A Heavy‐atom‐free Molecular Motif Based on Symmetric Bird‐like Structured Tetraphenylenes with Room‐Temperature Phosphorescence (RTP) Afterglow over 8 s

In recent years, pure organic room-temperature phosphorescence (RTP) with highly efficient and long-persistent afterglow has drawn substantial awareness. Commonly, spin-orbit coupling can be improved by introducing heavy atoms into pure-organic molecules. However, this strategy will simultaneously increase the radiative and non-radiative transition rate, further resulting in dramatic decreases in the excited state lifetime and afterglow duration. Here in this work, a highly symmetric bird-like structure tetraphenylene (TeP), and its three symmetrical halogenated derivatives (TeP-F, TeP-Cl and TeP-Br) are synthesized, while their RTP properties and mechanisms are systematically investigated by both theoretical and experimental approaches. As the results, the rigid, highly twisted conformation of TeP restricts the non-radiative processes of RTP and gives rise to the enhancement of electron-exchange, which can contribute to the RTP radiation process. Despite the faint RTP of the bromine and chlorine-substituted ones (TeP-Br, TeP-Cl), the fluoro-substituted TeP-F exhibited a long phosphorescent lifetime up to 890 ms, corresponding to an extremely long RTP afterglow over 8 s, which could be incorporated into the best series of non-heavy-atom RTP materials reported in previous literature.