Alignment of a Columnar Hexagonal Discotic Liquid Crystal on Self-Assembled Monolayers

The alignment of the columnar hexagonal (Col(h)) phase of the discotic liquid crystal 2,3,6,7,10,11-hexakis(undecyloxy)triphenylene on well-characterized surfaces comprised of thiol-on-gold self-assembled monolayers (SAMs) has been studied using polarizing optical microscopy in combination with evanescent wave ellipsometry. Homeotropic alignment is preferred for both COOH- and CH3- functionalized SAMs (both high and low energy surfaces). Even when the SAM layer is comprised of edge-on oriented triphenylene units, evanescent wave ellipsometry shows that the liquid crystal aligns predominantly in a homeotropic (face-on) manner. These results confirm the view that the surface acts primarily as a mechanical barrier and although the chemical nature of the surface can enhance the ease or quality of the alignment of the Col(h) phase, it is of secondary importance.