Swallow‐Tailed Polycatenars: Controlling Complex Liquid Crystal Self‐Assembly and Mirror Symmetry Breaking at the Lamellae‐Network Cross‐Over

Srinatha M.K., Poppe S., Shanker G., Alaasar M., Tschierske C.

Herein we report the synthesis and investigation of a new type of liquid crystalline trimers. These trimers were designed and synthesized by connecting a 2,3,4-trihydroxy benzonitrile to three 4-cyanobiphenyl units via ether linkages with aliphatic spacers having a variable number of carbon atoms ( n = 5–12). Their phase behaviour has been characterized by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). It was found that all the synthesized materials exhibit nematic phases that exist over wide temperature ranges and are even stable around ambient temperature. Smectic phases are suppressed and unique fiber formation was found for the nematic phases of these low molecular mass materials. Therefore, this report represents a step forward towards the design and synthesis of single component room temperature nematic materials with unique properties for potential applications. The first examples of low symmetry trimers and these materials provide access to unique low molar mass nematics capable of forming freely suspended fibers. • The first examples of low symmetry trimers forming room temperature nematics • Unique low molar mass nematics capable of forming freely suspended fibers • In comparison with CBs, these trimers provide wider nematic ranges. • Reduced symmetry and dynamic network formation suppress smectic and crystallization.

Reppe T., Poppe S., Tschierske C.

AbstractSpontaneous development of chirality in systems composed of achiral molecules is important for new routes to asymmetric synthesis, chiral superstructures and materials, as well as for the understanding of the mechanisms of emergence of prebiotic chirality. Herein, it is shown that the 4,4′‐diphenylbenzil unit is a universal transiently chiral bent building block for the design of multi‐chained (polycatenar) rod‐like molecules capable of forming a wide variety of helically twisted network structures in the liquid, the liquid crystalline (LC) and the crystalline state. Single polar substituents at the apex of tricatenar molecules support the formation of the achiral (racemic) cubic double network phase with Ia d symmetry and relatively small twist along the networks. The combination of an alkyl chain with fluorine substitution leads to the homogeneously chiral triple network phase with I23 space group, and in addition, provides a mirror symmetry broken liquid. Replacing F by Cl or Br further increases the twist, leading to a short pitch double gyroid Ia d phase, which is achiral again. The effects of the structural variations on the network structures, either leading to achiral phases or chiral conglomerates are analyzed.

Chen C., Poppe M., Poppe S., Tschierske C., Liu F.

AbstractLiquid state self‐assembly is important for the understanding of the complex structures developed in abiogenesis and biogenesis as well as for numerous potential technological applications. Herein we report the first body‐centered cubic liquid crystalline phase with 8‐connected network topology and open octahedral network structure. It is formed by dynamic soft self‐assembly of X‐shaped polyphiles with oligo(para‐phenylene‐ethynylene) cores. The π‐conjugated rods with perfluorinated inner benzene rings form networks conjoined by eight‐way junctions, which are formed by nano‐segregated spheres involving hydrogen‐bonded polar end groups, while the branched aliphatic chains at opposite sides of the cores fill the continuum. This novel cubic phase is based on the I‐WP minimal surface separating the frameworks of polyaromatic cores from the most disordered chain segments. It can also be considered as a dense sphere packing. Such liquid organic frameworks, representing hybrids of sphere packings and networks could be of interest for organic photonics and other technologies.

Alaasar M., Poppe S., Cao Y., Chen C., Liu F., Zhu C., Tschierske C.

A series of achiral tricatenar rod-like molecules with a 3,5-disubstitution pattern at one end and a single alkyl chain at the other end of a rod-like azobenzene derived core is reported. Depending on temperature and alkyl chain length, these Y-shaped compounds self-assemble into different types of liquid crystalline (LC) phases, ranging from non-tilted and synclinic tilted hexatic, via non-tilted and anticlinic tilted smectic and bicontinuous cubic LC phases, to a spontaneous mirror symmetry broken isotropic liquid (Iso1[*]) or a related achiral liquid network phase (Iso1). An additional tilted, but uniaxial smectic phase was observed at the transition between anticlinic and synclinic tilt correlation and was investigated by soft resonant X-ray scattering with respect to possible helix formation. This work provides a new concept for the design of technological interesting azobenzene based LC materials with anticlinic tilted smectic C phases (SmCa) and with azobenzene units organized in the long range or short range helical network structures of bicontinuous cubic and chiral isotropic liquid phases, respectively. Core fluorination removes all lamellar phases, leaving only the cubic phase over wide temperature ranges, even at ambient temperature.

Dressel C., Reppe T., Poppe S., Prehm M., Lu H., Zeng X., Ungar G., Tschierske C.

AbstractBicontinuous cubic liquid crystalline phases of π‐conjugated molecules, representing self‐assembled 3D‐ordered interpenetrating networks with cubic symmetry, are receiving increasing attention due to their capacity for charge transport in all three dimensions and their inherent spontaneous helicity. Herein, a robust general design concept for creating bicontinuous cubic phases is reported. It is based on a nonsymmetric‐substituted π‐conjugated 5,5′‐diphenyl‐2,2′‐bithiophene platform with one end containing three out‐fanning flexible chains and with a range of substituents at the other end (the apex). The cubic phases are stable over broad temperature ranges, often down to ambient temperature, and tolerate a wide range of apex substitution patterns, allowing structural diversity and tailoring of the cubic phase type and application‐relevant properties. With an increasing number and size of apex substituents, a sequence of three different modes of cubic self‐assembly is observed, following an increasing helical twist. Thus, two ranges of the achiral double network Iad phase range can be distinguished, a long pitch and a short pitch, with the chiral triple network I23 cubic phase in the intermediate pitch range. The findings provide a new prospect for the directed design of cubic phase‐forming functional materials based on spontaneously formed helical network liquid crystals with tunable application specific properties.

Cao Y., Alaasar M., Nallapaneni A., Salamończyk M., Marinko P., Gorecka E., Tschierske C., Liu F., Vaupotič N., Zhu C.

The bicontinuous double gyroid phase is one of the nature's most symmetric and complex structures, the electron density map of which was established long ago. By utilizing small-angle x-ray scattering, resonant soft x-ray scattering at the carbon K edge and model-dependent tensor-based scattering theory, we have not only elucidated morphology but also identified molecular packing in the double gyroid phases formed by molecules with different shapes, i.e., rodlike vs taper shaped, thus validating some of the hypothetical packing models and disproving others. The spatial variation of molecular orientation through the channel junctions in the double gyroid phase can be either continuous in the case of anisotropic channels or discontinuous in the case of isotropic channels depending on the molecular structure and shape.

Tschierske C., Dressel C.

Recent progress in mirror symmetry breaking and chirality amplification in isotropic liquids and liquid crystalline cubic phases of achiral molecule is reviewed and discussed with respect to its implications for the hypothesis of emergence of biological chirality. It is shown that mirror symmetry breaking takes place in fluid systems where homochiral interactions are preferred over heterochiral and a dynamic network structure leads to chirality synchronization if the enantiomerization barrier is sufficiently low, i.e., that racemization drives the development of uniform chirality. Local mirror symmetry breaking leads to conglomerate formation. Total mirror symmetry breaking requires either a proper phase transitions kinetics or minor chiral fields, leading to stochastic and deterministic homochirality, respectively, associated with an extreme chirality amplification power close to the bifurcation point. These mirror symmetry broken liquids are thermodynamically stable states and considered as possible systems in which uniform biochirality could have emerged. A model is hypothesized, which assumes the emergence of uniform chirality by chirality synchronization in dynamic “helical network fluids” followed by polymerization, fixing the chirality and leading to proto-RNA formation in a single process.

Hu Y., Li W., Jia P., Wang X., Xu L., Yang H.

AbstractOver the past few years, the fabrication of artificial light‐harvesting systems with aggregation‐induced emission (AIE) has attracted significant attention because of the wide applications of these systems in organic chemistry, supramolecular chemistry, energy chemistry, and even materials science. This progress report focuses on recent advances in the design and preparation of artificial light‐harvesting systems with AIE. In addition, the properties, functions, and applications of these systems are discussed.

Cao Y., Feng C., Jakli A., Zhu C., Liu F.

• Theoretical consideration of resonant scattering from form factor to signal intensity calculation is introduced. • The practical experimental environment and sample preparation procedure are briefly introduced to show the current experimental capacity and limitation of resonant scattering. • Studies on 1D/3D chiral structures, via soft and tender resonant X-ray scattering, are thoroughly reviewed. The helical pitch recognition and the structural elucidation ability of resonant scattering are properly summarized and explained. • The potential applications and future improvements of resonant scattering are clearly stated in perspective of both researcher and facility developer. Chirality is an essential research topic in material science, especially in soft materials including liquid crystals, polymers and biomaterials. There have been many discussions about chiral structures since people discovered and studied this particular property. Yet, due to the lack of effective characterization methods, many questions on helical structures of chiral materials remain unsolved. Here, we review recent applications of resonant soft (RSoXS) and tender resonant X-ray scattering (TReXS) on chirality study in soft materials, with a focus on liquid crystals phases. In the end, a perspective is presented on the potential applications and future improvements of resonant scattering.

Vaupotič N., Salamończyk M., Matraszek J., Vogrin M., Pociecha D., Gorecka E.

We have studied properties of novel thermotropic mesogenic materials that exhibit both an achiral double gyroid (Ia3[combining macron]d symmetry) and chiral cubic phase (previously assigned the Im3[combining macron]m symmetry). We argue that in the chiral cubic phase molecules form micelles and channels arranged into continuously interconnected hexagons. From the X-ray diffraction experiment supported by modelling, exact positions of hexagons and their connections were deduced and showed to be embedded on a WP (degenerated Neovius) minimal primitive surface. The elastic energy of such a structure is close to the one of the double gyroid phase, which is in agreement with a very low enthalpy change observed at the phase transition. We also argue that the chirality of the phase is related to the lack of mirror symmetry of non-flat hexagons accompanied by an alternating inclination of molecules in the neighbouring segments of hexagon; the chirality of individual hexagon is amplified on the whole hexagon network by steric effects.

Söderman O., Henriksson U.

Extensive deuterium NMR relaxation data are presented for two specifically deuterium labelled surfactants forming bicontinuous cubic phases with water. 2H spin-lattice (R1) and spin-spin (R2) relaxation rates were measured over an extended frequency range from 2 - 60 MHz. The data are interpreted with an existing theoretical framework for spin relaxation in bicontinuous cubic phases, which takes its starting point in the description of bicontinuous phases using periodic minimal surfaces. We show that the theory succeeds in accounting for the data and that the defining parameters of the theory, correlation times and order parameters, are in agreement with related data in other surfactant phase situations. Specifically, we obtain the surfactant self-diffusion coefficient over the minimal surface in one unit cell and show that it is in agreement with the corresponding macroscopic NMR diffusion data. By measuring two additional NMR relaxation parameters for each carbon on the surfactant hydrocarbon tail, we demonstrate how order parameter and correlation time profiles can be obtained. Finally, we analyse published molecular dynamics trajectories for a bicontinuous cubic phase. The analysis provides further support for the theoretical framework used to interpret relaxation data.

Reppe T., Poppe S., Cai X., Cao Y., Liu F., Tschierske C.

Benzil (diphenylethane-1,2-dione), which is a long known example for an achiral molecule crystallizing in a chiral space group, can also show mirror symmetry breaking in the fluid state if it is suitably functionalized. For some of the new benzil derivatives even three different subsequent mirror symmetry broken soft matter states with a chiral conglomerate structure can be observed. One is an isotropic liquid, the second one a cubic liquid crystal with a complex network structure and the third is a soft crystalline solid. Chirality develops by helical self-assembly combined with dynamic network formation, thus allowing macroscopic chirality synchronization. These achiral molecules, combining a transiently chiral bent core with multiple alkyl chains, provide a unique link between the mirror symmetry breaking phenomena observed for polycatenar and bent-core mesogens. The homogeneously chiral networks are of interest for application as chiral materials, and as templates for chiral recognition, separation and enantioselective catalysis.

Tsutsui Y., Zhang W., Ghosh S., Sakurai T., Yoshida H., Ozaki M., Akutagawa T., Seki S.

AbstractUse of organic molecules as lasing media has much potential to develop next‐generation optical devices as soft‐matter photonics with wideband tunability and large coherence area. Although mirrorless lasing was theoretically predicted and practically demonstrated in helical cholesteric liquid crystalline (LC) phases of organic compounds, recent studies on optical confinement have been much focused into hard‐crystalline phases of the molecules. Aggregation‐induced emission (AIE) and enhancement (AIEE) provides one of the optimal molecular systems for light amplification in condensed phases, and herein AIEE activity and excited‐state intramolecular proton transfer (ESIPT) are successfully coupled in a room temperature (RT) nematic LC. Suppressing the effect of concentration quenching/self‐absorption and attaining four‐level system for population inversion by the combination of AIEE and ESIPT in LC phases, lead to the amplified spontaneous emission (ASE) from a newly designed molecule: C5Ph‐HBT dispersed in RT LC matrix with the pumping energy threshold of 20 mJ cm–2. Moreover, the nematic LC allows the orientation of the molecular dipoles in response to external electric fields. Hence, the fluorescence as well as the ASE switching is attained at RT, demonstrating the potential of these composite materials as “switchable” ASE media for technological progress.

Feng H., Lam J.W., Tang B.Z.

Self-assembly is an essential process in nature and important in the field of material science, biology and chemistry. Study on self-assembly has lasted for more than 100 years and has resulted in the emergence of many specific research areas, such as marriage of self-assembly to the burgeoning aggregation-induced emission (AIE). Thanks to their fascinating photophysical properties, AIE luminogens (AIEgens) have shown great potential in this area. In this review, we mainly highlight recent advances in monocomponent self-assembly of AIEgens, including self-assembly of amphiphilic AIEgens and chiral AIEgens, host–guest interaction and their applications in circularly-polarized luminescence and light-harvesting platform. Due to the AIE effect, functional AIEgens possess inherent advantage to visualize the fine process of self-assembly. Moreover, attention is also given to well-defined architectures, such as spherical vesicles, helical nanostructures, and nanofibers obtained through self-assembly of AIEgens. The present review may entice readers to realize the importance of self-assembly and attract much more chemists to give their contribution to this area.

Zeng X., Ungar G.

A new molecular-level model is proposed for the “Smectic-D” liquid crystal whose structure has remained controversial since the 1960s. The phase has a body-centred cubic lattice, and all previous structural models assumed an Imm space group. However, this contradicts the recent discovery that the phase is always chiral, even in non-chiral compounds. The new model has the non-centrosymmetric space group I23, and consists of three interpenetrating networks, with 3-way planar network junctions like in the double gyroid phase. Rafts of 3–4 parallel molecules stack with an 8° twist on top of each other, forming spontaneously chiral columnar network segments. Homochirality throughout the network is enforced by matching the helical sense of all confluent segments at junctions. The findings indicate that coordinated helicity, previously unrecognized, is a key driving force responsible for the formation of a number of chiral and achiral, cubic and non-cubic bicontinuous phases of rod-like molecules.

Alaasar M., Cao Y., Neumann T., Tan T., Liu F., Giese M.

Helical networks and lamellar and nematic phases of bithiophene-based polycatenars. Functional tricatenars were investigated for fluorescence behaviour.

Liu N., Liu X., Liang Y., Hu X., Gao H., Xiao Y.

Four novel bisthienylethene-based rod-like liquid crystals containing different numbers of terminal fluorine atoms were designed and the effect of the terminal fluorination on liquid crystalline self-assembly, optical characteristics in solid state and mechanochromic behaviors was explored. The transition from non-liquid crystal via monotropic liquid crystal with smectic A phase to enantiotropic liquid crystal with smectic A phase can be achieved and the temperature ranges of smectic A phase gradually increase by increasing the terminal fluorination, which might be due to the increase in intermolecular interactions by increasing the number of terminal fluorine atom. All the compounds exhibited AIE characteristic and distinct solid-state fluorescence with yellow or orange color. In addition, the non-fluorosubstituted and difluorosubstituted bisthienylethene-based liquid crystals exhibited reversible mechanochromism, whereas the monofluorosubstituted and trifluorosubstituted bisthienylethene-based liquid crystals exhibited extremely weak mechanochromism due to different intermolecular interaction, molecular symmetry and molecular conformation. These investigations implied that the incorporation of fluorine atom could result in interesting self-assemblies and photophysical properties in different states, which also provided the understanding of the relationship between chemical structures and properties.

Murad A., Alaasar M., Darweesh A.F., Eremin A.

Fullerene doping allows for tuning the conductivity and photovoltaic properties of cubic bithiophene-based liquid crystalline organic semiconductors.

Zeng C., Deng W., Zhao K., Redshaw C., Donnio B.

AbstractLipophilic biphenylthiophene‐ and phenanthrothiophene‐triazine compounds, BPTTn and CPTTn, respectively, were prepared by a tandem procedure involving successive Suzuki‐Miyaura coupling and Scholl cyclodehydrogenation reactions. These compounds display photoluminescence in solution and in thin film state, solvatochromism with increasing solvent's polarity, as well as acidochromism and metal ion recognition stimuli‐responsive fluorescence. Protonation of BPTT10 and CPTT10 by trifluoroacetic acid results in fluorescence quenching, which is reversibly restored once treated with triethylamine (ON‐OFF switch). DFT computational studies show that intramolecular charge transfer (ICT) phenomena occurs for both molecules, and reveal that protonation enhances the electron‐withdrawing ability of the triazine core and reduces the band gap. This acidochromic behavior was applied to a prototype fluorescent anti‐counterfeiting device. They also specifically recognize Fe3+ through coordination, and the recognition mechanism is closely related to the photoinduced electron transfer between Fe3+ and BPTT10/CPTT10. CPTTn self‐assemble into columnar rectangular (Colrec) mesophase, which can be modulated by oleic acid via the formation of a hydrogen‐bonded supramolecular liquid crystal hexagonal Colhex mesophase. Finally, CPTTn also form organic gels in alkanes at low critical gel concentration (3.0 mg/mL). Therefore, these star‐shaped triazine molecules possess many interesting features and thus hold great promises for information processing, liquid crystal semiconductors and organogelators.

Alaasar M., Darweesh A.F., Anders C., Iakoubovskii K., Masafumi Y.

π-Conjugated molecules are of special interest for designing new materials with potential optoelectronic applications. Herein, we report the synthesis of new functional non-symmetric substituted triple-chain π-conjugated 5,5′-diphenyl-2,2′-bithiophenes. They have a...

Eremin A., Murad A., Alaasar M.

We report an unusually strong flow-induced birefringence in an optically isotropic cubic phase occurring below the isotropic chiral conglomerate phase formed by a low-molecular-weight polycatenar mesogen. The transition into the birefringent state occurs thresholdless and the induced birefringence is comparable with that observed in polymeric systems. We suggest that the flow-induced deformation of the cubic structure is responsible for the strong rheo-optical response.

Ya-ping L., Jiang-tao S., Chun-yang L., Zi-yun Z., Mei T., Dan-shu Y.

Alaasar M., Cai X., Cao Y., Liu F.

Tuning from 1D to 3D mesophases by alkyl chain engineering. Multichain π-conjugated hockey-stick molecules form lamellar SmA and meso-structure Ia3̄d with continuous networks. The effect of the position of the central bent-core unit on helical self-assembly is discussed.

Alaasar M., Poppe S.

• First examples of photo-sensitive hockey-stick polycatenar liquid crystals are reported. • The liquid crystal self-assembly was characterized by DSC, PLM and XRD. • Network formation in addition to transition from 1D to 3D liquid crystalline phases are observed. • Fast and reversible photo switching between different types of liquid crystalline phases is achieved. Photo switchable liquid crystalline (LC) materials are of great interest for optical and photonic applications. Herein we report the design, synthesis, and molecular self-assembly of the first examples of photosensitive hockey-stick (HS) polycatenars. Therefore, two new series of HSLCs derived from 4-cyanoresorcinol bent-core unit connected to a short azobenzene-based side arm with one variable alkoxy chain and a long ester-based wing terminated with two alkoxy chains at 3 and 5 positions of the terminal benzene ring are reported. They differ from each other in the length of the terminal chains connected to the long arm. The LC self-assembly of these HSLCs was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electro optical investigations. Depending on length of terminal chains a transition from one dimensional (1D) tilted and non-tilted smectic phases to three dimensional (3D) achiral bicontinuous cubic phases with Ia 3 ¯ d symmetry (Cub bi / Ia 3 ¯ d ) upon chain elongation is observed. Moreover, achiral isotropic liquid networks were observed for medium and long chain homologues. Most of mesophases are room temperature LCs phases with wide ranges as observed in the cooling cycles, where once they are formed on heating, no sign of crystallization is detected down to ambient temperature. Finally, UV light irradiation results in fast and reversible photoinduced transformation between different types of LCs phases as well as between LC phase and isotropic liquid.

Shivanna J.M., Alaasar M., Hegde G.

• Polycatenars based azobenzenes show liquid crystalline properties. • Nematic phases with cybotactic clusters in addition to Smectic C phase. • All compounds exhibit fast thermal back relaxation time. • Caterpillar model is used to explain the possible reason. We report the photo switching behaviour of azobenzene-based rod-like polycatenar molecules having three hexyloxy chains at one terminus and a variable chain at the other terminus. The latter was varied with OC 6 H 13 , SC 6 H 13 , C 7 H 13 , OC 10 H 21 and O(CH 2 ) 4 C 6 F 13 chains. All molecules are liquid crystalline exhibiting wide ranges of nematic phases with cybotactic clusters in addition to Smectic C phase. The effect of variation of terminal chains on the photo-switching properties of the above series of azo compounds upon light irradiation and their kinetic reactions are presented. Surprisingly, trans - cis conversion in solutions takes around 85 s, while relaxing back i.e. cis - trans conversion takes ~30–60 min depending on the type of the terminal chain. On the other hand, irrespective of the nature of the terminal chain all of them exhibit fast thermal back relaxation time. A possible reason for the observed phenomena is presented based on a caterpillar model. Moreover, the potential of one selected example in optical storage device is elucidated. Therefore, this work shows the structure-property relationship in such type of polycatenar molecules related to their photo switching properties.