Double helicene possessing B–N dative bonds built on 1,4-dihydropyrrolo[3,2-b]pyrrole core

Abstract

Just four steps are required to transform 2-nitrobenzaldehyde into centrosymmetric, quadrupolar N,B-doped nanographenes possessing two nitrogen–boron dative bonds. A convergent fragment coupling strategy allowed rapid access to key intermediates bearing the 1,4-dihydropyrrolo[3,2-b]pyrrole core. 2,6-Di-tert-butylpyridine turned out to be the best base for the formation of B←N bonds. This synthetic strategy can be extended to encompass double helicenes possessing two [7]helicene units bearing four five-membered rings. The size of the peripheral arm influences the reaction output: in the case of replacing benzene with dibenzothiophene, the yield decreases from 75% to 16%. Interestingly only two enantiomers and not meso form are formed in the latter case. The obtained double helicene containing 14 fused rings, exhibits green emission characterized by reasonable fluorescence quantum yield reaching 0.38. This dye has average circularly polarized luminescence brightness (B_CPL) of about 15 M⁻¹ cm⁻¹. The analysis of the electronic structure of the dyes with quantum chemical methods reveals highly-delocalized excited states with the core of the dye acting as a electron-donating moiety.