Phase transition studies in symmetric dimeric liquid crystals

As a part of the systematic studies on symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, (referred to as m.OnO.m with m = 3, 4, and 5; and n = 8, 9, and 10), we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A (N–SmA) phases exhibited by the just mentioned compounds. The methods employed are differential scanning calorimetry and dilatometry. The compounds studied were 3.O8O.3, 4.O8O.4, and 5.O8O.5; and 3.O9.03, 5.O9O.5, 3.O10O.3, 4.O10O.4, and 5.O10O.5. Different from the case of their corresponding monomers, all these compounds exhibit a nematic phase only with the exception of 5.O8O.5 which exhibits a SA phase in addition to the nematic phase. The phase transitions viz., isotropic–nematic transitions studied in all these compounds were confirmed to be of first-order nature, whereas the N–SmA transition exhibited by the compound 5.O8O.5 only was found to be of second-order nature. We also report in this article the calculated density jumps, thermal expansion coefficient maxima, and pressure dependence of transition temperatures which are analyzed in the light of the available literature data.     

Heterocyclic pyridine-based liquid crystals: synthesis and mesomorphic properties

A series of new calamitic liquid crystals, 4-{[(pyridin-4-yl)methylidene]amino}phenyl 4-alkoxybenzoates comprising a heterocyclic (pyridine) and two phenyl rings core system, terminal alkoxy chain, imine and ester linkers were synthesised and characterised. This series consists of nine members wherein the members differ by the length of alkoxy chain (C_nH_2n+1O–, where n = 2, 4, 6, 8, 10, 12, 14, 16, 18). Spectral analysis results were in accordance with the expected structure. Their thermotropic behaviours were studied by using differential scanning calorimetry, optical polarising microscopy and powder X-ray diffraction techniques. A single mesophase (nematic) was observed for the first three members of the series (n = 2, 4 and 6). As the alkoxy chain increased to n = 8 and n = 10, the nematic phase appeared together with an additional smectic A (SmA) phase. When moving from n = 12 until the highest members (n = 18), the nematic phase disappeared and these compounds only exhibited a single mesophase (SmA).     

Synthesis,characterisation and phase transition studies in N-(-4-ethyloxybenzylydene)-4’-alkoxyanilines

Synthesis and characterisation are carried out in N-(-4-ethyloxybenzylidene)-4?-alkoxyanilines, 2O.Om liquid crystalline (LC) compounds with m = 3, 4 and 6–10. All the compounds exhibit monovariant nematic phase except with m = 10; the LC material shows bivariant nematic-smectic-C in addition to the nematic phase. Further, the variation in density with temperature in all these seven compounds is studied. The density and thermal expansion coefficient results reveal that the phase transitions, namely isotropic to nematic and nematic to SmC, present in these compounds show first-order nature as expected and that all the parameters show characteristic change in the vicinity of the phase transformation. The results are discussed in the light of the data available on other nO.Om compounds.     

Influence of terminal substituent on non-linear S-shaped oligomers consisting of azobenzene moieties at the peripheral arm: Synthesis,characterisation and phase transition behaviour

A series of symmetrical S-shaped mesogens based on 4,4′-bis-(6-bromohexyloxy)biphenyl as a central unit containing two 2-{6-[4-(4-substitutedphenylazo)phenoxy]hexyloxy}phenol as side-chain groups has been successfully synthesised. The terminal substituent was varied from halogen (X = F, Cl, Br and I) to non-halogen (X = C₂H₅ and OC₂H₅). The oligomers with C₂H₅ and OC₂H₅ substituents exhibit predominantly the monotropic nematic (N) phase. The OC₂H₅-containing derivatives possess long-range stability of N phase than its C₂H₅-containing analogue in which it has small range of N phase stability. As for halogen-containing analogues, oligomer with F exhibits monotropic N phase whilst oligomers with Cl and Br exhibit monotropic N and smectic A (SmA) phases. In addition, homologue with Br shows additional phase which is smectic B (SmB) phase upon further cooling. However, the oligomers in which F, Cl and Br were substituted by I exhibits purely monotropic SmA and SmB phases. X-ray diffraction measurements reveal that the smectic phase is inclined to the monolayer structure.     

Reentrant nematic phase in 4-alkyl-4′-cyanobiphenyl (nCB) binary mixtures

The appearance of reentrant nematic phase was identified in nCB/8CB (n = 1–6) binary mixtures using differential scanning calorimetry, small-angle X-ray diffraction and polarising microscopy with photomonitor. Phase diagrams can roughly be unified if plotted against the average number of alkyl chain length. Distinction was recognised for the phase boundaries between nematic and smectic A phases of even and odd n of the minor component nCB. The character of 8CB as ‘SmA former’ is briefly discussed.     

Overall analysis of first‐order phase transitions in some alkyloxycyanobiphenyl liquid crystals

Measurements of pressure, molar volume and specific heat as functions of temperature in the isotropic (I) phase as well as in the smectic A (SmA) and nematic (N) mesophases of some alkyloxycyanobiphenyl compounds (nOCB, n = 6–10) were carried out using differential thermal analysis under pressure, densitometry, X‐ray powder diffraction and modulated differential scanning calorimetry. Thermodynamic properties, such as latent heats and volume jumps at the different phase transitions, were determined. The coherence of this whole set of data was tested using pressure–temperature data through the slopes associated to their phase transitions, extrapolated at normal pressure in the light of the Clausius–Clapeyron equation.     

Synthesis,characterisation and liquid crystalline properties of some Schiff base and cinnamate central linkages involving 1,3,5-trisubstituted pyrazolone ring system and their Cu(II) complexes

Two new mesogenic homologous series, each containing 1,3,5-trisubstituted pyrazolone derivatives, 4-n-alkoxyphenyl and Schiff base–cinnamate central linkages, have been synthesised to give 4-[(5-hydroxy-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl) methyleneamino] phenyl 3-(4-n-alkoxyphenyl)acrylate [Series-A] and 4-[(5-hydroxy-3-methyl-1-p-tolyl-4,5-dihydro-1H-pyrazol-4-yl)methyleneamino] phenyl 3-(4-n-alkoxyphenyl)acrylate [Series-B] and their Cu(II) complexes have also been synthesised. These compounds were characterised by elemental analysis, Fourier transform infrared (FT-IR), proton nuclear magnetic resonance (¹H NMR), carbon-13 NMR (¹³C NMR) and ultraviolet (UV)-visible and mass spectral studies. Their mesomorphic behaviour was studied by polarising optical microscope (POM) with a heating stage. POM data were compared with differential scanning calorimetry thermograms. In Series-A and -B, all compounds exhibit mesomorphism. Series-A compounds exhibit an enantiotropic nematic mesophase except propyl derivative, while a smectic A (SmA) mesophase is observed from the heptyl derivative and persists up to the last member of the homologous series. n-Heptyloxy derivative is monotropic for SmA phase. Series-B compounds also exhibit the enantiotropic nematic mesophase, while the SmA mesophase is observed from the heptyl derivative and persists up to the last member of the homologous series. n-Dodecyloxy derivative exhibits monotropic SmA and nematic mesophases. The mesomorphic properties of both series are compared with each other and the other structurally related compounds. The study reveals that cinnamate linkage containing liquid crystals have higher thermal stability compared to structurally related series containing chalcone linkage. In case of complex series, only one compound from each series gives nematic mesophase.     

Structure property and mesomorphic behaviour of S-shaped liquid crystal oligomers possessing two azobenzene moieties

A new series of symmetrical S-shaped oligomers 4,4?-bis(ω-2-(ω-[(bromophenyl)diazenyl]-alkoxy)phenoxy)hexylbiphenyl consisting of two different spacers (inner -(CH₂)₆- and outer -(CH₂)_n-) have been synthesised. Their physical, thermal and texture observation over various transition temperatures are reported. The outer spacers for these compounds vary from n = 4 to n = 9. The oligomers with even number of members exhibit monotropic phase in which the compound with n = 4 shows nematic (N) phase whilst those with members n = 6 and 8 exhibit N and smectic A (SmA) phases. However, the homologs with odd number of members display enantiotropic phase in which the compound with n = 5 exhibits N and smectic phases whereas the members with n = 7 and 9 are predominantly smectogenic. The temperature range of N phase for even-numbered member decreased with elongation of the outer spacer. The smectic phase stability among the members in the present series increases when the outer spacer n is increased from 5 to 8.     

Novel oxazepinedione-derived symmetric dimers: synthesis and mesophase characterisation of seven-membered heterocyclic compounds

The synthesis and characterisation of three sets of symmetric dimeric compounds composed of seven-membered oxazepinedione heterocyclic rings were carried out. All the dimers possess the tetradecyl- (n = 14) alkyl side chain attached to the nitrogen atom of the oxazepinedione core. The oxazepinedione core in turn was connected with varied connecting spacers (n = 4, 6, 8, 10 and 12). The dimers were spectroscopically characterised by FT-IR, ¹H-NMR, ¹³C-NMR and elemental analysis techniques. The compounds were investigated for liquid crystalline properties using differential scanning calorimetry and polarising optical microscopy with heating assembly. The precursor imines 2a–e itself started exhibiting liquid crystalline SmA/tilted hexatic mesophase. Further fusion of 2a–e with maleic anhydride, succinic anhydride and phthalic anhydride gave the novel oxazepinedione-derived symmetric dimers 3a–e, 4a–e and 5a–e respectively. The dimers 3a–e and 4a–e did not exhibit any liquid crystal (LC) properties. However, the phthalic anhydride-fused oxazepinediones 5a–e show monotropic nematic liquid crystalline phase. The results indicate that the formation of mesophase is dependent on the type of fused oxazepinedione ring.     

Low-temperature nematic phase in asymmetrical 1,3,4-oxadiazole bent-core liquid crystals possessing lateral methoxy group

Asymmetrical bent-core molecules based on 1,3,4-oxadiazole bent-core unit have been synthesised as a new design with a lateral methoxy group at outer phenyl ring of the molecule. These new asymmetrical bent-core molecules resemble hockey-stick shaped due to the presence of two different arms of different lengths. One arm of these molecules is elongated having two phenyl rings and possesses a 4-n-alkyloxy chain of a different number of carbon atoms (n = 4, 8, 12 and 18) and other arm is short and has one phenyl ring with fixed 4-n-octyloxy chain. The bent-core molecules possess a lateral polar methoxy group at the elongated arm of the molecule. These bent-core compounds exhibited fluorescence emission in the UV wavelength region (~377–386 nm) whereas in acetonitrile and dimethylformamide, solvent displays blue emission peak with a large stoke shift.The bent-core molecules with the number of carbon atoms (n = 4, 8 and 12) at the elongated arm exhibited monotropic nematic phase at low temperature, while the 4-n-octadecyloxy chain at the elongated arm displayed smectic A phase. Dielectric studies were performed in the nematic phase of the bent-core mesogens confirm the formation of the cybotactic cluster in the nematic mesophases.     

Mesomorphic and photophysical behaviour of 1,3,4-oxadiazole based hockey stick reactive mesogens

New hockey stick mesogens derived from 1,3,4-oxadiazole as a bent-core unit have been synthesised. The molecules resemble hockey stick shape due to the presence of two arms containing a different number of phenyl rings attached with the 1,3,4-oxadiazole bending unit. The shorter arm of the molecule consists of one phenyl ring and 4-n-alkyloxy terminal chains whereas the long arm of the molecule possesses containing two phenyl rings which are linked via imine linkage and reactive 4-n-undecenyloxy as a terminal chain. The thermal stabilities of the newly synthesised compounds were carried out by thermogravimetric analysis (TGA). The mesomorphic behaviour was investigated by polarising optical microscopy (POM) and differential scanning calorimetry (DSC). All the compounds exhibit enantiotropic nematic phase along with smectic phases (SmA and SmC phases). Interestingly, the compounds with lower 4-n-alkyloxy terminal chains (n = 4 and 6) exhibit a wide range of optically isotropic DC phase. On increasing, the terminal 4-n-alkoxy chain length the DC phase disappears. The photophysical properties of the compounds were investigated in different solvents and in the solid state. It was observed that the compound exhibit absorption in UV region and emission in the green region.     

λ-Shaped liquid crystal trimers with dual terminal cholesteryl moieties: synthesis and concomitant of N*, SmA and cholesteric glassy phases

Two homologous series of λ-shaped chiral liquid crystal trimers composed of a laterally substituted benzylidene-aniline as the central core armed by two cholestryl ester moieties via odd-even alkyl spacer are synthesised. All the compounds are mesogenic exhibiting both chiral nematic (N*) and SmA phases except for trimers bearing long spacers (n = 9–10). A pronounced odd-even effect is observed on the phase transition temperatures and clearing enthalpies when the spacer length is varied in which the even-parity members show higher value. The widening of N* phase upon elongation of the alkyl spacer can be interpreted as a result of the destabilisation of SmA phase. Full recrystallisation from the cholesteric phase upon cooling is not observed for all the compounds bearing long spacer, especially oxydecanoyl spacer. Instead, the anisotropic fluid vitrified to form the cholesteric glassy state characteristic of polymer at low temperature. This finding is also evident from the polarising optical microscope whereby non-crystalline texture which resembles the Grandjean texture with various reflection colours is observed upon cooling to low temperature. The effect of the lateral substituents on the liquid-crystalline behaviour is also discussed.     

Synthesis,characterisation, mesomorphic investigation and temperature-dependent Raman study of a novel calamitic liquid crystal: methyl 4-(4′-n-alkoxybenzylideneamino)benzoate

A new series of Schiff base calamitic liquid crystal; methyl 4-(4′-n-alkoxybenzylideneamino)benzoate (MABAB), H_2n+1C _n OC₆H₄C(H)=NC₆H₄COOCH₃ (n = 6, 8, 10, 12, 14, 16) has been synthesised and characterised by elemental analyses, Fourier transform infrared spectroscopy (FT-IR), ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy. The mesomorphic properties of these compounds were studied by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). All members of the series exhibit enantiotropic smectic A (SmA) mesophase. Temperature-dependent micro-Raman study of one of the members, MABAB-10 has been employed to identify phase transitions and the molecular rearrangement therein. Analysis of Raman marker bands; C–H in-plane bending, C–C stretching of phenyl rings and –C(H)=N– linking group of core confirms the transitions clearly as observed through DSC and POM. An in situ Raman measurement of C–H in-plane bending mode has also been performed to visualise the molecular changes more clearly. The Raman study gives an evidence of induced co-planarity of rings at Cr→SmA phase transition. The density functional theoretical (DFT) optimisation of monomer, dimer and rotational conformer of MABAB-10 also support the induced co-planarity at Cr→SmA phase transition.     

Dielectric studies on binary mixtures of systems exhibiting intermediate nematic phase

Two binary phase diagrams of the liquid crystals (4-n-pentylphenyl-4-n-hexyloxybenzoate (PPHB) with 4-nitrophenyl-4-n-pentylbenzoate (NPPB) and 4-n-hexyloxyphenyl-4-n-decyloxybenzoate (HPDB) with NPPB) have been studied. PPHB shows only a nematic phase where as for HPDB trimorphism, with the SmC, SmA and nematic phases, was observed. Substance NPPB which has a strong polar nitro group is non-mesogenic one. Both the phase diagrams show an induction and stabilization of SmA phase and appearance of the nematic state in the high concentration range of polar component. The dielectric measurements confirm the phase transition temperatures and show changes in the short range interaction at the phase transitions.     

Synthesis and characterisation of asymmetrical mesogenic materials based on 2,5-disubstituted-1,3,4-oxadiazole

New mesogenic homologous series bearing 1,3,4-oxadiazole ring with a nitro terminal group, 4-(5-(4-nitrophenyl)-1,3,4-oxadiazol-2-yl)phenyl 4-((4-methoxybenzylidene)amino)benzoate (G₁–G₁₁), were synthesised. Their chemical structures are identified by fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (¹H-NMR) and elemental analysis. The liquid crystalline properties of the series G_n and their precursory F_n were screened by differential scanning calorimetry and optical polarising microscopy (OPM). The compounds of the series G_n were screened by thermogravimetric analysis to observe their thermal stability. The target compounds (G_n) in this study, were displayed different liquid crystalline mesophase, the first two homologous (G₁ and G₂) did not show any liquid crystalline behaviour, the homologous (G₃–G₁₀) which have an alkoxy terminal group (n = 3–10) exhibited nematic phase, whilst the last derivative of the series (G₁₁), n = 12, displayed SmA phase. The mesomorphic properties of these derivatives were affected by the presence of the nitro group at the end of the molecules which was classified as a strong polar group. Also, the role of alkoxy terminal chain and the bent heterocyclic ring (1,3,4-oxadiazole) in the liquid crystalline properties of these molecules were debated.     

High pressure studies of the static permittivity tensor components in the nematic phase of 6CB

The dielectric permittivity tensor components, ε_II and ε_⊥, in the nematic phase of 6CB (4‐n‐hexyl‐4′‐cyanobiphenyl) were measured in the pressure range 0.1–130 MPa and the temperature range 12–58°C. The dielectric anisotropy, Δε(p, V, T) = ε_II ‐ ε_⊥, was analysed in isothermal, isobaric and isochoric conditions taking into account the pVT data and the well known Maier and Meier equation. On that basis the nematic order parameter S(p, V, T) was determined. This was used to calculate the parameter Γ relating the interaction potential with the volume (density). Its value Γ = 4.1 agrees very well with other estimates.     

Visco-elastic properties of reentrant nematic liquid crystalline mixtures

The visco-elastic properties for binary mixtures of 4-n -hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) are investigated in detail by a light scattering technique. The mixtures exhibit a reentrant nematic (RN) phase between the smectic A (SmA) and crystal phases in the range 22.0–29.5 wt % of 6OCB. The viscosity and the elastic constant increase with cooling in both the nematic and reentrant nematic phases. It is also found that the mixtures exhibit anomalously large values of viscosity and elastic constant near the phase transition, i.e. a pretransitional phenomenon can be observed. The activation energy for viscosity is smaller in the RN phase than in the ordinary nematic phase appearing at higher temperatures, while the normalized elastic constant (defined as the ratio of the elastic constant to the square of the dielectric anisotropy) is larger. Moreover, the visco-elastic behaviour depends on the specific time during which the sample has been kept in the SmA phase. These results confirm that the SmA phase has a strong influence on the formation of molecular aggregates, which plays an important role in the reentrant phenomenon of these systems.     

Bromine-terminated azobenzene liquid crystals

A series of liquid crystals (LCs) of bromine-terminated azobenzene were synthesised and characterised. The LCs were composed of an azobenzene core, an alkyl chain and a flexible spacer with a bromine atom at a remote position. Mesomorphic properties were dependent on both the alkyl chain length and the relative position of the bromine atom at the end of the spacer group. The nematic phase was favoured over the smectic A phase for alkyl chains with one and seven carbon atoms. However, the SmA mesophase was dominant for compounds with 10-carbon alkyl chains. The remote bromine atom in the spacer group favoured SmA for homologous with n-decyl chains and the nematic phase for n-heptyl and methyl groups. Molecular modelling showed that the azobenzene LCs tended to adopt the all trans-conformation in the gas phase as the number of carbon atoms increased. For short spacer groups, bent conformations contributed to the level population proportion of conformers. For the non-LC 5a, the gauche conformation became the most stable with a torsional angle of –68.9°. X-ray experiments showed a monolamellar SmA mesophase in an antiparallel arrangement. Absorption maxima at 360 and 440 nm were assigned to π–π* and n–π* transitions, respectively.     

Synthesis and mesomorphic properties of four-ring fluorinated liquid crystals with trifluoromethyl group

The discovery of nematic phase in 3-fluoro-4-trifluoromethyl-4?-(4?-alkyl [trans, trans-1,1?-bicyclohexyl]-4-yl) 1,1?-biphenyls (n = 2, 3, 4, 5) is an accidental affair. A convenient method was used for the synthesis of these target compounds. After the preparation of the intermediate with trifluomethyl group, the target compounds were synthesised by Suzuki cross-coupling reaction. Then, these phase transition temperatures were measured by polarising textural observation, which confirmed the existence of single nematic phase. These temperatures were also measured by differential scanning calorimetry, which showed that the clearing point of compound (n = 3) is 267℃. The possible mechanism of occurrence of unique nematic phase was discussed on the basis of the principles of fluorine chemistry.     

Raman spectra of magnetic field induced cholesteric → nematic phase transition in doped p-octoxybenzoic acid

Raman spectroscopy is employed to investigate helical twist formation in nematic and smectic C phases of p-n-octoxybenzoic acid (OBA) doped with a small amount of Cholesteric Nonanoate (CN). A cholesteric→ nematic phase transition in OBA/CN is induced by an external magnetic field in the temperature range 135–148°C. The threshold field (H_c) is equal to 8.0 kGauss for 0.33 % by weight CN/OBA, and no hysteresis is observed.