New optically active imine dopants derived from (R)-(+)-1-(1-naphthyl)ethylamine: Relation between their large helical twisting power and internal rotation potential profiles

The imine based aromatic dopants derived from (R)-1-phenylethylamine show the largest helical twisting power (HTP) among the known chiral compounds having chiral centers doped in nematic liquid crystals. In order to clarify the relationship between molecular structure and HTP, conformational search for imine based model molecules was carried out. The computational results indicated that the model compound, (R)-N-benzylidene-1-phenylethanamine (M1) is characterized by having one energy minimum conformation. In (R)-N-benzylidene-1-(naphthalen-1-yl)ethanamine (M2) where benzene ring in M1 is replaced by naphthalene ring, the conformation was found to exhibit small number of local minima and relatively high rotational barrier around the single bond between chiral center and its aromatic end group. Experimental results showed that HTP of M2 (13 μm−1) is fairly higher than that of M1 (3 μm−1). Further, several dopants derived from R-(+)-1-(1-naphthyl)ethylamine also showed the markedly high HTP of around 60 μm−1 which is larger by 20 μm−1 than those of the corresponding R-(+)-1-phenylethylamine homologues. It was thus concluded that the chiral center neighboring to the mesogenic part is the most important factor for high HTP of the imine based dopants.