Influence of branched chains on the mesomorphic properties of symmetrical and unsymmetrical triphenylene discotic liquid crystals

A series of novel symmetrical and unsymmetrical triphenylene‐based discotic liquid crystalline materials with one or six branched peripheral alkoxy chains have been prepared. These materials have been compared with analogous known symmetrical and unsymmetrical compounds to reveal a balance between steric and space‐filling effects of the peripheral branched chains, which significantly affects intermolecular forces of attraction and packing, and hence affects melting and isotropisation temperatures of the liquid crystalline materials. The desired result of reduction of melting points and enhancement of isotropisation temperatures has been accomplished by use of branched alkoxy chains in both symmetrical and unsymmetrical materials.     

δ‐Methyl Branching in the Side Chain Makes the Difference: Access to Room‐Temperature Discotics

Although discotic liquid crystals are attractive functional materials, their use in electronic devices is often restricted by high melting and clearing points. Among the promising candidates for applications are [15]crown‐5 ether‐based liquid crystals with peripheral n‐alkoxy side chains, which, however, still have melting points above room temperature. To overcome this problem, a series of o‐terphenyl and triphenylene [15]crown‐5 ether derivatives was prepared in which δ‐methyl‐branched alkoxy side chains of varying lengths substitute the peripheral linear alkoxy chains. The mesomorphic properties of the novel crown ethers were studied by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction. δ‐Methyl branching indeed lowers melting points resulting in room‐temperature hexagonal columnar mesophases. The mesophase widths, which ranged from 87 to 30 K for o‐terphenyls, significantly increased to 106–147 K for the triphenylenes depending on the chain lengths, revealing the beneficial effect of a flat mesogen, due to improved π–π interactions.     

Unsymmetrical alkoxy-substituted triphenylenes: the dependence of mesomorphism on molecular shape

Several series of unsymmetrical hexa-alkoxytriphenylenes have been prepared. For almost every series a maximum in the Colh-I clearing point is found for the most symmetrical triphenylene, i.e. when all six alkoxy chains are of equal length. A similar trend is found for the melting points (Tm), although the effect is not so pronounced. A minimum length is required for formation of a columnar mesophase (Col hd). After this critical value has been reached for short alkoxy chains, much lower Colh-I temperatures are then observed for longer chains. Ordered hexagonal columnar phases required for efficient photoluminescence and electroluminescence have been found for several asymmetrical homologues with comparable TCol-I to those observed for the symmetrical homologues, but also with lower Tm. This may facilitate the preparation of mixtures of triphenylenes with the desired ordered hexagonal columnar phases with a TCol-I and a Tm below room temperature.     

苯并菲盘状液晶的合成、亲氟效应及分子对称性对介晶性的影响

New symmetrical and asymmetrical triphenylene-containing discotic liquid crystals with two different peripheral alkyl chains, known as sym-TP(OC6H13)3(OR)3 and asym-TP(OC6H13)3(OR)3, were synthesized. Their thermotropic liquid crystal properties were investigated through polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses. The asyrranetdcal discogens are 2,6,11-rialkoxy-3,7,10-trihexyloxytriphenylenes, with the alkyl chain carbon numbers varying from 3-10, 12, and 14, while the symmetrical compounds are 2,6,10-trialkyloxy-3,7,11-trihexyloxytriphenylene. Two fluoroalkoxy substituted triphenylene discogens, 2,6,10-td(4,4,4-trifluorobutoxy)-3,7,11-trihexyloxytriphenylene and its asymmetrical isomer 2,6,11-tri(4,4,4-trifluorobutoxy)-3,7,10-trihexyloxytdphenylene were prepared. These two compounds show higher melting and clearing points than their alkoxy analogs, which implies that fluorophilic effect exists in the formation and stabilization of discotic columnar mesophase. The triphenylene derivatives TP(OC6H13)3(OR)3 with two different peripheral chains, symmetrically or asymmetrically attached on triphenylene cores, have lower melting points and clearing points than those of the higher symmetrical compounds TP(OR)6 with the same total chain carbon numbers. The mixed-chain-triphenylenes with longer alkoxy chains (n=9,10,12,14) show columnarmesophase at room temperature.     

Asymmetric banana-shaped liquid crystals with two different terminal alkoxy chains

Two series of asymmetric banana-shaped compounds have been synthesized and studied. In the 1,3-phenylene bis[4-(4'-alkoxybenzoyloxy)]benzoate series the lack of symmetry was derived solely from the difference in length of the two terminal alkoxy chains. In the 3,4'-biphenylene bis[4-(4'-alkoxybenzoyloxy)]benzoate series the asymmetric nature originates from the 3,4'-substitution of the central biphenyl group and from the difference in length of the two terminal chains. All the melting points of the asymmetrical compounds in the series with the central phenyl unit are lower than those of the symmetrical compounds. The liquid crystalline B₁ or B₂ phase was retained in all cases. In the series with the central biphenyl unit the compounds with the shortest chain attached to the para-position of the central biphenyl unit have the lowest melting points. A significant lowering of the melting points in comparison with the symmetrically substituted compounds, however, could not be achieved. All the compounds of both series show a layer spacing which is comparable to those of the symmetrically substituted parent compounds. The observed switching behaviour of both the symmetric and asymmetric compounds with a B₂ phase was antiferroelectric.     

Structural anisotropy study by resonance Raman scattering in nematics with different dyes

Ionic liquid crystals based on congruent ion pairs composed of mesogenic cations and anions of similar shape provide an attractive tool for the tuning of mesophase properties. Here, the effect of the number and symmetry of lipophilic side chains and the type of head group on the phase type and thermal mesophase properties was probed by the synthesis and investigation of two series of novel guanidinium and imidazolium sulphonates and compared with the corresponding iodides. Their mesomorphic properties were examined via differential scanning calorimetry, optical polarising microscopy and X-ray diffraction. While derivatives bearing only one alkoxy chain in either cation or anion with up to three alkoxy chains in total within the ion pairs display smectic A mesophases, hexagonal columnar mesophases were observed for all other compounds with four or five alkoxy chains totally irrespective of the head group. However, with increasing steric bulk, i.e. with a total of six alkoxy chains, the symmetry of the aryl moiety in the anion as well as the type of head group becomes relevant, resulting in hexagonal columnar or plastic phases for guanidinium sulphonates with symmetrical anions, while those with unsymmetrical anions were non-mesomorphic. In contrast, the corresponding imidazolium sulphonates displayed cubic phases for the combinations of symmetrical cation/symmetrical anion and symmetrical cation/unsymmetrical anion. If both ions are unsymmetrically substituted, the imidazolium sulphonate displayed a hexagonal columnar phase. The results further demonstrate the utility of the congruent ion pairs for tailor-made ionic liquid crystals.     

Fabrication of vertical alignment liquid crystal cell without forming a conventional polyimide-alignment layer

Two series of asymmetric banana-shaped compounds have been synthesized and studied. In the 1,3-phenylene bis[4-(4′-alkoxybenzoyloxy)]benzoate series the lack of symmetry was derived solely from the difference in length of the two terminal alkoxy chains. In the 3,4′-biphenylene bis[4-(4′-alkoxybenzoyloxy)]benzoate series the asymmetric nature originates from the 3,4′-substitution of the central biphenyl group and from the difference in length of the two terminal chains. All the melting points of the asymmetrical compounds in the series with the central phenyl unit are lower than those of the symmetrical compounds. The liquid crystalline B₁ or B₂ phase was retained in all cases. In the series with the central biphenyl unit the compounds with the shortest chain attached to the para-position of the central biphenyl unit have the lowest melting points. A significant lowering of the melting points in comparison with the symmetrically substituted compounds, however, could not be achieved. All the compounds of both series show a layer spacing which is comparable to those of the symmetrically substituted parent compounds. The observed switching behaviour of both the symmetric and asymmetric compounds with a B₂ phase was antiferroelectric.     

Liquid crystalline properties of salicylaldimine-based dimers: influence of terminal alkyl chain length and central part

The synthesis and thermotropic properties of four homologous series of salicylaldimine-based dimer liquid crystals are reported. Two 4-(4-alkoxy-2-hydroxybenzylideneamino)benzoyloxy groups are connected to a central part consisting of a 1,3-phenylene, 1,5-pentylene, 2,2-dimethyl-1,5-pentylene or 3,3-dimethyl-1,5-pentylene unit. The terminal alkoxy chains have been varied from 4 to 16 carbon atoms in length. All the compounds exhibit liquid crystalline phases whose behaviour depends on the nature of the central part and the length of the alkoxy terminal chains. All compounds of the series with the central phenyl part exhibit enantiotropic B-phases, and the sequence B₆-B₁-B₂ on increasing terminal chain length was observed. Replacement of the phenyl group with a pentyl central group partly suppresses the formation of B-phases. The longer homologues of this series show the B₁ phase, while the shorter exhibit an intercalated SmC_c mesophase. The introduction of methyl substituents to the pentyl spacer causes the melting points to fall dramatically and the formation of B-phases is totally suppressed. The compounds with the long tails show intercalated SmA_c phases and those with short tails show intercalated SmC_c phases.     

Influence of peripheral alkyl chain length on the mesomorphic behaviours of hexasubstituted triphenylene 2,3-dicarboxylic esters

A comparative study of our established synthetic approaches to hexasubsituted triphenylenes 2,3-dicarboxylic esters containing four identical β-alkoxy and two adjacent β-alkoxycarbonyl side chains shows that the phase behaviours of small-sized discotic liquid crystals can be tailored over a wide range by simply varying the length of the peripheral alkyl chains. All the prepared esters in two series were observed to form a single hexagonal columnar phase, except for Tp4-1 having four β-butyloxy and two adjacent β-methoxycarbonyl chains which displays two columnar mesophase behaviours with a transition from the columnar plastic phase to hexagonal columnar phase. A significant difference between the two mesophase was observed in the variable temperature X-ray diffraction studies, and the mesophase assignment was also confirmed by polarising optical microscopy and differential scanning calorimetry. Moreover, the prepared esters in each series display the general trend of decreasing clearing temperature upon increasing alkoxy or alkoxycarbonyl chains length. The intermediate triphenylene 2,3-dicarboxylic acids were also found not only to exhibit columnar hexagonal mesophase over a narrower temperature range by maintaining high melting and clearing points but also to form organogel on mixing with toluene or dichloromethane with the assistance of hydrogen bonding.     

Lowering melting points in asymmetrically substituted Salen-copper(II) complexes exhibiting mesomorphic behavior. Structure of the mesogen Cu(5-hexyloxySalen)

In comparison with their symmetrical analogues, unsymmetrically substituted Cu-Salen complexes show mesophases with lowered melting points. For terminally substituted complexes, symmetrical ones (R(1) = R(2)) have only an S(A) phase, while for unsymmetrical alkoxy substitution a monotropic S(E) phase occurs and the melting temperature decreases with no loss in mesophase stability. Lateral substitution, when it is symmetrical (R(3) = R(4)), lowers mesophase stability but not melting temperature, and when it is unsymmetrical, it greatly lowers both mesophase stability and melting temperature compared with the parent compound. Substitution at the imine carbon (R(5), R(6)) also lowers chemical stability (decomposition) of the compounds. The structure of the 5-hexyloxy complex (R(1) = R(2) = OC(6)H(13), R(3) = R(4) = R(5) = H) shows the pre-mesophasic arrangement likely adopted after melting.     

Investigations on liquid crystalline partially fluorinated alkyl and succinimidyl benzoates

The synthesis and mesophase properties of partially fluorinated alkoxy‐substituted benzoic alkyl and succinimidyl (NHS) esters with one, two and three perfluoroalkyl alkoxy chains are reported. The mesophases were studied using differential scanning calorimetry (DSC), polarizing optical microscopy and X‐ray diffraction of non‐oriented samples. The SmA phases of the one‐chain methyl esters are monotropic, while those of the one‐chain NHS esters are enantiotropic. The more wedge‐shaped two‐ and three‐chain alkyl esters do not form mesophases, whereas the succinimidyl analogues exhibit hexagonal columnar phases. Their enhanced mesophase stability is caused by the higher polarity of the succinimidyl ring with respect to the alkyl ester groups. Aggregation of the dipolar succinimidyl groups, together with the microsegregation of the lipophilic and fluorophilic segments of the partially fluorinated alkoxy chains, is assumed to lead to a threefold structured morphology in both the SmA and the Col_h phases. This threefold structuring can be regarded as analogous to known morphologies of ABC triblock copolymers.     

Synthesis and study the liquid crystalline properties of compounds containing benzoxazole core and terminal vinyl group

Terminal vinyl-based benzoxazole liquid crystalline compounds, 2-(3-fluoro-4?-alkoxy-1,1?-biphenyl ?4-yl)-5-(2-propenyloxymethyl)-benzoxazole (nPPF(2)BP), were synthesised and their structures were confirmed by infrared (IR) spectra, proton nuclear magnetic resonance (¹H-NMR) spectra, gas chromatography with electron impact-mass spectrometry (GC/EI-MS), matrix-assisted laser desorption/ionisation-time of flight (MALDI-TOF) mass spectrometry and elemental analysis (EA). The compounds show enantiotropic smectic/nematic phases with mesophase ranges are 71–97 °C and 87–136°C on heating and cooling processes for nPPF(2)BP, respectively. They give low melting points due to lateral fluoro substituent and flexible terminal 2-propenyloxymethyl chain. It is found that the compounds nPPF(2)BP with shorter alkoxy chain (n = 3, 4) exhibit a wide range of nematic mesophase, which is ascribed to enhanced π–π interaction caused by terminal vinyl moiety, whereas further elongation of the terminal alkoxy chain results in supressing nematic phase and increasing smectic mesophase. Compared with methyl terminated analogues, 2-propenyloxymethyl terminated compounds nPPF(2)BP display much lower melting points and wider or comparable mesophase range both in heating and cooling.     

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.     

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.     

Trimetallic Nickel–Lanthanum and Nickel–Gadolinium Metallomesogens

Adducts were formed between a mesomorphic Ni(salen) complex [salen=2,2′-N,N′-bis(salicylidene)ethylenediamine] with six terminal alkoxy chains and a lanthanide nitrate (Ln=La, Gd). Different alkoxy chain lengths were used: OC¹²H²⁵, OC¹⁴H²⁹, OC¹⁶H³³ and OC¹⁸H³⁷. Trinuclear nickel–lanthanum and nickel–gadolinium complexes [Ln(NO³)³{Ni(salen)}²] were obtained. The compounds exhibit a wide-temperature-range hexagonal columnar mesophase (Col^H) with rather low melting points. The mesophase stability ranges of both the parent nickel complexes and the nickel–lanthanide complexes decrease with increasing chain length. A decrease in the mesophase stability range over the lanthanide series was also observed. The results are compared with those of similar copper–lanthanide complexes. A marked difference is the higher thermal stability of the nickel–lanthanide complexes in comparison with the copper–lanthanide complexes.     

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.     

Dependence of Mesomorphic Behaviour of Methylene‐Linked Dimers and the Stability of the NTB/NX Phase upon Choice of Mesogenic Units and Terminal Chain Length

Twelve symmetrical dimeric materials consisting of a nonamethylene (C9) spacer and either phenyl 4‐(4′‐alkylphenyl)benzoate, phenyl 4‐(4′‐alkylcyclohexyl)benzoate or phenyl 4‐(4′‐alkylbicyclohexyl)carboxylate mesogenic units were prepared and their mesogenic behaviour characterised by POM, DSC and XRD. All of the materials exhibited nematic phases with clearing points in excess of 200 °C. Four compounds were found to exhibit the twist‐bend nematic phase, with one material exhibiting a transition from the N_TB phase into an anticlinic smectic ‘X’ phase. Across all three series of compounds the length of terminal chain is seen to dictate, to some degree, the type of mesophase formed: shorter terminal chains favour nematic and N_TB mesophases, whereas longer terminal aliphatic chains were found to promote smectic phases.     

Hydrogen-bonded block mesogens derived from semiperfluorinated benzoic acids and the non-mesogenic 1,2-bis(4-pyridyl)ethylene

Thermal properties of benzoic acids carrying one or two semiperfluorinated alkoxy tails on the aromatic core have been investigated in binary mixtures with the non-liquid crystalline bidirectional trans-1,2-bis(4-pyridyl)ethylene. The hydrogen bonded complexes built from the complementary molecular species show a significantly enhanced mesophase stability compared with the fluorinated acids in their pure states. The mesophase morphologies of the complexes are governed mainly by the number of the partially fluorinated chains grafted to the acid component. Mixed systems comprising the one-chain acids exhibit a smectic C phase followed by a smectic A phase at more elevated temperatures. Incorporation of a second semiperfluorinated chain into the acid leads to the formation of columnar mesophases. These columnar phases of the H-bonded complexes should represent ribbon phases resulting from the collapse of the smectic layers.     

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.     

Unusual thermal behaviour of mesogens containing an N,N-dialkylaminomethylene lateral substituent

An N,N-dialkylaminomethylene lateral substituent can be used to reduce the melting temperatures of nematogens having a long rigid core. However, compared with a single lateral alkoxy chain, this lateral substituent does not enhance the thermodynamic stability of the mesophase. DSC curves and optical microscopy show that after melting, a new solid appears in the liquid crystal phase or in the isotropic phase of these compounds. NMR spectra of the nematic phase and of the solid phase show that a thermal reaction releases dialkylamine into the medium and produces a disubstituted 2 H -indazole in rather good yield. Further heating of each individual mixture leads to a second wide range enantiotropic nematic phase. The purified disubstituted 2 H-indazole has a larger nematic range and the nematic phase is stable at high temperature, indicating that the 2 H -indazole motif can be used to build new mesogenic structures. The temperature dependence of the 13C chemical shifts of one of the initial compounds was obtained. The results indicate that the two chains on the nitrogen atom are equivalent and more or less folded back in the same way along the mesogenic core.