Pure Organic Thermally Activated Delayed Fluorescence Afterglow Polymers via Dopant Isomerization

<i>N</i>-(<i>o</i>-Cyanophenyl)carbazole can be dimerized at different positions, which may change excited state behaviors. Herein, 2,3'-dicyano-3,4'-di(carbazol-9-yl)biphenyl (D34C) is designed and synthesized and doped into polymers. However, we find that D34C does not exhibit room temperature phosphorescence but emits fluorescence (FL) and bright thermally activated delayed fluorescence (TADF) with lifetimes of hundreds of milliseconds, which is observed in diverse matrices such as PMMA, MBS, ABS, PS, HIPS, and SIS. The simple positional isomerization makes the abundant triplet excitons undergo only reverse intersystem crossing rather than room temperature phosphorescence (RTP) radiation, which is rather rare in organic doped polymers. Since the production of TADF afterglow requires a certain excitation time, the generally indistinguishable FL and TADF efficiencies are separated for the first time. This work not only provides novel TADF afterglow polymers with diverse mechanical properties but also will evoke the subtle design of conjugated organic molecules to dramatically change photoexcitation and emission behaviors.