Energy Transfer in Dendritic Macromolecules:  Molecular Size Effects and the Role of an Energy Gradient

Perylene-terminated monodendrons 1 − 7 and phenyl-terminated reference monodendrons 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 ability of these compounds increases with increasing generation due to the increase in molar extinction coefficient. However, the efficiency of the energy transfer decreases with increasing generation in this series. With increasing generation, the photoluminescence intensity from the perylene core still increases and the expected level-off in the photoluminescence intensity has not been reached in this series of compounds. Dendrimer 1 is unique in that the energy transfer in this molecule 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 to monodendrons 2 − 7, 1 possesses a variable monomer type at each generation that creates an energy funnel. The ultrafast energy transfer in this system is best explained by the presence of this energy gradient.