Achieving a Record Photoluminescence Quantum Yield in Green Light-Emitting Carbon-Centered Radicals with Nanosecond Emission Lifetimes.

Organic luminescent radicals possess considerable potential for applications in organic light-emitting diodes (OLEDs)-based visible light communication owing to their intrinsic advantages of nanosecond emission lifetimes and spin-allowed radiative transitions. However, the inherently narrow energy bandgap and multiple nonradiative channels of organic radicals make it difficult to achieve efficient green and blue light-emitting, which is not conducive to applying visible light communication in diverse fields. In this study, a series of carbon-centered radicals derived from N-heterocyclic carbenes are designed and synthesized, some of which exhibiting hybrid local and charge-transfer (HLCT) states that resulting in efficient green emission. The results of photophysical characterizations and theoretical calculations demonstrate that the luminescence efficiency is closely related to their emission states. This relationship inhibits the nonradiative channels while simultaneously opening the radiative channels of organic radicals exhibiting HLCT states but not those with locally excited states. Intriguingly, a high photoluminescence quantum yield value of up to 70.1% at 534 nm is observed, which is the highest among green light-emitting carbon-centered radicals reported to date. Based on this exceptional result, an OLED device is fabricated and achieved an external quantum efficiency of 8.8%. These results demonstrate its potential application in electroluminescent devices.