Water‐Resistant and Multicolor Room Temperature Phosphorescence from Cellulose

ABSTRACT Developing sustainable room temperature phosphorescent (RTP) materials with intrinsic water resistance and multicolor afterglow remains a significant challenge. Herein, we present cellulose‐based materials exhibiting stable multicolor RTP in water via a matrix rigidification strategy. The reconstruction of multiple hydrogen bonds and enhancement of non‐bonded interactions effectively suppress non‐radiative decay and triplet exciton quenching, thereby facilitating efficient RTP emission under aqueous conditions. To the best of our knowledge, this is the first demonstration of cellulose‐based materials that exhibit hydrophilicity while maintaining water‐resistant RTP. By tuning the energy level of the lowest triplet state, the materials simultaneously achieve multicolor RTP, high phosphorescence efficiency (36.56%), and a prolonged lifetime (723 ms). Additionally, they exhibit excellent processability into films, fibers, and hydrogels, along with good recyclability. These features enable applications in diverse areas such as multilevel information encryption and marine rescue, significantly broadening the practical utility of sustainable cellulose‐based RTP materials, particularly in aqueous environments.