Photoinduced Electron-Transfer within Free Base and Zinc Porphyrin Containing Poly(Amide) Dendrimers

Abstract

The synthesis and photophysical characterization of a series of free base and zinc porphyrin containing, Newkome-type dendrimers terminated with anthraquinone groups (FbP-Ga-AQn and ZnP-Ga-AQn) and ethyl groups (FbP-Ga-Etn and ZnP-Ga-Etn) 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-Ga-AQn were shown to exhibit substantial quenching (58−75%) of the porphyrin fluorescence as measured against the analogous ethyl-terminated dendrimers (FbP-Ga-Etn) in steady-state fluorescence experiments. The zinc porphyrin containing dendrimers ZnP-Ga-AQn 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-Ga-AQn 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 kET values in the range of 3.77 × 107 s-1 to 2.28 × 108 s-1 that were dependent upon both dendrimer generation number and solvent. Similar experiments on ZnP-Ga-AQn 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.