On the importance of the headgroup substrate bond in thiol monolayers: A study of biphenyl-based thiols on gold and silver

Self-assembled monolayers of a series of omega-(4'-methyl-biphenyl-4-yl)-al kanethiols (CH3-C6H4-C6H4-(CH2)(m)-SH, m = 1-6) formed on polycrystalline g old and silver surfaces were characterized in detail by contact angle measu rements, optical ellipsometry, X-ray photoelectron spectroscopy (XPS), refl ection absorption infrared spectroscopy (IRRAS), and near-edge X-ray absorp tion fine structure spectroscopy (NEXAFS). The orientation of the biphenyl moiety, determined by combining the results from IRRAS and NEXAFS, exhibits a pronounced dependence on the number of methylene groups. Similar to n-al kanethiols an odd-even effect is observed which on silver is opposite to th at on gold. For m = odd on gold and m = even on silver the arrangement of t he aromatic moieties agrees well with the bulk structure of biphenyl, and t he bonding of the thiols to the substrate is in agreement with an sp(3) hyb ridization of the sulfur on gold and sp on silver, respectively. In the opp osite case of m = even on gold and m = odd on silver, the biphenyl moieties adopt a significantly more canted orientation which, as a consequence, res ults in a lower coverage. The odd-even behavior of the coverage is in sharp contrast to that seen for n-alkanethiols. The experiments provide evidence that a significant driving force exists to pertain the sp3 and sp hybridiz ation of sulfur on gold and silver, respectively. In the case of gold subst rates the experimental results are in conflict with available bending poten tials derived from ab initio calculations.