The dynamics of electronic energy transfer in novel multiporphyrin functionalized dendrimers: A time-resolved fluorescence anisotropy

The dynamics of electronic energy transfer (EET) for a series of spherical porphyrin arrays based on different generations of poly(propylene imine) de ndrimers have been investigated using time-resolved fluorescence anisotropy measurements (TRAMS) in a glass environment. The first, third, and fifth g eneration dendrimers consisting of 4, 16, and 64 porphyrin chromophores, re spectively, are investigated in this study. We observe a depolarization of the fluorescence in all three dendrimers as compared to the monoporphyrin m odel compound, indicating that EET takes place between the chromophores wit hin the dendrimers. The experimental TRAMS results were compared to computa tionally simulated data obtained from the Pauli master equation. For the fi rst generation dendrimer, we find the rate of energy transfer is well descr ibed by Forster theory. Anomalous behavior is observed in the third generat ion dendrimer where the limiting anisotropy value suggests that energy tran sfer is confined to only the porphyrins contained within a dendron. Interde ndron porphyrin EET is thus unfavorable due to dendron segregation. In the fifth generation dendrimer, the TRAMS data is best explained by a model whi ch includes independent and simultaneous rapid EET between porphyrins conta ined on the surface of the dendrimer sphere and slow EET between porphyrins in adventitious dendrons found probably either outside or inside of the sp here.