Synthesis and Photocatalytic CO 2 Reduction of a Cyclic Zinc(II) Porphyrin Trimer with an Encapsulated Rhenium(I) Bipyridine Tricarbonyl Complex.

We previously reported a cyclic Zn(II) porphyrin trimer in which three Zn porphyrins are alternately bridged by three 2,2'-bipyridine (bpy) moieties, enabling the encapsulation of metal complexes within the nanopore formed by the Zn porphyrins. In this study, we introduced a [Re(CO) 3 Br] fragment into one of the bpy moieties of the cyclic trimer to form the catalytic Re(4,4'-R 2 -bpy)(CO) 3 Br center (R=methyl ester). The ester groups (R) play an important role in the synthesis of the cyclic structure. However, it was observed that these ester groups significantly deactivated the photocatalytic CO 2 reduction reaction. Therefore, we converted the ester groups with a suitable reducing reagent into hydroxymethyl groups, followed by acetylation to form acetoxymethyl groups. This modification remarkably enhanced the photocatalytic activity of the cyclic trimer=Re complex system for CO 2 reduction. Moreover, in the modified system, the presence of the Re complex induced room-temperature phosphorescence of the Zn porphyrin. The phosphorescence was significantly quenched by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole, indicating that efficient electron transfer mediated by the excited triplet state of the Zn porphyrin occurs during the photocatalytic CO 2 reduction.