Long-range electron transfer in porphyrin-containing [2]-rotaxanes: Tuning the rate by metal cation coordination

A series of [2]-rotaxanes has been synthesized in which two Zn(II)-porphyrins (ZnP) electron donors were attached as stoppers on the rod. A macrocycle attached to a Au(III)-porphyrin (AuP⁺) acceptor was threaded on the rod. By selective excitation of either porphyrin, we could induce an electron transfer from the ZnP to the AuP⁺ unit that generated the same ZnP^.+-AuP^. charge-transfer state irrespective of which porphyrin was excited. Although the reactants were linked only by mechanical or coordination bonds, electron-transfer rate constants up to 1.2 × 10¹⁰ s^-1 were obtained over a 15-17 Å edge-to-edge distance between the porphyrins. The resulting charge-transfer state had a relatively long lifetime of 10-40 ns and was formed in high yield (>80%) in most cases. By a simple variation of the link between the reactants, viz. a coordination of the phenanthroline units on the rotaxane rod and ring by either Ag⁺ or Cu⁺, we could enhance the electron-transfer rate from the ZnP to the excited ³AuP⁺. We interpret our data in terms of an enhanced super-exchange mechanism with Ag⁺ and a change to a stepwise hopping mechanism with Cu⁺, involving the oxidized Cu(phen)₂²⁺ unit as a real intermediate. When the ZnP unit was excited instead, electron transfer from the excited ¹ZnP to AuP⁺ was not affected, or even slowed, by Ag⁺ or Cu⁺. We discuss this asymmetry in terms of the different orbitals involved in mediating the reaction in an electron-and a hole-transfer mechanism. Our results show the possibility to tune the rates of electron transfer between noncovalently linked reactants by a convenient modification of the link. The different effect of Ag⁺ and Cu⁺ on the rate with ZnP and AuP⁺ excitation shows an additional possibility to control the electron-transfer reactions by selective excitation. We also found that coordination of the Cu⁺ introduced an energy-transfer reaction from ¹ZnP to Cu(phen)₂⁺ (k = 5.1 × 10⁹ s^-1) that proceeded in competition with electron transfer to AuP⁺ and was followed by a quantitative energy transfer to give the ³ZnP state (k = 1.5 × 10⁹ s^-1).