Sustainable Cellulose Chiral Photonic Films Enable Simultaneous Ultralong Room‐temperature Phosphorescence and High Luminescence Dissymmetry With Multicolor Tunability

Simultaneously achieving high luminescence dissymmetry factors ( g lum ) and ultralong lifetimes in circularly polarized room‐temperature phosphorescence (CP–RTP) materials remains a fundamental challenge, as it requires reconciling well‐defined chiral ordering, efficient triplet‐state stabilization, and matched energy‐level alignment within a single material system. Here, we report a versatile bio‐based strategy to overcome this trade‐off by incorporating phosphorescent chromophores and polymer matrices into chiral cellulose nanocrystal films via one‐step co‐assembly. The resulting bio‐based materials exhibit an ultralong room‐temperature phosphorescence lifetime of up to 4.242 s. Through targeted optical path engineering using a chiral photonic crystal filter, these long‐lived films achieve a large g lum value of −0.7093, without compromising phosphorescence performance. To expand the emission scope, guest fluorophore doping enables broad spectral tuning of the chiral emission via Förster resonance energy transfer (FRET), yielding multicolor CP–RTP and controllable white‐light emission. Benefiting from their switchable, multimodal optical properties, these films show strong potential for advanced dynamic optical anti‐counterfeiting. This work addresses the core trade‐off between ultralong lifetime and high g lum in CP–RTP systems, and establishes a sustainable, general platform for the design of next‐generation chiral luminescent materials.