Intramolecular "Through-Bond" and "Through-Space" Electron Transfer Pathways in Covalently Linked Porphyrin-Quinone Molecules

Article abstract of DOI:10.1246/bcsj.69.1329

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 A. 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 π-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.