Triphenylene-based discotic liquid crystals as self-assembled monolayers

A series of omega-thiol-substituted 2,3,6,7,10,11-hexaalkoxytriphenylenes w as synthesised for the purpose of forming self-assembled monolayers (SAMs) on gold in which the self-organizing, discogenic headgroups have a known or ientation. In principle, these headgroups can be oriented either with their short axes perpendicular (disc "face-on") on their short axes parallel (di sc "edge-on") to the surface, depending on the number and position of the t ethering points. To obtain a "face-on" orientation of the headgroups a trit hiol (three points of attachment) was synthesized and to obtain an "edge-on " orientation both monothiol and ortho-dithiol derivatives (one or two poin ts of attachment) were made. Within the SAMs it was expected that the headg roups 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 t o the surface. The kinetics of formation and the structures of the monolaye rs were characterized by contact angle measurements, ellipsometry, and scan ning tunneling microscopy (STM). In the case of the SAMs obtained from the "edge-on" materials, these confirmed that the desired orientation of the he adgroups had been achieved. STM images of a SAM obtained from one of the "e dge-on" materials also clearly showed the desired organization of the headg roups into columnar aggregates running parallel to the surface. Clearer STM images were obtained after the SAM had been immersed into a solution of TN F (trinitrofluorenone), a compound known to interdigitate between the triph enylene nuclei. In the case of the SAMs obtained from the "face-on" materia l, scanning tunneling microscopy gave no indication of the expected hexagon al ordering and ellipsometry suggested that these systems form multilayer r ather than monolayer structures.