Photoexcitation and One-Electron Reduction Processes of a CO2 Photoreduction Dyad Catalyst Having a Zinc(II) Porphyrin Photosensitizer

We have explored the photophysical properties and one electron reduction process in the dyad photocatalyst for CO2 photoreduction, ZnP-phen=Re, in which the catalyst of fac-[Re(1,10-phenanthoroline)(CO)3Br] (phen=Re) is directly connected with the photosensitizer of zinc (II) porphyrin (ZnP), using time-resolved infrared spectroscopy, transient absorption spectroscopy, and quantum chemical calculations. We revealed the photophysical properties that (1) the intersystem crossing occurs with a time constant of ~20 ps, which is more than 50 times faster than that of zinc (II) porphyrin, and (2) the charge density in the excited singlet and triplet states is mainly localized on ZnP, which means the excited state is assignable to the π -π* transition in ZnP. The one electron reduction using the reductant, 1,3-dimethyl-2-,3-dihydro-1H-benzo[d]imidazole (BIH), occurs via the triplet excited state with time constant of ~170 ns and directly from the ground state by the deprotonated BIH with the time constant of ~3 μs. The charge in the one electron reduction species spans ZnP and the phenanthroline ligand and the dihedral angle between ZnP and the phenanthroline ligand is rotated by ~24° with respect to that in the ground state, which presumably offers an advantage for proceeding to the next CO2 reduction reaction step. These findings on the initial processes of CO2 photoreduction would help us to design novel dyad photocatalysts using porphyrin photosensitizers.