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 center dot 4'MPyP)] (CF3SO3) [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 (Delta G = -0.05 eV), the introduction of electron-withdrawing ethyl ester groups on the bpy ligand significantly improves the thermodynamics of the process (Delta G = -0.42 eV). As a consequence, in the new dyad fac-[Re(CO)(3)(4,4'-DEC-bpy)(Zn center dot 4'MPyP)] (CF3SO3) (4; 4,4'-DEC-bpy = 4,4'-diethoxycarbonyl-2,2'-bipyridine), an efficient 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 effect on the photophysics of the system. For dyad 4, the time constants for the charge-separation and charge-recombination processes in solvents of different polarity (PrCN, DCM, and toluene) were measured by an ultrafast time-resolved absorption technique (lambda(exc) = 560 nm).