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Nanosized Molecular Switch Controls Liquid-Crystal Colors

pH-Dependent switch could be a prototype for detecting explosives, spoiled foods

by Carmen Drahl
September 2, 2013 | A version of this story appeared in Volume 91, Issue 35

In response to pH, this nanoscale switch changes a liquid crystal’s color from purple to green (micrographs of liquid crystal shown).
A pair of line structures show the two configurations that switch with the addition of acid or base.
In response to pH, this nanoscale switch changes a liquid crystal’s color from purple to green (micrographs of liquid crystal shown).

Chemists have developed enough molecular switches to populate a molecular hardware store. Yet it’s rare for switching on the atomic scale to translate into an effect at the macroscopic level. Now, Xin Su, Ivan Aprahamian, and colleagues at Dartmouth College have developed a molecular switch that can change a liquid crystal’s color in response to pH (Angew. Chem. Int. Ed. 2013, DOI: 10.1002/anie.201305514). The switch is made from two cholesterol molecules connected by a hydrazone linker. In the presence of trifluoroacetic acid, the hydrazone turns like a rotor, changing the relative orientation of the cholesterol units. The orientation changes back in the presence of potassium carbonate base. By doping this switch into a liquid crystal called nematic phase 5, Aprahamian’s team rendered the material chiral and capable of developing a helical architecture that dictates color. They showed that adding acid changes the liquid crystal’s color from purple to green, which can be reversed by adding base. Aprahamian is now studying additional switches to produce other color changes, which could lead to detectors for the basic amines characteristic of spoiled foods. He also aims to reduce salt accumulation on the liquid crystal—a problem that currently gums up the works after just three cycles.


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