Abstract
ABSTRACTSecond-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.
Highlights
IntroductionBent-core molecules have emerged as an exciting class of liquid crystals (LCs)
In recent years, bent-core molecules have emerged as an exciting class of liquid crystals (LCs)
Using a description of second-harmonic generation (SHG) in dielectric media, incorporating intermolecular interactions,[50,51,52] collective effects [19,53] and surface interactions, [17,18] we examine the sources of the SH signal in the nematic phase
Summary
Bent-core molecules have emerged as an exciting class of liquid crystals (LCs). One area where little work has been done in bent-core materials is that of second-harmonic generation (SHG). This is perhaps surprising given that the unique shape of bent-core molecules offers the possibility to generate a more intense second-harmonic (SH) signal than from rod-like molecules. Bentcore molecules have an electron donor–acceptor system along each rod-like segment of the molecule giving them a higher hyperpolarisability than calamitic molecules.[11,12] Second, they have reportedly greater flexoelectric coefficients [5,13,14,15] which could be expected to lead to a greater flexoelectric polarisation and, SH signal in the nematic phase. Bent-core molecules with their transverse dipole moments provide a very good example of such molecules
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