The synthesis and evaluation of nitrile-containing ferroelectric chiral liquid crystal dopants

• Main Author: Reddy, Viswanath
• Other Authors: Lewis, Rob; Hird, Michael
• Published: University of Hull 2014
• Subjects: 540; Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.646831

Ferroelectric Liquid Crystal Displays offer fast switching times in the sub millisecond region and a high contrast ratio. There is a need for faster refresh rates to cope with fast moving images and the possibility of sequential red-green-blue illumination that will enable higher definition quality. This has led to a renewed interest in the ferroelectric liquid crystal display mode which employs a smectic C* material generated via addition of a chiral dopant to an achiral SmC host. This thesis first reviews liquid crystals and their properties, the smectic C* phase in particular and previous work on chiral dopants. Ferroelectricity and its use in an LCD display mode is explained. The aim is to prepare a stable chiral dopant that is compatible with a difluoroterphenyl based host FLC mixture and to provide a moderate spontaneous polarisation, a long N* pitch length and maintain the broad temperature range of the achiral host. This report first describes the synthesis of (S)-2-((3’-fluoro-4’-(octyloxy)-[1,1’- biphenyl]-4-yl)oxy)propanenitrile as a chiral dopant. The compound has a similar structure to a standard biphenyl ester dopant BE8OF2N, but replaces an ester link to the stereogenic centre with an ether link, with the main aim of increasing chemical stability. The key part of the synthesis involves an SN2 reaction between lactamide tosylate and a hydroxybiphenyl. The non-mesogenic dopant was formulated as 5, 7 and 10% w/w mixtures with an FLC host mixture (KCHM211) that contains only dialkyldifluoroterphenyls. Liquid crystal phase transitions and N* pitch lengths are reported and the phase transitions are compared with that from the BE8OF2N with KCHM211 host mixture. The spontaneous polarisation at room temperature was found to be 20 nC cm−² and the tilt angle of the SmC* phase temperature range is reported. This result lead to the synthesis of three mesogenic dopants and the synthesis is described, employing the same chiral 2-cyanoethyl group linked by oxygen to the difluoroterphenyl core used in the FLC host mixture. The derivatives with an ortho diflluorphenyl group in the middle ring or the end ring remote from the chiral centre have blue phases, but those with the fluoro-substituents adjacent to the chiral group possess an N* phase. All the mesogenic dopants were formulated in a 7% w/w mixture with KCHM211. LC phase behaviour, and electrooptic data is reported. The results are compared with those from the non-mesogenic dopant. Dopant reduce the SmC*-SmA* by only 3 ◦C from that of the host mixture, but has a low Ps (maximum 6 nC cm−²). The Ps of the terphenyl dopants is lower than expected, and molecular modelling and single crystal X-ray diffraction studies were used in an attempt to rationalise this observation. The electron distribution and dipole moments were calculated, but they follow a different trend to that of the Ps data. The single crystal structure also shows a different conformation of the chiral nitrile relative to the aromatic fluoro-substituents than that obtained by modelling. These results indicate the complexity in predicting or explaining the magnitude of Ps in ferroelectric liquid crystals.