Ionic Gating of Liquid‐Crystal‐Like Pteridine Assemblies Enables Tunable Light Scattering

The bottom‐up assembly of small organic molecules into dynamic optical materials remains a major challenge in materials chemistry. Biological systems, however, demonstrate that hierarchical organization of such molecules can generate diverse optical functionalities. Here we show that zebrafish pigment cells undergo a developmentally regulated phase transition in which pteridine‐rich organelles, known as pterinosomes, transform from disordered ultraviolet (UV)‐absorbing compartments into highly reflective, liquid‐crystal‐like scattering structures. Cryogenic STEM diffraction mapping and electron microscopy reveal that this transition corresponds to the reorganization of a disordered fibrous matrix into a hierarchical, concentrically ordered mesophase. We show that pterinosomes are built from a composite pteridine material whose relative composition is developmentally tuned. We further identify ionic gating as a key regulator of this process: potassium (K + ) depletion enables π – π stacking‐driven ordering, thereby decoupling metabolite accumulation from crystallization. Optical modeling indicates that the resulting mesoscale organization generates strong dielectric anisotropy and refractive‐index contrast, enabling efficient broadband scattering through collective effects in organelle arrays. Our findings establish ionic gating and hierarchical assembly as strategies for controlling the phase behavior of confined small‐molecule systems and provide design principles for tunable bio‐inspired photonic materials.