Effect of End Group Position on the Formation of a Single Porphyrin Molecular Junction

We have investigated the electric conductance of Au nanocontacts in solutions containing tetrakis-5,10,15,20-(2-acetylthiomethylphenyl)porphyrin (2SCP) or tetrakis-5,10,15,20-(4-acetylthiomethylphenyl)porphyrin (4SCP). The conductance histogram showed a broad feature around 1G0(2e2/h) together with a large background in the 4SCP solution, while the conductance histogram showed a sharp 1G0 peak in the 2SCP solution. After breaking the Au atomic contact, a single Au/4SCP/Au molecular junction could be formed in the 4SCP solution. The conductance of the single Au/4SCP/Au molecular junction was determined to be 5 × 10−4 (±1 × 10−4)G0. In contrast, the conductance of the Au/2SCP/Au molecular junction could not be determined. The difference in the conductance behavior could be explained by the adsorption configuration of the molecules on Au surfaces, defined by the position of the end groups.