Isomeric rotaxanes

This separation was surprisingly easy using Ctg-bonded silica plates and a watenmethanol (70 30 v/v) solvent mixture[10], A similar mixture of isomeric rotaxanes was obtained by the same treatment starting from compound 5+[9,10]. This work afforded the first examples of truly asymmetric CD-based rotaxanes. The positive charge on the... 

Dodziuk, H., Nowinski, K.S. Topological isomerism should rotaxanes, endohedral fullerene complexes and in-out isomers of perhydrogenated fullerenes be considered as such Tetrahedron 54 (1998), 2917-2930. 

For the solvent-dependence of the translational isomerism associated with some catenanes and rotaxanes, see (a) Ashton,... 

The first example of a photoresponsive [2]rotaxane, published in 1997 by Nakashima and co-workers, is one of those cases [61]. Molecular shuttle E/Z-224+ consists of an a-cyclodextrin macrocycle, and a tetracationic thread containing an azobiphenoxy moiety, very closely related to azobenzene, and two bipyridinium stations. The well-known E-Z isomerizations of azobenzenes and the ability of cyclodextrins to bind lipophylic compounds in water are exploited in this system to achieve shuttling. When the azobiphenoxy station is in its trans form, E-224+, the cyclodextrin encapsulates it preferentially over the more hydrophilic bipyridinium station (Scheme 12). 

Fig. 32 Transport of a diiodomethane droplet up a 12° incline on a surface with physisorbed, photoswitchable bistable [2]rotaxanes [228]. (a) Cartoon and photograph of the surface in the ground state. The macrocycle preferentially encircles an unfluorinated recognition site, (b) The same surface in the photostationary state (after UV irradiation). Olefin isomerization in the preferred recognition site causes some macrocycles to shuttle to the fluorinated site, changing the polarophobicity of the surface and moving the droplet several millimeters uphill...

Figure 4.25 Formation of the isomeric redox-active rotaxanes 61a and 61b ...

Photoisomerizations can often occur by several different mechanisms. Systems that isomerize via a controlled mechanism are potential candidates for molecular machines [184]. Energy in the form of light is absorbed and converted to controlled mechanical force on the molecular scale. Examples of a mono-directional rotor [185, 186], a switchable rotor [187], and a molecular shuttle [188] have been demonstrated. These systems are light-controlled, but there are also examples of systems which control molecular motion based on electro- and/or chemical modulation, such as the threading/imthreading of (pseudo)rotaxanes [189-196]. 

It should be mentioned here that very interesting constitutional and translational isomerism is observed in the series of catenanes and rotaxanes which contain phenol derivatives such as macrocycUc phenylene-crown components as well as phenolic polyether chains (see also Lehn s recently published book ). 

Fig. 16A-D. Mechanical switching in rotaxanes. A Rotaxanes may exist in isomeric states by the movement of the ring component between dissymmetric sites on the string component. B A redox- or pH-switchable [2]rotaxane. While the cyclophane complexes the native benzidine site (spectrum, curve a), the reduced or protonated benzidine repels the cyclophane, causing it to move to the dioxybiphenylene site (spectrum, curve b). C An azobenzene-based switchable [2]rotaxane. The cyclodextrin ring complexes the azobenzene site in the trans-state, but it is repelled from the ds-azobenzene. The state of the system is measurable by circular dichroism (plot). D A pH-switchable rotaxane. When the amine on the string component is protonated, it complexes the crown ether ring by hydrogen-bonding interactions (40a). When the amine is deprotonated, however, the ring component moves to the bipyridinium unit, where it is complexed by n donor-acceptor interactions (40b). The plots in B and C are adapted from [67] and [69], respectively, with permission...

Lewis and his co-workers have continued their investigations into DNA hairpins with a variety of stilbene diether linkages. In the present report they have examined systems based on the linkers (3) and (4). The rotaxane formed between the stilbene derivative (5) and a-cyclodextrin (CD) does not undergo ,Z-isomerization. Contrary to this the complex of (6) with the p-CD isomerizes slowly on irradiation. The behaviour of the stilbene (6) is different and irradiation in a-CD brings about isomerization with a quantum yield of 0.06 for the E,Z process. The quantum yield for the reverse isomerization is 0.71. These values are to be compared with 0.17 and 0.79 for the isomerization in solution. The authors suggest that the irradiation of (6) in the CD brings about a movement... 

The (Z)-stilbene (6) does not form a rotaxane with dibenzo-24-crown-8 but, when the stilbene is isomerized (sensitized by benzil) to the E form, a pseudo-rotaxane is formed.Lewis and Crompton have reported that the photoacidity of the stilbene derivatives (7) and (8) is dependent on the position of the hydroxy group on the stilbene, and as a consequence the meta derivative undergoes isomerism.The influence of the nitrogen atoms on the photochemistry of the 1,4-distyrylbenzene analogue (9) and related compounds has been assessed. The photophysics and conformational analysis of the stilbene dimer (10) have been studied. Modelling of the optical properties of the stilbenophane (11) has been reported. 

Fig. 6 A photochemically driven translation isomerism in a [2]rotaxane supramolecular machine according to Ashton et al. The flash photolysis data, together with electrochemistry observations, allowed the authors to obtain a complete electronic picture of the processes governing the switching mechanism in this [2Irotaxane, from both energy and kinetic perspectives.

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