Photo-Induced Charge Transfer in Single-Molecule p-n Junctions.

We measured photo-induced charge separation in isolated individual C60 tethered 2,5 dithienylpyrrole triad (C60 triad) molecules with submolecular resolution using a custom built laser assisted scanning tunneling microscope. Laser illumination was introduced evanescently into the tunneling junction through total internal reflection and the changes in tunneling current and electronic spectra caused by photoexcitation were measured and spatially resolved. Photo-induced charge separation was not detected for all C60 triad molecules, indicating that the conformations of the molecules may affect the excitation probability, lifetime, and/or charge distribution. Photo induced signal was not observed for dodecanethiol molecules in the surrounding matrix nor for control molecules without C60 moieties, as neither absorbs incident photons at this energy. This spectroscopic imaging technique has the potential to elucidate detailed photo-induced carrier dynamics, which are inaccessible via ensemble-scale (i.e., averaging) measurements, that can be used to direct the rational design and optimization of molecular p-n junctions and assemblies for energy harvesting.