Transient dynamics in molecular junctions: Coherent bichromophoric molecular electron pumps

The possibility of using single-molecule junctions as electron pumps for energy conversion and storage is considered. It is argued that the small dimensions of these systems allow the use of unique intramolecular quantum coherences in order to pump electrons between two leads and to overcome relaxation processes which tend to suppress the pumping efficiency. In particular, we demonstrate that a selective transient excitation of one chromophore in a bichromophoric donor-bridge-acceptor molecular junction model yields currents which transfer charge (electrons and holes) unevenly to the two leads in the absence of a bias potential. The utility of this mechanism for charge pumping in steady-state conditions is proposed.