Ultrafast photoinduced electron transfer between porphyrinic subunits within a bis(porphyrin)-stoppered rotaxane

Abstract

A rotaxane has been built around a central copper(I) bis(1,10-phenanthroline) complex with gold(III) and zinc(II) porphyrins acting as terminal stoppers. Upon selective excitation of either porphyrin, rapid electron transfer occurs from the zinc porphyrin to the appended gold porphyrin. The copper(I) complex donates an electron to the resultant zinc porphyrin [pi]-radical cation, and the ground-state system is restored by relatively slow electron transfer from the gold porphyrin neutral radical to the copper(II) complex. The rates of the various electron-transfer steps observed with the rotaxane are compared to those occurring in closely related systems, and it is concluded that the copper(I) complex mediates photoinduced electron transfer between the terminal porphyrinic subunits but not the reverse reaction. A qualitative understanding of the electron-transfer rate is presented in terms of a simple orbital energy diagram involving through-bond electron or hole transfer. 39 refs., 3 figs., 2 tabs.