Rotaxane structure, donor-acceptor complexes

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. 

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