Understanding the Self‐Assembly of Cellulose Nanocrystals—Toward Chiral Photonic Materials

Over millions of years, animals and plants have evolved complex molecules and structures that endow them with vibrant colors. Among the sources of natural coloration, structural color is prominent in insects, bird feathers, snake skin, plants, and other organisms, where the color arises from the interaction of light with nanoscale features rather than absorption from a pigment. Cellulose nanocrystals (CNCs) are a biorenewable resource that spontaneously organize into chiral nematic liquid crystals having a hierarchical structure that resembles the Bouligand structure of arthropod shells. The periodic, chiral nematic organization of CNC films leads them to diffract light, making them appear iridescent. Over the past two decades, there have been many advances to develop the photonic properties of CNCs for applications ranging from cosmetics to sensors. Here, the origin of color in CNCs, the control of photonic properties of CNC films, the development of new composite materials of CNCs that can yield flexible photonic structures, and the future challenges in this field are discussed. In particular, recent efforts to make flexible photonic materials using CNCs are highlighted.