Electron transfer in molecular binuclear complexes and relation with electron transport through nanojunctions

Intramolecular electron transfer in binuclear mixed valence complexes where the metal atoms are linked by a bridging ligand illustrates a long-known problem: is it a two-step process with a temporary oxidation or reduction of the bridging ligand, or a one-step process best described by the super-exchange mechanism? In the latter case, information on the effective coupling between metal sites, Vab, can be obtained from the intensity of the intervalence transition. It is thus possible to study the decay of the coupling with distance, and also to show the possibility of molecular switching by using a photoactive ligand.However an integrated description of both mechanisms is possible by the explicit introduction of the bridging ligand nuclear degrees of freedom. This is achieved in a three-center model where the usual Hush–Marcus parabolas are replaced by three paraboloids in interaction. It is thus possible to compare different chemical systems and visualize the two limiting mechanisms.An interesting parallel can be made with the case of nanojunctions of the metal–molecule–metal type, which are increasingly available from scanning tunneling microscopy of molecules deposited on a suitable substrate. Such systems present an additional flexibility because the position of one electrode and the potentials can be changed. Thus one can probe again the decay of the electronic effect with distance and the possibility of molecular switching. But in addition, new effects can be observed. A remarkable experiment is the “charge trapping” on a single atom or a single molecule through the application of a voltage pulse. This can be considered as a redox reaction, the charge being stabilized by nuclear relaxation, the same process as the one occurring in polaron formation or in the two-step process invoked above. Finally, by moving the STM tip laterally above a molecule weakly coupled to its substrate, one can image in some cases molecular orbitals.