Phenylazomethine dendrimers hearing a cobalt porphyrin core act as catalysts for CO2 reduction in the presence of a strong Lewis acid such as lanthanide trifluoromethanesulfonate (Ln(OTf)(3)). We investigated the catalytic activity using electrochemical measurements (cyclic voltammetry) on a glassy carbon electrode in a DMF solution. Dissolving CO2 gas into the solution, the cyclic voltammograms displayed an irreversible increase of the cathodic current. This result suggests the catalytic reduction of CO2. The redox potential (-1.3 Vversus Fc/Fc(+)) at which the catalytic behavior was observed is 1.1 V higher than that catalyzed by cobalt tetraphenylporphyrin (CoTPP). The generation number (n) dependence of the dendrimer catalysts showed the maximum activity at n = 3. A significant decrease of the activity for the largest dendrimer (n = 4) indicates a steric effect, which prevents transmission of the substrate CO2 molecule) and electrons to the catalytic center (cobalt porphyrin core). For more efficient catalysis, a novel open-shell dendrimer having a pocket on one side of the molecule was designed and synthesized. Because the accessibility to the core in the opened shell improved, this dendrimer exhibited the highest catalytic activity. These results suggest that tuning of the local domain around the cobalt porphyrin center would lead to a decisive solution for further activation of the CO2 molecule. (c) 2006 Wiley Periodicals, Inc.