The low cost and low power requirements of electronic paper displays promise to make them suitable for applications from variable price tags to wearable health monitors. But for the technology to fully realize this potential, the electronic ink employed must be completely reversible, must offer high contrast and must be available in three colors.Progress in contrast and reversibility has been steady. And now scientists have proposed a compact, three-color electronic ink for use in electronic paper displays.Researchers at California Institute of Technology in Pasadena and at the University of California, Los Angeles, have designed a single dye compound that can display red, green or blue depending upon the applied voltage. It features bistable catenane, a molecule consisting of two interlocking rings. One ring is cyclobis(paraquat-p-phenylene) (CBPQT4+). The other, larger ring is composed of spacer regions separating molecular components called TTF and DNP.A voltage applied to the molecular substrate changes the oxidation state of the TTF and DNP. The optical absorption of catenane is determined by which segment of the larger ring is closer to the CBPQT4+. With no applied voltage, the CBPQT4+ is nearer to the TTF unit, which results in a peak absorbance at 854 nm and a green reflectance. Applying a specific voltage oxidizes the TTF, pushing the CBPQT4+ ring to the DNP and giving the dye an absorption peak at 473 nm and red reflectance.Modifying the bistable molecule to get a tristable, three-color dye requires a balancing act, as the oxidation potential, binding energy and molecular orbital energy must allow for switchability and reversibility while providing the absorption peak appropriate for blue reflectance. William A. Goddard III of Caltech and his colleagues have calculated that adding another component, difluorinated benzidine, to the large ring should result in an electrochemically selectable three-color ink.Using ab initio calculations and parametric fits verified by measurements on the bistable compound, Goddard’s team predicts that the new compound will have an absorption peak at 601 nm, providing blue reflectance. Because the catenane design provides three colors, each physical pixel in an electronic paper display using the dye would be an image pixel, as opposed to designs that require separate red, green and blue elements. Operation of the bistable catenane has been demonstrated in a polymer matrix, and the researchers see no reason why this would not be possible for the three-color molecule.