Hikaru Ishii

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, we systematically investigate the photochemical behavior of Cz-TTM in solutions, together with the influence of impurities on the photophysical properties of the compound. We 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.