Simultaneous multi-resonant thermally activated delayed fluorescence and room temperature phosphorescence from biluminescent nitrogen-containing indolocarbazoles

Organic biluminescence, the simultaneous emission from both the singlet and triplet excited state manifolds, is a rare and incompletely understood emission process. However, biluminescent compounds have wide-reaching applications, such as in sensing, anti-counterfeiting, and optoelectronics, owing to the complex interplay of excited states having distinct spectral profiles and lifetimes. Herein, the biluminescence of a family of polycyclic aromatic heterocycles known as nitrogen-containing indolocarbazoles (NICz) is described. As 1 wt.% doped films in polymethylmethacrylate (PMMA), these compounds exhibit dual fluorescence/room temperature phosphorescence (RTP) with λ_PL in the near-UV (≈375 nm) and green (≈500 nm), respectively, and remarkably long phosphorescence lifetimes extending into the multi-second regime. This RTP is shown to persist even at doping concentrations as low as 0.1 wt.%. Additionally, two of the emitters exhibit multi-resonant thermally activated delayed fluorescence (MR-TADF)/RTP biluminescence, which, to the best of knowledge, would be the first examples of such behavior. Finally, insight is provided into the dependence of these competing emission pathways on the temperature and concentration, with supporting wavefunction-based computations.