Structural Forces in Self-Assembled Monolayers:  Terphenyl-Substituted Alkanethiols on Noble Metal Substrates

Self-assembled monolayers (SAMs) formed from 4,4‘-terphenyl-substituted alkanethiols C 6 H 5 (C 6 H 4 ) 2 (CH 2 ) n SH (TP n, n = 1−6) on polycrystalline (111) gold and silver substrates have been characterized by X-ray photoelectron spectroscopy (XPS), high-resolution XPS, near-edge X-ray absorption fine structure spectroscopy, ellipsometry, and water contact angle measurements. The packing density of the SAM constituents and the orientation of the terphenyl moieties exhibited a pronounced odd−even variation with the number ( n ) of methylene groups in the aliphatic (CH 2 ) n linker, which was opposite on silver as compared to gold. A higher packing density and a corresponding smaller inclination of the terphenyl moieties was observed for odd numbers of the methylene units in TP n on Au, and for even numbers of these units in TP n on Ag. The observed odd−even effects are explained by the strong dependence of the bending potentials in the metal−S−C bond on the deviation of the metal−S−C bond angle from an optimal value of ≈104° (Au) and ≈180° (Ag), respectively. The optimal metal−S−C angles are determined by the hybridization of the sulfur in the metal−thiolate bond, which is proposed to have sp 3 hybridization on Au and an sp hybridization on Ag, respectively.