Isomer-Dependent Franck−Condon Blockade in Weakly Coupled Bipyridine Molecular Junctions

Franck−Condon blockade is an attractive
functionality of molecular junctions, but its tunability is still
a challenge that has not been fully addressed. We show here
from first-principles calculations that the electron−vibration
coupling strength of a weekly coupled bipyridine molecular
junction can be largely tuned from weak to strong coupling
regime through isomerization. Electron transport properties of
four bipyridine isomers, namely 2,6′-bipyridine, 2,4′-bipyridine, 2,2′-bipyridine, and 4,4′-bipyridine, have been exclusively examined. A very strong Franck−Condon blockade is found to
be associated with 2,2′-bipyridine and 4,4′-bipyridine molecules and should be observable experimentally. A gate-controlled
conductance switch is proposed for a molecular junction with a 4,4′-bipyridine molecule. Our calculations have clearly
demonstrated that bipyridine isomers are excellent candidates for the experimental study of vibration-mediated transport
properties in a single molecule.