  We discuss the nature of charge transfer in molecular conductors upon
connecting to two metallic contacts and imposing a voltage bias across them.
The sign of the charge transfer (oxidation vs. reduction) depends on the
position of the metal Fermi energy with respect to the molecular levels. In
addition, the charge transfer depends on the strength of the coupling
(chemisorption vs. physisorption) with the contacts. A convenient way to
establish the nature and onset of the charge transfer and the corresponding
features in the I-V is to draw an energy level diagram for each spin species.
Starting from such a level diagram, we argue that transport in the Tour-Reed
switching molecules, which consist of a central phenyl ring with a nitroamine
redox center, involves the oxidation of a highest occupied molecular orbital
(HOMO)-based level.
