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First-principles calculations reveal that upon adsorption to the Cu(111) surface, the C–C single bonds within the p-benzoquinonemonoimine zwitterion (ZI) contract by about 6%. A detailed analysis reveals that the bond shortening is primarily a result of backdonation from Cu orbitals of s and d symmetry to the lowest unoccupied orbital (LUMO) of the ZI. This LUMO is π*-antibonding across the molecule and π-bonding across the C–C bond that shortens. We illustrate that the level alignment between the Fermi level of the surface and the frontier molecular orbitals of the ZI, the topology of the LUMO, and the distance between the substrate and the adsorbate are important factors enabling bond strengthening via backdonation. An extended transition state–natural orbitals for chemical valence (ETS-NOCV) analysis is applied to molecular models for this system, and it confirms that the surface → LUMO backdonation on Cu(111) is larger than on Ag(111) and Au(111).

The synthesis of 2D H-bonded cocrystals from the room-temperature ferroelectric organics croconic acid (CA) and 3-hydroxyphenalenone (3-HPLN) is demonstrated through self-assembly on a substrate under ultrahigh vacuum. 2D cocrystal polymorphs of varied stoichiometry were identified with scanning tunneling microscopy, and one of the observed structural building blocks consists of two CA and two 3-HPLN molecules. Computational analysis with density functional theory confirmed that the experimental (CA)2(3-HPLN)2 tetramers are lower in energy than single-component structures due to the ability of the tetramers to pack efficiently in two dimensions, the promotion of favorable electrostatic interactions between tetramers, and the optimal number of intermolecular hydrogen bonds. The structures investigated, especially the experimentally found tetrameric building blocks, are not polar. However, it is demonstrated computationally that cocrystallization can, in principle, result in heterogeneous structures with dipole moments that exceed those of homogeneous structures and that 2D structures with select stoichiometries could favor metastable polar structures.

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...

A study of the two-dimensional crystallization of rhodizonic acid on the crystalline surfaces of gold and copper is presented. Rhodizonic acid, a cyclic oxocarbon related to the ferroelectric croconic acid and the antiferroelectric squaric acid, has not been synthesized in bulk crystalline form yet. Capitalizing on surface-assisted molecular self- assembly, a two-dimensional analogue to the well-known solution-based coordination chemistry, two-dimensional structures of rhodizonic acid were stabilized under ultrahigh vacuum on Au(111) and Cu(111) surfaces. Scanning tunneling microscopy, coupled with fi rst-principles calculations, reveals that on the less reactive Au surface, extended two- dimensional islands of rhodizonic acid are formed, in which the molecules interact via hydrogen bonding and dispersion forces. However, the rhodizonic acid deprotonates into rhodizonate on Cu substrates upon annealing, forming magic clusters and metal − organic coordination networks with substrate adatoms. The networks show a 2:1 distribution of rhodizonate coordinated with 3 and 6 Cu atoms, respectively. The stabilization of crystalline structures of rhodizonic acid, structures not reported before, and their transition into metal − organic networks demonstrate the potential of surface chemistry to synthesize new and potential useful organic nanomaterials.

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.