Molecular Topology and the Surface Chemical Bond: Alternant Versus Nonalternant Aromatic Systems as Functional Structural Elements

Abstract

The interaction of carbon-based aromatic molecules and nanostructures with metals can strongly depend on the topology of theirπ-electron systems. This is shown with a model system using the isomers azulene, which has a nonalternantπsystem with a 5-7 ring structure, and naphthalene, which has an alternantπsystem with a 6-6 ring structure. We found that azulene can interact much more strongly with metal surfaces. On copper (111), its zero-coverage desorption energy is 1.86 eV, compared to 1.07 eV for naphthalene. The different bond strengths are reflected in the adsorption heights, which are 2.30 Å for azulene and 3.04 Å for naphthalene, as measured by the normal incidence x-ray standing wave technique. These differences in the surface chemical bond are related to the electronic structure of the molecularπsystems. Azulene has a low-lying LUMO that is close to the Fermi energy of Cu and strongly hybridizes with electronic states of the surface, as is shown by photoemission, near-edge x-ray absorption fine-structure, and scanning tunneling microscopy data in combination with theoretical analysis. According to density functional theory calculations, electron donation from the surface into the molecular LUMO leads to negative charging and deformation of the adsorbed azulene. Noncontact atomic force microscopy confirms the deformation, while Kelvin probe force microscopy maps show that adsorbed azulene partially retains its in-plane dipole. In contrast, naphthalene experiences only minor adsorption-induced changes of its electronic and geometric structure. Our results indicate that the electronic properties of metal-organic interfaces, as they occur in organic (opto)electronic devices, can be tuned through modifications of theπtopology of the molecular organic semiconductor, especially by introducing 5-7 ring pairs as functional structural elements.

Alternate abstract:

Plain Language Summary

In organic electronic devices, such as modern displays with organic light-emitting diodes, the organic materials connect to metal electrodes. The resulting metal-organic interfaces determine important performance parameters such as rates of charge-carrier injection. Precise control over the interface properties, especially the wave-function overlap and the energy-level alignment, is therefore critical for rational improvement of organic electronic devices. Here, we show that the properties of metal-organic interfaces depend strongly on the linking pattern of the atoms in the organic material.

In organic semiconductors, the carbon atoms are typically laid out in a honeycomblike sheet of abutting six-sided rings. If the sheet contains no odd-numbered rings, it is described as an “alternant topology.” Researchers rarely consider nonalternant topologies, which occur when the structure contains, for example, five- or seven-sided rings. To elucidate the influence of the topology on the interaction with a metal surface, we compare the aromatic hydrocarbon naphthalene with its nonalternant isomer, azulene, and study their interactions with a copper surface.

We find that azulene forms a much stronger and shorter bond to copper than naphthalene. Spectroscopic analysis of the electronic structure reveals that azulene forms a true chemical bond and receives negative charge from the surface, whereas naphthalene bonds only weakly and does not exchange charge. Theoretical analysis reveals that the influence of the topology on the electronic structure, especially the lowest unoccupied molecular orbital, is responsible for the different behavior.

We propose that the incorporation of nonalternant structural elements can be used to control and optimize performance-related properties of functional metal-organic interfaces.