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 x 10(10) s(-1) were obtained over a 15-17 Angstrom 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 (AuP)-Au-3. We interpret our data in terms of an enhanced superexchange mechanism with Ag+ and a change to a stepwise hopping mechanism with Cu+, involving the oxidized Cu(phen)(2)(2+) unit as a real intermediate, When the ZnP unit was excited instead, electron transfer from the excited (ZnP)-Zn-1 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 (ZnP)-Zn-1 to Cu(phen)(2)(+) (k = 5.1 x 10(9) s(-1)) that proceeded in competition with electron transfer to AuP+ and was followed by a quantitative energy transfer to give the (ZnP)-Zn-3 state (k = 1.5 x 10(9) s(-1)).