Chiral/Achiral Copolymers of Biphenylylacetylenes Bearing Various Substituents: Chiral Amplification through Copolymerization, Followed by Enhancement/Inversion and Memory of the Macromolecular Helicity

Synthesis and applications of helical polymers with dynamic and static memories of helicity

This review mainly highlights our studies on the synthesis of one-handed helical polymers with a static memory of helicity based on the noncovalent helicity induction with a helical-sense bias and subsequent memory of the helicity approach that we developed during the past decade. Apart from the previous approaches, an excess one-handed helical conformation, once induced by nonracemic molecules, is immediately retained ("memorized") after the complete removal of the nonracemic molecules, accompanied by a significant amplification of the asymmetry, providing novel switchable chiral materials for chromatographic enantioseparation and asymmetric catalysis as well as a highly sensitive colorimetric and fluorescence chiral sensor. A conceptually new one-handed helix formation in a racemic helical polymer composed of racemic repeating units through the deracemization of the pendants is described.

Control of One-Handed Helicity in Polyacetylenes: Impact of an Extremely Small Amount of Chiral Substituents

Controlling the one-handed helicity in synthetic polymers is crucial for developing helical polymer-based advanced chiral materials. We now report that an extremely small amount of chiral biphenylylacetylene (BPA) monomers (ca. 0.3-0.5 mol %) allows complete control of the one-handed helicity throughout the polymer chains mostly composed of achiral BPAs. Chiral substituents introduced at the 2-position of the biphenyl units of BPA positioned in the vicinity of the polymer backbones contribute to a significant amplification of the helical bias, as interpreted by theoretical modeling and simulation. The helical structures, such as the helical pitch and absolute helical handedness (right- or left-handed helix) of the one-handed helical copolymers, were unambiguously determined by high-resolution atomic force microscopy combined with X-ray diffraction. The exceptionally strong helix-biasing power of the chiral BPA provides a highly durable and practically useful chiral material for the separation of enantiomers in chromatography by copolymerization of an achiral functional BPA with a small amount of the chiral BPA (0.5 mol %) due to the robust helical scaffold of the one-handed helical copolymer.

Multi-chiral materials comprising metallosupramolecular and covalent helical polymers containing five axial motifs within a helix

Supramolecular and covalent polymers share multiple structural effects such as communication mechanisms among monomer repeating units, which are related to their axial helical structure. Herein, a unique multi-helical material combining information from both metallosupramolecular and covalent helical polymers is presented. In this system, the helical structure described by the poly(acetylene) (PA) backbone (cis-cisoidal, cis-transoidal) guides the pendant groups in a fashion where a tilting degree emerges between a pendant and the adjacent ones. As a result, a multi-chiral material is formed comprising four or five axial motifs when the polyene skeleton adopts either a cis-transoidal or cis-cisoidal configuration: the two coaxial helices-internal and external-and the two or three chiral axial motifs described by the bispyridyldichlorido Pt^II complex array. These results show that complex multi-chiral materials can be obtained by polymerizing appropriate monomers that combine both point chirality and the ability to generate chiral supramolecular assemblies.

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.

Screw sense excess and reversals of helical polymers in solution

The helix reversal is a structural motif found in helical polymers in the solid state, but whose existence is elusive in solution. Herein, we have shown how the photochemical electrocyclization (PEC) of poly(phenylacetylene)s (PPAs) can be used to determine not only the presence of helix reversals in polymer solution, but also to estimate the screw sense excess. To perform these studies, we used a library of well folded PPAs and different copolymers series made by enantiomeric comonomers that show chiral conflict effect. The results obtained indicate that the PEC of a PPA will depend on the helical scaffold adopted by the PPA backbone and on its folding degree. Then, from these studies it is possible to determine the screw sense excess of a PPA, highly important in applications such as chiral stationary phases in HPLC or asymmetric synthesis.

An attempt to consider cooperativity in helical-sense preferences induced in fused macrocycles

We synthesized bis and tris(macrocycle)s in which a two- or three-fold macrocycle was fused and each adopted twisted forms with M- or P-helicity. According to the sense of twisting in each element, diverse conformations can be generated in a molecule. We present two types of conformational preferences. One is the innate preference for a helical form with an identical sense of twisting throughout the entire molecule. The other is the helical-sense preference for a particular sense of twisting. We were interested in the relationship between K_n and (K₁)ⁿ, where K_n is an equilibrium constant for the conformational interconversion between two helical forms (MM and PP, MMM and PPP) and n is the number of elements, since we considered that the relationship could be a metric to see the interinfluence among these macrocyclic elements in a single molecule. Through VT measurements in ¹H NMR and CD spectroscopy, we attempted to quantify the helical-sense preferences induced in the fused macrocycles (n = 2 and 3) to compare K_n and (K₁)ⁿ.

Ultra-fast One-Handed Helix Induction and Its Static Helicity Memory in a Poly(biphenylylacetylene) with a Catalytic Amount of Chiral Ammonium Salts

We report an ultra-fast helix induction and subsequent static helicity memory in poly(biphenylylacetylene) (PBPA-A) assisted by a catalytic amount of nonracemic ammonium salts comprised of non-coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF^- ) as a counter anion. The remarkable acceleration of the helix-induction rate in PBPA-A accompanied by the significant amplification of the asymmetry relies on the two methoxymethoxy groups of the biphenyl pendants, which can gain access to enfold the chiral ammoniums in a crown-ether manner in specific aromatic solvents, leading to ultra-fast helicity induction, which is completed within 30 s. In aromatic solvents, helicity memory is lost rapidly, but is quite stable in long-chain hydrocarbons. The best use of specific solvents for helicity induction and static helicity memory, respectively, provides a highly sensitive chirality sensing system toward a small amount of chiral amines and amino acids when complexed with BArF^- .

Metal-Enhanced Helical Chirality of Coil Macromolecules: Bioinspired by Metal Coordination-Induced Protein Folding

Although the supramolecular helical structures of biomacromolecules have been studied, the examples of supramolecular systems that are assembled using coils to form helical polymer chains are still limited. Inspired by enhanced helical chirality at the supramolecular level in metal coordination-induced protein folding, a series of alanine-based coil copolymers (poly-(l-co-d)-ala-NH₂) carrying (l)- and (d)-alanine pendants were synthesized as a fresh research model to study the cooperative processes between homochirality property and metal coordination. The complexes of poly-(l-co-d)-ala-NH₂ and metal ions underwent a coil-to-helix transition and exhibited remarkable nonlinear effects based on the enantiomeric excess of the monomer unit in the copolymers, affording enhanced helical chirality compared to poly-(l-co-d)-ala-NH₂. More importantly, the synergistic effect of amplification of asymmetry and metal coordination triggered the formation of a helical molecular orbital on the polymer backbone via the coordination with the d orbital of copper ions. Thus, the helical chirality enhancement degree of poly-(l-co-d)-ala-NH₂/Cu²⁺ complexes (31.4) is approximately 3 times higher than that of poly-(l-co-d)-ala-NH₂/Ag⁺ complexes (9.8). This study not only provides important mechanistic insights into the enhancement of helical chirality for self-assembly but also establishes a new strategy for studying the homochiral amplification of asymmetry in biological supramolecular systems.

Photostability and Dynamic Helical Behavior in Chiral Poly(phenylacetylene)s with a Preferred Screw‐Sense

Helical polymers such as poly(phenylacetylene)s (PPAs) are interesting materials due to the possibility of tuning their helical scaffold (sense and elongation) once they have been prepared and by the presence of external stimuli. The main limitation in the application of PPAs is their poor photostability. These polymers degrade under visible light exposure through a photochemical electrocyclization process. In this work, it was demonstrated, through a selected example, how the photochemical degradation in PPAs is directly related to their dynamic helical behavior. Thus, while PPAs with dynamic helical structures show poor photostability under UV/Vis light exposure, poly‐(R)‐1, bearing an enantiopure sulfoxide group as pendant group and designed to have a quasi‐static helical behavior, shows a large photostability due to the restricted conformational composition at the polyene backbone, needed to orient the conjugated double bonds prior to the photochemical electrocyclization process and the subsequent degradation of the material.

Catalytic One-Handed Helix Induction and Subsequent Static Memory of Poly(biphenylylacetylene)s Assisted by a Small Amount of Carboxy Groups Introduced at the Pendants

A dynamically racemic helical copolymer composed of an achiral biphenylylacetylene (BPA) bearing methoxymethoxy groups at the 2,2′-positions and 1 mol % of an achiral BPA carrying 2-carboxy-2′-methoxymethoxy groups at the biphenyl pendants was found to fold into an excess one-handed helix with significant amplification of the helicity in the presence of a small amount of optically active amines. The induced macromolecular helicity was retained (“memorized”) after removal of the chiral amines. The copolymer had a significant sensitivity for detecting the chirality of chiral amines with a sensitivity more than 10000-fold higher than that of the corresponding homopolymers with no carboxy group, thus showing Cotton effects even in the presence of a 0.01 equiv of an optically active amine. The effects of the substituents at the 4′-position of the biphenyl pendants of the copolymers and the structures of the chiral amines on the macromolecular helicity induction were also investigated.

Photochemical Electrocyclization of Poly(phenylacetylene)s: Unwinding Helices to Elucidate their 3D Structure in Solution

Photochemical electrocyclization of poly(phenylacetylene)s (PPAs) is used for the structural elucidation of a polyene backbone. This method not only allows classification of PPAs in cis-cisoidal (ω₁ <90°) or cis-transoidal structures (ω₁ >90°), but also approximating ω₁ . A PPA solution is illuminated with visible light and monitoring the photochemical electrocyclization of the PPA helix by measuring the ECD spectra at different times. PPAs with a cis-cisoidal structure show a reduction of the ECD signal of at least 50 % before 30 min of irradiation, while cis-transoidal helices need much longer time because the transoidal bond must be isomerized. The different cis-cisoidal and cis-transoidal helices require different times to decrease their ECD signal by 50 % (t_1/2 ), depending on the degree of compression or stretching of the helix, establishing a relationship between the secondary structure adopted by PPA (ω₁ ) and the time required to lose the ECD vinylic signal by light irradiation.

Racemic Monomer-Based One-Handed Helical Polymer Recognizes Enantiomers through Auto-Evolution of Its Helical Handedness Excess

A racemic monomer-based optically inactive polyacetylene folds into a one-handed helix assisted by a nonracemic alcohol, which can separate various enantiomers as a chiral stationary phase in chromatography. The chiral-resolving power is virtually identical to that of the enantiopure monomer-based one-handed helical polyacetylene. Because of its unique static memory of the induced helicity, the original racemic polyacetylene expresses an auto-evolution of its helical handedness over time, and at the same time, chirality of the nonracemic alcohol is discriminated accompanied by successive enhancement of its optical purity enantioselectively adsorbed on the helical polyacetylene owing to the chiral filter effect as directly monitored by NMR, which contributes to further enhancing the helix-sense-excess of the helical polyacetylene.

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.