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This article reviews the surface adsorption of p-benzoquinonemonoimine zwitterions from the class of N-alkyldiaminoresorcinones (or 4,6-bis-dialkylaminobenzene-1,3-diones, i.e. C6H2(···NHR)2(···O)2), where R = H, R = C2H5, n-C4H9), on a variety of conducting surfaces. These small molecules with a large electrical dipole exhibit molecular orientation and packing on surfaces depending on the deposition methodology (from solution or from the vapor) as well as the substrate. What is very clear from the investigations of this class of molecules is that inter-molecular interaction is not simply driven by dipolar interactions alone, but frontier orbital symmetry as well. This is illustrated by the reversible adsorption of di-iodobenzene on molecular films of (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate C6H2(···NHR)2(···O)2 where R = n-C4H9, where it is clear that absorption is isomer specific.

The role of dipole-dipole interactions in the self-assembly of dipolar organic molecules on surfaces is investigated. As a model system, strongly dipolar model molecules, p-benzoquinonemonoimine zwitterions (ZI) of type C6H2(center dot center dot center dot NHR)(2)(center dot center dot center dot O)(2) on crystalline coinage metal surfaces were investigated with scanning tunneling microscopy and first principles calculations. Depending on the substrate, the molecules assemble into small clusters, nano gratings, and stripes, as well as in two-dimensional islands. The alignment of the molecular dipoles in those assemblies only rarely assumes the lowest electrostatic energy configuration. Based on calculations of the electrostatic energy for various experimentally observed molecular arrangements and under consideration of computed dipole moments of adsorbed molecules, the electrostatic energy minimization is ruled out as the driving force in the self-assembly. The structures observed are mainly the result of a competition between chemical interactions and substrate effects. The substrate's role in the self-assembly is to (i) reduce and realign the molecular dipole through charge donation and back donation involving both the molecular HOMO and LUMO, (ii) dictate the epitaxial orientation of the adsorbates, specifically so on Cu(111), and (iii) inhibit attractive forces between neighboring chains in the system ZI/Cu(111), which results in regularly spaced molecular gratings. (C) 2015 AIP Publishing LLC.

The structure and ordering of two-dimensional sheets of croconic acid (CA) on Au(111) were studied with scanning tunneling microscopy and first-principles calculations. Extended porous honeycomb networks are formed that differ from those that were recently reported to form on Ag(111) and density functional theory (DFT) calculations are used to help rationalize how the two substrates, Au(111) and Ag(111), influence the CA networks. The CA network that is synthesized on Au(111) corresponds to the 2D structure that has the lowest ground-state DFT energy when modeled without the substrate and uses a different isomer of CA than the one that is found within the Ag(111) network and the bulk three-dimensional crystal; this is attributed to the weaker CA–CA and CA–substrate interactions on Au(111). The Au(111) network has no net dipole moment, but a hydrogen transfer mechanism which forms a metastable polar network seems plausible if the latter remains dynamically stable. The formation of the Ag(111) network is at...

This article reviews the surface adsorption of p-benzoquinonemonoimine zwitterions from the class of N-alkyldiaminoresorcinones (or 4,6-bis-dialkylaminobenzene-1,3-diones, i.e. C6H2(···NHR)2(···O)2), where R = H, R = C2H5, n-C4H9), on a variety of conducting surfaces. These small molecules with a large electrical dipole exhibit molecular orientation and packing on surfaces depending on the deposition methodology (from solution or from the vapor) as well as the substrate. What is very clear from the investigations of this class of molecules is that inter-molecular interaction is not simply driven by dipolar interactions alone, but frontier orbital symmetry as well. This is illustrated by the reversible adsorption of di-iodobenzene on molecular films of (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate C6H2(···NHR)2(···O)2 where R = n-C4H9, where it is clear that absorption is isomer specific.

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.