Rotaxane

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Graphical representation of a rotaxane.
Graphical representation of a rotaxane.
Crystal structure of a rotaxane with a cyclobis(paraquat-p-phenylene) macrocycle reported by Stoddart and coworkers in the Eur. J. Org. Chem. 1998, 2565-2571.
Crystal structure of a rotaxane with a cyclobis(paraquat-p-phenylene) macrocycle reported by Stoddart and coworkers in the Eur. J. Org. Chem. 1998, 2565-2571.
Crystal structure of a rotaxane with an α-cyclodextrin macrocycle reported by Anderson and coworkers in Chem. Commun. 2001, 493-494.
Crystal structure of a rotaxane with an α-cyclodextrin macrocycle reported by Anderson and coworkers in Chem. Commun. 2001, 493-494.

A rotaxane is a mechanically-interlocked molecular architecture consisting of a dumbbell-shaped molecule that is threaded through a macrocycle or ring-like molecule. The two components are kinetically trapped as the two end-groups of the dumbbell (often called stoppers) are larger than the internal diameter of the ring, and thus prevent dissociation (unthreading) since this would require significant distortion of the covalent bonds. The name, rotaxane, is derived from the Latin for wheel (rota) and axle (axis).

They are conceptually related to other mechanically-interlocked molecular architectures such as catenanes, molecular knots or borromean ring. The synthesis of such entangled architectures has been made efficient through the combination of supramolecular chemistry with traditional covalent synthesis, however mechanically-interlocked molecular architectures have properties that differ from both “supramolecular assemblies” and “covalently-bonded molecules”. Recently the terminology "mechanical bond" has been coined to describe the connection between the two components of rotaxanes. Although mechanically-interlocked molecular architectures, such as rotaxanes, are an emerging area of research many examples have been found in biological systems including: cystine knots, cyclotides or lasso-peptides such as microcin J25 are protein, and a variety of peptides with rotaxane substructure.

Accepted nomenclature is to designate the number of components of the rotaxane in brackets as a prefix.[1] Therefore the cartoon rotaxane displayed to the right would be a [2]rotaxane as it consists of a single dumbbell and a single macrocycle.

Contents

[edit] Synthesis

Rotaxanes are most commonly constructed by preorganizing or threading the parts utilizing hydrogen bonding, metal coordination, hydrophobic forces, covalent bonds, or coulombic interactions. Examples are crown ethers with a wide variety of structures, cyclodextrins with molecular wires and dyes and a rotaxane based on cucurbituril and hexamethylene diamine. Rotaxanes can also be synthesized by a clipping mechanism.

[edit] Potential applications

Rotaxane-based molecular machines have been of initial interest for their potential use in molecular electronics as logic switching elements.[2][3] These molecular machines are usually based on the movement of macrocycle on the dumbbell. The macrocycle can rotate around the axis of the dumbbell like a wheel and axle or it can slide along its axis from one site to another. Controlling the position of the macrocycle allows the rotaxane to function as molecular switch with each possible location of the macrocycle corresponding to a different state. Such systems have also been demonstrated as molecular muscles.

Potential application as long lasting dyes is based on the enhanced stability of the inner portion of the dumbbell shaped molecule.[4][5] Studies with cyclodextrin protected rotaxane azo dyes established this characteristic. More reactive squaraine dyes have also been shown to have enhanced stability by preventing nucleophilic attack of the inner squaraine moiety.[6] The enhanced stabilities of rotaxane dyes is attributed to the insulating effect of the macrocycle which is able to block interactions with other molecules.

In a nanorecording application[7] a certain rotaxane is deposited as a Langmuir-Blodgett film on ITO coated glass. When a positive voltage is applied with the tip of a scanning tunneling microscope probe, the rotaxane rings in the tip area switch to a different part of the dumbbell and the resulting new conformation makes the molecules stick out from the surface by 0.3 nanometer and this height difference turns out to be sufficient for a memory dot. It is not yet possible to erase such a nanorecording film.

[edit] References

  1. ^ www.polyacs.org
  2. ^ On the Way to Rotaxane-Based Molecular Motors: Studies in Molecular Mobility and Topological Chirality Christoph A. Schalley, Kaweh Beizai, and Fritz Vögtle Acc. Chem. Res.; 2001; 34(6) pp 465 - 476; (Article) DOI: 10.1021/ar000179i Abstract
  3. ^ Transition Metal-Containing Rotaxanes and Catenanes in Motion: Toward Molecular Machines and Motors Jean-Pierre Sauvage Acc. Chem. Res.; 1998; 31(10) pp 611 - 619; (Article) DOI: 10.1021/ar960263r Abstract
  4. ^ Rotaxane-encapsulated cyanine dyes: enhanced fluorescence efficiency and photostability Jonathan E. H. Buston, James R. Young and Harry L. Anderson Chemical Communications, 2000, (11), 905 - 906 Abstract
  5. ^ Rotaxane-Encapsulation Enhances the Stability of an Azo Dye, in Solution and when Bonded to Cellulose Michael R. Craig, Michael G. Hutchings, Tim D. W. Claridge, Harry L. Anderson Angewandte Chemie International Edition Volume 40, Issue 6 , Pages 1071 - 1074 2001 [1]
  6. ^ www.nd.edu
  7. ^ Stable, Reproducible Nanorecording on Rotaxane Thin Films Min Feng, Xuefeng Guo, Xiao Lin, Xiaobo He, Wei Ji, Shixuan Du, Deqing Zhang, Daoben Zhu, and Hongjun Gao J. Am. Chem. Soc.; 2005; 127(44) pp 15338 - 15339; (Communication) DOI: 10.1021/ja054836j Abstract Detailed molecular model

[edit] External links

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