Columnar Rectangular Mesophases Research Articles

Investigation of mesomorphic, photophysical and gelation behavior in aroylhydrazone based liquid crystals: Observation of mesophase crossover phenomena

Five series of substituted aroylhydrazones having two alkoxy chains at 3- and 5- positions at amide end of the molecule have been synthesized; wherein each series differ from each other in terms of number and position of alkoxy chains at imine end. The compounds of first series with the single alkoxy chain at 4-position at imine end self-assembled into columnar hexagonal mesophase. The lower homologues of the series with alkoxy chains at 3- and 5- positions at imine end showed a transition from columnar hexagonal to columnar rectangular mesophase in cooling cycle while the higher homologues exhibited columnar hexagonal mesophase. The series with alkoxy chains at 3- and 4- positions exhibited columnar rectangular mesophase. All the homologues of series with alkoxy chains at 2-, 3- and 4- positions showed a transition from columnar rectangular to columnar oblique mesophase in cooling cycle. The series with alkoxy chains at 3-, 4- and 5- positions exhibited transition in columnar hexagonal mesophase. The mesophase crossover from one columnar mesophase to the other was found to be dependent on the chain length as well as on the temperature during the self-assembly process in these molecules. The temperature dependent columnar mesophase crossover has been observed for the first time in these systems. The substitution pattern showed an influence on the photophysical properties. The number and position of alkoxy chains played a critical role in the formation of gel in these molecules. These liquid crystalline gelators have application potential in organic optoelectronic functional materials and devices.

Liquid crystalline oxovanadium(IV) and copper(II) complexes of halogen-substituted salphen ligands: role of metal and spacer substituents

ABSTRACT Three new series, each of hemi-discoid oxovanadium(IV) and copper(II) complexes based on halogen-substituted ‘salphen’ type ligands; H2L = [N,N-Bis(4-n-alkoxysalicylidene)-4-fluoro/chloro/bromo-1,2-diaminobenzene] (n = 12, 14, 16) have been accessed and their mesomorphic properties investigated. The complexes have been characterised by elemental analyses, FT-IR and absorption spectroscopy. The fluoro and the chloro substituted oxovanadium(IV) complexes display thermotropic columnar rectangular mesophases with a p2mm symmetry. On the contrary, the bulky bromo substituted oxovanadium(IV) complexes decompose at a high temperature precluding any mesomorphic study. The analogous copper(II) complexes also undergo decomposition on heating and are non-mesomorphic. Molecular arrangements in the mesophases have been proposed based on variable temperature powder X-ray diffraction (PXRD) study. The influence of metal’s coordination and electronegativity and/or steric hindrance of the spacer’s substituent on the mesomorphic properties of the complexes is discussed. Density functional theory (DFT) calculations carried out using the GAUSSIAN 09 program on representative oxovanadium(IV) and copper(II) complexes indicated a distorted square-pyramidal geometry around the VO(IV) centre and a distorted square planar geometry around the Cu(II) centre in their complexes.

Effect of alkoxy chain density on the mesogenic properties of aroylhydrazone based liquid crystals: synthesis, characterisation, photophysical and gelation behaviour

ABSTRACT Three series of ester linkage containing aroylhydrazone based compounds have been synthesised where density and length of alkoxy chains are varied sequentially at ester end, keeping amide end of the molecule the same to investigate the effect of alkoxy chain density. The series of the compounds with single alkoxy chain at 4- position at ester terminal show nematic mesophase. The series with two alkoxy chains at 3- and 5- positions exhibit columnar rectangular mesophase and the series with three alkoxy chains at 3-, 4- and 5- positions show columnar oblique mesophase. It was observed that increasing the number of alkoxy chains at ester terminal, self-assembly of molecules changes and leads to a change from nematic to columnar mesophase and also significant enhancement in the mesomorphic range. Temperature dependent Raman studies confirm the presence of intermolecular hydrogen bonding which assist the self-assembly and gelation properties of molecules. All the series show similar photophysical properties despite the fact that the series vary in their molecular structures. Representative compounds of each series exhibited gelation in hydrocarbon solvents. FESEM images of the gels showed interwoven network of nanofibers of several micrometres length. These supergelators may be good candidates for applications in electronic devices.

Effect of regioisomerism on the self-assembly, photophysical and gelation behavior of aroylhydrazone based polycatenars: Synthesis and characterization

In this paper we report five series of aroylhydrazone based polycatenars differing from each other in number and position of alkoxy chains on one end of the core; keeping other end the same. A systematic variation in the structures was carried out to understand the effect of position and chain length on the mesophase behavior. In the case of single alkoxy chain at 4-position, there is a transition from Smectic C to Smectic A mesophase during cooling and Smectic A to Smectic C transition in heating cycle in all the homologues except n = 6 & n = 8. In the case of two alkoxy chains at 3-, 5-positions, the compounds are non-mesogenic in nature. On changing the position of alkoxy chains from 3-, 5-positions to 3-, 4-positions, intermolecular forces weaken and these compounds become liquid crystalline and show columnar rectangular mesophase resulting from the effect of regioisomerism. Moving on to the series with three alkoxy chains, it was observed that 2-, 3-, 4-trialkoxy substituted compounds exhibit columnar rectangular mesophase except lower chain length (n = 6 & 8) while 3-, 4-, 5-trialkoxy substituted aroylhydrazones show columnar rectangular mesophase; except n = 10 which shows columnar oblique mesophase. Thus it was established that on increasing density of alkoxy chains around one terminal, mesophase changes from smectic to columnar mesophase. Temperature dependent Raman studies confirm the presence of intermolecular hydrogen bonding. Further, effect of substitution pattern did not show much influence on the photophysical properties of the mesogenic series in solid and solution state. However, the non-mesogenic series showed a different absorption and emission transition from the mesogenic series in solid and solution state. Solid state fluorescence studies show almost similar emission maxima in all the series. However, it was also noticed that non-mesogenic series show maximum red shifted emission maxima in solid state as compared to that of solution. All the series showed good gelation properties with less than 1% CGC (wt%) suggesting the strong ability of molecules to form gel. These H-bonded liquid-crystalline gels have immense potential for applications in emissive displays.

Unsymmetrically substituted room temperature discotic liquid crystals based on hexa–peri–hexabenzocoronene core

AbstractThe first discotic system based‐on alkoxy (tri‐ & di‐) substituted hexa‐peri‐hexabenzocoronene (HBC) that self‐organizes into room temperature columnar structure is reported. Tri‐ and di‐ substituted HBC derivatives showed reddish and yellowish green fluorescence under long wavelength (365 nm) UV light illumination in solution as well as in thin films in the liquid crystalline (LC) state at room temperature, respectively. Thermotropic LC behavior of these compounds was studied by differential scanning calorimetry, polarized optical microscopy and small‐ and wide‐angle X‐ray scattering. Tri‐alkoxy derivative was found to self‐organize into a highly ordered columnar rectangular mesophase, while the other (di‐) possessed a columnar hexagonal mesophase, indicating that in spite of their non‐planarity, the aromatic cores in the mesophase are tightly packed to give highly ordered phases. This system is a promising candidate as an active component of organic light emitting devices, due to its facile processability, low band gap, luminescence and exceptionally higher ordered self‐organization behavior in the mesophase derived from X‐ray scattering studies.

Triphenylene-Based Room-Temperature Discotic Liquid Crystals: A New Class of Blue-Light-Emitting Materials with Long-Range Columnar Self-Assembly.

A straightforward synthesis of multialkynylbenzene-bridged triphenylene-based dyad systems (via flexible alkyl spacers) that self-organize into room-temperature columnar structures over a long range is reported. The compounds with spacer lengths (n) of 8 and 10 exhibit a columnar rectangular mesophase whereas a compound with n = 6 shows a columnar rectangular plastic phase. Interestingly, the later compound (n = 6) shows the formation of well-nucleated spherulites of about several hundred micrometers that suggest the existence of a long-range uniform self-assembly of columns. All of these compounds show blue luminescence in solution and in the thin-film state under long-wavelength (365 nm) UV light. These compounds fulfill the described demands such as long-range columnar self-assembly at room temperature, a good yield with high purity, and blue-light emitters under the neat condition for possible potential applications in semiconductor devices. They also match the criteria of facile processing from the isotropic state because of their low isotropization temperature. This new class of materials is promising, considering the emissive nature and stabilization of the columnar mesophase at ambient temperature.

Syntheses and Properties of Graphyne Fragments: Trigonally Expanded Dehydrobenzo[12]annulenes

We present herein the synthesis and properties of the largest hitherto unknown graphyne fragment, namely trigonally expanded tetrakis(dehydrobenzo[12]annulene)s (tetrakis-DBAs). Intramolecular three-fold alkyne metathesis reactions of hexakis(arylethynyl)DBAs 9 a and 9 b using Fürstner's Mo catalyst furnished tetrakis-DBAs 8 a and 8 b substituted with tert-butyl or branched alkyl ester groups in moderate and fair yields, respectively, demonstrating that the metathesis reaction of this protocol is a powerful tool for the construction of graphyne fragment backbones. For comparison, hexakis(arylethynyl)DBAs 9 c-g have also been prepared. The one-photon absorption spectrum of tetrakis-DBA 8 a bearing tert-butyl groups revealed a remarkable bathochromic shift of the absorption cut-off (λcutoff ) compared with those of previously reported graphyne fragments due to extended π-conjugation. Moreover, in the two-photon absorption spectrum, 8 a showed a large cross-section for a pure hydrocarbon because of the planar para-phenylene-ethynylene conjugation pathways. Hexakis(arylethynyl)-DBAs 9 c-e and 9 g and tetrakis-DBA 8 b bearing electron-withdrawing groups aggregated in chloroform solutions. Comparison between the free energies of 9 e and 8 b bearing the same substituents revealed the more favorable association of the latter due to stronger π-π interactions between the extended π-cores. Polarized optical microscopy observations, DSC, and XRD measurements showed that 8 b and 9 e with branched alkyl ester groups displayed columnar rectangular mesophases. By the time-resolved microwave conductivity method, the columnar rectangular phase of 8 b was shown to exhibit a moderate charge-carrier mobility of 0.12 cm(2) V(-1) s(-1) . These results indicate that large graphyne fragments can serve as good organic semiconductors.

Wedge-shaped 1,2-diamidobenzenes forming columnar mesophases via hydrogen bonding

Two series of o-diamidobenzenes, bearing long-chain alkyl substituted gallic ester groups, have been synthesised. The 3,4,5,6-unsubstituted amides 7 with a minimum chain length of C9 exhibited columnar rectangular mesophases of p2mm symmetry with Z = 2. The liquid crystalline phases spread over phase width of extremely wide temperature intervals from -50 to 118°C. However, the phase width slightly decreased with an increase in the length of alkyl substituents. Despite their very similar molecular structure, 1,2-dimethoxy-4,5-diamides 10 revealed a different packing behaviour in their mesophases, due to steric factors. The diamides 10 formed columnar rectangular mesophases with p2mm symmetry, but with Z = 3. In a comparison with unsubstituted analogues 7, compounds 10 displayed even broader mesophases (with temperature intervals up to 220°C).

Photoconductive Nile red cyclopalladated metallomesogens

The synthesis and the electrochemical, photophysical and photoconductive properties of two new cyclometallated metallomesogens are described. These two complexes were respectively obtained by covalent metallation of Nile red, or a polyalkylated Nile red derivative, to a palladium(II) ion and by using a polyalkylated Schiff base as an ancillary ligand to complete the coordination sphere of the metal centre. The introduction of long alkyl chains on either one or both ligands leads to disk-shaped metallomesogens with ordered room temperature columnar organisation induced, in both cases, by the formation of dimeric pairs. As shown by powder X-Ray diffraction studies, both complexes exhibit a columnar rectangular mesophase over a wide temperature range. These complexes show a redox solution behaviour characterised by one reversible single-electron reduction and two consecutive reversible oxidation waves, which give them an electrochemical amphoteric character. Energy levels and distributions of frontier orbitals have been estimated. Photoconductivity of the mesophase at room temperature has been observed from UV-Vis to near IR wavelength. The high thermal, electrochemical and mesophase stabilities of the studied Nile red complexes confirm that cyclometallated Pd(II) complexes form a new class of photoconductors with features suitable for molecular semiconductor applications.

Counterion Effects on the Columnar Mesophases of Triphenylene‐Substituted [18]Crown‐6 Ethers: Is Flatter Better?

The twisted lateral tetraalkyloxy ortho-terphenyl units in dibenzo[18]crown-6 ethers 1a-f were readily converted into the flat tetraalkyloxytriphenylene systems 2a-f by oxidative cyclization with FeCl(3) in nitromethane. Reactions of the latter with potassium salts gave complexes KX.2, which displayed mesomorphic properties. The aromatization increased both the clearing and melting points; the mesophase stabilities, however, were mainly influenced by the respective anions upon complexation with various potassium salts. In contrast, the alkyl chain lengths played only a secondary role. Among the potassium complexes of triphenylene-substituted crown ethers KX.2, only those with the soft anions I(-) and SCN(-) displayed mesophases with expanded phase temperature ranges of 93 degrees C and 132 degrees C (for KX.2e), respectively, as compared to the corresponding o-terphenyl-substituted crown ether complexes KI.1e (DeltaT=51 degrees C) and KSCN.1e (plastic crystal phase). Anions such as Br(-), Cl(-), and F(-) decreased the mesophase stability, and PF(6)(-) led to complete loss of the mesomorphic properties of KPF(6).2 although not for KPF(6).1. For crown ether complexes KX.2 (X=F, Cl, Br, I, BF(4), and SCN), columnar rectangular mesophases of different symmetries (c2mm, p2mg, and p2gg) were detected. In contrast to findings for the twisted o-terphenyl crown ether complexes KX.1, the complexation of the flat triphenylene crown ethers 2 with KX resulted in the formation of organogels. Characterization of the organogel of KI.2e in CH(2)Cl(2) revealed a network of fibers.

Desymmetrization of 3,3′‐Bis(acylamino)‐2,2′‐bipyridine‐Based Discotics: The High Fidelity of Their Self‐Assembly Behavior in the Liquid‐Crystalline State and in Solution

Two novel nonsymmetrical disc-shaped molecules 1 and 2 based on 3,3'-bis(acylamino)-2,2'-bipyridine units were synthesized by means of a statistical approach. Discotic 1 possesses six chiral dihydrocitronellyl tails and one peripheral phenyl group, whereas discotic 2 possesses six linear dodecyloxy tails and one peripheral pyridyl group. Preorganization by strong intramolecular hydrogen bonding and subsequent aromatic interactions induce self-assembly of the discotics. Liquid crystallinity of 1 and 2 was determined with the aid of polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. Two columnar rectangular mesophases (Col(r)) have been identified, whereas for C(3)-symmetrical derivatives only one Col(r) mesophase has been found. In solution, the molecularly dissolved state in chloroform was studied with (1)H NMR spectroscopy, whereas the self-assembled state in apolar solution was examined with optical spectroscopy. Remarkably, these desymmetrized discotics, which lack one aliphatic wedge, behave similar to the symmetric parent compound. To prove that the stacking behavior of discotics 1 and 2 is similar to that of reported C(3)-symmetrical derivatives, a mixing experiment of chiral 1 with C(3)-symmetrical 13 has been undertaken; it has shown that they indeed belong to one type of self-assembly. This helical J-type self-assembly was further confirmed with UV/Vis and photoluminescence (PL) spectroscopy. Eventually, disc 2, functionalized with a hydrogen-bonding acceptor moiety, might perform secondary interactions with molecules such as acids.

Guanidinium Chlorides with Triphenylene Moieties Displaying Columnar Mesophases

Novel guanidinium chlorides 6a−f which are tethered to a pentaalkyloxytriphenylene unit have been prepared by a five-step sequence from hexaalkyloxytriphenylenes 1a−f. The mesomorphic properties of 6a−f were studied by DSC, POM, and X-ray diffraction. Columnar rectangular mesophases were found for derivatives 6b−f with long alkyl chains. Detailed SAXS experiments revealed a change of the common Colr (C2/m) phase to Colr (P2m) phase with increasing chain length. Derivative 6a with pentyl side chain displayed only unidentified crystalline or plastic crystalline properties.

Liquid Crystalline Metal-Free Phthalocyanines Designed for Charge and Exciton Transport

A joint theoretical and experimental study of the electronic and structural properties of liquid crystalline metal-free phthalocyanines bearing a strong potential for charge and exciton transport has been performed. The synthesis of such compounds has been triggered by quantum chemical calculations showing that: (i) hole transport is favored in metal-free phthalocyanines by their extremely low reorganization energy (0.045 eV) and large electronic splittings; and (ii) the efficiency of energy transfer along the one-dimensional discotic stacks is weakly affected by rotational disorder due to the two-dimensional character of the molecules. We have synthesized two metal-free phthalocyanines with different branched aliphatic chains on the gram scale to allow for a full characterization of their solid-state properties. The two compounds self-organize in liquid crystalline mesophases, as evidenced by optical microscopy, differential scanning calorimetry, X-ray powder diffraction, and molecular dynamics simulations. They exhibit a columnar rectangular mesophase at room temperature and a columnar hexagonal mesophase at elevated temperature.

Liquid crystalline polymers containing discotic metallomesogens in the main chain

Metallomesogenic polymers having discotic units in the main chain have been synthesized using β-diketonates of Cu(II) and Pd(II) as monomers. The corresponding homologues of low molecular weight have also been synthesized as models of the polymers. All the materials prepared exhibit columnar mesophases. The model compounds exhibit a columnar hexagonal mesophase. In the case of the polymers, the more flexible polymers 2 a and 2 b show a columnar hexagonal mesophase whereas the more rigid polymers 1 a and 1 b exhibit a columnar rectangular mesophase. Fibers of these polymers have an orientation of the polymeric chains along the stretching direction and the disk-like units are arranged in columns perpendicular to the fiber axis.