Significance of the connecting position between Zn(ii) porphyrin and Re(i) bipyridine tricarbonyl complex units in dyads for room-temperature phosphorescence and photocatalytic CO2 reduction: unexpected enhancement by triethanolamine in catalytic activity

Abstract

We synthesized three new dyads composed of a Zn porphyrin and fac-Re(bpy)(CO)₃Br (bpy = 2,2′-bipyridine) units, ZnP-nBpyRe^Br (n = 4, 5, and 6), in which the porphyrin is directly connected at the meso-position through the 4-, 5-, or 6-position of the bpy. We investigated the relationships between the connecting positions and the photophysical properties as well as catalytic activity in the CO₂ reduction reaction. The dyad connected through the 6-position, ZnP-6BpyRe^Br, showed obvious phosphorescence with a lifetime of 280 μs at room temperature, in N,N-dimethylacetamide (DMA), whereas the other two dyads showed almost no phosphorescence under the same conditions. The photocatalytic CO₂ reduction reactions in DMA using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the electron donor and the three dyads ZnP-nBpyRe^Br selectively produced CO with similar initial rates, but the durabilities were low. The addition of triethanolamine (TEOA) suppressed the decomposition of dyads, improving their durabilities and reaction efficiencies. In particular, ZnP-5BpyRe^Br was remarkably improved—it gave the highest durability and reaction efficiency among the three dyads; the reaction quantum yield reached 24%. The reason for this significant activity is no accumulation of electrons on the Zn porphyrin in ZnP-5BpyRe^Br, which would be caused by dual interactions of TEOA with the Re and Zn ions in the dyad. As the highest catalytic activity was observed in ZnP-5BpyRe^Br among the three dyads, which had no room-temperature phosphorescence (RTP), the catalytic activities and RTP properties are considered independent, but they are greatly influenced by the connecting positions on the bpy ligand in ZnP-nBpyRe^Br.