  We study theoretically the transport properties of a molecular two level
system with large electron-vibron coupling in the Coulomb blockade regime. We
show that when the electron-vibron coupling induces polaron states, the
current-voltage characteristic becomes strongly asymmetric because, in one
current direction, one of the polaron state blocks the current through the
other. This situation occurs when the coupling between the polaron states is
smaller than the coupling to the leads. We discuss the relevance of our
calculation for experiments on C_140 molecules.
