Photoinduced processes in porphyrin-stoppered [3]-rotaxanes

Energy and electron transfer processes taking place in [3]-rotaxanes contai ning various photo- and electro-active metal-complex fragments, such as zin c(II)- and gold(III)-porphyrin stoppers, copper(I)- and silver(I)-bisphenan throline complex moieties, were studied by spectroscopic steady state and t ime resolved techniques. Energy transfer from the zinc porphyrin excited si nglet state to the MLCT excited state of the Cu(I) complex fragments was ob served in the [3]-rotaxane [PZnCuCuPZn](2+), with a lifetime of 180 ps. In the rotaxane [PZnCu-PZn](+) energy transfer occurred only from the porphyri n (tau = 490 ps) close to the Cu(I) complex, while the lifetime of the dist ant zinc porphyrin was unaffected. Quenching of the distant porphyrin was p artly restored in [PZnCuAgPZn](2+), while the one close to the Cu(I) comple x displayed a luminescence lifetime of 300 ps. In the case of the free [3]- rotaxane [PZn--PAu](+), the data suggest that a very slow electron transfer process takes place between the porphyrin stoppers probably through the ex tended "necklace" spacer.