Reversible change of luster color from pale yellow to wine red in microcrystalline film by photochromic diarylethene having a naphthyl group†

Recently, metal-free organic materials that change their luster color in response to external stimuli have been reported. The development of such materials could have a significant impact on applications in the decorative and ink industries. Here we report a reversible change in luster color from pale yellow to wine red in microcrystalline film by using a photochromic diarylethene having thiazole rings and a naphthyl group. The diarylethene used can easily form thin plate-like crystals by recrystallization. In using these crystals, they were stacked in the same plane, forming a microcrystalline film. The combination of specular reflections from the surface of the plate-like crystals and multiple reflections from their stacked structure brings about a pale yellow luster effect. This film showed a reversible change in luster color from pale yellow to wine red due to the photocyclization–cycloreversion reactions of diarylethene. Such metallic-appearing luster materials, which reversibly control the luster color in widely different color coordinates depending on the light, could have a significant impact on decorative and ink applications.