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 a nd 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 q uinone rings are in the range of 2.5-4.2 Angstrom. Therefore, it was e xpected that there are two possible intramolecular ET pathways in the compound, i.e., through bond and through space. The ET rates for charg e separation process (k(cs)) were obtained on the basis of fluorescenc e 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 betwe en the redox pair is not used as a stepping stone in ET pathways; in o ther words, ET in the present molecules takes place in a through-bond mechanism. To compete with fast ET with through-bond pathways in coval ently linked donor-acceptor systems where the two ET pathways are poss ible, through-space ET may function under quite limited conditions.