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metal-carbene complexes for the synthesis of molecular switches and devices

Final Report Summary - ORGANOMETALLICSWITCH (metal-carbene complexes for the synthesis of molecular switches and devices)

The area of molecular electronics has thus far been dominated by either organic or purely inorganic materials. For example, metallic GaAs and GaP nanowires have been constructed for semiconductor applications.[1] While these materials show excellent electron mobility properties, their tuning potential is limited. In contrast, organic semiconductors such as conjugated oligomers or polymers display promising properties,[2] though their electron carrier properties are intrinsically lower than those of metallic materials. While organic electronic devices are ready to be commercialized, they aim at low-cost applications rather than high performance. Hybrid organic-inorganic materials might overcome such limitations and combine the advantages of purely inorganic and organic materials. Organometallic complexes may become especially useful as they comprise tunable organic ligand moieties linked by relatively strong M–C bonds to transition metal centers. An appropriate choice of the metal center, e.g. redox-active iron or cobalt, is expected to enhance electron mobility. Moreover, the accessibility of different oxidation states of such metal centers may provide an additional switching function.
As ligands, we considered imidazolium-derived N-hetercyclic carbenes (NHCs) to be particularly promising, as the potential for M=C carbene-type π bonding may further stimulate electronic communication between the metal center and remote donor/acceptor sites located at the ligand periphery (Fig. 1). In addition, the strong donor ability and covalent character of the ligand-metal bond should provide further advantages for the fabrication of materials, such as increased stability of the coordinated metal center.

The objective of the research project was to evaluate the applicability of NHC-complexes in molecular electronics, i.e. for the synthesis of molecular switches, electroactive materials and ultimatively devices.
Despite the evidence for carbene-type π bonding in such complexes,[3] preliminary tests surprisingly suggested low electron mobilities through ditopic delocalised NHC linkers.[4] This was reflected by very weak electronic coupling of the linked metal sites in complexes of the kind represented in Figure 2 (left). It was reasoned that the geometry around the complex does not allow an optimal orbital overlap at the metal-ligand interface.

In order to investigate this assumption, we synthesised model compounds with a octahedral coordination sphere and 2-pyridyl wingtip groups (Figure 2 - right) which ensure a beneficial mutual orientation of the linker and the metal sites. Electrochemical and spektro-electrochemical analysis proved enhanced electrochemical communication and remarkable stability, which renders these compounds useful as multi-state switches. In conclusion, these results suggest that NHCs can indeed be useful building blocks for molecular electronics but it is critical to pay attention to the coordination geometry.
During the elaborate synthesis of above model compounds, we ensured facile functionalisation of the ditopic ligand. Efforts to exploit the electrochemical properties of these complexes in self-assembled materials are under way.

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