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Efficient ultralong organic phosphorescent materials have potential applications in some fields, such as bioimaging, anti‐counterfeiting, and sensors. Nevertheless, phosphorescence efficiencies of metal‐free organic materials are low due to weak spin–orbit coupling and vigorous nonradiative transitions under ambient conditions. Here a chemical strategy to improve phosphorescence efficiency with intermolecular π‐type halogen bonding construction via isomerism is presented. X‐ray single crystal analysis reveals that different halogen bonding is formed among p‐BrTCz, m‐BrTCz, and o‐BrTCz crystals. Phosphorescence efficiency of m‐BrTCz in solid can reach 13.0%, seven times of o‐BrTCz in solid owing to effective π‐type halogen bonding, which is further confirmed by theoretical calculations. However, ultralong phosphorescence lifetimes are little affected, 155, 120, and 156 ms for p‐BrTCz, m‐BrTCz, and o‐BrTCz in the solid state, respectively. Furthermore, a simple pattern for data encryption and decryption is first demonstrated under sunlight. This result will provide an approach for improving the phosphorescent efficiency of metal‐free organic phosphors with ultralong luminescence.

Smart materials with ultralong phosphorescence are rarely investigated and reported. Herein we report on a series of molecules with unique dynamic ultralong organic phosphorescence (UOP) features, enabled by manipulating intermolecular interactions through UV light irradiation. Our experimental data reveal that prolonged irradiation of single-component organic phosphors of PCzT, BCzT, and FCzT under ambient conditions can activate UOP with emission lifetimes spanning from 1.8 to 1330 ms. These phosphors can also be deactivated back to their original states with short-lived phosphorescence by UV irradiation for 3 h at room temperature or through thermal treatment. Additionally, the dynamic UOP was applied successfully for a visual anti-counterfeiting application. These findings may provide unique insight into dynamic molecular motion for optical processing and expand the scope of smart-response materials for broader applications.