Effects of Dendrimer Generation on Site Isolation of Core Moieties:  Electrochemical and Fluorescence Quenching Studies with Metalloporphyrin Core Dendrimers

Abstract

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.

Dendrimer Complexes Based on Fine-Controlled Metal Assembling

“…This type also has one limited hybrid structure and conformation similar to that of the ''type A'' dendrimers. 77,78 The dendritic backbones, which can be compared to branches in a tree, are important for characterization of the dendrimer properties such as electrochemical response [79][80][81][82][83][84][85][86] or photo-reactivity, [87][88][89][90][91][92][93][94][95][96][97] although the backbones do not usually play a central role as the functional center. Modification of the backbones by metal ions should result in drastic changes in the entire conformation, in the environment around the core and in the electronic properties of the dendrimers.…”

Macromolecular semi-rigid nanocavities for cooperative recognition of specific large molecular shapes

“…Other types of phenylazomethine dendrimer (RuG4a, ZnG4b and ZnG4c) were newly synthesized as described in the Supplementary Methods. A benzylether (Fréchet type) dendrimer with a zinc porphyrin core was synthesized as described in the literature 57 . All pyridine derivatives used as guest molecules were purchased or synthesized as described in the Supplementary Methods.…”

Polyester Dendrimers

“…Porphyrins were selected as core molecules because of their potential applications in many areas [156][157][158][159]. Reports dealing with porphyrins decorated with dendrimers had previously appeared [112,[160][161][162][163][164][165][166][167][168]. After investigating three different synthetic strategies for this study, it was concluded that a spacer was required to be attached to the porphyrin to increase the hydrolytic stability and allow synthesis of higher generations.…”