Rotaxanes stability

Fu N, Gassensmith JJ, Smith BD (2009) Effect of stopper size on squaraine rotaxane stability. Supramol Chem 21 118-124... 

More recently, Kim et al. synthesized dendritic [n] pseudorotaxane based on the stable charge-transfer complex formation inside cucurbit[8]uril (CB[8j) (Fig. 17) [59]. Reaction of triply branched molecule 47 containing an electron deficient bipyridinium unit on each branch, and three equiv of CB[8] forms branched [4] pseudorotaxane 48 which has been characterized by NMR and ESI mass spectrometry. Addition of three equivalents of electron-rich dihydrox-ynaphthalene 49 produces branched [4]rotaxane 50, which is stabilized by charge-transfer interactions between the bipyridinium unit and dihydroxy-naphthalene inside CB[8]. No dethreading of CB[8] is observed in solution. Reaction of [4] pseudorotaxane 48 with three equiv of triply branched molecule 51 having an electron donor unit on one arm and CB[6] threaded on a diaminobutane unit on each of two remaining arms produced dendritic [ 10] pseudorotaxane 52 which may be considered to be a second generation dendritic pseudorotaxane. 

In this kind of pseudorotaxanes, rotaxanes, and catenanes, the stability of a specific (supra)molecu-lar structure is a result, at least in part, of the CT interaction. In order to cause mechanical move ments, such a CT interaction has to be destroyed. 

Slipping. The slipping method exploits the kinetic stability of the rotaxane. Provided the end groups of the dumbbell are of appropriate size, they are able to reversibly thread through the macrocycle at higher temperatures, but by lowering the temperature the dynamic complex becomes kinetically trapped as a rotaxane. 

The relatively nonpolar squaraine rotaxane 14c was found to interact with cells in a very similar way to the well-known lipophilic dye Nile Red this probe rapidly accumulates at lipophilic sites inside a living cell, such as the endoplasmic reticulum and intracellular lipid droplets [55], The red emission band for probe 14c is quite narrow and permits the acquisition of multicolor images. It displayed high chemical stability and low toxicity. 

With the example of stained E. coli cells, the squaraine rotaxane 15b containing a zinc(II)-dipicolylamine (Zn-DPA) ligand, which is known to selectively associate with the anionic surfaces of bacterial cells, was found to be almost 100 times more photostable as compared to Cy5-Zn-DPA [55]. This can be attributed to stronger cell-surface affinity of 15b, leading to a slower off rate for the probe. The remarkable stability of 15b permits fluorescence imaging experiments that are impossible with probes based on conventional NIR cyanine dyes such as Cy5. Squaraine rotaxanes are likely to be superior substitutes for conventional cyanine dyes for biomedical imaging applications that require NIR fluorescent probes. 

An important feature of the rotaxane in Scheme 9.12 is that the translocation of the macrocycle is also achieved by the reduction of the fullerene to its trianion, which is both effected and observed by cyclic voltammetry. In DMSO, the proximity of the macrocycle to the fullerene stabilized substantially the electrogenerated trianion (A i/2 = 46 mV) through n—n interactions. Surprisingly, in THF where the macrocycle is preferentially positioned on the peptide station, a similar behavior was... 

Besides their topology, rotaxanes and catenanes are also appealing systems for the construction of molecular machines because (i) the mechanical bond allows a large variety of mutual arrangements of the molecular components, while conferring stability to the system, (ii) the interlocked architecture limits the amplitude of the intercomponent motion in the three directions, (iii) the stability of a specific... 

When rotaxanes and catenanes contain redox-active units, electrochemical techniques are a very powerful means of characterization. They provide a fingerprint of these systems giving fundamental information on (i) the spatial organization of the redox sites within the molecular and the supramolecular structure, (ii) the entity of the interactions between such sites, and (iii) the kinetic and thermodynamic stabilities of the reduced/oxidized and charge-separated species. 

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. 

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