Superexchange rotaxanes

Kinetic studies have demonstrated that photo-induced electron transfer between the zinc and the gold porphyrin occurs at a rate of (1.7 ps)-1 in Cu(I)-complexed [2]-rotaxane 102, which is much higher than in the case of the free rotaxane 107 (36 ps)-1.73 The higher photoinduced electron transfer rate in the Cu(I) complex 102 than in the demetallated system 107 was explained also in terms of a superexchange mechanism. 

The studies described in this section were started shortly after the X-ray crystal structure of the RC of Rh. viridis was disclosed [73]. During these years, the role of the so-called accessory bacteriochlorophyll BCh was under debate [79]. In particular, the possibility was considered that it could play the role of a superexchange relay between SP and BPh (see Figure 22). In this respect, the copper(I)-complexed [2]rotaxane Cu.20+ represented a functional artificial model of the SP/BPh/BCh triad, the central Cu complex fragment between the Zn porphyrin donor and the Au porphyrin acceptor mimicking the function of BCh between SP and BCh. However, the kinetic scheme shown in Figure 22a has been revised, being now quite firmly established that (at least at room temperature) BCh is directly involved in the electron transfer reaction the transfer from the electronically excited special pair SP to BCh takes about 3 ps, and the next transfer step to the BPh, 0.65 ps [80]. In the earlier experiments, detection of the intermediate state SP+BCh was prevented by its relatively slow population and fast decay. 

I" ", were interpreted as confirmation of the superexchange mechanism for reaction (1) [54]. This was further supported by the results obtained with 12", the free rotaxane, where photophysical properties very similar to I were evidenced, Scheme 1. The experimental kinetic data collected for these structures (Table 1) allowed us to derive a good correlation between the rates of electron transfer and the reciprocal of the energy gap between the relevant orbitals on the porphyrin excited state and on the phenanthroline ligand, confirming a superexchange mechanism for electron transfer, for reactions (1),... 

Andersson M, Linke M, Chambron JC, Davidsson J, Heitz V, Sauvage JP and Hammarstrom L. Porphyrin-Containing 2-Rotaxanes Metal Coordination Enhanced Superexchange Electron Transfer between Noncovalently Linked Chromophores. J. Am. Chem. Soc. 2000 122 3526-... 

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