Relationship between the ability to control liquid crystal alignment and wetting properties of calix[4]resorcinarene monolayers

Abstract

This paper describes the correlation between the ability to control nematic liquid crystal (LC) alignment and wetting properties of mixed monolayers formed by coadsorption of two O-octacarboxymethylated calix[4]resorcinarenes (CRA-CMs) with either perfluorooctyl- or octylazobenzene units. Photoirradiation of CRA-CM monolayers induces reversible photoisomerization reaction of the surface azobenzenes, which causes changes in LC alignment and wettability of the monolayers. The photogenerated LC alignments are considerably influenced by the surface compositions of the mixed monolayers. When LC cells fabricated with substrate plates modified with single-component monolayers of CRA-CM with p-octylazobenzenes are subjected to oblique irradiation with non-polarized UV light, the orientation of LC molecules is changed from homeotropic to homogeneous alignments tilting toward the direction of light propagation. In contrast, single-component and mixed monolayers of CRA-CM with p-perfluorooctylazobenzenes cause homeotropic alignments and tilted alignments with high pretilt angles, respectively. The level of photoisomerization and surface free energy of the CRA-CM monolayers are not a sufficient condition to cause the contrasting alignment behaviors. We conclude that the molecular-level morphology and/or fluidity of the monolayer surfaces of CRA-CMs, which is deduced from the contact angle hysteresis of anisotropic liquids for the surfaces, is the most significant factor to induce the LC alignment alterations observed.