Abstract
2,9-Dichloroquinacridone is an industrially important bluishred pigment which falls in the category of hydrogen-bonded pigments. In addition to red single crystals, we have recently isolated black single crystals from the vapor phase. Structure analysis of both crystals revealed that the red phase is typically characterized by NH…O intermolecular hydrogen bonds and corresponds to the commercial product; whereas, in the black phase, there exist no intermolecular hydrogen bonds, quite contrary to our expectation. Furthermore, the molecule in the black phase is heavily deformed and has a higher heat of formation than that in the red phase. Because of this, the black phase can easily be transformed into the red phase by heating at about 530 K for several minutes. Additionally, molecular distortion upon crystallization is found to largely displace the absorption band toward longer wavelengths, giving rise to a black color in the solid state.
Published Version
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