Dyes and pigments play a crucial role as the colorants in the imaging areas. In the period of black & white copies, organic pigments have extensively been studied as photoconductors for electrophotographic photoreceptors. However, nowadays, pigments have more and more attracted attention as colorants, not only for color copies based upon the electrophotographic process, but also for inkjet printers. The color of the dyestuff is mostly determined by its molecular structure; whereas that of pigments arises from the molecular structure as well as intermolecular interactions in the solid states. Therefore, the color of pigments can be influenced by molecular arrangement, i.e., crystal structure. In present keynote, I begin with a brief introduction about the difference between dyes and pigments in general and then talk about the intermolecular interactions in the solid state from the standpoint of the crystal structure. I will also introduce you a variety of polymorphs in pigments that are isolated as single crystals and discuss some representative factors which induce phase transition, leading to different colors.
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