Single electron transistor with a single conjugated molecule

We report on a single electron transistor (SET) where the electronic levels of a single pi-conjugated molecule control the electron transport properties. The molecule can be in several distinct charged states from +3 to -4. The experiment closely resembles electrochemical measurements allowing us to report on quantitative measurements of the redox potentials (or ionization potentials) of a single molecule in a solid state device. The molecular excitation energies extracted from the SET measurements are strongly correlated with electrochemical data. In contrast, the HOMO-LUMO gap is strongly reduced in a solid state environment, as compared to solution. We suggest that this surprising effect may be caused by image charges generated in the source and drain electrodes. They will result in a strong localization of the charges on the molecule. (C) 2004 Elsevier B.V. All rights reserved.