<i>para</i> ‐Substituent Effects on Radical Fate in Anthracene‐Substituted Triarylmethyl Radicals

Comprehensive Summary 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.