Dual room-temperature phosphorescence derived from the reversible homolysis in the high proportion n-electron organic...

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 × 10⁵ 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 × 10¹³ 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.