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Presto, Instant Sunglasses!


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Researchers have developed a material that almost instantaneously changes from clear to dark blue when exposed to ultraviolet (UV) light, and it just as quickly reverts to clear when the light is turned off. The new material, one of a class called photochromics, could be useful in optical data storage as well as in super-fancy sunglasses.

For more than a decade, chemical engineer Jiro Abe and colleagues at Aoyama Gakuin University in Japan have been studying the light-sensitive properties of photochromic materials, particularly those derived from a compound called hexaarylbiimidazole (HABI). In its natural state, HABI is colorless, but when ultraviolet light breaks one of the bonds in the molecule, it produces a version that is dark blue. The problem has been that the transformation takes tens of seconds or longer, so the only commercial application has been sunglasses that slowly darken.

When Abe’s team began analyzing HABI’s chemical structure through simulations and laboratory experiments, they found that by adding naphthalene to the compound, they could accelerate the color change to about 180 milliseconds. Adding a compound called cyclophane instead of naphthalene improved the clear-to-blue conversion even more–to about 30 milliseconds. Better still, Abe and colleagues report in the current issue of the Journal of the American Chemical Society, the cyclophane version of HABI reverts just as rapidly to its colorless state when the UV light source is turned off. And the compound is so stable that the reactions can be repeated thousands of times.

If added to Plexiglas or other lens materials, the modified version of HABI would allow sunglasses to instantaneously darken in sunlight and turn clear equally rapidly whenever their wearer moves indoors. HABI’s properties could also make it a prime candidate for a new generation of optical data-storage devices, in which its color on/off ability could substitute for the magnetic on/off switches that underlie today’s electronic data-storage devices.

The new version of HABI developed by Abe and colleagues shows “remarkable stability and rapid switching,” says chemist Devens Gust of Arizona State University, Tempe. These are qualities that few other photochromic materials have, he says, so this compound is a “significant step forward” in harnessing these materials for data processing and storage.