Chemistry

Switching Hands

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Science  24 Mar 2000:
Vol. 287, Issue 5461, pp. 2117
DOI: 10.1126/science.287.5461.2117c

Some synthetic polymers can be switched reversibly between right-handed and left-handed helical conformations. Such two-state switchable materials have potential applications in data storage, transmission, and display devices, but the desired helical inversion behavior remains rare.

Koe et al. have synthesized poly(diarylsilylene) copolymers—of interest because of their electroluminescence properties—with enantiopure chiral side chains. By modifying the composition of the copolymers, they obtained a compound with a thermally driven right-left transition in solution at 10°C. Li et al. have been able to regulate the helical sense of polyisocyanates, polymers which usually form populations containing equal proportions of left-handed and right-handed helical backbones. They attached small amounts of a racemic mix of optically switchable chiral ligands to the polymers. Irradiation with circularly polarized light then generated a small enantiomeric excess in the chiral ligands. This slight excess was amplified by the polymer, and the population developed an overwhelming preponderance of one-handedness. Reversible switching could be achieved by flipping the polarity of the light.

The degree of helicity in biopolymers that naturally form helices can be modified by changing external conditions, such as the solvent and the temperature, or by aggregation. Higashi et al. have grafted oligopeptide units onto the surface of a dendrimer, a hyperbranched macromolecule with a high density of surface functional groups. Alignment of the peptide groups onto the surface of the dendrimer leads to enhanced helicity.—JU

Chem. Commun.2000, 389 (2000); J. Am. Chem. Soc., in press; Chem. Commun.2000, 361 (2000).

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