjugation. A supramolecular rectangle was obtained by coordination to Cu(I) and assembly with a dicyano stilbene. This coordination-driven supramolecular assembly contains a total of four aza[4]helicene moieties and displays two types of π-π stacking interactions in the solid state, that is, between two helicene moieties and between one helicene and a bridging dicyano ligand. All the supramolecular arrangements are discussed by comparing them with previous work on the parent 3,4-butano-1-phenyl-2,5-bis(2-pyridyl)phosphole.

Chiral hexacoordinated phosphates: From pioneering studies to modern uses in stereochemistry

Chiral cationic molecular structures and supramolecular assemblies can be effectively analyzed, resolved and stereocontrolled through an asymmetric ion pairing with enantiopure hexacoordinated phosphate anions like TRISPHAT or BINPHAT. After an historical perspective on the development of these anionic reagents, this account describes their synthesis and their use as chiral NMR solvating, resolving and asymmetry-inducing reagents.

Molecular structure modulated properties of azobenzene-substituted polydiacetylene LB films: Chirality formation and thermal stability

Langmuir–Blodgett (LB) films of three novel azobenzene-substituted diacetylene monomers (DA1, DA2 and DA3) were fabricated and their optical and chiroptical properties were investigated in detail by ultraviolet–visible (UV–vis) spectra and circular dichroism (CD) spectra. Achiral DA1 molecules could form chiral LB films through overcrowded packing of the azobenzene moiety, while achiral DA2 and DA3 molecules not. When exposed to left-or right-handed circular polarized UV light (CPUL), striking left- or right-handed (opposite) CD signals for azobenzene chromophores and polydiacetylene chains were observed for the polymerized DA1 (PDA1) and DA2 (PDA2) LB films. However, DA3 LB films could hardly be polymerized in this case, and only striking opposite CD signals for azobenzene chromophores could be observed. It was demonstrated that the intermolecular steric hindrance and irregular arrangement of azobenzene chromophores were not favorable for the topo-polymerization and chirality formation of polydiacetylenes backbone. Further, the effects of thermal treatment on the supramolecular chirality of above three LB films were studied. Strong collective noncovalent interactions (π–π stacking) were believed to be responsible for the thermal stability of chiral supramolecular assemblies.

Enantioselective Catalysis of the Aza-Cope Rearrangement by a Chiral Supramolecular Assembly

The chiral supramolecular catalyst Ga(4)L(6) [L = 1,5-bis(2,3-dihydroxybenzoylamino)naphthalene] is a molecular tetrahedron that catalyzes the 3-aza-Cope rearrangement of allyl enammonium cations. This catalysis is accomplished by preorganizing the substrate in a reactive conformation within the host. This work demonstrates that through the use of enantiopure assembly, its chiral cavity is capable of catalyzing the 3-aza-Cope rearrangement enantioselectively, with yields of 21-74% and enantiomeric excesses from 6 to 64% at 50 degrees C. At lower temperatures, the enantioselectivity improved, reaching 78% ee at 5 degrees C. This is the highest enantioselectivity to date induced by the chiral cavity of a supramolecular assembly.

Influence of Counteranion on the Chiral Supramolecular Assembly of Alkynylplatinum(II) Terpyridyl Metallogels That Are Stabilised by Pt⋅⋅⋅Pt and π–π Interactions

Helical superstructures: A series of luminescent chiral alkynylplatinum(II)-terpyridyl complexes has been successfully synthesised and shown to be capable of forming metallogels that show helical fibrous nanostructures. The chiral supramolecular structure and the spectroscopic properties are found to depend on the extent of aggregation through Pt...Pt and pi-pi interactions in various counteranions, as revealed by UV/Vis, circular dichroism (CD), and luminescence studies (see picture).

Chiral Specificity of Doubly Resonant Sum-Frequency Generation in An Anisotropic Thin Film

The chiral specificity of infrared-visible sum-frequency generation (SFG) is investigated under doubly resonant (DR) conditions. An anisotropic thin film made of stacks of chiral supramolecular assemblies is investigated as a function of incoming and outgoing light polarizations. SFG experiments, crossed with in situ polarized absorption measurements, show that the double resonance process leads to a selection of the nonvanishing components of the chiral second-order susceptibility tensor. Specifically, it is demonstrated that a chiral component overcomes the achiral ones. This result is linked to the model for DR-SFG and to the selection rules of the vibration modes and electronic transition involved in the process.

A multivalent PyBox asterisk ligand

A discrete multivalent PyBox ligand was investigated. An efficient synthesis and the properties of 1 are reported (UV–visible, cyclic voltammetry, NMR, MALDI-Tof). It incorporates a sulfur-rich persulfurated benzene core which was compatible with a metal-catalyzed reaction in spite of donating and oxidizable sulfur atoms. Metal-catalysis with a persulfurated aromatic ligand was demonstrated for the first time in a model reaction: the Rh-enantioselective hydrosilylation of acetophenone. The interesting features were the reactivity and the enantiocatalytic behavior while varying the metal content. This work promotes new thoughts toward chiral supramolecular assemblies or metal nanoparticles stabilized with chiral multivalent ligands.

Chiral Supramolecular Assemblies of a Squaraine Dye in Solution and Thin Films: Concentration‐, Temperature‐, and Solvent‐Induced Chirality Inversion

We prepared novel cholesterol-appended squaraine dye 1 and model squaraine dye 2 and investigated their aggregation behavior in solution and thin films using photophysical, chiroptical, and microscopic techniques. Investigations on the dependence of aggregation on solvent composition (good/poor, CHCl3/CH3CN) demonstrated that squaraine dye 1 forms two novel H-type chiral supramolecular assemblies with opposite chirality at different good/poor solvent compositions. Model compound 2 formed J-type achiral assemblies under similar conditions. The supramolecular assembly of 1 observed at lower fractions of the poor solvent could be assigned to the thermodynamically stable form, while a kinetically controlled assembly is formed at higher fractions of the poor solvent. This assignment is evidenced by temperature- and concentration-dependent experiments. With increasing temperature, the chirality of the kinetically controlled aggregate was lost and, on cooling, the aggregate with the opposite chirality was formed. On further heating and cooling the aggregates thus formed resulted in no significant changes in chirality, that is they are thermodynamically stable. Similarly, at lower concentrations, the thermodynamically stable form exists, but at higher concentration aggregation was found to proceed with kinetic control. Based on these observations it can be assumed that formation of the kinetically controlled assembly might be largely dependent on the presence of the nonpolar cholesterol moiety as well as the amount of poor solvent present. However, under solvent-free conditions, structurally different aggregates were observed when drop cast from solutions containing monomer, whereas a left-handed CD signal corresponding to the thermodynamically controlled assemblies was observed from pre-aggregated solutions.

Chiral Spaces in Supramolecular Assemblies

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Scope and Mechanism of the C−H Bond Activation Reactivity within a Supramolecular Host by an Iridium Guest: A Stepwise Ion Pair Guest Dissociation Mechanism

A chiral self-assembled supramolecular M(4)L(6) assembly has been shown to be a suitable host for a series of reactive monocationic half-sandwich iridium guests 1, 3, and 4 that are capable of activating C-H bonds. Upon encapsulation, selective C-H bond activation of organic substrates occurs. Precise size and shape selectivity are observed in the C-H bond activation of aldehydes and ether substrates. The reactions exhibit significant kinetic diastereoselectivities. Thermodynamic studies have shown that the iridium starting materials and products are bound strongly by the host assembly. The encapsulation process is largely entropy-driven. Kinetic investigations with water-soluble phosphine traps and added salts have provided evidence for a unique stepwise mechanism of guest dissociation for [4 subset Ga(4)L(6)]. Iridium guest 4 first dissociates from the host cavity to form an ion pair with the host exterior. This species then fully dissociates from the host exterior into the bulk solution. Model ion pair intermediates were characterized directly with (1)H NMR NOESY techniques. The rate of iridium guest dissociation is slower than the rate observed for the C-H bond activation processes, indicating that the selective C-H bond activation reactivity occurs within the cavity of the supramolecular host.

Chiral supramolecular assemblies from some achiral Schiff bases without alkyl chain through the organization at the air/water interface

Two novel Schiff bases, 1-benzthiazolyliminomethyl-2-naphthol (TSN) and 1-(6′-chlorobenzthiazolyliminomethyl)-2-naphthol (ClTSN), were synthesized and their molecualr assemblies at the air/water interface were investigated. Both of the compounds could be spread on water surface although they have no alkyl chains. The Schiff base molecules formed J- or H-aggregate in the Langmuir–Schaefer (LS) films transferred from water surface. In AFM, a multilayer film formation was observed. It is interesting to note that the LS films of achiral TSN and ClTSN transferred from water surface showed chirality. The supramolecular chirality in these LS films was suggested to be due to a cooperative stereoregular π–π stacking of the functional groups in a helical sense.

Amplification of chirality in liquid crystals

The amplification of molecular chirality by liquid crystalline systems is widely applied in investigations towards enantioselective solvent-solute interactions, chiral supramolecular assemblies, smart materials, and the development of liquid crystal displays. Here we present an overview of recent achievements in the development of new chiral dopant systems for the generation of cholesteric liquid crystalline phases. Based on a distinction between shape-persistent and bistable dopants, several dopant classes will be discussed.

Polymorphism in supramolecular chiral structures of R- and S-alanine on Cu(1 1 0)

A comprehensive study of the local and supramolecular adsorption structures created by the chiral R- and S-enantiomers of alanine on the Cu(1 1 0) surface has been conducted using a multi-technique approach, including reflection absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). Over the entire 300–470 K temperature range studied, the amino acid is found to adsorb as an alaninate species with a local chiral adsorption motif. However, this singular preference of local chemical form contrasts sharply with the supramolecular organisation at the surface where polymorphism is exhibited. This polymorphic behaviour arises from subtle and dynamic changes in the bonding, orientation and adsorption footprints of individual molecules, leading to alterations in the molecule–metal, intermolecular and metal–metal interactions that dictate self-assembly. Thus, at low coverage, a single disordered phase is observed but at higher coverage, three other temperature dependent phases occur. At room temperature, a two-dimensional equivalent of a ‘nematic’ phase is constructed from short single- and double-chain chiral assemblies that possess a preferred chiral orientation but no long range periodicity. This ‘nematic’ phase acts as a precursor to a highly ordered chiral supramolecular assembly, created at 430 K, that consists of regular arrays of size- and shape-defined chiral clusters. This phase possesses global organisational chirality with only one chiral domain observed for each enantiomer. For both the ‘nematic’ and the highly ordered chiral phase, the organisation for the R-enantiomer is the mirror image of that seen for the S-enantiomer, i.e., there is chirality transfer from the nanoscale to the macroscale. By 470 K, both R- and S-alanine form an achirally organised (3 × 2) structure that appears to be the thermodynamically favoured phase for the alanine/Cu(1 1 0) system. The supramolecular organisation and chirality of the various structures are discussed in terms of the molecular chirality and footprint chirality of the alaninate, together with possible intermolecular interactions and reconstructions of the underlying metal surface atoms. A number of candidate models for the system are suggested, but it is clear that a full understanding of this complex adsorption system will only emerge from further careful, high level experimental and computational efforts that currently remain a challenge.

Supramolecular Assemblies of a Series of 2‐Arylbenzimidazoles at the Air/Water Interface: In Situ Coordination, Surface Architecture and Supramolecular Chirality

The spreading behavior and supramolecular assemblies of some arylbenzimidazoles with 2-substituted aromatic groups such as phenyl, naphthyl, anthryl and pyrenyl on water surface and the subphase containing AgNO3 were investigated. It was observed that although these compounds lack long alkyl chains, they showed surface activity when spread from chloroform solution on water surface and formed the supramolecular assemblies. When AgNO3 was present in the subphase, a coordination between the imidazole group of the compounds and Ag(I) occurred in situ in the spreading film, which was verified by the surface pressure/area (pi-A) isotherms and UV/Vis absorption spectra. Both the spreading films from water and the aqueous AgNO3 subphase were transferred onto solid substrates and their surface morphologies as well as properties were characterized by AFM, UV/Vis absorption and CD spectra. Various surface morphologies such as nanoparticles, block domains and nanoutensils were observed depending on the substituted aromatic groups. Interestingly, although all of these compounds were achiral, supramolecular chirality was obtained for some of the arylbenzimidazole films assembled from either the water surface or the subphase containing AgNO3. It was revealed that chiral assemblies could be obtained from water surface for the benzimidazoles which have pyrenyl or alpha-naphthyl groups. For benzimidazole derivative with anthryl group, chiral assemblies could be obtained when spreading on the aqueous AgNO3 subphase. For the benzimidazoles with phenyl or beta-naphthyl groups, no chirality was obtained. It was suggested that both the overcrowded stacking of the aromatic groups and the cooperative arrangement of the molecules on water surface or aqueous AgNO3 subphase play a crucial role in forming the chiral supramolecular assemblies.

Formation of supramolecular cavitands on copper electrode surfaces.

The adsorption of 1,1'-dibenzyl-4,4'-bipyridinium molecules (dibenzyl-viologen or DBV(2+) for the sake of simplicity) on chloride precovered Cu(100) has been studied in an electrochemical environment by means of cyclic voltammetry and in situ scanning tunneling microscopy. DBV(2+) spontaneously forms a highly ordered phase on the chloride c(2 x 2) adlayer at potentials close to the onset of the copper dissolution reaction when the pure supporting electrolyte (10 mM HCl/5 mM KCl) is exchanged by one also containing DBV(2+). This ordered phase can be described by a ( radical 53 x radical 53)R15.9 degrees unit cell relating the organic adlayer to the chloride c(2 x 2) structure underneath or alternatively by a ( radical 106 x radical 106)R29.05 degrees unit cell relating the organic layer to the Cu(1 x 1) substrate structure. Thus, the negatively charged chloride layer acts as a template for the adsorption and phase formation of DBV(2+). Compared to the copper-chloride interaction, the DBV(2+)-chloride interaction appears to be weaker since the organic layer can be easily removed from the surface by the tunneling tip when drastic tunneling conditions (low bias voltage, high tunneling current) are applied. A key structural element of the DBV(2+) adlayer is an assembly of four individual DBV(2+) molecules forming square-shaped supramolecular units with pronounced cavities in their center. Characteristically, the supramolecular assemblies reveal a preferential rotational orientation resulting in the appearance of two chiral forms of these assemblies. Furthermore, these two chiral supramolecular assemblies occur in two mirrored domains of the ( radical 53 x radical 53)R15.9 degrees structure. It can be assumed that these viologen-based supramolecular architectures can be used as potential host cavitands for the inclusion of smaller organic molecules.

Reversible optical transcription of supramolecular chirality into molecular chirality.

In nature, key molecular processes such as communication, replication, and enzyme catalysis all rely on a delicate balance between molecular and supramolecular chirality. Here we report the design, synthesis, and operation of a reversible, photoresponsive, self-assembling molecular system in which molecular and supramolecular chirality communicate. It shows exceptional stereoselectivity upon aggregation of the molecules during gel formation with the solvent. This chirality is locked by photochemical switching, a process that is subsequently used to induce an inverted chiral supramolecular assembly as revealed by circular dichroism spectroscopy. The optical switching between different chiral aggregated states and the interplay of molecular and supramolecular chirality offer attractive new prospects for the development of molecular memory systems and smart functional materials.

Encapsulation of cationic ruthenium complexes into a chiral self-assembled cage.

A chiral supramolecular assembly encapsulates the two cationic ruthenium sandwich complexes [CpRu(eta(6)-C(6)H(6))](+) and [CpRu(p-cymene)](+). The host-guest complexes K(11)[CpRu(eta(6)-C(6)H(6)) subset Ga(4)L(6)] (2) and K(11)[CpRu(p-cymene) subset Ga(4)L(6)] (3) were characterized by one- and two-dimensional NMR techniques as well as by electrospray mass spectrometry. Encapsulation of the prochiral complex [CpRu(p-cymene)](+) by the chiral host renders enantiotopic protons diastereotopic as evidenced by (1)H NMR spectroscopy.