Single-electron transistor of a single organic molecule with access to several redox states

A combination of classical Coulomb charging, electronic level spacings, spin, and vibrational modes determines the single-electron transfer reactions through nanoscale systems connected to external electrodes by tunnelling barriers1. Coulomb charging effects have been shown to dominate such transport in semiconductor quantum dots2, metallic3 and semiconducting4 nanoparticles, carbon nanotubes5,6, and single molecules7,8,9. Recently, transport has been shown to be also influenced by spin—through the Kondo effect—for both nanotubes10 and single molecules8,9, as well as by vibrational fine structure7,11. Here we describe a single-electron transistor where the electronic levels of a single π-conjugated molecule in several distinct charged states control the transport properties. The molecular electronic levels extracted from the single-electron-transistor measurements are strongly perturbed compared to those of the molecule in solution, leading to a very significant reduction of the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. We suggest, and verify by simple model calculations, that this surprising effect could be caused by image charges generated in the source and drain electrodes resulting in a strong localization of the charges on the molecule.