Alkyl Cyanobiphenyl Research Articles

Influence of multifluorophenyloxy terminus on the mesomorphism of the alkoxy and alkyl cyanobiphenyl compounds in search of new ambient nematic liquid crystals and mixtures

ABSTRACT A series of cyanobiphenyl (CB) and cyano-p-terphenyl (CT) derivatives containing a variety of highly fluorinated aryloxyl termini and different connecting bridges were efficiently synthesized via aromatic nucleophilic substitution (SNAr) and their mesogenic properties described. Comparison with the non-fluorinated analogues indicates that the terminal multifluoroaryloxy group generally decreases the crystal to nematic phase transistion temperature and enhances the supercooling of these mesogens. Furthermore, several binary LC mixtures formed by the multifluoroaryloxy tail-terminated compounds were found exhibiting promising wide room temperature nematic phases ranges comparable to the commercial quaternary mixture E7. Our binding free energy (GBE) calculations predict that the fluorinated aryloxy terminated molecules tend to assume homeotropic orientation on Al(ClO4)3 and Ni(ClO4)2 metal salt decorated surfaces, which is consistent with the observed anchoring behavior. As such, these materials are promising candidates for chemoresponsive sensor devices which display a rapid response to a variety of analytes.

New room temperature nematogens by cyano tail termination of alkoxy and alkylcyanobiphenyls and their anchoring behavior on metal salt-decorated surface

ABSTRACTA series of cyano tail-terminated alkoxy and alkyl cyanobiphenyl compounds and some cyano-p-terphenyl derivatives were synthesized and mesogenic properties described. Comparison with the K series and M series indicates that the terminal cyano group generally enhances the supercooling of the molecules. Furthermore, several binary LC mixtures formed by the cyano tail-terminated compounds were found exhibiting promising room temperature nematic phases ranges comparable to the commercial quaternary mixture E7. The equimolar binary mixture of CN5OCB and CN7OCB shows homeotropic ordering at the metal salts-decorated surfaces and planar ordering at the free surface, which is consistent with GBE values we calculated. As such, these materials are promising candidates for sensor devices which display a rapid response to a variety of analytes.

Synthesis and properties of fluorine tail-terminated cyanobiphenyls and terphenyls for chemoresponsive liquid crystals

ABSTRACTA series of fluorine tail-terminated alkoxy and alkyl cyanobiphenyl compounds and some cyano-p-terphenyl derivatives were synthesised and mesogenic properties described. Comparison with the non-fluorinated K series and M series indicates that the terminal fluorine atom generally decreases the transition temperatures and, more interestingly, depresses the formation of a smectic phase. Several binary LC mixtures formed by the fluorine tail-terminated compounds were found exhibiting promising room temperature nematic phases with wide ranges. The mixture F7OCB and F8OCB shows homeotropic ordering at the metal salts-decorated surfaces and planar ordering at the free surface, which may have potential application in designing a more sensitive and faster LC system to targeted analytes.

Structural, electrochemical and optical properties of 4′-n-alkyl-4-cyanobiphenyl (nCB) – A computational approach

ABSTRACTThe structures of nCB (n = 6 & 7 where n is the number of carbon atoms in the alkyl chain) have been optimized using the Becke3-Lee-Yang-Parr (B3LYP) hybrid functional with 6–31G+(d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been computed using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the DFT method. The features of electronic transitions and excited states have been calculated via configuration interaction singles (CIS) with the semiempirical Hamiltonian Zerner intermediate neglect of differential overlap (ZINDO). The photo sensitivity of liquid crystalline alkyl cyanobiphenyl has been presented on ultraviolet (UV) absorption based approach through Density functional theory (DFT) calculations. The structural and electrochemical properties such as HOMO (H), LUMO (L), and energy gap (Eg = EL – EH) have been investigated. A comparison of dimers during the different modes of interactions suggests an absorption maxima at longer wavelength for 7CB, indicating the high photo sensitivity. Further, the 6CB dimers exhibit a lower band gap; hence its conductivity is high in comparison with the 7CB dimers.

Effect of Molecular Interactions and Homologue Number on UV Absorption Spectra of Liquid Crystalline Alkyl Cyanobiphenyl Dimers: DFT Calculations

The ultraviolet (UV) absorption spectra of the liquid crystalline dimer complexes of 4′-n-alkyl-4-cyanobiphenyls (nCB: n = 3, 4, 5 where n is the number of carbon atoms in the alkyl chain) have been presented. The nCB structures have been optimized using the density functional Becke3-Lee-Yang-Parr (B3LYP) hybrid functional with 6-31+G (d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been computed using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the density functional theory (DFT). The features of electronic transitions and excited states have been calculated via configuration interaction singles (CIS) with the semiempirical Hamiltonian Zerner intermediate neglect of differential overlap (ZINDO). The various modes of molecular interactions and the homologue number are found to be structural parameters affecting the formation of mesophases, UV absorption spectral characteristics, and photo stability of the compounds. These results offer a hint for the calculations involving the different modes of molecular interactions, and separations between dimers, to model photo stability or in tuning the absorbing chromophore to match the wavelength of desired application.

Impedance spectroscopy investigation of electrophysical characteristics of the electrode-liquid crystal interface

The behavior of frequency dependences of the impedance of a capacitive measuring cell with a liquid crystal has been investigated in the frequency range from 10−1 to 105 Hz. A method for determining electrophysical characteristics of the liquid crystal in the bulk and at the liquid crystal-metal electrode interface has been proposed and tested for liquid crystals of the alkyl cyanobiphenyl series, which are doped with ionic surfactants. The method is based on the use of an equivalent electrical circuit, which makes it possible to approximate the impedance spectra with the required accuracy, and also on the determination of the frequency at the singular point in the impedance spectra, at which the reactive component of the electric current flowing through the liquid-crystal cell is negligible compared to the active component.

Impedance spectroscopy investigation of liquid crystals doped with ionic surfactants

The effect of a direct-current (dc) electric field on the electrophysical characteristics of nematic liquid crystals of the alkyl cyanobiphenyl series nCB (n = 6–8) and the multicomponent liquid-crystal mixture MB-1, which are doped with ionic surfactants, has been investigated using impedance spectroscopy. It has been found that the impedance spectra depend substantially on the dc bias voltage applied to the sample. The approximation of the measured spectra with specially developed equivalent circuits of the measuring cell with a sample has made it possible to determine the capacitance of the electric double layer and the electrical conductivity of the samples, as well as the mobility, concentration, and diffusion coefficient of the ions. It has been shown that a region of the space charge is formed in the frequency range f < 100 Hz near the electrodes of the measuring cell with a liquid-crystal sample, which leads to an increase in the active (resistive) and reactive (capacitive) components of the impedance.

Role of molecular interactions and end chain length on the photosensitivity of liquid crystalline alkyl cyanobiphenyl dimers – UV absorption-based approach through DFT calculations

In this paper, the photosensitivity of liquid crystalline alkyl cyanobiphenyl (nCB: n = 6, 7; n is the number of carbon atoms in the alkyl chain) dimers has been presented through density functional theory (DFT) calculations. The nCB structures have been optimized using the Becke, three-parameter, Lee–Yang–Parr hybrid functional with the 6-31+G (d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been computed using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the DFT method. The excited states have been calculated via configuration interaction singles with the semi-empirical Hamiltonian Zerner intermediate neglect of differential overlap. The influence of molecular interactions and the end chain length on ultraviolet absorption spectral characteristics and the photosensitivity of the compounds has been discussed. These results offer a hint for the protection of various optical devices from the intense light induced damages, and to model photosensitivity.

Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series

AbstractExperimental results for the temperature dependence of the density of propyl – cyanobiphenyl (3CB), butyl – cyanobiphenyl (4CB) and hexyl – cyanobiphenyl (6CB) are presented. The results are compared with previous results for temperature dependence of other members of the alkyl cyanobiphenyl series (nCB). The deviation of the density from linear temperature dependence is discussed in terms of the recently discovered corresponding rule for nematic liquid crystals. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Virtual isotropic‐nematic transitions in alkyl cyanobiphenyls

Pretransitional fluctuations in the isotropic phase of liquid crystalline and non‐liquid crystalline alkyl cyanobiphenyls have been investigated using light scattering and magnetic birefringence measurements. We find evidence for a virtual isotropic‐nematic phase transition in short‐chain alkyl cyanobiphenyls with no observable nematic phase. The measured temperature dependence of fluctuations is well‐described by mean‐field theory. Virtual phase transition temperatures extrapolated from separate light scattering and magnetic birefringence experiments are in good agreement. Landau–de Gennes model parameters for the compounds investigated are calculated from the experimental results.

The dynamic electric birefringence of homeotropic aligned liquid crystals

Dynamic Kerr effect behaviour and transient molecular reorientation were investigated in thin layer of some alkyl cyanobiphenyl liquid crystals in the nematic phase at several temperatures and in an electro-optical system based on homeotropic-homogeneous alignment. All of the Kerr effect transients were obtained using homeotropically aligned samples. The variation of the relaxation decay time with temperature is very small, and low activation energies are obtained for the cooperative reorientation of the whole samples. The shapes of the Kerr decay transient for large retardations and the static Kerr constants of the liquid crystals were also studied using a computer program of MathCad 4.0.

Thermal- and light-induced nanocluster formation in silver–mesogenic cyanophenyl condensed films

Thermal- and light-induced formation of nanoclusters has been studied by ESR spectroscopy in silver–mesogenic cyanobiphenyl films obtained via low temperature co-deposition of metal and organic component vapors—long chain alkyl(alkoxy)cyanobiphenyls (CB): C n H 2 n+1 –C 6H 5–C 6H 5–CN, where n=5 (5CB), 8 (8CB) and C n H 2 n+1 O–C 6H 5–C 6H 5–CN, n=8 (8OCB). The combination of the results of IR, UV–Vis and ESR spectroscopy of silver–cyanobiphenyl co-condensate films shows the formation of metastable Ag(CB) 2 complexes at 90–200 K. Both thermal degradation of the complexes by annealing of the film samples up to 200–300 K and by light irradiation at lower temperatures lead to the formation of silver nanoclusters and their further aggregation in anisotropic matrix resulted in the production of anisotropic metal–mesogenic nanostructures.

Effect of surface and molecular structure on the dielectric behaviour in thick layers of cyanobiphenyl liquid crystals

The static dielectric studies of hexoxy heptoxy and octoxy cyanobiphenyl liquid crystals (6OCB, 7OCB and 8OCB) and the euteric nematic mixture of 6OCB/7OCB indicate that molecular alignment can be induced by the steel electrod surface of a dielectric thick cell with an electrode gap of about 1 mm. Whereas the permittivity values of alkyl cyanobiphenyls (6CB, 7CB and 8CB) in the nematic phase indicate imperfect alignment of the liquid crystal molecules. The good parallel molecular alignment of alkoxy cyanobiphenyls was to existence of oxygen atom in their molecular structures. These observations demonstrated a sensitive relationship between the molecular alignment, molecular structure and surface effect. Analysis of the dielectric data re-confirmed that the dipole components of the alkoxy cyanobiphenyls along the long molecular axes have a strong tendency for anti-parallel alignment which increases with increasing nematic order, whereas the dipole components perpendicular to the long molecular axes indicate a tendency for parallel alignment.

Thermal behavior of silver-containing mesogenic cyanobiphenyl films

Abstract Formation of nanostructures in metal-mesogenic co-condensate films have been studied during low temperature codeposition of metal and organic component vapors—long chain alkyl(alkoxy)cyanobiphenyls (CB): C n H 2 n +1 →C n H 2 n +1, where n =5 (5CB), 8 (8CB), and C n H 2 n +1 OC 6 H 5 C 6 H 5 CN, n =8 (8OCB). The existence of low temperature Ag–CB complexes was shown by IR, UV-visible and ESR techniques. Thermal degradation of the complexes by annealing of the samples up to 300 K led to the formation of nanosize (15–30 nm) silver particles incorporated in anisotropic CB-structure and their further aggregation in mesogenic matrices.

Surface ordering above the isotropic-smectic-A transition at a silane-treated substrate.

Surface ordering in a homologous series of alkyl cyanobiphenyl (nCB) liquid crystals having a direct isotropic-smectic-A (I-A) transition was investigated using evanescent-wave ellipsometry. The liquid crystal was bounded by a solid substrate treated with a silane surfactant which induced homeotropic (perpendicular) ordering of the liquid crystal molecules in the smectic-A phase. In the isotropic phase, one of the liquid crystals (10CB) partially wet the interface with an orientationally ordered, homeotropically aligned layer. The ordered interfacial layer grew without layering transitions but remained finite in thickness as the bulk I-A transition was approached. The interfacial layer has significantly lower orientational order than is observed in the smectic phase, indicating the possibility that the surface region of 10CB may be in a surface-induced, nonspontaneous nematic phase. The other liquid crystals (11 and 12CB) showed no surface ordering behavior whatsoever. Models describing the ordered surface layer of 10CB are presented. The results can be interpreted as a sharp transition in the surface ordering behavior as the chain length of the liquid crystal is varied, at the I-A transition of a liquid crystal.

The Effect of Dopants on the Twist Elastic Constant and the Rotational Viscosity Coefficient for Nematics

The twist elastic constant, K2, and the rotational viscosity coefficient, γ1, are of importance when the response time for the in-plane switching mode is studied. Since adding dopants is one technique to improve the response characteristics, the effect of dopants on these physical properties is significant. The effect on K2 and γ1 of adding alkyl(alkoxy) phenylcyclopentenones and alkyl(alkoxy) cyanobiphenyls to the base mixture ZLI-4792 together with their temperature dependence have been investigated using different temperature scales. The reduced temperature scale showed the effect of these dopants on K2 is small. On the other hand, the temperature dependence of γ1 depends on both the absolute temperature scale and the reduced temperature scale. Therefore, it is clear that the choice of temperature scale with which to compare γ1 for different systems raises fundamental questions which way not have a unique answer.

Synthesis and properties of metal–mesogenic nanostructures

Formation of metal–organic nanostructures in co-condensate films have been studied during low temperature codeposition of metal and organic component vapours using a new class of mesogenic ligands-long chain alkyl(alkoxy)cyanobiphenyls (CB): C n H 2 n+1 –C 6H 5–C 6H 5–CN, where n=5 (5 CB), 8 (8 CB), and C n H 2 n+1 O–C 6H 5–C 6H 5–CN, n=8 (8 OCB). The existence of low temperature Ag-5CB complexes was shown by IR–UV–Visible and ESR techniques. By performing ab initio quantum chemistry calculations including partial geometry optimization and vibrational analysis for the sandwich-like cyanobenzene–silver complexes, it is shown that the observed red shifts in the CN stretching region of the IR spectra of silver–cyanobiphenyl condensate films is a consequence of the formation of the π-complexes. Thermal degradation of the complexes by annealing of the samples at 200–300 K led to the formation of nanosize (15–30 nm) silver particles incorporated in anisotropic CB-structure and their further aggregation in mesogenic matrices.

Polarized ATR-IR Studies of Surface-Field-Induced Anchoring of Alkyl Cyanobiphenyls

We report initial studies of the use of polarized ATR-IR spectroscopy to obtain information on molecular alignment, on the mesoscopic scale, in the interfacial region of alkyl cyanobiphenyl liquid crystals in contact with self-assembled monolayers (SAMs). Specific mesoscopic alignment was induced by variation of the surface field, via the ω-functional group of self-assembled monolayers of alkanethiol derivatives. Our experimental approach involved the deposition of a thin gold layer onto a ZnSe prism. The presence of this layer strongly attenuates the s-polarized field leading to an “effective surface selection rule”, which can be used to determine the average orientation of the liquid crystal molecules within the attenuation depth of the evanescent field. A frequency shift of the C⋮N stretching band was seen in the case of homeotropic anchoring.

Studying topography and sub-surface structure of 8CB liquid crystal films with shear-force microscopy

Scanning shear-force microscopy, which is usually employed to track scanning near-field optical microscopy probes across samples, was used to observe the sub-surface smectic layer structure through the thickness of a film of the alkyl cyanobiphenyl liquid crystal 8CB on a highly oriented pyrolytic graphite substrate from force-distance measurements. With the noncontact imaging shear-force microscopy supplies, it was possible to follow, in the topography, the movement of a monolayer of the 8CB molecules. This has enabled the diffusion of molecules in a precursor film of 8CB to be observed without significantly disrupting the free liquid surface.

Study of molecular motions in alkyl cyanobiphenyls using positron lifetime spectroscopy

Positron lifetime measurements have been carried out in two liquid-crystal-forming homologues of alkyl cyanobiphenyls, 7CB and 10CB. In each of these two compounds measurements were performed during the heating cycle of samples prepared either by quenching or by slow cooling from the respective liquid crystalline phase. In both compounds, the behavior of quenched and slow-cooled samples is found to be different. Unlike the slow-cooled sample, the material in the quenched sample seems to have transformed into a glassy solid. Theo-Ps pick-off lifetime in the quenched sample exhibits a strong temperature dependence. In each case, its value exhibits six broad peaks at various characteristic temperatures which have been ascribed to various motions associated with the molecules of these compounds. The quenched samples of 7CB and 10CB exhibit glass transitions at ∼268 and ∼266 K, respectively. The present work demonstrates an interesting application of positron lifetime spectroscopy (PLS).