Electronic decoupling of a cyclophane from a metal surface

Abstract

Electronic self-decoupling of an organic chromophore from a metal substrate is achieved using a naphtalenediimide cyclophane to spatially separate one chromophore unit of the cyclophane from the substrate. Observations of vibronic excitations in scanning tunneling spectra demonstrate the success of this approach. These excitations contribute a significant part of the tunneling current and give rise to clear structure in scanning tunneling microscope images. We suggest that this approach may be extended to implement molecular functions at metal surfaces.electron transport | functional molecules at surfaces | scanning tunneling microscopy | vibronic states P redictions are difficult, especially about the future. We hope that this truism will be valid for the future of surface science, too. Nevertheless, it appears to be safe to predict that functional molecules at surfaces will be one of the foci of surface science over the next decade. The opportunities in this field are virtually unlimited and range from technological issues such as controlling interface effects in organic devices to adventurous endeavors such as building molecular machines at surfaces. Although these topics are most challenging, the present state of the experimental and theoretical methods of surface science is good reason for optimism that significant scientific progress will be made in this field.This article focuses on the electronic coupling between molecules and a metal substrate. At the interface, charge transfer and hybridization affect the levels of an adsorbed molecule, which may significantly modify its properties. To recover and use the intrinsic molecular properties, which may be taylored over a wide range, a degree of decoupling from the metallic surface may be desirable. Effective molecular decoupling has been achieved using multilayers of molecules (1) or ultrathin insulating layers (2-5). An alternative approach is to chemically modify a molecule using spacer groups in order to lift a particular subunit from the substrate. With this aim, for instance, bulky groups have been used to preserve switching capability of an azobenzene derivative (6-11). Previously, this approach was used to obtain an electrical insulation of a model molecular wire (12). However, these molecules turned out to be too flexible and thus deformed upon adsorption at metal substrates (13).Here we use designed cyclophanes to achieve decoupling of one chromophore from a metal surface. These cyclophanes consist of two rigidly separated parallel π-systems from which only one adsorbs to the surface, whereas the second one is expected to remain separated from the metal. Thus, cyclophanes represent a class of molecules that are particularly interesting for investigating columnar π-stacking and through-space or through-bond electronic conductance (14-16). A naphthalenediimide (NDI) cyclophane (Fig. 1) was chosen as a model compound for the present study. It is demonstrated that this organic molecule, with its height of a few angstroms on a metallic substrate, provide...

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