Color polymorphism in organic crystals

Colour polymorphism is a rare phenomenon that occurs when a compound possesses packing or conformational polymorphs of different colours. Here the authors review representative chemical systems that display colour polymorphism and explore the origins of this property. Color polymorphism is an interesting property of chemical systems which present crystal polymorphs of different colors. It is a rare phenomenon, with only a few examples reported in the literature hitherto. Nevertheless, systems exhibiting color polymorphism have many potential applications in different domains, such as pigment, sensor, and technology industries. Here, known representative chemical systems showing color polymorphism are reviewed, and the reasons for them to present such property discussed. Also, since some of the concepts related to color polymorphism have been frequently used imprecisely in the scientific literature, this article provides concise, systematic definitions for these concepts.