Distance dependence of photoinduced electron transfer in metalloporphyrin dimers

To study the dynamics and mechanism of intramolecular photoinduced electron transfer (PET) reactions, a series of (Zn-II-Fe-III), meso-tetraarylmetall oporphyrin dimers were synthesized and the kinetics of their PET reactivity was measured. Molecular building blocks were prepared by selective nucleop hilic aromatic substitution of a para fluorine on tetraarylporphyrins conta ining a single pentafluorophenyl group. This synthetic approach allows a wi de variety of systematic modifications such as type and length of spacer, m etal center, and redox-potential difference between donor and acceptor. The edge-to-edge distance between the two porphyrins varies from 14.4 to 27.3 Angstrom. Into a symmetric dimer, with two identical porphyrins covalently linked by a rigid partly saturated bridge, one zinc(II) and one iron(III) c an be inserted. From measurements of fluorescence lifetimes the rate consta nts for PET from the electronically excited state of the zinc porphyrin to the bis(imidazole)iron porphyrin cation were evaluated. The electron-transf er rate decreases by a factor of only 165 when the distance increases by 13 Angstrom. This small decrease is indicative of a surprisingly weak attenua tion of the electronic coupling with distance.