Triphenylene-based discotic liquid crystals as self-assembled monolayers

A series of omega-thiol-substituted 2,3,6,7,10,11-hexaalkoxytriphenylenes was synthesised for the purpose of forming self-assembled monolayers (SAMs) on gold in which the self-organizing, discogenic headgroups have a known orientation. In principle, these headgroups can be oriented either with their short axes perpendicular (disc "face-on") on their short axes parallel (disc "edge-on") to the surface, depending on the number and position of the tethering points. To obtain a "face-on" orientation of the headgroups a trithiol (three points of attachment) was synthesized and to obtain an "edge-on" orientation both monothiol and ortho-dithiol derivatives (one or two points of attachment) were made. Within the SAMs it was expected that the headgroups of the "face-on" discogen would self-organize into a hexagonal array and those of the "edge-on" discogens into columns that would run parallel to the surface. The kinetics of formation and the structures of the monolayers were characterized by contact angle measurements, ellipsometry, and scanning tunneling microscopy (STM). In the case of the SAMs obtained from the "edge-on" materials, these confirmed that the desired orientation of the headgroups had been achieved. STM images of a SAM obtained from one of the "edge-on" materials also clearly showed the desired organization of the headgroups into columnar aggregates running parallel to the surface. Clearer STM images were obtained after the SAM had been immersed into a solution of TNF (trinitrofluorenone), a compound known to interdigitate between the triphenylene nuclei. In the case of the SAMs obtained from the "face-on" material, scanning tunneling microscopy gave no indication of the expected hexagonal ordering and ellipsometry suggested that these systems form multilayer rather than monolayer structures.