Substitution pattern controlled charge transport in BN-embedded aromatics-based single molecule junctions.

The understanding of charge transport at a single molecule level is a prerequisite for the fabrication of molecular devices. Here, the relationship between molecular conductance, substitution pattern and stimuli response in BN-embedded aromatics was systematically investigated using the break junction technique. It was found that the para-phenylthioether-anchored BN molecule (p-BN-p) shows the highest conductance of 10-4.86G0, and the meta-phenylthioether-anchored BN molecule (m-BN-m) exhibits the lowest conductance which is lower than the instrument detection limit (<10-6.0G0). The m-BN-p and p-BN-m molecules, with both para- and meta-substituted anchor groups on two termini, show moderate conductances of 10-5.50G0 and 10-5.45G0, respectively. The conductance difference is interpreted as a distinct quantum interference effect caused by the substitution pattern of the anchoring groups. Notably, their conductance changes slightly upon coordination with a fluoride ion, in spite of the distinct change of their frontier orbital energy levels. These results demonstrate that, in addition to the frontier orbital energy levels, the anchors play an important role in the design of stimuli-responsive molecular electronic devices with a high on/off current ratio.