Pseudorotaxanes structures

Product 129 containing a cyclopentene-based photochromic fragment was proposed as an axis of pseudorotaxane structures, which has potential for use in the construction of a prototype of molecular machines active as a functional stopper (08DP294). 

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. 

Figure 1. Schematic representation of a pseudorotaxane structure formed from a macrocyclic host and a linear guest.

Figure 7. Self-assembly between a wire-type compound and a macrocyclic ring leading to an inclusion complex not showing the expected pseudorotaxane structure [16],...

As we have seen above, several pseudorotaxane structures are stabilized—at least in part—by CT interactions. In these systems, if the CT interactions are destroyed or weakened, dethreading can occur. This can be done by reduction of an electron-acceptor unit or by oxidation of an electron-donor unit contained in the thread or in the raacrocycle. In most cases, the CT interactions can be restored by an opposite redox process, which thus promotes a reverse mechanical movement leading to the original structure. 

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... 

This example nicely demonstrates how different gas-phase experiments provide more than one piece of evidence. The assignment of pseudorotaxane structure for the 1 1 complex can now firmly be based on (i) the qualitative binding strengths, (ii) size compatibility arguments, and (iii) the kinetic behavior observed for guest exchange reactions. 

DPT calculations at the B3LYP/6-31G(d,p) level of theory confirmed the presence in the 2 cla pseudorotaxane structure of two NH - O hydrogen bonds (Fig. 30.5, left). Analogously, in the H NMR spectrum of the 3a cla pseudo[2] rotaxane structure two sets of shielded and unshielded benzyl signals can be always observed (Fig. 30.4a) corresponding to their exo- or ent/o-cavity disposition. 

The extension of the endo-aSsy rule allowed the formation of the first examples of oriented handcuff pseudorotaxane architectures with a predefined stereochemistry [27]. With respect to the use of a single calixarene, a further synthetic challenge is represented by the use of a double-calixarene like 20 (Fig. 30.13), because of the inherent difficulty in controlling the relative orientation of the wheels within the entire pseudorotaxane system. In fact, the threading of 20 with a bis-ammonium axle (e.g., 14 and 17 ) could give rise again to the three stereoisomeric handcuff pseudorotaxane structures, [27] with the two calix-wheels in three different relative orientations, head-to-head HJd) (I), tail-to-tail (T,T) (II), and head-to-tail (H,T) (III) (Fig. 30.13). 

A close inspection of the H NMR spectrum of the mixture clearly evidenced the absence of signals of the known (7 ,7 )-13 cla2 pseudo[3]rotaxane as weU as that of any heteropseudorotaxane [34]. In contrast, (//,//)-13 Cl02 (Scheme 30.5) was clearly present. In conclusion, from the ten possible pseudorotaxane structures, the / e /- )-/ /homo-pseudo[3]rotaxane was self-sorted as the only... 

It has long been known that crown ethers can form adducts with organic ammonium ions [25]. As mentoned above (Section 2.2.2), it has been found that suitable "wire-type" ammonium ions can thread through crown ethers to give pseudorotaxane structures [16,26a]. Accordingly, we have recently verified that the "wire-type" (9-anthracenyl)benzylammonium ion, AH, and the DB24C8 crown ether give rise to... 

The quenching of the luminescence of the molecular components when they are threaded in the pseudorotaxane structure can be used to estimate their association constant. In fact, the residual luminescence of 1 " and 2 in a solution containing... 

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