Electronic Structure of a Spin Crossover Molecular Adsorbate

We have investigated the occupied and unoccupied electronic structure of ultrahigh vacuum (UHV) evaporated molecular thin films of the spin crossover [Fe­(H₂B­(pz)₂)₂(bipy)] complex (with H₂B­(pz)₂ = bis­(hydrido)­bis­(1<i>H</i>-pyrazol-1-yl)­borate and bipy = 2,2′-bipyridine) by ultraviolet photoelectron spectroscopy (UPS), inverse photoemission (IPES), and X-ray absorption spectroscopy (XAS). A bandgap of 2–3 eV is deduced from combined UPS and IPES measurements of the molecular films on Au substrates. The matching Fe XAS and IPES spectra indicate that the electronic unoccupied states have a significant Fe weight. The shift of the unoccupied density of states seen in inverse photoemission is consistent with the thermally induced spin crossover transition for [Fe­(H₂B­(pz)₂)₂(bipy)] deposited on the organic ferroelectric copolymer poly­(vinylidene fluoride) with trifluoroethylene (PVDF–TrFE)

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

Altering the Static Dipole on Surfaces through Chemistry: Molecular Films of Zwitterionic Quinonoids

The adsorption of molecular films made of small molecules with a large intrinsic electrical dipole has been explored. The data indicate that such dipolar molecules may be used for altering the interface dipole screening at the metal electrode interface in organic electronics. More specifically, we have investigated the surface electronic spectroscopic properties of zwitterionic molecules containing 12π electrons of the <i>p</i>-benzoquinonemonoimine type, C₆H₂(<u>···</u>NHR)₂(<u>···</u>O)₂ (R = H (<b>1</b>), <i>n</i>-C₄H₉ (<b>2</b>), C₃H₆–S–CH₃ (<b>3</b>), C₃H₆–O–CH₃ (<b>4</b>), CH₂–C₆H₅ (<b>5</b>)), adsorbed on Au. These molecules are stable zwitterions by virtue of the meta positions occupied by the nitrogen and oxygen substituents on the central ring, respectively. The structures of <b>2</b>–<b>4</b> have been determined by single crystal X-ray diffraction and indicate that in these molecules, two chemically connected but electronically not conjugated 6π electron subunits are present, which explains their strong dipolar character. We systematically observed that homogeneous molecular films with thickness as small as 1 nm were formed on Au, which fully cover the surface, even for a variety of R substituents. Preferential adsorption toward the patterned gold areas on SiO₂ substrates was found with <b>4</b>. Optimum self-assembling of <b>2</b> and <b>5</b> results in ordered close packed films, which exhibit n-type character, based on the position of the Fermi level close to the conduction band minimum, suggesting high conductivity properties. This new type of self-assembled molecular films offers interesting possibilities for engineering metal–organic interfaces, of critical importance for organic electronics