Applications of Rotaxanes

Numerous proposals of other applications are scattered throughout this book. To name but a few, CyD-based optical sensors, nanowires, and biosensors are presented in Sections 10.3 and 10.6 and in Table 14.3, respectively. In this chapter a few applications in food and drinks, in cosmetics and toiletry, in the textile and wrapping industries, and in agrochemistry are shown, while the applications of rotaxanes (discussed in Chapter 12) in molecular devices are briefly discussed at the end. 

For any application of rotaxanes, as. for example, the constuction of machines at a molecular scale, stability is a major issue. Stoppers too small in size permit the wheel to deslip from the axle so that the rotaxane decays into its two free components (Fig. 3). - A region of metastable rotaxanes exists between pseudorotaxanes without stopper groups on one side and stable rotaxanes on the other, where the barrier for the deslipping process is so high that cleavage of a covalent bond in the axle or wheel would be energetically favored. In some cases, the transition from stable rotaxanes to their quickly deslipping... 

An overview of the synthesis, structure, photophysical properties, and applications of squaraine rotaxanes as fluorescent imaging probes and chemosensors is provided in a recent review [67]. Although a variety of squaraine dyes form rotaxanes with the molecular cage 25 or with a tetralactam macrocyclic system introduced by Leigh and co-workers [16, 17], there is no evidence in the literature that conventional cyanine dyes can be embedded in these macrocycles. 

Polyethers, either involving aromatic groups or not, have shown wide application in the formation of this class of rotaxanes. In the case of the former type, the interaction of the electron-rich aromatic groups with the bipyridinium units is the most often used as the means to form the precursor and is demonstrated by rotaxane 95 in which dendritic stoppers are used <1996JA12012>. In the case of the latter type, the interaction of the ether oxygen atoms in the ring... 

The application of this template to the clipping of acyclic dumbbell-shaped molecules to form rotaxanes [14] has received less intensive scrutiny but involves a similar synthetic scheme and conditions. A few representative examples of [2]rot-axanes synthesized in this manner (16-20) are shown in Figure 10.2 [15c, 18-21]. The use of a 1,5-dihydroxynaphthalene core in these cases usually results in higher yields of the desired interlocked products [18,21]. 

So far we discussed the application of the graph theory in studies of electronic structure of molecules. In this chapter we shall describe some topological concepts that appeared in stereochemistry in connection with the investigations of a new remarkable type of chemical compounds, catenanes, rotaxanes, knots. 

There are reviews including two comprehensive articles of Gibson [1] and Stoddart [2] on the polyrotaxanes and polycatenanes [3-12], in addition to a lot of review articles and books on the rotaxanes and catenanes [13-28]. Short reviews on the applications of polyrotaxanes are also reported [29-38]. 

Polymerized [2]rotaxanes were claimed as good chemical vapour sensors [75]. In thin film form they were sensible to phenol vapours as well as to other H-bond donors such as p-nitrophenol or 2,2,2-trifluoroethanol. The observed phenomena was reversible and resulted in fluorescence quenching accompanied by a slight bathochromic shift. It was also found that the polymers were apt to metal bonding due to the presence of the tetrahedral pockets. This fact manifested itself in the appearance of an additional absorption band. The sensitivity of a given polymer thin film was proportional to the film porosity defined by steric properties of the R-substituant. The studies and applications of these molecules are just at the beginning. 

In this chapter we focus our attention on the use of rotaxanes as the framework for the assembly of switchable molecules. This is indeed only one of the many different types of molecules and molecular assemblies that have been proposed in the literature as candidates for the development of novel devices. However, the chemistry of rotaxanes has developed considerably, mostly through the efforts of Professor Stoddart and his coworkers [4]. We will describe some of our own collaborative work with the Stoddart group and venture into some relatively unexplored aspects of the physical chemistry of rotaxanes. We will also address in detail some of the real problems that hamper the practical use of molecular devices in technological applications. Finally, we will discuss the use of metal and semiconductor nanoparticles as a possible new route for the development of new types of molecular devices. 

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