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 (τ(1/2)) in air-saturated solution under ambient conditions were also determined. Surprisingly, all three radicals showed remarkable stabilities: τ(1/2) = 7, 3.5, and 43 days.