Assignment of the quadrupolar splittings in fully deuteriated alkyl chains of liquid crystalline compounds The case of 4-n-hexyloxy-4'-cyanobiphenyl

There have been disagreements about the assignment of the quadrupolar splittings in the deuteriated n-hexyl chain in 4-n-hexyloxy-4'-cyanobiphenyl (6OCB); these are resolved by comparing the splittings of the deuteron resonances in the fully deuteriated chain compound with those in the material with a selectively deuteriated chain. The methods available for assigning the quadrupolar splittings in fully deuteriated chains are then applied to 6OCB and it is concluded that measurement of spin-lattice relaxation times, T₁, is the best method.     

The structure and conformation of a mesogenic compound between almost zero and almost complete orientational order

The conformational distributions in molecules that form liquid crystalline phases are predicted to depend strongly on orientational order. Results are presented here to test this hypothesis. The mesogen 4-hexyloxy-4'-cyanobiphenyl (6OCB) has been studied by NMR spectroscopy in the isotropic phase and in the nematic phase. In the isotropic phase the field-induced orientational ordering produces small dipolar couplings between ¹³C and ¹H nuclei, which were determined from the ¹³C spectra. Couplings between ¹H nuclei were also obtained using 2D selective refocusing experiments. In the nematic phase, both ¹H-¹H dipolar couplings and quadrupolar splittings for deuterium nuclei were measured for partially-deuterated samples. Both proton and deuterium spectra were also obtained for 6OCB in an equimolar mixture with 4-(ethoxybenzylidene)-4'-butylaniline (EBBA). This mixture exhibits SmA and SmB phases. The data obtained from these experiments has been analysed to yield the probability distribution of the conformations in this molecule generated by rotations about bonds. It is found that there is a substantial influence of the orientational order of the molecules on these distributions.     

The influence of director fluctuations on molecular reorientation of a small probe molecule in a liquid-crystalline environment

The influence of director fluctuations on a probe molecule dissolved in a nematic liquid crystal is calculated to all orders using the gaussian properties of the director field. Consequences for the N.M.R. spectrum and relaxation through quadrupolar interaction of the probe molecule are indicated. It is found that the equilibrium distribution of molecular orientations, and consequently quadrupolar spectral splittings are virtually unchanged by fluctuations in the director field. The spectral densities are also obtained and it is shown that in addition to J₀₁ also J₀₀ and J₀₂ become frequency dependent in the N.M.R. frequency range. The frequency dependence of J₀₀ and J₀₂ has a logarithmic contribution; the magnitude of the frequency dependent contribution to J₀₂ is, however, insufficient to explain experimental results.     

Study of internal ring rotation and molecular reorientation in the liquid crystal 5O.7 by deuterium N.M.R. spectroscopy

The spectral densities of motion were determined by deuterium N.M.R. relaxation measurements in the nematic, smectic A and smectic C phases of 4-n-pentyloxybenzylidene-d₁-4'-heptylaniline and 4-n-pentyloxybenzylidene-4'-heptylaniline-2,3,5,6-d₄. By examining two atomic sites on a 5O.7 molecule, we were able to gain information on the reorientation motion and internal rotation of the aniline ring. It was also found that director fluctuations make some contribution to the spectral density J₁ (ω). We use the superimposed rotations model to account for the internal ring motion and the small step rotational diffusion model for the molecular reorientation. The derived rotational diffusion constants for the spinning and tumbling motions appear to give physically plausible activation energies in the mesophases of 5O.7.     

The structure and conformation of a mesogenic compound between almost zero and almost complete orientational order

The conformational distributions in molecules that form liquid crystalline phases are predicted to depend strongly on orientational order. Results are presented here to test this hypothesis. The mesogen 4‐hexyloxy‐4′‐cyanobiphenyl (6OCB) has been studied by NMR spectroscopy in the isotropic phase and in the nematic phase. In the isotropic phase the field‐induced orientational ordering produces small dipolar couplings between ¹³C and ¹H nuclei, which were determined from the ¹³C spectra. Couplings between ¹H nuclei were also obtained using 2D selective refocusing experiments. In the nematic phase, both ¹H–¹H dipolar couplings and quadrupolar splittings for deuterium nuclei were measured for partially‐deuterated samples. Both proton and deuterium spectra were also obtained for 6OCB in an equimolar mixture with 4‐(ethoxybenzylidene)‐4′‐butylaniline (EBBA). This mixture exhibits SmA and SmB phases. The data obtained from these experiments has been analysed to yield the probability distribution of the conformations in this molecule generated by rotations about bonds. It is found that there is a substantial influence of the orientational order of the molecules on these distributions.     

Dielectric relaxation studies of 6OCB/8OCB mixtures with the nematic-smectic A-nematic re-entrant phase sequence

The low frequency relaxation process was studied for 6OCB/8OCB mixtures with three concentrations (27.0, 27.3 and 27.5 wt %) exhibiting the isotropic-nematic-smectic A-nematic re-entrant-crystalline phase sequence and four mixtures (28.5, 30.0, 35.0 and 40.0 wt %) with the isotropic-nematic-crystalline phase sequence. In the liquid crystalline phases, all dielectric spectra could be excellently described by the Debye equation. The relaxation time tau passes smoothly through the phase transitions separating the liquid crystalline phases. The activation barriers hindering the molecular rotations around the short axes are practically the same in the nematic and smectic phases and become larger in the re-entrant nematic phase. Smaller values of the barrier in the nematic phase of mixtures in comparison with those obtained recently for pure 6OCB and 8OCB are explained as an effect of weakening of the molecular interactions caused by increased dipole-dipole associations between molecules in mixtures in relation to pure substances. The slightly larger activation barrier in the nematic re-entrant phase indicates stronger molecular associations in this phase. d     

Dielectric relaxation studies of 6OCB/8OCB mixtures with the nematic-smectic A-nematic re-entrant phase sequence

The low frequency relaxation process was studied for 6OCB/8OCB mixtures with three concentrations (27.0, 27.3 and 27.5 wt %) exhibiting the isotropic-nematic-smectic A-nematic re-entrant-crystalline phase sequence and four mixtures (28.5, 30.0, 35.0 and 40.0 wt %) with the isotropic-nematic-crystalline phase sequence. In the liquid crystalline phases, all dielectric spectra could be excellently described by the Debye equation. The relaxation time tau passes smoothly through the phase transitions separating the liquid crystalline phases. The activation barriers hindering the molecular rotations around the short axes are practically the same in the nematic and smectic phases and become larger in the re-entrant nematic phase. Smaller values of the barrier in the nematic phase of mixtures in comparison with those obtained recently for pure 6OCB and 8OCB are explained as an effect of weakening of the molecular interactions caused by increased dipole-dipole associations between molecules in mixtures in relation to pure substances. The slightly larger activation barrier in the nematic re-entrant phase indicates stronger molecular associations in this phase. d     

Order parameters of dye molecules in lyotropic nematic mesophases

The alignment in a magnetic field of two similar azo dyes in binary nematic lyotropic phases (C₇F₁₅CO₂NH₄ and C₈F₁₇CO₂NH₄ in D₂O) has been studied. The order 5 parameter for the dyes was determined by measuring quadrupolar splittings (deuterium N.M.R.), the anisotropy of the electric conductivity and dichroism. The order parameter of the dye fixed in the disc-like perfluoro micelle with positive diamagnetic anisotropy was determined as a function of magnetic field strength, surfactant concentration, temperature and chain length. In addition the time constants for orientation and disorientation were determined.     

Nuclear spin relaxation of sodium cations in bacteriophage Pf1 solutions

The nuclear magnetic resonance (NMR) spectra for the I=3/2 23Na cation dissolved into filamentous bacteriophage Pf1 solutions display line splittings and relaxation times consistent with an interaction between the 23Na nuclear quadrupole moment and the electric field gradient produced by the negatively charged Pf1 particles. The 23Na NMR line splittings and relaxation rates corresponding to magnetization recovery and single, double, and triple quantum coherence decays are measured in Pf1 solutions and compared to theoretical values. The deviation of the observed dc spectral density J0 from the equal first harmonic J(omega0) and second harmonic J(2omega0) values as J(omega0)=J(2omega0) not equal to J0 in these solutions suggests that ion migration in the electric field gradient of the Pf1 particles produces an anisotropic relaxation mechanism. Correlation functions and thus spectral densities for this process are calculated from solutions to the Fokker-Planck equation for radial motion in an electric potential and used to estimate measured relaxation rates. Appropriate electric potentials are generated from the solutions to the Poisson-Boltzmann equation for a charged Pf1 particle in aqueous phase, functions that lead to theoretical estimates of NMR line splittings consistent with experimental observations.     

Textural,thermal, optical and electrical properties of Iron nanoparticles dispersed 4′-(Hexyloxy)-4-biphenylcarbonitrile liquid crystal mixture

In this work, the effect of Iron nanoparticles (Fe NPs) dispersion in 4′-(Hexyloxy)-4-biphenylcarbonitrile (6OCB) nematic liquid crystal properties has been studied. Inclusion of Fe NPs (0.25 wt. %) in 6OCB liquid crystal (LC) on textures, isotropic–nematic transition temperature (T_I–N), electro-optical and dielectric properties have been investigated in planar aligned cell. The threshold voltage (V_th) and T_I–N decrease after dispersion of Fe NPs. Dielectric spectroscopy in nematic phase show that relaxation frequency (f_r) also decreases after dispersion of Fe NPs in 6OCB. The observed relaxation mode is due to the flip-flop motion of LC molecules about their short axis. The band gap and AC conductivity in case of 6OCB-Fe sample increase over pure 6OCB sample. A decrease in activation energy is also noticed.     

Transitions between the B2 phase and more usual smectic phases in binary systems of banana-shaped with calamitic mesogens

Deuterium NMR spectroscopy is used to study a ring-deuteriated chiral liquid crystal 4-(2-methylbutyl)oxycarbonylphenyl 4-(10-undecenyloxy)benzoate. The quadrupolar and proton-deuteron dipolar splittings, and deuteron quadrupolar and Zeeman spin-lattice relaxation times were measured as a function of temperature in the smectic A phase at two different Larmor frequencies. The derived spectral densities of motion at different temperatures were analysed simultaneously using a rotational diffusion model which also includes internal ring rotations. Motional parameters (D _⊥, D _∥, D _R) and order parameter tensors (S_zz , S_xx -S_yy ) were obtained. Although the present data seem insufficient to draw a definitive conclusion, we believe that it is possible for this particular chiral molecule to have D_⊥ >D_∥ , which is different from non-chiral rod-like liquid crystals.     

Study of the surface and interfacial tensions in systems containing a low molar mass liquid crystal

The surface tension of a low molar mass liquid crystal (LMMLC), 4-cyano-4'-n-heptyloxybiphenyl (70CB), was measured as a function of temperature using the pendant drop method, forming drops of different volumes ranging from 5 to 11 mm³. Contact angles formed by drops of 70CB in the nematic and isotropic phases on plates of polystyrene (PS) and of a liquid crystal polymer (LCP), VECTRA A910, were also measured. Only large drops could be used for surface tension analysis. It was shown that in the nematic phase the surface tension of 70CB decreases with increasing temperature, and that in the isotropic phase the surface tension increases with increasing temperature. Using the values of contact angle and of surface tension of 7OCB it was possible to evaluate the interfacial energy between 7OCB and PS and between 7OCB and VECTRA. The interfacial energy between 7OCB and PS, and between 7OCB and VECTRA, decreased with increasing temperature for ranges of temperatures corresponding to both phases of 70CB.     

Coupled motional model for spin relaxation in 5CB liquid crystal

Using the master equation method we examine the effects of correlated internal rotations in an alkyl chain on its deuterium spectral densities of motion. The overall reorientation of molecules is coupled to the internal motions in their alkyl end chains by the use of conformation-dependent diffusion axes. The pentyl chain that is attached to a cylindrical biphenyl core in 4-n-pentyl-4′-cyanobiphenyl is studied and the results are compared with the previously published decoupled diamond-lattice model.     

Dielectric studies of nematic-smectic A-reentrant nematic transition in the 8OCB/6OCB mixtures

The results of measurement of the anisotropies of the electric permittivity and conductivity for pure 8OCB and 6OCB together with their mixtures are presented. It was found that (i) the dielectric properties of mesophases composed of molecules with a strong tendency to antiparallel association in the nematic phase (8OCB) and molecules which do not exhibit such a tendency (6OCB) undergo a drastic change in mixtures with a relatively low concentration of 6OCB, (ii) the effect of smectic density modulation in the N-S_A-N_R sequence is very subtle, and (iii) for concentrations higher than a critical value the nematic phase exhibits a smectic-like ordering which vanishes with increasing concentration of 6OCB.     

Deuterium N.M.R. of the n-octyl-β-D-glucopyranoside-solvent liquid-crystalline systems

Deuterium quadrupolar splittings were measured as a function of concentration and temperature for deuteriated water, benzene-d₆, toluene-d₈ and acetonitrile-d₃, dissolved in n-octyl-β-D-glucopyranoside (OG). The splittings and temperature dependence are larger for the apolar solvents. Water forms rather rigid structures with OG as indicated by its large linewidths. OG can accommodate both water and benzene (or toluene) together and the results for such systems are also reported. Some additional results for the hydroxyl deuteriated OG and for a system containing a lower homologue of OG are reported.     

Conformational analysis of 1,2-dimethoxyethane and 1,2-diphenyloxyethane incorporated in nematic liquid crystals

The conformations of flexible chain molecules incorporated in a nematic environment have been investigated. The phase behaviours and orientational characteristics of 1,2-dimethoxyethane (DME) dissolved in 4'-methoxybenzylidene-4-n-butylaniline at low solute mol fraction have been reported in our previous paper. In this work, proton-proton and carbon-carbon dipolar coupling constant measurements were attempted in addition to ²H NMR observations of quadrupolar splittings. These conformation-dependent properties were analysed according to the rotational isometric state (RIS) simulation scheme previously proposed. Our treatment rests on the assumption that the molecular axis of the chain should tend to align along the nematic field. Studies were further extended to a mixture of 1,2-diphenyloxyethane (DPE) with a nematic liquid crystal, 4,4'-azoxyanisole. Replacement of the terminal methyl groups in DME by phenyl groups leads to DPE. The results of the analysis indicate that the fraction of elongated conformers such as ttt tends to increase significantly on going from the isotropic to the liquid crystalline solution: DME; 9·4 to 15·6 per cent and DPE; 12·3 to 20·0 per cent. The tgt form (tg⁺ t or tg^-t) is the lowest-energy arrangement of these molecules. The fraction of this conformer also increases in DPE (26·8 to 40·0 per cent), while it decreases in DME (25·7 to 18·8 per cent). The conformational distributions of chain molecules were found to remain invariant over the range of concentration (0·5-6·0 mol per cent) and temperature (22·0-44·5°C) studied. It has been concluded that the observed variation of proton dipolar and deuterium quadrupolar couplings with concentration and temperature mainly arises from the orientational order of the molecular axis, which varies sensitively with the alignment of the surrounding solvent molecules. These results suggest that flexible chains are also participating in the nematic interaction by adjusting their configurations so as to enhance favourable interactions and suppress unfavourable steric repulsions when accommodated in an anisotropic potential field.     

Detailed study of a two component smectic 4TPB-8OCB system with a nematic gap. Phase diagram and viscosity study

The phase diagram of a two component system composed of two smectic compounds: 4-octyloxy-4'-cyanobiphenyl (8OCB) and 4-isothiocyanatophenyl 4-butylbenzoate (4TPB) was investigated. Three Miesowicz viscosity coefficients η₁, η₂, η₃ and the refractive indices at different temperatures as well as the enthalpies of the phase transitions were measured. It was stated, that the properties of the induced nematic phase, for example, the nematic phase existing between two smectic regions, are the same as the ones observed in the case of low viscosity nematic mesogens.     

The dynamics of the CH3CN molecule in the isotropic phase and in a nematic solution

NMR lineshape studies of acelonitrile in the isotropic and the liquid-crystalline nematic phase of PCH have been performed. The scalar relaxation of the second kind due to the presence of the ¹⁴N quadrupolar nucleus has been confirmed as the most important relaxation mechanism for this molecule in both the isotropic and the anisotropic phase. It has been found largely responsible for the selective broadening on ¹³C and ¹H transitions. A minor contribution arising from intramolecular dipolar relaxation mechanism has also been investigated. Linewidth analysis of the NMR spectra allowed us to determine the quadrupolar relaxation time T_IN of the ¹⁴N nucleus. This is connected to the correlation time for rotational diffusion perpendicular to the molecular symmetry axis. A possible explanation of a residual selective broadeining which effects the ¹³C and ¹H NMR transitions and is not taken into account by this mechanism, is also given.     

NMR relaxation of rigid molecules in the nematic phase of a discotic liquid crystal

Frequency and temperature dependent NMR relaxation measurements were performed on deuteriated benzene, pyrene and triphenylene dissolved in the nematic phase of a discotic liquid crystal. The results show a strong frequency dependence of the spectral densities. Based on the symmetries of the system and the usual model for director fluctuations this frequency dependence should be equal for J₁ and J₂. From fitting the commonly used model of rotational diffusion and director fluctuations to the data we see that this is not the case for benzene and triphenylene, even though the fits themselves are satisfactory. Values for the elastic constants, effective viscosity and translational diffusion in similar discotic liquid crystals do not account quantitatively for the frequency dependence of benzene. For both pyrene and triphenylene quantitative comparison was impossible due to lack of translational diffusion data. We also find that the so-called cut-off wave-length is of the order of the dimensions of the liquid crystal molecules, just as in ordinary nematics.     

Phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D2O system as determined by 2H and 14N NMR

The high resolution phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D₂O system has been mapped out using ²H and ¹⁴NNMR spectroscopy. The ¹⁴N quadruple splittings are more than an order of magnitude larger than corresponding ²H splittings, while the line widths are only two to three times larger. Thus, ¹⁴NNMR offers an order of magnitude improvement over ²H NMR in the resolution of the spectra from coexisting phases. The ²H spectra of samples in biphasic regions are often complicated by chemical exchange of D₂O molecules between coexisting phases, particularly at low TMAHFN concentrations. Analysis of the ²H line shapes of a TMAHFN/D₂O sample with a weight fraction of TMAHFN of 0.230 obtained at various times following cooling of the sample into the isotropic/nematic biphasic region shows that the mean diameter for the dispersed nematic droplet grows from about 7 to about 26 μm over a period of 2 h. At a mean droplet size of 7 μm the exchange of TMA⁺ ions between the coexisting phases is slow on the NMR time-scale and exchange effects are not observed in ¹⁴N spectra. The TMAHFN/D₂O phase diagram exhibits the generic form of those of the CsPFO/water and APFO/D₂O systems, which are the only other systems composed of stable discotic micelles for which high resolution phase diagrams are currently available, but the nematic phase is displaced to smaller TMAHFN concentrations. Specifically, a discotic nematic phase N_D⁺, intermediate between an isotropic micellar phase I and a lamellar phase L, exists for weight fractions of TMAHFN between 0.149 (φ = 0.105) and 0.420 (φa = 0.325) and temperatures between 277.3 and 327.6 K.