High-Spin Diradical Dication of Chiral π‑Conjugated Double Helical Molecule
journal contributionposted 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 bisdiazahelicene 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 bisdiazahelicene, 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 bisdiazahelicene, 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 bisdiazahelicene and its diradical dication show strong chirooptical properties that are comparable to helicene or carbon–sulfur 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.