Enantiotropic Smectic Mesophases Research Articles

Molecular Order of Topologically Variant Flexible Mesogens by 13C Nuclear Magnetic Resonance.

13C nuclear magnetic resonance investigations in the nematic phase of mesogens comprising a rod-like core with three phenyl rings connected to a fourth phenyl ring via a flexible spacer are reported. The molecules are abbreviated as monomer, dimer, and trimer as they comprised one, two, and three pairs of core and spacer combinations linked to ring IV, respectively. Hot-stage optical polarizing microscopy and differential scanning calorimetry studies confirmed that all of them exhibit an enantiotropic nematic phase with additional monotropic or enantiotropic smectic mesophases. Large values of 13C-1H dipolar couplings of the order of 11 kHz are observed for all the cases for the terminal carbon C1 of the core unit. These high values indicated that the corresponding CH vector is collinear with the long axis of the molecule, which itself is aligned parallel to the magnetic field. In contrast, the terminal carbon of the ring IV (C19/C17) exhibits a relatively smaller value in the range of 2.0-2.5 kHz, reflecting the divergent local dynamics at different sections of the mesogens. The orientational order parameters of the phenyl rings computed from the 13C-1H dipolar couplings have been used to obtain the conformation of the mesogens in the nematic phase. It is concluded that the dimer and trimer exhibit C2 and C3 symmetry with the ring IV connected by spacers tilted away from the symmetry axis by 35.9 and 90° for the two cases, respectively. This leads to the interesting tripod-like molecular shape for the trimer in the nematic phase rather than the planar representation of the λ shape.

Preparation and properties of 1-methyl-1H-benzimidazole-based mesogenic compounds incorporating ethynyl moiety

ABSTRACT Heterocyclic mesogenic compounds are one of the interesting liquid crystals. Here, a series of 1-methyl-1H- benzimidazole-based mesogenic compounds incorporating ethynyl moiety, 2-[4-[2-[4-(alkoxy)phenyl]ethynyl]phenyl]-1-methyl-1H-1,3-benzimidazole derivatives (nPEPMx) with terminal groups of H, CH3 and NO2 (coded as nPEPMH, nPEPMM and nPEPMN, respectively), are synthesised and their properties are evaluated. It is noted that the compounds nPEPMx with short alkoxy chain less than 9 carbon atoms display enantiotropic nematic mesophases, whereas corresponding reference compounds without ethynyl moiety have enantiotropic smectic mesophases. The mesophase ranges of compounds nPEPMx 50–70°C and 55–108°C on heating and cooling for nPEPMH, 76–106°C and 98–142°C for nPEPMM, and 78–132°C and 130–166°C for nPEPMN, respectively. Besides, compounds nPEPMx show obviously large birefringence of 0.501–0.641, suggesting a potential application in liquid crystal mixture. The high birefringence may result from its large π-conjugated molecule structure incorporating ethynyl and benzimidazole π-conjugated mesogenic unit.

Palladium (II) Catalyzed Arylation and Methylene Oxidation of 2, 7‐Dibromo Fluorenes with Heteroaryl Esters: Synthesis of Mesogenic2‐Heteroaryl and 2, 7‐Diheteroaryl‐9‐fluorenones

AbstractA facile and unusual palladium catalyzed synthesis of 2‐aryl and 2,7‐diheteroaryl‐9‐fluorenones via a one‐pot CH‐activation‐coupling‐oxidation of 2‐bromo and 2,7‐dibromofluorenes with heteroaryl esters have been achieved. Plausible mechanism for the formation of title compounds invoking palladium mediated direct arylation followed by methylene oxidation is provided. Significantly, among the synthesized compounds, 2,7‐dithiophene‐9‐fluorenones showed enantiotropic smectic mesophases and also optical emission.

Preparation and mesomorphic properties of 1-methyl-1H-benzimidazole-based compounds

ABSTRACTA series of 1-methyl-1H-benzimidazole-based compounds, 2-(4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-1-methyl- 1H-1,3-benzimidazole derivatives (nPPMx-M) with terminal hydrogen, methyl and nitro moieties (coded as nPPMH-M, nPPMM-M and nPPMN-M, respectively), were prepared and their structures were characterised. The compounds display enantiotropic smectic mesophases for hydrogen and methyl terminated compounds (nPPMH-M and nPPMM-M), and enantiotropic nematic mesophases for nitro terminated compounds (nPPMN-M) with short alkoxy chain below than 10 carbon atoms, where the mesophase ranges are 24–72°C and 74–104°C on heating and cooling processes for nPPMH-M, 90–119°C and 110–135°C for nPPMM-M, and 102–129°C and 113–207°C for nPPMN-M, respectively. It is noted that the compounds nPPMx-M exhibit much lower melting points and much wider mesophase range both in heating and cooling than non-1-methyl substituted analogs, which are ascribed to the disruption of hydrogen bonding among the molecules caused by methyl substitution at 1-position of benzimidazole. Meanwhile, among the compounds nPPMx-M, much wider mesophase ranges are obtained for nPPMM-M and nPPMN-M, indicating a much high mesophase stability for the compounds bearing terminal moiety (CH3 and NO2).

Preparation and characterisation of laterally monofluorinated mesogenic benzimidazole-based compounds

ABSTRACTSeries of laterally monofluorinated compounds, 2-(4ʹ-alkoxy-3-fluorobiphenyl-4-yl)-1H-benzimidazole derivatives (nPPF(3)Mx) bearing different substituents (H, CH3, NO2, coded as nPPF(3)MH, nPPF(3)MM and nPPF(3)MN, respectively) at 5-position, were prepared and their structures were characterised. According to the results from differential scanning calorimetry and polarising optical microscopy, the present compounds nPPF(3)Mx exhibit enantiotropic smectic mesophases, for which the mesophase ranges are 13–67 and 47–111°C on heating and cooling for nPPF(3)MH, 84–112 and 126–154°C for nPPF(3)MM, and 23–102 and 49–117°C for nPPF(3)MN, respectively. Compared to non-fluorinated analogues, monofluorinated nPPF(3)Mx have low melting/clearing points and display enhanced mesophase range both in heating and cooling, which are attributed to the disruption of the side-to-side intermolecular packing caused by the ortho lateral fluoro substituents and the increased dipole–dipole interaction between the polar fluoro-substituted molecules, respectively. It is noted that nPPF(3)MM and nPPF(3)MN show a much wider mesophase range than nPPF(3)MH, which suggest that the substituent at benzimidazole moiety can improve the mesophase stability.

Benzoate liquid crystals with direct isotropic–smectic transition and antipathogenic activity

A smectogen liquid crystal based on benzoate units has been synthesized and structurally characterized by FTIR and 1 H NMR spectroscopy. Besides, its structure and supramolecular arrangement in the crystalline state was demonstrated by single crystal X-ray diffraction measurements. The thermotropic behaviour, monitored by differential scanning calorimetry and polarized light microscopy, consists in the formation of an enantiotropic smectic mesophase with a direct first order transition from the isotropic to smectic mesophase, and with its thermal stability range superposed on human body temperature. The smectogen liquid crystal has moderate wettability – suitable for biocompatible materials and presents good antipathogenic activity against gram positive and gram negative bacteria and fungus. All these properties recommend the understudied liquid crystal to be used in biochemical and biological applications.

New Mesogenic Compounds Containing a Terminal-Substituted Benzoxazole Unit

A series of novel mesogenic 2-(4-alkoxyphenyl-1-yl)-benzoxazole derivatives bearing different substituents (H, NO2, CH3, Cl, coded as nPB-H, nPB-N, nPB-M, and nPB-C, respectively) at the 5-position were prepared and characterized. nPB-N, nPB-M, and nPB-C exhibited enantiotropic smectic mesophases with the mesophase ranges 3 °C–32 °C and 3 °C–82 °C on heating and cooling processes, whereas nPB-H showed no mesophases. The substituents with the stronger electron withdrawing effect let to the wider mesomorphic temperature domain. The nPB-M, nPB-C, and nPB-H displayed intense emission in CH2Cl2 solutions with λmax peaks of the photoluminescence spectra at 350–355 nm when excited at their absorption maxima.

Synthesis and mesomorphic properties of 2-(4′-alkoxybiphenyl-4-yl)-1H-benzimidazole derivatives

Three series of 2-(4′-alkoxybiphenyl-4-yl)-1H-benzimidazole derivatives (nM-x), which possessed 5-nitrobenzimidazole (nM-N series), benzimidazole (nM-H series) or 5-methylbenzimidazole (nM-M series) units at the end of the molecule, were synthesised and characterised by infrared, 1H- and 13C-nuclear magnetic resonance spectra, electrospray ionisation-mass spectrometry and elemental analysis. Their phase transition behaviour was investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. All the compounds exhibited enantiotropic smectic mesophases with wide temperature domains for a carbon number in the alkoxy chain from 6 to 16, where the mesophase ranges were 14–91°C and 17–99°C during heating and cooling processes for the nM-N compounds, 7–25°C and 8–49°C for the nM-H compounds and 48–81°C and 52–85°C for the nM-M compounds, respectively. The effect of the length of alkoxy chain on mesomorphic properties was discussed. The nM-N and nM-M exhibited a much wider mesophase range whether during heating or cooling process than the corresponding nM-H series, especially for the longer terminal chain (n > 8), which indicated that the substituent in the benzimidazole moiety was helpful in increasing the mesophase stability.

Synthesis and properties of substituted benzoxazole-terminated liquid crystals

The synthesis and characterisation, and mesomorphic and photophysical properties of a new type of benzoxazole-terminated heterocyclic compounds, 2-(4ʹ-alkoxybiphenyl-4-yl)-benzoxazole derivatives (nB-x), are reported. For better understanding of the structure–property relationships, compounds bearing a variety of polar substituents in the 5-position of the benzoxazole moiety were prepared, including H (nB-H), NO2 (nB-N), CH3 (nB-M) and Cl (nB-C). Their phase transition behaviour was investigated by differential scanning calorimetry and polarising optical microscopy. In the case of carbon atoms in the alkoxy chain between 2 and 10 they exhibited enantiotropic smectic mesophases, for which the mesophase ranges were 101–175°C and 159–182°C on heating and cooling for nB-N, 5–63°C and 19–63°C for nB-H, 81–90°C and 78–96°C for nB-M, and 131–132°C and 131–132°C for nB-C, respectively. The effect of the length of the alkoxy chain on mesomorphic properties was studied. Compounds nB-N, nB-M and nB-C exhibited a much wider mesophase range either on heating or cooling than the corresponding nB-H series, which indicated that the substituent on the benzoxazole moiety was helpful in increasing the mesophase stability. Substituents with stronger electron-withdrawing properties gave a wider mesomorphic temperature domain, in the order NO2 > Cl > CH3 > H, which was similar to the effect of the substituents on the UV absorption bands of nB-x. The opposite effect was found for the corresponding Schiff base liquid crystals (nS-x). With the exception of nB-N, the nB-x series displayed intense photoluminescence emission in methylene chloride solution, with absorption maxima within the range 395–404 nm.

Synthesis and characterization of novel thermotropic liquid crystalline copoly(ester imide)s

Novel liquid crystalline copoly(ester imide)s were synthesized via polyesterification of triethyleneglycol bis(4-carboxyphenyl) ether (le), diacetoxybiphenyl, and diacids with imide moieties. The effects of composition on the changes of T g , T m , and T i were examined by global TSC and DSC. Thermal gravimetric analyses (TGA) found that 4a-d and 5a-g possess higher thermal stability. Strong stir opalescence phenomenon and observations from polarized optical microscopy identified that 2b-e and 3a-d possess the typical schlieren texture of an enantiotropic nematic mesophase. The birefrigent melts of 4a-d and 5a-g, however, displayed particular liquid crystalline behavior. Copolymers with higher aromatic imide ring content (4a-d, 5a-g) form a layered structure and an enantiotropic smectic mesophase in the melting state. The melt viscosity of the semetic mesophase was higher than the nematic mesophase which was observed by capillary rheometer.

Side chain liquid crystalline polymers containing siloxane spacer: 7. Syntheses and properties of polyimides and polyamides containing a mesogenic side chain connected with a disiloxane linkage

Preparations of polyimides and polyamides containing a mesogenic group in the side chain connected with a siloxane bond in the spacer component were carried out. A 3,5-diaminobenzyloxy-type monomer having a mesogen consisting of a p'-substituted biphenyl benzoate group was prepared by means of hydrosilylation of 1-[3-43,5-dinitrobenzyloxy)propyl]-1,1,3,3-tetramethyldisiloxane with a mesogenic allyloxy compound, followed by catalytic reduction of the two nitro groups. The polyimides were synthesized by polycondensation of the diamine monomers with 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), followed by chemical imidization. The polyamides were also synthesized by polycondensation of the diamine monomers with dicarboxylic dichloride compounds, i.e., terephthaloyl dichloride, oxalyl dichloride, malonyl dichlonride, succinyl dichloride, glutaryI dichloride, adipoyl dichloride, pimeloyI dichloride and azelaoyl dichloride. The structural effects on the thermal properties of the polymers were investigated. The obtained polyamides having an aliphatic group in the main chain exhibited an enantiotropic smectic mesophase at around 100-150C, while no mesophase was observed for the polyimides and a polyamide having an aromatic backbone.

Molecular Engineering of Liquid Crystal Polymers by Living Polymerization. XIX. Synthesis and Characterization of Poly[2-(4-Biphenyloxy)ethyl Vinyl Ether

Abstract The synthesis and living cationic polymerization of 2-(4-biphenyloxy)ethyl vinyl ether (BEVE) are described. Polymers and oligomers with degrees of polymerization from 3.8 to 22.1 and narrow molecular weight distribution were synthesized and characterized by differential scanning calorometry (DSC) and thermal optical polarized microscopy. When determined from their first DSC heating scans, poly(BEVE)s exhibit only a crystalline phase over the entire range of molecular weights. When determined from the second and first cooling scans, poly(BEVE) with a degree of polymerization of 3.8 exhibits an enantiotropic smectic mesophase and a crystallization process on the heating scan, while the oligomers with degrees of polymerization from 4.7 to 8.1 exhibit only an enantiotropic smectic mesophase. The polymers with higher degrees of polymerization do not exhibit any mesophase. An explanation for this behavior is provided.

Exhibition of Induced Mesophases in the Binary Systems where One or Both the Components are Non-Mesogenic

Abstract A number of binary systems with induced smectic and/or nematic mesophases where one or both the components are mesogenic, have been reported by us.1,2 To quantify the results, eight more binary systems of the types were studied. In the three binary systems of both non-mesogenic components, induced nematic and smectic mesophages are observed in number of compositions. In the binary system where one of the components is monotropic nematic, an induced smectic phase is observed and the monotropic nematic phase is converted to an enantiotropic phase. The remaining binary systems where one component exhibits a monotropic smectic mesophase, an enantiotropic smectic mesophase is observed at comparatively lower temperatures. The terminal nitro substituent is responsible for the induced mesomorphic behaviour in the binary systems of present investigation. The phase diagrams of the present study are discussed in detail and the common features emerging from the overall study are outlined to envisage such properties in the other systems.

Diacetylenic Liquid Crystalline Polymers II: Derivatives of 10,12-Docosadiyne-1,22-Dioic Acid

Abstract Polymers of 10,12-docosadiyne-1,22-dioic acid with different mesogenic groups have been prepared. The mesogenic groups were diphenyl (Polymer I); p-diphenylazoxy (Polymer 11); 2,2′-dimethyldiphenylazoxy (Polymer III); 2-methyldiphenylazomethine (Polymer IV) and α-methylstilbene (Polymer V). All polymers crystallized extensively at room temperature, but only polymers I and V have displayed an enantiotropic smectic mesophase. The X-ray diffractograms were consistent with a smectic H mesophase for both polymers. Polymers I and V differed also from polymers 11-IV by enhanced reactivity of the diacetylenic moiety. A model for chain packing consistent with the X-ray diffraction pattern of the crystalline and the smectic phase is proposed.