Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

Abstract

Unusual behaviour of the dark conglomerate (DC) phase seen in an oxadiazole-based achiral bent-core liquid crystal, which has not previously been reported for the DC phase of other liquid crystals, is described. Under polarizing optical microscopy, we see no domains of opposite handedness in the ground state of the DC phase. However, it shows unusual transformations when an electric field is applied to the system: On increasing the electric field, at first the domains of opposite handedness become visible and then they grow in size and slowly the sample transforms to a monochiral or single handed form which is followed by a nonchiral state at very high fields. The threshold electric fields required to achieve these changes are temperature dependent and the transformations are seen irrespective of the frequency of the applied electric field (100 Hz to 5 kHz), type of the waveform (sine, square and triangular) and the thickness (1.5 µm to 15 µm) or the geometry (planar and twisted) of the device used. Further, there is no field-induced high birefringence texture observed even though sufficiently large electric field (~22 V/µm) has been applied across the devices. The nature of the behaviour is investigated by various techniques such as optical microscopy, conoscopy, circular dichroic and Raman spectroscopies, electro-optics and dielectric spectroscopy. The possible physical phenomena behind these changes are discussed in detail.

Anomalously low twist and bend elastic constants in an oxadiazole-based bent-core nematic liquid crystal and its mixtures; contributions of spontaneous chirality and polarity

“…In a similar way vanishing of the twist constant may induce a spontaneous chiral symmetry-breaking when the system is split into macroscopic chiral domains of the opposite handedness, as observed in compound 2 and other oxadiazole NLCs at low temperatures. 5,8 This may provide a long-awaited explanation of why many similar non-chiral bent-core materials exhibit chiral SmC*-like phases or the dark conglomerate phase, which are also composed of chiral domains.…”

“…The differences include increased flexoelectricity 1,2 and the potential to exhibit exotic phases such as the twist-bend nematic phase 3,4 and the dark conglomerate phase. 5,6 The remarkable spontaneous segregation of chiral domains of opposite handedness has also been reported in the nematic phase formed from bent-core molecules. 7,8 The elastic properties of NLCs are critical parameters, influencing their behaviour in devices as they define both the threshold voltage and steepness of the electro-optic response.…”

Dark conglomerate phases of bent-core liquid crystals

“…However, below is an example of a DC phase observed in an oxadiazole based bent-core liquid crystal in which the electro-optic behaviour of the DC phase under an electric field corresponds to a number of structural changes [53]. They lead to unusual changes in physical properties and transformation where the overall sponge-like structure of the DC phase remains intact even under high electric fields approaching the dielectric breakdown limit [54].…”

Electrically tunable refractive index in the dark conglomerate phase of a bent-core liquid crystal

“…25 However, the X-ray investigations carried out to date do not indicate any such molecular association in the DC phase. 26 The other possibility is that a spectral blue shift occurs on reducing the temperature, instigating a decrease in the average refractive index. In the latter case, we expect a strong change in the Sellmeier coefficient, B, but, as mentioned, no significant change in B is observed as the n avg decreases.…”

“…26 In short, the above mentioned electric field driven transformations indicate that in the ground state the DC phase of OC12-Ph-ODBP Ph-C5 bent-core liquid crystal has SmC a P A FIG. 4.…”