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 Å), 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 Å).
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 (∼4.0
Å) and minimizing the overlap of triazinyls.