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 ω-(4‘-methyl-biphenyl-4-yl)-alkanethiols (CH3−C6H4−C6H4−(CH2)m−SH, m = 1−6) formed on polycrystalline gold and silver surfaces were characterized in detail by contact angle measurements, optical ellipsometry, X-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (IRRAS), and near-edge X-ray absorption 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-alkanethiols an odd−even effect is observed which on silver is opposite to that on gold. For m = odd on gold and m = even on silver the arrangement of the aromatic moieties agrees well with the bulk structure of biphenyl, and the bonding of the thiols to the substrate is in agreement with an sp3 hybridization of the sulfur on gold and sp on silver, respectively. In the opposite case of m = even on gold and m = odd on silver, the biphenyl moieties adopt a significantly more canted orientation which, as a consequence, results 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 hybridization of sulfur on gold and silver, respectively. In the case of gold substrates the experimental results are in conflict with available bending potentials derived from ab initio calculations.