Phase behaviour of star-shaped binary mixtures of triphenylbenzenes,triphenylboroxines and triphenyltriazines

ABSTRACT

Star-shaped nonaalkoxy-1,3,5-triphenyltriazines were prepared and their mesomorphism was investigated by differential scanning calorimetry (DSC), polarised optical microscopy (POM) and X-ray diffraction (WAXS and SAXS). Compounds with chain lengths C₉ – C₁₂ displayed enantiotropic hexagonal columnar mesophases over a wide range of temperature. Their similarity in molecular size and mesomorphism prompted us to study their binary mixtures with previously reported planar 1,3,5-triphenylboroxines and propeller-shaped 1,3,5-triphenylbenzenes. All three compounds are fully miscible in their hexagonal columnar mesophases, but mixtures of planar with propeller-shaped compounds generated columnar stacks that contain only one type of molecule to avoid their shape incompatibility. However, the two types of columnar stacks were fully miscible which circumvented macroscopic phase separation.     

CPI induction of liquid crystal behaviour in triphenylenes with a mixture of hydrophobic and hydrophilic side chains

2,3,6,7,10,11-Hexasubstituted triphenylenes have been synthesized that contain a mixture of hydrophobic (C₆H₁₃O) and hydrophilic (CH₃OCH₂CH₂OCH₂CH₂O) side chains. At one extreme HAT6 (1a) (six hydrophobic chains) shows thermotropic behaviour and at the other TP6EO2M (1e) (six hydrophilic chains) shows lyotropic behaviour. Of the triphenylenes with a mixture of hydrophobic and hydrophilic side chains, only the triphenylene with one hydrophilic side chain and five hydrophobic side chains (1b) gives a thermotropic columnar phase. None of the others show liquid crystal behaviour. However, all of these triphenylenes form binary 1:1 compounds when mixed with PDQ9 (2a) and with PTP9 (2b). These CPI (complimentary polytopic interaction) stabilized compounds give thermotropic hexagonal columnar phases over wide temperature ranges.     

Investigating the effects of side chain length on the AIE properties of water-soluble TPE derivatives

The emissive properties of fluorophores in aggregated state are important for the development of bio-sensors or bio-imaging reagents. So three water-soluble TPE derivatives with different lengths of side chains have been synthesized and we investigated the effects of side chains on aggregation-induced emission (AIE) properties in the aggregated states. The results indicate that side chains on the fluorophores play a pivotal role in their emission in aggregated state mediated by heparin or solid state, because the coplanarity of these TPE derivatives was affected by side chains. The rates of radiative decay k_f and non-radiative decay k_nr have been obtained through the quantum yields and lifetime, and a larger k_f and smaller k_nr were present for compound TPE-C4N, suggesting that the aggregated TPE-C4N should posses the most remarkable fluorescent property.     

Steric effect on the formation of columnar phases in β-diketonate copper(II) complexes

A systematic study of the mesomorphic properties of three series of copper(II) complexes based on β-diketonate ligands containing branched side chains is reported. These disc-like compounds have four, six and eight flexible alkoxy side chains appended to the central core, in which two or four side chains were substituted by bulkier secondary alkoxy groups: 1-methylbutyloxy R ' = C₅(2°) or 1-methylheptyloxy R ' = C₈(2°). The mesomorphic results indicated that at least eight side chains are required to form stable columnar mesophases; other compounds with four or six side chains are not mesogenic regardless of the combination of the carbon length on the alkoxy or secondary alkoxy groups of the side chains. The compounds 3 with shorter R ' = C₅(2°) side chains were all non-mesogenic regardless of the carbon length of three alkoxy side chains (R = C₈, C₁₀, C₁₂) used. However, when the longer 1-methylheptyloxy side chain R ' = C₈(2°) was substituted, the compounds 3b-3e with various alkoxy groups (R = C₆, C₇, C₈, C₁₀, C₁₂) exhibited columnar phases. The mesophases were characterized and identified as columnar hexagonal phases (Col_h), as expected, by thermal analysis and optical polarized microscopy. The presence of the introduced secondary alkoxy groups apparently appeared to influence the formation of columnar phases. The clearing points were relatively lower than other similar copper(II) compounds not substituted by secondary alkoxy side chains.     

Ionic liquid crystals with 1,2,3-triazole + tolane core and a pendant imidazolium unit: mesophases from one- to three-dimensional molecular organisation by the variation of alkoxy chains number and spacer length

Ionic liquid crystals (ILCs) are a class of materials that present a large and varied form of applicability and whose properties can change dramatically with small modifications in their molecular structure. It is, therefore, of great importance to improve the knowledge about the property–molecular structure relationship. In this study, we present the synthesis and characterisation of 7 new rod- and wedge-shaped ILCs, whose liquid crystalline behaviour variation is related to the variations in the number of alkoxy chains and/or in the spacer chain length. All compounds have a rigid core constituted by a tolane group and a 1,2,3-triazole heterocycle, which is connected to a pendant 1-methylimidazolium through a alkyl spacer chain. The mesomorphic properties were studied by DSC, thermogravimetric analysis, polarising optical microscopy, wide angle X-ray diffraction and SAXS with synchrotron radiation. The increase in the number of long alkoxy chains changed the molecular self-assembly from bilayered SmA₂ and SmC₂ to hexagonal columnar, passing through oblique columnar, bicontinuous cubic and hexagonal close-packed mesophase. On the other hand, the increase in the alkyl spacer chain length has an especially significant effect for the compounds with two terminal alkoxy chains, where a Col_o/Col_h – Cub_v – HCP phase sequences were observed.     

Mesomorphic properties of cyanobiphenyl dimers with a central malonate unit

A series of cyanobiphenyl dimers attached via alkoxy spacers to a central malonate were prepared, and the mesomorphic behaviour was studied by differential scanning calorimetry (DSC), polarised optical microscopy and X-ray diffraction (Wide-angle X-ray scattering and Small-angle X-ray scattering). Depending on spacer lengths and substitution of the malonate, nematic and smectic A mesophases with pronounced odd–even effect were observed. C-2-unsubstituted malonates formed nematic phases for chain lengths C₆–C₁₄, while C₁₂ and C₁₄ homologues displayed additional smectic A phases. In contrast, malonates with fluorinated tails at C-2 displayed exclusively smectic A phases. Remarkably, the X-ray diffraction profile of the smectic A phase of the C-2-unsubstituted C₁₂ malonate showed a fundamental (001) and the corresponding third-order (003) diffraction peak, but no (002) reflection. Using Fourier analysis, the diffraction pattern was converted to an electron density profile, which was in good agreement with the proposed packing model of the SmA mesophase based on a horseshow- or hairpin-like conformation of the malonate.     

Ionic Liquid Crystals Formed by Self‐Assembly around an Anionic Anthracene Core

We have designed and synthesised a series of modular, mesogenic complexes based on anthracene‐2,6‐disulfonate and trialkoxybenzyl‐functionalised imidazolium cations. Each complex contains a central, rigid, dianionic anthracene core and two flexible monocations bearing paraffin chains anchored on imidazolium rings. Anthracene‐2,6‐disulfonate can be crystallised with various simple alkylammonium ions and, in the case of ⁺N(CH₃)₂(C₁₆H₃₃)₂, a crystal structure determination has shown that the long paraffinic chains are intercalated between the anthracene moieties. The dianion forms columnar mesophases with trialkoxybenzylimidazolium cations, as identified by polarising optical microscopy and X‐ray scattering measurements. Differential scanning calorimetry studies confirmed mesomorphic behaviour from room temperature to about 200 °C for alkyl chains containing 8, 12 and 16 carbon atoms. The strong luminescence of anthracene is maintained in the mesophase and fluorescence measurements confirmed the presence of J aggregates in all cases. The new functional materials described herein provide an easy access to stable and luminescent mesomorphic materials engineered by an ionic self‐assembly process.     

具有不同柱状结构的苯并菲盘状液晶化合物的合成及自组装

合成了3种含有不同长度烷基链的苯并菲盘状液晶化合物; 通过1H NMR 和 MALDI-TOF MS对其结构进行了表征; 利用差示扫描量热法(DSC)、热台偏光显微镜(POM)和小角X射线散射实验(SAXS)对3种液晶化合物的自组装行为进行了研究. 结果表明, 烷基链的长度对苯并菲盘状液晶化合物自组装结构的影响显著. 柔性链为辛基的苯并菲盘状液晶化合物自组装成六方柱状液晶相; 柔性链为十二烷基的化合物自组装成倾斜柱状液晶相; 而柔性链为十六烷基的化合物则未形成液晶相.     

AIE Fluorescent Gelators with Thermo‐, Mechano‐, and Vapochromic Properties

A series of aggregation‐induced emission (AIE) fluorescent gelators (TPE‐C_n‐Chol) were synthesized by attaching tetraphenylethylene (TPE) to cholesterol through an alkyl chain. The properties of the gel, nano‐/microaggregate, and condensed phases were studied carefully. TPE‐C_n‐Chol molecules form AIE fluorescent gels in acetone and in DMF. Their fluorescence can be reversibly switched between the “on” and “off” states by a gel–sol phase transition upon thermal treatment. The AIE properties of aggregated nano‐/microstructures in acetone/water mixtures with different water fractions were studied by using fluorescence spectrometry and scanning electron microscopy (SEM). In different acetone/water mixtures, the TPE‐C_n‐Chol molecules formed different nano‐/microaggregates, such as rodlike crystallites and spherical nanoparticles that showed different fluorescence colors. Finally, the condensed phase behavior of TPE‐C_n‐Chol was studied by using polarizing microscopy (POM), differential scanning calorimetry (DSC), fluorescence spectrometry, fluorescence optical microscopy, and wide‐angle X ray scattering (WAXS). The clover‐shaped TPE unit introduced into the rodlike cholesterol mesogen inhibits not only the formation of a liquid‐crystal phase but also recrystallization upon cooling from the isotropic liquid phase. Very interestingly, TPE‐C_n‐Chol molecules in the condensed state change their fluorescence color under external stimuli, such as melting, grinding, and solvent fuming. The phase transition is the origin of these thermo‐, mechano‐, and vapochromic properties. These findings offer a simple and interesting platform for the creation of multistimuli‐responsive fluorescent sensors.     

An example of an unsymmetrically substituted,monofunctionalized anthraquinone-based discotic liquid crystal

We report here the synthesis and characterization of an unsymmetrically substituted, monofunctionalized anthraquinone-based discotic liquid crystal, namely 5-hydroxy-1-(4-nitrobenzyloxy)2,3,6,7-tetrapentyloxy-9,10-anthraquinone. Interestingly, this compound, which has only four alkyloxy chains attached to the anthraquinone core, shows a hexagonal columnar phase (Col_h) of much wider temperature range compared with analogues having six alkyloxy chains. Incidentally, among the anthraquinone-based disc-like molecules exhibiting columnar phases known so far, this is the first molecule, which does not possess any C₂-axis of symmetry. Besides its unsymmetrical structure and interesting mesomorphic behaviour, monofunctionalized monomers of this kind, unlike the available C₂-symmetric difunctionalized tetraethers, can serve as potential synthons for obtaining dimers in a unique way. In addition, various anthraquinone-based oligomers, metallomesogens, and side group liquid crystalline polymers can be obtained from these monomers.     

Luminescent Ionic Liquid Crystals from Self‐Assembled BODIPY Disulfonate and Imidazolium Frameworks

A series of modular mesogenic salts based on the combination of anionic 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (F‐BODIPY) 2,6‐disulfonate dyes and trialkoxybenzyl‐functionalised imidazolium cations has been designed and synthesised. Each salt contains a rigid dianionic BODIPY core associated with two imidazolium cations functionalised by 1,2,3‐trialkoxybenzyl (alkyl=n‐C₈, n‐C₁₂ or n‐C₁₆) units or, in one case, with imidazolium cations functionalised by a trialkylgallate (3,4,5‐trialkoxybenzoate) unit in which the 3,5‐dialkyl groups are terminated with a polymerisable acrylate entity. All these compounds were highly fluorescent in solution with quantum yields ranging from 54 to 62 %. In the solid state, the width of the emission band observed at around 650 nm is a clear signature of aggregation. With the trialkoxybenzylimidazolium cations, polarised optical microscopy (POM) and X‐ray scattering experiments showed that columnar mesophases were formed. Differential scanning calorimetry (DSC) studies confirmed the mesomorphic behaviour from room temperature to about 130 °C for salts with alkyl chains containing 8, 12 and 16 carbon atoms. The strong luminescence of the BODIPY unit was maintained in the mesophase and fluorescence measurements confirmed the presence of J aggregates in all cases. The salt containing the gallate‐functionalised imidazolium cations showed no mesomorphism but the acrylate terminal units could be used to engender photoinitiated polymerisation thereby allowing the material to be immobilised on glass plates. The polymerisation process was followed by FTIR spectroscopy and the fixed and patterned films were highly fluorescent with a solid‐state emission close to that of the complex in the solid state.     

Polycatenar liquid crystals based on bent-shaped chalcone and cyanopyridine molecules

In this study, the synthesis, structural characterisation and mesomorphic and optical properties of seven new bent-shaped and polycatenar bent-shaped compounds derived from chalcone and cyanopyridine are reported. The mesomorphic behaviour was investigated by differential scanning calorimetry (DSC), polarised optical microscopy (POM) and X-ray diffraction (XRD) and correlated with the molecular structure. Two bent-core hexacatenars molecules (Ic and IIc) presented liquid crystalline properties, showing a hexagonal columnar (Col_h) phases at room temperature, being each disc constituted by two mesogens. Optical studies were also performed for the final molecules, being conducted by ultraviolet-visible and fluorescence spectrometry. The cyanopyridine derivatives show moderate luminescence quantum yields, ranging between 18% and 27%, with emission maxima around 371 nm. It is also shown that while the chalcone central unit favours a calamitic liquid crystalline behaviour in molecules with lower number of aliphatic chains, a polycatenar structure with cyanopyridine as the central unit favours a Col_h arrangement, also providing luminescence properties to the molecule.     

Peripheral chain length-dependent complex phase behaviour in C3-symmetric hexa-alkylated liquid crystals

We report the thermal and self-assembly properties of C₃-symmetric liquid crystalline (LC) molecules consisting of a conformationally tunable triazole-based mesogen and six-fold alkyl chains. Unlike the LC compound (1) based on non-crystallisable octyl chains, 2 and 3, which have crystallisable dodecyl and tetradecyl chains, respectively, exhibit a cold crystallisation which only takes place under slow heating conditions (2°C/min). In contrast with the vertically interdigitated lamellar crystalline phase of 1, a laterally interdigitated bilayered lamellar structure driven by the crystallisation of the dodecyl or tetradecyl chains is observed in the cold crystallisation temperature range. In addition to their crystalline morphology, 2 and 3 show LC morphological behaviour distinct from that of 1, 2 and 3 exhibit a hexagonal columnar LC phase consisting of T-shaped conformers rather than the lamellar LC phase of 1. The morphological transformation from the lamellar (1) to the columnar phase (2 and 3) can be rationalised by the alleviation of the conformational energy of the longer alkyl chains. Consequently, the simple variation of alkyl chain length in the C₃-symmetric LC system results in contrasting thermal and assembly properties in the crystalline and LC phases.     

Tyrosine‐Based Ionic Liquid Crystals: Switching from a Smectic A to a Columnar Mesophase by Exchange of the Spherical Counterion

Synthetic strategies were developed to prepare l ‐tyrosine‐based ionic liquid crystals with structural variations at the carboxylic and phenolic OH groups as well as the amino functionality. Salt metathesis additionally led to counterion variation. The liquid‐crystalline properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X‐ray diffraction (WAXS, SAXS). The symmetrical ILC chlorides bearing the same alkyl chain at both the ester and ether but either an acyclic or cyclic guanidinium group displayed enantiotropic SmA₂ mesophases with phase widths of 31–88 K irrespective of the head group. It was particularly the replacement of chloride in the acyclic guanidinium ILC by hexafluorophosphate that induced a phase change from SmA₂ to Col_r. This phase change was attributed to a higher curvature of the interface due to the larger anion, which increased the effective head group cross‐sectional area of the amphiphilic ILC. The unsymmetrical acyclic guanidinium chlorides, bearing a constant C₁₄ ester and variable alkyl chains on the phenolic position, formed enantiotropic SmA₂ phases. The derivative with the largest difference in chain lengths, however, displayed a Col_r phase, resulting from discoid aggregates of the cone‐shaped guanidinium chloride. The results are discussed in terms of the packing parameters, which indicate that the phase behaviour of the thermotropic tyrosine‐based ILCs shows analogies to those of lyotropic liquid crystals.     

Bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments: synthesis and characterization of mesomorphic behaviour

A series of bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments have been synthesized and their mesomorphic behaviour characterized. In an attempt to understand structure–property relationships, the lengths of the terminal alkoxy chains have been varied from C₂ to C₁₂, C₁₆ and C₁₈, resulting in 13 new bent-core V-shaped molecules. The thermal behaviour of these new compounds has been investigated by optical microscopy, calorimetry and X-ray diffraction studies. In general the compounds show conventional mesophases similar to those shown by calamitic LCs. The materials exhibit good thermal stability, even though their melting and clearing transition temperatures are high as a result of the presence of intramolecular hydrogen bonding between the H-atom of the hydroxyl group and the N-atom of the imine functionality. The phase appearing in the first member of the series with ethoxy chains is a nematic, while the C₃ to C₆ derivatives exhibit a smectic A phase as well as a nematic phase. The higher homologues, C₇ to C₁₂, C₁₆ and C₁₈, show only the smectic A phase. X-ray studies reveal that the SmA phase has a partially bilayer (interdigitated) structure. Remarkably, in some cases, the smectic A phase supercools well below room temperature. It is apparent from our studies that increasing the length of the alkoxy chains promotes smectic behaviour, in agreement with the general observation made for such bent-core molecules.     

Discotic Metallomesogens: Effects of Sidechains Density in Transition Metal Bis(β-Diketonate) Complexes

The preparation and mesogenic properties of a series of discotic β-diketonate metal complexes are reported. The results show that the density of side chains, positions of side chains, and the geometries of the metal centers play important roles in determining the mesomorphic behaviors and thermodynamic stability of these complexes. In the series of copper complexes 3, all these disc-like molecules with eight alkoxy side chains exhibit columnar hexagonal disordered (D_hd) mesophases. In the series of copper complexes 2 with six side chains, only compounds substituted with longer alkoxy chains (n = C₁₄ or C₁₆) exhibit discotic columnar mesophase. However, in the series of complexes 1, only crystal-to-isotropic transitions were observed. The results showed that induction of liquid crystallinity not only depends on the numbers of side chains (i.e. side chain density), but also on the degree of distribution over the central core. Palladium complexes analogs exhibit similar discotic mesophases, and due to their greater core-core organization, they also have higher clearing points and wider temperature range of mesophases than copper complexes.     

Tunable Phosphorescence/Fluorescence Dual Emissions of Organic Isoquinoline-Benzophenone Doped Systems by Alkoxy Engineering

Dual/multi-component organic doped systems with room-temperature phosphorescence (RTP) properties have been developed. However, the unknown luminescence mechanism still greatly limits the development of the doped materials. Herein, a new doped system exhibiting phosphorescence/fluorescence dual emission (Φ_phos=4–24 % and τ_phos=101–343 ms) is successfully constructed through prediction and design. A series of isoquinoline derivatives with different alkoxy chains were selected as the guests. Benzophenone was chosen as the host owing to the characteristics of low melting point and good crystallinity. The alkoxy chain lengths of the guests are first reported to be used to control the fluorescence and phosphorescence intensities of the doped materials, which results in different prompt emission colors. Additionally, the doped ratio of the guest and host can also control the luminous intensities of the materials. In particular, the doped materials still exhibit phosphorescent properties even if the ratio of the guest/host is as low as 1:100 000.     

Synthesis,Structure, and Physico‐optical Properties of Manganate(II)‐Based Ionic Liquids

Several ionic liquids (ILs) based on complex manganate(II) anions with chloro, bromo, and bis(trifluoromethanesulfonyl)amido (Tf₂N) ligands have been synthesized. As counterions, n‐alkyl‐methylimidazolium (C_nmim) cations of different chain length (alkyl=ethyl (C₂), propyl (C₃), butyl (C₄), hexyl (C₆)) were chosen. Except for the 1‐hexyl‐3‐methylimidazolium ILs, all of the prepared compounds could be obtained in a crystalline state at room temperature. However, each of the compounds displayed a strong tendency to form a supercooled liquid. Generally, solidification via a glass transition took place below ?40 °C. Consequently, all of these compounds can be regarded as ionic liquids. Depending on the local coordination environment of Mn²⁺, green (tetrahedrally coordinated Mn²⁺) or red (octahedrally coordinated Mn²⁺) luminescence emission from the ⁴T(G) level is observed. 1 The local coordination of the luminescent Mn²⁺ centre has been unequivocally established by UV/Vis as well as Raman and IR vibrational spectroscopies. Emission decay times measured at room temperature in the solid state (crystalline or powder) were generally a few ms, although, depending on the ligand, values of up to 25 ms were obtained. For the bromo compounds, the luminescence decay times proved to be almost independent of the physical state and the temperature. However, for the chloro‐ and bis(trifluoromethanesulfonyl)amido ILs, the emission decay times were found to be dependent on the temperature even in the solid state, indicating that the measured values are strongly influenced by nuclear motion and the vibration of the atoms. In the liquid state, the luminescence of tetrahedrally coordinated Mn²⁺ could only be observed when the tetrachloromanganate ILs were diluted with the respective halide ILs. However, for [C₃mim][Mn(Tf₂N)₃], in which Mn²⁺ is in an octahedral coordination environment, a weak red emission from the pure compound was found even in the liquid state at elevated temperatures.     

Mesophase behaviour and thermal stability of octa-alkoxy substituted phthalocyaninatocobalt (II) complexes

Cobalt (II) phthalocyanines substituted with eight alkoxy chains in the peripheral (2, 3, 9, 10, 16, 17, 23, 24) positions were prepared. The alkoxy chain length was varied between n-butyloxy (C₄H₉O) and n-octadecyloxy (C₁₈H₃₇O). Studies by polarizing optical microscopy and high temperature X-ray diffraction revealed that all the complexes are liquid crystalline and that they exhibit a hexagonal columnar mesophase (Col_h). Transition enthalpies were determined by differential scanning calorimetry. The clearing point could only be observed for compounds with a chain length longer than C₁₃H₂₇O. Both the melting and clearing points decrease with increasing chain length. The transition temperatures of these discotic metallomesogens are higher than those of the corresponding metal-free phthalocyanines, but are comparable with those of the corresponding copper (II) compounds. The thermal decomposition of the compounds was studied by thermogravimetry.     

Effects of L-tyrosine esters on quenching and photosensitized charge separation using polymers containing ruthenium(II) complex

A series of ruthenium(II) complex-containing partially quaternized poly(1-vinylimidazole)s with various alkyl side chains such as hexyl (C₆RuQPIm), dodecyl (C₁₂RuQPIm), and hexadecyl (C₁₆RuQPIm) were synthesized. The effects of L -tyrosine esters with hexyl (C₆Tyr), octyl (C₈Tyr), and dodecyl (C₁₂Tyr) on the quenching with methylviologen and photosensitized charge separation reactions were investigated using these metallopolymers as polymer photosensitizers. The quenching reaction took place through both a dynamic quenching process and a static quenching one mediated by the L -tyrosine esters. The kinetic parameters for these processes were obtained from a computed curve fitting using a Stern–Volmer equation derived from a combination of dynamic quenching and static quenching. The parameters had a significant dependence on the lengths of the alkyl groups in the L -tyrosine esters and the alkyl side chains on these metallopolymers. During photosensitized charge separation, the reaction proceeded through these quenching processes, referred to as direct and mediated processes. The initial rates of methylviologen radical formation also depended on these lengths; they increased when the lengths of the alkyl side chain on these metallopolymers and alkyl groups in the L -tyrosine esters were long. Such a dependence was caused by a change in the balance of some effects such as the electrostatic and steric effects. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4360–4367, 1999