Single-molecule spectroscopy of molecular aggregates at low temperature

We have conducted single-molecule spectroscopy of a fluorescent polyphenylene dendrimer consisting of four peripheral perylenemonoimides which serve as energy donors and a central terrylenediimide which is the energy acceptor. After selective excitation of the donors the low-temperature emission spectra of single dendrimers show the purely electronic zero-phonon line as the most prominent feature of the acceptor. These sharp emission lines are subjected to appreciable spectral shifts. Fluorescence excitation spectroscopy of individual dendrimers in the spectral region of the donor absorption allows to extract energy transfer rates for single donors within the dendrimer. Although the energy transfer from perylenemonoimide to terrylenediimide is quite efficient, energy transfer between two proximate perylenemonoimides might be a competing process. This is shown by experiments with molecular dimers, in which two perylenemonoimides are held at a short distance by a benzil spacer.