Intramolecular energy transfer in bis-porphyrins containing diimine chelates of variable geometry as spacers

Free-base porphyrin dimers bridged by either 6,6'-diphenyl-2,2'-bipyridine or 2,9-diphenyl-1,10-phenanthroline spacers were synthesized and characteri zed. Metallation of one of the porphyrins with Zn-II allowed us to study in tramolecular energy-transfer processes between the Zn-II-porphyrin photorec eptor and the free-base porphyrin energy acceptor by time-correlated single -photon counting. The rate of energy transfer was shown to depend strongly on the nature of the bridge, being enhanced by a factor of nearly 6 when co mparing the (cis) phenanthroline bridge with the trans-bipyridine bridge. A detailed analysis of the fluorescence decays suggests the presence of two slowly interconverting conformers for the 6,6'-djphenyl-2,2'-bipyridine spa cer.