Rotaxane unthreading

More recently, Harada et al. applied the complexation process to side-chain systems via Method 6 (Figure 10), in which the guest sites were introduced as pendant groups and thereafter the CD was threaded onto them [104, 105]. Different types of hydrocarbon chain as pendant groups were studied for their compatibility with different CDs. As the cyclic was not blocked, the products can be viewed as side-chain poly(pseudo rotaxane)s of Type 9. Probably because of the rapid exchange process between threaded and unthreaded forms, the isolation of the solid-state polyrotaxane was not reported. 

Fig. 14 Principle of the synthesis of rotaxane 16 +(see Figure 15). (i) Formation of a monostoppered axle, (ii) Threading ofthe axle through the wheel, (iii) Unthreading ofthe macrocycle is prevented by adding a second blocking group.

Light-driven Unthreading Reaction in Rotaxane with a [Ru(diimine)3]2+ Core... 

A special type of ABA block copolymer is formed when dendrimers are used as stoppers on rotaxanes. Rotaxanes are linear polymers onto which one or several large macrocycles are threaded. In order to prevent unthreading, sufficiently bulky groups must be attached to the two ends of the rotaxane. The [G-3] po-ly(benzyl ether) dendrimer proved to be highly suited for trapping [128] (see Scheme 16). The linear polymer is made of stiff bipyridinium groups that spontaneously assemble with diparaphenylene-34-crown-10 to form the rotaxane. The dendrimers are attached via quaternation of pyridine with benzyl bromide... 

Rotaxanes are molecular assemblies in which a linear component is mechanically threaded through one or more cyclic molecules - much like beads on a string -with the ends of the linear component terminated with bulky groups to prevent unthreading of the structure. The concept of a rotaxane (Latin rota, wheel axis, axle) is not new, with the idea of a rotaxane being put forward as early as 1961. Following this, several early syntheses were reported in which the rotaxane was typically obtained in very low yield. ... 

An investigation has shown that the photodecomposition of the [3]rotaxane (180) is slower than the naked dumb-bell this may be of significance for a new approach to insulated molecular wires. The pseudorotaxane formed between (9-anthrylmethyl)methylammonium hexafluorophosphate and dibenzo-24-crown-8 can be unthreaded by addition of NBu4" Cl", and rethreaded by addition of NHBua Pp6". Such modifications cause profound changes in their luminescence spectra, and it has been suggested that such a system has potential use as a sensor for chloride ions. 

The simplest rotaxanes are molecules made from a macrocycle threaded onto a dumbbell-shaped molecule consisting of an axle bearing bulky end groups (or stoppers) to prevent macrocycle unthreading. The name rotaxane was coined in 1967 by Schill and Zollenkopf [3] and supplanted the term hooplane proposed the same year by Harrison and Harrison [4]. As a matter of fact, these authors described quite simultaneously two very different rotaxane syntheses. 

Copper(I)- free [2]rotaxane 12" (PF6 ) was obtained by treatment of 9 " (PF6 )2 with excess KCN (Fig. 6). The fact that demetalation of Cu" -complexed rotaxane 9 " afforded a single product 12 and the absence of release of the bis-porphyrin by macrocycle 3 is proof that the compound synthesized is a true rotaxane. Linking is provided only by steric crowding of the porphyrin moieties, preventing unthreading of the bis-porphyrin from macrocycle 3. 

Necessary Controls Prepare unthreaded 1 1 complex of axle and wheel and repeat experiment under the same conditions examine fragments of free axle and free wheel anions to identify which fragments of rotaxane originate from which component. 

As depicted in Fig. 12, rotaxanes are composed of a dumbbellshaped component, in the form of a rod and two bulky stopper groups, around which there are encircling macrocyclic component s) [310]. The stoppers of the dumbbell prevent the macro-cycle(s) from unthreading from the rod. 

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