Reversible Intramolecular Electron Transfer at the Single-Molecule Level

Investigation of the photophysical and photochemical processes that occur in single molecules by means of fluorescence microscopy has rapidly developed over the last decade.[1,2] We have focused on the photophysics of synthetic multichromophore systems and have shown evidence for intramolecular (excitation) energy migration, that is, energy hopping[3,4] and directional F%rster energy transfer.[5] Photoinduced electron transfer is another fundamental physical process, but it is more difficult to monitor by single-molecule spectroscopy. Electron transfer is usually an efficient mechanism for fluorescence quenching which leaves no signal to be detected at the single-molecule level. In some special cases, chargetransfer fluorescence may be observed after photoinduced electron transfer,[6] but its quantum yield is usually quite low and therefore less suited for single-molecule detection. Despite this difficulty some reports on forward electron transfer in single molecules exist in the literature. Interfacial electron transfer from a single molecule of a cresyl violet dye to the indium–tin oxide surface on which it was physisorbed was shown by Lu and Xie.[7] Weaker fluorescence and a decreased fluorescence lifetime with respect to the same molecule on a glass surface demonstrated the occurrence of electron transfer. In a similar way electron transfer between a guanine base and a dye linked to a DNA molecule could also be monitored, and conformational changes of DNA molecules were observed.[8–10] Intramolecular electron transfer in a synthetic system was recently demonstrated by Zang et al.[11] in a compound in which a perylenediimide dye that serves as the electron acceptor upon excitation is connected to an aniline donor. On a sufficiently polar substrate, no fluorescence from single molecules could be observed, but after protonation of the amine by treatment of the sample with hydrogen chloride, the molecules could be visualized. This experiment nicely dem