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 ∼ 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.

The photophysical and hydrodynamic properties of dendrimers (GnPZn and GnTPPH2) with zinc porphyrin (PZn) and tetraphenylporphyrin (TPP) cores are studied in tetrahydrofuran (THF) and dimethylformamide (DMF). UV−vis absorption spectra of GnPZn exhibit a small red shift of the Soret band upon increasing the generation as a result of interactions between the dendrons and the core. All fluorescence decays obtained from global analysis show a monoexponential profile. The intrinsic viscosity obtained for GnPZn from the hydrodynamic volume (V h) passes through a maximum as a function of generation (G) in agreement with earlier experimental findings and calculations suggesting that the internal density profile of dendrimers decrease monotonically outward from the center of the molecule. Within the investigated range (G = 1−3), GnTPPH2 exhibits an approximately constant intrinsic viscosity due to the linear dependence between the hydrodynamic volume and the molecular weight. The differences observed between GnPZn and GnTPPH2 are correlated to structural differences in their cores. The additional phenyl group of the TPP in GnTPPH2 increases the distance between the branches and the porphyrin moiety compared to GnPZn, resulting in a more flexible structure. The enhanced flexibility allows the terminal groups to sample more conformational space and therefore decreases the volume of the dendrimer as compared to the theoretical fully extended structure where V h ∝ G 3. A comparison of the results obtained from analysis of fluorescence anisotropy decays with previously reported viscometry measurements shows a dependence of the structural collapse on the core size.

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