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 Ag_n-pyrazine-Ag_n molecular junctions on the inverse of the size (1/n) of the linear metal chains. The density of charge (q_eff = q/n) in the metal-to-metal charge transfer excited states (CT_MM: Ag_n^q-pyrazine-Ag_n^-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 (S_1,Pz). In enlarged junctions, pyrazine bonds preferably to one of the Ag_n clusters and this weak adsorption produces a significant unexpected asymmetry for forward and reverse charge transfer processes.