Generalizing the effects of chirality on block copolymer assembly

Hierarchical assembly of thermoresponsive helical dendronized poly(phenylacetylene)s through photo-crosslinking of the thermal aggregates

Supramolecular assembly of helical homopolymers to form stable chiral entities in water is highly valuable for creating chiral nanostructures and fabricating chiral biomaterials. Here we report on thermally induced supramolecular assembly of helical dendronized poly(phenylacetylene)s (PPAs) in aqueous solutions, and their in-situ photo-crosslinking at elevated temperatures to afford crosslinked nano-assemblies with hierarchical structures and stabilized helicities. These helical dendronized homopolymers carry cinnamate-cored dendritic oligoethylene glycol (OEG) pendants, which exhibit characteristic thermoresponsive behavior. Their thermal aggregation confers hexagonal packing of the polymer chains, and simultaneously resulting in enhancement of their chiralities. Assisted by radial amphiphilicity and worm-like molecular geometry, these dendronized PPAs form supramolecular twisted fibers, spheroid particles or toroids via thermal aggregation. Through UV photoirradiation above their cloud points (Tcps), cycloaddition of cinnamate moieties from the dendritic pendants promotes intermolecular crosslinking of dendronized PPA chains within the thermal aggregates, and simultaneously, the dynamic morphologies and supramolecular chirality from the dendronized PPAs through thermally induced aggregation can be fixed. In addition, photo-crosslinking can be occurred solely within individual aggregates due to the protection of densely packed dendritic OEGs. Therefore, various crosslinked assemblies from the dendronized homopolymers with tailorable morphologies and stabilized chirality are fabricated by tuning their thermally induced dynamic aggregations followed by in-situ photo-crosslinking. We believe that this work paves a convenient route to fabricate chiral assemblies with stabilized morphologies and fixed chiralities from dynamic helical homopolymers through intermolecular crosslinking, which can be promising for various chiral applications.

Chiral Luminophore Guided Self-Assembly of Achiral Block Copolymers for the Amplification of Circularly Polarized Luminescence

This work aims to examine the effect of self-assembly on the chiroptic responses of the achiral block copolymer (BCP) polystyrene-b-poly(ethylene oxide) (PS-b-PEO) associated with chiral luminophores, (R)- or (S)-1,1'-bi-2-naphthol ((R)- or (S)-BINOL), through hydrogen bonding. With the formation of a well-ordered helical phase (H*), significantly induced circular dichroism (ICD) signals for the PEO block in the mixture can be found. Most interestingly, a remarkable amplification with an extremely large dissymmetry factor of luminescence (glum) from 10-3 to 0.3 (i.e., induced circular polarized luminescence (iCPL) behavior) for the chiral BINOLs in the mixture can be achieved by the formation of the helical phase (H*) via mesochiral self-assembly. As a result, by taking advantage of BCP for mesochiral self-assembly, it is feasible to create a nanostructured monolith with substantial optical activities, offering promising applications in the design of chiroptic devices.

Coil-Helix Block Copolymers Can Exhibit Divergent Thermodynamics in the Disordered Phase

Chiral building blocks have the ability to self-assemble and transfer chirality to larger hierarchical length scales, which can be leveraged for the development of novel nanomaterials. Chiral block copolymers, where one block is made completely chiral, are prime candidates for studying this phenomenon, but fundamental questions regarding the self-assembly are still unanswered. For one, experimental studies using different chemistries have shown unexplained diverging shifts in the order-disorder transition temperature. In this study, particle-based molecular simulations of chiral block copolymers in the disordered melt were performed to uncover the thermodynamic behavior of these systems. A wide range of helical models were selected, and several free energy calculations were performed. Specifically, we aimed to understand (1) the thermodynamic impact of changing the conformation of one block in chemically identical block copolymers and (2) the effect of the conformation on the Flory-Huggins interaction parameter, χ, when chemical disparity was introduced. We found that the effective block repulsion exhibits diverging behavior, depending on the specific conformational details of the helical block. Commonly used conformational metrics for flexible or stiff block copolymers do not capture the effective block repulsion because helical blocks are semiflexible and aspherical. Instead, pitch can quantitatively capture the effective block repulsion. Quite remarkably, the shift in χ for chemically dissimilar block copolymers can switch sign with small changes in the pitch of the helix.

Controllable Circularly Polarized Luminescence with High Dissymmetry Factor via Co-Assembly of Achiral Dyes in Liquid Crystal Polymer Films

Circularly polarized luminescence (CPL) materials are highly demanded due to their great potential in optoelectronic and chiroptical elements. However, the preparation of CPL films with high luminescence dissymmetry factors (glum ) remains a formidable task, which impedes their practical application in film-based devices. Herein, a facile strategy to prepare solid CPL film with a high glum through exogenous chiral induction and amplification of liquid crystal polymers is proposed. Amplification and reversion of the CPL appear when the films are annealed at the chiral nematic liquid crystalline temperature and the maximal glum up to 0.30 due to the enhancement of selective reflection. Thermal annealing treatment at different liquid crystalline states facilitates the formation of the chiral liquid phase and adjusts the circularly polarized emission. This work not only provides a straightforward and versatile platform to construct organic films capable of exhibiting strong circularly polarized emission but also is helpful in understanding the exact mechanism for the liquid crystal enhancement of CPL performance.

Photonically active bowtie nanoassemblies with chirality continuum

Chirality is a geometrical property described by continuous mathematical functions^1-5. However, in chemical disciplines, chirality is often treated as a binary left or right characteristic of molecules rather than a continuity of chiral shapes. Although they are theoretically possible, a family of stable chemical structures with similar shapes and progressively tuneable chirality is yet unknown. Here we show that nanostructured microparticles with an anisotropic bowtie shape display chirality continuum and can be made with widely tuneable twist angle, pitch, width, thickness and length. The self-limited assembly of the bowties enables high synthetic reproducibility, size monodispersity and computational predictability of their geometries for different assembly conditions⁶. The bowtie nanoassemblies show several strong circular dichroism peaks originating from absorptive and scattering phenomena. Unlike classical chiral molecules, these particles show a continuum of chirality measures² that correlate exponentially with the spectral positions of the circular dichroism peaks. Bowtie particles with variable polarization rotation were used to print photonically active metasurfaces with spectrally tuneable positive or negative polarization signatures for light detection and ranging (LIDAR) devices.

The Promise of Soft-Matter-Enabled Quantum Materials

The field of quantum materials has experienced rapid growth over the past decade, driven by exciting new discoveries with immense transformative potential. Traditional synthetic methods to quantum materials have, however, limited the exploration of architectural control beyond the atomic scale. By contrast, soft matter self-assembly can be used to tailor material structure over a large range of length scales, with a vast array of possible form factors, promising emerging quantum material properties at the mesoscale. This review explores opportunities for soft matter science to impact the synthesis of quantum materials with advanced properties. Existing work at the interface of these two fields is highlighted, and perspectives are provided on possible future directions by discussing the potential benefits and challenges which can arise from their bridging.

Induced Circular Dichroism and Circularly Polarized Luminescence for Block Copolymers with Chiral Communications

Many sophisticated chiral materials are found in living organisms, giving specific functions and required complexity. Owing to the remarkable optical properties of chiral materials, they have drawn significant attention for the development of synthetic materials to give optical activities for appealing applications. In contrast to a top-down approach, the bottom-up approach from self-assembled systems with chiral host-achiral guest and achiral guest-chiral host for induced circular dichroism and induced circularly polarized luminescence has greatly emerged because of its cost-effective advantage with easy fabrication for mesoscale assembly. Self-assembled hierarchical textures with chiral sense indeed give significant amplification of the dissymmetry factors of absorption and luminescence (g_abs and g_lum ), resulting from the formation of well-ordered superstructures and phases with the building of chromophores and luminophores. By taking advantage of the microphase separation of block copolymers via self-assembly, a variety of well-defined chiral nanostructures can be formed as tertiary superstructures that can be further extended to quaternary phases in bulk or thin film. In this article, a conceptual perspective is presented to utilize the self-assembly of chiral block copolymers with chiral communications, giving quaternary phases with well-ordered textures at the nanoscale for significant enhancement of dissymmetry factors.

Breaking Mirror Symmetry of Double Gyroids via Self-Assembly of Chiral Block Copolymers

Significant enhancement of segment-scale chirality, as measured by vibrational circular dichroism (VCD), is observed in the helical phase (H*) of polylactide-based chiral block copolymers (BCPs*) due to the mesoscale chirality of the microphase-separated domains. Here, we report a weaker, yet meaningful, enhancement on the VCD signal of a double gyroid phase (DG) as compared to a double diamond phase (DD) and disordered phase from the same diblock BCPs*. Residual VCD enhancement indicates a weak degree of chiral symmetry breaking, implying the formation of a chiral double gyroid (DG*) instead of the canonical achiral form. Calculations on the basis of orientational self-consistent field theory, comparing coupling between the segmental-scale preference of an intradomain twist and morphological chirality, show that a transition between DG and DG* takes place above the critical chiral strength, driving a weak volume asymmetry between the two enantiomeric single networks of DG*. The formation of nanostructures with controllable mesoscale chiral asymmetry indicates a pathway for the amplification of optical activity driven by self-assembly.

Hierarchical self-assembly into chiral nanostructures

One basic principle regulating self-assembly is associated with the asymmetry of constituent building blocks or packing models. Using asymmetry to manipulate molecular-level devices and hierarchical functional materials is a promising topic in materials sciences and supramolecular chemistry. Here, exemplified by recent major achievements in chiral hierarchical self-assembly, we show how chirality may be utilized in the design, construction and evolution of highly ordered and complex chiral nanostructures. We focus on how unique functions can be developed by the exploitation of chiral nanostructures instead of single basic units. Our perspective on the future prospects of chiral nanostructures via the hierarchical self-assembly strategy is also discussed.

Reversal of Handedness of Ionic liquid based Chiral Block Copolymers via Self-Assembly in Solution and Bulk Phase

Polymerized ionic liquid (PIL) based ionic chiral block copolymers (BCPs*) were synthesized by functionalization of poly(4-vinyl pyridine) segment in poly(styrene)-block-poly(4-vinyl pyridine) (PS-b-P4VP) block copolymer. Owing to the ease of ion...

Confined Self-Assemblies of Chiral Block Copolymers in Thin Films

Self-assembly of chiral block copolymers (BCPs*) can give rise to ordered chiral nanostructures, that is, a helical phase (H* phase), via chirality transfer from the molecular level to mesoscale. In the present work, we reported the self-assembly of BCPs* under one-dimensional spatial confinement. The morphological dependence of self-assembled BCPs* on the molecular weights and the film thickness was investigated. As chiral nanostructures, the H* phase can be formed in bulk, nonchiral nanostructures that were observed in the thin films. Also, the topology effect of self-assembly of BCPs* was examined. The self-assembly of BCPs* with a star-shaped topology exhibited a distinct morphology compared with that of linear BCPs*. The present work provides new insight into the chirality transfer of macromolecules under spatial confinement.

Microphase separation in helix-coil block copolymer melts: computer simulation

By means of molecular dynamics simulation, the process of the microphase separation in the melts of diblock helix-coil copolymers comprising a flexible and a helical block was studied. The resulting microstructures were examined, and the spatial distribution of the blocks and molecular packing were investigated. The phase diagram was built in terms of the fraction of the helical block and the incompatibility parameter of the blocks. The comparison of the diagrams for helix-coil and the classic coil-coil copolymer blends was carried out. It was shown that the total region where the ordering into distinctive microstructures takes place is similar for both diagrams. But for the helix-coil copolymers the area of the cylinders splits into the region of those with circular and elliptical cross-sections; the bicontinuous phase area is much wider; in the lamellar phases, the helical blocks were oriented precisely perpendicular to the lamellar interface, forming a cohesive interlocked structure of densely packed helices.

Reversible Micrometer-Scale Spiral Self-Assembly in Liquid Crystalline Block Copolymer Film with Controllable Chiral Response

The spiral is a fundamental structure in nature and spiral structures with controllable handedness are of increasing interest in the design of new chiroptical materials. In this study, micrometer-scale spiral structures with reversible chirality were fabricated based on the assembly of a liquid crystalline block copolymer film assisted by enantiopure tartaric acid. Mechanistic insight revealed that the formation of the spiral structures was closely related to the liquid crystalline properties of the major phase of block copolymer under the action of chiral tartaric acid. The chiral spiral structures with controllable handedness were easily erased under ultraviolet light irradiation and restored via thermal annealing. This facile thermal treatment method provides guidance for fabrication of chiral micrometer-scale spiral structures with adjustable chiral properties.

Supramolecular Chirality Transfer toward Chiral Aggregation: Asymmetric Hierarchical Self-Assembly

Self-assembly, as a typical bottom-up strategy for the fabrication of functional materials, has been applied to fabricate chiral materials with subtle chiral nanostructures. The chiral nanostructures exhibit great potential in asymmetric catalysis, chiral sensing, chiral electronics, photonics, and even the realization of several biological functions. According to existing studies, the supramolecular chirality transfer process combined with hierarchical self-assembly plays a vital role in the fabrication of multiscale chiral structures. This progress report focuses on the hierarchical self-assembly of chiral or achiral molecules that aggregate with asymmetric spatial structures such as twisted bands, helices, and superhelices in different environments. Herein, recent studies on the chirality transfer induced self-assembly based on a variety of supramolecular interactions are summarized. In addition, the influence of different environments and the states of systems including solutions, condensed states, gel systems, interfaces on the asymmetric hierarchical self-assembly, and the expression of chirality are explored. Moreover, both the driving forces that facilitate chiral bias and the supramolecular interactions that play an important role in the expression, transfer, and amplification of the chiral sense are correspondingly discussed.

Helical polymer self-assembly and chiral nanostructure formation

This review surveys recent progress towards robust chiral nanostructure fabrication techniques using synthetic helical polymers, the unique inferred properties that these materials possess, and their intricate connection to natural, biological chirality.

Networks with controlled chirality via self-assembly of chiral triblock terpolymers

Nanonetwork-structured materials can be found in nature and synthetic materials. A double gyroid (DG) with a pair of chiral networks but opposite chirality can be formed from the self-assembly of diblock copolymers. For triblock terpolymers, an alternating gyroid (G^A) with two chiral networks from distinct end blocks can be formed; however, the network chirality could be positive or negative arbitrarily, giving an achiral phase. Here, by taking advantage of chirality transfer at different length scales, G^A with controlled chirality can be achieved through the self-assembly of a chiral triblock terpolymer. With the homochiral evolution from monomer to multichain domain morphology through self-assembly, the triblock terpolymer composed of a chiral end block with a single-handed helical polymer chain gives the chiral network from the chiral end block having a particular handed network. Our real-space analyses reveal the preferred chiral sense of the network in the G^A, leading to a chiral phase.

Spiral Hierarchical Superstructures from Twisted Ribbons of Self-Assembled Chiral Block Copolymers

Spiral hierarchical superstructures were found in the self-assembly of chiral block copolymers (BCPs*) composed of a chiral poly(l-lactide) (PLLA) and an achiral polystyrene (PS) as major and minor blocks, respectively. The PLLA helical chain with semiflexible rod-like character as compared to the random coil of PS results in self-assembly with a conformational asymmetry effect overwhelming the compositional one. Consequently, instead of the forming PS cylinder microdomains in the PLLA matrix, a smectic liquid-crystal-like bilayer sandwiched with PLLA and PS microdomains will be formed. Owing to twisting and bending due to the chiral cholesteric liquid-crystal-like force field combined with steric hindrance at the chiral interface, the forming bilayers (twisted ribbon) will develop into either a concentric lamellar texture from scrolling or roll-cake textures from spiraling. This study might bring a concept for the formation of spiral hierarchical superstructures from self-assembled bilayers for device application.

Spiranthes sinensis-Inspired Circular Polarized Luminescence in a Solid Block Copolymer Film with a Controllable Helix

Chiral materials with circular polarized luminescence (CPL) have attracted much interest because of their extensive optical information and remarkable sensitivity. Inspired by the helical template in Spiranthes sinensis, we propose here a general and flexible method for fabricating solid CPL materials using a block copolymer-formed helix as a template. A chiral arrangement of various nonchiral fluorescent molecules was obtained in the block copolymer-based hybrid film. An excimer chiralty rule was found for the CPL emission of nonchiral fluorescent molecules: a right-handed helix induced left-handed CPL emission and a left-handed helix induced right-handed CPL emission. A dissipative particle dynamics simulation showed that such an antihelical effect is related to the length between the adjacent interacting points of nonchiral fluorescent molecules along the helical structure. Furthermore, the fluorescent films had a high dissymmetric factor for CPL emission, and thus, the films provide a general and flexible platform for designing and preparing advanced functional chiroptical materials.

Lasing from an Organic Micro-Helix

Organic solid-state semiconductor lasers are attracting ever-increasing interest for their potential application in future photonic circuits. Despite the great progress made in recent years, an organic laser from 3D chiral structures has not been achieved. Now, the first example of an organic nano-laser from the micro-helix structure of an achiral molecule is presented. Highly regular micro-helixes with left/right-handed helicity from a distyrylbenzene derivative (HM-DSB) were fabricated and characterized under microscope spectrometers. These chiral micro-helixes exhibit unique photonic properties, including helicity-dependent circularly polarized luminescence (CPL), periodic optical waveguiding, and length-dependent amplified spontaneous emission (ASE) behavior. The successful observation of laser behavior from the organic micro-helix extends our understanding to morphology chirality of organic photonic materials and provides a new design strategy towards chiral photonic circuits.

Insensitivity of Sterically Defined Helical Chain Conformations to Solvent Quality in Dilute Solution

The interplay between polymer-polymer and polymer-solvent interactions as well as interactions that impose secondary structures determines the conformation of polymer chains in dilute solution. Polypeptoids-poly(N-substituted glycines) have been shown to form helical secondary structures primarily driven by steric interactions from chiral, bulky side chains, while polypeptoids with a racemic mixture of the same side chains lead to unstructured coil chains with a shorter Kuhn length. Small-angle neutron scattering (SANS) of the polypeptoids in dilute solution reveals that the helical polypeptoids are only locally stiffer than the coil chains formed from the racemic analogue, but exhibit overall flexibility. We show that chain conformations of both helical and coil polypeptoids (in terms of radius of gyration, R_g) are insensitive to solvent quality (parametrized by the second virial coefficient, A₂). Potential effects from the bulky, chiral/racemic side chains dominating chain conformations are excluded by comparison with an achiral polypeptoid lacking side chain chirality. The specific interactions between polypeptoid segments are likely dominating the chain conformations in this type of polypeptoids as opposed to polymer-solvent interactions or energetic contributions from the helical secondary structure.

Mesochiral phases from the self-assembly of chiral block copolymers

Self-assembly of block copolymers with chiral sense gives mesochiral phases possessing helical sense. With the controlled chirality of the helical cylinder and chiral network, it is appealing to fabricate chiral materials for applications.

Irreversible topological transition of a stretched superhelix: the interplay of chiralities

Helical complexes with certain intramolecular constraints on their topological state, exhibit nontrivial conformational changes in response to an external tension. We consider a generic right-handed helix and construct identical right and left-handed superhelixes. By using molecular dynamics simulations, we discover that under an external tension, the interplay between the helix chirality and the chirality of the superhelix leads to a fundamental topology transition of the stretched right-handed superhelix to its left-handed counterpart. We characterize and rationalize this irreversible phenomenon for a wide range of intramolecular angular stiffnesses and determine the associated phase diagram at different external loadings.