Cholesteric Pitch Near the Smectic-C Phase

The temperature dependence of the cholesteric pitch of the chiral ester 4-n-hexyloxyphenyl-4'-(2”-methylbutyl)biphenyl-4-carboxylate has been measured in detail using the method of selective reflection. This compound has a cholesteric to smectic-C transition at 79°C. Because of the first-order nature of the transition, the pretransitional unwinding of the pitch is considerably less pronounced than that in cholesteryl esters near the smectic-A transition. The wavelength of maximum reflection increases from 440 nm at 110°C to 560 nm at the transition. When fitted to a power-law temperature dependence, the data imply an exponent of 0.82 ± 0.15, which is consistent with de Gennes’ theory in the meanfield approximation.