Optical energy transport and interactions between the excitations in a coumarin-perylene bisimide dendrimer

Energy transfer properties of novel coumarin-perylene bisimide dendrimer are studied by means of steady state and time-resolved UV/vis spectroscopy. At low donor excitation density fast (transfer rate approximately 10 ps(-1)) and efficient (quantum yield approximately 99.5%) donor-acceptor energy transfer is observed. The random distributions of donor-acceptor orientations and distances result in nonexponential energy transfer kinetics. The energy transfer remains independent of excitation density up to densities corresponding to one absorbed photon per 10 dendrimer molecules. At higher excitation densities the transfer rate is found to increase due to excitation of multiple donors per dendrimer. Control of the donor-acceptor energy transfer rate is achieved by pre-excitation of the acceptor and monitored by prepump-pump-probe experiments, which show that the energy transfer rate can be decreased by a factor of 2. The relative orientations of transition dipole moments in the donor and acceptor molecules are found to be one of the key factors determining the energy transfer dynamics at high excitation densities.