Self-Assembled Monolayers of Discotic Liquid Crystalline Thioethers, Discoid Disulfides, and Thiols on Gold:  Molecular Engineering of Ordered Surfaces

This paper describes a comparative study of self-assembled monolayers of different discoid molecules containing thioether, thiol, or disulfide substituents on gold. Discotic liquid crystalline hexaalkylthio-substituted triphenylenes and tricycloquinazolines, and derivatized triphenylenes with a single long ω-thioalkyl or ω-dithioalkyl substituent, are shown to self-assemble from dilute solution onto gold surfaces. Monolayers are formed owing to the known strong interaction of these functional groups with the gold. The characterization of the monolayers includes contact angle, polarized grazing incidence Fourier transform infrared spectroscopy (FTIR), quartz crystal microbalance (QCM), and atomic force microscopy (AFM) measurements. The discotic hexaalkyl thioethers assemble essentially face-on and without detectable lateral order onto the gold substrate. The alkyl chains do not lie flat on the gold but cover the aromatic cores. In contrast, the aromatic cores of the molecules in the monolayers of the discoid monosubstituted triphenylenes with a single thiol or disulfide substituent are shown to be in edge-on orientation. Furthermore, AFM reveals lateral order in these monolayers. Ordered areas, in the form of columns of edge-on oriented molecules, extend over length scales exceeding 100 nm. Domains of different orientation can be observed.