To study the dynamics and mechanism of intramolecular photoinduced electron transfer (PET) reactions, a series of (Zn II -Fe III ) meso-tetraarylmetalloporphyrin dimers were synthesized and the kinetics of their PET reactivity was measured. Molecular building blocks were prepared by selective nucleophilic aromatic substitution of a para fluorine on tetraarylporphyrins containing a single pentafluorophenyl group. This synthetic approach allows a wide variety of systematic modifications such as type and length of spacer, metal 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 Å. Into a symmetric dimer, with two identical porphyrins covalently linked by a rigid partly saturated bridge, one zinc(II) and one iron(III) can be inserted. From measurements of fluorescence lifetimes the rate constants for PET from the electronically excited state of the zinc porphyrin to the bis(imidazole)iron porphyrin cation were evaluated. The electron-transfer rate decreases by a factor of only 165 when the distance increases by 13 Å. This small decrease is indicative of a surprisingly weak attenuation of the electronic coupling with distance.

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