Columnar Discotic Liquid Crystals Research Articles

Thermo-Induced Bathochromic Emission in Columnar Discotic Liquid Crystals Realized by Intramolecular Planarization.

Most organic thermochromic fluorescent materials exhibit thermo-induced hypsochromic emission due to the formation of excimers in ordered molecular solids; however, it is still a challenge to endow them with bathochromic emission despite its significance in making up the field of thermochromism. Here, a thermo-induced bathochromic emission in columnar discotic liquid crystals is reported realized by intramolecular planarization of the mesogenic fluorophores. A three-armed discotic molecule of dialkylamino-tricyanotristyrylbenzene was synthesized, which preferred to twist out of the core plane to accommodate ordered molecular stacking in hexagonal columnar mesophases, giving rise to bright green monomer emission. However, intramolecular planarization of the mesogenic fluorophores occurred in isotropic liquid increasing the conjugation length, and as a result led to thermo-induced bathochromic emission from green to yellow light. This work reports a new concept in the thermochromic field and provides a novel strategy to achieve fluorescence tuning from intramolecular actions.

Electroluminescent Aggregation-Induced Emission-Active Discotic Liquid Crystals Based on Alkoxy Cyanostilbene-Functionalized Benzenetricarboxamide with Ambipolar Charge Transport

There is significant demand for molecular functional materials with tailored light-emissive and charge transport properties for their utilization in organic optoelectronic devices. Motivated by such promising properties, we present the synthetic design and emissive and semiconducting properties of aggregation-induced emission (AIE)-active columnar discotic liquid crystals based on cyanostilbene-modified benzenetricarboxamide derivatives (1a, 1b, and 1c). Enantiotropic mesomorphic behavior over a wide temperature range, including room temperature, with columnar hexagonal self-assembly was observed for all of the compounds. The space-charge limited current (SCLC) technique revealed the ambipolar charge transport for reported materials with balanced electron and hole transport of the order of 10–3 cm2/(V s). On the other hand, all of the compounds 1a–c were tested as emitter materials in solution-processed organic light-emitting devices at different concentrations with several hosts, viz., poly(vinylcarbazole) (PVK), 1,3-bis(N-carbazolyl)benzene (mCP), bis[3,5-di(9H-carbazol-9-yl)phenyl]diphenylsilane (SimCP2), and (carbazolyl)-1,10-biphenyl (CBP). The maximum luminance of 1255 cd/m2 corresponding to sky-blue emission was observed for compound 1a at 3.0 wt % with the CBP host.

Luminescent columnar discotics as highly efficient emitters in pure deep-blue OLEDs with an external quantum efficiency of 4.7.

Development of materials that serve as efficient blue emitters in solution-processable OLEDs is challenging. In this study, we report three derivatives of C3-symmetric 1,3,5-tris(thien-2-yl)benzene-based highly luminescent room temperature columnar discotic liquid crystals (DLCs) suitable as solid-state emitters in OLED devices. When employed in solution-processed OLEDs, one of the derivatives having the highest photoluminescence quantum yield exhibited a maximum EQE of 4.7% and CIE chromaticity of (0.16, 0.05) corresponding to the ultra deep-blue emission. The finding is sufficiently significant in the field of DLC-based deep blue emitters.

Engineering fibre morphology in self-assembled physical gels of a prototypical discotic liquid crystal

ABSTRACT Supramolecular self-assembly of functional soft matter is key for next-generation optoelectronic devices. Columnar discotic liquid crystals (DLCs) are an ordered class of mesogenic materials, and their supramolecular assembly into fibres can lead to physical gel formation. However, the morphology of the fibres and factors affecting gel formation in DLCs are not well understood. In this work, we focus on prototypical symmetric hexakis(alkyloxy)triphenylene (HATn) n: 4–7 DLCs. The length of the peripheral alkyl chain of the HATn DLC gelator has a significant role in gel phase thermal stability and fibre morphology, and varying alkyl chain length from C4H9 to C7H13 increases the clearing temperature from 35°C to 60°C. Polarised micro-Raman spectroscopy of DLC fibres reveals optical activity. A novel DLC fibre templated, nematic-LC backfilling technique is utilised to study the alignment of 4′-Pentyl-4-biphenylcarbonitrile (5CB), revealing insights about molecular orientation at the surface on HATn fibres. To study correlation between DLC physical gel fibre morphology and optoelectronic device performance, DLC physical-gel Dye Sensitised Solar Cells (DSSCs) are fabricated. Effects of varying gelator concentration and incorporating an amphiphilic Ruthenium-based dye (Z907) are studied. Concentration and interfacial compatibility of the dye/DLC interface show strong correlation with DSSC photovoltaic performance.

Enhanced Conductivity and Thermochromic Luminescence in Hydrogen Bond-Stabilized Columnar Liquid Crystals.

Developing discotic columnar liquid crystals (LCs) with both high electrical conductivity and strong luminescence remains a challenge because the intracolumnar interdisc π-π stacking usually in ordered discotic columnar LCs is essential to generate charge transport pathways but normally detrimental to light emissions. We here present tricyanotristyrylbenzene-based quasi-discotic LCs upon bearing three wedge-shaped alkyl tails for addressing this issue. The resulted columnar materials displayed both high electrical conductivity and strong luminescence, especially for the ones stabilized by multivalent hydrogen-bonding interactions. Besides, an interesting thermochromic luminescence tuning behavior in a smooth manner was observed over a wide wavelength range for the hydrogen bond-stabilized columnar LCs. This study will lead to the future design and application of new multifunctional optoelectronic materials by integrating excellent conductivity and luminescence tuning behaviors.

High Hole Mobility and Efficient Ambipolar Charge Transport in Heterocoronene-Based Ordered Columnar Discotics.

Heterocoronene, a new redox-active core fragment, is utilized for the synthesis of room-temperature columnar discotic liquid crystals (DLCs). Three wedge-shaped side chains having different lengths of alkyl tails are introduced at the periphery of the heterocoronene core to prepare three kinds of discotic molecules, 1 (R = C10H21), 2 (R = C12H25), and 3 (R = C14H29). X-ray diffraction (XRD) analysis confirmed the packing variation in the columnar lattices regulated by alkyl chains of discrete length and steric bulk. When used in space charge limited current devices, compound 1 exhibits a high hole mobility value of 8.84 cm2/V s at ambient temperature, whereas compounds 2 and 3 show efficient ambipolar charge transport behavior with maximum hole (μh) and electron (μe) mobilities of 0.70 and 3.59 cm2/V s, respectively, for compound 3. The mobility values (μh = 8.84 cm2/V s for 1 and μe = 3.59 cm2/V s for 3) are remarkable and the highest ever disclosed for any DLC-based organic semiconductor, promising to deliver a good balance between mobility and processability in devices. The grazing incidence small- and wide-angle X-ray scattering experiments are employed to quantify the extent of alignment in the film state, which correlates with the observed trend of mobility values.

Single crystal texture by directed molecular self-assembly along dual axes.

Creating well-defined single-crystal textures in materials requires the biaxial alignment of all grains into desired orientations, which is challenging to achieve in soft materials. Here we report the formation of single crystals with rigorously controlled texture over macroscopic areas (>1 cm2) in a soft mesophase of a columnar discotic liquid crystal. We use two modes of directed self-assembly, physical confinement and magnetic fields, to achieve control of the orientations of the columnar axes and the hexagonal lattice along orthogonal directions. Field control of the lattice orientation emerges in a low-temperature phase of tilted discogens that breaks the field degeneracy around the columnar axis present in non-tilted states. Conversely, column orientation is controlled by physical confinement and the resulting imposition of homeotropic anchoring at bounding surfaces. These results extend our understanding of molecular organization in tilted systems and may enable the development of a range of new materials for distinct applications.

Fluorophore core-engineered supramolecular discotic columnar liquid crystals with tunable fluorescent behavior

Tunable fluorescent behaviour was achieved for hydrogen-bonded supramolecular discotic columnar liquid crystals via a fluorophore core engineered approach.

Supramolecular columnar discotic nematic liquid crystal by hydrogen bonding: Synthesis and characterization

The design, synthesis, and characterization of a star shaped 2,4,6-tris-(4`-carboxyphenoxy)-1,3,5-triazine liquid crystalline with columnar discotic mesophase properties establish H-bond interactions with 3,5-dialkoxypyidine were reported. The structures of the synthesized compounds were actually determined by elementary analysis, and FT-IR, 1HNMR, 13CNMR, and mass spectroscopy. The mesomorphic properties of these mesogens were examined using differential scanning calorimetry (DSC) and optical polarizing microscopy (OPM). The synthesized molecules exhibited enantiotropic hexagonal columnar liquid crystal, which depends for the H- bond complex in a 1:3 ratio.

Discotic liquid crystals of transition metal complexes 56: Synthesis of mesogenic phthalocyanine-fullerene dyads and influence of the substitution position of alkoxy chains and the kind of terminal groups on appearance of the helical supramolecular structure

We have synthesized twelve novel discotic columnar liquid crystals based on a phenoxy-group-substituted phthalocyaninato copper(II) complex having the same alkoxy chain of C[Formula: see text]H[Formula: see text]O at different positions in the phenoxy group: the parent compounds {0a~0c-16}and the OH-substituted compounds {3a~3c-16}, the OFBA-substituted compounds {2a~2c-16}and the C[Formula: see text]-substituted dyads {1a~1c-16}. The letters of a, b and c mean substitution positions of C[Formula: see text]H[Formula: see text]O group at [Formula: see text], [Formula: see text] and [Formula: see text], respectively. We have investigated the influence of both substitution position of the alkoxy chains and the kind of terminal groups (OH, OFBA and C[Formula: see text] on the mesomorphism and the helical supramolecular structure, by using DSC, POM and temperature-variable small angle X-ray diffraction measurements. As a result, an additional big peak (Peak H) tends to appear at around 2[Formula: see text] = 1.1[Formula: see text] in the X-ray diffraction patterns only for the dyads {1a~1c-16}but not for the other compounds, {0a~0c-16}, {3a~3c-16}and {2a~2c-16}, regardless of the substitution positions of the alkoxy group. Moreover, we revealed that both the m-substituted derivative 1a-16 and the m,p-substituted derivative 1c-16 gave Peak H, but that only the p-substituted derivative 1b-16 did not give Peak H among these three dyads {1a~1c-16}. From the temperature-variable small angle X-ray diffraction measurements for the m,p-substituted derivative 1c-16 using two different sample preparation methods, we proved that the Peak H originates from a helical pitch of fullerenes. We also pointed out that the m-substituted long alkoxy chains are at least needed to form the helical supramolecular structure in the present (PhO)[Formula: see text]PcM-C[Formula: see text]-based dyads.

Homeotropic Self-Alignment of Discotic Liquid Crystals for Nanoporous Polymer Films.

Nanostructured polymer films with continuous, membrane-spanning pores from polymerizable hexagonal columnar discotic liquid crystals (LCs) were fabricated. A robust alignment method was developed to obtain homeotropic alignment of columns between glass surfaces by adding a small amount of a tri(ethylene glycol) modified analogue of the mesogen as a dopant that preferentially wets glass. The homeotropic LC alignment was fixated via a photoinitiated free radical copolymerization of a high-temperature tolerant trisallyl mesogen with a divinyl ester. Removal of the hydrogen-bonded template from the aligned columns afforded a nanoporous network with pores of nearly 1 nm in diameter perpendicular to the surface, and without noticeable collapse of the nanopores. The effect of pore orientation was demonstrated by an adsorption experiment in which homeotropic film showed a threefold increase in the initial uptake rate of methylene blue compared to planarly aligned films.

Excellent superhydrophobic surface and anti-corrosion performance by nanostructure of discotic columnar liquid crystals

A series of high and low symmetry hexa-peri-hexabenzocoronene derivatives, compounds 1–3, was successfully synthesized and applied for corrosion resistance on iron surfaces. In particular, compound 1 exhibited excellent anti-corrosion behavior, as demonstrated by electrochemical impedance spectroscopy (EIS) and contact angle (CA) measurements. A high resistance of 186.7 kΩ/cm2 and excellent hydrophobicity (CA ∼143.9°) with brine solution were obtained. The discotic columnar liquid crystals (DCLCs) coating on the iron surface provide protection under various environments, including exposure to air and solutions with different pH values. This characteristic is attributed to the excellent hydrophobicity and self-healing properties of discotic columnar liquid crystals.

Thermodynamic study of a plastic columnar discotic material 2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene dispersed with gold nanoparticles under elevated pressure

We report a systematic work on the effect of pressure on thermodynamic properties of a plastic columnar discotic liquid crystal (DLC), namely 2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene (HAT4) dispersed with gold nanoparticles up to 1000 MPa by using a high-pressure differential scanning calorimeter. The goal is to determine how elevated pressure can be used to modify the physical properties of plastic columnar DLC and their nanocomposites via changes in inter- and intra-molecular distances of discotics. We find that the columnar hexagonal plastic to isotropic liquid (Colhp–IL) phase transition temperatures and enthalpy decrease with increasing pressure. However, crystal to plastic columnar hexagonal (Cr–Colhp) transition temperature remains almost unchanged for pure and their nanocomposites. The effect of pressure on the phase behaviour of DLC has been studied to elucidate the nature of the mesophase behaviour.

Optical Retardation and Dispersion of Developable Domains in Columnar Hexagonal Liquid Crystal

It’s well known that the optical retardation measurement depends directly on the birefringence of the liquid crystal. We are interested by the simplest distorted texture of discotic columnar liquid crystals (DCLC) where the columns curve circularly their structures around the core line. The present opportunity is to use a hexagonal columnar structure where the topology of columns hemispheres is well known. We have observed the DCLC interference fringes from a half drop in the hexa-n-hexyloxytriphenylene close to the isotropic transition. Using polychromatic spectral lamp, the optical retardation symmetry has been used to describe the spherical texture of the columns. The present work shows that the optical retardation exhibits a noticeable difference between the birefringence measurement Δn versus the wavelength λ. The birefringence is given by the relationship: , (Δno=-0,196 and B=0,023 μm2).

Easily Accessible Thermotropic Hydrogen-Bonded Columnar Discotic Liquid Crystals from Fatty Acid- Tris-Benzoimidazolyl Benzene Complexes.

We report the formation of easily accessible hydrogen‐bonded columnar discotic liquid crystals (LCs) based on tris‐benzoimidazolyl benzene (TBIB) and commercially available fatty acids. By increasing the length of the fatty acid, the temperature range of liquid crystallinity was tuned. Introducing double bonds in octadecanoic acid lowered the crystallization temperature and increased the temperature range of the mesophase. Surprisingly, dimerized linoleic acid also forms an LC phase. When using branched aliphatic acids with the branching point close to the acid moiety, the mesophase was lost, whereas phosphonic acid or benzenesulfonic acid derivatives did have a mesophase, showing that the generality of this approach extends beyond carboxylic acids as the hydrogen‐bond donor. Furthermore, a polymerizable LC phase was obtained from mixtures of TBIB with a methacrylate‐bearing fatty acid, providing an approach for the fabrication of nanoporous polymer films if the methacrylate groups are polymerized. Finally, the higher solubility of methyl‐TBIB was used to suppress phase separation in stoichiometric mixtures of the template molecule with fatty acids.

Effect of high concentration of colloidal gold nanoparticles on the thermodynamic, optical, and electrical properties of 2, 3, 6, 7, 10, 11-hexabutyloxytryphenylene discotic liquid crystalline material

ABSTRACTIn this work, colloidal gold nanoparticles (GNPs) have been dispersed in a plastic columnar discotic liquid crystal (DLC), namely 2,3,6,7,10,11-hexabutyloxytryphenylene (HAT4). Thermodynamic, dielectric, and optical properties of nanocomposites verify the incorporation of the nanoparticles (NPs) into columnar matrix. Due to addition of GNPs in the triphenylene-based DLC (HAT4), columnar hexagonal to isotropic liquid phase, transition temperature appreciably decreases; however, the crystal to columnar hexagonal phase transition temperature does not change drastically. Interestingly, the ionic conductivity of nanocomposite shows an increment by two orders of magnitude due to doping of GNPs. Dielectric permittivity measured parallel to the column axis in the homeotropic aligned sample has improved. Optical study suggests that band gap has decreased due to dispersion of GNPs.

Effect of five-membered ring and heteroatom substitution on charge transport properties of perylene discotic derivatives: A theoretical approach

Density functional theory calculations were carried out to investigate the evolvement of charge transport properties of a set of new discotic systems as a function of ring and heteroatom (B, Si, S, and Se) substitution on the basic structure of perylene. The replacement of six-membered rings by five-membered rings in the reference compound has shown a prominent effect on the electron reorganization energy that decreases ∼0.2 eV from perylene to the new carbon five-membered ring derivative. Heteroatom substitution with boron also revealed to lower the LUMO energy level and increase the electron affinity, therefore lowering the electron injection barrier compared to perylene. Since the rate of the charge transfer between two molecules in columnar discotic systems is strongly dependent on the orientation of the stacked cores, the total energy and transfer integral of a dimer as a disc is rotated with respect to the other along the stacking axis have been predicted. Aimed at obtaining a more realistic approach to the bulk structure, the molecular geometry of clusters made up of five discs was fully optimized, and charge transfer rate and mobilities were estimated for charge transport along a one dimensional pathway. Heteroatom substitution with selenium yields electron transfer integral values ∼0.3 eV with a relative disc orientation of 25°, which is the preferred angle according to the dimer energy profile. All the results indicate that the tetraselenium-substituted derivative, not synthetized so far, could be a promising candidate among those studied in this work for the fabrication of n-type semiconductors based on columnar discotic liquid crystals materials.

Synthesis and Properties of Alkoxy-Bridged Triphenylene and Perylene Monoimide Diesters Dyads

Columnar discotic liquid crystals have high charge carrier mobility, and donor-bridge-accepter-based supra-molecular compounds have photoinduced intramolecular electron transfer behavior. In order to make the organic materials possess these two performances, dyads composed of hexaalkoxy triphenylene unit and perylene monoimide diesters unit were prepared in this work. In the dyads, the flexible alkoxys were used as bridges, the triphenylene units having six electron-donating alkoxy tails acted as electron donors, and the perylene monoimide diesters units having four electron-withdrawing carbonyls acted as electron acceptors. Their structures were established by proton nuclear magnetic resonance (H NMR), infrared spectroscopy (IR), mass spectrometry (MS) and elemental analysis (EA). The photophysical properties were characterized by means of UV-Vis absorption spectroscopy and fluorescence spectroscopy. The results showed that in dilute dichloromethane solutions the absorbance strength of these dyads was the sum of that of their monomers, hexakishexyloxy triphenylene (HAT6) and N-hexyl-perylene monoimide dihexyl esters (PMD6), and not interfered by the length of flexible bridges. When excited at 475 nm, the strength of the fluorescence of the dyads decreased when the spacers shortened from dodecyloxy, decyloxy, hexyloxy to ethoxy groups. Actually, when the spacer was ethoxy group, the fluorescence of the dyad was almost quenched completely. This is attributed to the photoinduced electron transfer properties (PET) between the donor and acceptor units. When excited at 280 nm, the strength of the fluorescence of the triphenylene units also became weaker when the spacers shortened from dodecyloxy to ethanyloxy. At the same time, the strength of the fluorescence of the perylene units became stronger. This is attributed to energy transfer from the triphenylene unit to the perylene unit. In addition, their liquid crystalline properties have been studied by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The results demonstrated that when the spacers were decyloxy and dodecyloxy the dyads possessed columnar liquid crystal behavior in the heating cir-

Supramolecular Discotic Columnar Liquid Crystals Built through Single Hydrogen Bonding between Carboxylic Acid and Pyridine Moieties

New self-assembled discotic columnar liquid crystals were prepared via single hydrogen bonding between the core molecule of 1,3,5-cyclohexanetricarboxylic acid and the peripheral units of stilbazole derivatives. The hydrogen-bonded liquid crystals exhibited rectangular columnar and hexagonal columnar mesophases depending on the number of alkyl chains in three arms of the discotic mesogen through dipole-dipole interactions between carbonyl groups along the column axis. The intermolecular hydrogen bonding between carboxylic acid and pyridine moieties provides an effortless methodology to prepare the ordered columnar liquid crystals for electronic applications.

Ordering properties of columnar discotic triazines containing three pendant triphenylenes with four or five fluorinated tails

Two series of discotic columnar liquid crystals were prepared and investigated, consisting of a triazine core to which three triphenylenes (HATs) are attached, connected via a flexible variable spacer containing a triazole group. The triphenylenes have five pentafluoropentyloxy tails or four pentafluoropentyloxy tails and one methoxy group. The compounds with four fluorinated tails on the HAT groups show a lamellar-columnar phase (Collam), whereas a compound with five fluorinated tails on the HAT groups shows a hexagonal-columnar (Colh) phase. Small differences in the steric properties and fluorophobic effects can therefore have a strong influence on the ordering of the molecules in the liquid crystalline phase.