Helical Polymers Research Articles

Coordination-Driven Self-Assembly of Complexes Constructed from Two Helical Ligands: Synthesis, Structures, and Selective Gas Adsorption Properties.

Two helical ligands (L1 and L2) were designed and synthesized by a Schiff base condensation reaction. Eight complexes, {[Zn(L1)I2]·H2O}n (1), [Cd2(L1)2I4(CH3OH)2] (2), [Hg2(L1)2I4] (3), [Ag(L1)NO3]n (4), [Ag2(L1)2(NO3)2DMSO]·H2O (5), {[Zn2(L2)2Cl4]·2CHCl3}n (6), {[Ag(L2)]·NO3}n (7), and {[Ag(L2)NO3]·CH3OH}n (8), were synthesized and characterized based on these two ligands. The crystal structures show that both Schiff base compounds exist as racemic ligands with equal amounts of P- and M-helicity, and the assembly of these racemic ligands with metal ions can lead to homochiral or heterochiral complexes via a chiral self-recognition or self-discrimination process. Complexes 2, 3, and 5 exist as heterochiral metallomacrocycles with a figure-eight conformation. Complexes 1, 6, and 8 exist as one-dimensional (1D) homochiral helical chain coordination polymers, while complexes 4 and 7 exist as 1D heterochiral helical chain coordination polymers. Furthermore, gas and vapor adsorption measurements show that all of the synthesized complexes exhibit good selective adsorption capacities toward methanol and ethanol vapor over N2, H2, and O2.

Selective synthesis of helical polymers

AbstractMany researches have been done on synthetic helical polymers because of their wide applications such as chiral recognition, enantiomeric separation, asymmetric catalysis, circularly polarized luminescence, liquid crystallization and so on. Synthetic helical polymers can be generally divided into static helical polymers and dynamic helical polymers on the basis of whether the helix inversion barriers are high or low. And this mini review mainly describes the three common selectively synthetic methods of static helical polymers such as polymerization with chiral monomers, helix‐sense‐selective polymerization and enantiomer‐selective polymerization. We hope that this mini review can provide more ideas on synthesis of chiral materials and encourage further development on novel synthetic methods of helical polymers.

Helical Molecular Springs with Varying Spring Constants

AbstractWe report a general synthetic route toward helical ladder polymers with varying spring constants, built with chirality‐assisted synthesis (CAS). Under tension and compression, these shape‐persistent structures do not unfold, but rather stretch and compress akin classical Hookean springs. Our synthesis is adaptable to helices with different pitch and diameter, which allowed us to investigate how molecular flexibility in solution depends on the exact geometry of the ladder polymers. Specifically, we showed with molecular dynamic simulations and by measuring the longitudinal 1H NMR relaxation times (T1) for our polymers at different Larmor frequencies, that increasing the helix diameter leads to increased flexibility. Our results present initial design rules for tuning the mechanical properties of intrinsically helical ladder polymers in solution, which will help inspire a new class of robust, spring‐like molecular materials with varying mechanical properties.

Helical Molecular Springs with Varying Spring Constants.

We report a general synthetic route toward helical ladder polymers with varying spring constants, built with chirality-assisted synthesis (CAS). Under tension and compression, these shape-persistent structures do not unfold, but rather stretch and compress akin classical Hookean springs. Our synthesis is adaptable to helices with different pitch and diameter, which allowed us to investigate how molecular flexibility in solution depends on the exact geometry of the ladder polymers. Specifically, we showed with molecular dynamic simulations and by measuring the longitudinal 1 H NMR relaxation times (T1 ) for our polymers at different Larmor frequencies, that increasing the helix diameter leads to increased flexibility. Our results present initial design rules for tuning the mechanical properties of intrinsically helical ladder polymers in solution, which will help inspire a new class of robust, spring-like molecular materials with varying mechanical properties.

N···I···N]+ Type Halogen-Bonding-Driven Supramolecular Helical Polymers with Modulated Chirality.

The [N···I···N]+ type halogen bond has been utilized to synthesize supramolecular architectures, while the applications in constructing helical motifs and modulating supramolecular chirality have been unexplored so far. In this work, the [N···I···N]+ halogen bond was introduced to drive the formation of supramolecular helical polymers via a Ag(I) coordination intermediate, showing tunable supramolecular chirality. Pyridine segments were conjugated to the asymmetric ferrocene skeleton, which show "open" and "closed" geometry depending on the sp2 N positions. Coordination with Ag(I) generated one-dimensional (1D) double helices and 2D helicates featured the [Ag(O)···I···Ag(O)]+ bond, which further stacked into 3D porous frameworks with chiral channels and adjustable pore sizes. Ionic exchange afforded 1D supramolecular helical polymers in solution phases driven by the [N···I···N]+ type halogen bonds, which was evidenced by the experimental results and density functional theory calculation. Fc2 exclusively demonstrated tunable supramolecular chirality in the formation of coordinated and halogen bonded polymers. In addition, solvent change would further inverse the helicity of halogen bonded supramolecular helical polymers depending on the rotation of the ferrocenyl core whose "closed" and "open" states were accompanied by the breakage of intramolecular hydrogen bonds. This work introduces a [N···I···N]+ type ionic halogen bond to prepare supramolecular helical polymers, providing multiple protocols in regulating helicity by ion exchange and solvent environments.

Full Control of the Chiral Overpass Effect in Helical Polymers: P/M Screw Sense Induction by Remote Chiral Centers After Bypassing the First Chiral Residue

AbstractIn helical polymers, helical sense induction is usually commanded by teleinduction mechanism, where the largest substituent of the chiral residue directly attached to the main chain is the one that commands the helical sense. In this work, different helical structures with different helical senses are induced in a helical polymer [poly‐(phenylacetylene)] when the conformational composition of two different dihedral angles of a pendant group with more than two chiral residues is tamed. Thus, while the dihedral angle at chiral residue1[(R)‐ or (S)‐alanine], attached to the backbone, produces an extended or bent conformation in the pendant resulting in two scaffolds with different stretching degree, the second dihedral angle at chiral residue2[(R)‐ or (S)‐methoxyphenylacetamide] places the substituents of this chiral center in a different spatial orientation, originating opposite helical senses at the polymer that are induced through a total control of the “chiral overpass effect”.

Full Control of the Chiral Overpass Effect in Helical Polymers: P/M Screw Sense Induction by Remote Chiral Centers After Bypassing the First Chiral Residue.

In helical polymers, helical sense induction is usually commanded by teleinduction mechanism, where the largest substituent of the chiral residue directly attached to the main chain is the one that commands the helical sense. In this work, different helical structures with different helical senses are induced in a helical polymer [poly-(phenylacetylene)] when the conformational composition of two different dihedral angles of a pendant group with more than two chiral residues is tamed. Thus, while the dihedral angle at chiral residue 1 [(R)- or (S)-alanine], attached to the backbone, produces an extended or bent conformation in the pendant resulting in two scaffolds with different stretching degree, the second dihedral angle at chiral residue 2 [(R)- or (S)-methoxyphenylacetamide] places the substituents of this chiral center in a different spatial orientation, originating opposite helical senses at the polymer that are induced through a total control of the "chiral overpass effect".

Synthesis, crystal structures, electrochemical and gas adsorption properties of two 1D silver(I) coordination polymers constructed from racemic helical ligands

Two helical ligands (L1 and L2) were designed and synthesized by Schiff base condensation reaction. Two Ag(I) coordination polymers (CPs), namely, {[Ag2(L1)2(NO3)2]·3H2O}n (1) and [AgL2NO3]n (2) were synthesized and characterized via elemental analysis, FT-IR spectra and single-crystal X-ray diffraction techniques. The crystal structures show that both Schiff base compounds exist as racemic ligands with equal amounts of P- and M-helicity. CP 1 exists as a 1D homochiral helical chain polymer via self-recognition process, and CP 2 exists as a 1D looped chain polymer. The cyclic voltammetric properties of the two CPs were investigated. Furthermore, gas and vapor adsorption measurements show that both CPs exhibit good adsorption capacity toward CH3OH over CO2 and N2.

Single-Molecule Unidirectional Processive Movement along a Helical Polymer Chain in Non-aqueous Medium.

In this work, a molecule “walking” along a single chain of a synthetic helical polymer, which is used as a rail on a substrate in an organic solvent at room temperature, is observed. The walking comprises the unidirectional processive movement of a short-chain molecule along a chiral helical chain in 3 nm steps, driven by Brownian motion and a tapping effect of the atomic force microscopy tip based on a flash ratchet mechanism. Furthermore, the rail consists of a long-chain substituted phenylacetylene polymer with pendant cholesteryl groups, along which the short-chain molecule can walk as a result of van der Waals interactions. The macromolecular motion is videoed using a fast-scanning atomic force microscope, and additionally, this phenomenon is also simulated by all-atom molecular dynamics calculations. On the basis of these results, we propose the principle of a polymer molecular motor. This is the first report of a synthetic walking machine of a chiral helical polymer driven by thermal fluctuation as an artificial life function.

The Anion-Directed Self-Assembly of tubular helical silver (I)-Based coordination polymers and the adsorption properties of hazardous substances

The controlled synthesis of topologically-structured helical coordination polymers (CPs) using versatile organic linkers and a metal centre, has recently attracted considerable attention. In this study, we design and synthesize a flexible tripodal ligand L1 (where L1 = 1, 3, 5-tris(2-pyridylsulfanylmethyl)-2, 4, 6-triethylbenzene) with an N, S cooperative chelation coordination geometry. In combination with silver salts in the presence of anions (nitrate and hexafluorophosphate), silver(I)-based tubular-shaped coordination polymers 1 ([Ag(NO3)(DMF)L12]n) and 2 ({[Ag(CH3CN)L12]•PF6}n) were precisely constructed through cooperative Ag-N/Ag-S and anion coordination interactions, which influenced the helical structures. Both single-stranded helical silver (I)-based CPs were characterised by NMR, PXRD, XPS, and FT-IR spectroscopy. CPs 1–2 have an irregular tube-like channel within their crystal structure which can capture iodine (adsorption capacity 0.27 g/g) and MnO4¯anions (0.25 eq. PF6¯ anions were exchanged) from their aqueous, or hexane, solution.

Multi-color Tunable and White Circularly Polarized Luminescent Composite Nanofibers Electrospun from Chiral Helical Polymer

Multi-color Tunable and White Circularly Polarized Luminescent Composite Nanofibers Electrospun from Chiral Helical Polymer

A strategy of constructing hybrid polymer helices promoted by polyacetylene chains in carbon nanotube

A strategy of constructing hybrid polymer helices promoted by polyacetylene chains in carbon nanotube

Intrahelical empty channels of nanoporous-crystalline α and β forms of PPO

A linear dichroism study of vibrational peaks of amorphous and semi-crystalline axially stretched poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films shows an unusual behaviour of co-crystalline and nanoporous-crystalline (α and β form) phases. In fact, signs of dichroism for all vibrational peaks of α and β phases are opposite with respect to those observed for amorphous phases. This behaviour indicates the occurrence in PPO α and β forms of large polymer helices, whose thermodynamic stability can be rationalized by placement of guest molecules in intrahelical channels. A large diameter helix, with chain periodicity of 1.06 nm, is compatible with layer-line position in WAXD patterns of axially stretched α form films. The occurrence for both α and β NC forms of PPO of intrahelical empty channels, can rationalize the very high guest diffusivity of semi-crystalline PPO films, which becomes particularly high when crystalline helical axes are preferentially perpendicular to the film plane.

Thermal Paper and Time Temperature Integrators Made From a Structural Colored Polymer Crosslinked With Hydrogen Bonded Cyclohexanoic Acid Derivatives

AbstractHydrogen‐bonded cyclohexanoic derivatives are applied in a structural colored cholesteric liquid crystal (CLC) helical polymer that demonstrates an extraordinary dual thermal response. Integration of the dynamic hydrogen‐bonded cyclohexanoic acid derivatives in the CLC polymer as supramolecular crosslinks yields an immediate and irreversible loss of the green structural color upon exposure to temperatures above the cholesteric‐to‐isotropic transition temperature (TNI) caused by the helical order loss. Exposure to temperatures below the TNI yields a gradual irreversible red color shift of the reflected wavelength over broad temperature and time ranges. Most likely due to polymerization confinement in combination with the dynamics of the carboxylic acid hydrogen bonds, helical unwinding takes place over time causing the red shift of the reflected wavelength. The temperature response can be programmed by altering the chemical composition of the liquid crystal mixture. Furthermore, the green, structural color of the polymer can also be easily changed making it possible to print different colored indicators combined with different respective temperature responses on a single substrate. The printed optical sensors can be easily delaminated by dissolving the non‐covalent crosslinked polymer. Such optical polymer materials are attractive for easy processable time–temperature integrators and thermal paper.

Circularly polarized luminescence of polymers with coil to helix transformation in water system triggered via metal coordination

Circularly polarized luminescence (CPL) of conjugated polymers is promising functional materials for a wide range application, particularly in the field of chiral optoelectronic devices. It is critical yet challenging to endow these materials against varied environment change, such as polarity of water, due to the destruction of the induced helicity of conjugated polymers by water hydrogen bonding. In this contribution, a series of aqueous phase CPL-active materials are constructed through the synergistic effect of hydrogen bonding and metal coordination. The fluorescent random coil amino acid-based poly( N -propargylamide) exhibiting obvious CD and CPL effects are induced by metal ions in aqueous solution. The conformational transition from coil to helix and the CPL performance are distinctly depicted via the CD, CPL signals and noticeably red-shifted UV–vis absorption. Moreover, the luminescence dissymmetry factor (| g lum |) values of the induced helical polymers were up to 8.07 × 10 −3 and 2.73 × 10 −2 , respectively, in aqueous solution and thin films. It was found that the successful induction of aqueous phase CPL-active polymer attribute to chiral amplification and aggregation-enhanced emission (AEE) effect of contracted cis-cisoidal helical main chain. These results indicated a general and simple approach for the construction of aqueous phase CPL materials, and a possible relationship between geometric structure of helical conformation and the CPL-activity induction of optically inactive polymers are simultaneously established. This contribution broadens practically chiroptical applications of CPL materials in water environment and enriches the synthetic types of CPL materials. • The CPL-active polymer based on cis-cisoid helix in aqueous solution and thin films states is prepared. • The | g lum | value of the prepared polymer in aqueous solution and films are as high as to 8.07 × 10 −3 and 2.73 × 10 −2 , respectively. • The successful trigger of CPL-activity attribute to chiral amplification and aggregation-enhanced emission (AEE) effect of one-handed helical structure.

Cover Feature: Supramolecular Ensembles Formed via Calix[5]arene‐Fullerene Host‐Guest Interactions (Chem. Asian J. 16/2022)

Calix[5]arenes have drawn attention due to their encapsulation capability for fullerenes. The high binding affinity between calix[5]arene and fullerene has enabled the development of various kinds of supramolecular fullerene polymers (such as linear fullerene polymers, helical fullerene polymers, and two-dimensionally extended fullerene networks) and the structural regulation of covalent polymers. Herein, supramolecular ensembles formed via calix[5]arene-fullerene complexation are reviewed in detail. More information can be found in the Review by Takehiro Hirao and Takeharu Haino.

Single-crystal-to-single-crystal translation of a helical supramolecular polymer to a helical covalent polymer

Polymers possessing helical conformation in the solid state are in high demand. We report a helical peptide-polymer via the topochemical ene-azide cycloaddition (TEAC) polymerization. The molecules of the designed Gly-Phe-based dipeptide, decorated with ene and azide, assemble in its crystals as β-sheets and as supramolecular helices in two mutually perpendicular directions. While the NH…O H-bonding facilitates β-sheet-like stacking along one direction, weak CH…N H-bonding between the azide-nitrogen and vinylic-hydrogen of molecules belonging to the adjacent stacks arranges them in a head-to-tail manner as supramolecular helices. In the crystal lattice, the azide and alkene of adjacent molecules in the supramolecular helix are suitably preorganized for their TEAC reaction. The dipeptide underwent regio- and stereospecific polymerization upon mild heating in a single-crystal-to-single-crystal fashion, yielding a triazoline-linked helical covalent polymer that could be characterized by single-crystal X-ray diffraction studies. Upon heating, the triazoline-linked polymer undergoes denitrogenation to aziridine-linked polymer, as evidenced by differential scanning calorimetry, thermogravimetric analysis, and solid-state NMR analyses.

In situ Synthesis of Supramolecular Polymers: Finding the Right Conditions when Combining Covalent and Non-Covalent Synthesis.

The combination of covalent and non‐covalent synthesis is omnipresent in nature and potentially enables access to new materials. Yet, the fundamental principles that govern such a synthesis are barely understood. Here, we demonstrate how even simple reaction mixtures behave surprisingly complex when covalent reactions are coupled to self‐assembly processes. Specifically, we study the reaction behavior of a system in which the in situ formation of discotic benzene‐1,3,5‐tricarboxamide (BTA) monomers is linked to an intertwined non‐covalent reaction network including self‐assembly into helical BTA polymers. This system shows an unexpected phase‐separation behavior in which an interplay of reactant/product concentrations, side‐products and solvent purity determines the system composition. We envision that these insights can bring us one step closer to how to design the synthesis of systems in a combined covalent/non‐covalent fashion.

In situ Synthesis of Supramolecular Polymers: Finding the Right Conditions when Combining Covalent and Non‐Covalent Synthesis

AbstractThe combination of covalent and non‐covalent synthesis is omnipresent in nature and potentially enables access to new materials. Yet, the fundamental principles that govern such a synthesis are barely understood. Here, we demonstrate how even simple reaction mixtures behave surprisingly complex when covalent reactions are coupled to self‐assembly processes. Specifically, we study the reaction behavior of a system in which the in situ formation of discotic benzene‐1,3,5‐tricarboxamide (BTA) monomers is linked to an intertwined non‐covalent reaction network including self‐assembly into helical BTA polymers. This system shows an unexpected phase‐separation behavior in which an interplay of reactant/product concentrations, side‐products and solvent purity determines the system composition. We envision that these insights can bring us one step closer to how to design the synthesis of systems in a combined covalent/non‐covalent fashion.

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.