Fine-Tuning the Colors of Natural Pigment Emodin with Superior Stability through Cocrystal Engineering

Abstract

Due to the increasing concern of toxicity when using synthetic pigments, naturally occurring materials have regained attention. However, the limited hues and instability restrict their applications. Emodin (EM) is considered as a naturally sourced pigment and has been added in chemic necessities for many years. In this work, continuous color changes can be achieved by careful selection of coformers via a cocrystal engineering approach. This fine color tuning could be attributed to the different π–π and charge-transfer interactions after cocrystal formation. Furthermore, EM cocrystals were found to have superior stability in formulations. Fine color modifications and better stability behaviors in cocrystals may provide a new pathway to develop naturally sourced pigments without chemical synthesis.