S. Sreehari Sastry

Induced Smectic-G Phase Through Intermolecular H-Bonding: Part III. Influence of Alkyl Chain Length of p-n-Alkoxybenzoic Acids on Thermal and Phase Behaviour

Abstract A novel series of intermolecular hydrogen bonding complexes have been synthesized by using p-n-alkoxybenzoic acids (alkyl chain length varies from propyl- to decyl and dodecyl-) (ABA) and a non-mesogenic moiety, 2-amino-5-chloro-pyridine (ACP), The thermal and phase behaviour of these complexes are studied by thermal microscopy (TM) and differential scanning calorimetry (DSC) techniques. The stabilization of intermolecular hydrogen bonding is studied by IR spectra. A detailed IR spectral investigation in solid and solution states suggest that both acid and pyridine nitrogens are complementary to one another by acting as proton donor and acceptor respectively. The comparative thermal studies with free benzoic acids and the corresponding analogous hydrogen bonded complexes reveal the inducement of a monotropic smectic-G phase with simultaneous quenching of nematic and smectic-C phases.

Order parameter studies on EPAP alkanoate mesogens

The refractive indices of p(p-ethoxyphenylazo) phenyl (EPAP) alkanoates, which are the nematic liquid crystals where the alkyl chain n is 5, 6, 7 and 11, were measured as a function of temperature at λ = 6328 Å using a wedge technique. The temperature variation of the orientational order parameter (S) is calculated at this wavelength using the Vuks model (with consideration of internal field) and by different methods excluding the internal field – Haller and Kuczynski models, and involving effective geometry parameter (αg). The intensity of light deflection in a uniaxial EPAP alkanoate with a particular molecular configuration and ordering by varying temperature is discussed. The temperature gradients of the ordinary (∂no /∂T) and extraordinary (∂ne /∂T) refractive indices are analysed. Due to a high clearing temperature with low molecular chain length, EPAP valerate exhibited low deviation of light with a large positive value of ∂no /∂T.

Investigations into effective order geometry in a series of liquid crystals

The effect of temperature variations on refractive index and molecular polarisability have been studied in liquid crystals based either on p,p′-alkoxyazoxybenzene derivatives, in which the alkyl group is n-pentyl or n-heptyl, or on p-alkoxybenzoic acids, in which it is n-hexyl or n-nonyl. The order parameter has been calculated using the Vuks, Neugebauer and Wu-modified equations, and also from birefringence measurements. In addition, an order parameter equation involving the effective geometry parameter has been derived and its influence on the deflection of light by the liquid crystalline compounds studied. At low temperatures, and at high values of order parameter and birefringence, the effective geometry parameter was low. In the light of the results obtained, a comparison of the order parameter derived by various methods is presented. The temperature variation of temperature gradient of the ordinary refractive index, dno /dT, and extraordinary refractive index, dne /dT, of the liquid crystals are sho...The effect of temperature variations on refractive index and molecular polarisability have been studied in liquid crystals based either on p,p′-alkoxyazoxybenzene derivatives, in which the alkyl group is n-pentyl or n-heptyl, or on p-alkoxybenzoic acids, in which it is n-hexyl or n-nonyl. The order parameter has been calculated using the Vuks, Neugebauer and Wu-modified equations, and also from birefringence measurements. In addition, an order parameter equation involving the effective geometry parameter has been derived and its influence on the deflection of light by the liquid crystalline compounds studied. At low temperatures, and at high values of order parameter and birefringence, the effective geometry parameter was low. In the light of the results obtained, a comparison of the order parameter derived by various methods is presented. The temperature variation of temperature gradient of the ordinary refractive index, dno /dT, and extraordinary refractive index, dne /dT, of the liquid crystals are shown. Among the compounds studied, p-n-heptyl–p′-azoxybenzene produced a large value for dno /dT, due to its lower clearing temperature.

Eigen value analysis studies on hydrogen-bonded mesogens

A new method is proposed for the physical investigation of hydrogen-bonded liquid crystals based on eigen value analysis and statistical covariance. Mesogenic p,n-alkyl benzoic acid (nBA) where n = 5–10 and non-mesogenic heptyl p-hydroxybenzoate (7HB) are used to obtain the hydrogen bonded liquid crystals. Intermolecular interactions result in the hydrogen bond between the proton donor (COOH) of nBA and proton acceptor (OH) of 7HB. Molecular interactions involving the various functional groups are studied by exploring the Fourier transform infrared spectral data to the maximum eigen value analysis. Furthermore, maximum eigen value of the liquid crystal textures recorded as a function of temperature is used to identify the mesomorphic characteristics of the samples. Results obtained from this method are in good agreement with literature.

Identification of phase transition temperatures by statistical image analysis

Analysis of the textures and phase transitions of thermotropic liquid crystals p-ethoxybenzylidene-p-n-butylaniline, cholesteryl myristate and cholesteryl decanoate was carried out using a polarising optical microscope under crossed polarisers attached with a hot stage and a high-resolution camera. Using MATLAB software, the statistical parameters, such as mean, standard deviation, root mean square, contrast and entropy of the textures as a function of temperature, are computed. These results show abrupt changes as a function of temperature, indicating the phase transition of the sample. The statistical image analysis of various liquid crystal samples are in good agreement with the literature results and the proposed methodology offers a reliable and very sensitive technique to identify mesophase transition temperatures.

Conformational studies of intermolecular hydorgen bonding through induced crystal G phase in nBA:7HB

This paper exploits the physical investigation on liquid crystal complexes obtained by self-organisation of p-n-alkyl benzoic acid (nBA) mesogens with non-mesogenic materials heptyl p-hydroxy benzoate (7HB). Intermolecular interactions of the molecules result the hydrogen bond between the proton donor (COOH) of nBA and proton acceptor (OH) of 7HB. The formation of hydrogen bond is attributed to the quenching of the nematic phase and inducement of crystal G phase in liquid crystal complex. A comparative study of phase abundance is presented with respect to the pure nBAs and other hydrogen bonded liquid crystal complexes of nBAs. Thermal and phase behaviour of the complexes are determined by polarising optical microscope (POM), differential scanning calorimetry (DSC) and image moments approach. Intermolecular interactions which result the hydrogen bond in complexes are investigated using Fourier transform infrared (FTIR) spectroscopy. Molecular structure of the liquid crystal complexes in the solid phase was elucidated using powder X-ray diffraction and proton nuclear magnetic resonance (1HNMR).

Novel approach to study liquid crystal phase transitions using Legendre moments

A novel approach is proposed to investigate the phase transitions of cholesteric liquid crystals using the Legendre moments. The textures of cholesteric liquid crystals (cholesteryl butyrate, cholesteryl n-valerate, cholesteryl decanoate, and cholesteryl myristate) are captured as a function of temperature using high-resolution camera attached to the arthroscopic mode of polarizing optical microscope with hot stage. A recurrence formula is used to compute the Legendre moments of the liquid crystal textures based on the Legendre polynomial using MATLAB software. The abrupt change in the values of Legendre moments as a function of temperature gives the phase transitions of liquid crystals. The investigated transition temperatures of cholesteric liquid crystals are compared with other techniques.

Inducement of smectic phases in hydrogen-bonded mesogenic systems

A new series of intermolecular hydrogen-bonded complexes has been synthesised using p-n-alkyl benzoic acids (nBA, where n = 5 to 10) and p-(p′-ethoxybenzylidene)-cyanoaniline (EBCA) moieties. The thermal and phase behaviour of the resulting compounds has been studied by polarising optical microscopy and differential scanning calorimetry. Thermal studies have also revealed the formation of the smectic G phase with a fine smooth multi-coloured mosaic texture. Moreover, the smectic A phase, with a focal conic fan texture, has also been observed, but only in the 10BA: EBCA compound. Detailed investigation of Fourier transform infrared and 1H nuclear magnetic resonance (NMR) spectra suggests that stabilisation is the result of intermolecular hydrogen bonding between the acid group of nBA and the cyano group of EBCA. Powder X-ray diffraction has confirmed the orthorhombic arrangement of the molecules in the hydrogen-bonded complexes.

Induced Smectic-G Phase Through Intermolecular Hydrogen Bonding, Part XIII: Impact of a Nematogen on Phase Behaviour of Hydrogen-Bonded Liquid Crystals

A series of hydrogen-bonded mesogens, p-( p′-methoxybenzylidene)-cyanoaniline:p-n-alkoxybenzoic acids (MBCA:nABA), is synthesised by using liquid-crystalline p-n-alkoxybenzoic acids (nABA) (where n represents alkoxy carbon numbers, 3-10 and 12) and a nematogen, p-( p′-methoxybenzylidene)-cyanoaniline (MBCA). The thermal and phase behaviour of these compounds is studied by thermal microscopy (TM) and differential scanning calorimetry (DCS) techniques. These studies reveal the induction of smectic-G phase in all the compounds. The structural elucidation pertaining to the formation and stabilisation of intermolecular hydrogen bonding is carried out by a detailed IR spectral investigation. The impact of imino group of the nematogen, MBCA, on the liquid-crystallinity of the present series is realised from the comparative studies made on the reported analogous series, p-aminobenzonitrile:p-n-alkoxybenzoic acids (ABN:nABA).

Induced Smectic-G Phase through Intermolecular Hydrogen Bonding, Part XII: Thermal and Phase Behaviour of p-aminobenzonitrile: p-n-alkoxybenzoic acids

New liquid crystalline compounds involving intermolecular hydrogen bonding between mesogenic p-n-alkoxybenzoic acids (nABA) (where n denotes the alkoxy carbon number varying from propylto decyl- and dodecyl-) and p-aminobenzonitrile (ABN) are synthesized. The thermal and phase behaviour of these materials is studied by Thermal Microscopy and Differential Scanning Calorimetry. Adetailed IR spectral investigation in solid and solution states confirms the formation of H-bonding between cyano and −COOH groups of ABN and nABA, respectively. Comparative thermal analyses of both free p-n alkoxybenzoic acids and H-bonded complexes suggest the induction of smectic-G phase in all the complexes.