Effects of dendrimer generation on site isolation of core moieties: Electrochemical and fluorescence quenching studies with metalloporphyrin core dendrimers

The ability of a dendritic shell to afford site isolation to a porphyrin core was evaluated using electron-transfer experiments with a series of porphyrin-core dendrimers. Cyclic voltammograms show that surrounding a porphyrin site with even a small generation (G similar to 2) dendrimer can significantly lower the rate of interfacial electron transfer, ostensibly by decreasing the proximity of the porphyrin core to the electrode surface. This inhibition of electron transfer is more pronounced when larger generation dendrimers are employed. While a significant measure of site isolation is achieved with respect to an electrode surface, no hindrance to penetration of a small molecule is afforded by the dendritic shell surrounding the porphyrin core, an encouraging result if dendrimers are to be designed as macromolecular hosts with a functioning catalyst at the core. Stern-Volmer analysis was used to investigate the accessibility of a small molecule, benzylviologen, to the porphyrin core. For generations 1-3, the dendritic structure surrounding the porphyrin core does not significantly inhibit the ability of the viologen to quench the fluorescence of the metalloporphyrin. When the porphyrin is surrounded by fourth-generation dendrons, a slight rate enhancement was observed with quenching being 33% faster. Absorption and fluorescence spectroscopies of solutions of the porphyrin-core dendrimers also suggest that the dendrimeric surroundings do not interfere electrochemically or photophysically with the porphyrin core. The characteristic wavelengths of absorption and emission of the porphyrin moiety did not change as the dendrimer generation was increased, indicating that the dendritic substituents do not appear to significantly affect the electrochemical and photophysical nature of the metalloporphyrin core.