K. L. Sandhya

Electrical conductivity and dielectric constant measurements of liquid crystal-gold nanoparticle composites

The behaviour of the anisotropic electrical conductivity of liquid crystal–gold nanoparticle (LC‐GNP) composites consisting of a commercially available room temperature nematic compound doped with alkylthiol‐capped GNPs has been investigated. The nematic–isotropic transition of the composite decreases nearly linearly with increasing X, the concentration of GNP (in weight %) at a rate of about 1°C /weight %. The inclusion of GNPs increases the electrical conductivity of the system with the value increasing by more than two orders of magnitude for X = 5%. However, the anisotropy in conductivity, defined as the ratio of the conductivity along (σ∥) and orthogonal (σ⊥) to the director shows a much smaller but definite decrease as X increases.

Photoinduced effects in nematic liquid crystals

Temperature, concentration of the solvent and pressure are the parameters that are well known to bring about phase transitions in liquid-crystalline systems. In recent years a new parameter has been added to this list: light. The principle behind these photoinduced transitions is the light-driven shape transformation of certain photoactive materials like, e.g., azobenzene. In this article, we present results of various aspects of our recent investigations on such photoinduced transitions in the nematic phase and highlight the feature that light is a new tool to study phase transitions and the associated critical phenomena.

Spacer parity dependence of photoinduced effects in liquid-crystalline dimers

Liquid crystals composed of photoactive groups like azobenzene are promising materials for optical switching and image storage applications. When exposed to ultraviolet radiation, such materials are known to induce a transition from a liquid-crystalline nematic phase, in which the azo molecules are in the trans form to an isotropic liquid phase when the azo molecules take the cis form. We report the observation of the influence of the parity and length of the flexible spacer of photoactive liquid-crystalline dimers in a guest host system on the photoinduced nematic-isotropic transition. A remarkable feature observed is that in this system, in which the guest dimers are present only in a small concentration (4.8 weight %) the odd–even parity of the spacer results in a striking alternation of the magnitude of the photoinduced shift in the nematic-isotropic transition temperature. Further, it is demonstrated that the shape of the photoactive molecule in its all-trans configuration can be used as a convenient...

Enhanced dynamic response of the photoinduced nematic–isotropic transition in a polymer matrix

We have investigated the dynamic response of the photoinduced nematic–isotropic transition and the thermal back relaxation process of a liquid crystal with photoactive molecules embedded in a polymer matrix. The results show that the presence of the polymer matrix leads to a dramatic improvement in the response time. Possible causes for this finding have been discussed.

Photoinduced Phase Transitions in Liquid Crystalline Systems

In recent years, a field of research that is growing steadily is the photoinduced phenomenon, in which the incident light itself brings about molecular ordering/disordering of the liquid crystalline system. This particular aspect of photonics, in which light can be controlled by light as a stimulus, is being proposed as the future technology for high-speed information processing and photo-chromic materials, in particular the azo compounds, have been found to be very promising candidates for optical data storage. A beautiful illustration of nature utilizing photoinduced effects is that of the biological photochrome bacteriorhodopsin found in the eye. Apart from their biological and technological importance, these systems exhibit photo-driven phase transitions, thus providing a new tool to study phase transitions and the associated critical phenomena. In this article we review some of our recent contributions to this field of photoinduced isothermal phase transitions in various liquid crystalline systems. In particular, we discuss the influence of the nanophase segregation to bring about a unique photo-driven disorder to order transition, local racemization phenomenon in bent-core banana B2 phase, the effect of high pressure and the dynamics of Sm A-Sm Cα* transition.

Anomalous increase of photocurrent anisotropy in a liquid crystalline binary mixture

We report photoconductivity measurements in a binary system of naphthalene-based liquid crystals. Under ultraviolet (365 nm) illumination we observe an anomalous increase in the photocurrent for the mixtures. For a particular concentration, the photocurrent anisotropy defined as the ratio of photocurrents orthogonal to and along the director in the Crystal E phase reaches a value of 850, the highest achieved under steady-state illumination conditions from a conventional source. We argue that when using such mixtures, it is possible to reach the limiting values predicted by models based on conduction dictated by charge-carrier hopping.

Dynamics of the two-dimensional melting transition of a liquid crystal confined in Anopore membranes

Dielectric measurements on a liquid crystal exhibiting the smectic A-crystal B transition and confined to Anopore membranes having 20 and 200 nm pore sizes are reported. The studies reveal that compared with the bulk, the confined material shows a decrease in transition temperature. More importantly, the confinement leads to a slowing of the relaxation mechanism by about three orders of magnitude.

Effect of pressure on liquid crystal dimers

We report high pressure investigations on a homologous series of liquid crystalline dimeric molecules in which the terminal chain length is kept constant but the length of the methylene spacer connecting the two mesogenic units is varied. We find that unlike the nematic–isotropic transition temperature and associated entropy change, there is no alternation in the slope of the nematic–isotropic phase boundary in the pressure–temperature plane as a function of the length of the spacer group in the molecule. By applying the Clausius–Clapeyron equation we conclude that the volume change at the transition should exhibit a strong odd–even effect. Measurements on the shortest homologue of the series, which is non-mesomorphic, show the expected result that the application of pressure induces mesomorphism in non-mesomorphic compounds.

Photoconductivity measurements in the discotic columnar phase of a few anthraquinone derivatives

It has recently been shown that the structure of the columnar phase of discotic liquid crystals is suitable for fast transport of photogenerated charge carriers. Although the magnitude of the charge carrier mobility exhibited by these systems is still small compared to those in organic single crystals, the fluidity and the resulting self-healing nature of the liquid crystalline columnar phases hold promises for potential applications. In this paper we report on steady-state photoconductivity measurements carried out on three anthraquinone derivatives exhibiting the columnar phase. Upon illumination of the sample with UV light the current through the sample increases abruptly and reaches a photo-stationary value within a short span; the fastest observed response time is 17 ms. It was found that the anisotropy of the photoconductivity, defined as the ratio of the photocurrent along and perpendicular to the column axis, reaches a value of about 450. We also discuss the relation between the chemical structure of the molecule and the maximum value of the photocurrent observed.