An approach to the electronic structure of molecular junctions with metal clusters of atomic thickness.

TD-DFT calculations predict a linear dependence of the energies of charge transfer states of Agn-pyrazine-Agn molecular junctions on the inverse of the size (1/n) of the linear metal chains. The density of charge (qeff = q/n) in the metal-to-metal charge transfer excited states (CTMM: Agnq-pyrazine-Agn-q) smoothly tunes the electronic structure of the junction, especially the metal-to-molecule charge transfer states (CT0 and CT1) and the first excited singlet of pyrazine (S1,Pz). In enlarged junctions, pyrazine bonds preferably to one of the Agn clusters and this weak adsorption produces a significant unexpected asymmetry for forward and reverse charge transfer processes.