In organic light-emitting diodes and related organic-electronic devices, molecular organic semiconductors are typically contacted by metal electrodes. The resulting metal/organic interfaces have a large impact on the performance of these devices, which makes the understanding of their chemical, electronic and geometric structure indispensable for the further rational development of organic electronics. Accordingly, model metal/organic interfaces with aromatic molecules have been a major research topic during the past decade. Up to now, the focus has been almost exclusively on aromatic systems with alternant topology such as pentacene, which have a uniform charge distribution and highly delocalized frontier orbitals. In contrast, aromatic systems with non-alternant topology have non-uniform charge distributions, which can result in considerable in-plane dipole moments, and more localized frontier orbitals. They also violate the Coulson-Rushbrooke pairing theorem and thus have shifted valence levels compared to the isomers with alternant topology. In this contribution, we present the first systematic studies of a non-alternant aromatic molecule on the (111) surfaces of Cu and Ag. In particular, we focus on naphthalene, which has as an alternant topology, and its non-alternant counterpart azulene (bicyclo[5.3.0]decapentaene). On the basis of extensive NEXAFS, PES, TPD, nc-AFM, STM and NIXSW studies, we show that the non-alternant topology results in much stronger interaction with metal surfaces, especially in the case of Cu(111), and that the interaction is more localized. Complementary periodic DFT calculations provide detailed insight into the surface chemical bond and charge redistribution between surface and molecule. A major part of the localized interaction is the donation of electron density into the molecular LUMO. This leads in turn to considerable in-plane and out-of-plane deformations of the adsorbed non-alternant species, while the alternant molecule remains largely undeformed.

Molecular topology and surface chemical bond: Alternant vs. non-alternant aromatic molecules

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.