Materials Science

Greasing the Color Switch

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Science  04 Mar 2005:
Vol. 307, Issue 5714, pp. 1379
DOI: 10.1126/science.307.5714.1379a

Spirooxazine and chromene are photochromic dye molecules that undergo a reversible color change when subject to irradiation. Switching between clear and colored states requires that half of the molecule undergo an approximately 90° rotation. In solution, switching and unswitching are fast processes, but when these molecules are embedded in a host matrix, the unswitching or color fade times are significantly longer and are strongly influenced by the viscosity of the matrix. Although a matrix with a lower glass transition temperature could be used to mitigate this problem, this would then compromise other properties of the lens.

Evans et al. have come up with a solution that was inspired by drug and gene delivery, where sensitive peptides or oligonucleotides are protected by a polymer conjugate. In this application, they covalently linked their dye molecules to low-glass transition temperature oligomers, such as poly(dimethylsiloxane) and poly(ethyleneglycol), which then shield the dye from the lower-viscosity matrix material. The attached oligomers do not alter the electronic character of the dyes, but they do act to lubricate the twisting motion, so that the color fade times were reduced by 40 to 99%. — MSL

Nature Mater. 10.1038/nmat1326 (2005).

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