Mechanically robust cellulose nanocrystal photonic films with propagation-switchable circularly polarized room-temperature phosphorescence.

The development of circularly polarized room-temperature phosphorescence (CPRTP) materials that simultaneously combine tunable photonic properties, long-lived emission, and mechanical robustness remains challenging. Here, we fabricate a series of poly(vinyl alcohol) (PVA)-reinforced cellulose nanocrystal (CNC) chiral photonic films through evaporation-induced helicoidal self-assembly followed by PVA infiltration. By regulating the helical pitch using α-D-glucose, the resulting films exhibit tunable photonic bandgaps and structural colors together with circularly polarized phosphorescence and luminescence. Benefiting from the synergistic rigidification effect of the CNC/Glu chiral matrix and PVA network, our PVA/CNC-Glu-X films combine an ultralong phosphorescence lifetime (1.09-1.36 s) with a high |glum| (0.16-0.51). In addition, the PVA/CNC-Glu-50 film enhanced mechanical performance with a tensile strength of 60 MPa and toughness of 2.40 MJ m-3. Notably, the asymmetric PVA/CNC architecture enables enantiomeric switching of CPL handedness through a propagation-direction-dependent reflection-transmission mechanism. Under top-side excitation, the selective reflection of L-CPL allows the detection of transmitted R-CPL, whereas bottom-side excitation leads to the detection of L-CPL, enabling dynamic chiroptical modulation and multilevel optical encryption. This work provides a feasible strategy for mechanically robust CPRTP photonic materials for advanced optical encoding and information security applications.