Sowndarya S V Shree, St John Peter C, Paton Robert S
Department of Chemistry, Colorado State University Fort Collins CO 80523 USA
Biosciences Center, National Renewable Energy Laboratory Golden CO 80401 USA
Chem Sci. 2021 Sep 6;12(39):13158-13166. doi: 10.1039/d1sc02770k. eCollection 2021 Oct 13.
Long-lived organic radicals are promising candidates for the development of high-performance energy solutions such as organic redox batteries, transistors, and light-emitting diodes. However, "stable" organic radicals that remain unreactive for an extended time and that can be stored and handled under ambient conditions are rare. A necessary but not sufficient condition for organic radical stability is the presence of thermodynamic stabilization, such as conjugation with an adjacent π-bond or lone-pair, or hyperconjugation with a σ-bond. However, thermodynamic factors alone do not result in radicals with extended lifetimes: many resonance-stabilized radicals are transient species that exist for less than a millisecond. Kinetic stabilization is also necessary for persistence, such as steric effects that inhibit radical dimerization or reaction with solvent molecules. We describe a quantitative approach to map organic radical stability, using molecular descriptors intended to capture thermodynamic and kinetic considerations. The comparison of an extensive dataset of quantum chemical calculations of organic radicals with experimentally-known stable radical species reveals a region of this feature space where long-lived radicals are located. These descriptors, based upon maximum spin density and buried volume, are combined into a single metric, the radical stability score, that outperforms thermodynamic scales based on bond dissociation enthalpies in identifying remarkably long-lived radicals. This provides an objective and accessible metric for use in future molecular design and optimization campaigns. We demonstrate this approach in identifying Pareto-optimal candidates for stable organic radicals.
长寿命有机自由基有望用于开发高性能能源解决方案，如有机氧化还原电池、晶体管和发光二极管。然而，能长时间保持不反应并可在环境条件下储存和处理的“稳定”有机自由基很少见。有机自由基稳定性的一个必要但不充分条件是存在热力学稳定作用，例如与相邻π键或孤对电子共轭，或与σ键超共轭。然而，仅热力学因素并不能产生具有较长寿命的自由基：许多共振稳定的自由基是瞬态物种，存在时间不到一毫秒。动力学稳定对于自由基的持久性也是必要的，例如抑制自由基二聚或与溶剂分子反应的空间效应。我们描述了一种定量方法来描绘有机自由基的稳定性，使用旨在捕捉热力学和动力学因素的分子描述符。将大量有机自由基的量子化学计算数据集与实验已知的稳定自由基物种进行比较，揭示了该特征空间中长寿命自由基所在的区域。这些基于最大自旋密度和埋藏体积的描述符被组合成一个单一指标——自由基稳定性得分，在识别寿命极长的自由基方面，该指标优于基于键解离焓的热力学标度。这为未来的分子设计和优化活动提供了一个客观且易于使用的指标。我们展示了这种方法在识别稳定有机自由基的帕累托最优候选物方面的应用。
