Observation of a Solid-State-Induced Thermally Populated Spin-Triplet State in Radical Regioisomers

High-spin organic polyradicals are commonly observed due to intramolecular spin–spin coupling, whereas high-spin monoradicals are rarely obtained and hard to be predicted. Herein we report the observation of solid-state-induced thermally populated spin-triplet state in tetraphenylethyene (TPE) based 1,2,4-benzotriazinyl monoradical regioisomers. Crystal packings play a crucial role for intermolecular spin–spin interactions in these systems. The packing modes vary from zigzag infinite chain (M1) to 1D slipped stacked columns (M2) and discrete centrosymmetric slipped dimers (M3). Importantly, solid-state ESR and SQUID studies reveal a thermally excited spin-triplet state in radicals M1 and M3, with small singlet–triplet energy gaps (ΔES–T ≤ −1.09 kcal/mol) and short spin–spin distance (∼4.1 A), as is proved by the observed forbidden transition signal Δms= ±2 from 140 to 330 K. In contrast, isomer M2 displays a paramagnetic behavior due to the longer spin–spin distance (>7.3 A). In addition, the theoretical ...

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