Biased Symmetry Breaking and Chiral Control by Self-Replicating in Achiral Tetradentate Platinum (II) Complexes

Obtaining homochirality from biased symmetry-breaking of self-assembly in achiral molecules remains a great challenge due to the lack of ingenious strategies and controlling their handedness. Here, we report the first case of biased symmetry breaking from achiral platinum (II) liquid crystals which self-organize into an enantiomerically enriched single domain without selection of handedness in twist grain boundary TGB ^[ *^] phase. Most importantly, the chiral control of self-organization can be achieved by using above the homochiral liquid crystal films with determined handedness (P or M) as a template. Moreover, benefiting from self-assembled superhelix, these complexes exhibit prominent circularly polarized luminescence with high |g_lum | up to 3.4×10^-3 in the TGB ^[ *^] mesophase. This work paves a neoteric avenue for the development of chiral self-assemblies from achiral molecules.

Nature-Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self-Assembly to DNA Mesophase and Nanocolloids

Liquid crystals (LCs) are omnipresent in living matter, whose chirality is an elegant and distinct feature in certain plant tissues, the cuticles of crabs, beetles, arthropods, and beyond. Taking inspiration from nature, researchers have recently devoted extensive efforts toward developing chiral liquid crystalline materials with self-organized nanostructures and exploring their potential applications in diverse fields ranging from dynamic photonics to energy and safety issues. In this review, an account on the state of the art of emerging chiral liquid crystalline nanostructured materials and their technological applications is provided. First, an overview on the significance of chiral liquid crystalline architectures in various living systems is given. Then, the recent significant progress in different chiral liquid crystalline systems including thermotropic LCs (cholesteric LCs, cubic blue phases, achiral bent-core LCs, etc.) and lyotropic LCs (DNA LCs, nanocellulose LCs, and graphene oxide LCs) is showcased. The review concludes with a perspective on the future scope, opportunities, and challenges in these truly advanced functional soft materials and their promising applications.

Сhiral and Racemic Fields Concept for Understanding of the Homochirality Origin, Asymmetric Catalysis, Chiral Superstructure Formation from Achiral Molecules, and B-Z DNA Conformational Transition

The four most important and well-studied phenomena of mirror symmetry breaking of molecules were analyzed for the first time in terms of available common features and regularities. Mirror symmetry breaking of the primary origin of biological homochirality requires the involvement of an external chiral inductor (environmental chirality). All reviewed mirror symmetry breaking phenomena were considered from that standpoint. A concept of chiral and racemic fields was highly helpful in this analysis. A chiral gravitational field in combination with a static magnetic field (Earth’s environmental conditions) may be regarded as a hypothetical long-term chiral inductor. Experimental evidences suggest a possible effect of the environmental chiral inductor as a chiral trigger on the mirror symmetry breaking effect. Also, this effect explains a conformational transition of the right-handed double DNA helix to the left-handed double DNA helix (B-Z DNA transition) as possible DNA damage.

Chiral Superstructure Mesophases of Achiral Bent-Shaped Molecules - Hierarchical Chirality Amplification and Physical Properties

Chiral mesophases in achiral bent-shaped molecules have attracted particular attention since their discovery in the middle 1990s, not only because of their homochirality and polarity, but also due to their unique physical/physicochemical properties. Here, the most intriguing results in the studies of such symmetry-broken states, mainly helical-nanofilament (HNF) and dark-conglomerate (DC) phases, are reviewed. Firstly, basic information on the typical appearance and optical activity in these phases is introduced. In the following section, the formation of mesoscopic chiral superstructures in the HNF and DC phases is discussed in terms of hierarchical chirality. Nanoscale phase segregation in mixture systems and gelation ability in the HNF phase are also described. In addition, some other related chiral phases of bent-shaped molecules are shown. Recent attempts to control such mesoscopic chiral structure and the alignment/confinement of HNFs are also discussed, along with several examples of their fascinating advanced physical properties, i.e. huge enhancement of circular dichroism, electro- and photo-tunable optical activities, chirality-induced nonlinear optics (second-harmonic-generation circular difference and electrogyration effect), enhanced hydrophobicity through the dual-scale surface morphological modulation, and photoconductivity in the HNF/fullerene binary system. Future prospects from basic science and application viewpoints are also indicated in the concluding section.

Supramolecular Helical Nanofibers Formed by Achiral Monomers and Their Reversible Sol-Gel Transition

Well-defined supramolecular helical nanofibers have been constructed by a rationally designed achiral monomer in aqueous solution based on the 1:2 host-guest combination between cucurbit[8]uril and a 4,4'-bipyridin-1-ium chloride (BPY⁺ ) salt derivative. The formed nanostructures could be adjusted by varying the concentration of monomer from helical nanofibers to a pH-responsive hydrogel.

Photomodulated Supramolecular Chirality in Achiral Photoresponsive Rodlike Compounds Nanosegregated from the Helical Nanofilaments of Achiral Bent-Core Molecules

We prepared a nonchiral mixture of achiral bent-core molecules and photoresponsive rodlike liquid crystalline (LC) molecules. With the help of the isothermal photochemical nematic (N)-isotropic (Iso) phase transition of the photoresponsive rodlike LC molecules, the corresponding phase transition from a dark conglomerate BX phase to another distinguishable dark conglomerate B4 phase took place in the mixture. A large circular dichroism (CD) signal originating from supramolecular chirality was detected in the initial BX phase. On the other hand, the detected CD signal was decreased in the B4 phase after UV irradiation. Interestingly, the decreased CD signal could be reverted to the initial CD signal with visible irradiation. This chiroptical process revealed in this work was stable and reversible and thus opens up the possibility of practical applications such as rewritable optical storage.

Multistep hierarchical self-assembly of chiral nanopore arrays

A series of simple hierarchical self-assembly steps achieve self-organization from the centimeter to the subnanometer-length scales in the form of square-centimeter arrays of linear nanopores, each one having a single chiral helical nanofilament of large internal surface area and interfacial interactions based on chiral crystalline molecular arrangements.

Dark conglomerate phases of azobenzene derived bent-core mesogens - relationships between the molecular structure and mirror symmetry breaking in soft matter

New 4-bromoresorcinol based bent-core molecules with peripheral fluoro substituted azobenzene wings have been synthesized and the liquid crystalline self-assembly was investigated by differential scanning calorimetry (DSC), optical polarizing microscopy (POM), electro-optic studies and X-ray diffraction (XRD). A new type of optically isotropic mesophase composed of chiral domains with opposite handedness (dark conglomerate phases, DC phases) is observed, which for some homologues with medium alkyl chain length is stable down to ambient temperature. It is proposed that these DC phases are formed by helical twisted nano-domains of limited size and composed of the crystallized aromatic cores which are separated by the disordered alkyl chains. This structure is distinct from the previously known soft helical nano-filament phases (HNF phases, B4 phases) formed by extended crystalline nano-filaments and also distinct from the fluid sponge phases composed of deformed fluid layers. Comparison with related bent-core molecules having H, F, Cl, I, CH3 and CN groups in the 4-position at the resorcinol core, either with or without additional peripheral fluorines, provided information about the effects of these substituents on the tendency to form DC phases. Based on these relationships and by comparison with the minimum energy conformations obtained by DFT calculations a hypothesis is provided for the formation of DC phases depending on the molecular structure.