Electron Transfer from the Porphyrin S2 State in a Zinc Porphyrin-Rhenium Bipyridyl Dyad having Carbon Dioxide Reduction Activity

fac-Re(bpy)(CO)(3)Cl (bpy = 2,2'-bipyridine) and similar complexes are known to act as photocatalysts for the reduction of CO2 to CO in the presence of electron donors. However, these complexes do not have significant absorption in the visible region. In a separate publication, we have reported the successful combination of fac-Re(bpy)(CO)(4)Cl with a zinc porphyrin that has strong absorption in the visible region to produce the ZnTMP-Re(bpy)(NHAc) dyad(5-{4-[N-4'-(rhenium(I)tricarbonylchloride-4-acetylamino-2,2'-bipyridyl)-carbamoyl]phenyl}-10,15,20-trimesityl-porphyrinatozinc(II)). In that paper we reported the photochemical reduction of CO2 using excitation of the porphyrin upper excited singlet state, S-2 state (visible region), in the dyad. The very wide visible light region is able to be utilized by the sensitizer. In the present work, as regards the detailed molecular mechanism, we have demonstrated the direct electron transfer from the Zn porphyrin S-2 state to the Re complex in the dyad by ultrafast transient spectroscopy. The fluorescence of the dyad was quenched by 67% compared to that of ZnTMP(COOMe) (5,10,15-trimesityl-20-[4-(methoxycarbonyl)phenyl]porphyrinatozinc) in DMF, measured from the steady-state fluorescence spectrum. The lifetime of the ZnTMP-Re(bpy)(NHAc) S-2 state was revealed to be 0.53 ps by time-resolved fluorescence and 0.56 ps by transient absorption decay, while that of ZnTMP(COOMe) was 1.6 ps. The porphyrin cation radical was actually identified in the time-resolved absorption spectrum. These are concrete evidence of electron transfer from the porphyrin S-2 state to the Re complex. The lifetime of the charge-separated state for ZnTMP-Re(bpy)(NHAc) was also esfimated to be 90 +/- 10 ps from the time-resolved absorption decay.