N-arylated 3,5-dihydro-4H-dinaphtho[2,1-c:1',2'-e]azepines: axially chiral donors with high helical twisting powers for nonplanar push-pull chromophores.

Axially chiral, N-arylated 3,5-dihydro-4H-dinaphtho[2,1-c:1',2'-e]azepines have been prepared by short synthetic protocols from enantiopure 1,1'-bi(2,2'-naphthol) (BINOL) and anilines. Alkynes substituted with two N-phenyldinaphthazepine donors readily undergo a formal [2+2] cycloaddition, followed by retro-electrocyclization, with tetracyanoethene (TCNE) to yield donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) featuring intense intramolecular charge-transfer (CT) interactions. A dicyanovinyl derivative substituted with one N-phenyldinaphthazepine donor was obtained by a "one-pot" oxidation/Knoevenagel condensation from the corresponding propargylic alcohol. Comparative electrochemical, X-ray crystallographic, and UV/Vis studies show that the electron-donor qualities of N-phenyldinaphthazepine are similar to those of N,N-dimethylanilino residues. The circular dichroism (CD) spectrum of a push-pull chromophore incorporating the chiral donor moiety features Cotton effects of exceptional intensity. With their elongated shape and the rigidity of the chiral N-aryldinaphthazepine donors, these chromophores are effective inducers of twist distortion in nematic liquid crystals (LCs). Thus, a series of the dinaphthazepine derivatives was used as dopants in the nematic LC E7 (Merck) and high helical twisting powers (beta) of the order of hundreds of microm(-1) were measured. Theoretical calculations were employed to elucidate the relation between the structure of the dopants and their helical twisting power. For the derivatives with two dinaphthazepine moieties, a strong dependence of the beta-values on the structure and conformation of the linker between them was found.