  Quantum transport through single molecules is very sensitive to the strength
of the molecule-electrode contact. Here, we investigate the behavior of a model
molecular junction weakly coupled to external electrodes in the case where
charging effects do play an important role (Coulomb blockade regime). As a
minimal model we consider a molecular junction with two spatially separated
donor and acceptor sites. Depending on their mutual coupling to the electrodes,
the resulting transport observables show well defined features such as
rectification effects in the I-V characteristics and nesting of the stability
diagrams. To be able to accomplish these results, we have developed a theory
which allows to explore the charging regime via the nonequilibrium Green
function formalism parallel to the widely used master equation technique. Our
results, beyond their experimental relevance, offer a transparent framework for
the systematic and modular inclusion of a richer physical phenomenology.
