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 macrocyclic ring, incorporating a 2,9-diphenyl-1,10-phenanthroline residue, threaded onto a second 2,9-diphenyl-1,10-phenanthroline residue with gold-(III) and zinc(II) porphyrins acting as terminal stoppers. The two chelating groups may be coordinated to copper(I) or zinc(II) cations 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 show a marked dependence on the molecular architecture and may be understood in terms of a frontier molecular orbital energy diagram involving through-bond electron or hole transfer. The coordinating cation modulates the energy of orbitals on the spacer and, thereby, affects the rate of electron transfer. The central metal complex may also be involved as a ''real'' intermediate in the electron-transfer pathway. Compared to the corresponding bis-porphyrin, rates of electron transfer at zero activation free energy change and the magnitude of electronic coupling between the porphyrins are significantly lower in the rotaxanes. This may be a consequence of subtle changes in the stereochemistry.