Fabrication of a carboxyl-terminated organic surface with self-assembly of functionalized terphenylthiols:: The importance of hydrogen bond formation

Self-assembled monolayers (SAMs) were prepared from different oligophenylthiol molecules (terphenylthiol, (C6H5C6H4C6H4SH, TPT), terphenylmethanethiol (C6H5C6H4C6H4CH2SH, TPMT), and carboxy terphenylmethanethiol (HOOCC6H4C6H4C6H4CH2SH, CTPMT)) on Au substrates. Compared to the more commonly used alkanethiols this class of organothiols has a much more rigid backbone. It is demonstrated that for organothiols with a strongly interacting function at the omega-position the larger mechanical rigidity significantly affects the molecular structure of the self-assembled films. Whereas the unsubstituted terphenylthiols, TPT and TPMT, form well-ordered monolayers with an average tilt angle of 27 degrees +/- 5 degrees, immersion into ethanolic solutions of the COOH-functionalized terphenylmethanethiol CTPMT yields highly oriented bilayers. The second layer is linked to the first layer by hydrogen bonds. This double-layer formation has not been observed for the corresponding alkanethiols. Single layers of CTPMT on gold could, however, be prepared by adsorption from a mixture with trifluoroacetic acid (TFA) or by immersion of the bilayers into TFA. The immersion of the bilayers into potassium hydroxide solutions leads to the intercalation of potassium ions into the bilayers without removal of the second layer.