Solid‐State Luminescence of Whitened Lignin in Clay Matrix with Prolonged Fluorescence Lifetime and Controllable Color

This study explores the luminescence properties of whitened lignin in organic solvent dispersions and self‐standing films composed of clay nanosheets. Lignin whitened by modifying bulky substituents in a polarity‐controlled solvent exhibits luminescence in liquid and solid media via electronic transitions parallel to the molecular plane of aromatic structures in lignin. Remarkably, the fluorescence lifetime of the whitened lignin is extended significantly from nanosecond order to submillisecond order (an increase of ≈ 10 5 times) by transitioning the dispersing medium from a liquid organic solvent to a solid clay nanosheet. Additionally, the luminescence properties of the whitened lignin in the clay nanosheet, such as the fluorescence color, can be tuned by altering the chemical structure of the whitened lignin and the clay nanosheet. This study underscores the potential of utilizing polymers derived from plants and minerals as functional materials to support a realization of decarbonized society. The observed luminescence behavior of lignin, a plant‐based aromatic polymer, within self‐standing films made from clay nanosheets, a mineral‐based inorganic polymer, highlights its promising application in photofunctional materials.