Dual room‐temperature phosphorescence derived from the reversible homolysis in the high proportion n‐electron organic crystal

The reversible transformation between organic molecules and radicals is an effective method for realizing dual emissions in a single material. Herein, in this work, we report a high proportion n‐electron [2,2′‐biisoindoline]‐1,1′,3,3′‐tetraone (4A2B) crystal with the reversible homolysis. The crystal exhibits dual room‐temperature phosphorescence (RTP) emissions at 400 nm ( τ = 114.28 μs) and 575 nm ( τ = 35.78 μs), originating from the 4A2B molecule and the corresponding radical ion pair generated by reversible homolysis of the weak N–N bond, respectively. Thanks to that, the radical ion pair performed a great photostability ( t 1/2 = 1.64 × 10 5 s) and an outstanding repeatability under 100 times on‐off excitation cycles. The RTP emission of the 4A2B molecule has a strong response on high energy excitation due to the ultrafast intersystem crossing process (1.22 × 10 13 s −1 ) within the high‐lying excited states. Furthermore, the 4A2B crystal with these two long‐lived RTP emissions is able to be applied in optical anti‐counterfeiting, optical encryption and high‐resolution bioimaging. This research elucidates that the incorporation of organic molecules and radical ion pairs may provide a new method to achieve dual emissions containing RTP emissions, fluorescence or thermally activated delayed fluorescence.