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

C. Scott Hartley, Nadia Kapernaum, Jeffrey C. Roberts, Frank Giesselmann, and Robert P. Lemieux*
J. Mater. Chem. 2006, 16, 2329–2337
https://doi.org/10.1039/B515313A

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 T − TC = +5 K. The extrapolated ec values of 3.07 and 2.28 deg µm V−1 are comparable to some of the highest ecvalues reported for neat SmA* materials. The electroclinic coefficient of a 4 mol% mixture of (R)-1in 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-PhPmixture 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 T − TC by small-angle X-ray scattering are consistent with this explanation. The results show that the variation in reduced layer spacing dA/dC with T − TC 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 T − TC 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.