X-Ray Diffraction Studies of Homologous Series of 4-Alkyl-4′ Cyanostilbene

Abstract

Various members of the series 4-alkyl-4′-cyanostilbene (n=4, …11) have been examined by x-ray diffraction using monodomain samples aligned in .2 T magnetic field. A moving-film technique has been employed with Fe Kα radiation (λ=1.937 Å) to record the zero level and upper levels in reciprocal space. Lower members (n = 4,5,6) form only a nematic phase; higher members (n = 10,11) form only a smectic-A phase; while intermediate members (n = 7,8,9) form both nematic and smectic-A phases. The equatorial reflections were used to establish effective side-chain separations while meridional reflections permitted the calculation of effective molecular lengths and layer thickness, respectively, in the nematic and smectic phases. These were found to be longer than the calculated molecular length in its most extended conformation. For the smectic-A phases, this implies some type of bilayer structure like that reported for other liquid-crystal compounds having polar end groups.

Key Words:

• 4-alkyl-4′-cyanostilbene
• x-ray diffraction
• smectic
• nematic
• moving-film technique
• partial bilayer