Spontaneous Mirror‐Symmetry Breaking in Isotropic Liquid Phases of Photoisomerizable Achiral Molecules

Spontaneous mirror‐symmetry breaking is of fundamental importance in science as it contributes to the development of chiral superstructures and new materials and has a major impact on the discussion around the emergence of uniform chirality in biological systems. Herein we report chirality synchronization, leading to spontaneous chiral conglomerate formation in isotropic liquids of achiral and photoisomerizable azobenzene‐based rod‐like molecules. The position of fluorine substituents at the aromatic core is found to have a significant effect on the stability and the temperature range of these chiral liquids. Moreover, these liquid conglomerates occur in a new phase sequence adjacent to a 3D tetragonal mesophase.     

Helical Nano‐crystallite (HNC) Phases: Chirality Synchronization of Achiral Bent‐Core Mesogens in a New Type of Dark Conglomerates

Spontaneous generation of macroscopic homochirality in soft matter systems by self‐assembly of exclusively achiral molecules under achiral conditions is a challenging task with relevance for fundamental scientific research and technological applications. Dark conglomerate phases (DC phases), being optically isotropic mesophases composed of conglomerates of macroscopic chiral domains and formed by some non‐chiral bent‐core mesogens, represent such a case. Here we report two new series of non‐symmetric bent‐core molecules capable of forming a new type of mirror symmetry broken DC phases. In the synthesized molecules, a bent 4‐bromoresorcinol core is connected to a phenyl benzoate wing and an azobenzene wing with or without additional peripheral fluorine substitution. The self‐assembly was investigated by DSC, polarizing microscopy, electro‐optical studies and XRD. Chiral and apparently achiral DC phases were observed besides distinct types of lamellar liquid crystalline phases with different degree of polar order, allowing the investigation of the transition from smectic to DC phases. This indicates a process in which increased packing density at first gives rise to restricted rotation and thus to growing polar order, which then leads to chirality synchronization, layer frustration and nano‐scale crystallization. Topological constraints arising from the twisted packing of helical conformers in lamellar crystals is proposed to lead to amorphous solids composed of helical nano‐crystallites with short coherence length (HNC phases). This is considered as a third major type of DC phases, distinct from the previously known liquid crystalline sponge phases and the helical nano‐filament phases (HNF phases). Guidelines for the molecular design of new materials capable of self‐assembly into these three types of DC phases are proposed.     

Inversion of the Supramolecular Chirality of Nanofibrous Structures through Co‐Assembly with Achiral Molecules

To understand the behavior of chiral nanostructures, it is of critical importance to study how achiral molecules regulate the chirality of such nanostructures and what the main driving forces for the regulation processes are. In this work, the supramolecular chirality of helical nanofibers consisting of phenylalanine‐based enantiomers is inverted by achiral bis(pyridinyl) derivatives through co‐assembly. This inversion is mainly mediated by intermolecular hydrogen bonding interactions between the achiral additives and the chiral molecules, which may induce stereoselective interactions and different reorientations for the assembled molecules, as confirmed by calculations. This work not only exemplifies a feasible method to invert the helicity of chiral nanostructures by the addition of achiral molecules, but also provides a method to explore their functions in environments where chiral and achiral molecules are in close proximity.     

Switching Chirophilic Self-assembly: From meso-structures to Conglomerates in Liquid and Liquid Crystalline Network Phases of Achiral Polycatenar Compounds

Spontaneous generation of chirality from achiral molecules is a contemporary research topic with numerous implications for technological applications and for the understanding of the development of homogeneous chirality in biosystems. Herein, a series of azobenzene based rod-like molecules with an 3,4,5-trialkylated end and a single n-alkyl chain involving 5 to 20 aliphatic carbons at the opposite end is reported. Depending on the chain length and temperature these achiral molecules self-assemble into a series of liquid and liquid crystalline (LC) helical network phases. A chiral isotropic liquid (Iso₁^[*^]) and a cubic triple network phase with chiral I23 lattice were found for the short chain compounds, whereas non-cubic and achiral cubic phases dominate for the long chain compounds. Among them a mesoscale conglomerate with I23 lattice, a tetragonal phase (Tet_bi) containing one chirality synchronized and one non-synchronized achiral network, an achiral double network meso-structure with Ia d space group and an achiral percolated isotropic liquid mesophase (Iso₁) were found. This sequence is attributed to an increasing strength of chirality synchronization between the networks, combined with a change of the preferred mode of chirophilic self-assembly between the networks, switching from enantiophilic to enantiophobic with decreasing chain length and lowering temperature. These nanostructured and mirror symmetry broken LC phases exist over wide temperature ranges which is of interest for potential applications in chiral and photosensitive functional materials derived from achiral compounds.     

Macroscopic Chirality of Supramolecular Gels Formed from Achiral Tris(ethyl cinnamate) Benzene‐1,3,5‐tricarboxamides

A C₃‐symmetric benzene‐1,3,5‐tricarboxamide substituted with ethyl cinnamate was found to self‐assemble into supramolecular gels with macroscopic chirality in a DMF/H₂O mixture. The achiral compound simultaneously formed left‐ and right‐handed twists in an unequal number, thus resulting in the macroscopic chirality of the gels without any chiral additives. Furthermore, ester–amide exchange reactions with chiral amines enabled the control of both the handedness of the twists and the macroscopic chirality of the gels, depending on the structures of the chiral amines. These results provide new prospects for understanding and regulating symmetry breaking in assemblies of supramolecular gels formed from achiral molecular building blocks.     

Dynamic Supramolecular Polymers Based on Benzene‐1,3,5‐tricarboxamides: The Influence of Amide Connectivity on Aggregate Stability and Amplification of Chirality

N‐Centred benzene‐1,3,5‐tricarboxamides (N‐BTAs) composed of chiral and achiral alkyl substituents were synthesised and their solid‐state behaviour and self‐assembly in dilute alkane solutions were investigated. A combination of differential scanning calorimetry (DSC), polarisation optical microscopy (POM) and X‐ray diffraction revealed that the chiral N‐BTA derivatives with branched 3,7‐dimethyloctanoyl chains were liquid crystalline and the mesophase was assigned as Col_ho. In contrast, N‐BTA derivatives with linear tetradecanoyl or octanoyl chains lacked a mesophase and were obtained as crystalline compounds. Variable‐temperature infrared spectroscopy showed the presence of threefold, intermolecular hydrogen bonding between neighbouring molecules in the mesophase of the chiral N‐BTAs. In the crystalline state at room temperature a more complicated packing between the molecules was observed. Ultraviolet and circular dichroism spectroscopy on dilute solutions of N‐BTAs revealed a cooperative self‐assembly behaviour of the N‐BTA molecules into supramolecular polymers with preferred helicity when chiral alkyl chains were present. Both the sergeants‐and‐soldiers as well as the majority‐rules principles were operative in stacks of N‐BTAs. In fact, the self‐assembly of N‐BTAs resembles closely that of their carbonyl (C?O)‐centred counterparts, with the exception that aggregation is weaker and amplification of chirality is less pronounced. The differences in the self‐assembly of N‐ and C?O‐BTAs were analysed by density functional theory (DFT) calculations. These reveal a substantially lower interaction energy between the monomeric units in the supramolecular polymers of N‐BTAs. The lower interaction energy is due to the higher energy penalty for rotation around the Ph? NH bond compared to the Ph? CO bond and the diminished magnitude of dipole–dipole interactions. Finally, we observed that mixed stacks are formed in dilute solution when mixing N‐BTAs and C?O BTAs.     

Role of Achiral Nucleobases in Multicomponent Chiral Self‐Assembly: Purine‐Triggered Helix and Chirality Transfer

Chiral self‐assembly is a basic process in biological systems, where many chiral biomolecules such as amino acids and sugars play important roles. Achiral nucleobases usually covalently bond to saccharides and play a significant role in the formation of the double helix structure. However, it remains unclear how the achiral nucleobases can function in chiral self‐assembly without the sugar modification. Herein, we have clarified that purine nucleobases could trigger N‐(9‐fluorenylmethox‐ycarbonyl) (Fmoc)‐protected glutamic acid to self‐assemble into helical nanostructures. Moreover, the helical nanostructure could serve as a matrix and transfer the chirality to an achiral fluorescence probe, thioflavin T (ThT). Upon chirality transfer, the ThT showed not only supramolecular chirality but also circular polarized fluorescence (CPL). Without the nucleobase, the self‐assembly processes cannot happen, thus providing an example where achiral molecules played an essential role in the expression and transfer of the chirality.     

The hydrogen‐bonding patterns of 3‐phenylpropylammonium benzoate and 3‐phenylpropylammonium 3‐iodobenzoate: generation of chiral crystals from achiral molecules

The crystal structures and hydrogen‐bonding patterns of 3‐phenylpropylammonium benzoate, C₉H₁₄N⁺·C₇H₅O₂⁻, (I), and 3‐phenylpropylammonium 3‐iodobenzoate, C₉H₁₄N⁺·C₇H₄IO₂⁻, (II), are reported and compared. The addition of the I atom on the anion in (II) produces a different hydrogen‐bonding pattern to that of (I). In addition, the supramolecular heterosynthon of (II) produces a chiral crystal packing not observed in (I). Compound (I) packs in a centrosymmetric fashion and forms achiral one‐dimensional hydrogen‐bonded columns through charge‐assisted N—H...O hydrogen bonds. Compound (II) packs in a chiral space group and forms helical one‐dimensional hydrogen‐bonded columns with 2₁ symmetry, consisting of repeating R₄³(10) hydrogen‐bonded rings that are commonly observed in ammonium carboxylate salts containing chiral molecules. This hydrogen‐bond pattern, which has been observed repeatedly in ammonium carboxylate salts, thus provides a means of producing chiral crystal structures from achiral molecules.     

Designing Liquid‐Crystalline Oligomers to Exhibit Twist‐Bend Modulated Nematic Phases

Arguably the most active area in liquid crystal research presently, the twist‐bend modulated nematic phase (TB) exhibits a nanoscale helical structure and therefore exhibits bulk chirality despite being formed of typically achiral materials. After a short introduction we address the structure‐property relationship that exists for this unique mesophase before introducing recent advances in strategies employed to prepare oligomeric materials that exhibit this state of matter. These methods afford fine control over the molecular substructure and potentially over the nanostructure of the exhibited mesophases.     

Three‐Ring‐Based Room‐Temperature Bent‐Core Nematic Compounds: Synthesis and Characterization

We report the synthesis and characterization of a new class of achiral three‐ring bent‐core compounds with an amide and ester linkage at the molecular bend, which are shown to exhibit nematic/phases in wide temperature ranges around room temperature (RT) and undulated SmC phases below RT. In contrast to previous studies, the compounds reported in this Communication show a true RT nematic phase with fluid physical appearance. They show strong photoluminescence in the mesophase and are found to display a one‐dimensional array of intermolecular hydrogen bonding. Furthermore, the nematic phases exhibited by these compounds show a good homeotropic alignment that can be exploited in applications such as optics and sensing. Considering the scarcity of bent‐core materials exhibiting an RT nematic mesophase, this new class of materials is promising.     

Self‐Assembly of 1,1′‐Biphenyl‐2,2′,6,6′‐tetracarboxylic Acid: Formation of an Achiral Grid with Chiral Compartments

D ₄ ‐symmetric chiral hydrogen‐bonded cyclotetramers (see the structure in the picture) are present in the self‐assembled achiral title compound in the solid state. The unilayered network set up from the chiral “square” blocks is achiral as a consequence of the crystal symmetry.     

Induced Circular Dichroism in Phosphine Gold(I) Aryl Acetylide Urea Complexes through Hydrogen‐Bonded Chiral Co‐Assemblies

Phosphine gold(I) aryl acetylide complexes equipped with a central bis(urea) moiety form 1D hydrogen‐bonded polymeric assemblies in solution that do not display any optical activity. Chiral co‐assemblies are formed by simple addition of an enantiopure (metal‐free) complementary monomer. Although exhibiting an intrinsically achiral linear geometry, the gold(I) aryl acetylide fragment is located in the chiral environment displayed by the hydrogen‐bonded co‐assemblies, as demonstrated by induced circular dichroism (ICD).     

Isolable Chiral Aggregates of Achiral π‐Conjugated Carboxylic Acids

The induced aggregation of achiral building blocks by a chiral species to form chiral aggregates with memorized chirality has been observed for a number of systems. However, chiral memory in isolated aggregates of achiral building blocks remains rare. One possible reason for this discrepancy could be that not much is understood in terms of designing these chiral aggregates. Herein, we report a strategy for creating such isolable chiral aggregates from achiral building blocks that retain chiral memory after the facile physical removal of the chiral templates. This strategy was used for the isolation of chiral homoaggregates of neutral achiral π‐conjugated carboxylic acids in pure aqueous solution. Under what we have termed an “interaction–substitution” mechanism, we generated chiral homoaggregates of a variety of π‐conjugated carboxylic acids by using carboxymethyl cellulose (CMC) as a mediator in acidic aqueous solutions. These aggregates were subsequently isolated from the CMC templates whilst retaining their memorized supramolecular chirality. Circular dichroism (CD) spectra of the aggregates formed in the acidic CMC solution exhibited bisignated exciton‐coupled signals of various signs and intensities that were maintained in the isolated pure homoaggregates of the achiral π‐conjugated carboxylic acids. The memory of the supramolecular chirality in the isolated aggregates was ascribed to the substitution of COOH/COOH hydrogen‐bonding interaction between the carboxylic acid groups within the aggregates for the hydrogen‐bonding interactions between the COOH groups of the building blocks and the chiral templates. We expect that this “interaction–substitution” procedure will open up a new route to isolable pure chiral aggregates from achiral species.     

Induction of chirality in an achiral monolayer at the liquid/solid interface by a supramolecular chiral auxiliary

An achiral oligo(p-phenylene vinylene) derivative with a ureido-triazine hydrogen bonding unit self-assembles into rows of hydrogen bonded dimers at the liquid/solid interface. Scanning tunneling microscopy reveals the formation of chiral domains, but overall, the surface remains racemic. Addition of a chiral auxiliary which is able to interact with the dimers through hydrogen bonding, showed that global organizational chirality could be achieved since a majority of the domains show the same handedness. After removing the chiral auxiliary with a volatile solvent, the global organizational chirality could be trapped, revealing a memory effect. With this straightforward supramolecular approach, we were able to create a chiral surface with preferred handedness composed of achiral molecules at the air/solid interface.     

Induction of Supramolecular Chirality in the Self‐Assemblies of Lipophilic Pyrimidine Derivatives by Choice of the Amino Acid‐Based Chiral Spacer

A new family of supramolecular organogelators, based on chiral amino acid derivatives of 2,4,6‐trichloro‐pyrimidine‐5‐carbaldehyde, has been synthesized. L ‐alanine was incorporated as a spacer between the pyrimidine core and long hydrocarbon tails to compare the effect of chirality and hydrogen bonding to that of the achiral analogue. The role of aromatic moiety on the chiral spacer was also investigated by introducing L ‐phenyl alanine moieties. The presence of intermolecular hydrogen‐bonding leading to the chiral self‐assembly was probed by concentration‐dependent FTIR and UV/Vis spectroscopies, in addition to circular dichroism (CD) studies. Temperature and concentration‐dependent CD spectroscopy ascribed to the formation of β‐sheet‐type H‐bonded networks. The morphology and the arrangements of the molecules in the freeze‐dried gels were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and X‐ray diffraction (XRD) techniques. Calculation of the length of each molecular system by energy minimization in its extended conformation and comparison with the small‐angle XRD pattern reveals that this class of gelator molecules adopts a lamellar organization. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) indicate that the solid state phase behavior of these molecules is totally dependent on the choice of their amino acid spacers. Structure‐induced aggregation properties based on the H‐bonding motifs and the packing of the molecule in three dimensions leading to gelation was elucidated by rheological studies. However, viscoelasticity was shown to depend only marginally on the H‐bonding interactions; rather it depends on the packing of the gelators to a greater extent.     

Identification of a Peripheral Substitution Symmetry Effect in Self‐Assembled Architectures

Two complementary classes of molecules based on a triphenylene core are synthesized. The two‐dimensional (2D) assemblies of these molecules deposited on a highly oriented pyrolytic graphite (HOPG) surface are identified with scanning tunneling microscopy (STM). Structures with large cavities are formed by symmetric molecules, while uniform and closely packed stripe‐assembled structures are obtained for asymmetric molecules. X‐ray diffraction (XRD) results support the observation of an ordered hexagonal columnar mesophase for symmetric molecules and a rectangular columnar mesophase for asymmetric molecules. The study demonstrates that the substitution symmetry has significant effects on the assembly characteristics of molecular architectures and also on the three‐dimensional (3D) macroscopic properties of the molecular materials.     

Water‐Dependent Optical Activity Inversion of Chiral DNA–Silica Assemblies

Chirality is widely found in nature and is expressed hierarchically in many organic–inorganic hybrid materials. Optical activity (OA) is the most fundamental attribute of these chiral materials. In this study, we found that the OA of impeller‐like chiral DNA–silica assemblies (CDSAs) was inverted with the addition of water. The state of DNA under dry and wet conditions, and the dual chirality of chiral DNA layers and twisted helical arrays of opposite handedness in CDSAs were considered to exert predominant effects on the OAs. The circular dichroism (CD) responses for the dry CDSAs were mostly attributed to the chiral arrangement of DNA layers, whereas the opposite CD responses for the wet CDSAs primarily originated from twisted helical arrays of DNA molecules. The observed CD signals were a super‐position of the two opposing OA responses. The increase in the longitudinal relation of DNA molecules due to the recovery of a double‐helical structure of DNA in the presence of water was considered to be the reason for the increase in intensity of the CD signals that originated from the twisted helical array, which led to the inversion of OA of the CDSAs. The inversion of the plasmon‐resonance‐based OAs for the chiral‐arranged achiral Ag nanoparticles (NPs) located in the channels of the CDSAs in dry and wet states further confirmed the dual chirality of DNA packing. Such research on DNA assemblies and metal NPs with dual, opposite chirality assists in the understanding of DNA hierarchical chirality in living systems and the creation of macroscopic ordered helical materials and biosensors.     

Chiral Amplification in Nature: Studying Cell‐Extracted Chiral Carotenoid Microcrystals via the Resonance Raman Optical Activity of Model Systems

Carotenoid microcrystals, extracted from cells of carrot roots and consisting of 95 % of achiral β‐carotene, exhibit a very intense chiroptical (ECD and ROA) signal. The preferential chirality of crystalline aggregates that consist mostly of achiral building blocks is a newly observed phenomenon in nature, and may be related to asymmetric information transfer from the chiral seeds (small amount of α‐carotene or lutein) present in carrot cells. To confirm this hypothesis, we synthesized several model aggregates from various achiral and chiral carotenoids. Because of the sergeant‐and‐soldier behavior, a small number of chiral sergeants (α‐carotene or astaxanthin) force the achiral soldier molecules (β‐ or 11,11′‐[D₂]‐β‐carotene) to jointly form supramolecular assemblies of induced chirality. The chiral amplification observed in these model systems confirmed that chiral microcrystals appearing in nature might consist predominantly of achiral building blocks and their supramolecular chirality might result from the co‐crystallization of chiral and achiral analogues.     

On‐Surface Evolution of meso‐Isomerism in Two‐Dimensional Supramolecular Assemblies

Chiral structures created through the adsorption of molecules onto achiral surfaces play pivotal roles in many fields of science and engineering. Here, we present a systematic study of a novel chiral phenomenon on a surface in terms of organizational chirality, that is, meso‐isomerism, through coverage‐driven hierarchical polymorphic transitions of supramolecular assemblies of highly symmetric π‐conjugated molecules. Four coverage‐dependent phases of dehydrobenzo[12]annulene were uniformly fabricated on Ag(111), exhibiting unique chiral characteristics from the single‐molecule level to two‐dimensional supramolecular assemblies. All coverage‐driven phase transitions stem from adsorption‐induced pseudo‐diastereomerism, and our observation of a lemniscate‐type (∞) supramolecular configuration clearly reveals a drastic chiral phase transition from an enantiomeric chiral domain to a meso‐isomeric achiral domain. These findings provide new insights into controlling two‐dimensional chiral architectures on surfaces.     

Self‐Assembly in Helical Columnar Mesophases and Luminescence of Chiral 1H‐Pyrazoles

Chiral polycatenar 1H‐pyrazoles self‐assemble to form columnar mesophases that are stable at room temperature. X‐ray diffraction and CD studies in the mesophase indicate a supramolecular helical organization consisting of stacked H‐bonded dimers. The liquid‐crystalline compounds reported are 3,5‐bis(dialkoxyphenyl)‐1H‐pyrazoles that incorporate two or four dihydrocitronellyl chiral tails. It can be observed that the grafting of these branched chiral substituents onto the 3,5‐diphenyl‐1H‐pyrazole core has a beneficial role in inducing mesomorphism, because isomeric linear‐chain compounds are not liquid crystalline; this is not the usual scheme of behavior. Furthermore, the molecular chirality is transferred to the columnar mesophase, because preferential helical arrangements are observed. Films of the compounds are luminescent at room temperature and constitute an example of the self‐organization of nondiscoid units into columnar liquid‐crystalline assemblies in which the functional molecular unit transfers its properties to a hierarchically built superstructure.