Air‐Stable Allyl Radicals and Heterocoupled Dimer: Synthesis, Characterization, and Near‐Infrared Emissions

Abstract Air‐stable luminescent monoradical remains rare. Herein, we report novel stable allyl radicals ( ARS ) achieved by conjugated extension of benzo[ b ]thiophene 1,1‐dioxide and steric protection of 1,3,5‐trichlorobenzene. The reactivity and photophysical properties of these allyl radicals are strongly dependent on protecting groups. Introducing p ‐tolyl group or triphenylamine afforded air‐stable monoradical ARS‐Ph and ARS‐TPA , respectively. ARS‐Ph displays doublet luminescence at 752 nm with negligible photoluminescent quantum yield (PLQY). ARS‐TPA exhibits NIR fluorescence at 860 nm with a PLQY of 11.7%. Whereas, chlorine‐protected ARS‐Cl proved inaccessible, an unsymmetric heterocoupled dimer ARS‐HD was isolated as a luminescent singlet diradicaloid with emission at 790 nm and a PLQY of 0.8%. Natural transition orbital analysis reveals the local excitation nature of ARS‐Ph , excited charge‐transfer characteristics of ARS‐TPA and ARS‐HD . Huang‐Rhys factor and reorganization energy analyses of ARS‐TPA disclose the critical effects of the vibrational decoupling and structure rigidity for its enhanced emission intensity and spectral sharpness. Remarkably, under continuous 365 nm light, ARS‐Ph , ARS‐TPA , and ARS‐HD exhibit excellent luminescent photostability in ambient conditions with half‐lives of 19.7, 42.6, and 21.4 h, respectively. Our study presents a straightforward strategy for developing air‐stable luminescent allyl radicals, expanding the family of luminescent radical materials.