Stabilization of Bicontinuous Cubic Phase and Its Two-Sided Nature Produced by Use of Siloxane Tails and Introduction of Molecular Nonsymmetry

A recent intriguing finding that a helical network arrangement forms the bicontinuous cubic phase is attracting great attention for the possibility of new routes to asymmetric synthesis by achiral molecules. However, the design of the molecular structure for the cubic phase is still unrevealed. In this work, a nonsymmetric core molecule with larger naphthalene and smaller benzene moieties at each side of the central linkage and the same disiloxanyldecyloxy terminal at both terminals is shown to be the first example of molecule forming both single-layered and double-layered core assembly modes in the Ia3d phase as a single molecule system. The molecule forms the former mode at high temperatures as a thermodynamically stable phase, similarly to the symmetric naphthalene core system, whereas, on cooling below a temperature (∼350 K), a metastable Ia3d phase forms a double-layered core state down to room temperature, which is common to the benzene core system. As another effect of the nonsymmetric core, the cubic phase is maintained at room temperature for more than 100 days with slight distortion. Infrared spectral studies and quantum chemical calculations suggested the easy transformation between the two core assembly modes. The core nonsymmetry can be a versatile fine-tuning of the core assembly mode and phase stability for the cubic phase molecules.

New complexes of liquid crystal discotic triphenylenes: induction of the double gyroid phase

Electron donor-acceptor liquid crystals have been attracting considerable attention due to possible applications in optoelectronics and photonics. The creation of such charge transfer complexes is a powerful and flexible instrument for modifying the structures and properties compared to those of the initial components. In the present work, such an approach is exemplified on new complexes formed via non-covalent interactions of triphenylene discotics, namely, 2,3,6,7,10,11-hexakis(pentyloxy) triphenylene (H5T) and 2-(acryloyloxypropyloxy)-3,6,7,10,11-pentapentylox-triphenylene (TPh-3A), with an electron acceptor, β-(2,4,7-trinitro-9-fluorenylideneaminooxy) propionic acid (TNF-carb). The structure of thin supported films of H5T, TPh-3A and their blends with TNF-carb was investigated by grazing-incidence wide-angle X-ray scattering using a synchrotron source. At room temperature, the pristine discotics crystallize in orthorhombic unit cells whereas the self-assembly of H5T and TPh-3A with TNF-carb results in a double gyroid and hexagonal phases, respectively. Formation of the double gyroid phase with the lattice parameter of 36.5 Å is driven by phase separation between the aromatic and alkyl regions of the system. It is supposed that the TNF-carb molecules of the complex are positioned in the nodes of the structure while the H5T molecules are located in the struts adjoining the nodes via triple junctions. For the hexagonal crystal of the TPh-3A/TNF-carb complex, the acceptor molecules are likely located in the interstices between the neighboring supramolecular columns of TPh-3A. The molecular structures of the blends were also explored by means of FTIR spectroscopy. A detailed FTIR spectra analysis illustrates fine changes in inter-molecular bonds. For example, the initially dimerized acceptor molecules totally disappear in the complex structures whereas in TPh-3A/TNF-carb additional H-bonds between the carboxylate group in TNF-carb and the ester group of TPh-3A form. The experimental data allows putting forward possible molecular models of the complex structures.

Molecular Packing in Double Gyroid Cubic Phases Revealed via Resonant Soft X-Ray Scattering

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.

Double-Gyroid Nanostructure Formation by Aggregation-Induced Atropisomerization and Co-Assembly of Ionic Liquid-Crystalline Amphiphiles

We report a new molecular‐design principle for creating double‐gyroid nanostructured molecular assemblies based on atropisomerization. Ionic amphiphiles containing two imidazolium rings close to each other were designed and synthesized. NMR data revealed that the rotation of the imidazolium rings is restricted, with an activation energy as high as 63 kJ mol⁻¹ in DMSO‐d₆ solution (DFT prediction for a model compound in the vacuum: 90–100 kJ mol⁻¹). Due to the restricted rotation, the amphiphiles feature “double” atropisomeric axes in their ionic segments and form three stable atropisomers: meso, R, and S. These isomers co‐organize into Ia3‾d ‐type bicontinuous cubic liquid‐crystalline mesophases through nanosegregation of the ionic and non‐ionic parts. Considering the intrinsic characteristic of Ia3‾d ‐type bicontinuous cubic structures that they are composed of intertwined right‐ and left‐handed single gyroids, we propose that the simultaneous presence of both R‐ and S‐atropisomers is an important contributor to the formation of double‐gyroid structures.

Bi-continuous orthorhombic soft matter phase made of polycatenar molecules

We report an observation of a new type of a continuous soft matter phase with an orthorhombic symmetry made of polycatenar molecules. The bi-continuous orthorhombic structure with the Pcab symmetry appears by deformation of a double gyroid cubic structure with the Ia3[combining macron]d symmetry.

Stereochemical Rules Govern the Soft Self-Assembly of Achiral Compounds: Understanding the Heliconical Liquid-Crystalline Phases of Bent-Core Mesogens

A series of bent-shaped 4-cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short-pitch heliconical lamellar liquid-crystalline phase with incommensurate 3-layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric-(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field-induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non-tilted and ferreoelectric switching lamellar phase, providing an alternative non-chiral way for the transition from anticlinic to synclinic tilt.

Molecular design of anti-spindle-like molecules by use of siloxanyl terminals for a thermotropic bicontinuous cubic phase

By modifying the three molecular moieties, siloxane terminal, alkyl spacer, and aromatic core part, the molecular requirements for the formation of bicontinuous cubic phases are presented.

Study of Liquid Crystals Showing Two Isotropic Phases by 1H NMR Diffusometry and 1H NMR Relaxometry

In this work, we report a study of two thermotropic liquid crystalline samples showing a not common mesophase behavior. The samples, namely a di-benzyloxy biphenyl derivative labelled 9/2 RS/RS, and a bimesogenic liquid crystal labelled L1, show a direct transition between two isotropic phases followed, at lower temperatures, by the optically isotropic, 3D structured, cubic phase. These systems have been investigated by means of 1H NMR diffusometry and 1H NMR relaxometry in order to characterize their isotropic–isotropic’–cubic mesophase behavior, mainly on the dynamic point of view. In particular, the temperature trend of the self-diffusion coefficients measured for both samples allowed us to significantly distinguish between the two isotropic phases, while the temperature dependence of the 1H spin-lattice relaxation time (T1) did not show significant discontinuities at the isotropic–isotropic’ phase transition. A preliminary analysis of the frequency-dependence of 1H T1 at different temperatures gives information about the main motional processes active in the isotropic mesophases.

Systematic exploitation of thermotropic bicontinuous cubic phase families from 1,2-bis(aryloyl)hydrazine-based molecules

Rational design of molecules that exhibit a thermotropic bicontinuous cubic (Cub) phase has been earnestly desired. In this work, we describe the suitable selection of a molecular motif that has enabled the systematic exploitation of eight new series of Cub-phase molecules with symmetric molecular cores, N-n (1), PB-n (2), S-n (3), and PEB-n (4), and unsymmetric cores, B-N-n (5), B-PB-n (6), B-S-n (7), and B-PEB-n (8). These eight series all originate from achiral chain-core-chain type rod-like molecules that exhibit two types of Cub phases, an achiral Ia3d phase, and a chiral phase. All the Ia3d phases formed were found to be isomorphous structures, with their cell dimensions being proportional to the core size, and the same was true for the latter chiral phase. We demonstrated that the formation is mainly governed by the segregation between core and alkyl moieties of the molecules, and thus, by the weight fraction of the core portion f_core. This work also demonstrates that the central dicarbonylhydrazine linkage bearing intermolecular hydrogen bonding ability exhibits a pinning effect that prevents slippage of π-stacks of molecules, which is critical for the formation of the two Cub phases that are composed of chiral networks with twisted molecular arrangements. In each series, the emergence of spontaneous chirality formation that occurred in the chiral phase was limited to between 0.36 and 0.50 in the range of f_core. An interesting insight was that the introduced unsymmetry of the molecular core strongly influenced the phase behavior, which lowered the temperature range of Cub phases to around that of the smallest core series B-n, while the high temperature limit (T_c) was roughly proportional to the core size, as determined by the strength of intermolecular π-π interactions.

Molecular dynamics simulation of thermotropic bolaamphiphiles with a swallow-tail lateral chain: formation of cubic network phases

T-shaped bolaamphiphiles (TBA) with a swallow-tail lateral chain have been found to provide a fertile platform to produce complex liquid crystalline phases that are accessible through changes of temperature and lateral chain length and design. In this work, we use molecular simulations of a simple coarse-grained model to map out the phase behavior of this type of molecules. This model is based on the premise that the crucial details of the fluid structure stem from close range repulsions and the strong directional forces typical of hydrogen bonds. Our simulations confirm that TBAs exhibit a rich phase behavior upon increasing the length of their lateral chain. The simulations detect a double gyroid phase and an axial-bundle columnar phase which bear some structural resemblance to those found in the experiment. In addition, simulations predict two cocontinuous phases with 3D-periodicity: the "single" diamond and the "single" plumber's nightmare phase. Our analysis of energetic and entropic contributions to the free energy of phases formed by TBA with either swallow-tail or linear side-chains suggest that the 3D-periodic network phases formed by the former are stabilized by the large conformation entropy of the side-chains.

A new cubic Ia3̅d crystal structure observed in a model single component system by molecular dynamics simulation

In our molecular dynamics simulations of the system of identical particles interacting through the harmonic-repulsive pair potential, we observed the formation of a cubic crystal structure that belongs to the Ia3̅d (#230) crystallographic space group. This crystal structure has not been previously seen either in experiments or in computer simulations, though its framework topology has been known from theoretical crystallographic considerations. Its unit cell contains 16 atoms, occupying only (16b) Wyckoff site, and arranged as two mutually intertwined unconnected networks with packing fraction of 0.37. The appearance of this structure is explained by the soft repulsive nature of the interaction potential. The observed Ia3̅d structure extends the small number of cubic structures formed in single component systems with spherically symmetric pair potentials in MD simulations. We speculate that materials with such structure could be found in soft matter systems or in selected crystals under high pressure.

Stabilization of the bicontinuous cubic phase in siloxane-terminated mesogens, 1,2-bis[4'-(n-(oligodimethylsiloxyl)alkoxy)benzoyl]hydrazine

The introduction of oligodimethyl siloxane segments at the termini of the alkyl tails has been employed to stabilize the bicontinuous cubic (Cub(bi)) phase of a chain-core-chain type molecule having a 1,2-bis(benzoyl)hydrazine central core with two chains attached at the 4' position of each benzoyl moiety. In this study, three silylated molecules, bis-C10Si2, bis-C10Si3, and C10Si2-C8C=C, were synthesized, where "CnSim" represents the number of carbon and silicon atoms in the chain and "bis" indicates the two chains being the same, whereas the last one is asymmetric with respect to the core. The phase behaviors were examined by using polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction techniques. All three compounds form Cub(bi) phases and their syntheses were compared including their parent compound bis-C18. It was clearly revealed that the introduction of oligodimethyl siloxane segments effectively suppresses the crystallization at low temperatures, and as a result stabilizes the Cub(bi) phases, in an extreme case down to room temperature. The semi-quantitative analyses in terms of lattice constant and three-dimensional electron density map help us to better understand the self-assembly process in the Cub(bi) phases. The study also revealed that the introduction of oligodimethyl siloxane segments is not only an alternative for the hydrocarbon segment but also is able to provide a versatile design strategy for obtaining stable Cub(bi) phases.

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.

Mirror symmetry breaking by mixing of equimolar amounts of two gyroid phase-forming achiral molecules

Mirror symmetry breaking was realised by mixing of two achiral molecules both of which form themselves achiral bicontinuous cubic phases.

A structural model of the chiral “Im3m” cubic phase

Assuming the twisted arrangement of rodlike molecules as the origin of the chirality as in the existing model, a new model of the molecular arrangement in the cubic “Im3m” phase is proposed.