Rotaxane porphyrin triads

Figure 13.62. Rotaxane porphyrin triads built around a bis(phenanthroline) complex.

Sandanayaka, A. S. D. et al. Axle length effect on photoinduced electron transfer in triad rotaxane with porphyrin, [60]fullerene, and triphenylarnine. J. Phys. Chem. A 114, 5242-5250, 2010. 

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. 

Undoubtedly, if not a 273 K model of the SP/BCh/BPh triad, the copper(I)-complexed [2]rotaxane Cu.20 remains a most elegant example of control by external complexation of the electron transfer rate between covalently linked donor and acceptor porphyrins. 

Inspired by previous rotaxane assemblies, Ito and Tanaka have reported the synthesis and the properties of triad 130 incorporating two zinc porphyrins and a mechanically attached Ceo electron acceptor. The behavior of the assembly depicted in Figure 13.70 is impressive in terms of lifetime of the CS state. The charge separation quantum yield is efficient (95%), and excitation of the zinc porphyrins leads to an eT process from their first singlet excited state of ZnP to... 

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