Symmetric Liquid Crystal Dimers Research Articles

Synthesis and mesomorphic properties of chiral liquid crystal dimers derived from azobenzene and substituted naphthol

Symmetrical liquid crystal dimers bis{2-alkyl-(S)-(+)-2-(6-[4-4′-decyloxyphenylazo)-benzoyloxy]-2-naphthyl)propinate} have conveniently been designed, synthesised and their mesomorphic properties were investigated. These dimers possess two identical mesogenic units with each of them attached to the terminal end of a flexible spacer (–C n H2n ; where n = 6–10). The respective mesogenic unit was made up from a naphthyl ring attached to the alkyloxylated azobenzene via the ester (COO–) bond. The chiral moiety attached to the 6-position of the naphthyl ring possesses a terminal carboxylic group COO– which linked up the mesogenic unit and the spacer. The thermal and optical properties for the ultimate compounds at different temperature were studied explicitly with the aids of differential scanning calorimetry and polarising optical microscopy. All dimers exhibit unambiguously the monotropic nematic and smectic A phases. The homeotropic alignment of the molecules within the anisotropic region was further confirmed through the application of homogeneously treated cell.

Synthesis and Mesomorphic Properties of Uncoordinated Liquid Crystal Dimers Bis[4-[(4′-Decyloxyphenyl)Carboxylate]Salicylideneimino] Alkanes and Their Copper(II) Complexes

Series of symmetrical liquid crystal dimers wherein the spacer connected to two mesogenic units of salicylideneimines consists of the aliphatic chain ‒(CH2)n‒ where n adopts even parity numbers from 2 to 10 have been synthesized and characterized. These dimers function as tetradentate ligands which possess strong affinity toward the Cu(II) ion leading to the formation of four-coordinated complexes with Cu‒N and Cu‒O modes. The uncoordinated dimers exhibit enantiotropic nematic as well as smectic C phases. However, the Cu(II) complexes behave predominantly as nematogens. Correlation study between the effect of variable spacer length and the mesomophic behaviors of these dimers along with their related Cu(II) complexes are reported. The Cu(II) complexes in which the ligands possess shorter spacers were seemed to exhibit higher thermal stability.

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.

Phase Transition Studies of 6.O12O.6 and 7.O6O.7 Using Density Measurements

The symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, show a rich variety of smectic polymorphism. We report here the nature of different transitions exhibited by two of these dimers using density measurements as a function of temperature. The compound 6.O12O.6 exhibits only nematic phase; while 7.O6O.7 exhibits smectic A and smectic F phases. All the phase transitions studied viz., isotropic – nematic, isotropic – smectic A, and smectic A – smectic F are confirmed to be of first order nature. The calculated density jumps, thermal expansion coefficient data and pressure dependence of transition temperatures are analysed in the light of available literature data.

Study of Molecular Polarizabilities and Orientational Order Parameter in the Nematic Phase of 6.O12O.6 and 7.O12O.7

As a part of our systematic study of the physical properties of the compounds of symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4’-oxy) alkanes (m.OnO.m), we report here an optical study of two nematogenic compounds of this series viz., 6.O12O.6 and 7.O12O.7 by carrying out the measurement of extraordinary and ordinary refractive indices using modified spectrometer. The molecular polarizability anisotropies are evaluated using Lippincott δ-function model, the molecular vibration method, Haller’s extrapolation method and scaling factor method. The molecular polrizabilities, αe and αo are calculated using Vuks’ isotropic field model and Neugebauer’s anisotropic field model. The orientational order parameter, S, is estimated by using the molecular polarizability values calculated from refractive index and denstity data as well as polarizability anisotropy values calculated as a function of temperature in the nematic phase of both of the dimers. These values are discussed with reference to the order parameter values obtained directly from the optical birefringence, Δn, data without assuming any internal field. A comparison of the order parameter values from all of these methods was carried out in the light of available literature data.

Synthesis and Mesomorphism of Rigid Bridge Triphenylene Discotic Liquid Crystal Dimers

Pd-Cu catalyzed homocoupling reaction has been used for the synthesis of symmetrical triphenylene discotic liquid crystal dimers, [C18H6(OCnH2n+1)5(O2CC6H4C≡C-)]2 (n = 4, 5, 6, 7, 8, 9), which connected two triphenylene discogens via a long rigid spacer. The purity and structure of monomers and dimers have been characterized with 1H NMR, IR and high resolution Mass. Their fine thermal stability has been confirmed by the results of thermogravimetry analysis (TGA). Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) results have shown that the monomers and dimers exhibit columnar mesophases (Col). The dimers display wide mesophase ranges and are room temperature liquid crystals, and their clearing points increased with the lengthening of peripheral alkyl chain.

Synthesis of Symmetric Liquid Crystal Dimers Based on Azo and Imine Groups and Investigation of Phase Behaviour by Varying Alkoxy Terminal Chain Length

With the objective to study the effect of alkoxy terminal chain length on mesomorphic properties of liquid crystals, we have synthesized two (Azo and Imine) different series of dimesogens by varying terminal alkoxy chain length (n = 6–12) with a short spacer unit in between two mesogens. Transition temperatures and phase characterization were studied by DSC, POM and XRD analysis. It was found that all the dimers show mesomorphic properties and the change in terminal alkoxy chain length has pronounced effect on the smectic phase window. In the DSC cooling scan, the smectic phase window of azo compounds increased from 6.4°C (2A6) to 16.0°C(2A12), whereas in Schiff base compounds, it increased from 9.2°C (2S6) to 31.0°C(2S12). Further, the 2A11 dimer was found to undergo photo induced configurational changes.

Synthesis and mesomorphic properties of chiral liquid crystal dimers containing lactate units

A series of symmetrical chiral ferroelectric liquid crystal dimers viz., alkyl bis{(R)-2-[4-(4′-decyloxyphenyl)benzoyloxyphenoxy]propionates}, ABDBP- n ( n = 6–10), containing lactate units were synthesized and characterized. The length of the methylene spacer was increased from 6 to 10 in between the two chiral carbons located at the inner side of dimer. All the synthesized dimers exhibited polymorphism by showing N ∗, SmA ∗, SmC ∗ and SmX ∗ (unidentified) mesophase sequence. The mesophases and their corresponding transition temperatures were identified by polarized microscopy and differential scanning calorimetric measurements. Electro-optic switching studies of these dimers in SmC ∗ phase has exhibited ferroelectric bistable switching characteristics with the maximum spontaneous polarization of about 60.74 nC cm −2. The tilt angles of the SmC ∗ phase was also measured to confirm the tilted phase characteristics.

Observation of intercalated smectic phases in symmetric liquid crystal dimers containing hydrazide groups

Dimeric hydrazide derivatives with nitro, phenyl, and methyl terminal subsistents were synthesized. The liquid crystalline properties were investigated by differential scanning calorimetry, polarizing optical microscopy and wide angle X‐ray diffraction. Interestingly, intercalated smectic phases were observed in these symmetric liquid crystal dimers. The effect of the substituents and the length of the spacer on the mesophase is discussed, confirming that intermolecular hydrogen bonding between the hydrazide groups was the driving force for the formation of the intercalated structures.

Experimental investigation of phase transitions exhibited by symmetric liquid crystal dimers

As a part of our systematic study of phase transitions in liquid crystal dimers we report here the variation of density across the phase transitions Isotropic – Smectic A and Smectic A – Smectic F exhibited by the Schiff base liquid crystal dimeric compounds of α,ω-bis(4-alkylanilinebenzylidene-4′-oxy)alkane (m.OnO.m) homologous series. The compounds studied are 6.O5O.6 and 6.O6O.6. The compound 6.O5O.6 exhibits only Smectic A phase, whereas the compound 6.O6O.6 exhibits Smectic A and Smectic F phases. The density jumps and peak in thermal expansion coefficient at the transitions confirm the first order nature of the transitions with a smaller density jumps compared to the monomeric liquid crystals. The calculated pressure dependance of the transition temperatures using Clauasius- Clapeyron equation is also reported. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase transition studies in mesogenic dimers

AbstractSchiff base liquid crystal dimers, both symmetric and non‐symmetric, in which two anisotropic groups are linked by a flexible spacer, exhibit a rich variety of smectic mesomorphism. The interest in this class of mesogens stems not only from their ability to act as model compounds for semi‐flexible mainchain liquid crystal polymers but also from their quite different properties compared to conventional low molar mass liquid crystals (monomers). We report here the phase transition studies on two examples of these schiff base liquid crystal dimers using the Differential Scanning Calorimetry and density measurements as a function of temperature. The symmetric liquid crystal dimer, α,ω‐bis (4‐n‐dodecylaniline benzylidene‐4′‐oxy)decane (10.O12O.10) exhibits a very rare Isotropic to G transition. Where as, the non‐symmetric dimer, α‐(4‐cyano biphenyl‐4′‐yloxy)‐ω‐(4‐n‐decylanilinebenzylidene‐4′‐oxy)decane (CB.O10O.10) exhibits a rare nematic to intercalated smectic A phase transition. The transitions studied, isotropic to nematic and isotropic to G exhibit a large density jump at the transition confirming their first order nature. Nematic to intercalated smectic A transition is found to be a second order transition whose behavior is similar to nematic to smectic A transition. Calculated values of pressure dependence of transition temperature and thermal expansion coefficient are also reported.

Methylene-linked liquid crystal dimers

A range of symmetric liquid crystal dimers which differ in the nature of the link, either ether or methylene, between the spacer and mesogenic units has been prepared and their transitional properties characterized. The nematic-isotropic transition temperature, T NI, and the associated entropy change, ΔS NI/R, are sensitive to the chemical nature of this link. Specifically, T NI falls on replacing ether links with methylene links for both odd and even members although this reduction is more pronounced for odd members. In comparison, ΔS NI/R increases on changing ether links for methylene links for even dimers, but decreases for odd-membered dimers. These observations are completely in accord with the predictions of a model developed by Luckhurst and co-workers in which the difference between the ether-linked and methylenelinked dimers rests exclusively in their shapes. Furthermore, the highly non-linear pentamethylenelinked dimers show a greater tendency to exhibit smectic behaviour; this is interpreted in terms of molecular packing giving rise to an alternating smectic phase.

Odd–even effects in the director dynamics of the nematic phases formed by symmetric and non-symmetric liquid crystal dimers. An ESR investigation

Liquid crystal dimers, in which two mesogenic groups are linked by a flexible spacer, have been found to exhibit a pronounced odd–even effect in many of their physical properties. These include the nematic–isotropic transition temperatures, transitional entropies and their orientational order. In contrast, there have been far fewer studies of the director dynamics within the nematic phases of liquid crystal dimers. Here we present results from investigations of the magnetic field-induced relaxation time for the director in the nematic phase, using ESR spectroscopy, for the non-symmetric α-(4′-cyanobiphenyl-4-yloxy)-ω-(4-alkylanilinebenzylidene-4′-oxy)alkane dimers and the symmetric α,ω-bis(4-alkylanilinebenzylidene-4′-oxy)alkane dimers. Unlike quantities such as the orientational order parameter it is not so apparent as to what temperature scale should be used to compare the relaxation time, which depends on the rotational viscosity coefficient γ1. We discuss various possibilities and show that for a certain choice the relaxation times do exhibit an odd–even effect as well as a dependence on the nature of the smectic phase following the nematic.

Symmetric and non-symmetric liquid crystal dimers with branched terminal alkyl chains: racemic and chiral

Andrew E. Blatch, I. D. Fletcher and Geoffrey R. Luckhurst, J. Mater. Chem., 1997, 7, 9 DOI: 10.1039/A602980I

Molecular conformation and packing of smectogenic dimeric liquid crystals in the crystalline state

Abstract The crystal structures of α,ω-bis(4-n-pentylanilinebenzylidene-4′-oxy)butane and α,ω-bis(4-n-pentylanilinebenzylidene-4′-oxy)pentane have been determined by single crystal X-ray analysis. These molecules have even and odd chain length spacers linking two mesogenic groups, respectively, and are members of a family of symmetric liquid crystal dimers which possess a rich smectic phase polymorphism, as well as pronounced odd—even effects in the transitional properties as the parity of the central spacer is varied.

Novel modulated Hexatic Phases in Symmetric Liquid Crystal Dimers

Homologues of the dimeric a, w-bis(4-n~alkylanilinebenzylidene-4'-oxy)alkanes (m.OnO.m) have been synthesised with spacer lengths n ranging from 9 to 12 methylene units and with terminal alkyl chain lengths m of10, 12 and 14 Characterisation of these materials has been carried out by X-ray diffraction, differential scanning calorimetry and optical microscopy. In six of these compounds a novel modulated tilted hexatic phase, denoted Si, has been identi- fied, in which the smectic layers have a periodic modulation in their structure, analogous to the St ribbon phase. This modulated hexatic phase is only observed for dimers with an odd parity spacer, suggesting that the average molecular shape, believed to be bent for the odd spacer dimers but linear for the even ones, is responsible for its formation. The compound 10.090.10 exhihits a second modulated hexatic phase denoted 52, which is monotropic, and which cannot be distinguished from the Si phase by polarizing microscopy or by calorimetry. These long chain dimeric liquid crystals have been found to have a strong tendency to form tilted hexatic phases; indeed two of the compounds studied here are the first dimers to undergo a transition directly from a tilted hexatic phase to the isotropic liquid.