Structural forces in self-assembled monolayers: Terphenyl-substituted alkanethiols on noble metal substrates

Self-assembled monolayers (SAMs) formed from 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)(2)(CH2)(n)-SH (TPn, 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 (CH2)(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 TPn on Au, and for even numbers of these units in TPn 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 approximate to104degrees (Au) and approximate to180degrees (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.