Conduction mechanism of Aviram-Ratner rectifiers with single pyridine-sigma-C(60) oligomers

We present the electron transport of pyridyl aza[60]fulleroid oligomers, abbreviated as C(60)NPy, which is based on the donor-barrier-acceptor (D-sigma-A) architecture, at a single molecular scale using scanning tunneling microscopy. A rectifying effect is observed in the current-voltage characteristics. The theoretical calculation shows that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are well localized either on the Py moiety (donor) or on the C(60) moiety (acceptor), indicating the sigma-bridge decouples the LUMO and the HOMO of the donor and the acceptor, respectively. This structure accords well with the unimolecular rectifying model proposed by Aviram and Ratner [Chem. Phys. Lett. 1974, 29, 277]. The mechanism of the rectifying effect is understood by analyzing in detail the electron transport through energy levels of the donor and the acceptor of the C(60)NPy molecules. By directly comparing the experimental conductance peaks and the calculated density of states of the C(60)NPy, we find that the observed rectification is attributed to the asymmetric positioning of the LUMOs and the HOMOs of both sides of the acceptor and the donor of the C(60)NPy molecules with respect to the Fermi level of the electrodes. When a main voltage drop is over the molecule-electrode vacuum junction but a small fraction over the molecule itself, the shift of the energy levels between the donor and the acceptor will be small. This behavior deviates from the original proposal by Aviram and Ratner in which a large shift of the energy level is expected.