Chiral Coalition in Helical Sense Enhancement of Copolymers: The Role of the Absolute Configuration of Comonomers

Stimuli-responsive synthetic helical polymers

Synthetic dynamic helical polymers (supramolecular and covalent) and foldamers share the helix as a structural motif. Although the materials are different, these systems also share many structural properties, such as helix induction or conformational communication mechanisms. The introduction of stimuli responsive building blocks or monomer repeating units in these materials triggers conformational or structural changes, due to the presence/absence of the external stimulus, which are transmitted to the helix resulting in different effects, such as assymetry amplification, helix inversion or even changes in the helical scaffold (elongation, J/H helical aggregates). In this review, we show through selected examples how different stimuli (e.g., temperature, solvents, cations, anions, redox, chiral additives, pH or light) can alter the helical structures of dynamic helical polymers (covalent and supramolecular) and foldamers acting on the conformational composition or molecular structure of their components, which is also transmitted to the macromolecular helical structure.

Thermo-responsive chiral micelles as recyclable organocatalyst for asymmetric Rauhut-Currier reaction in water

Developing eco-friendly chiral organocatalysts with the combined advantages of homogeneous catalysis and heterogeneous processes is greatly desired. In this work, a family of amphiphilic one-handed helical polyisocyanides bearing phosphine pendants is prepared, which self-assembles into well-defined chiral micelles in water and showed thermo-responsiveness with a cloud point of approximately 38.4 °C. The micelles with abundant phosphine moieties at the interior efficiently catalyze asymmetric cross Rauhut-Currier reaction in water. Various water-insoluble substrates are transferred to target products in high yield with excellent enantioselectivity. The yield and enantiomeric excess (ee) of the product generated in water are up to 90% and 96%, respectively. Meanwhile, the yields of the same R-C reaction catalyzed by the polymer itself in organic solvents is <16%, with an ee < 72%. The homogeneous reaction of the chiral micelles in water turns to heterogeneous at temperatures higher than the cloud point, and the catalyst precipitation facilitates product isolation and catalyst recovery. The polymer catalyst is recycled 10 times while maintaining activity and enantioselectivity.

Dynamic monitoring of self-assembly by confining conformational changes of butterfly-motion-based molecules

A simple dynamic monitoring strategy for chiral self-assembly is achieved by confining the bent-to-planar evolution observed in N,N'-diphenyl-dihydrodibenzo[a,c]phenazine derivatives (DPAC-R/S-GLD). Besides, this approach provides a facile pathway to fabricate architectures with circularly polarized luminescence (CPL) properties.

Conformationally supramolecular chirality prevails over configurational point chirality in side-chain liquid crystalline polymers

In nature, the communication of primary amino acids in the polypeptides influences molecular-level packing, supramolecular chirality, and the resulting protein structures. In chiral side-chain liquid crystalline polymers (SCLCPs), however, the hierarchical chiral communication between supramolecular mesogens is still determined by the parent chiral source due to the intermolecular interactions. Herein, we present a novel strategy to enable the tunable chiral-to-chiral communication in azobenzene (Azo) SCLCPs, in which the chiroptical properties are not dominated by the configurational point chirality but by the conformationally supramolecular chirality that emerged. The communication of dyads biases supramolecular chirality with multiple packing preference, thereby overruling the configurational chirality of the stereocenter. The chiral communication mechanism between the side-chain mesogens is revealed through the systematic study of the chiral arrangement at the molecular level, including mesomorphic properties, stacking modes, chiroptical dynamics and further morphological dimensions.

Diastereomeric multi-chiral pendant groups: Their key role in stimuli-responsive polymeric responses

Chiral information transmission in helical polymers bearing multi-chiral pendant groups is usually determined by the absolute configuration of the first chiral center. The second chiral residue usually has low-to-null influence in the macromolecular handedness of the polymer, due to its remote position respect to the polyene main chain. Here, we demonstrate how the stimuli responsive properties of diastereomeric polymers, obtained by changing the absolute configuration of the second chiral center, are different due to the unlike properties of diastereoisomers.

Stereochemical dominance in hierarchically formed helicates

The competition of different chiral ligands in the control of stereochemistry of hierarchically formed helical coordination compounds is investigated. It is found that sterically demanding chiral units can dominate the chiral induction of the helix even if they are present as a minor species. Hereby the relative strength of stereoinduction of different chiral units can be evaluated.

Supramolecular Helical Assemblies of Dirhodium(II) Paddlewheels with 1,4-Diazabicyclo[2.2.2]octane: A Remarkable Substituent Effect on the Helical Sense Preference and Amplification of the Helical Handedness Excess of Metallo-Supramolecular Helical Polymers

We report unique coordination-driven supramolecular helical assemblies of a series of dirhodium(II) tetracarboxylate paddlewheels bearing chiral phenyl- or methyl-substituted amide-bound m-terphenyl residues with triethylene glycol monomethyl ether (TEG) or n-dodecyl tails through a 1:1 complexation with 1,4-diazabicyclo[2.2.2]octane (DABCO). The chiral dirhodium complexes with DABCO in CHCl₃/n-hexane (1:1) form one-handed helical coordination polymers with a controlled propeller chirality at the m-terphenyl groups, which are stabilized by intermolecular hydrogen-bonding networks between the adjacent amide groups at the periphery mainly via a cooperative nucleation-elongation mechanism as supported by circular dichroism (CD), vibrational CD, and variable-temperature (VT) absorption and CD analyses. The VT visible-absorption titrations revealed the temperature-dependent changes in the degree of polymerization. The columnar supramolecular helical structures were elucidated by X-ray diffraction and atomic force microscopy. The helix sense of the homopolymer carrying the bulky phenyl and n-dodecyl substituents is opposite those of other chiral homopolymers despite having the same absolute configuration at the pendants. A remarkably strong "sergeants and soldiers" (S&S) effect was observed in most of the chiral/achiral copolymers, while the copolymers of the bulky chiral phenyl-substituted dirhodium complexes with n-dodecyl chains displayed an "abnormal" S&S effect accompanied by an inversion of the helix sense, which could be switched to a "normal" S&S effect by changing the solvent composition. A nonracemic dirhodium complex of 20% enantiomeric excess bearing the less bulky chiral methyl substituents with n-dodecyl chains assembled with DABCO to form an almost one-handed helix (the "majority rule" (MR) effect), whereas the three other nonracemic copolymers showed a weak MR effect.

Merging Supramolecular and Covalent Helical Polymers: Four Helices Within a Single Scaffold

Supramolecular and covalent polymers share multiple structural effects such as chiral amplification, helical inversion, sergeants and soldiers, or majority rules, among others. These features are related to the axial helical structure found in both types of materials, which are responsible for their properties. Herein a novel material combining information and characteristics from both fields of helical polymers, supramolecular (oligo(p-phenyleneethynylene) (OPE)) and covalent (poly(acetylene) (PA)), is presented. To achieve this goal, the poly(acetylene) must adopt a dihedral angle between conjugated double bonds (ω1) higher than 165°. In such cases, the tilting degree (Θ) between the OPE units used as pendant groups is close to 11°, like that observed in supramolecular helical arrays of these molecules. Polymerization of oligo[(p-phenyleneethynylene)n]phenylacetylene monomers (n = 1, 2) bearing L-decyl alaninate as the pendant group yielded the desired scaffolds. These polymers adopt a stretched and almost planar polyene helix, where the OPE units are arranged describing a helical structure. As a result, a novel multihelix material was prepared, the ECD spectra of which are dominated by the OPE axial array.

Brightening up Circularly Polarized Luminescence of Monosubstituted Polyacetylene by Conformation Control: Mechanism, Switching, and Sensing

The first example of luminescent monosubstituted polyacetylenes (mono-PAs) is presented, based on a contracted cis-cisoid polyene backbone. It has an excellent circularly polarized luminescence (CPL) performance with a high dissymmetric factor (up to the order of 10^-1 ). The luminescence stems from the helical cis-cisoid PA backbone, which is tightly fixed by the strong intramolecular hydrogen bonds, thereby reversing the energy order of excited states and enabling an emissive energy dissipation. CPL switches are facilely achieved by the solvent and temperature through reversible conformational transition. By taking advantages of fast response and high sensitivity, the thin film of mono-PAs could be used as a CPL-based probe for quantitative detection of trifluoroacetic acid with a wider linear dynamic range than those of photoluminescence and circular dichroism. This work opens a new avenue to develop novel smart CPL materials through modulating conformational transition.

Hierarchical communication of chirality for aromatic oligoamide sequences

The communication of chirality at a molecular and supramolecular level is the fundamental feature capable of transmitting and amplifying chirality information. Yet, the limitation of one-step communication mode in many artificial systems has precluded the ability of further processing the chirality information. Here, we report the chirality communication of aromatic oligoamide sequences within the interpenetrated helicate architecture in a hierarchical manner, specifically, the communication is manipulated by three sequential steps: (i) coordination, (ii) concentration, and (iii) ion stimulus. Such approach enables the information to be implemented progressively and reversibly to different levels. Furthermore, the chiral information on the side chains can be accumulated and transferred to the helical backbones of the sequences, resulting in that one of ten possible diastereoisomers of the interpenetrated helicate is finally selected. The circular dichroism experiments with a mixture of chiral and achiral ligands demonstrate a cooperative behavior of these communications, leading to amplification of chiral information.

Communication of chirality at a molecular level is the fundamental for transmitting chirality information but one-step communication modes in many artificial systems limits further processing the chirality information. Here, the authors report chirality communication of aromatic oligoamide sequences within interpenetrated helicate architecture in a hierarchical manner.

Insights into the co-assemblies formed by different aromatic short-peptide amphiphiles

The mechanism of co-assembly of different aromatic dipeptides has been studied using a combination of microscopy and spectroscopy techniques. At an equimolar ratio, the kinetics of the process is favored giving rise to alternate copolymers.

Biasing the Screw-Sense of Supramolecular Coassemblies Featuring Multiple Helical States

By enchaining a small fraction of chiral monomer units, the helical sense of a dynamic polymer constructed from achiral monomer units can be disproportionately biased. This phenomenon, known as the sergeants-and-soldiers (S&S) effect, has been found to be widely applicable to dynamic covalent and supramolecular polymers. However, it has not been exemplified with a supramolecular polymer that features multiple helical states. Herein, we demonstrate the S&S effect in the context of the temperature-controlled supramolecular copolymerization of chiral and achiral biphenyl tetracarboxamides in alkanes. The one-dimensional helical structures presented in this study are unique because they exhibit three distinct helical states, two of which are triggered by coassembling with monomeric water that is codissolved in the solvent. The self-assembly pathways are rationalized using a combination of mathematical fitting and simulations with a thermodynamic mass-balance model. We observe an unprecedented case of an "abnormal" S&S effect by changing the side chains of the achiral soldier. Although the molecular structure of these aggregates remains elusive, the coassembly of water is found to have a profound impact on the helical excess.

Chiral information harvesting in helical poly(acetylene) derivatives using oligo(p-phenyleneethynylene)s as spacers

A chiral harvesting transmission mechanism is described in poly(acetylene)s bearing oligo(p-phenyleneethynylene)s (OPEs) used as rigid achiral spacers and derivatized with chiral pendant groups. The chiral moieties induce a positive or negative tilting degree in the stacking of OPE units along the polymer structure, which is further harvested by the polyene backbone adopting either a P or M helix.

Supramolecular Assembly of Thiophene-Based Oligomers into Nanostructured Fluorescent Conductive and Chiral Microfibers

The implementation of nano/microelectronic devices requires efficient strategies for the realization of supramolecular structures with desired function and supported on appropriate substrates. This article illustrates a strategy based on the synthesis of thiophene oligomers having the same "sulfur-overrich" quaterthiophene inner core (non bonding interactional algorithm) and different terminal groups. Nano/microfibers are formed on surfaces having a morphology independent of the nature of the deposition substrate and displaying a wide tuning of properties that make the fibers optoelectronically suitable for application in devices.

Chiroptical Sensing: A Conceptual Introduction

Chiroptical responses have been an essential tool over the last decades for chemical structural elucidation due to their exceptional sensitivity to geometry and intermolecular interactions. In recent times, there has been an increasing interest in the search for more efficient sensing by the rational design of tailored chiroptical systems. In this review article, advances made in chiroptical systems towards their implementation in sensing applications are summarized. Strategies to generate chiroptical responses are illustrated. Theoretical approaches to assist in the design of these systems are discussed. The development of efficient chiroptical reporters in different states of matter, essential for the implementation in sensing devises, is reviewed. In the last part, remarkable examples of chiroptical sensing applications are highlighted.

Induced Circular Dichroism of Isotactic Poly(2-vinylpyridine) with Diverse and Tunable "Sergeants-and-Soldiers" Type Chiral Amplification

Facile and efficient construction of the helical structure with diverse and tunable chiral amplification from an achiral polymer is attractive but remains a challenge to develop multiple functional materials. We report herein a macromolecular acid-base complex of highly isotactic poly(2-vinylpyridine) (mmmm > 99%), (+)-camphorsulfonic acid, and dodecylbenzensulfonic acid. The asymmetric induction of the chiral additive through the ionic interactions between pyridinium pendants and acid ions drives the polymer backbone to twist in a preferred direction in CHCl₃ and its mixture with CH₃CN. The sign and intensity of induced circular dichroism rely on the base to acid ratio, the chiral acid content, and the solvent nature. By systematically tuning the solvent composition, four distinct types of "sergeants-and-soldiers" mode chiral amplification are achieved for the first time within a single system owing to the solvent dependent bias of chiral-chiral and chiral-achiral ion pairs.

Homochiral nanotubes from heterochiral lipid mixtures: a shorter alkyl chain dominated chiral self-assembly

It is an important topic to achieve homochirality both at a molecular and supramolecular level. While it has long been regarded that "majority rule" guides the homochiral self-assembly from an enantiomer mixture, it still remains a big challenge to manipulate the global homochirality in a complex system containing chiral species that are not enantiomers. Here, we demonstrate a new example wherein homochiral nanotubes self-assembled from a mixture of heterochiral lipids that deviated from the "majority rule". We have found that when two heterochiral lipids with mirror headgroups but a 2-methylene discrepancy in alkyl chain length are mixed, homochiral nanotubes are always formed regardless of their mixing ratio. Remarkably, the helicity of the nanotube is exclusively controlled by the molecular chirality of the lipids with shorter alkyl chains, i.e., the chiral self-assembly was dominated by the lipid with the shorter alkyl chain. MD simulation reveals that the match of both the alkyl chain length and hydrogen-bonding between two kinds of lipids plays an important role in the assembly. This work provides a new insight into the supramolecular chirality of complex systems containing multi chiral species.

Chiral-to-Chiral Communication in Polymers: A Unique Approach To Control Both Helical Sense and Chirality at the Periphery

A novel approach to the classical Sergeants and Soldiers effect, using chiral Sergeants and chiral Soldiers, allows control over both helical and external chirality in helical polymers. In the systems reported here, it is possible to induce the same helical sense ( M or P) from either of the two enantiomers of a chiral pendant group ["chiral Soldier", major component; i.e., ( R)- or ( S)-1] when it faces a single enantiomer of an appropriate "chiral Sergeant" [minor component; i.e., ( S)-2]. For instance, the copolymer series poly[( R)-1 _r- co-( S)-2_{(1- r)}], poly[( S)-1 _r- co-( S)-2_{(1- r)}], and poly[( rac)-1 _r- co-( S)-2_{(1- r)}] adopt the same P helix even though the major component shows the opposite absolute configuration. This chiral-to-chiral communication effect is transmitted by the stabilization of different conformations in each enantiomeric form of the Soldier. As a result, this groundbreaking approximation to the Sergeants and Soldiers effect allows the preparation of a single-handed helix-which depends only on the Sergeant's configuration-with different chiralities on the helix periphery. Thus, a P helix can be decorated with the R isomer, S isomer, or even a racemic mixture of the chiral Soldier. A change in the absolute configuration of the Sergeant affords the opposite M helix, which can also be decorated with the R isomer, S isomer, or racemic mixture of the chiral Soldier.

Unraveling Mechanisms of Chiral Induction in Double-Helical Metallopolymers

Self-assembled helical polymers hold great promise as new functional materials, where helical handedness controls useful properties such as circularly polarized light emission or electron spin. The technique of subcomponent self-assembly can generate helical polymers from readily prepared monomers. Here we present three distinct strategies for chiral induction in double-helical metallopolymers prepared via subcomponent self-assembly: (1) employing an enantiopure monomer, (2) polymerization in a chiral solvent, (3) using an enantiopure initiating group. Kinetic and thermodynamic models were developed to describe the polymer growth mechanisms and quantify the strength of chiral induction, respectively. We found the degree of chiral induction to vary as a function of polymer length. Ordered, rod-like aggregates more than 70 nm long were also observed in the solid state. Our findings provide a basis to choose the most suitable method of chiral induction based on length, regiochemical, and stereochemical requirements, allowing stereochemical control to be established in easily accessible ways.