  Combining insights from quantum chemistry calculations with master equations,
we discuss a mechanism for negative differential resistance (NDR) in molecular
junctions, operated in the regime of weak tunnel coupling. The NDR originates
from an interplay of orbital spatial asymmetry and strong electron-electron
interaction, which causes the molecule to become trapped in a non-conducting
state above a voltage threshold. We show how the desired asymmetry can be
selectively introduced in individual orbitals in e.g., OPE-type molecules by
functionalization with a suitable side group, which is in linear conjugation to
one end of the molecule and cross-conjugated to the other end.
