Mechanistic Origin of Photodegradation and Impurity Effects in Donor–Acceptor Luminescent Radicals

Abstract Organic luminescent radicals have emerged as promising open‐shell emitters for advanced optoelectronic and sensing applications. Among them, the carbazole‐substituted tris(2,4,6‐trichlorophenyl)methyl radical (Cz‐TTM) exhibits remarkable photostability and emission efficiency compared with the parent TTM radical. Nevertheless, Cz‐TTM can be visibly degraded by indoor light irradiation and its photodegradation mechanism is poorly understood. Here, the study systematically investigates the photochemical behavior of Cz‐TTM in solutions, together with the influence of impurities on the photophysical properties of the compound. It is found that emissive and non‐emissive impurities are introduced depending on the purification methods and the light irradiation conditions. Impurities cause underestimation of photoluminescence quantum yield through competitive absorption. Photodegradation under continuous irradiation undergoes a multi‐step reaction via metastable intermediates, which react with dissolved oxygen upon local excitation of the TTM radical moiety. Quantum‐chemical calculations indicate a ring‐closure followed by dechlorination within the TTM moiety as the key degradation pathway. The superior photostability of Cz‐TTM to TTM originates from its stabilized first excited doublet energy level, which increases the activation energy for ring‐closure reaction in Cz‐TTM. These combined experimental and theoretical insights reveal the fundamental factors governing the photostability of TTM‐type luminescent radicals and offer design principles for next‐generation organic luminescent materials.