Effect of the Bending Potential on Molecular Arrangement in Alkaneselenolate Self-Assembled Monolayers

Abstract

Self-assembled monolayers (SAMs) of hybrid 4,4′-biphenyl-substituted alkaneselenolates, CH3(C6H4)2(CH2)nSe(BPnSe) with a variable length of the aliphatic part (n ) 1-6, 10, 11) have been prepared on (111) gold and silver substrates and characterized by a variety of complementary experimental techniques. The packing density of the SAM constituents and the orientation of the biphenyl moieties were found to exhibit a pronounced “odd-even” variation with the number (n) of methylene units in the aliphatic linker of BPnSe moieties, which was opposite on silver as compared to gold. In particular, a smaller inclination and a corresponding higher packing density of the biphenyl moieties was observed for odd numbers of the methylene units in BPnSe/Au and for even numbers of these units in BPnSe/Ag. The observed odd-even effects were explained by a significant bending potential, favoring definite orientation of the metal-Se-C bond and entering the balance of the structure-building interactions either cooperatively or competitively. The existence of this bending potential is supposed to be closely related to the exact bonding configuration of the headgroup atom, with the optimal substrate-Se-C angles being different for Au and Ag substrates. In view of the analogous behavior of thiolate-based systems, one can assume a common origin of the observed phenomena in chalcogen-based SAMs, in both of which the bonding configuration of the headgroup seems to be an important or even deciding factor in the balance of structure-building interactions.