Rotaxanes, characteristics

The properties of these rotaxane dendrimers are quite different from those of the individual rotaxanes or dendrimers and often a blend of both. In view of the versatile characteristics that a dendron or dendrimer can manifest, several new properties can be imparted to the rotaxanes. For example, the solubility of rotaxanes in organic solvents as well as in water can be significantly improved when large dendrimer units are appended enhancing the prospects of their use as molecular machines. The dendritic units can also influence the photo/electro-chemical properties of the rotaxanes. Employing photo-receptive dendron units, photo chemically driven molecular machines may be developed, where the dendrons act as antenna for photo-harvesting [62]. 

Stoddart and co-workers have made use of Frechet-type dendrons as dendritic stoppers for self-assembled [n]rotaxanes [65]. The solubility enhancement that results from incorporating dendritic wedges at the termini facilitated the purification of these materials by column chromatography despite the polycationic nature of their bipyridinium backbone. Again, the dendritic wedges did not alter the electrochemical characteristics of the viologen subunits. Flowever, the enhanced solubility resulting from the presence of the dendritic components en-... 

Table 4 Spectral characteristics of squaraines and squaraine rotaxanes in chloroform [56, 58]...

Table 5 Photophysical characteristics of squaraine dyes, squaraine rotaxanes, and IgG conjugates [60]...

Table 7 Spectral characteristics of squaraine 23a and rotaxane [23a C 25 D Na2][2C104] in acetonitrile [63]...

Chiu SH, Rowan SJ, Cantrill SJ, Glink PT, Garrell RL, Stoddart JF (2000) A rotaxane-like complex with controlled-release characteristics. Org Lett 2 3631-3634... 

The cyclic voltammetry behavior of the Cu(II) rotaxane, 4(5)2+ (Fig. 14.8b), is very different from that of 4, t l +. The potential sweep for the measurement was started at - 0.9 V, a potential at which no electron transfer should occur, regardless of the nature of the surrounding of the central Cu(II) center (penta- or tetracoordinate). Curve i shows two cathodic peaks a very small one, located at + 0.53 V, followed by an intense one at —0.13V. Only one anodic peak at 0.59 V appears during the reverse sweep. If a second scan ii follows immediately the first one i, the intensity of the cathodic peak at 0.53 V increases noticeably. The main cathodic peak at —0.15 V is characteristic of pentacoordinate Cu(II). Thus, in 4(5)2+ prepared from the free rotaxane by metalation with Cu(II) ions, the central metal is coordinated to the terdentate terpyridine of the wheel and to the bidentate dpp of the axle. On the other hand, the irreversibility of this peak means that the pentacoordinate Cu(I) species formed in the diffusion layer when sweeping cathodically is transformed very rapidly and in any case before the electrode potential becomes again more anodic than the potential of the pentacoordinate Cu2 + /Cu+ redox system. The irreversible character of the wave at —0.15 V and the appearance of an anodic peak at the value of + 0.53 V indicate that the transient species, formed by reduction of 4(5)2 +, has undergone a complete reorganization, which leads to a tetracoordinate copper rotaxane. The second scan ii, which follows immediately the first one i, confirms this assertion. 

While rotaxanes are composed of wires and rings, catenanes consist of two or more interlocked rings. The word catenan comes from the Latin word catena , which means linked chains. Although the interlocked rings in catenanes are not bonded together by covalent bonds, they cannot be separated from each other. The molecule is stabilized simply by spatial interlocking. This characteristic is different to other supermolecules, where specific interactions play crucial roles when fixing the structures of complexes. 

CT interactions and electron transfer processes play a fundamental role in the chemistry of rotaxanes and catenanes. CT interactions are often responsible for the driving forces that lead to the syntheses of these compounds such interactions live on when the components have been interlocked, and therefore contribute to determine the actual structure of the resulting compound. Because of the presence of CT interactions, the electronic absorption and emission spectra, as well as the electrochemical behavior, of many rotaxanes and catenanes exhibit characteristic features, quite different from those exhibited by the separated components. 

One of the structural features seen in polyrotaxanes is the absence of any covalent binding between cyclic compounds and a linear polymeric chain capped with bulky end-groups at both terminals [1]. It looks like a necklace the cyclic compounds are mechanically locked by the linear polymeric chain. The cyclic compounds in a poly-rotaxane can slide and/or rotate along the axial polymeric chain if the polyrotaxane is soluble in a certain solvent. Furthermore, such mechanical locking between the cyclic compounds and the linear polymeric chain will be opened once one of the terminal bulky end-groups is cleaved by any external conditions. These characteristics are only observable in and specific to polyrotaxanes and are never seen in conventional polymeric architectures (Fig. 1). 

The characteristics of dyeing i-PP with the azo-dye-a-CD-rotaxane are presented in Fig. 26. Not shown in the figure, however, is that the dyed i-PP yarns are both light and wash fast. 

Finally, even though the matter is beyond the aim of the present topic, a short mention is due to pseudorotaxanes, rotaxanes, catenanes and similar compounds (Refs. 299-303 and therein), to which conventional and/or unconventional hydrogen bridges confer peculiar characteristics making them extremely important in supramolecular and nanotechnologies chemistry for constructing molecular machines. 

The occupation of each 7i-donor station in the thread of rotaxane 24+ by the macrocyclic bead gives rise to characteristic charge transfer bands. Therefore,... 

Before going on to discuss molecular electronic machines, it will be useful to describe their structural foundation at a molecular level, namely those based on interlocked molecules. Interlocked molecules can take on a variety of forms, the most common being catenanes, rotaxanes, knots [16], and carceplexes [17]. For the purpose of this review, only catenanes, rotaxanes and their geometrically related complexes - pseudorotaxanes [18] - will be discussed. When conferred with the ability to undergo some mechanical motion as a result of an applied stimulus - be it chemical, electrochemical, or photochemical - these interlocked molecular and interpenetrated supramo-lecular systems often take on the characteristics of molecular machines [19]. 

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