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

We report an air-stable diradical dication of chiral D 2 -symmetric conjoined bis[5]diazahelicene with an unprecedented high-spin (triplet) ground state, singlet triplet energy gap, Δ E ST = 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 D 2 -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.