Synthesis and Chiroptical Properties of Chiral Boron Complexes and Nitrogen-Containing Helicenes

In recent years, chiral dyes capable of showing circularly polarized luminescence (CPL) have received considerable attention due to their potential applications to 3D displays, security paints, information storages, and bioimaging. CPL was usually evaluated by a dissymmetry factor, glum, and chiral dyes showing high glum values have been developed. Attachment of chromophores to a chiral platform is one of the convenient methods to prepare chiral dyes, and pyrenes, BODIPYs, and porphyrins linked with axially chiral binaphthyls and cyclodextrins showed CPL in the region of the chromophores. On the other hand, helicenes, ortho-fused aromatics, show intense CPL originating from the inherent chirality. We developed boron dyes with carbazole-1,3-azole ligands showing solid-state fluorescence, and later we found the Et2AlCl-mediated reaction to give chiral boron dyes with (R)- or (S)-BINOL showing CPL both in solution and solid states. We also developed azahelicene-fused boron dyes, whose azahelicene framework was newly synthesized via the intramolecular oxidative coupling of 3,6-bis(biphenyl-2-yl)carbazole. The short-step synthesis of the azahelicene allowed for the preparation of a variety of derivatives including cyclic azahelicene dimers, π-extended helicenes, and multiple helicenes. Here the synthesis and chiroptical properties of these chiral boron complexes and azahelicenes.