The doublet-spin nature of radical emitters is advantageous for applications in organic light-emitting diodes, as it avoids the formation of triplet excitons that limit the electroluminescence efficiency of non-radical emitters. However, radicals generally show low optical absorption and photoluminescence yields. Here we explain the poor optical properties of radicals based on alternant hydrocarbons, and establish design rules to increase the absorption and luminescence yields for donor–acceptor-type radicals. We show that non-alternant systems are necessary to lift the degeneracy of the lowest energy orbital excitations; moreover, intensity borrowing from an intense high-lying transition by the low-energy charge-transfer excitation enhances the oscillator strength of the emitter. We apply these rules to design tris(2,4,6-trichlorophenyl)methyl–pyridoindolyl derivatives with a high photoluminescence quantum yield (>90%). Organic light-emitting diodes based on these molecules showed a pure-red emission with an over 12% external quantum efficiency. These insights may be beneficial for the rational design and discovery of highly luminescent doublet emitters.

自由基发射体的双重自旋特性有利于有机发光二极管的应用，因为它避免了三重态激子的形成，从而限制了非自由基发射体的电致发光效率。然而，自由基通常显示出低的光吸收和光致发光产率。在这里，我们解释了基于交替烃的自由基的较差的光学性能，并建立了设计规则以增加供体-受体型自由基的吸收和发光产率。我们表明，非替代系统对于解除最低能量轨道激发的简并性是必要的。此外，通过低能电荷转移激发从强烈的高处跃迁中借用的强度提高了发射极的振荡器强度。我们将这些规则应用于设计tris（2,4，具有高光致发光量子产率（> 90％）的6-三氯苯基）甲基-吡啶并吲哚基衍生物。基于这些分子的有机发光二极管显示出纯红色发射，外部量子效率超过12％。这些见解对于合理设计和发现高发光双峰发射器可能是有益的。