Rotaxane



   

A rotaxane is a macrocycle" (see graphical representation). The name is derived from the Latin for wheel (rota) and axle (axis). The two components of a rotaxane are kinetically trapped since the ends of the dumbbell (often called stoppers) are larger than the internal diameter of the ring and prevent disassociation (unthreading) of the components since this would require significant distortion of the covalent bonds.

Much of the research concerning rotaxanes and other mechanically-interlocked molecular architectures, such as catenanes, has been focused on their efficient synthesis. However, examples of rotaxane 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.

Synthesis

The earliest reported synthesis of a rotaxane in 1967 relied on the statistical probability that if two halves of a dumbbell shaped molecule were reacted in the presence of a macrocycle that some small percentage would connect through the ring.[2] To obtain a reasonable quantity of rotaxane the macrocycle was attached to a hydrophobic forces, covalent bonds, or coulombic interactions. The three most common strategies to synthesize rotaxane are "capping", "clipping", and "slipping",[3] though others do exist.[4][5]

Capping

  Synthesis via the capping method relies strongly upon a thermodynamically driven template effect; that is the "thread" is held within the "macrocycle" by non-covalent interactions. This dynamic complex or pseudorotaxane is then converted to the rotaxane by reacting the ends of the threaded guest with large groups preventing disassociation.

Clipping

The clipping method is similar to the capping reaction except that in this case the dumbbell shaped molecule is complete and is bound to a partial macrocyle. The partial macrocyle then undergoes a ring closing reaction around the dumbbell shaped molecule forming the rotaxane.

Slipping

The method of slipping is one which exploits the kinetic stability of the rotaxane. If the end groups of the dumbbell are an appropriate size it will be able to reversibly thread through the macrocycle at higher temperatures. By cooling the dynamic complex it becomes kinetically trapped as a rotaxane at the lower temperature.

Potential applications

 

Molecular machines

Rotaxane-based molecular machines have been of initial interest for their potential use in macrocycle allows the rotaxane to function as molecular switch with each possible location of the macrocycle corresponding to a different state. These rotaxane machines can be manipulated both by chemical and photochemical inputs.[9] Rotaxane based systems have also been demonstrated as molecular muscles.

Ultrastable dyes

Potential application as long lasting dyes is based on the enhanced stability of the inner portion of the dumbbell shaped molecule.[10][11] Studies with macrocycle which is able to block interactions with other molecules.

Nanorecording

In a nanorecording application[13] a certain rotaxane is deposited as 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.

Nomenclature

Accepted nomenclature is to designate the number of components of the rotaxane in brackets as a prefix.[14] Therefore the graphical representation of rotaxane displayed above would be a [2]rotaxane as it consists of a single dumbbell and a single macrocycle.

See also

References

  1. ^ J. A. Bravo, F. M. Raymo, J. F. Stoddart, A. J. P. White, D. J. Williams (1998). "High Yielding Template-Directed Syntheses of [2]Rotaxanes". Eur. J. Org. Chem. (11): 2565-2571. doi:10.1002/(SICI)1099-0690(199811)1998:11%3C2565::AID-EJOC2565%3E3.0.CO;2-8.
  2. ^ I. T. Harrison and S. Harrison (1967). "Synthesis of a stable complex of a macrocycle and a threaded chain". J. Am. Chem. Soc. 89 (22): 5723-5724. doi:10.1021/ja00998a052.
  3. ^ F. Aricó, J. D. Badjic, S. J. Cantrill, A. H. Flood, K. C.-F. Leung , Y. Liu, and J. F. Stoddart (2005). "Templated Synthesis of Interlocked Molecules". Topics in Current Chemistry 249: 203-259. doi:10.1007/b104330.
  4. ^ I. Yoon, M. Narita, T. Shimizu, and M. Asakawa (2004). "Threading-Followed-by-Shrinking Protocol for the Synthesis of a [2]Rotaxane Incorporating a Pd(II)-Salophen Moiety". J. Am. Chem. Soc. 126 (51): 16740 -16741. doi:10.1021/ja0464490.
  5. ^ N. Kameta, K. Hiratani and Y. Nagawa (2004). "A novel synthesis of chiral rotaxanes via covalent bond formation". Chem. Commun. (51): 466-467. doi:10.1039/b314744d.
  6. ^ C. A. Stanier, M. J. O Connell, H. L. Anderson and W. Clegg (2001). "Synthesis of fluorescent stilbene and tolan rotaxanes by Suzuki coupling". Chem. Commun. (5): 493-494. doi:10.1039/b010015n.
  7. ^ C. A. Schalley, K. Beizai, and F. Vögtle (2001). "On the Way to Rotaxane-Based Molecular Motors: Studies in Molecular Mobility and Topological Chirality". Acc. Chem. Res. 34 (6): 465-476. doi:10.1021/ar000179i.
  8. ^ J. P. Sauvage (1998). "Transition Metal-Containing Rotaxanes and Catenanes in Motion: Toward Molecular Machines and Motors". Acc. Chem. Res. 31 (10): 611-619. doi:10.1021/ar960263r.
  9. ^ V. Serreli, C.-F. Lee, E. R. Kay and D. A. Leigh (2007). "Exercising Demons: A Molecular Information Ratchet". Nature 445: 523-527. doi:doi:10.1038/nature05452.
  10. ^ J. E. H. Buston, J. R. Young and H. L. Anderson (2000). "Rotaxane-encapsulated cyanine dyes: enhanced fluorescence efficiency and photostability". Chem. Commun. (11): 905-906. doi:10.1039/b001812k.
  11. ^ M. R. Craig, M. G. Hutchings, T. D. W. Claridge, H. L. Anderson (1998). "Rotaxane-Encapsulation Enhances the Stability of an Azo Dye, in Solution and when Bonded to Cellulose". Angew. Chem. Int. Ed. 40 (6): 1071-1074. doi:10.1002/1521-3773.
  12. ^ E. Arunkumar, C. C. Forbes, B. C. Noll, and B. D. Smith (2005). "Squaraine-Derived Rotaxanes: Sterically Protected Fluorescent Near-IR Dyes". J. Am. Chem. Soc. 127 (10): 3288-3289. doi:10.1021/ja042404n. free copy
  13. ^ M. Feng, X. Guo, X. Lin, X. He, W. Ji, S. Du, D. Zhang, D. Zhu, and H. Gao (2005). "Stable, Reproducible Nanorecording on Rotaxane Thin Films". J. Am. Chem. Soc. 127 (44): 15338 - 15339. doi:10.1021/ja054836j.
  14. ^ E. S. Wilks Macromolecular Nomenclature Note No. 24[1]
 
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