Multiporphyrinic rotaxanes: Control of intramolecular electron transfer rate by steering the mutual arrangement of the chromophores

A [2]-rotaxane Zn-2-Au+ made from a dumbbell component ended by Zn(II) porp hyrin stoppers and a ring component incorporating a Au(III) porphyrin has b een assembled in 13% yield using the transition metal templating route. H-1 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 th e templating metal resulted in a changeover of the molecule, the threaded m acrocycle undergoing a pirouetting motion placing the Au(III) porphyrin in the deft formed by the two Zn(II) porphyrin stoppers. At room temperature, the changeover could be either complete or partial, depending on the solven t 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 f ree rotaxane and its Cu(I) complex, Zn2Cu+Au+. In the former case, the phot oinduced electron transfer process could be clearly resolved for an extende d 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 Zn-2-Aurotaxanes, the charge-separated state, in which the Zn(II) porphyrin is a c ation radical and the Au(III) porphyrin a neutral radical, was generated at a rate of 5 x 10(9) s(-1) and disappeared at a rate of 2 x 10(8) s(-1). In the case of Zn2Cu+Au+, the primary step is very Likely energy transfer fro m the Zn(II) porphyrin singlet excited state to the MLCT state of the centr al Cu(I) complex, followed by an electron transfer from the excited Cu(I) u nit to the Au(III) porphyrin and a successive charge shift from the Zn(ll) porphyrin to the oxidized Cu(Ll) complex. [2]-rotaxane Zn-2-Au+, in which n o bond pathway can be identified between the donor and the, acceptor, is a typical case of electron transfer involving molecular fragments connected b y mechanical bonds.