Multiporphyrinic Rotaxanes: Control of Intramolecular Electron Transfer Rate by Steering the Mutual Arrangement of the Chromophores

Abstract

A [2]-rotaxane Zn2−Au+ made from a dumbbell component ended by Zn(II) porphyrin stoppers and a ring component incorporating a Au(III) porphyrin has been assembled in 13% yield using the transition metal templating route. 1H NMR studies show that its conformation in solution is very different from those of its complexes with Cu+, Ag+, and Li+. In particular, removal of the templating metal resulted in a changeover of the molecule, the threaded macrocycle undergoing a pirouetting motion placing the Au(III) porphyrin in the cleft formed by the two Zn(II) porphyrin stoppers. At room temperature, the changeover could be either complete or partial, depending on the solvent used. Photoinduced electron transfer from one of the Zn(II) porphyrins to the Au(III) porphyrin of the macrocycle was evidenced in the case of the free rotaxane and its Cu(I) complex, Zn2Cu+Au+. In the former case, the photoinduced electron transfer process could be clearly resolved for an extended conformation that is characterized by the Zn(II) porphyrins pointing far from the Au(III) porphyrin electron acceptor, and accounting for 30% of the total in acetonitrile at room temperature. In both Zn2Cu+Au+ and Zn2−Au+ rotaxanes, the charge-separated state, in which the Zn(II) porphyrin is a cation radical and the Au(III) porphyrin a neutral radical, was generated at a rate of 5 × 109 s-1 and disappeared at a rate of 2 × 108 s-1. In the case of Zn2Cu+Au+, the primary step is very likely energy transfer from the Zn(II) porphyrin singlet excited state to the MLCT state of the central Cu(I) complex, followed by an electron transfer from the excited Cu(I) unit to the Au(III) porphyrin and a successive charge shift from the Zn(II) porphyrin to the oxidized Cu(II) complex. [2]-rotaxane Zn2−Au+, in which no bond pathway can be identified between the donor and the acceptor, is a typical case of electron transfer involving molecular fragments connected by mechanical bonds.