Synthesis and Photocatalytic CO2 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)3Br] fragment into one of the bpy moieties of the cyclic trimer to form the catalytic Re(4,4’‐R2‐bpy)(CO)3Br 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 CO2 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 CO2 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 CO2 reduction. We introduced a [Re(CO)3Br] fragment into the trimer to create a new heterobimetallic photocatalyst. The presence of ester groups in the Zn porphyrin trimer significantly deactivated CO2 reduction. After converting the ester groups to acetoxymethyl groups, the photocatalytic activity was remarkably enhanced. The modified trimer exhibited Re‐induced phosphorescence, which was quenched by an electron donor, indicating efficient electron transfer during CO2 reduction.