EFFECT OF THE SPACER MOIETY ON THE RATES OF ELECTRON-TRANSFER WITHIN BIS-PORPHYRIN-STOPPERED ROTAXANES

A set of rotaxanes has been constructed consisting of a 30-membered ma crocyclic ring, incorporating a 2,9-diphenyl-1,10-phenanthroline resid ue, threaded onto a second 2,9-diphenyl-1,10-phenanthroline residue wi th gold-(III) and zinc(II) porphyrins acting as terminal stoppers. The two chelating groups may be coordinated to copper(I) or zinc(II) cati ons or left free. Upon selective excitation of either porphyrin, rapid electron transfer occurs from the zinc porphyrin to the appended gold porphyrin and the ground state is restored by relatively slow reverse electron transfer. The rates of the various electron-transfer steps s how a marked dependence on the molecular architecture and may be under stood in terms of a frontier molecular orbital energy diagram involvin g through-bond electron or hole transfer. The coordinating cation modu lates the energy of orbitals on the spacer and, thereby, affects the r ate of electron transfer. The central metal complex may also be involv ed as a ''real'' intermediate in the electron-transfer pathway. Compar ed to the corresponding bis-porphyrin, rates of electron transfer at z ero activation free energy change and the magnitude of electronic coup ling between the porphyrins are significantly lower in the rotaxanes. This may be a consequence of subtle changes in the stereochemistry.