ENERGY-TRANSFER IN DENDRITIC MACROMOLECULES - MOLECULAR-SIZE EFFECTS AND THE ROLE OF AN ENERGY GRADIENT

Perylene-terminated monodendrons 1-7 and phenyl-terminated reference m onodendrons 8-14 have been synthesized, and the intramolecular energy transfer has been studied using steady-state as well as time-resolved fluorescence spectroscopy. In the series 2-7, the light-harvesting abi lity of these compounds increases with increasing generation due to th e increase in molar extinction coefficient. However, the efficiency of the energy transfer decreases with increasing generation in this seri es. With increasing generation, the photoluminescence intensity from t he perylene core still increases and the expected level-off in the pho toluminescence intensity has not been reached in this series of compou nds. Dendrimer 1 is unique in that the energy transfer in this molecul e occurs at a very fast rate. The rate constant for energy transfer in 1 is at least 2 orders of magnitude larger than in 2-7. In contrast t o monodendrons 2-7, 1 possesses a variable monomer type at each genera tion that creates an energy funnel. The ultrafast energy transfer in t his system is best explained by the presence of this energy gradient.