Dependence of molecular polarizability on the orientational order in nematic liquid crystals

The dispersion dependences of refractive indices in the visible range were used to obtain experimental values of the Lorentz tensor components L _j and the mean molecular polarizability $ \bar \gamma $ \bar \gamma for five nematic liquid crystals belonging to two homological series. The dependence of L _j components on the homologue number, mesophase temperature, birefringence value, and the orientational order of molecules in the nematic phase and upon a nematic-smectic A phase transition was revealed. The effect of the isotropization of the Lorentz tensors and the local field tensor with decreasing birefringence and molecular polarizability anisotropy Δγ was confirmed. The quadratic dependence $ \bar \gamma $ \bar \gamma (S) on the molecular orientational order parameter S in the nematic phase was found. It was invariant with respect to the nematic-smectic A transition. The dependences $ \bar \gamma $ \bar \gamma (S) and Δγ(S) are explained within molecular statistical theory as consequences of the correlation between orientational and conformational degrees of freedom of molecules. These conformational degrees of freedom are related to the internal rotation of molecular fragments, which affects the electronic conjugation of the fragments and the oscillator strengths of molecular transitions.     

Mean polarizability of molecules and anisotropy of the Lorentz tensor upon a nematic-smectic a phase transition: Their behavior in a homologous series

Experimental values of the mean polarizability of molecules, [`(g)]\bar \gamma , and components of the Lorentz tensor, L <sub>j</sub> , in the nematic and smectic A phases are obtained for a homologous series of n-alkyl-p-(4-ethoxybenzylideneamino)-α-methylcinnamates. Dependences of the [`(g)]\bar \gamma and L _j values on the mesophase temperature, the orientational order parameter S of molecules, and the number n in the homologous series are revealed. The quadratic dependence of [`(g)]\bar \gamma (S) in the nematic and smectic phases is established that is invariant with respect to the nematic-smectic A transition. Polarizability densities of the molecular core and the alkyl chain are found from the monotone decreasing dependence [`(g)]\bar \gamma (n)/v (where v is volume per one molecule) in the smectic phase. The presence (or absence) of the odd-even alternation of L _j (n) in the nematic (smectic) phase is shown. A monotone decrease in the Lorentz tensor anisotropy L with an increase in n is revealed in the smectic phase, and limiting values L _j (n → ∞) are determined.     

A study of the induced helical pitch in a reentrant nematic mixture

Abstract

The temperature dependence of the induced helical pitch is reported for cholesteric and reentrant cholesteric phases of liquid crystal systems comprising 4-n-hexyloxy and 4-n-octyloxy-4′-cyanobiphenyl with a non-mesogenic optically active dopant. It was found that on adding small quantities of the dopant that the temperature range of the S_A phase is narrowed and subsequently disappears, while short range smectic fluctuations persist, influencing the helical twisting features. Critical index values were determined from the temperature dependence of the pitch.     

207Pb N.M.R. of lead(II) soaps in solid,liquid-crystalline and liquid phases

Abstract

²⁰⁷Pb N.M.R. data are reported for a number of even chain length lead(II) carboxylates (soaps) at various temperatures. At room temperature, the solid lead(II) decanoate and tetradecanoate show similar spectra, with a single metal ion site, and modest shielding anisotropy. As the temperature is increased, the soaps (hexanoate to octadecanoate) all form a highly ordered smectic phase, which gives a very broad ²⁰⁷Pb signal of linewidth comparable to that of the solid phase. At higher temperatures, the hexanoate to dodecanoate soaps form a lamellar L _α (smectic A) phase, whilst the longer chain length carboxylates melt directly to the liquid phase. Both the lamellar L _α and liquid phase give fairly sharp, isotropic signals, whose chemical shifts and linewidths are strongly temperature dependent. Possible explanations for this effect include paramagnetic contributions to the shielding tensor from low-lying electronic states of Pb(II), and contributions to the observed signal from different coordination species produced in the lead(II) carboxylate system. Although there are discrete changes in chemical shift at the phase transition, the magnitudes observed in all the phases are similar, suggesting that there are no dramatic changes in the metal coordination environment.     

Synthesis and mesomorphism of novel multi-arm liquid crystals with cholic acid as chiral centre linking Schiff base moieties as mesogens

A new series of multi-arm chiral liquid crystals (LCs) D1–D3 were synthesised and characterised. Cholic acid was used as the core and ω-[4-(p-alkoxybenzoloxy)phenoxycarbonyl]valeric acid (B1–B3) was used as the mesogenic arms, containing different terminal substituent X (B1: X= -OCH₃, B2:X= -CH₃, B3: X= -Cl). Their structures and mesomorphic properties were investigated by Fourier-transform infrared spectroscopy, nuclear magnetic resonance hydrogen spectrometer, differential scanning calorimetry, polarised optical microscopy and X-ray diffraction, respectively. The mesogenic B1–B3 displayed smectic B phase. The multi-arm LC D1 displayed cholesteric, while D2 and D3 exhibited nematic phase. The formation of cholesteric phase of D1 was affected by both the chiral core – the bulky cholic acid and the polarity of the terminal substituent of the mesogenic arm. That D1 displayed cholesteric phase but D2–D3 did not indicated that the stronger polarity of the terminal group OCH₃ of D1 played an important part in stabilising the cholesteric phase. The multi-arm LCs D1–D3 all showed ultraviolet activity. The wavelength of maximum absorption of D1–D3 was affected by the terminal substituent of the mesogenic arm.     

Role of the Polymer Matrix on the Photoluminescence of Embedded CdSe Quantum Dots

The photoluminescence (PL) of CdSe quantum dots (QDs) that form stable nanocomposites with polymer liquid crystals (LCs) as smectic C hydrogen‐bonded homopolymers from a family of poly[4‐(n‐acryloyloxyalkyloxy)benzoic acids] is reported. The matrix that results from the combination of these units with methoxyphenyl benzoate and cholesterol‐containing units has a cholesteric structure. The exciton PL band of QDs in the smectic matrix is redshifted with respect to QDs in solution, whereas a blueshift is observed with the cholesteric matrix. The PL lifetimes and quantum yield in cholesteric nanocomposites are higher than those in smectic ones. This is interpreted in terms of a higher order of the smectic matrix in comparison to the cholesteric one. CdSe QDs in the ordered smectic matrix demonstrate a splitting of the exciton PL band and an enhancement of the photoinduced differential transmission. These results reveal the effects of the structure of polymer LC matrices on the optical properties of embedded QDs, which offer new possibilities for photonic applications of QD–LC polymer nanocomposites.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

Synthesis and magnetooptical properties of mesomorphic complexes of lanthanides with β-aminovinyl ketones

Complexes of the composition L(LH)₂Ln(NO₃)₂ (Ln = La, Dy, Gd, Er, Eu, or Tb) were obtained by the reaction of -aminovinyl ketone (LH) with rare earth metal nitrates. All the compounds synthesized are thermotropic liquid crystals having the smectic S_A phase. The values of magnetic anisotropy of these complexes measured by magnetic birefringence are fairly high.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2331–2333, September, 1996.     

Dielectric studies and critical behaviour in the vicinity of nematic–isotropic phase transition of a smectogenic binary mixture showing induced nematic phase

The phase diagram of a binary mixture composed of compounds, one having NCS terminal group (4DBT, showing smectic A₁ phase) and the other with CN terminal group (11OCB, showing smectic A_d phase), exhibiting induced nematic phase in a certain concentration range (0.100 < x_4DBT < 0.951) is reported here. Results of the static dielectric parameters measurement on this binary system within the entire mesomorphic range are presented. Evidence of strong pretransitional behaviour near the nematic–isotropic (N–I) phase transition, indicating the influence of tricritical behaviour, is observed. Precise determination of discontinuity (ΔT) and the critical exponent (α) of N–I phase transition have been carried out. Moreover, the order parameter critical exponent β is correctly predicted by the tricritical hypothesis through the dielectric anisotropy data for all the investigated mixtures.     

The effect of locations of triple bond at terphenyl skeleton on the properties of isothiocyanate liquid crystals

Three series of new tolanyl benzene isothiocyanate liquid crystals A, B and C were synthesised with lateral fluorine substituent at different positions and 3,5-difluoro terminal groups. Series D and compound E4 with carbon–carbon triple bond connected with the isothiocyanated benzene were also prepared as reference. Their structures were confirmed using infrared spectrometry, nuclear magnetic resonance and mass spectrometry. The mesomorphic properties, birefringence, dielectric anisotropy and miscibility of these compounds were investigated. Series A exhibits higher clearing points than that of the corresponding compounds D. Series B displays the lowest melting points, the broadest nematic phase temperature range and no smectic phase with increase of the carbon number of alkyl chain. Lateral fluorine in the position of Y (series C) results in an increase of smectic phase. The birefringence (~0.454–0.490) of series A is almost the same as that of the corresponding reference compounds D, while series B shows a decrease of Δn about 0.05 compared with compounds D. For comparison, mixtures formulated by A and B exhibit higher clearing points and better low temperature stability than those of the mixtures based compounds D in the same fraction.     

Electrooptical properties of new type fluorinated phenyl-tolane isothiocyanate liquid crystal compounds

A series of 4?-(4-alkyl-phenyl)-2?,6?-difluorotolane isothiocyanate liquid crystal (LC) compounds were synthesised, and their phase transitions and electrooptical properties were investigated. These compounds exhibit high birefringence, about 0.47–0.52. As the number of carbon atoms in the alkyl chains increases, these LC compounds can exhibit smectic phases. When these LCs were mixed into the commercial LCs, the birefringence values of LC mixtures become higher than pure commercial LCs, and the visco-elastic coefficients of five LC mixtures are very close to each other at every test temperature. The results of response properties indicate that the compounds with a tri-fluorine substitution and n-propyl end group possess better response performance than the others. These LC compounds are particularly attractive for achieving fast response times in LC optic devices.     

Thermal tuning band gap in cholesteric liquid crystals

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.     

Principal component analysis of dipole moment derivative signs of chloroform

The use of principal components as a basis for a graphical procedure to analyze polar tensor data is proposed. Molecular orbital and experimental polar tensor data for all possible sign combinations of the ? p /?Q_j of CHCl₃ and CDCl₃ are represented graphically as principal component scores facilitating sign selection for the ? p /?Q_j. The graphs are particularly useful in analyzing an apparent conflict in ? p /?Q_j sign choices based on the isotopic invariance criterion and molecular orbital results for the A₁ symmetry species of these molecules. The numerical impacts of individual sign ambiguities for the ? p /?Q_j on the polar tensor data are measured by the variances associated with the principal components. Assuming the ? p /?Q_j sign sets with indeterminate signs provide replicated results for the polar tensor elements, their errors are estimated and compared with errors obtained previously by propagating intensity uncertainties through the polar tensor equations.     

Influence of the bias field on dielectric properties of the SmCA* in the vicinity of the SmC*–SmCA* phase transition

In a large class of smectic mixtures prepared at our University, the phase transition between chiral ferroelectric smectic C (SmC*) and chiral antiferroelectric smectic C (SmC_A*) phases can be observed on cooling. Under bias field the temperature of the phase transition SmC*?SmC_A* decreases (ca. 100°C in the investigated mixture). The transition is called: unwound SmC*?twisted SmC_A* phase transition. The Goldstone mode in SmC* phase is reduced by a direct current field while two modes (P_H and P_L) in the SmC_A* phase are amplified. The amplitude of the fast X mode observed in the SmC_A* phase is reduced. The aim of this paper is to show how parameters of the modes in SmC_A* phase (calculated from Cole–Cole model) change with bias voltage—when twisted structure in SmC_A* phase is gradually unwound. The character of the modes observed in SmC_A* is discussed. A new effect is shown: a high value of dielectric loss is detected in the unwound SmC* phase, which is very close to SmC_A*.     

A nematic phase created in mixtures of polar smectics The effect of smectic layer properties

Phase diagrams of binary mixtures composed of compounds with the NCS terminal group (n-DBT, n-PBT, n-TPB (smectic A₁) or n-BT (smectic E₁)) and n-OCB, n-CB (smectics A_d) are presented. It is shown that the width of the nematic gap that separates the A₁ or E₁ phase region from the smectic A_d phase is related to the interaction energy of the molecules in the smectic layers and to the difference in the smectic layer spacings.     

Tilt and temperature dependence of the pitch in the chiral smectic C phase

Abstract

The chirality of the constituent molecules in the chiral smectic phase induces a helical structure with a pitch, p ₀. Because of the tilt and chirality there is a spontaneous polarization and a bend deformation which act upon the induced helix. The resulting pitch is described as a function of p ₀ using the phenomenological theory of a chiral smectic C phase. The pitch, p ₀, is then calculated using a molecular theory of the cholesteric phase. The results obtained explain the experimental observations, at least qualitatively.     

Effect of composition on the pressure-induced re-entrant cholesteric phase behaviour of mixtures of non-polar cholesteryl n-alkanoates

Abstract

Recently we reported a pressure-induced re-entrant cholesteric phase for ternary systems of non-polar cholesteryl n-alkanoates. In these systems one component contains a shorter n-alkyl chain than the other two. We now show that the former produces a positive excess volume in the smectic phase, which is probably responsible for the pressure-induced re-entrant phase behaviour. The maximum temperature of the cholesteric/smectic A phase boundary is found to decrease drastically with decrease of the n-alkyl chain length of this particular component.     

New isothiocyanato liquid crystals containing thieno[3,2-b]thiophene central core

New isothiocyanato liquid crystal (LC) materials based on thieno[3,2-b]thiophene core have been synthesised and characterised. Their mesomorphic and physical properties were evaluated. The results indicate that most of the new compounds exhibit only enantiotropic smectic A phases with high clearing points, and the difluorinated compounds (A3 and A6), in particular, also display a stable nematic phase with low melting points. Compared with the corresponding biphenyl and thiophene-phenyl analogues, these thieno[3,2-b]thiophene-based compounds possess increased birefringence (the highest value found to be 0.40) and mesogenic phase intervals. These new thieno[3,2-b]thiophene-based LCs exhibit a high birefringence and a high clearing point and are of potential use as components of high birefringence mixtures.     

Auto-Organisation in Silica-Based Organic-Inorganic Gels Obtained by Sol-Gel Process

A new type of hybrid material is prepared through sol-gel processing by the polycondensation of (MeO)₃Si–R–Si(OMe)₃ units containing a rigid organic anisotropic group R. The hybrid covalently bounded organic-inorganic gel shows an unexpected structural birefringence n (n = 2 × 10^–3). This birefringence is induced by a strain field anisotropy during gel aging as revealed by simultaneous dynamic light scattering and birefringence measurements. To better address the role of the strain field, we use a free interface sol-gel/air to control strain anisotropy and to measure the gel optical axis and its birefringence. We find that the birefringence is associated to a long-range orientational order of organic moieties induced by the strain anisotropy during the gel aging.     

The effect of different arms on the properties of chiral branched-arm liquid crystals based on isosorbide as the chiral core

A new series of two-arm ( MA₁ , MA₂ ), four-arm ( MA₃ ) and six-arm ( MA₄ ) chiral liquid crystalline compounds containing isosorbide as the chiral core were synthesised. Four precursors of branched-arm A₁–A₄ were first obtained, and then were esterised separately with isosorbide to obtain four chiral branched-arm liquid crystals ( MA₁–MA₄ ). The structure and properties have been characterised. The results show that MA₁ was smectic–cholesteric phase with a fan-like and oily streak texture; MA₂–MA₄ showed a cholesteric phase with oily streak texture, or lined texture and finger print texture. Isosorbide successfully induced a cholesteric phase. The melting point and clearing point values for MA₁–MA₄ first increased and then decreased. The branched-arm and chiral core quantity affected the liquid crystalline properties. At the same time, the difference in side-arms resulted in different directions of rotation. MA₁–MA₂ showed right-handed rotation and had selective reflection; MA₃–MA₄ demonstrated left-handed rotation and did not have selective reflection.