  Sequential tunneling model is used to study electron transport in molecular
rectifiers based on structures of donor-bridge-acceptor type. The device is
made of two metallic electrodes connected by a molecule, which contains
acceptor and donor subunits, separated by insulating bridge. Both subunits are
modeled as quantum dots with discrete energy levels, isolated from each other
by potential barrier and weakly coupled to both electrodes through tunnel
junctions. Intervalence donor-acceptor tunneling process is treated as a
superexchange mechanism or equivalently through the use of Marcus theory.
Analytic formula for the current is found in the case of the Aviram-Ratner
ansatz of rectification and current-voltage characteristic is obtained at an
arbitrary strength of the potential drop over the tunneling region. It is shown
that rectification current depends on the position of the acceptor's LUMO and
donor's HOMO levels with respect to the Fermi energy of the electrodes before
bias is applied, and their shift due to the bias voltage.
