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).
