para-Substituent Effects on Radical Fate in Anthracene-Substituted Triarylmethyl Radicals

Abstract

Triarylmethyl radicals are representative carbon-centered open-shell species whose persistence and reactivity are highly sensitive to molecular structure. Incorporation of an anthracene unit offers an attractive strategy for stabilizing such radicals because its extended π-framework can promote spin delocalization while simultaneously providing steric protection. However, spin delocalization onto the anthracene framework may also generate coupling-prone peripheral sites, making the resulting radicals susceptible to dimerization and photodegradation. Herein, we systematically investigate how para-substitution on the anthracene unit governs the radical fate of anthracene-substituted bis(2,4,6-trichlorophenyl)methyl radicals. A series of derivatives bearing no substituent, a triisopropylsilylethynyl group, a 4-tert-butylphenyl group, or a mesityl group at the anthracene para-position were prepared and compared. Strikingly, different outcomes were observed after oxidation. The unsubstituted system undergoes direct intermolecular coupling followed by further oxidation to afford quinonoid closed-shell dimer 2C. Introduction of a triisopropylsilylethynyl substituent enables formation of radical 4R in solution, but crystallization induces irreversible σ-dimerization to give 4D. The 4-tert-butylphenyl-substituted radical 5R exhibits improved but still limited persistence. In contrast, the para-mesityl-substituted derivative 6R can be isolated as a persistent radical and structurally characterized by single-crystal X-ray diffraction. Combined crystallographic, UV–vis, EPR, electrochemical, and theoretical studies reveal that these divergent outcomes originate from the interplay between spin-density redistribution and local steric shielding at the coupling-prone anthracene para-position. Consistently, the photostability of the radicals increases progressively from 4R to 5R and 6R, with half-lives of approximately 57, 79, and 222 min, respectively, under continuous 365 nm irradiation. This work establishes para-substituent engineering as an effective strategy for controlling dimerization versus persistence in anthracene-substituted triarylmethyl radicals and provides a useful design principle for persistent π-conjugated carbon-centered radicals.

Keywords

Organic radical / Triarylmethyl radical / Radical persistence and reactivity / Spin delocalization / Dimerization / X-ray crystallography / DFT calculation / Electron paramagnetic resonance

Rights & permissions

2026 SIOC, CAS, Shanghai, & WILEY-VCH GmbH