Rotaxane
A rotaxane (from
Much of the research concerning rotaxanes and other mechanically interlocked molecular architectures, such as
Synthesis
The earliest reported synthesis of a rotaxane in 1967 relied on the
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, for example rotaxinations with cyclodextrin macrocycles involve exploitation of the hydrophobic effect. This dynamic complex or pseudorotaxane is then converted to the rotaxane by reacting the ends of the threaded guest with large groups, preventing disassociation.[8]
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 macrocycle. The partial macrocycle then undergoes a
Slipping
The method of slipping is one which exploits the thermodynamic[10] 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.
Snapping
snapping involves two separate parts of the thread, both containing a bulky group. one part of the thread is then threaded to the macrocycle, forming a semi rotaxane, and end is closed of by the other part of the thread forming the rotaxane.
"Active template" methodology
Leigh and co-workers recently began to explore a strategy in which template ions could also play an active role in promoting the crucial final covalent bond forming reaction that captures the interlocked structure (i.e., the metal has a dual function, acting as a template for entwining the precursors and catalyzing covalent bond formation between the reactants).
Potential applications
Molecular machines
Rotaxane-based molecular machines have been of initial interest for their potential use in
Ultrastable dyes
Potential application as long-lasting dyes is based on the enhanced stability of the inner portion of the dumbbell-shaped molecule.[20][21] 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.[22] The enhanced stability of rotaxane dyes is attributed to the insulating effect of the macrocycle, which is able to block interactions with other molecules.
Nanorecording
In a nanorecording application,
Nomenclature
Accepted nomenclature is to designate the number of components of the rotaxane in brackets as a prefix.[24] Therefore, the a rotaxane consisting of a single dumbbell-shaped axial molecule with a single macrocycle around its shaft is called a [2]rotaxane, and two cyanostar molecules around the central phosphate group of dialkylphosphate is a [3]rotaxane.
See also
- Catenane
- Mechanically interlocked molecular architecture
- Molecular Borromean rings
- Molecular knots
- Polyrotaxane
References
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