Columnar Liquid Crystal Phase Research Articles

Simulating the Lyotropic Phase Behavior of a Partially Self-Complementary DNA Tetramer.

DNA oligomers in solution have been found to develop liquid crystal phases via a hierarchical process that involves Watson-Crick base pairing, supramolecular assembly into columns of duplexes, and long-range ordering. The multiscale nature of this phenomenon makes it difficult to quantitatively describe and assess the importance of the various contributions, particularly for very short strands. We performed molecular dynamics simulations based on the coarse-grained oxDNA model, aiming to depict all of the assembly processes involved and the phase behavior of solutions of the DNA GCCG tetramers. We find good quantitative matching to experimental data at both levels of molecular association (thermal melting) and collective ordering (phase diagram). We characterize the isotropic state and the low-density nematic and high-density columnar liquid crystal phases in terms of molecular order, size of aggregates, and structure, together with their effects on diffusivity processes. We observe a cooperative aggregation mechanism in which the formation of dimers is less thermodynamically favored than the formation of longer aggregates.

Fluorescent columnar liquid crystals based on BTD hexacatenars with salicylaldimine and benzaldimine core: Self-assembly and multifunctional properties

Two series of hexacatenars SB/n and BB/n containing a benzothiadiazole (BTD) central core with triple alkyl chains at both ends via salicylaldehyde or benzaldehyde derivated Schiff base links, respectively, were prepared. Different Schiff base links lead to different self-assembly and photophysical behaviors. Both series can self-assemble into columnar liquid crystal (LC) phases but with different lattices (p6mm for SB/n, p4mm for BB/n) in bulk states, and SB/n can additionally form gels in organic solution. Interestingly, SB/n series were characterized with excited state intramolecular proton transfer (ESIPT) and aggregation-induced emission enhancement (AIEE) effects, and displayed high fluorescence in their solid, LC and gel states with large Stokes shifts and environmental sensitive unique spectra patterns. The effect of photoisomerization driven by ESIPT process on LC state of SB/n was observed. The applications of SB/n as fingerprint imaging device, Al3+ fluorescence chemosensor, white light emitting diodes (WLED) as well as light-emitting liquid crystal displays (LE-LCD) have been successfully realized.

Microfluidics-Based Drying-Wetting Cycles to Investigate Phase Transitions of Small Molecules Solutions.

Drying-wetting cycles play a crucial role in the investigation of the origin of life as processes that both concentrate and induce the supramolecular assembly and polymerization of biomolecular building blocks, such as nucleotides and amino acids. Here, we test different microfluidic devices to study the dehydration-hydration cycles of the aqueous solutions of small molecules, and to observe, by optical microscopy, the insurgence of phase transitions driven by self-assembly, exploiting water pervaporation through polydimethylsiloxane (PDMS). As a testbed, we investigate solutions of the chromonic dye Sunset Yellow (SSY), which self-assembles into face-to-face columnar aggregates and produces nematic and columnar liquid crystal (LC) phases as a function of concentration. We show that the LC temperature-concentration phase diagram of SSY can be obtained with a fair agreement with previous reports, that droplet hydration-dehydration can be reversibly controlled and automated, and that the simultaneous incubation of samples with different final water contents, corresponding to different phases, can be implemented. These methods can be further extended to study the assembly of diverse prebiotically relevant small molecules and to characterize their phase transitions.

Charge carrier transport in perylene-based and pyrene-based columnar liquid crystals.

Electron and hole transport characteristics were evaluated for perylene-based and pyrene-based compounds using electron-only and hole-only devices. The perylene presented a columnar hexagonal liquid crystal phase at room temperature with strong molecular π-stacking inside the columns. The pyrene crystallizes bellow 166 °C, preserving the close-packed columnar rectangular structure of the mesophase. Photophysical analysis and numerical calculations assisted the interpretation of positive and negative charge carrier mobilities obtained from fitting the space charge limited regime of current vs voltage curves. The pyrene-based material demonstrated an electron mobility two orders of magnitude higher than the perylene one, indicating the potential of this class of materials as electron transporting layer.

Design, synthesis and properties of mesogen jacketed liquid crystalline polymers with fluorinated triphenylene pendants

Discotic liquid crystalline polymers (DLCPs) combining the advantages of discotic liquid crystals and polymers yet have been less developed compared to the calamitic counterparts mainly due to lack of DLCP materials. Here, mesogen-jacketed liquid crystalline polymers with pendant fluorinated triphenylene were successfully synthesized via proper polymerizations of the corresponding monomers respectively. Nuclear magnetic resonance spectroscopy (both 1H NMR and 13C NMR), time-of-flight mass spectrometry (MALDI-TOF MS), Fourier transform Infrared (FT-IR) spectroscopy and other characterization methods were used to confirm the structure of the products. The mesomorphic properties of the obtained triphenylene monomers with unsaturated flexible tails and resulting polymers were investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffractometer (XRD). The propargylene-containing monomer shows a smectic liquid crystals phase and the pentynylene monomer exhibit a highly ordered columnar helical liquid crystal phase. The three side chain polymers containing fluorinated triphenylene have good thermal stability (decomposition temperature of 350 °C or above). At the same time, the polymers prepared by the polymerization of propargylene-containing monomers have both hexagonal columnar phase (Colh) and rectangular columnar phases (Colr). The polymers with long spacer chains derived from Methyl acrylate have a hexagonal columnar structure (Colh). The fluorinated triphenylene monomers and polymers with ordered liquid crystal phases synthesized in this work provide useful inspiring for structural modifications of DLCPs and are promising for applications in organic optoelectronics.

Coatable Negative Dispersion Retarder: Kinetically Controlled Self-Assembly Pathway of Butterfly-Shaped Molecular Building Blocks for the Construction of Nanocolumns.

Disc-shaped building blocks with columnar phases have attracted attention for their potential in optical applications, including a retarder. However, to achieve coatable high-performance optical films, it is essential to understand a subtle interaction balance between building blocks and relevant self-assembled behaviors during material processing. Herein, we studied a self-assembled nanocolumn evaluation of linear butterfly-shaped dendrons (T-A3D) consisting of thiophene-based conjugated core and flexible alkyl dendron. X-ray diffraction provided insight into the unique hexagonal columnar liquid crystal phase of T-A3D, driven by intermolecular hydrogen bonding and coplanarity of the thiophene-based conjugated core. The formation of a self-assembled nanocolumn with high mobility enabled the uniaxial orientation of butterfly-shaped T-A3D on the aligned rod-shaped nematic reactive mesogens, resulting in a transparent and colorless two-layered negative retarder. The self-assembled nanocolumn consisting of butterfly-shaped molecule would break a new ground for developing advanced optical thin films.

Enantiomers Self-Sort into Separate Counter-Twisted Ribbons of the Fddd Liquid Crystal─Antiferrochirality and Parachirality.

The recently discovered orthorhombic liquid crystal (LC) phase of symmetry Fddd is proving to be widespread. In this work, a chiral hydroxybutyrate linkage is inserted into the molecular core of hexacatenar rodlike compounds, containing a thienylfluorenone fluorophore. In addition to more usual tools, the methods used include grazing-incidence X-ray scattering, modulated differential scanning calorimetry (DSC), flash DSC with rates up to 6000 K/s, and chiro-optical spectroscopies using Mueller matrix method, plus conformational mapping. Although pure R and S enantiomers form only a strongly chiral hexagonal columnar LC phase (Colh*), the racemic mixture forms a highly ordered Fddd phase with 4 right- and 4 left-handed twisted ribbon-like columns traversing its large unit cell. In that structure, the two enantiomers locally deracemize and self-sort into the columns of their preferred chirality. The twisted ribbons in Fddd, with a 7.54 nm pitch, consist of stacked rafts, each containing ∼2 side-by-side molecules, the successive rafts rotated by 17°. In contrast, an analogous achiral compound forms only the columnar phase. The multiple methods used gave a comprehensive picture and helped in-depth understanding not only of the Fddd phase but also of the "parachiral" Colh* in pure enantiomers with irregular helicity, whose chirality is compared to the magnetization of a paramagnet in a field. Unusual short-range ordering effects are also described. An explanation of these phenomena is proposed based on conformational analysis. Surprisingly, the isotropic-columnar transition is extremely fast, completing within ∼20 ms. A clear effect of phase on UV-vis absorption and emission is observed.

Rack-gear structures in columnar liquid crystal phases of trialkoxybenzyl pentafluorobenzoates and their influences on intercolumnar interdigitation and macroscopic morphologies

ABSTRACT In this study, 3,4,5-trialkoxybenzyl pentafluorbenzoates with alkyl chains of various lengths were synthesised, and the phase transition behaviours, liquid crystallinities, and molecular packing structures of their columnar phases were investigated. The results showed that discs comprising two half-discoid molecules stack up to form columns with rack-gear structures, generating columnar liquid crystal phases. From powder and single crystal X-ray diffraction investigation, the mechanism of the self-assembly of molecules into columns and their intercolumnar interdigitation was postulated. Furthermore, it was revealed that the combination of alkyl groups of the benzyl groups was closely related to the type of columnar liquid crystal phases generated and the macroscopic morphologies of the liquid crystal domains.

Discotic liquid crystals 45 years later. Dendronized discs and crowns increase liquid crystal complexity to columnar from spheres, cubic Frank-Kasper, liquid quasicrystals and memory-effect induced columnar-bundles

In 1977 Chandrasekhar laboratory reported the first examples of a new class of thermotropic liquid crystals self-organized from planar disc-like molecules substituted with alkyl groups. X-ray analysis demonstrated that these disc-like molecules stack on top of each other at irregular spacing in columns located in the paraffin melt of their alkyl groups. These supramolecular columns self-organize in a hexagonal array defined as the new thermotropic discotic or columnar hexagonal liquid crystal phase. This new thermotropic liquid crystal phase has translational periodicity in two dimensions and liquid-like disorder in the third one, along the column axis. In 2002 Bushby and Lozman dedicated a brief review article to the 25 anniversary of discotic liquid crystals. This review also included a brief discussion of supramolecular discs and columns self-organized from self-assembling dendrons and dendrimers. The current publication is not another review but a brief perspective on new liquid crystal phases self-organized from dendronized rigid planar and conformationally flexible discs as well as from crown-like molecules. Crown-like molecules are the most stable conformation of a transient unstable disc. All dendronized disc-like and crown molecules adopt a crown conformation that self-assemble helical columns and spherical helices. A large diversity of new liquid crystalline phases self-organize from these helical columns and spherical helices. They include helical columnar hexagonal phases from crowns and from supramolecular spheres, Frank-Kasper A15 (known as cubic phase of space group Pm3¯n), σ phase (also known as tetragonal of space group P42/mnm), 12-fold quasi liquid crystals (QLC) and supramolecular orientational memory (SOM) derived complex bundles of supramolecular columnar hexagonal arrays that are not yet completely elucidated. The impact of Chandrasekhar on the discovery of self-assembling dendrons and dendrimers via his biaxial nematic liquid crystal concept will also be discussed. This perspective is dedicated to the 45 anniversary of the discovery of discotic liquid crystals and to the memory of Professor Sivaramakrishna Chandrasekhar, a modest, respectful and great scientist.

Induction of a Columnar Liquid Crystal Phase at Low Temperature by Replacing Stearyl Groups with Oleyl Groups in a Discoid Molecule, and Efficient Chiral Amplification in the Liquid Crystal Phase

Abstract Replacement of stearyl groups with oleyl groups in a discoid molecule induced a rectangular columnar phase at low temperature. Although the molecule has no hydrogen bonding sites, the induced liquid crystal phase showed a highly efficient chiral amplification. The helical packing structure was postulated based on its X-ray diffraction profile and circular dichroism spectra.

Synthesis and Strong π-π Interaction of Hexaazatriphenylene Derivatives with Alternating Electron-Withdrawing and -Donating Groups.

Hexaazatriphenylene (HAT) derivatives have attracted wide attention because of their electron-deficient nature and unique self-assembly properties. In this work, a facile synthesis method for obtaining HAT derivatives with alternating electron-withdrawing nitrile and electron-donating alkoxy groups (HATCNOCn) is proposed. Crystal structure analysis indicated that HATCNOCn forms a one-dimensional columnar structure via strong π-π interactions. Density functional theory calculations revealed that the edge of HATCNOCn is divided into positively and negatively charged sites owing to the presence of alternating nitrile and alkoxy groups, which would induce strong π-π interactions. Thermal analysis and polarizing optical microscopy revealed that HATCNOCn exhibits columnar liquid-crystal phases. Time-resolved microwave conductivity measurements further demonstrated the photoconductive nature of HATCNOCn. The proposed strategy could provide a new strategy for the design of novel organic semiconductive materials.

Lyotropic isotropic to columnar phase transition in RNA solutions

ABSTRACT RNA oligomers exhibit lyotropic chiral nematic and columnar liquid crystal phases. The lyotropic chiral nematic and columnar phases with special emphasize on the isotropic to columnar phase transition in RNA solutions are described based on Landau theory. A free energy is constructed based on the order parameters to describe the chiral nematic and columnar liquid crystal phases and isotropic to columnar phase transition. The lyotropic isotropic to columnar phase transition is found to be always first order. The birefringence has been calculated in the isotropic phase of isotropic to columnar phase transition. Theoretical observation of the existence of the lyotropic isotropic to columnar phase transition in RNA solutions is confirmed with experimental results.

Semi-flexible star-shaped molecules: conformational analysis of nano-segregated mesogens forming columnar liquid-crystal phases

Abstract The structural prerequisites are investigated, which make star-shaped molecules suitable precursors for the formation of columnar liquid-crystalline phases. Electronic structure calculations on smaller mesogens show that not all conformers exhibit the atomistic structure, the stability against distortion, and additional dipole moments, which favour columnar stacking. For the presently studied compounds with short terminating alkyl chains, the calculations indicate that the steric factor becomes dominant with increasing star size. Thus, the optimised geometric structures were employed to generate a simplified mathematical model of the structures, which accounts only for the steric interaction in the larger stars. With the help of these diagrams, the most common conformers of star-shaped molecules can be derived in a systematic fashion.

Properties tuning of supramolecular discotics by non-mesogenic triazines and acids

In this work, a new series of supramolecular discotic liquid crystals induced by hydrogen bonding between triazines T x and aromatic acids A y are presented, most of which exhibit columnar mesomorphic properties. The design strategy of combining the hydrogen bond parallel to the molecular plane with the π-stacking trend of triazine derivatives and aromatic acids was utilized, which has been widely confirmed by nematic, smectic or columnar liquid crystal phases. The formation of hydrogen bonded complexes was proven by FT-IR and 1H NMR spectroscopy and their stability studied by variable temperature FT-IR techniques. All prepared hydrogen bonded complexes displayed mesogenic properties and their liquid crystalline properties were investigated by means of DSC, POM and XRD. This work provides useful information to assist our further design of hydrogen bonded supramolecular liquid crystals for optoelectronic applications.

A thermo-birefringence switchable columnar liquid crystalline compound

In the field of columnar liquid crystals, a new type of thermo-optical switching material that shows a birefringence inversion without changing the columnar alignment by controlling the temperature was synthesized. The urea molecules generate one-dimensional linear molecular aggregates using urea-urea intermolecular hydrogen bonding and exhibit rectangular and hexagonal columnar phases. In polarized light optical microscopy, the texture perfectly disappears during the transition between the two columnar phases because it passes through the point where birefringence becomes zero. The inversion mechanism is explained by the conformational change of each molecule in the column. In this study, we propose a new switchable optical material in columnar liquid crystal phases.

Chiral Self-Sorting and the Realization of Ferroelectricity in the Columnar Liquid Crystal Phase of an Optically Inactive N,N'-Diphenylurea Derivative Possessing Six (±)-Citronellyl Groups.

An axially polar-ferroelectric columnar liquid crystal (AP-FCLC) phase that exhibits both switching and maintenance of the macro-polarity in the column axis direction has been achieved in an N,N′-bis(3,4,5-trialkoxyphenyl)urea compound (rac-1) prepared from (±)-citronellyl bromide. Although it had been thought that chirality is necessary to achieve the AP-FCLC phase from our previous study, the optically inactive compound which is a mixture of 21 stereoisomers, generated an AP-FCLC phase. We confirmed its ferroelectricity and investigated the mechanism for realizing the AP-FCLC phase using optoelectronic experiments, X-ray diffraction, and circular dichroism spectroscopy. As a result, it was suggested that chiral self-sorting occurs in the columnar liquid crystal phase, in which molecules with a similar stereochemistry form a one-handed helical column, and columns with the same helicity gather together to form a chiral domain. Accordingly, we conclude that the optically inactive compound rac-1 also indicates ferroelectricity similar to that of an optically pure urea compound because of chiral self-sorting.

Electric-Field-Induced Chirality in Columnar Liquid Crystals.

We describe a novel class of tetraphenylbenzene-based discotic molecules with exceptional self-assembling properties. Absorption and fluorescence studies confirmed the formation of J-type aggregates in solution. The discotic mesogens also show an enhancement of the emission upon aggregation. Interestingly, these discotic molecules displayed enantiotropic hexagonal columnar liquid crystal (LC) phases that can be switched into a helical columnar organization by application of an electric field. The helical columns arise from the electric-field-induced tilt of the polar fluorobenzene ring that directs all of the peripheral phenyl groups into a propeller-like conformation with respect to the central benzene core. A cooperative assembly process of these propeller-shaped molecules resolves into a helical columnar organization, in which the preferred helical sense is obtained from the stereogenic center proximate to the polar carbon-fluorine bond. The ease of inducing chirality in columnar LCs by an electric field presents opportunities to create next-generation chiral materials for a variety of applications.

Chemical Design and Physical Properties of Dynamic Molecular Assemblies

Abstract The thermally activated motional freedom of protons (H+), ions (M+), and molecules can be controlled using supramolecular approaches. In single crystals, motional freedom is enabled because of the small size of H+ and M+ (e.g., Li+ and Na+), and the thermally activated motion of small molecular units can yield molecular rotator structures in electrically conducting and magnetic crystals. The design of hydrogen-bonded networks and rotator–stator structures is a rational method to form functional dynamic molecular assemblies, and the thermally activated motional freedom of alkylamide (–CONHCnH2n+1) chains in discotic hexagonal columnar (Colh) and lamellar (La) liquid crystal phases enables the dipole inversion of polar N–H⋯O= hydrogen-bonded chains, enabling a ferroelectric response to an applied external electric field. The thermally activated rotational freedom of neutral radicals in plastic crystals results in multifunctional dielectric, magnetic, and optical properties at the order–disorder phase transition. In hydrogen-bonded host–guest molecular crystals, dynamic structural transformations are coupled with highly reversibly guest adsorption–desorption in the crystalline state. Further, changes in the fluorescence colour of excited-state intramolecular proton transfer (ESIPT) systems can be exploited for solid-state molecular sensing, in which both dynamic molecular rotation and conformational transformations drastically affect the fluorescent responses.

Highly Ordered Organic Piezoresponsive Materials Obtained by Cross-linking Electroresponsive Columnar Liquid Crystal Compounds

Abstract An N,N′-diphenylurea derivative possessing six oleyloxy groups exhibited hexagonal columnar liquid crystal phases, and the columnar molecular aggregates were cross-linked with applying an electric field to produce a highly-ordered thin film which has a polarity in the thickness direction. The resulting aromatic polyurea film showed a relatively large vertical piezoelectric constant (d33 of 6.1 pm/V).

Construction of a liquid crystalline double helix supramolecular structure and its electro-responsive behaviour

ABSTRACT A chiral urea compound, N,N’-bis(4-(3,4,5-trialkoxyphenylmethyl)phenyl)urea (1a), possessing a long core and six chiral alkyl chains ((S)-citronellyl groups) was designed to construct a double helix supramolecular structure in a liquid crystal phase. As a result, the urea compounds exhibited a hexagonal columnar liquid crystal phase, and the molecules were self-organised into a double helix structure by intermolecular interactions (dipole-dipole interactions, hydrogen bonds, and π-π interactions). Furthermore, the compound’s responsive behaviour to an applied external electric field was investigated, and its phase transition behaviour, column alignment, columnar structures, and electro-responsiveness were compared with those of an achiral urea compound (1b) possessing six n-decyl groups to investigate the effect on introducing the chirality. These results indicated that higher-order structures such as double helices are effective in suppressing thermal motion and delaying polarisation relaxation.