Real-Space Investigation of Intermolecular Energy Transfer by Scanning Tunneling Luminescence Spectroscopy

Abstract

Energy transfer is an essential process in photosynthesis and various energy-harvesting devices. So far, nanoscale features of energy transfers are unknown because of the limited spatial resolution of conventional methods. We conducted a molecular-level investigation of energy transfers in molecular dimers consisting of a free-base phthalocyanine and magnesium phthalocyanine (H₂Pc and MgPc) by scanning tunneling luminescence spectroscopy. Local excitation of an MgPc with the tunneling current of a scanning tunneling microscope give rise to luminescence from a nearby H₂Pc as a result of energy transfer. We demonstrate the possibility of a single-molecule valve device for energy transfer, which is revealed by a blinking behavior of energy transfer. Moreover, a back-and-forth energy transfer was observed, where the energy was transferred from the second singlet state (so-called Q_y state) of H₂Pc to the first singlet state (Q state) of MgPc and finally to the first singlet state (Q_x state) of H₂Pc.