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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. [ABSTRACT FROM AUTHOR]