Deuterated Radical Emitters Achieve 10.8% EQE at 800 nm with Remarkably Enhanced Operational Stability.

Near-infrared (NIR) radical emitters are promising for organic light-emitting diodes (OLEDs) but are limited by low emission efficiency and operational stability. Here, we demonstrate that C-H perdeuteration markedly enhances the performance of both the benchmark TTM-TPA radical and a red-shifted analogue, PyBTM-TPA. Photoluminescence quantum yields increase from 24.0% to 30.7% for TTM-TPA and from 7.3% to 10.4% for PyBTM-TPA. Deuterated devices also exhibit improved performance, achieving record maximum external quantum efficiencies (EQEmax) of 10.81% at peak wavelength 800 nm and 3.56% at 865 nm for perD-TTM-TPA and perD-PyBTM-TPA, respectively, together with extended operational lifetimes. We further reveal that the isotope effect is governed by molecular reorganization energy, determined by both the effective Huang-Rhys factor and the distribution of vibronic coupling among individual vibrational modes. These findings establish deuterium engineering as a general strategy for improving radical-based NIR-OLEDs.