Molecular wires of zinc (II) octaethylporphyrin using 4,4’ bipyridine as linker

Q. Ferreiraa, J. Morgadoa,b

a Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal
b Departamento de Engenharia Química e Biológica, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal


Metal-porphyrins are excellent building blocks to form highly ordered self-assembled arrays, which allows these molecules to have potential applications in nano/molecular electronics. Porphyrins have high affinity with Highly Ordered Pyrolitc Graphite (HOPG), leading to the formation of rigid and stable supramolecular structures due to π-π stacking interactions. On the other hand, the reactivity of the metal centre allows the possibility to fabricate long molecular wires without loss of conductivity(1), (2).
In this work we demonstrate the preparation of very stable molecular wires made of zinc (II) octaethylporphyrin (ZnOEP) linked by 4,4’-bipyridine (4,4’-BP) on HOPG. These molecular wires were prepared step-by-step, at the tetradecane/HOPG interface, being the deposition of each monolayer characterised by scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS). Under these conditions, molecular wires with up to 5 monolayers: ZnOEP/ 4,4’-BP / ZnOEP/ 4,4’-BP / ZnOEP were prepared, being each monolayer characterised with respect to molecular organization and electrical properties. STM images and I/V measurements showed that the packing density is well maintained upon growth of the molecular wire and the electrical transport occurs across the molecular wire. Figure 1 show the average of 30 I/V curves measured for each ZnOEP monolayer, with a reduction of conductance upon increase of the molecular wire length.