Influence of Structural Fluctuations, Proton Transfer, and Electric Field on Polarization Switching of Supported Two-Dimensional Hydrogen-Bonded Oxocarbon Monolayers

The structural alignment, proton transfer, and molecular dipole under an electric field and as a function of simulation time have been investigated computationally for experimentally observed two-dimensional sheets of croconic acid (CA) on Ag(111) surface and rhodizonic acid (RA) molecules on Au(111) surface at room temperature. Depending on their local environment, some of the OH···O bonds in the CA monolayer exhibit spontaneous proton transfer especially for those bonds that are part of a trimer unit within the hydrogen-bonding network. In stark contrast, the RA molecules exhibit little proton transfer. It is found that thermal structural fluctuations of the molecular layers translate into considerable fluctuations of the polarization vector within the film plane, and even polarization reversal, at room temperature, which even can mask additional contributions to the polarization from the spontaneous and electric field induced proton transfer in CA monolayer. A common feature for both supported CA and RA monolayers is their constant polarization normal to the film plane

Magic Electret Clusters of 4‑Fluorostyrene on Metal Surfaces

We report a combined experimental and theoretical study of the adsorption and assembly of a simple dipolar molecule, 4-fluorostyrene, on both Cu and Au surfaces. Self-assembly occurs in the form of small highly polar electrets with discrete (“magic”) sizes that depend on the surface metal. Charge transfer between the molecule and surface results in a ∼90° reorientation of the electric dipole moment as compared to the gas-phase molecule and a doubling of its magnitude. The magic size can be understood in terms of a balance between attractive interactions in the form of both directional C–H···F hydrogen bonding and van der Waals interactions, as well as repulsive forces from Columbic interaction between the charged molecules. While this work illustrates the importance of interfacial charge transfer in molecular dipole engineering at surfaces, it offers unique chiral systems that are highly regular and dipolar with which to study and understand charge- and spin-transfer across metal–organic interfaces

Rhodizonic Acid on Noble Metals: Surface Reactivity and Coordination Chemistry

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 first-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

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

The large intrinsic electric dipole of about 10 D of a <i>p</i>-benzoquinonemonoimine compound from the class of <i>N</i>-alkyldiaminoresorcinone (or 4,6-bisdialkylaminobenzene-1,3-diones, i.e., C₆H₂(<u>···</u> NHR)₂(<u>···</u> 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

Charge-Transfer-Induced Magic Cluster Formation of Azaborine Heterocycles on Noble Metal Surfaces

We report a combined experimental and theoretical study of the adsorption and assembly of a nitrogen–boron-containing heterocycle, 1,2-dihydro-1,2-azaborine, on Au(111) and Cu(111). Despite the inherent molecular dipole moment, the self-assembly behavior is found to be highly surface dependent, with isolated molecules prevalent on Cu(111) and discrete (“magic”) clusters on Au(111). The ability to form clusters of a particular size can be understood in terms of a balance between attractive intermolecular interactions, including directional B–H···H–N dihydrogen bonding, and repulsive forces from Coulombic interactions between the charged molecules dictated by differences in the charge transfer and Pauli repulsion between the adsorbate and the surface. This work highlights the importance of metal–molecule charge transfer in the adsorption and assembly of dipolar molecules on surfaces and demonstrates that their surface-bound properties cannot be predicted a priori from gas-phase dipole moments alone

Nitrogen-Doping Induced Self-Assembly of Graphene Nanoribbon-Based Two-Dimensional and Three-Dimensional Metamaterials

Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties