Excitation energy transfer in dendritic host-guest donor-acceptor systems.

We report on a study of a physically formed host-guest system, which was designed to be investigated by fluorescence energy transfer. All donor and acceptor molecules used were cyanine dyes. Investigation was performed at the ensemble level as well as at the single-molecule level. The ensemble measurements revealed a distribution of binding sites as well for the donor as for the acceptor. Accordingly, we found a distribution of the energy transfer efficiency. At the single-molecule level, these distributions are still present. We could discriminate entities that show very efficient energy transfer, some that do not show any energy transfer and systems whose energy transfer efficiency is only about 50%. The latter allowed the time-resolved detection of energy transfer of single entities through the acceptor decay. Finally, we discuss the observation that the energy transfer efficiency fluctuates as a function of time.