Photoinduced processes in porphyrin-stoppered [3]-rotaxanes

Energy and electron transfer processes taking place in [3]-rotaxanes containing various photo- and electro-active metal-complex fragments, such as zinc(II)- and gold(III)-porphyrin stoppers, copper(I)- and silver(I)-bisphenanthroline complex moieties, were studied by spectroscopic steady state and time resolved techniques. Energy transfer from the zinc porphyrin excited singlet state to the MLCT excited state of the Cu(I) complex fragments was observed in the [3]-rotaxane [PZnCuCuPZn](2+), with a lifetime of 180 ps. In the rotaxane [PZnCu-PZn](+) energy transfer occurred only from the porphyrin (tau = 490 ps) close to the Cu(I) complex, while the lifetime of the distant zinc porphyrin was unaffected. Quenching of the distant porphyrin was partly restored in [PZnCuAgPZn](2+), while the one close to the Cu(I) complex 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 extended "necklace" spacer.