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Optical and electro-optical properties of bimesogenic organosiloxane antiferroeletric liquid crystals with molecular tilt approaching 45 degrees

Journal article
Authors M.N. Pivnenko
P. Lehmann
Lachezar Komitov
H.J. Coles
Published in Liquid Crystals
Volume 32
Pages 173-181
ISSN 0267-8292
Publication year 2005
Published at Department of Physics (GU)
Pages 173-181
Language en
Keywords bimesogenic, siloxane, liquid crystals, ferroelectric, antiferroelectric
Subject categories Molecular physics, Optical physics, Condensed Matter Physics, Organic synthesis, Physical organic chemistry, Material physics with surface physics

Abstract

We have studied the optical and electro-optical properties of three different bimesogenic siloxane materials. The dimers are symmetrical and the two mesogenic groups are connected by a siloxane unit containing three Si atoms and methylene spacers. The mesogens contain F, Cl and Br substituents, respectively, laterally attached to the phenyl ring lying closest to the chiral centre of the mesogen. These materials each exhibited a broad temperature range antiferroelectric phase and large molecular tilt, which is almost temperature-independent. We have shown that, at low temperatures, the molecular tilt in the antiferroelectric phase is the same as the apparent molecular tilt in the field-induced ferroelectric state and is close to 45°. It was found that the antiferroelectric phase of these compounds, aligned in such way that the dimeric molecules were lying in a plane parallel to the confining substrates, exhibited very low in-plane birefringence. For this reason, samples of the compounds inserted between crossed polarizers in the antiferroelectric state exhibited an extremely dark state, with contrast ratios as high as 1000:1, which did not change noticeably on rotating the sample between the crossed polarizers. It was shown that the optically isotropic siloxane moiety did not contribute to the optical properties, viz. birefringence or tilt angle, but served to enhance the promotion of the antiferroelectric phase and the temperature independent properties. We also showed the existence of a linear electro-optic response in the pretransitional region of the antiferroelectric phase of these compounds over a moderate range of fields.

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