Color Change\nof Indole Squaraine between Blue and\nYellow with Golden Luster

Color change of materials by external stimulus has attracted\nmuch\nattention for various applications. We synthesized 1,3-bis(3,3-dimethyl-2-methylene\nindolenine)squaraine (ISQ), which was a yellow powder with golden\nluster (ISQ(gold)) and whose color changed to blue (ISQ(blue)) by\nbeing ground using mortar and pestle. Scanning electron microscopy\nindicated that ISQ(gold) consisted of laminated plate-like crystals\nwith a large flat surface, which was attributable to (2 0–2)\nfacets of monoclinic crystals. Measurements of the diffuse reflectance\nspectra revealed that J- and H-type aggregates of ISQ were formed\nby increasing the weight percent of ISQ(blue) in the mixture with\nBaSO<sub>4</sub>. A thin film, which was prepared by a spin-coating\nof ethanol with dissolved ISQ(blue) on glass substrates, looked blue\non a white paper, whereas it looked yellowish on a black paper. By\nrepeating the coating process, the film thickness increased, and the\nthick film exhibited yellow with golden luster regardless of the background\ncolor. These results indicate that the color depends on whether the\ntransmitted light through ISQ or the reflected light on the ISQ surface\nmainly contributes to color vision. To neglect the effect of the transmitted\nlight and intensify the reflection on the surface, a pellet with a\nsmooth surface was prepared by pressing ISQ(gold) or ISQ(blue) at\n20 kg/cm<sup>2</sup>. The ISQ(blue) pellet consisted of the laminated\nsheets and was dark blue-green. However, under light irradiation,\nthe ISQ(blue) pellet exhibited a yellow color with golden luster,\nwhose specular reflection spectrum was similar to that of the ISQ(gold)\npellet. Our results indicate that the color of ISQ is tunable by controlling\nthe transmission and the reflection of the light, including the specular\nand diffuse reflections.