Photo‐Switchable FRET‐Enabled Flexible Luminescent Films for Multilevel Optical Security

ABSTRACT Flexible optical materials with switchable multichannel outputs are highly desirable for advanced anti‐counterfeiting and information security applications; yet, it remains challenging to integrate photochromism, luminescence, and mechanical robustness into a single platform. Here, we report a photo‐switchable luminescent composite film by incorporating a terbium complex donor (Tb‐Phen) and a spiropyran derivative acceptor (SP‐OH) into a polyurethane matrix. The dual‐ligand Tb‐Phen complex shows strong green emission, high quantum yield, and good stability, whereas SP‐OH provides reversible photochromic conversion. Upon UV irradiation, SP‐OH is transformed into its merocyanine form, which activates an efficient fluorescence resonance energy transfer (FRET) process from Tb‐Phen to the acceptor, leading to pronounced quenching of green emission and emergence of a red emission channel. The optimized composite film shows a high FRET efficiency of 80.7%, a tensile strength of 20.12 MPa, an elongation at break of 494%, good thermal stability, and stable reversible switching behavior under repeated UV/visible‐light cycles. Benefiting from the coupled photochromic and luminescent responses, the film enables multilevel anti‐counterfeiting, wavelength‐selective information encryption, and rewritable optical data storage. This work provides an effective strategy for constructing flexible photo‐responsive FRET materials with integrated optical and mechanical functions.