Rotaxanes defined

In this review, we tried to cover all the supramolecular species that maybe classified as rotaxane dendrimers. We classified them by their structures - where in dendrimer rotaxane-hke features are introduced. Several different types of macrocycles have been employed as a ring component in the templated synthesis of rotaxane dendrimers. While the synthesis of Type I and II rotaxanes dendrimers is relatively straightforward, that of well-defined Type III rotaxane dendrimers, particularly those of second and higher generations, is still challenging. 

Fullerene stoppers have also been introduced in rotaxanes as a way to probe the motion of the ring thanks to their well-defined photophysical and electrochemical properties.54 In the rotaxane shown in Scheme 9.12, the hydrogen bonding station (a glycylglycine template) was placed far away from the fullerene by a triethylene glycol spacer.55... 

Numerous examples of catenanes 1, rotaxanes 2, and trefoil knots 3 (Scheme 1) have been previously reported in the literature and are still attracting considerable attention (see Chapters 4 and 6-8) [1-5]. These aesthetically appealing molecules have in common that the topological bonds occurring in catenanes 1 and trefoil knots 2 and the mechanical bonds connecting the component parts of rotaxanes 3 are defined at a molecular scale without ambiguity [1, 2, 4]. 

Different types of polyrotaxanes, depending on how the cyclic and the linear units are connected, have been conceived [6-8, 12], According to the location of the rotaxane unit, polyrotaxanes can be defined as main-chain systems, Types 4, 5, 6, 7, and 8 (rows one and two in Table 1), and side-chain systems, Types 9, 10, 11, and 12 (rows three and four in Table 1). In main-chain polyrotaxanes the rotaxane unit is part of the main chain. In side-chain polyrotaxanes, the rotaxane moiety is located in the side chain as a pendant group. Polyrotaxanes can also be classified as polypseudorotaxanes and true polyrotaxanes, depending on their thermal stability toward dethreading. Polypseudorotaxanes are those without BG (column one in Table 1), in which the rotaxane components can be disassociated from each other by external forces. True polyrotaxanes are those with BG at the chain ends or as in-chain units (column two in Table 1), in which the rotaxane units are thermally stable unless one or more covalent bonds is/are broken. 

Therefore, polyrotaxanes can be simply defined as polymeric materials containing rotaxane units. They are different from conventional linear homopolymers because they always consist of two components, a cyclic species mechanically attached to a linear species. They also differ from polymer blends as the individual species are interlocked together and from block copolymers since the two components are noncovalendy connected. Thus new phase behavior, mechanical properties, molecular shapes and sizes, and different solution properties are expected for polyrotaxanes. Their ultimate properties depend on the chemical compositions of the two components, their interaction and compatibility. This review is designed to summarize the syntheses of these novel polymers and their properties. 

In addition to the types of structure and chemical compositions, the properties of a polyrotaxane are determined by the amount of cyclic incorporated. To define such quantities, the min value was introduced [7, 12], Min, file threading efficiency, was defined for systems of Types 4-6, 9, and 10 as the average number of cyclic molecules per repeat unit [7, 12]. However, this definition seems a little awkward for polyrotaxanes of Types 7, 8, 11, and 12, because in these polyrotaxanes the linear component penetrates through the polymeric cyclic instead of the cyclic being threaded on to the linear species. To fit all the types in Table 1, we redefine min as the proportion of rotaxane repeat units in the polymer. 

Any artificial rotaxane-based mimic of the natural ATP synthase motor must bear a chiral element that helps to define the direction of rotation. Chirality can, of course, be implemented in such molecules by adding chiral groups as the stoppers or the wheel. Rotaxanes with cyclodextrins as the wheels have been described [11], and rotaxanes with glucose-containing stoppers are known [14]. Rotaxanes with elements of planar chirality have also been realized [15]. 

The simplest rotaxane morphology, as defined in the Introduction and represented in Figure 2.1, is noted as [2]-rotaxane to express that it is made up with two components a ring and a dumbbell.13 More generally, an [n]-rotaxane contains n — 1 rings threaded onto a dumbbell component. Figure 2.4 shows several relatively simple rotaxane morphologies that have been realized. 

Threading of two cyclodextrins onto a symmetrical dumbbell can occur head-to-head, head-to-tail, or tail-to-tail, defining a new class of diastereoisomeric [3]-rotaxanes, as shown schematically in Figure 2.12d. Anderson and co-workers have shown that end-capping of 4,4,-bis(diazonio)azobenzene chloride 43 with 2,6-dimethylphenol 44 in the presence of a-CDX produced the [3]-rotaxane 45 in 12% yield, in addition to the [2]-rotaxane 46 in 9% yield, and free dumbbell 47 in trace amounts (Figure 2.18).42 The stereochemistry of the [3]-rotaxane species is remarkable because the two cyclodextrin beads have their smallest rims facing to each other. Therefore the threading reaction was stereoselective. The reasons for the exclusive formation of the tail-to-tail stereoisomer are not clearly established. 

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. 

Until now, we considered the simplest rotaxane morphology, as defined in the introduction and represented in Figure 1. This molecular system is noted as [2]-... 

From a supramolecular perspective, cyclodextrins offer a preformed cavity of defined size within a water-soluble macrocycle. The interaction between guest molecules, or substituents to the cyclodextrin itself, leads to some useful results. Hydrophobic fluorescent substituents will be self-included by the macrocycle in aqueous solution until they are displaced by guests with even greater affinities [6]. The act of displacement will generate a fluorescent response that lends itself to sensor applications. It is also possible to use the hydrophobic core to induce rotaxane formation. This approach has been used by many groups, for example, Liu has recently used it to prepare water-soluble gold-polypseudorotaxanes that capture fullerenes such as C60 [7]. 

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