Improving the Efficiency of the Photoinduced Charge-Separation Process in a Rhenium(I)–Zinc Porphyrin Dyad by Simple Chemical Functionalization

We demonstrate here that, whereas the rhenium(I) − zinc porphyrin dyad fac-[Re(CO) 3 (bpy)(Zn · 4 ′ MPyP)](CF 3 SO 3 ) [1; 4 ′ MPyP = 5-(4 ′ -pyridyl)-10,15,20-triphenylporphyrin] shows no evidence for photoinduced electron transfer upon excitation in the visible region because the charge-separated state ZnP + − Re − is almost isoenergetic with the singlet excited state of the zinc porphyrin ( Δ G = − 0.05 eV), the introduction of electron-withdrawing ethyl ester groups on the bpy ligand signi fi cantly improves the thermodynamics of the process ( Δ G = − 0.42 eV). As a consequence, in the new dyad fac-[Re(CO) 3 (4,4 ′ -DEC-bpy)(Zn · 4 ′ MPyP)](CF 3 SO 3 ) (4; 4,4 ′ - DEC-bpy = 4,4 ′ -diethoxycarbonyl-2,2 ′ -bipyridine), an e ffi cient and ultrafast intramolecular electron-transfer process occurs from the excited zinc porphyrin to the rhenium unit upon excitation with visible light. Conversely, the introduction of electron-donor tert-butyl groups on the meso-phenyl moieties of the zinc porphyrin has a negligible e ff ect on the photophysics of the system. For dyad 4, the time constants for the charge-separation and charge- recombination processes in solvents of di ff erent polarity (PrCN, DCM, and toluene) were measured by an ultrafast time-resolved absorption technique ( λ exc = 560 nm).