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January 21, 2011

Helical molecules that contract reversibly when oxidized pave the way to new single-molecule electrochemical switches

o-Phenylene oligomers can be envisaged as springy chairs. When oxidized (red), the molecule is contracted and less dynamic than its neutral counterparts (white).

New, small spring-like polymer chains, or oligomers, from organic compounds called o-phenylenes have been created by Eisuke Ohta, Takanori Fukushima, Takuzo Aida and colleagues at RIKEN Advanced Science Institute in Wako1. These oligomers consist of benzene rings that connect to each other at a sharp angle, leading to their helical structure. The team’s oligomers can change shape and become more rigid when subjected to an electrochemical signal. They could soon serve as single-molecule machines for application in molecular computers.

The degree of twisting of natural helical structures, such as the DNA double-helix, plays an essential role in many important biological functions. Because of their twisted architecture, artificial helices can facilitate the separation and the synthesis of chiral compounds—asymmetric molecules that cannot be superimposed with their mirror image.



Naure Chemistry - Redox-responsive molecular helices with highly condensed π-clouds

Helices have long attracted the attention of chemists, both for their inherent chiral structure and their potential for applications such as the separation of chiral compounds or the construction of molecular machines. As a result of steric forces, polymeric o-phenylenes adopt a tight helical conformation in which the densely packed phenylene units create a highly condensed π-cloud. Here, we show an oligomeric o-phenylene that undergoes a redox-responsive dynamic motion. In solution, the helices undergo a rapid inversion. During crystallization, however, a chiral symmetry-breaking phenomenon is observed in which each crystal contains only one enantiomeric form. Crystals of both handedness are obtained, but in a non-racemic mixture. Furthermore, in solution, the dynamic motion of the helical oligomer is dramatically suppressed by one-electron oxidation. X-ray crystallography of both the neutral and oxidized forms indicated that a hole, generated upon oxidation, is shared by the repeating o-phenylene units. This enables conformational locking of the helix, and represents a long-lasting chiroptical memory

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