Discotic Dimers Research Articles

Useful synthetic artifacts? The impact of ubiquitous linker-adjacent groups on the self-assembly of discotic dimers.

Although discotic dimers commonly feature bulky ether substituents adjacent to the linking group, the impact of these chains on self-assembly remains unclear. A series of dibenzo[a,c]phenazine dimers with alkoxy groups ortho to the linker were prepared and their solution conformational dynamics and liquid crystalline properties examined. The presence of a methoxy substitutent adjacent to the bridging group increased the phase stability, whereas longer chains dramatically decreased clearing temperatures. NMR solution studies indicated that adjacent groups increased the preference of dimers to adopt unfolded conformers. DFT models indicated that the unfolded structures were nonplanar and hence less compatible with columnar ordering, leading to a destabilization of the mesophases.

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.

Semirigid discotic dimers: flexible but not flexible enough?

Two diastereomeric dibenzo[a,c]phenazine 1,2-cyclohexyl bridged diesters were prepared and their phase properties examined. While the trans dimer exhibited a broad columnar liquid crystal phase, the cis dimer was amorphous at all temperatures studied. This difference was attributed to the conformational dynamics of the two systems. NMR and DFT studies indicate that both dimers adopt folded and unfolded conformers in solution. While their folded geometries were similar, the trans dimer adopts an extended, largely planar structure, whereas the cis dimer is limited to non-planar unfolded structures and likely disrupts columnar ordering. Geometric constraints imposed by the cylcic linker were also important for columnar stability, with the trans dimer clearing 40 °C lower than the corresponding acyclic 2R,3R-butyl linked dimer, likely because the cyclohexyl group hinders π-π stacking in the unfolded conformation of the former.

Strategies for promoting discotic nematic phases.

A series of discotic dimers were synthesized in an attempt to obtain nematic discotic liquid crystal phases. Although initial target compounds were nonmesogenic, the incorporation of bulky groups adjacent to the ester bridge, in conjunction with peripheral chains of mixed length, were used to promote the formation of an enantiotropic nematic phase.

Synthesis and Mesophase Properties of Triphenylene Dimers Linked through Linear and Bent Alkyne‐Aryl‐Alkyne Bridges

AbstractNematic mesophase formation in disc‐like systems remains relatively rare compared to higher‐order columnar organisation, but it is observed in some rigid dimeric systems. Mesophase formation in rigid discotic dimers has been sequentially probed and is shown to be predictably controlled through structural choice of the bridge. Moderately bent bridges shorten the discogen separation and reduce conformational freedom. Severely bent dimers (1,2‐phenylene bridge) show no mesophases but, as the angle is widened, columnar and nematic organisation becomes possible and progressively more favoured. The linear arrangement gives the most stable columnar mesophase and a narrow‐range nematic mesophase.

Supramolecular Self‐Assembly of Ionic Discotic Liquid Crystalline Dimer with DNA at Interfaces

AbstractNanoarchitectonics through ionic self‐assembly at interfaces is an attractive approach to obtain advanced functional materials. Here, a novel discotic dimer‐DNA complex hybrid system has been investigated at air‐water and air‐solid interfaces. The ionic discotic liquid crystalline dimer consisted of two triphenylene cores linked via alkyl spacer with a imidazolium moiety. At air‐water interface, the dimer formed a stable monolayer with a reversible collapse. The surface manometry results suggested that the condensed phase of the monolayer consisted of molecules arranged in an edge‐on conformation. To understand the folding behavior of the molecules, DFT calculations were carried out which showed that the quantum chemically optimized folded‐form of the molecule was electronically more stable than its unfolded‐form. Upon adding DNA to the subphase, a complex monolayer was formed with enhanced stability as was indicated by increased collapse pressure and decreased limiting area. Importantly, this complexation enabled an efficient and stable multilayer formation on silicon substrates with layers as many as 40. Since both DNA and discotic dimer molecules share common properties of one‐dimensional charge transport with compatible structures, this complex film could serve as a model system for organic electronics.

Competition and promotion of different mesophases in a series of novel unsymmetrical discotic dimers via subtle modification of spacers and peripheral side chains

The superlattice structure of the columnar plastic phase in unsymmetrical triphenylene-based dimers.

Mesophase Tuning in Discotic Dimers π-Conjugated Ionic Liquid Crystals through Supramolecular Interactions and the Thermal History

π-Conjugated ionic liquid crystals are a very interesting class of salts where the coupling of ionic and electronic functions with the anisotropy typical of liquid crystals may give rise to materials with advanced bulk properties. Defect-free active thin films obtainable by simple mesophase self-assembly of these materials may be exploited in a number of different electro-optical devices. Here we show that the rich mesomorphism of the thienoviologen salts 4,4′-(2,2′-bithiophene-5,5′-diyl)bis(1-alkylpyridinium), which includes smectic, columnar, and nematic phases, significantly depends on the counterion type and the length of the promesogenic alkyl chains, highlighting the delicate balance among ion/ion, π–π stacking, and hydrophobic interactions. These salts show notable fluorescence properties in the bulk, strongly dependent on the self-assembling.

A systematic study on the influences of linkage length on phase behaviours and charge carrier mobilities of discotic dimers

ABSTRACTTwo series of discotic dimers T3Dn and T5Dn based on hexapropoxytriphenylene (HAT3) and hexapentyloxytriphenylene(HAT5), respectively, with polymethylene linkage O(CH2)nO (n = 3–12) have been synthesised. Their mesomorphism was investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The results showed that side chains induced a phase transition from colhp phase to colh phase, namely dimers T3Dn (n = 6–12) based on HAT3 exhibiting a single colhp phase, dimers T4Dn (n = 6, 7, 11, 12) based on HAT4 showing a highly ordered colhp phase in low-temperature region and a colh phase in high-temperature region, dimers T4Dn (n = 8–10) based on HAT4 displaying a single colhp phase and dimers T5Dn (n = 6–11) based on HAT5 indicating a single colh phase. Dimers T4Dn showed a phase competition between colh phase and colhp phase induced by linkages. Surprisingly, a unique phenomenon was found by us, that is, those compounds in which linkage lengths were twice those of side chains showed the largest enthalpies, the smallest intracolumnar spacings and the highest charge-carrier mobilities among their homologues, respectively, which implied that they formed the most highly ordered phase among their homologues.

Applications of liquid crystals in biosensing and organic light-emitting devices: future aspects

ABSTRACTThis article summarises recent advances made in our laboratory towards the development of new technological applications, such as biosensors and organic light-emitting diodes (OLEDs) based on liquid crystals (LCs) other than LC displays. The study of biomolecular interaction using LC material relies on the specific interaction between the LC and the biomolecule of interest at interfaces that permit the biomolecular events to be amplified into easily measured signals for various sensing applications. In the first part, we emphases recent studies in the design and modulation of LC-based interfaces based on robust colloidal LC gels for biological amplification, qualitative and quantitative understanding of important biomolecular interactions at LC–aqueous interfaces for diagnostic and laboratory applications and design of LC droplets that hold promise to act as a marker for cells and cell-based interactions. In the second part, we described design of organic materials for application in OLEDs on various discotic monomers, dimers and oligomers. These molecules have the ability to transport charges, holes and electrons. In addition, because of the high conductivity and π–π stacking, they are considered as the advanced materials for practical applications. The technological advances in our laboratory using discotic LCs will be briefly presented in this article.

Folded discotic dimers.

We report the synthesis and properties of a dibenzophenazine discotic dimer, which was found to adopt a folded conformation in solution. This stable π-stacked structure persists at elevated temperatures and in all solvents examined. Folding allows the dimer to assemble into a columnar liquid crystal phase, despite its short linking group.

Synthesis and mesomorphism of triphenylene-based dimers with a highly ordered columnar plastic phase

A series of triphenylene-based discotic dimers have been synthesized, each of the triphenylene nuclei bears five β-OC4H9 substituents, which are linked through the remaining β-positions by a flexible O(CH2)nO polymethylene chain (n = 2–12). Their chemical structures were confirmed by proton nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, high-resolution mass spectrometry and elemental analysis. The mesomorphic properties of these compounds were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. It was found that these dimers for which n > 4 formed a highly ordered columnar plastic phase and some of them (n = 6, 7, 10, 11, and 12) exhibited multiple mesophases. This is the first time that the rectangular columnar plastic phase was defined by us as a novel liquid crystal phase of compound 4d (n = 5). The introduction of a single interconnecting chain between adjacent molecules does not significantly perturb the formation of a columnar plastic phase but enriches the mesophases of discotic molecules and hinders the crystallization; the mesophases of 4e–k (n = 6–12) can be supercooled into a glass state in which the self-assembly columnar structures are retained.

Lamellar organization of discotic dimer enforced by steric manipulation

Dibenzo[a,c]phenazine dimer mesophases exhibited a lamellar assembly as demonstrated by XRD diffraction. This is a new successful molecular design to achieve a hierarchical supramolecular assembly for dimeric discotic liquid crystals. All the mesophases are readily frozen into a glassy state at room temperature.

Enforced liquid crystalline properties of dibenzo[a,c]phenazine dimer and self assembly

Dibenzo[a,c]phenazine dimers with the following features: (i) short spacer, (ii) peripheral chains shorter and longer than the spacers, and (iii) reduced symmetrical structure of the disc were synthesized. The dibenzo[a,c]phenazine disc has enforced the self-assembly of discotic dimers connected through six methylene spacers to form mesophases through a folded conformation in the mesophase. An X-ray diffraction study pinpoints the preferential intra-molecular bridging conformation of these dimers at the mesophase. These new discotic dimers showed polymesomorphism and glass formation. Interestingly, these discotic dimers form organogels with non-polar hydrocarbon solvents and long chain aliphatic alcohols. A superstructure arising from association of the nanofibers was observed with transmission electron microscopy.

Non-symmetrical discotic liquid crystalline dimers: molecular design, synthesis and mesomorphic properties‡

The synthesis and mesomorphic properties of novel non-symmetrical discotic dimers have been investigated. Dimers have been prepared by the combination of electron-deficient (n-type) anthraquinone and electron-rich (p-type) triphenylene discotic monomers. The mesophases have been characterised using polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. Most of the dimers have been shown to exhibit a rectangular columnar mesophase and one has a nematic columnar mesophase also. Charge transfer behaviour has been studied by UV–Vis spectroscopy.

Novel banana-discotic hybrid architectures

Here we present the design and synthesis of novel banana-discotic dimers and banana-bridged discotic dimers. The chemical structures have been characterized by spectral techniques and elemental analysis. The thermal behaviors of the compounds have been investigated by polarizing optical microscopy and differential scanning calorimetry. None of these synthesized compounds exhibit any liquid crystalline property probably because of the incompatibility of the bent-core with the discotic core.

Novel Benzene-Bridged Triphenylene-Based Discotic Dyads

We report the synthesis and characterization of two series of novel triphenylene-based benzene-bridged symmetric discotic dimers. Two triphenylene discotics have been connected to a rigid benzene ring via flexible methylene spacers. In one series, triphenylene moiety was tethered with benzene via an ester linkage, while in the second series it is via an ether linkage. Within each series, the orientation of the linkage of the triphenylene core around the benzene core has been changed by substituting the benzene ring at o-, m-, and p-positions. These materials have been characterized from their spectral and elemental analysis. The thermotropic liquid crystalline properties were investigated by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction studies. All the virgin compounds do not display any mesomorphism; however, their charge transfer complexes with trinitrofluorenone, an electron acceptor, exhibited columnar mesophases. The direct current (dc) conductivity of charge transfer complexes at two ratios has been studied at variable temperature. The small conductivity value demonstrates that long spacers as well as connectivity to the rigid benzene ring dilute the column packing, hence making pi-pi interaction less efficient for the entire column length. A hexagonal assembly of triphenylene-benzene dimer 12 on a highly oriented pyrolytic graphite (HOPG) surface has been visualized with scanning tunneling microscopy (STM).

Synthesis of Triphenylene Discotic Liquid Crystal Dimers: Click Chemistry As an Efficient Tool

Click chemistry between alkynes and azides high efficiently produces triazoles and would be a convenient tool for synthesis of discotic liquid crystal dimers, oligomers and polymers. In this report, triphenylenes containing terminal alkynes and azides were prepared, and the cycloaddition reaction (click chemistry) between them catalyzed by CuI-Et3N in toluene produced discotic liquid crystal dimers in high yield and good selectivity. The dimers and intermediates have been fully characterized by 1H NMR and high resolution MS. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) results show the dimers possess columnar mesophase in wide temperature range from very low temperature. The triazole moiety in the dimer can act as electron and hydrogen bond acceptor, and ionic forming part, rich chemical and physical properties can be expected from these new structure discotic dimers.

A computer simulation of model discotic dimers

We set up a model for discotic liquid crystal dimers and study, by means of Monte Carlo simulations, their phase behaviour and self–assembling properties, in comparison with the simpler monomeric case. Each discotic dimer is described by two oblate Gay–Berne ellipsoids connected by a flexible spacer, modelled by a harmonic “spring” of three different lengths. We find that dimerization in general yields produces a significant change on the phase behaviour, with an increase of the columnar–nematic transition temperature, a widening of the nematic region and the apparent suppression of the crystalline phase in favour of the columnar phase up to very low temperatures. Longer spacers prove to ease the formation of columns and to increase the orientational order.

Comparative Study of Planar Motion in Monomeric and Dimeric Discotics

Deuterium NMR spectra are reported in powder samples of discotic monomer and dimer as a function of temperature in their column Col(ho) phases. To simulate the observed powder patterns, a threefold jump model is used in the monomer, while in the related dimer the libration motion of the monomeric core is described using the infinitesimal jump method under a restricting potential due to the spacer. By comparing the diffusive rates for the two samples, it is concluded that the planar motion in the dimer is at least 30 times smaller than that of the monomer. This could lead to an enhancement of charge and energy transport in discotic dimer systems.