A Bowl‐Shaped ortho‐Semiquinone Radical Anion: Quantitative Evaluation of the Dynamic Behavior of Structural and Electronic Features

Abstract

Organic radical ions play increasingly important roles in a wide range of research fields from biochemistry to materials science. [1] o-Semiquinone is a typical organic radical anion with multistage redox ability that can form chelate salts and complexes with many kinds of metal cations. Investigation of a variety of transition-metal complexes of o-semiquinone radical could help not only to increase general knowledge of their fundamental chemistry but also to develop functional materials based on valence tautomerism phenomena. [2,3] In contrast, the alkali-metal salts have been extensively studied, and much attention has been focused on the dynamic behavior of their structure and of the electronic features of the ion pair in solution. [4,5] Solution-phase ESR spectroscopy measurements of some alkali-metal salts show significant temperature dependence in their hyperfine coupling constants (hfccs). [4b,d] The origin of this behavior has been only qualitatively interpreted as temperature-dependent positional change or migration of the alkali-metal cation to the oxygen atoms of the o-semiquinone radical. [4e,h] A quantitative discussion of such dynamic behavior has not been carried out to date.Recently, we synthesized and isolated corannulene (1) [6] based stable neutral mono-and diradical derivatives with bowl-shaped non-alternant p-conjugated systems. [7] Their three-dimensional spin-delocalized nature and intra-and intermolecular magnetic interactions were experimentally illustrated in terms of geometrical and topological aspects. These studies have inspired us to design a novel bowl-shaped radical anion system 2C À based on corannulene with the osemiquinone moiety. Notably, 2C À is the first bowl-shaped semiquinone radical anion with non-alternant p conjugation. [8] Herein we report the synthesis of a sodium salt of 2C À (Na + ·2C À ), which is stable in an oxygen-free solution and even in the solid state under nitrogen atmosphere. Thanks to the high stability imparted by the bulky tert-butyl groups, we could evaluate 3D molecular and electronic structures of Na + ·2C À using the curvature and electronic spin and charge distribution by ESR spectroscopy and 1 H and 23 Na ENDOR/ TRIPLE measurements as well as by DFT calculations. We emphasize that the temperature dependence of the hfccs, related to the bowl-shaped structure, can for the first time allow for a quantitative discussion of the dynamic behavior of the ion pair in a solution of Na + ·2C À with the help of sophisticated DFT calculations. These studies demonstrate the salient structural and electronic features of a bowl-shaped semiquinone radical salt with the temperature-dependent positional change of a countercation, which originates in the concave-convex dynamics of the bowl-shaped skeleton.A synthetic route for Na + ·2C À is depicted in Scheme 1. Our modified experimental conditions of the reported penta-tertbutylation of 1 [9] gave tetra-tert-butyl corannulene 3 as the major product. By bromination and subsequent coupling reaction, [10] we obtained ...

Electronic Structures and Chiroptical Properties of Post‐functionalized Helicene Quinones

“…rac ‐1,2‐Dimethoxy‐4‐phenyl[6]helicene (±)‐ 8 could be isolated in moderate yields and was deprotected by BBr 3 in CH 2 Cl 2 in the next step to give catechol (±)‐ 9 . After separation of the enantiomers with the help of preparative HPLC on a chiral stationary phase, enantiopure catechols ( P )‐(+)‐ 9 and ( M )‐(−)‐ 9 were oxidized by AgO in CH 3 CN in excellent yields to form o ‐quinones ( P )‐(+)‐ 3 and ( M )‐(−)‐ 3 , respectively . In contrast to the conversion of 5 into 6 , which proceeded at 22 °C, the oxidation of 9 to 3 required a reaction temperature of 90 °C over 2 h.…”

Corannulene derivatives for organic electronics: From molecular engineering to applications

“…Over the past several decades, chemists have been fascinated by its unique topological structure, and endeavor to its functionalization and properties [3][4][5][6]. These derivatives have been applied in the fields of supramolecular chemistry [7][8][9][10], liquid crystals [11,12], chiroptical activity [7,13,14], bottom-up synthesis of carbon nanotubes [15], radicals [16][17][18][19][20] and lithium-ion cells [21]. Although its semiconducting properties are also interesting and have attracted considerable attention on the synthesis and basic characterization, the device performance based on corannulene derivatives has not been demonstrated until recently [22][23][24][25][26][27].…”