Photoexcitation\nand One-Electron Reduction Processes\nof a CO<sub>2</sub> Photoreduction Dyad Catalyst Having a Zinc(II)\nPorphyrin Photosensitizer

We\nhave explored the photophysical properties and one-electron\nreduction process in the dyad photocatalyst for CO<sub>2</sub> photoreduction, <b>ZnP-phen=Re</b>, in which the catalyst of <i>fac</i>-[Re(1,10-phenanthoroline)(CO)<sub>3</sub>Br] is directly connected\nwith the photosensitizer of zinc(II) porphyrin (<b>ZnP</b>),\nusing time-resolved infrared spectroscopy, transient absorption spectroscopy,\nand quantum chemical calculations. We revealed the following photophysical\nproperties: (1) the intersystem crossing occurs with a time constant\nof ∼20 ps, which is much faster than that of a <b>ZnP</b> single unit, and (2) the charge density in the excited singlet and\ntriplet states is mainly localized on <b>ZnP</b>, which means\nthat the excited state is assignable to the π–π*\ntransition in <b>ZnP</b>. The one-electron reduction by 1,3-dimethyl-2-phenyl-2,3-dihydro-1<i>H</i>-benzo[<i>d</i>]imidazole occurs via the triplet\nexcited state with the time constant of ∼100 ns and directly\nfrom the ground state with the time constant of ∼3 μs.\nThe charge in the one-electron reduction species spans <b>ZnP</b> and the phenanthroline ligand, and the dihedral angle between <b>ZnP</b> and the phenanthroline ligand is rotated by ∼24°\nwith respect to that in the ground state, which presumably offers\nan advantage for proceeding to the next CO<sub>2</sub> reduction process.\nThese insights could guide the new design of dyad photocatalysts with\nporphyrin photosensitizers.