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)](CF3SO 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 (ΔG = -0.05 eV), the introduction of electron-withdrawing ethyl ester groups on the bpy ligand significantly improves the thermodynamics of the process (ΔG = -0.42 eV). As a consequence, in the new dyad fac-[Re(CO)3(4,4′-DEC-bpy)(Zn·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 (λexc = 560 nm). © 2013 American Chemical Society.