Single Molecule Conductance of Dipyridines with Conjugated Ethene and Nonconjugated Ethane Bridging Group

We report single molecule conductance measurement of 4,4‘-bipyridine (BPY), 1,2-di(pyridin-4-yl)ethene (BPY-EE), and 1,2-di(pyridin-4-yl)ethane (BPY-EA), in which the two pyridine rings sandwich the conjugated ethene group and the nonconjugated ethane group, respectively. The conductance measurements of the molecules bridged in two Au electrodes have been carried out in aqueous solutions using the scanning tunneling microscopy-based break junction (STM-BJ) technique. All three molecules show two sets of conductance values with the high conductance (HC) values about 8−10 times higher than the low conductance (LC) values. The HC values for BPY, BPY-EE, and BPY-EA are (4.7 ± 1.1) × 10 -3 G 0, (2.3 ± 0.9) × 10 -3 G 0, and (4.5 ± 2.2) × 10 -4 G 0, respectively, in an order of BPY > BPY-EE > BPY-EA. The decrease of conductance of BPY-EE can be attributed mainly to the length increase of the molecule, while the decrease of the conductance of BPY-EA reveals the degree of electron decoupling upon insertion of one CH 2 CH 2 group between the two pyridyl rings in the molecule. The present work demonstrates that with pyridines as the end groups, it is possible to study the conductance of very short alkane chains.