Isolable chiral aggregates of achiral π-conjugated carboxylic acids

The induced aggregation of achiral building blocks by a chiral species to form chiral aggregates with memorized chirality has been observed for a number of systems. However, chiral memory in isolated aggregates of achiral building blocks remains rare. One possible reason for this discrepancy could be that not much is understood in terms of designing these chiral aggregates. Herein, we report a strategy for creating such isolable chiral aggregates from achiral building blocks that retain chiral memory after the facile physical removal of the chiral templates. This strategy was used for the isolation of chiral homoaggregates of neutral achiral π-conjugated carboxylic acids in pure aqueous solution. Under what we have termed an "interaction-substitution" mechanism, we generated chiral homoaggregates of a variety of π-conjugated carboxylic acids by using carboxymethyl cellulose (CMC) as a mediator in acidic aqueous solutions. These aggregates were subsequently isolated from the CMC templates whilst retaining their memorized supramolecular chirality. Circular dichroism (CD) spectra of the aggregates formed in the acidic CMC solution exhibited bisignated exciton-coupled signals of various signs and intensities that were maintained in the isolated pure homoaggregates of the achiral π-conjugated carboxylic acids. The memory of the supramolecular chirality in the isolated aggregates was ascribed to the substitution of COOH/COOH hydrogen-bonding interaction between the carboxylic acid groups within the aggregates for the hydrogen-bonding interactions between the COOH groups of the building blocks and the chiral templates. We expect that this "interaction-substitution" procedure will open up a new route to isolable pure chiral aggregates from achiral species.     

Mechanism of diastereoisomer-induced chirality of BiOBr

Chiral molecule-driven asymmetric structures are known to be elusive because of the intriguing chirality transfer from chiral molecules to achiral species. Here, we found that the chiral assembly of BiOBr is independent of the chirality of the organic molecular inducer but dependent on geometric structural matching between the inducer and inorganic species. Diastereoisomeric sugar alcohols (DSAs) with identical numbers of carbon chiral centers and functional groups but with different R/S configurations and optical activities (OAs) were chosen as symmetry-breaking agents for inducing chiral mesostructured BiOBr films (CMBFs) under hydrothermal conditions. Multiple levels of chirality with different handedness were identified in the CMBFs. Density functional theory (DFT) calculations and molecular dynamics (MD) simulations suggest that asymmetric defects in the Br–Bi tetragonal cone caused by physically adsorbed DSAs on the surfaces of the BiOBr crystals are the geometric basis for triggering the chiral twist in the BiOBr monolayer. Our findings provide new insights for understanding the origin of chirality and the chiral transfer mechanism underlying the assembly of achiral species.

The chirality transfer is dependent on geometrical matching between the chiral inducer and inorganic species.     

Dynamic chirality, chirality transfer and aggregation behaviour of dithienylethene switches

The synthesis and characterisation of a series of chiral and achiral low molecular weight organogelators (LMWGs) based on bis-amide substituted dithienylethene photochromic switches is reported. The LMWGs gelate a range of solvents depending on the specific functionalisation of the hydrogen bonding amide groups. In mixtures of chiral and achiral LMWGs the stereochemical outcome of the chiral aggregation is determined by the chiral LMWG molecules in most cases. However, for the first time we demonstrate that the stereochemical outcome of the aggregation can be influenced by the achiral LWMG molecules in some cases. Furthermore specific π-π (and/or van der Waals) interactions of chiral LMWGs 1-3o with the solvent allow the solvent to influence the control of chirality of aggregation. This influence of the solvent has a dramatic effect on whether four- or two-gel states are available.     

Insight into the chiral induction in supramolecular stacks through preferential chiral solvation

Preferred handedness in the supramolecular chirality of self-assembled achiral oligo(p-phenylenevinylene) (OPV) derivatives is induced by chiral solvents and spectroscopic probing provides insight into the mechanistic aspects of this chiral induction through chiral solvation.     

Chiral assemblies of achiral rigid-flexible molecules at the air/water interface induced by silver(I) coordination

Chiral films are obtained from achiral rigid-flexible molecules. Due to hydrogen bonding and steric constraints, these molecules can self-assemble into chiral assemblies at the air/water interface upon compression. When the molecules are spread on a subphase containing AgNO(3), they form a stable monolayer through coordination with Ag(I) ions, as confirmed by surface pressure-area isotherms, UV/Vis and FTIR spectroscopy, and AFM. More interestingly, macroscopic chirality was detected in the Ag(I)-coordinated films by circular dichroism measurements. The formation mechanisms of the two kinds of chiral films are briefly discussed.     

手性超分子组装研究进展

介绍了超分子手性的基本构筑方式及其特点,分别从手性分子组装、手性分子诱导非手性分子及非手性分子组装等3个方面对最近几年来在手性超分子组装领域内的重要成果及最新进展进行了综述,并对这一领域的发展前景作了展望。     

Structural rules for the chiral supramolecular organization of OPE-based discotics: induction of helicity and amplification of chirality

A systematic study on the structural rules that regulate the chiral supramolecular organization of oligo(phenylene ethynylene) (OPE)-based discotics is presented. This study is based on the chirooptical properties of two different series of triangular shape OPEs. The first of them is composed by OPE-based trisamides with a variable number of chiral side chains (compounds 1) that self-assemble following a cooperative mechanism. The CD experiments carried out with these desymmetrized trisamides demonstrate that only one stereogenic center is sufficient to achieve a helical organization with a preferred handedness. However, the ability to amplify the chirality decreases upon decreasing the number of stereocenters at the peripheral side chains. The second series is constituted by triangular shape OPEs with a variable number of ether and amide functional groups and constant absolute configuration of the stereogenic centers at all of the peripheral chains (compounds 2). These compounds do not self-assemble into helical aggregates as demonstrated by the corresponding CD studies. The amplification of chirality observed in the mixtures of some of the components of both series has been investigated. The combination of chiral trisamide 1d with chiral but nonhelical 2b or 2c does not produce an amplification of chirality most probably due to the mismatch between the stereogenic centers of both components. However, the combination of achiral trisamide 1a with chiral but nonhelical bisamide 2c generates, in a cooperative manner, helical structures with a preferred handedness in a process involving the transfer of helicity from 1a to 2c and the transfer of chirality from 2c to 1a. The structural features of the OPE discotics also exert a strong influence on the columnar aggregates. Thus, while achiral 1a bundles into thick filaments to form an organogel, the gelation ability of these triangular OPEs decreases upon increasing the number of stereogenic centers, being totally canceled for compounds 2 in which the amide functionalities are replaced by ether linkages. Finally, we have also registered AFM images of the helical aggregates obtained from the mixture of 1a+2c, which implies an efficient transfer of the chiral objects from solution to surfaces. The study presented herein increases the understanding of the structural rules that regulate the chiral supramolecular organization of discrete molecules in general and, more specifically, those based on π-conjugated oligomers.     

Amplification of Dissymmetry for Circularly Polarized Photodetection by Cooperative Supramolecular Polymerization

Circularly polarized photodetectors require chiral light absorption materials with high sensing efficiency and low costs. Here readily accessible point chirality has been introduced to dicyanostilbenes as the chiral source, facilitating remote chirality transfer to the π-aromatic core by cooperative supramolecular polymerization. The single-handed supramolecular polymers display powerful circularly polarized photodetection capability with a dissymmetry factor value as high as 0.83, superior to those of π-conjugated small molecules and oligomers. Strong chiral amplification occurs between the enantiopure sergeants and the achiral soldiers. The resulting supramolecular copolymers exhibit comparable photodetection efficiency to those of the homopolymeric ones, with a 90 % decrease in the enantiopure compound consumption. Therefore, cooperative supramolecular polymerization provides an effective yet economical avenue toward circularly polarized photodetection applications.     

Chiral expression at the solid-liquid interface: a joint experimental and theoretical study of the self-assembly of chiral porphyrins on graphite

The chiral organization of an enantiopure functional molecule on an achiral surface has been studied with the aim of understanding the influence of stereogenic centers on the self-assembly in two dimensions. A chiral tetra meso-amidophenyl-substituted porphyrin containing long hydrophobic tails at the periphery of the conjugated pi-electron system was prepared for this purpose. Scanning tunneling microscopy (STM) images of the compound at the graphite-heptanol interface reveal a chiral arrangement of the molecules, with the porphyrin rows tilted by 13 degrees with respect to the normal to the graphite axes. In terms of molecular modeling, a combination of molecular dynamics simulations on systems constrained by periodic boundary conditions and on unconstrained large molecular aggregates has been applied to reach a quantitative interpretation on both the density of the layer and its orientation with respect to the graphite surface. The results show clearly that (i) the methyl groups of the stereogenic point toward the graphite surface and (ii) the porphyrin molecules self-assemble into an interdigitated structure where the alkyl chains align along one of the graphite axes and the porphyrin cores are slightly shifted with respect to one another. The direction of this shift, which defines the chirality of the monolayer, is set by the chirality of the stereogenic centers. Such an arrangement results in the formation of a dense chiral monolayer that is further stabilized by hydrogen bonding with protic solvents.     

Inversion of the Supramolecular Chirality of Nanofibrous Structures through Co‐Assembly with Achiral Molecules

To understand the behavior of chiral nanostructures, it is of critical importance to study how achiral molecules regulate the chirality of such nanostructures and what the main driving forces for the regulation processes are. In this work, the supramolecular chirality of helical nanofibers consisting of phenylalanine‐based enantiomers is inverted by achiral bis(pyridinyl) derivatives through co‐assembly. This inversion is mainly mediated by intermolecular hydrogen bonding interactions between the achiral additives and the chiral molecules, which may induce stereoselective interactions and different reorientations for the assembled molecules, as confirmed by calculations. This work not only exemplifies a feasible method to invert the helicity of chiral nanostructures by the addition of achiral molecules, but also provides a method to explore their functions in environments where chiral and achiral molecules are in close proximity.     

Expression of Chirality by Achiral Coadsorbed Molecules in Chiral Monolayers Observed by STM

The supramolecular packing mode of physisorbed monolayers built up by chiral isophthalic acid derivatives and coadsorbed achiral solvent molecules was imaged at the liquid/graphite interface with scanning tunneling microscopy (STM). The picture on the right shows the submolecularly resolved STM image of an enantiomorphous domain composed of the R enantiomer of the isophthalic acid derivative studied and 1-heptanol molecules; the latter express the chirality of the monolayer. Upon adsorption a racemic mixture is separated into enantiomorphous domains.     

Herringbone and Helical Self-Assembly of π-Conjugated Molecules in the Solid State through CH/π Hydrogen Bonds

Self-organization of organic molecules through weak noncovalent forces such as CH/π interactions and creation of large hierarchical supramolecular structures in the solid state are at the very early stage of research. The present study reports direct evidence for CH/π interaction driven hierarchical self-assembly in π-conjugated molecules based on custom-designed oligophenylenevinylenes (OPVs) whose structures differ only in the number of carbon atoms in the tails. Single-crystal X-ray structures were resolved for these OPV synthons and the existence of long-range multiple-arm CH/π interactions was revealed in the crystal lattices. Alignment of these π-conjugated OPVs in the solid state was found to be crucial in producing either right-handed herringbone packing in the crystal or left-handed helices in the liquid-crystalline mesophase. Pitch- and roll-angle displacements of OPV chromophores were determined to trace the effect of the molecular inclination on the ordering of hierarchical structures. Furthermore, circular dichroism studies on the OPVs were carried out in the aligned helical structures to prove the existence of molecular self-assembly. Thus, the present strategy opens up new approaches in supramolecular chemistry based on weak CH/π hydrogen bonding, more specifically in π-conjugated materials.     

Tuning Achiral to Chiral Supramolecular Helical Superstructures

The design and synthesis of achiral organic functional molecules which can assemble into a chiral with selective handedness in the absence of chiral substances is an important in understanding the role chirality plays within these systems. In this review, we described general approaches towards supramolecular chiral molecules the synthesis and self‐assembly of achiral molecule to active chiral molecules to investigate controlled supramolecular chiral nanostructures with their photoluminescent properties for rapid, sensitive and selective detection of analytes of choice. Various small molecules have been discussed for achiral to chiral along with induction of chirality and controlled chiral helical structures in detail. We discussed few examples where stimuli used to control the chirality such as temperature, pH etc. Finally, we will also explore on the photo responsive helicity properties of the aggregation induced emission active molecule such as tetraphenylethene conjugates.     

Laser-operated chiral molecular switch: quantum simulations for the controlled transformation between achiral and chiral atropisomers

We report quantum dynamical simulations for the laser controlled isomerization of 1-(2-cis-fluoroethenyl)-2-fluorobenzene based on one-dimensional electronic ground and excited state potentials obtained from (TD)DFT calculations. 1-(2-cis-fluoroethenyl)-2-fluorobenzene supports two chiral and one achiral atropisomers, the latter being the most stable isomer at room temperature. Using a linearly polarized IR laser pulse the molecule is excited to an internal rotation around its chiral axis, i.e. around the C-C single bond between phenyl ring and ethenyl group, changing the molecular chirality. A second linearly polarized laser pulse stops the torsion to prepare the desired enantiomeric form of the molecule. This laser control allows the selective switching between the achiral and either the left- or right-handed form of the molecule. Once the chirality is "switched on" linearly polarized UV laser pulses allow the selective change of the chirality using the electronic excited state as intermediate state.     

Synthesis of complementary double-stranded helical oligomers through chiral and achiral amidinium-carboxylate salt bridges and chiral amplification in their double-helix formation

A series of complementary molecular strands from 2-mer to 5-mer that are composed of m-terphenyl units bearing chiral/achiral amidine or achiral carboxyl groups linked via Pt(II) acetylide complexes were synthesized by sequential stepwise reactions, and their chiroptical properties on the double-helix formation were investigated by circular dichroism (CD) and (1)H NMR spectroscopies. In CHCl(3), the "all-chiral" amidine strands consisting of (R)- or (S)-amidine units formed preferred-handed double helices with the complementary achiral carboxylic acid strands through the amidinium-carboxylate salt bridges, resulting in characteristic induced CDs in the Pt(II) acetylide complex regions, indicating that the chiral substituents on the amidine units biased a helical sense preference. The Cotton effect patterns and intensities were highly dependent on the molecular lengths. The complementary double-helix formation was also explored using the chiral/achiral amidine strands with different sequences in which a chiral amidine unit was introduced at the center (center-chiral) or a terminus (edge-chiral) of the amidine strands. The effect of the sequences of the chiral and achiral amidine units on the amplification of chirality (the "sergeants and soldiers" effect) in the double-helix formation was investigated by comparing the CD intensities with those of the corresponding all-chiral amidine double helices with the same molecular lengths. Variable-temperature CD experiments of the all-chiral and chiral/achiral amidine duplexes demonstrated that the Pt(II)-linked complementary duplexes are dynamic and their chiroptical properties including the chirality transfer from the chiral amidine unit to the achiral amidine ones are significantly affected by the molecular lengths, sequences, and temperatures. On the basis of the above results together with molecular dynamics simulation results, key structural features of the Pt(II)-linked oligomer duplexes and the effect of the chiral/achiral amidine sequences on the amplification of chirality are discussed.     

Chiral Deep Eutectic Solvents Enable Full-Color and White Circularly Polarized Luminescence from Achiral Luminophores

Herein, chiral deep eutectic solvents (DES) are prepared by lauric acid as hydrogen bond donors (HBD) and chiral menthol as hydrogen bond acceptors (HBA). When achiral fluorescent molecules are dopedin the menthol-based chiral DES, they emit circularly polarized luminescence (CPL) with handedness controlled by the molecular chirality (l or d ) of menthol. Remarkably, the strategy is universal and a series of achiral fluorescent molecules can be endowed with CPL activity, showing a full-color and white CPL upon appropriate mixing, which paves the way to prepare white CPL materials. Interestingly, CPL appears only in a certain temperature range in the DES. Variable-temperature spectra and other characterization methods reveal that the H-bond network in the chiral DES plays an important role in inducing CPL. This work unveils how the interior structure as well as the hydrogen-bond network of a chiral DES can transfer its chirality to achiral luminophores for the first time and realizes a full-color and white CPL in a DES.     

Compression induced chiral symmetry breaking of monolayers comprised of banana-shaped achiral molecules at an air-water interface: Williams-Bragg approach

Chirality of monolayers comprised of banana-shaped achiral molecules at an air-water interface was investigated theoretically, and a forming mechanism of chiral structure as an assembly of achiral molecules was argued. A model of such monolayers was constructed taking into account the short-range repulsive interaction between constituent banana-shaped achiral molecules, and the free energy density functional of the model was derived as a generalization of Williams-Bragg approach. It was predicted that chiral symmetry breaking occurs by monolayer compression, where two-dimensional characteristics of monolayers at an interface plays an important role in the formation of chiral structure by banana-shaped achiral molecules.     

Hydrogen Bonding Directed Supramolecular Polymerisation of Oligo(Phenylene-Ethynylene)s: Cooperative Mechanism, Core Symmetry Effect and Chiral Amplification

The design of supramolecular motifs with tuneable stability and adjustable supramolecular polymerisation mechanisms is of crucial importance to precisely control the properties of supramolecular assemblies. This report focuses on constructing π-conjugated oligo(phenylene ethynylene) (OPE)-based one-dimensional helical supramolecular polymers that show a cooperative growth mechanism. Thus, a novel set of discotic molecules comprising a rigid OPE core, three amide groups, and peripheral solubilising wedge groups featuring C(3) and C(2) core symmetry was designed and synthesised. All of the discotic molecules are crystalline compounds and lack a columnar mesophase in the solid state. In dilute methylcyclohexane solution, one-dimensional supramolecular polymers are formed stabilised by threefold intermolecular hydrogen bonding and π-π interactions, as evidenced by (1) H?NMR measurements. Small-angle X-ray and light scattering measurements reveal significant size differences between the columnar aggregates of C(3) - and C(2) -symmetrical discotics, that is, the core symmetry strongly influences the nature of the supramolecular polymerisation process. Temperature-dependent CD measurements show a highly cooperative polymerisation process for the C(3) -symmetrical discotics. In contrast, the self-assembly of C(2) -symmetrical discotics shows a smaller enthalpy release upon aggregation and decreased cooperativity. In all cases, the peripheral stereogenic centres induce a preferred handedness in the columnar helical aggregates. Moreover, one stereogenic centre suffices to fully bias the helicity in the C(2) -symmetrical discotics. Finally, chiral amplification studies with the C(3) -symmetrical discotics were performed by mixing chiral and achiral discotics (sergeants-and-soldiers experiment) and discotics of opposite chirality (majority-rules experiment). The results demonstrate a very strong sergeants-and-soldiers effect and a rather weak majority-rules effect.     

Quantified chirality, molecular similarity, and helical twisting power in lyotropic chiral nematic guest/host systems

Lyotropic liquid crystals can exhibit phase chirality. The mechanism behind the transfer of chirality between a chiral dopant and a liquid crystalline host phase is still under discussion. Our own recent results and proposals are the following. Lyotropic phase chirality can exist even at very low concentrations of chiral dopants, with less than 1 chiral dopant per 50 micelles. There is evidence for an intramicellar double twist which could be due to the induction of chiral conformations in the achiral surfactant chains. The chirality of arbitrary molecules can be quantified by means of the 'Hausdorff distance'. Increasing chirality of a dopant does not necessarily imply increasing helical twisting power, and molecular similarity between chiral guest and achiral host is essential for effective chirality transfer.     

Frustrated ostwald ripening in self-assembled monolayers of cruciform pi-systems

This study details a scanning tunneling microscopy investigation into the mechanism of chiral grain growth in highly ordered, self-assembled monolayer films composed of cruciform pi-systems. Although the molecules themselves are achiral, when they adsorb from solution onto graphite, they adopt a gear-like conformation that, by virtue of the surface, is chiral. These handed subunits arrange themselves into enantiomeric two-dimensional domains. The unique finding from this study is that Ostwald ripening is frustrated between domain boundaries that are of opposite chirality because direct interconversion between the chiral units on the surface is energetically inhibited.