High‐Efficiency Deep‐ NIR Radical OLEDs via Suppression of Non‐Radiative Loss ^†

Comprehensive Summary

Efficient organic light‐emitting diodes (OLEDs) operating in the deep near‐infrared (NIR, >800 nm) region remain fundamentally constrained by severe non‐radiative losses dictated by the energy‐gap law. Here, we report that introducing sterically demanding and weakly electron‐donating methyl groups into radical emitters suppresses non‐radiative decay, enabling efficient deep‐NIR electroluminescence. Compared with conventional π‐extension approaches, the introduction of methyl substituents, combined with a moderate reduction in donor π‐conjugation, effectively restricts structural relaxation and attenuates low‐frequency vibrational coupling, while preserving high‐frequency vibrational decoupling, leading to further red‐shifted emission without sacrificing radiative efficiency. Transient absorption spectroscopy reveals accelerated host–guest energy transfer, underpinning efficient exciton utilization. As a result, OLEDs based on the radical emitter deliver deep‐NIR electroluminescence at 840 nm with a maximum EQE of 6.1%, representing a record performance for metal‐free emitters beyond 800 nm. This work highlights the importance of excited‐state relaxation management via steric modulation for deep‐NIR organic optoelectronic materials.