Physics

OLED

Nanotechnology

Organic chemistry

Quantum efficiency

Optics

Astronomy

Radical

Luminescence

Quantum yield

Exciton

Chemistry

Materials science

Condensed matter physics

Layer (electronics)

Spectral line

Photoluminescence

Electroluminescence

Optoelectronics

Photochemistry

Fluorescence

Oscillator strength

Nature Materials

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 (&gt;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. An investigation on the electronic transitions of organic radicals allows us to identify design rules to increase the oscillator strength of these emitters and obtain efficient radical-based light-emitting diodes operating in the visible range.

Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes