Working the Links, Threading the Needle

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Science  21 Apr 2000:
Vol. 288, Issue 5465, pp. 401
DOI: 10.1126/science.288.5465.401f

Molecules that are physically interlocked, either like links on a chain (catenanes) or a bead on a string (rotaxanes), are potentially useful in electronics or micromechanics, but significant challenges remain in designing their synthetic routes and in interrogating their physical state.

Kawaguchi and Harada have explored the use of charged end groups to hold the “bead” of a rotaxane, a cyclodextrin molecule, onto its “string,” a long alkyl chain. Nuclear magnetic resonance spectra reveal that end groups with multiple positive charges function to slow down the rate at which the string is threaded through the bead as well as to stabilize the assembled structure. In essence, these end groups act as knots.

Balzani et al. have tackled the problem of reading out the state of “translational isomers” of catenated links. In their system, two electron-rich hydroquinone groups are positioned on opposite sides of one ring, whereas in the other ring electron-deficient diazapyrenium and bipyridinium groups are in opposition. Rotation of the second ring with respect to the first determines which group is sandwiched between the hydroquinone moieties. They can drive this rotational isomerization by adding a small molecule that preferentially complexes with the diazapyrenium group, and they show that the two isomers have different colors and redox properties.

Jimenez et al. have synthesized a molecule consisting of an eyelet, a linker, and an aglet, with the two end components each containing one-half of a copper-chelating site. When copper(I) is added, two molecules form a complex, with each aglet threading through the eyelet of the other molecule. The monomer units are oriented antiparallel to each other in the threaded structure. Further modification of the monomer units may lead to more extended threaded chains, which would be capable of extending or contracting in response to an external signal with applications, for example, in sensor technology.—PDS, JU

J. Am. Chem. Soc., in press; J. Am. Chem. Soc.122, 3542 (2000); Angew. Chem. Int. Ed.39, 1295 (2000).

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