Dipole driven bonding schemes of quinonoid zwitterions on surfaces

Abstract

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.

Self-assembly of strongly dipolar molecules on metal surfaces

“…Also, the EZI are typically observed to form linear chains, although we have reported 2D island formation in an earlier study. 3 The reason for 1D chain formation is simple: hydrogen bond formation involves the oxygen-and amine groups of the molecules, and for EZI and BZI, this is only possible for alternating molecular orientation due to the constraining R groups. A 2D bonding scheme, where all molecules point in the same direction, is only possible with the NHR groups if R= =H.…”

“…Instead, molecules of similar dipole moment can repel each other on one surface 9 and attract each other on another. 11 Molecules were observed to arrange their dipoles into linear chains, 7 one-dimensional chains of alternating dipole orientation, 3 rings, 8 domains of parallel dipole alignment, 4 porous two-dimensional (2D) networks with 90 • dipole alignment, 7 and honeycomb networks of more complex moment alignment. 12 Multiple phases were often observed on the same sample.…”

“…In some cases, these interactions were shown to dominate over forces resulting from chemical bonds between the molecules. Various examples can be found in published studies of organics under ambient atmospheric conditions, 1 at the liquid-solid interface 2 and under ultrahigh vacuum conditions, [3][4][5][6][7][8] with the molecular dipole aligned perpendicular to the surface 9 or within the surface plane, 3,4 and for various molecule sizes ranging from small molecules such as quinonoid zwitterions 3 and styrenes 4 to larger ones such as anthracene. 10 In reviewing those reported assemblies of dipolar molecules, it becomes evident that most of them do a) Electronic mail: ezurek@buffalo.edu b) Electronic mail: aenders2@unl.edu not correspond to a configuration of lowest electrostatic energy.…”

“…Under UHV conditions on the (111) faces of the coinage metals Ag, Au, and Cu, molecule-substrate interactions play a deterministic role in what self-assembled structures quinonoid zwitterions form. 3,19 An important consideration in this discussion is that the dipole moment of a molecule adsorbed onto a metal substrate deviates, often drastically, from the dipole moment of a free molecule. In an earlier study, we have reported that the magnitude of the dipole moment of a ZI molecule adsorbed to an Ag or Cu cluster (a finite model for the substrate-adsorbate system) yields a dipole moment that is approximately one sixth of the free molecule value or approximately 1.5 D. 3 This change in dipole results from a substrate-mediated charge exchange between the HOMO and the LUMO of the molecules.…”

“…3,19 An important consideration in this discussion is that the dipole moment of a molecule adsorbed onto a metal substrate deviates, often drastically, from the dipole moment of a free molecule. In an earlier study, we have reported that the magnitude of the dipole moment of a ZI molecule adsorbed to an Ag or Cu cluster (a finite model for the substrate-adsorbate system) yields a dipole moment that is approximately one sixth of the free molecule value or approximately 1.5 D. 3 This change in dipole results from a substrate-mediated charge exchange between the HOMO and the LUMO of the molecules. But the opposite is also observed: styrene molecules are nearly non-polar as free molecules but acquire a significant in-plane dipole when adsorbed to Ag(100) but not on Ag(111).…”