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David Bradley ISSUE #70
November 2007
Double Vision With Coordination Polymers
Daniel Leznoff

Calcite crystals can make you see double. You don't ingest them to achieve some kind of mind warp effect; they are simply birefringent, having essentially two focal points. I say this simply, but designing new materials with the property of birefringence is quite complicated issue.

Now, one of the most birefringent solid materials known has been produced by researchers at Simon Fraser University in Canada using a coordination polymer based on the terpyridine ligand, and bridged metal complexes based on the linear and asymmetric chemical group, the dicyanoaurate anion, rather than a mineral such as calcite.

Daniel Leznoff and Zuo-Guang Ye point out that the modular nature of coordination polymers offers great scope for fine-tuning their optical properties for particular applications, such as dual-focus lenses for optical disk readers.

Credit: Leznoff et al/Angew/Wiley

"The ability to judiciously choose a metal center, a chelating ligand, and a bridging ligand to tailor the overall physical and optical properties of the resulting polymer affords an exceptional design freedom in materials science," the researchers say.

Birefringent optical components are usually made of calcite. Critically, they have a refractive index that is different in two directions within the crystal?this is what makes them birefringent. Light appears to bend when it enters a transparent material?picture a swimming pool net plunged below the surface. With birefringent materials, the beam of light is split into two perpendicularly polarized beams, which then exit the material shifted relative to each other.

Leznoff and Ye explain the advantages of their approach as providing almost limitless design possibilities simply by varying the individual components?the metal center, the chelating groups, and the bridging ligands. Each can be selected and linked in numerous combinations to produce desired properties in the final material. The team predicts that improved optical data storage and data transfer in communications technology are possible applications for such highly birefringent materials.

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Angew Chem Int Edn, 2007, 46, 8804-8807