Anchoring transition of bent-rod liquid crystal dimers on different surfaces

Non-symmetrical dimers have been synthesised, in which bent- and rod-shaped mesogenic molecules are linked by a flexible polymethylene spacer -(CH2)n-. It has been shown that compounds in which n = 5, 6 or 7 exhibit one (nematic), two (nematic and smectic), and no mesogenic phases, respectively. These exhibit a variety of anchoring transition behaviours depending on the mesogenic molecules (n = 5 or 6) and surfaces (polyimide surfaces for homeotropic and planar alignment, or a perfluoropolymer surface). Discontinuous and continuous planar-to-homeotropic orientational changes take place with decreasing temperature. These differences may originate from different dielectric anisotropy, and steric interactions caused by the directional relationship of bent and rod mesogens, depending on the length of the spacer. A possible explanation is suggested on the basis of surface-molecule interactions.