Design and Synthesis of New Stable Fluorenyl-Based Radicals

Organic neutral radicals have long
fascinated chemists with a fundamental
understanding of structure–reactivity relationships in organic
reactions and with applications as new functional materials. However,
the elusive nature of these radicals makes the synthesis, isolation,
and characterization very challenging. In this work, the synthesis
of three long-lived, fluorenyl-based radicals are reported. The geometry
and electronic structures of these radicals were systematically investigated
with a combination of various experimental methods, besides density
functional theory (DFT) calculations, which include X-ray crystallographic
analysis, electron spin resonance (ESR), electron nuclear double resonance
(ENDOR), cyclic voltammetry, and UV–vis–NIR measurements.
Their half-life periods (τ1/2) in air-saturated solution
under ambient conditions were also determined. Surprisingly, all three
radicals showed remarkable stabilities: τ1/2 = 7,
3.5, and 43 days.

有机中性自由基长期以来一直受到化学家的关注，既为人们理解有机反应中的结构-反应性关系提供了基础理论支撑，也可作为新型功能材料获得实际应用。然而，这类自由基固有的难捉摸特性使其合成、分离与表征极具挑战。本工作报道了三种长寿命芴基自由基的合成。除密度泛函理论（DFT）计算外，研究团队结合多种实验手段系统探究了这些自由基的几何结构与电子结构，包括X射线晶体衍射分析、电子自旋共振（ESR）、电子核双共振（ENDOR）、循环伏安法以及紫外-可见-近红外（UV–vis–NIR）测试。此外，本研究还测定了这三种自由基在空气饱和溶液、常温常压环境下的半衰期。令人意外的是，所有三种自由基均展现出优异的稳定性，其半衰期τ₁/₂分别为7天、3.5天与43天。