Helicene Radicals: Molecules Bearing a Combination of Helical Chirality and Unpaired Electron Spin.

Chiral helicenes that have helical π-conjugated frameworks comprised of ortho-fused aromatic rings have been intensively investigated for several decades owing to their attractive properties and high potential for a variety of applications. In contrast, studies concerning helicene radicals (open-shell helicenes) formed by redox reactions have been severely limited due to inherent high reactivity of organic radicals. However, open-shell helicenes possessing a combination of chirality and unpaired electron spin(s) delocalized on helical π-conjugated orbital(s) are promising for novel synergic magnetic functions. Recently, several kinds of isolable air-stable helicene radicals have been prepared to enable their detailed analysis of the crystal structures and physicochemical properties. This Minireview describes the recent advances and future prospects of organic-based helicene radicals categorized into three groups: 1) ionic radicals, 2) neutral radicals, and 3) diradicals.