Odd‐even effects in the optical properties of chiral twin liquid‐crystalline cholesteryl ω‐(cyanobiphenylyloxy) alkanoates

A series of novel cholesteryl w ‐(cyanobiphenylyloxy)alkanoate esters with a varying number (2–7 and 10) of methylene units in the alkanoate spacer was synthesized and their mesogenic properties were investigated by polarization microscopy and differential scanning calorimetry. All compounds show a broad range of enantiotropic liquid‐crystalline behavior. Both the cholesteric‐isotropic and the smectic‐cholesteric transition temperatures show a pronounced odd‐even effect as a function of spacer length. The selective reflection wavelength of the cholesteric phases has been determined as a function of spacer length and temperature. For the first time, we have shown that the reflection wavelength also shows a pronounced odd‐even effect. The compounds with an even number of methylene units reflect at much lower wavelengths than the compounds with an odd number of methylene units. The reflection wavelength of the latter compounds decreases strongly with temperature, whereas the reflection wavelength of the compounds with the even spacers is almost independent of the temperature. The odd‐even effect in the optical properties attenuates with increasing spacer length.