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

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.