Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles

Abstract

Second-harmonic generation (SHG) in the nematic phase of bent-core oxadiazole-based liquid crystals (LCs) was studied and compared to that for the rod-like compound 4-cyano-4ʹ-n-octylbiphenyl (8CB). Weak, isotropically scattered second-harmonic (SH) light was observed for all materials, consistent with SHG by nematic director fluctuations. The SH intensity produced by the bent-core materials was found to be up to~3.4 times that of 8CB. We discuss this result in terms of the dependence of SH intensity on temperature, elastic constants and flexoelectric coefficients. We have calculated the latter by using a molecular field approach with atomistic modelling, thus demonstrating how molecular parameters contribute to the flexoelectric coefficients and illustrating the potential of this method for predicting the flexoelectric behaviour of bent-core LCs. We show that the increased SH signal in the bent-core compounds is partly due to their nematic phases being at a much higher temperature, and also potentially due to them having greater flexoelectric coefficients, up to~1.5 times those of 8CB. These estimates are consistent with reports of increased flexoelectric coefficients in bent-core compounds in comparison to rod-like compounds.ARTICLE HISTORY

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

“…The nematic phase that is most commonly used is formed from rod-like (calamitic) molecules, but markedly different behaviour is observed in the nematic phase formed from bent-core mesogens. 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.…”

Anomalies in the twist elastic behaviour of mixtures of calamitic and bent-core liquid crystals

“…These methods of determining the elastic constants do not take into account any contributions from flexoelectric effects. However, it has been found previously that in compound 2, the flexoelectric coefficients are only a factor of 2 greater than calamitic LCs [1,2] so this omission is justified. All measurements of the dielectric anisotropy and elastic constants were taken on cooling and are presented as a function of reduced temperature T-T NI .…”

“…The anisotropy, together with their electro-optical responsiveness has led to NLCs being used widely in devices. Most NLCs that are used in devices tend to be rod-like (calamitic) in shape but more recently nematic phases formed from bent-core (BC) molecules have garnered interest, due to properties including increased flexoelectricity [1,2] as well as a reversal of the relationship between the splay (K 11 ) and bend (K 33 ) elastic constants, ie. K 11 > K 33 [3][4][5][6][7][8][9].…”

The 2016 Luckhurst-Samulski Prize

“…The fourth paper by James Addis and colleagues entitled Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles [13] shows that the second harmonic intensity produced by bent-core nematic liquid crystals is greater that that found for rod-like nematogens. This observation is discussed in terms of the dependence of the second harmonic intensity on temperature, elastic constants and flexoelectric coefficients.…”