Color Change of Indole Squaraine between Blue and Yellow with Golden Luster.

Color change of materials by external stimulus has attracted much attention for various applications. We synthesized 1,3-bis(3,3-dimethyl-2-methylene indolenine)squaraine (ISQ), which was a yellow powder with golden luster (ISQ(gold)) and whose color changed to blue (ISQ(blue)) by being ground using mortar and pestle. Scanning electron microscopy indicated that ISQ(gold) consisted of laminated plate-like crystals with a large flat surface, which was attributable to (2 0-2) facets of monoclinic crystals.