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High-Spin Diradical Dication of Chiral π‑Conjugated Double Helical Molecule

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journal contribution
posted on 2019-10-17, 19:47 authored by Chan Shu, Hui Zhang, Arnon Olankitwanit, Suchada Rajca, Andrzej Rajca
We report an air-stable diradical dication of chiral D2-symmetric conjoined bis[5]­diazahelicene with an unprecedented high-spin (triplet) ground state, singlet triplet energy gap, ΔEST = 0.3 kcal mol–1. The diradical dication possesses closed-shell (Kekulé) resonance forms with 16 π-electron perimeters. The diradical dication is monomeric in dibutyl phthalate (DBP) matrix at low temperatures, and it has a half-life of more than 2 weeks at ambient conditions in the presence of excess oxidant. A barrier of ∼35 kcal mol–1 has been experimentally determined for inversion of configuration in the neutral conjoined bis[5]­diazahelicene, while the inversion barriers in its radical cation and diradical dication were predicted by the DFT computations to be within a few kcal mol–1 of that in the neutral species. Chiral HPLC resolution provides the chiral D2-symmetric conjoined bis[5]­diazahelicene, enriched in (P,P)- or (M,M)-enantiomers. The enantiomerically enriched triplet diradical dication is configurationally stable for 48 h at room temperature, thus providing the lower limit for inversion barrier of configuration of 27 kcal mol–1. The enantiomers of conjoined bis[5]­diazahelicene and its diradical dication show strong chirooptical properties that are comparable to [6]­helicene or carbon–sulfur [7]­helicene, as determined by the anisotropy factors, |g| = |Δε|/ε = 0.007 at 348 nm (neutral) and |g| = 0.005 at 385 nm (diradical dication). DFT computations of the radical cation suggest that SOMO and HOMO energy levels are near-degenerate.

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