peri-Benzo-Diindenotetracenyl: Helically π-Extended Allyl Radical with Robust Stability.

Here is described the synthesis and characterization of a stable hydrocarbon radical, peri-benzo-diindenotetracenyl, with a helical structure. Although the helical π-radical has no peripheral substituents, it was stable in the solid and solutions. According to the X-ray diffraction analysis and quantum chemical calculations, the radical was best described as an allyl radical fused by five Clar's sextets. The optically resolved enantiomers exhibited mirror image CD spectra with |g CD | of 2.4×10 -4 at 522 nm. The racemization barrier was determined to be 95.9 kJ/mol at 298 K, which is compatible with that of [5] helicene (108 kJ/mol).