Issue 24, 2006

Electroclinic effect in chiral SmA* liquid crystals induced by atropisomeric biphenyl dopants: amplification of the electroclinic coefficient using achiral additives

Abstract

The atropisomeric compound (R)-2,2′,6,6′-tetramethyl-3,3′-dinitro-4,4′-bis[(4-nonyloxybenzoyl)oxy]biphenyl ((R)-1) was doped in the achiral liquid crystal hosts 2-(4-butoxyphenyl)-5-octyloxypyrimidine (2-PhP) and 4-(4′-heptyl[1,1′-biphen]-4-yl)-1-hexylcyclohexanecarbonitrile (NCB76), and electroclinic coefficients ec were measured as a function of the dopant mole fraction x1 in the chiral SmA* phase at TTC = +5 K. The extrapolated ec values of 3.07 and 2.28 deg µm V−1 are comparable to some of the highest ec values reported for neat SmA* materials. The electroclinic coefficient of a 4 mol% mixture of (R)-1 in 2-PhP is amplified by achiral 2-phenylpyrimidine additives (5 mol%) that are longer than 2-PhP; in the best case, ec is amplified by a factor of 3.2 with 5-(tetradecyloxy)-2-(4-(tetradecyloxy)phenyl)pyrimidine (3g), which is almost twice as long as 2-PhP. However, no amplification is observed in a 4 mol% mixture of (R)-1 in NCB76 using the same series of additives. A correlation between ec values and the temperature range of the SmA* phase suggests that the amplification of ec with increasing length of the additive 3 in the (R)-1/2-PhP mixture is due primarily to a decrease in the tilt susceptibility coefficient α as the second-order SmA*–SmC* phase transition moves away from the tricritical point. Measurements of smectic layer spacing as a function of TTC by small-angle X-ray scattering are consistent with this explanation. The results show that the variation in reduced layer spacing dA/dC with TTC for the pure host 2-PhP fits to a square-root law, which indicates that the second-order SmA–C transition is nearly tricritical. On the other hand, the corresponding variation in dA/dC with TTC for a 5 mol% mixture of 3g in 2-PhP fits to a linear relation, which indicates that the second-order SmA–C transition approaches typical mean-field behavior.

Graphical abstract: Electroclinic effect in chiral SmA* liquid crystals induced by atropisomeric biphenyl dopants: amplification of the electroclinic coefficient using achiral additives

Supplementary files

Article information

Article type
Paper
Submitted
27 Oct 2005
Accepted
29 Mar 2006
First published
24 Apr 2006

J. Mater. Chem., 2006,16, 2329-2337

Electroclinic effect in chiral SmA* liquid crystals induced by atropisomeric biphenyl dopants: amplification of the electroclinic coefficient using achiral additives

C. S. Hartley, N. Kapernaum, J. C. Roberts, F. Giesselmann and R. P. Lemieux, J. Mater. Chem., 2006, 16, 2329 DOI: 10.1039/B515313A

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