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 (Delta ES-T <= -1.09 kcal/mol) and short spin-spin distance (similar to 4.1 angstrom), as is proved by the observed forbidden transition signal Delta m(s) = +/- 2 from 140 to 330 K. In contrast, isomer M2 displays a paramagnetic behavior due to the longer spin-spin distance (>7.3 angstrom). In addition, the theoretical calculations provide the insight of orbital overlap between molecules. This work gives a unique strategy to access the solid-state-induced thermally populated spin-triplet state in radicals by tuning the intermolecular spin-spin distance (similar to 4.0 angstrom) and minimizing the overlap of triazinyls.