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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. [ABSTRACT FROM AUTHOR]

Reaction of Cu(ClO4)2⋅6 H2O with a tripodal 2N2O ligand, H2Me2NL, having a p‐(dimethylamino)phenol moiety, in CH2Cl2/MeOH (1:1 v/v) under basic conditions under an inert gas atmosphere gave [Cu(Me2NL)(H2O)] (1). The same reaction carried out under aerobic conditions gave [Cu(Me2NL)(MeOH)]ClO4 (2), which could be obtained also from the isolated complex 1 by reaction with O2 in CH2Cl2/MeOH. The X‐ray crystal structures of 1 and 2 revealed similar square‐pyramidal structures, but 2 showed the (dimethylamino)phenoxyl radical features. Complex 1 exhibits characteristic CuII EPR signals of the dx2-y2 ground state in CH2Cl2/MeOH at 77 K, whereas 2 is EPR‐silent. The EPR and X‐ray absorption fine structure (XAFS) results suggest that 2 is assigned to the CuII–(dimethylamino)phenoxyl radical. However, complex 1 showed different features in the absence of MeOH. The EPR spectrum of the CH2Cl2 solution of 1 exhibits distortion from the dx2-y2 ground state and a temperature‐dependent equilibrium between the CuII–(dimethylamino)phenolate and the CuI–(dimethylamino)phenoxyl radical. From these results, CuII–phenoxyl radical complex 2 is concluded to be formed by the reaction of 1 with O2 via the CuI–phenoxyl radical species. [ABSTRACT FROM AUTHOR]

In order to gain new insights into the effect of the π–π stacking interaction of the indole ring with the CuII–phenoxyl radical as seen in the active form of galactose oxidase, we have prepared a CuII complex of a methoxy‐substituted salen‐type ligand, containing a pendent indole ring on the dinitrogen chelate backbone, and characterized its one‐electron‐oxidized forms. The X‐ray crystal structures of the oxidized CuII complex exhibited the π–π stacking interaction of the indole ring mainly with one of the two phenolate moieties. The phenolate moiety in close contact with the indole moiety showed the characteristic phenoxyl radical structural features, indicating that the indole ring favors the π–π stacking interaction with the phenoxyl radical. The UV/Vis/NIR spectra of the oxidized CuII complex with the pendent indole ring was significantly different from those of the complex without the side‐chain indole ring, and the absorption and CD spectra exhibited a solvent dependence, which is in line with the phenoxyl radical–indole stacking interaction in solution. The other physicochemical results and theoretical calculations strongly support that the indole ring, as an electron donor, stabilizes the phenoxyl radical by the π–π stacking interaction. [ABSTRACT FROM AUTHOR]