Intramolecular ''through-bond'' and ''through-space'' electron transfer pathways in covalently linked porphyrin-quinone molecules

In order to understand electron transfer (ET) pathways through bonds and through space, a porphyrin-spacer-benzoquinone molecule, where the spacer is spiro[4.4]nonane and a phenyl group is inserted between the space of the redox pair, was prepared. On the basis of MM2 calculation, the edge-to-edge distances between the phenyl and the porphyrin or quinone rings are in the range of 2.5-4.2 Angstrom. Therefore, it was expected that there are two possible intramolecular ET pathways in the compound, i.e., through bond and through space. The ET rates for charge separation process (k(cs)) were obtained on the basis of fluorescence lifetime. The k(cs) value of the above compound is almost identical with that of a reference compound in which no inserted phenyl ring is present. Therefore, it was concluded that the inserted pi-system between the redox pair is not used as a stepping stone in ET pathways; in other words, ET in the present molecules takes place in a through-bond mechanism. To compete with fast ET with through-bond pathways in covalently linked donor-acceptor systems where the two ET pathways are possible, through-space ET may function under quite limited conditions.