Structural and photophysical characterization of multichromophoric pyridylporphyrin-rhenium(I) conjugates

Four porphyrin-Re(I) conjugates, in which a pyridylporphyrin chromophore is directly coordinated to the electron-acceptor fragment [fac-Re(CO)(3)(bipy)](+), were prepared: the dimeric and pentameric compounds [fac-Re(CO)(3)(bipy)(4'MPyP)](CF3SO3) (1) (4'MPyP = 4'-monopyridylporphyrin) and [fac-Re(CO)(3)(bipy)(4)(mu-4'TPyP)]-(CF3SO3)(4) (2) (4'TPyP = 4'-tetrapyridylporphyrin), and the corresponding compounds with 3' rather than 4' porphyrins, [fac-Re(CO)(3)(bipy)(3'MPyP)](CF3SO3) (3) and [fac-Re(CO)(3)(bipy)(4)(mu-3'TPyP)](CF3SO3)(4) (4). These adducts proved to be very stable in solution and were also structurally characterized in the solid state by X-ray crystallography. A detailed photophysical study was performed on the zincated adducts of the conjugates 1-3, labeled 5, 6, and 7, respectively. In all adducts the typical fluorescence of the zinc-porphyrin unit was reduced in intensity and lifetime by the presence of the peripheral rhenium-bipy fragment(s) (heavy-atom effect). For the dyads 5 and 7 the photoinduced charge transfer process from the zinc-porphyrin to the Re(I)-bipy unit is only slightly exoergonic. Ultrafast spectroscopy experiments showed no evidence for electron transfer quenching in the dyads as such, whereas the addition of pyridine (that binds axially to zinc and thus affects the porphyrin redox potential) led to a moderately efficient photoinduced electron transfer process. In perspective, an appropriate functionalization of the bipy ligand and/or of the porphyrin chromophore might improve the thermodynamics and, thus the efficiency, of the photoinduced electron transfer process.