Triphenylene-based Discotic Liquid Crystals Research Articles

Enhancing Charge Transport Using Boron and Nitrogen Substitutions into Triphenylene-Based Discotic Liquid Crystals.

The substitution of p-block heteroatoms into polyaromatic hydrocarbons offers the potential for introducing enhanced molecular properties and advancing material development for electro-optical applications. Using density functional theory, we characterize the substitution of boron and nitrogen atoms into a 2,3,6,7,10,11-hexakis(hexathiol)triphenylene (TTP) core, a precursor for a material with a discotic liquid crystal phase, to determine the strength of exciton dissociation and the influence doping has on the formation of a heterojunction with graphene. The substitution of nitrogen and boron into the TTP motif enables tunability of both electron and hole coupling between hetero- and homodyads. The coupling is found to far exceed that of TTP and varied transport behavior with different combinations of doped cores of nitrogen-TTP and boron-TTP is reported. Heterodyads of nitrogen-TTP with boron-TTP appear to be ambipolar in electron/hole coupling, whereas heterodyads of boron- or nitrogen-TTP with TTP form strong electron coupling dyads and homodyads of nitrogen-TTP and boron-TTP form strong hole coupling. Finally, we describe the heterojunction of nitrogen- or boron-TTP with monolayer graphene and observe Ohmic contacts with large hole transport barriers. The presence of induced dipoles occurs at the interface in all heterojunctions, suggesting the possibility of tuning the junction with external potentials and improving exciton dissociation.

Influence of monohalogenated flexible tails on the mesomorphism of triphenylene-based discotics

A series of triphenylene-based discotic liquid crystals (DLCs) modified by fluorination or chlorination at the end of peripheral flexible chains were examined in this work. DLC derivatives containing one mono fluorine or chlorine-substituted flexible chain 4a-e and six mono fluorine or chlorine-substituted flexible chains 8a & 8b, 10a & 10b were mainly synthesized and molecular structures confirmed by 1H NMR, 19F NMR and MALDI-TOF MS. The mesogenic properties of the obtained compounds were further studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). Impressively, compounds 4a-e were found exhibiting hexagonal columnar phases and the π-π stacking distances of 4a-e were lower than that of the parent molecule HAT5. Due to the influence of atomic radius and molecular symmetry, the mesophase stability of the fluorinated triphenylene compounds was greater than the chlorinated ones. Through the modification of the side chain, the self-assembly behavior of triphenylene was stronger and the intracolumnar distance was decreased, which may give rise to a higher charge transport mobility. Compound 10b showed self-assembly behavior and formed a spherulite texture that could also remain orderly until room temperature. This is rare in reported DLCs and it might be applied as active components of in electronic devices. These ordered stacked discotic molecules may exhibit improved charge carrier mobility as semiconducting materials.

Room-temperature triphenylene-based discotic liquid crystal dimers: effects of amide linkage and central fluoromethylene spacer

ABSTRACT Two series of triphenylene-based dimers both having a central fluoromethylene spacer of four or six carbon atoms and ester or amide linkages were synthesised and their phase behaviour investigated. All the synthesised dimers are room-temperature discotic liquid crystals and exhibit hexagonal columnar mesophases that supercool into a stable anisotropic columnar glass. The length of the fluoromethylene spacer has a minor effect on the phase transition temperatures of these dimers. However, dimers 1a and 2a with a (CF2)4 spacer exhibit approximately 40°C higher clearing points and wider mesomorphic phase ranges compared with their hydrocarbon analogs 1c and 2c with a (CH2)4 spacer respectively. Compared to the corresponding ester-linked dimers 1a-c, three amide-linked dimers 2a-c exhibit 20°C higher clearing points upon cooling. The observation indicates not only that the introduction of the fluoromethylene spacer stabilised the mesophase in these discotic dimers, but also that the two interactions have synergistic effects on columnar packing in this system. Moreover, only amide-linked dimers exhibit good gelation property in a mixture of toluene and petrol ether, with a lower critical gelation concentration of 1.5 wt/vol%, thus indicating that gel formation occurred under the collaboration of hydrogen bonding and intrinsic π interactions in these dimers.

The steric hindrance effect of bulky groups on the formation of columnar superlattices and optoelectronic properties of triphenylene-based discotic liquid crystals

Molecular conformations and electrostatic potential isosurfaces of 2- and 3,6-position bulky group substituted triphenylene discotic carboxylic esters calculated using the Gaussian 09 package.

Multiple glassy dynamics in dipole functionalized triphenylene-based discotic liquid crystals revealed by broadband dielectric spectroscopy and advanced calorimetry – assessment of the molecular origin

A selected series of dipole functionalized triphenylene-based discotic liquid crystals (DLCs) was synthesized and investigated in a systematic way to reveal the phase behavior and molecular dynamics. The later point is of particular importance to understand the charge transport in such systems which is the key property for their applications such as organic field-effect transistors, solar cells or as nanowires in molecular electronics, and also to tune the properties of DLCs. The mesomorphic properties were studied by polarizing optical microscopy, X-ray diffraction, and differential scanning calorimetry, which were compared to the corresponding unfunctionalized DLC. The molecular dynamics were investigated by a combination of state-of-the-art broadband dielectric spectroscopy (BDS) and advanced calorimetry such as fast scanning calorimetry (FSC) and specific heat spectroscopy (SHS). Besides localized fluctuations, surprisingly multiple glassy dynamics were detected for all materials for the first time. Glassy dynamics were proven for both processes unambiguously due to the extraordinary broad frequency range covered. The α1-process is attributed to fluctuations of the alky chains in the intercolumnar space because a polyethylene-like glassy dynamics is observed. This corresponds to a glass transition in a confined three-dimensional space. The α2-process found at temperatures lower than α1-process, is assigned to small scale rotational and/or translational in plane fluctuations of the triphenylene core inside distorted columns. This can be considered as a glass transition in a one-dimensional fluid. Therefore, obtained results are of general importance to understand the glass transition, which is an unsolved problem of condensed matter science.

Supramolecular self-assembly and physical-gel formation in disc-like liquid crystals: a scalable predictive model for gelation and an application in photovoltaics.

The application of triphenylene-based discotic liquid crystal derivatives as physical gelators is investigated. In particular, we focus on 2,3,6,7,10,11-hexakis-pentyloxytriphenylene (HAT5) and the longer alkyl chain homologue (HAT6). The driving mechanisms behind and parameter space of non-covalent physical gel formation is studied. A Hansen solubility parameter (HSP) approach is used to predict physical gelation of these disc-like liquid crystalline molecules in a variety of common organic and inorganic solvents important to electrochemical devices. Our results show that HSP analysis is very useful for the prediction of gel formation. The results are transferrable and can form the basis for future investigations into liquid crystalline physical gels. Furthermore, we use acetonitrile as a solvent and apply the gels as electrolytes in dye sensitized solar cells. It is observed that using a binary mixture of gelators results in average photovoltaic power conversion efficiencies as high as 7.21%, compared to a 5.9% reference electrolyte. This is attributed to a reduction in electron recombination at the n-type interface and provides further insight about hybrid gelators. Coupled with an increase in device stability, the results are promising for gel-based dye sensitized solar cells as well as potentially other electrolytic devices such as batteries and supercapacitors.

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.

Synthesis and Mesophase of a Triphenylene-Based Discotic Liquid Crystal with Plastic Columnar Phase

A novel triphenylene-based discotic liquid crystal 2, 3, 6, 7, 10, 11-hexabutylcarboxyltriphenylene was synthesized. The chemical structure was determined by 1HNMR and FT-IR spectrum, and its mesophase was investigated by DSC and POM methods. The compound showed classical texture of columnar plastic phase under POM at the mesophase temperature from175°C-185°C during cooling.

Effect of dispersion of gold nanoparticles on the optical and electrical properties of discotic liquid crystal

Dispersion of gold nanoparticles (GNPs) in mononitro-substituted triphenylene-based discotic liquid crystal (DLC), which shows columnar plastic phase (Colp) along with columnar hexagonal phase (Colh) at room temperature, was studied by differential scanning calorimetry, polarising optical microscopy, X-ray diffraction, dielectric spectroscopy, visible absorbance spectroscopy and IR dichroic technique. The experimental results show that inclusion of GNPs in the DLC matrix strains the columnar matrix. More ordered Colp which possesses high charge carrier mobility because of the three-dimensional positional order is still preserved even by insertion of GNPs into the columnar matrix. However, we observed a decrease in orientational order parameter (S) and increase in relaxation time (τ) for disc motion with GNPs in Colp. But there is enhancement in dc electrical conductivity by several orders of magnitude at ambient conditions with GNPs. These GNP–DLC composite systems hold great promises in many organic semiconductor device applications.

Vibrational density of states of triphenylene based discotic liquid crystals: dependence on the length of the alkyl chain

The vibrational density of states of a series of homologous triphenylene-based discotic liquid crystals HATn (n = 5, 6, 8, 10, 12) depending on the length of the aliphatic side chain is investigated by means of inelastic neutron scattering. All studied materials have a plastic crystalline phase at low temperatures, followed by a hexagonally ordered liquid crystalline phase at higher temperatures and a quasi isotropic phase at the highest temperatures. The X-ray scattering pattern for the plastic crystalline phase of all materials shows a sharp Bragg reflection corresponding to the intercolumnar distance in the lower q-range and a peak at circa 17 nm(-1) related to intracolumnar distances between the cores perpendicular to the columns as well as a broad amorphous halo related to the disordered structure of the methylene groups in the side chains in the higher q-range. The intercolumnar distance increases linearly with increasing chain length for the hexagonal columnar ordered liquid crystalline phase. A similar behaviour is assumed for the plastic crystalline phase. Besides n = 8 all materials under study exhibit a Boson peak. With increasing chain length, the frequency of the Boson peak decreases and its intensity increases. This can be explained by a self-organized confinement model. The peaks for n = 10, 12 are much narrower than for n = 5, 6 which might imply the transformation from a rigid system to a softer one with increasing chain length. Moreover the results can also be discussed in the framework of a transition from an uncorrelated to a correlated disorder with increasing n where n = 8 might be speculatively considered as a transitional state.

Triphenylene-based discotic liquid crystals: recent advances

Since the early work of Chandrasekhar and his co-workers on hexaesters of benzene published in 1977, discotic liquid crystals (DLCs), in particular, triphenylene-based DLC materials have been investigated intensively, especially over the last decade. The first successful commercialisation of triphenylene-based DLCs has been accomplished in Fuji ‘Wide-View’ optical compensation films. DLCs represent a broad well understood class of soft matter which possess the ability to self-organise into highly anisotropic and ordered structures such as columns that function not only as organic anisotropic semiconductors, but also contribute to the development of new smart materials in the field of organic electronics for many device applications such as photovoltaic devices, light-emitting diodes, field-effect transistors, memory elements, and sensors. Over the last 35 years, more than 1000 triphenylene derivatives have been synthesised and investigated starting from structure-properties to structure-device performance relationships. The very first review by Cammidge and Bushby followed by Kumar summarised the chemistry and physical properties of triphenylene-based discotics up to 2003. In this review, progress in the research of triphenylene DLC materials since 2004 is comprehensively outlined.

Synthesis of Symmetrical and Unsymmetrical Triphenylene Discotic Liquid Crystals Using Antimony(V)Chloride Under Scholl Oxidation

Triphenylene-based discotic liquid crystals, useful in studying the energy and charge migration in self-organized systems, are the most widely synthesized and studied discotic liquid crystals. In this paper, we report an efficient synthetic procedure for the preparation of symmetrical and unsymmetrical triphenylene discotic liquid crystals using antimony pentachloride as a novel reagent. Scholl oxidative trimarization of 1,2-dialkoxybenzenes with SbCl5 yields hexaalkoxytriphenylenes in good yield, while the oxidative coupling of a 3,3’,4,4’-tetraalkoxybiphenyl with a 1,2,3-trialkoxybenzene affords an unsymmetrically substituted heptaalkoxy-triphenylene derivative. The potential of this new reagent was compared with the other known reagents for the synthesis of alkoxytriphenylenes.

Triphenylene-based discotic liquid crystals: star-shaped oligomers and branched-chain polymers

Liquid crystalline main-chain polymers based on a repeat hexaalkoxytriphenylene moiety have previously been made by the reaction of dihydroxytetraalkoxytriphenylenes with α,ω-dibromoalkanes catalysed by caesium carbonate in N-methyl pyrrolidone. We now show that the molecular weights of these polymers can be increased by adding ˜5 wt% of a tribromide (a 1,3,5-trialkoxybenzene or trialkoxymethylbenzene with three ω-bromoalkyl substituents) to the reaction mixture. Studies of model oligomer systems show that, provided the ω-bromoalkyl chains are long enough, the presence of these branching points should not affect the formation of the Colh mesophase. Indeed, if ˜5 wt% of the tribromide is used in the synthesis of the polymer, it remains liquid crystalline. However, if >20 wt% of the tribromide is used, introducing a high degree of branching and cross-linking, the liquid crystal behaviour disappears.

Discotic Ionic Liquid Crystals of Triphenylene as Dispersants for Orienting Single‐Walled Carbon Nanotubes

Orient and conduct: Triphenylene-based discotic ionic liquid crystals (ILCs) with six imidazolium ion pendants can disperse pristine single-walled carbon nanotubes (SWNTs). When the ILC is columnarly assembled, doping with SWNTs results in macroscopic homeotropic columnar orientation. Combination of shear and annealing treatments gives rise to three different orientation states, which determine the anisotropy of electrical conduction.

Synthesis and Mesomorphism of a Novel Triphenylene-Based Discotic Liquid Crystal with Chiral Chain

A novel triphenylene-based discotic liquid crystal compound was synthesized in this study. The chemical structures of compounds were characterized by 1HNMR spectra, FT-IR spectra and were confirmed. The phase transition behavior was studied in the combination of texture observation by DSC and POM equipped with a temperature-controllable hot-stage. The object compound showed liquid crystal phase and the results showed that columnar liquid crystal phase between 90.1°C and 138.1 °C during heating.

Twofold Orientation of Triphenylene-Based Discotic Liquid Crystals on Gold

Self-organization of first adsorbed monolayer of discotic liquid crystal, 2,3,6,7,10,11-hexapentyloxytriphenylene on the Au(111) surface was investigated by scanning tunneling microscopy at a solid/liquid interface. Visualizing the structure of the interface between organic molecules and the surface of the electrode is crucial for understanding the physics of potential devices, like organic light-emitting diodes or organic photovoltaic cells. We present images recorded with molecular resolution that reveal presence of ordered domains of hexagonal symmetry. Mutual distances between the centers of H5T lattice, combined with the orientation of the unit-cell suggest commensurability with the underneath metal surface. Two types of H5T lattices have been evidenced, coexisting on the same Au(111) substrate. They correspond to (7 × 7)R30 and (7 × 7)R44 structure of H5T molecules.

A DIONE APPROACH TO MODIFY THE OPTICAL AND MESOPHASE PROPERTIES OF DISCOTIC TRIPHENYLENE DERIVATIVES

Two discotic liquid crystalline compounds, 6,7,10,11-tetra-pentyloxytriphenylene-1,4-dione and 6,7,10,11-tetra-heptyloxy- triphenylene-1,4-dione were synthesized and characterized. The synthesis consisted of two key steps: the preparation of 1,4-dimethoxy-6,7,10,11-tetra-alkoxytriphenylene and the oxidation to form the target compounds. In the latter step, the reaction was significantly improved using sodium nitrite in trifluoroacetic acid as an oxidant, resulting in an enhanced overall reaction yield and dramatically reduced reaction time. Both products show liquid crystalline phases, which were investigated using polarized optical microscopy and thermal analysis. The product with a longer side chain displayed a broader liquid crystal phase temperature range and favored a faster structure formation at low temperature. In addition to absorption at 272 nm, which is commonly observed in triphenylene-based discotic liquid crystals, the optical absorption of both compounds extends to the visible range, with extra absorption observed at 630 nm.

Triazole-Modified Triphenylene Derivative: Self-Assembly and Sensing Applications

Triphenylene-based discotic liquid crystal 3 bearing 1,2,3-triazole groups has been synthesized using "click" chemistry. Discotic mesogen 3 has good thermal stability, and incorporation of triazole groups results in stabilization of columnar mesophases down to room temperature and formation of organogels in cyclohexane and mixed solvents such as hexane and dichloromethane (4:1 v/v). Characterization of the organogel of 3 in cyclohexane revealed a porous network. However, presence of Cd(2+) ions in solution obstructed the self-assembly of this derivative due to preferred interactions between Cd(2+) ions and triphenylene units over π-π interactions among triphenylene groups. Further, strong emission of derivative 3 in its nonaggregated form makes it a promising fluorescence sensory material for nitroaromatic compounds.

Mesophase Studies and Charge Transportation Properties of 3,6-dicarbethoxy-2,7,10,11-tetra-pentyloxytriphenylene

In this study, a triphenylene-based discotic liquid crystal 3,6-dicarbethoxy-2,7,10,11-tetra-pentyloxytriphenylene was synthesized. The chemical structure was characterized by FTIR, 1HNMR.The mesophases of the material synthesized here was characterized by using polarizing optical microscopy (POM), differential scanning calorimetry (DSC). It was found that the compound displayed columnar phase during cooling at 164.7 °C and the columnar phase can be preserved even at room temperature. The charge transportation properties of the compound were measured by using time-of-flight method.

Synthesis and Mesophases Study on 2-(8-bromooctanoic acid ester)-3,6,7,10,11-penta-pentyloxytriphenylene

A novel triphenylene-based discotic liquid crystal compound 2-(8-bromooctanoic acid ester)-3,6,7,10,11-penta-pentyloxytriphenylene was synthesized in this study.The chemical structure of the compound was characterized by1HNMR spectra, FT-IR spectra and we confirmed its structure. The mesophases were investigated by using different scanning calorimetry DSC and polarizing optical microscopy POM, the results showed that the target product had Liquid crystal properties and the temperature range of 90-101 °C, and showed a clear columnar phase. The carrier transport mobility was measured using time-of-flight TOF method, μ=1.09*10-2cm2V-1s-1.