Molecular tube

Can you imagine atoms connected together to form a molecule shaped like a minuscule soccer ball How about connections that result in molecular tubes Remarkably, the element carbon can form these molecular shapes. Perhaps even more remarkably, chemists did not discover this until late in the twentieth century. 

Bucky balls (football molecules) were only discovered in 1985 and named fullerene after the architect Buckminster-Fuller. The Nobel Prize for chemistry in 1996 was awarded for this new carbon chemistry. Molecular tubes with this structure have particularly interesting properties. 

The molecular tube was soluble in water, DMF, and DM SO, though the unbridged polyrotaxane was insoluble in the first two. Its H NMR spectra in D20 and DMSO-d6 and its 13C NMR showed the presence of both bridged and unbridged CDs. The H NMR peak was broader for the bridged CD than for the unbridged one, and this was an additional indication that the tube is polymeric. 

Ghadiri, R.M. Cyclic Homodetic Peptides with Repeating d-l Chirality, Employable for Assembling and Disassembling Molecular Tubes, 96-632444 6613875 (2003). 

Such a molecule can be stabilized by a system of delocalized Ji-electrons, which is closed into a toroid of 10 aromatic rings. Reactive sites are four CH groups, which are at the ends of this molecular tube. Such substances belong apparently to a new class of organic compounds, which is intermediate between planar polycyclic aromatic hydrocarbons and three-dimensional fullerenes, nanotubes. Quantum-chemical calculations of the electronic and spatial structure of C32H8 and some other molecules indicate that they have an increased reactivity and semiconductor properties. 

In all the discrete structures described above, the axial sites are either open or bound to a weak donor ligand. A number of extended structures have been made by reacting [ cri-Rh2(DAniF)2 (02CCH2C02)]2 and [ cri-Rh2(DAniF)2 (02CC02)]4 with the polypyridine or polynitrile ligands. The resulting structures include one, two-, and three-dimensional molecular tubes that are described in detail elsewhere. ... 

Figure 4.50. Molecular tube (J.-H. Fuhrhop, P. Blumtritt, C. Lehmann, P. huger, J. Am. Chem. Soc., 113, 7437 (1991))...

The authors prepared a molecular tube by crossUnking adjacent a-CD units in a polyrotaxane (molecular necklace) [126,127] (Scheme 4). The molecular necklace was dissolved in 10% NaOH, to which epichlorohydrin was then added. Epichlorohydrin was allowed to react with the hydroxyl groups of the threaded a-CD to link up each a-CD. 2,4-Dinitrofluorobenzene moieties, bulky stoppers, at both ends were removed by the treatment with a strong base (25% NaOH). The yield of molecular tube was 92% and the average molecular weight was estimated to be ca. 2 x 10" by GPC. The molecular tube could accommodate Is" ions in an aqueous solution of KI-I2 efficiently, whereas a-CD could not. 

Crosslinking of a-CD with epichlorohydrin in aqueous solution under well controlled conditions furnished hyperbranched water-soluble a-CD polymers [96,97]. On the other hand, template directed crosslinking of a-CD, threaded on PEG, gave rise to linear CD polymers with two to three bridges between every two neighboring rings, or so-called molecular tubes, as shown in Fig. 2 [98-101],... 

Fig. 2 Template directed synthesis of molecular tubes (MT) Harada et al.

Harada et al. constructed a molecular tube out of a-cyclodextrin-based polyrotaxane (Scheme 25) [156]. The polyrotaxane was synthesized from a... 

Yui s group analyzed the thermodynamics on the inclusion complexation between the a-cyclodextrin-based nanotube and sodium alkyl sulfonate [148]. They prepared a supramolecular hydrogel utilizing enthalpy-driven complexation between the molecular tube and an amphiphilic molecule [147]. They carried out the thermodynamic analysis of inclusion complexation between a-cyclodextrin-based molecular tube and poly(ethylene oxide)-Wocfc-poly(tetrahydrofuran)-b/oc/c-poly(ethylene oxide) triblock copolymer in terms of isothermal titration calorimetry [157]. Furthermore, they incorporated the tube into gels that could recognize the length of alkyl chain [158]. 

Liu et al. synthesized a bis(molecular tube) from organoselenium-bridged /J-cyclodextrin-Pt(IV) complex and propylene glycol, utilizing Harada s protocol and they characterized the tube by TEM and AFM [159]. 

An insulated molecular wire was prepared by Ito s group from the a-cy-clodextrin-based molecular tube and polyaniline with emeraldine base [164]. They found that the insulated wire was moved and cut off by manipulating with the cantilever tip of AFM [165]. 

Lengthening of the axle can lead to the threading of several CDs to form polypseudorotaxanes which beeome poly-rotaxanes when bulky end groups are attached at either end of the axle. An interesting example of such a polyrotax-ane is shown in Figure 26 where the polyethyleneglycol-a-CD polyrotaxane 100 is reacted with epoxide 101 in 10% NaOH in water to form two or three links between the primary and secondary hydroxyls of the ends of adjacent a-CDs to make a molecular tube in the [2]-rotaxane 102. Treatment with 25% NaOH in water hydrolyzes the dini-trophenyl end groups from the axle to release the molecular tube 103. 

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