Electron Transfer Processes in Rotaxanes and Catenanes

Electron transfer processes in rotaxanes and catenanes 01MI44. 

Electron transfer processes in metal-assisted catenanes, rotaxanes, and knots 01MI45. 

Research on rotaxanes and catenanes has developed exponentially during the past few years [1-10]. Before discussing the electron transfer processes taking place in rotaxanes and catenanes, we will briefly recall the synthetic approaches to these fascinating compounds. The general way to indicate a rotaxane or a catenane is [n]rotaxane and [n]catenane, where n is the number of interlocked components. 

CT interactions and electron transfer processes play a fundamental role in the chemistry of rotaxanes and catenanes. CT interactions are often responsible for the driving forces that lead to the syntheses of these compounds such interactions live on when the components have been interlocked, and therefore contribute to determine the actual structure of the resulting compound. Because of the presence of CT interactions, the electronic absorption and emission spectra, as well as the electrochemical behavior, of many rotaxanes and catenanes exhibit characteristic features, quite different from those exhibited by the separated components. 

For space reasons, in this chapter we will only review recent advances in the field of molecular-level machines operating by means of photoinduced electron-transfer processes. Since such machines are based on pseudorotax-anes, rotaxanes, and catenanes, we will first recall some important features of these kinds of supramolecular systems. 

For the sake of space, in this chapter we will only discuss examples of molecular-level machines based on photoinduced electron transfer processes. An extensive review on artificial molecular machines [3c] and more detailed discussions on electron-transfer processes involving pseudorotaxanes [23a], and rotaxanes and catenanes [23b] are reported elsewhere. 

Abstract The approach based on the copper(I)-templated synthesis of porphyrin catenanes and rotaxanes developed by the authors group is here reviewed. Zn(II) porphyrins and gold(III) porphyrins were chosen as electron donors and electron acceptors, respectively, to constitute the electro- and photoactive parts of the present systems. The processes—energy and electron transfer reactions—occiuring in the interlocked structures upon light absorption in the presence or absence of Cu(I) are presented, their rates and efficiencies critically compared and discussed with respect to properties of the components and of the ensemble. A detailed examination of differences and analogies in photoreactivity between the present and closely related systems reported by others is presented. 

Detailed electron- and energy-transfer studies have been carried out, on the copper(I) complexes, on the demetalated species and, in some cases, on complexes obtained by exchanging the copper center for another cationic metal (Li, Ag" "). The photophysical properties have been studied in different solvents, however, being solvents of similar polarity, the results for different systems can be considered comparable. Time-resolved measurements allowed us to analyze in detail several electron and energy transfer processes occurring in cascade in some of the highly multicomponent catenanes and rotaxanes of the present article. In addition, it has been possible to identify very distinct conformers in solution, some of them containing two chromophores located close to one another and others with remote chromophores. 

An intriguing series of electro- and photoactive porphyrin catenanes and rotaxanes, obtained by the authors through copper(I)-templated synthesis, is reviewed in the sixth chapter by Lucia Flamigni, Valerie Heitz, and Jean-Pierre Sauvage. The photo-induced processes - energy and electron transfer reactions - occurring in the interlocked structures upon light absorption are discussed in detail and critically compared to closely related systems reported by others. 

A classic example of the formation of a macrocycle by a neutral template is that of the versatile host compound and component of molecular machines, the so-called blue box, or cyclobis paraquat-para-phenylene. Reaction of the horseshoe precursor with dibromo-para-xylene leads to the formation of a tricationic intermediate that is capable of binding the template molecule (Scheme 3), which closes the macrocycle to form the tetracationic cyclophane. The jT-ir interactions of the charge-transfer variety (the complex of the product and template is colored, whereas the components are not) assisted by the charge on ihe product are a major driving force in the process, as revealed in X-ray structures of complexes. It should be noted that the interaction is of the jr-n type assisted by the complementary positive charge on the bipyridinium residues and r-electron-rich nature of the template. This supramolecu-lar synthon can be used for other cyclophanes, catenanes, and rotaxanes (see Self-Assembly of Macromolecular Threaded Systems, Self-Assembled Links Catenanes, and Rotaxanes—Self-Assembled Links, Self-Processes). 

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