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 1.2x10(10) x s(-1) were obtained over a 15-17 A 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 superexchange 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 = 5.1x10(9) x 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 = 1.5x10(9) x s(-1)).
Andersson
Mikael
M
Department of Physical Chemistry, Uppsala University, Box 532, S-751 21 Uppsala, Sweden.
Linke
Myriam
M
Chambron
Jean-Claude
JC
Davidsson
Jan
J
Heitz
Valérie
V
Hammarström
Leif
L
Sauvage
Jean-Pierre
JP
D002412:Cations; D004583:Electrons; D006046:Gold / Q000737:chemistry*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008665:Metalloporphyrins / Q000138:chemical synthesis / Q000737:chemistry*; D008968:Molecular Conformation; D018716:Molecular Mimicry; D010777:Photochemistry; D043862:Rotaxanes; D013329:Structure-Activity Relationship; D015032:Zinc / Q000737:chemistry*
D002412:Cations; D008665:Metalloporphyrins; D043862:Rotaxanes; D006046:Gold; D015032:Zinc