Reestablishing Odd-Even Effects in Anthracene-Derived Monolayers by Introduction of a Pseudo-C2v Symmetry

A series of anthracene[2,3-d]oxazolyl-2-alkylthioacetates (AOxCn-SAc) with n = 2-6 methylene groups in the alkyl chain were designed and synthesized to investigate the influence of the substitution along the long axis of the molecule on the structural behavior of the respective self-assembled monolayers (SAMs) on Au(111). While in previous work anthracene-terminated alkanethiols, in which the alkyl group was attached to the off-axis 2-position of the acene, showed an exceptionally small influence of the number of methylene groups (n) in the aliphatic linker, the new system exhibits a strong dependence of almost all monolayer properties on the length of the aliphatic linker, with the parity of n being the decisive parameter. These so-called odd-even effects are a result of the bond angle at the anchoring S atom as well as the all-trans conformation of the alkyl linkers, which in particular affect the packing density and the orientation of the aromatic part. Thus, using a variety of complementary experimental techniques, distinct odd even variation of molecular inclination (by 10-11 degrees) and the packing density (by 20%) were found, with the packing density of the odd-numbered systems being even as high as the ones of the well-established alkanethiolate systems. The high quality and well-defined character of these SAMs, along with a low band gap of only 3.0 eV, make them relevant for application in organic and molecular electronics.