Dielectric studies of liquid crystals under high pressure III. Low frequency relaxation process in 4-(trans-4′-n-hexylcyclohexyl)isothiocyanato benzene

Abstract

Dielectric studies of 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) were performed in the pressure range 0·1–150 MPa, the frequency range 1 kHz–13 MHz and the temperature range 295–325 K. The temperature and pressure dependencies of the static permittivity ?_0∥ and of the relaxation time τ_∥ are analysed and compared with the analogous data obtained recently for 4-n-pentyl-4′-cyanobiphenyl (5CB) (Parts I and II of this series). Marked differences in the dielectric properties of the nematic phases of the two substances are observed. They are interpreted as a result of varying degrees of molecular association in particular compounds. It is concluded that in the nematic phase of 6CHBT dipole–dipole correlations do not exist or are very weak, whereas for 5CB they are easily broken by a relatively low pressure.     

Dielectric anisotropy in the nematic phase of 8PCH as a function of temperature and pressure

The dielectric permittivity components, ε_∥ and ε_⊥, in the nematic phase of 8PCH (trans-4-n-octyl(4-cyanophenyl)cyclohexane) were measured at 1 atm as a function of temperature (T), and at two temperatures as a function of pressure (p). A close similarity of the temperature and pressure behaviours of the dielectric anisotropy, δε = ε_∥ - ε_⊥, was established. It is argued that p and T are equivalent quantities in the formation of the nematic state. The well known Maier and Meier equations describe the dielectric parameters under both p = constant and T = constant conditions fairly well.     

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.     

Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CBnCB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CBnOCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBOnOCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBOnOCB dimer and lowest for the CBnCB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (T_NTBN) is higher for the CBnOCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBOnOCB dimers, a virtual value of T_NTBN was estimated for CBO3OCB and T_NTBN was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CBnCB or CBnOCB series. The dependence of T_NTBN on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.     

Why chiral tartaric imide derivatives give large helical twisting powers in nematic liquid crystal phases: substituent-effect approach to investigate intermolecular interactions between dopant and liquid crystalline molecules

Novel C₂-symmetric chiral dopant derivatives, namely, N-substituted (2R, 3R)-2,3-bis(4-(4-octyloxyphenyl)benzoyloxy)succinimides1a-h, were synthesised, and the effects of the N-substituent and imide-carbonyl groups on the helical twisting powers (HTPs) were investigated in two nematic liquid crystalline compounds, 4-n-pentyl-4?-cyanobiphenyl (5CB) and N-(4-ethoxybenzylidene)-4-n-butylaniline (EBBA). As a result, it was clarified that the bulkiness of the N-substituents has a significant correlation with the HTPs, and the imide-carbonyl group interacts strongly with the cyano group of 5CB to give high HTPs in the nematic phases. However, it is assumed that the imide-carbonyl groups of the dopants do not have strong electrostatic attractive interactions with EBBA molecules in the nematic phase to afford the moderate HTPs.     

The effect of olefinic terminal chains on the mesomorphic properties of 4,4′-disubstituted diphenyldiacetylenes

New diphenyldiacetylenes of the type with A, B = H and/or F; m = 0, 1; n = 1–4; and X = C _n H_2n+1, F, CF₃ or CN were synthesized and their mesomorphic properties determined by hot stage polarizing microscopy and DSC. When m = 0, all of these compounds showed only a nematic phase except when X = CF₃ when both nematic and smectic A phases were seen. Both clearing and melting temperatures were higher than those reported for substitution with the corresponding alkyl chains but the much larger increase in clearing temperatures produced considerably wider nematic phases. Eutectic mixtures of a few of these olefins yielded nematic materials also having much wider temperature ranges and higher clearing temperatures than the eutectic mixtures of the alkyl compounds, while retaining their high birefringence and low viscosities. Such materials are of interest for beam-steering devices. Four of the diacetylenes with m = 1 (A, B = H) were also prepared (X = C₆H₁₃, F, n= 2, 3). When X was C₆H₁₃ (n=2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X=F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X=C₆H₁₃ and n=2 had a melting temperature below room temperature.     

Dielectric studies of liquid crystals under high pressure II. Low frequency relaxation process in 4-n-pentyl-4′-cyanobiphenyl in relation to theories of the nematic state

Abstract

The results of high pressure dielectric studies of 4-n-pentyl-4′-cyanobiphenyl (5CB) are analysed in terms of theories of the nematic state. The retardation factor g∥ = τ∥/τ₀ and the effective, single-particle potential of mean torque were calculated at the nematic–isotropic transition temperature T _NI and along the isothermal, isobaric and isochoric paths within the nematic phase of 5CB. The potential of mean torque is compared with the order parameter known for the same conditions. The values of parameter γ relating the potential to the volume is discussed.     

The effect of inversion of the ester group on the mesophase behaviour of some azo/ester compounds

Five homologous series of 4-substituted phenyl 4′-(4″-alkoxy phenylazo) benzoates (In_a–?e) were prepared in which, within each homologous series, the length of the terminal alkoxy group varies between 8, 10, 12, 14 and 16 carbons, while the other terminal substituent, X, is a polar group that alternatively changed from CH₃O, CH₃, H, Br, and CN groups. Compounds prepared were characterised by infrared, mass, and H¹-NMR spectroscopy and their mesophase behaviour investigated by differential scanning calorimetry (DSC) and polarised light microscopy (PLM). The results were discussed in terms of mesomeric and polarisability effects. Only for the lower group of compounds, I8_a-e, that showed a nematic phase, the nematic-to-isotropic transition temperatures (T_N–I) were successfully correlated to the polarisability anisotropy of bonds to the substituent X. A comparative study was made between the investigated compounds and two previously prepared isomeric groups. In the first group of isomers, 4-(4′alkoxy phenylazo) phenyl 4″-substituted benzoates (IIn_a–e), the ester groups are inverted. While in the second, 4-(4′-substituted phenylazo) phenyl 4″-alkoxy benzoates (IIIn_a–e), two modifications were made, inversion of the COO group, and exchange of the two wing substituents     

Electron beam irradiation-induced transformations in the electrical properties of 4’-octyl-4-cyanobiphenyl (8CB)

Changes in the dielectric and thermodynamical properties of electron beam-irradiated 4′-octyl-4-cyanobiphenyl (8CB) were studied. Irradiation-induced changes in the phase transition temperature, dielectric anisotropy, relaxation frequency and activation energy of an observed non-collective relaxation mode corresponding to molecular rotation about the short axis were determined in both nematic and smectic A_d phases. In the nematic phase, dielectric anisotropy increased for a small dose but decreased for a relatively high dose, whereas the relaxation frequency increased due to the irradiation. The pure and irradiated samples were characterised by UV–visible spectroscopy, Fourier transform infrared spectroscopy, gas chromatography, gas chromatography coupled with mass spectroscopy and pulse radiolysis. The observed changes in the dielectric parameters are related to the detachment of the CN group from some of the 8CB molecules due to the electron beam irradiation.     

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.     

A cyanide-bridged 1-D helical chain involving both four- and six-coordinate nickel(II)

A cyanide-bridged coordination polymer, {[Ni(tren)][Ni(CN)₄]} _n (tren?=?tris(2-aminoethyl)amine), has been synthesized by self-assembly of cis-[Ni(tren)]²⁺ and [Ni(CN)₄]^2? building blocks. In the molecular structure, the paramagnetic cis-[Ni(tren)]²⁺ cations are bridged by diamagnetic [Ni(CN)₄]^2? anions through two cis-cyanides to form a 1-D helical chain of {[Ni(tren)][Ni(CN)₄]} _n . The compound crystallizes with a centrosymmetrical space group, P2₁ /n, in which the helical chains are packed in alternating right- and left-handed chiralities with a helical pitch of 10.2566(3)?Å (equal to the length of the b-axis), leading to the formation of a racemic compound. The crystal packing is stabilized by moderately strong hydrogen-bonding between primary amines of tren and nitrogens of terminal cyanide.     

Molecular dynamics in the nematic,SA and SC phases of 8S5 and 9S5 as studied by dielectric methods

Abstract

Two experimental techniques—time domain spectroscopy (TDS) and a steady state frequency method—have been used to study dielectric spectra for the isotropic, nematic, S_A, S_C and S_Y phases of two thioesters (4-n-pentylphenyl-4′-n-octyloxy-and 4′-n-nonyloxythiobenzoates (C _n H_2n+1 O?C₆ H₄?COS?C₆H₄?C₅H₁₁, where n = 8 and n = 9) known as 8S5 and 9S5, in the frequency range from 10 Hz to 10 GHz. In the case of 8S5 a deuteriated sample (8S5-d ₂₈) has been used to study relaxation processes in the nematic and smectic phases.     

New hockey stick compounds with a lateral methyl group showing nematic,synclinic and anticlinic smectic C phases

New hockey stick-shaped mesogens with a lateral methyl group inserted between the m-alkyloxy-chain and the azomethine connecting group, 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl) phenyl-4-n-alkyloxycinnamates have been synthesised. The interesting feature of these hockey stick-shaped mesogens is the appearance of the nematic phase in addition to two polymorphic tilted smectic phases – the synclinic smectic C (SmC_s) and the anticlinic SmC_a phase. Physical characterisation of these mesogens has been carried out using polarising optical microscopy, differential scanning calorimetry, optical birefringence, X-ray diffraction (XRD), electro–optical and dielectric spectroscopy measurements. The SmC_s–SmC_a phase transition is accompanied by only a small calorimetric signal but causes a pronounced change in the optical textures. Orientational order parameters determined from the birefringence measurements have been compared with those estimated from XRD patterns. Dielectric measurements point to the formation of small ‘soft’ ferroelectric clusters responsible for a low frequency absorption range.     

Elastic,dielectric and optical constants of the nematic mixture E49

The Merck nematic mixture E49 exhibits a large nematic interval (0–100 °C) and a large dielectric anisotropy. Both of these features make E49 interesting for applications and basic physics. Unfortunately, no systematic measurements of the material constants of this mixture and their temperature dependence have been reported in the literature. In this paper we report experimental measurements of the splay and bend elastic constants (K ₁₁ and K ₃₃) of the ordinary and the extraordinary refractive indices (n _ort and n _par) at the wavelength λ?=?632.8 nm and of the two elastic constants parallel and orthogonal to the director (ε_par and ε_ort) at the frequency ν?=?5?kHz. The temperature dependence of all of these parameters is found in the temperature range 25–99 °C. The measurements of the elastic constants are performed using both a dielectric and an optical method simultaneously on the same nematic sample. The results obtained using the two methods are in a satisfactory agreement between them within the estimated experimental uncertainty. The ordinary and the extraordinary indices are measured using the prism method.     

Synthesis and physico-chemical studies of novel mixed-ligand cyanonitrosyl {CrNO}5 complexes of chromium with benzyl-, benzoyl- and acetyl-pyridines

Summary Novel mixed-ligand cyanonitrosyl complexes of chromium(I), [Cr(NO)(CN)₂(L)₂(H₂O)], (where L=2-, 3-, or 4-benzylpyridine, 2-(4-chlorobenzyl)pyridine, 2-, 3-, or 4-benzoylpyridine, 3-(4-methylbenzoyl)pyridine, or 2-, 3-, or 4-acetylpyridine) have been prepared by the interaction of potassium pentacyanonitrosylchromate(I) monohydrate, K₃[Cr(NO)(CN)₅]·H₂O with L. The complexes, characterized by elemental analyses, magnetic measurements, e.s.r. and i.r. spectral studies, contain chromium(I) in a low-spind ⁵-configuration. An octahedral structure, where CN istrans to CN, L istrans to L, and NO istrans to water is proposed for all the complexesEnemark and Feltham⁽¹⁾ have proposed circumventing the problem of the non-innocent nature of the nitrosyl ligand by considering nitrosyls as containing the {MNO} ⁿ group, wheren is the number of electrons of M, plus the number of electrons in the ^*-orbital of the NO (or more convenientlyn is the number of d-electrons if nitrosyl is regarded as being coordinated as NO⁺)     

Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4?-[1,ω-alkanediylbis(thio)]bis-[1,1?-biphenyl]-4-carbonitriles (CBSnSCB), and 4?-({ω-[(4?-cyano[1,1?-biphenyl]-4-yl)oxy]alkyl}thio)[1,1?-biphenyl]-4-carbonitriles (CBSnOCB) are described. The odd members of both series show twist-bend nematic and nematic phases, whereas the even members exhibit only the nematic phase. An analogous cyanoterphenyl-based dimer, 3⁴-{6-[(4?-cyano[1,1?-biphenyl]-4-yl)thio]-hexyl}[1¹,2¹:2⁴,3¹-terphenyl]-1⁴-carbonitrile (CT6SCB), is also reported and shows enantiotropic N_TB and N phases. The transitional properties of these dimers are discussed in terms of molecular curvature, flexibility and biaxiality. The same molecular factors also influence the birefringence of nematic phases. Resonant X-ray scattering studies of the twist-bend nematic phase at both the carbon and sulfur absorption edges were performed, which allowed for the determination of critical behaviour of the helical pitch at the transition to the nematic phase, the behaviour was found to be independent of molecular structure. It was also observed that despite the different molecular bending angle and flexibility, in all compounds the helical pitch length far from the N-N_TB transition corresponds to 4 longitudinal molecular distances.     

The nematic-isotropic two phase region in mixtures of quasi-spherical solutes in alkylcyanobiphenyls: An E.S.R. study using tempone as probe

The transitional behaviour of binary mixtures, each containing a non-mesomorphic quasi-spherical solute at a low mole fraction in a nematogenic solvent, has been investigated by E.S.R. spectroscopy using 4-oxo-2,2,6,6-tetramethyl-4-piperidinyl-1-oxy (tempone) as a spin probe. This approach makes use of the smaller coupling constant obtained for the spin probe in the orientationally ordered nematic phase relative to that obtained in the isotropic phase. The solutes included Et₄C (tetraethylmethane) and R ₄Sn (R = C^b2bH₅, n-C_{3 H 7} and nC_b4H₉). The solvents were 4-n-pentyl-4′-cyanobiphenyl (5CB) 4-n-hexyl-4′-cyanobiphenyl (6CB) and 4-n-heptyl-4′-cyanobiphenyl (7CB). This fast approach compares favourably with other studies. In addition it provides the tempone spectral parameter, f, which is a relative measure of the fraction of the nematic phase at different temperatures within the two phase region.     

Effect of a rigid,non-polar solute on the dielectric anisotropy,the splay and bend elastic constants,and on the rotational viscosity coefficient of 4-4′-n-heptylcyanobiphenyl

The effect of mixing a rigid, non-polar, non-mesogenic solute, biphenyl (C₆H₅-C₆H₅), in the nematic solvent 7CB (4,4′-n-heptylcyanobiphenyl) is investigated. The solute is found to reduce the nematic order and a two-phase region appears. We report measurements of the transition temperatures, dielectric anisotropy, and splay and bend elastic constants, as well as the rotational viscosity coefficient by the method of electric field-induced Fréedericksz transition for biphenyl concentrations up to 8.0%.     

Role of weak interactions in controlling the topology of coordination polymeric chains in [Pt(CN)4] bridged Cu(II) complexes: Syntheses, crystal structure and magnetic studies

Four coordination polymeric complexes, [{Cu₂(aepn)₂Pt(CN)₄·H₂O}(H₂O){Pt(CN)₄}]_n, (1), [{Cu(dpt)₂Pt(CN)₄}]_n (2), [{Cu(dien)Pt(CN)₄}]_n (3) and [{Cu(iprdien)Pt(CN)₄}]_n (4) (where aepn = N-(2-aminoethyl)-1,3-propanediamine, dpt = 3,3′-imino bispropylamine, dien = diethylenetriamine, iprdien = N′-isopropyldiethylenetriamine), have been synthesized and characterized by X-ray single crystal structure determination and variable temperature magnetic measurements. The formation of the supramolecular assemblies has been rationalized and it has been shown that competition between coordinative forces and hydrogen bonding interactions is crucial in the determination of final solid-state packing. A zig-zag/helical 1D chain (2 and 3) appears when the chelating amine at the Cu(II) center is capable of forming the maximum number of hydrogen bonds, on the other hand when one or more of the amine sites are blocked, due to the prevention of the maximization of hydrogen bonding, (4,2) ribbons (1 and 4) appear. Variable temperature magnetic measurement shows the presence of weak antiferromagnetic interactions in all the complexes.     

Simulation studies of dipole correlation in the isotropic liquid phase

The Kirkwood correlation factor g₁ determines the preference for local parallel or antiparallel dipole association in the isotropic phase. Calamitic mesogens with longitudinal dipole moments and Kirkwood factors greater than 1 have an enhanced effective dipole moment along the molecular long axis. This leads to higher values of Δ? in the nematic phase. This paper describes state-of-the-art molecular dynamics simulations of two calamitic mesogens 4-(trans-4-n-pentylcyclohexyl)benzonitrile (PCH5) and 4-(trans-4-n-pentylcyclohexyl)chlorobenzene (PCH5-Cl) in the isotropic liquid phase using an all-atom force field and taking long range electrostatics into account using an Ewald summation. Using this methodology, PCH5 is seen to prefer antiparallel dipole alignment with a negative g₁ and PCH5-Cl is seen to prefer parallel dipole alignment with a positive g₁; this is in accordance with experimental dielectric measurements. Analysis of the molecular dynamics trajectories allows an assessment of why these molecules behave differently.