Fast Photochromism

Bespectacled folks once marveled at lenses that darken in sunlight and return to their untinted state indoors, but the sluggish transition from tinted to transparent left some feeling less than enlightened. Scientists are now getting closer to lenses that shift in an instant, thanks to a new photochromic compound. Jiro Abe and coworkers at Japan's Aoyama Gakuin University have created a hexaarylbiimidazole derivative that changes from colorless to moss green when irradiated with UV light (Org. Lett., DOI: 10.1021/ol801135g). The light homolytically cleaves the C–N bond that links the molecule's imidazole rings, generating a pair of 2,4,5-triphenylimidazolyl radicals (shown). A naphthalene moiety tethers these two radicals together, thereby preventing them from diffusing away from one another. In the absence of UV light, the radicals quickly recombine. At room temperature, this rapid thermal bleaching returns the molecule to its colorless form in a matter of milliseconds. The coloring and decoloring process is so fast that Abe's team can use a UV-light-emitting diode to make a green-colored cloud zip around a solution of the compound.