  The concept of single molecule rectifiers proposed in a theoretical work by
Aviram and Ratner in 1974 was the starting point of the now vibrant field of
molecular electronics. In the meantime, a built-in asymmetry in the conductance
of molecular junctions has been reported at the experimental level. In this
contribution, we present a theoretical comparison of three different types of
unimolecular rectifiers: i) systems where the donor- and acceptor-part of the
molecules are taken from charge-transfer salt components; ii) zwitterionic
systems; and iii) Tour wires with nitro substituents. We conduct an analysis of
the rectification mechanism in these three different types of asymmetric
molecules on the basis of parameterized quantum-chemical models as well as with
a full non-equilibrium Greens function / density functional theory (NEGF-DFT)
treatment of the current/voltage characteristics of the respective
metal/molecule/metal junctions. We put a particular emphasis on the prediction
of rectification ratios (RR), which are crucial for the assessment of the
technological usefulness of single molecule junctions as diodes. We also
compare our results with values reported in the literature for other types of
molecular rectification, where the essential asymmetry is not induced by the
structure of the molecule alone but either by a difference in the electronic
coupling of the molecule to the two electrodes or by attaching alkyl chains of
different lengths to the central molecular moiety.
