Photoinduced electron-transfer within free base and zinc porphyrin containing poly(amide) dendrimers

The synthesis and photophysical characterization of a series of free base and zinc porphyrin containing, Newkome-type dendrimers terminated with anthraquinone groups (FbP-G(a)-AQ(n) and ZnP-G(a)-AQ(n)) and ethyl groups (FbP-G(a)-Et-n and ZnP-G(a)-Et-n) are described. These dendrimers were designed for use as mimics of the photosynthetic reaction center. Red-shifts in the absorption spectra, particularly in the anthraquinone-terminated series, were interpreted as resulting from backfolding of the dendrimer branches. Dendrimers FbP-G(a)-AQ(n) were shown to exhibit substantial quenching (58-75%) of the porphyrin fluorescence as measured against the analogous ethyl-terminated dendrimers (FbP-G(a)-Et-n) in steady-state fluorescence experiments. The zinc porphyrin containing dendrimers ZnP-G(a)-AQ(n) exhibited nearly complete quenching (96-99.5%) of the porphyrin fluorescence. An intramolecular electron-transfer mechanism is proposed for the substantial decrease in fluorescence in both series of dendrimers. Time-resolved fluorescence experiments for FbP-G(a)-AQ(n) were fit to 2-3 exponentials and indicated that multiple orientations of the porphyrin and anthraquinone groups contribute to the electron-transfer event. These results were in good agreement with the steady-state fluorescence results. From the time-resolved fluorescence data, the electron-transfer rate constants were calculated, indicating k(ET) values in the range of 3.77 x 10(7) s(-1) to 2.28 x 10(8) s(-1) that were dependent upon both dendrimer generation number and solvent. Similar experiments on ZnP-G(a)-AQ(n) also indicated that multiple zinc porphyrin anthraquinone conformations were likely responsible for the electron-transfer. Dramatic differences between the steady-state and time-resolved fluorescence data in the zinc porphyrin dendrimers were interpreted in terms of ligation of the terminal anthraquinone groups with the zinc porphyrin that results in either a nonemissive state or an ultrafast electron-transfer.