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.

高自旋有机多自由基（High-spin organic polyradicals）因分子内自旋-自旋耦合而普遍存在，而高自旋单自由基则鲜有被观测到且难以预测。本文报道了基于四苯乙烯（tetraphenylethyene, TPE）的1,2,4-苯并三嗪基单自由基区域异构体中，固态诱导的热布居自旋三重态的观测结果。晶体堆积在该类体系的分子间自旋-自旋相互作用中发挥关键作用。其堆积模式可分为锯齿形无限链（M1）、一维滑移堆叠柱列（M2）以及离散的中心对称滑移二聚体（M3）三类。值得注意的是，固态电子顺磁共振（Electron Spin Resonance, ESR）与超导量子干涉器件（Superconducting Quantum Interference Device, SQUID）研究表明，自由基M1与M3存在热激发自旋三重态，其单重态-三重态能隙较小（ΔE_S–T ≤ −1.09 kcal/mol），自旋-自旋距离较短（约4.1 Å）；这一点可通过140 K至330 K范围内观测到的禁阻跃迁信号Δms=±2得以证实。与之形成对比的是，异构体M2因自旋-自旋距离较长（>7.3 Å）而表现出顺磁行为。此外，理论计算揭示了分子间轨道重叠的内在机制。本工作通过调控分子间自旋-自旋距离（约4.0 Å）并减小三嗪基的轨道重叠，为实现自由基中固态诱导的热布居自旋三重态提供了一种独特策略。