Electron Transfer Processes in Pseudorotaxanes

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. 

Pseudorotaxanes may be involved in electron transfer processes from three different viewpoints (i) the recognition process between the thread and the macrocycle may result from a charge-transfer interaction, which implies the appearance of characteristic spectroscopic and electrochemical properties (ii) the pseudorotaxane structure can be dethreaded/rethreaded by chemically, electrochemically, and pho-tochemically induced electron transfer processes, which leads to the concept of molecular machines and (iii) dethreading/rethreading of pseudorotaxanes can control the occurrence of charge transfer and electron transfer processes, which offers a route to information processing at the molecular level. 

As we have seen above, CT interactions introduce low-energy excited states responsible for the absorption bands in the visible region (Figure 11). Light excitation in these CT absorption bands formally leads to the transfer of an electron from the donor to the acceptor component (optical electron transfer). As a consequence— particularly when this process leads to formation of charges of the same sign in the two components—one can expect destabilization of the pseudorotaxane structure followed by dethreading. In practice, however, this simple approach does not work because the back electron transfer process is much faster than the separation of the molecular components—a process which requires extended nuclear motions and... 

In the previous section we have described pseudorotaxane systems in which electron transfer inputs govern dethreading/threading processes, opening the way to the control of nuclear movements (molecular machines). In this section, we will see that, in their turn, nuclear movements induced by an appropriate stimulation (e.g., an acid/base reaction) can govern the occurrence of electron transfer processes or CT interactions. This aspect of pseudorotaxane chemistry can be exploited for the construction of electronic devices for information processing at the molecular level. 

Figure 33. Supramolecular system that mimics the function played by a macroscopic extension [140]. The two pseudorotaxane-type connections between the three molecular components can be controlled independently by acid/base and red/ox stimulation, respectively. In the fully assembled [3]pseudorotaxane, a photoinduced electron transfer process occurs from the excited state of the [RLiibpy) moiety of 35 to the bipyridinium unit of 36H +, which, in turn, is plugged into crown ether 2.

Fig. 8. Processes that can occur upon light excitation of a [2]pseudorotaxane in its CT band. Although the original structure is destabilized, dethreading does not occur because it requires nuclear motions that are slower than the back electron-transfer process [25]...

To really achieve photoinduced dethreading, a different approach has been devised [98, 99], based on the use of an external electron transfer photosensitizer (P) and a sacrificial reductant (Red), as illustrated in Figure 22. The photosensitizer must be able to (i) absorb light efficiently and (ii) have a sufficiently long-lived and reductant excited state, so that its excitation (process 1) in the presence of the pseudorotaxane will lead (process 2) to the transfer of an electron to a bipyridinium unit of the cyclophane. The relatively fast back electron transfer from the reduced cyclophane component to the oxidized photosensitizer is prevented by the sacrificial reductant, which, if... 

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