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.

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We report on the synthesis of a new thiol-modified terpyridine ligand 4‘-(5-mercaptopentyl)-2,2‘:6‘,2‘‘-terpyridinyl (tpy − SH) as well as of its complexes with Co, Cr, and Os. We have prepared complexes of the type [M(tpy − SH)2] n + and [M(tpy − SH)L] n + (L = terpyridine, tetrapyridylpyrazine (tppz)). The free ligand as well as the tpy − SH-containing metal complexes adsorb strongly onto gold electrode surfaces, and in the case of the metal complexes, they retain their redox-active responses at potentials very close to those of nonadsorbing analogs in homogeneous solution. In the case of [Co(tpy − SH)(tpy)]2+ adsorbed onto a gold electrode surface, the cyclic voltammetric response exhibits an aging process where it sharpens significantly over long times (days), suggesting a slow reorganization of the adsorbed complex on the electrode surface. Reaction with CoCl2 of a gold electrode whose surface had been previously modified with the tpy − SH ligand ostensibly gives rise to Au/S−tpy−CoCl2. Upon cycling the potential of such a modified electrode over the potential region of +0.40 to −0.40 V, we observe a voltammetric wave at about −0.20 V which initially decreases rapidly with time but much more slowly afterward. Concomitantly, a new reversible voltammetric wave appears at +0.13 V which we ascribe to the formation of the surface-immobilized [Co(tpy − SH)2]2+ complex via a surface chelate formation (Au−S−tpy−Co−tpy−S−Au). The presence of sharp isopotential points suggests a simple transformation between these two surface species. Gold electrodes could also be modified with [Os(tpy − SH)(tpy)]2+ which, as in the case of the analogous cobalt complex, exhibited a redox wave associated with the surface immobilized complex at a potential very similar to that of [Os(tpy)2]2+ in solution. Reaction of a gold electrode surface, previously modified with [Os(tpy − SH)tppz]2+, with [Co(tppz)(Cl)2] gives rise to the appearance of a new voltammetric wave at +0.26 V, which we ascribe to binding of the cobalt to the pendant tppz to give rise to a structured interface of the type Au−S−tpy−Os−tppz−Co−tppz. An electrode modified with a layer of [Cr(tpy − SH)2]3+ exhibits electrocatalytic behavior toward the reduction of nitric oxide (NO) in solution.