Engineering the electronic structure of organics through interface manipulation, particularly the interface dipole and the barriers to charge carrier injection, is of essential importance to improve organic devices. This requires the meticulous fabrication of desired organic structures by precisely controlling the interactions between molecules. The well-known principles of organic coordination chemistry cannot be applied without proper consideration of extra molecular hybridization, charge transfer and dipole formation at the interfaces. Here we identify the interplay between energy level alignment, charge transfer, surface dipole and charge pillow effect and show how these effects collectively determine the net force between adsorbed porphyrin 2H-TPP on Cu(111). We show that the forces between supported porphyrins can be altered by controlling the amount of charge transferred across the interface accurately through the relative alignment of molecular electronic levels with respect to the Shockley surface state of the metal substrate, and hence govern the self-assembly of the molecules.

The permanent dipole of quinonoid zwitterions changes significantly when the molecules adsorb on Ag(111) and Cu(111) surfaces. STM reveals that sub-monolayers of adsorbed molecules can exhibit parallel dipole alignment on Ag(111), in strong contrast with the antiparallel ordering prevailing in the crystalline state and retrieved on Cu(111) surfaces, which minimizes the dipoles electrostatic interaction energy. DFT shows that the rearrangement of electron density upon adsorption is a result of donation from the molecular HOMO to the surface, and back donation to the LUMO with a concomitant charge transfer that effectively reduces the overall charge dipole.

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 ⟨1̅01⟩ directions of the Au(111) surface, but otherwise their arrangement is only weakly perturbed by the Au(111) (23 × √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.