Coverage-Dependent Interactions at the Organics-Metal Interface: Quinonoid Zwitterions on Au(111)

The large intrinsic electric dipole of about 10 D of a p-benzoquinonemonoimine compound from the class of N-alkyldiaminoresorcinone (or 4,6-bisdialkylaminobenzene-1,3-diones, i.e., C6H2(- NHR)(2)(- O)(2), where R = H) zwitterions is reduced considerably upon adsorption on Au( 111) substrates. Scanning tunneling microscopy images reveal parallel alignment of adsorbed molecules within extended islands, leading to the formation of polarized domains. This is in contrast to the typical antiparallel alignment found in the bulk. High-resolution images show that the molecules form rows along the <(1) over bar 01 > directions of the Au(111) surface, but otherwise their arrangement is only weakly perturbed by the Au(111) (23 x root 3) herringbone surface reconstruction. Density functional theory calculations show that upon increasing the molecular density the strength of the interaction between the zwitterions and the Au(111) surface decreases. Thus, the charge redistribution, which occurs at the interface as a result of molecular adsorption, and therefore the interfacial dipole is coverage dependent. The weakening of the interaction at the organic-metal interface with increasing coverage is experimentally observed as a contraction of the intermolecular bond length. Moreover, it is the strong adsorbate-adsorbate interactions (and not the interactions between the adsorbate molecules and the surface) which determine the molecular arrangement within the 2D network the zwitterions form.