Coherent charge transport through molecular wires: Exciton blocking and current from electronic excitations in the wire

We consider exciton effects on current in molecular nanojunctions, using a model comprising a two two-level sites bridge connecting free-electron reservoirs. Expanding the density operator in the many-electron eigenstates of the uncoupled sites, we obtain a 16 x 16 density matrix in the bridge subspace whose dynamics is governed by Liouville equation that takes into account interactions on the bridge as well as electron injection and damping to and from the leads. Our consideration can be considerably simplified by using the pseudospin description based on the symmetry properties of Lie group SU(2). We study the influence of the bias voltage, the Coulomb repulsion, and the energy-transfer interactions on the steady-state current and, in particular, focus on the effect of the excitonic interaction between bridge sites. Our calculations show that in case of noninteracting electrons this interaction leads to reduction in the current at high voltage for a homodimer bridge. In other words, we predict the effect of exciton blocking. The effect of exciton blocking is modified for a heterodimer bridge and disappears for strong Coulomb repulsion at sites. In the latter case the exciton type interactions can open new channels for electronic conduction. In particular, in the case of strong Coulomb repulsion, conduction exists even when the electronic connectivity does not exist.