Relationship between Chemical Structure and Helical Twisting Power in Optically Active Imine Dopants Derived from ($R$)-(+)-1-(1-Naphthyl)ethylamine

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In this study, we prepared a series of imine-based chiral dopants derived from ($R$)-(+)-1-(1-naphthyl)ethylamine, which are soluble in liquid crystal solvent, and examined their helical twisting power (HTP) for a typical nematic host, 4$'$-hexyloxy-4-biphenylcarbonitrile (6-OCB). The chiral dopants basically consist of the mesogenic aromatic part, which is directly bound to the chiral naphthylethyl part by imine linkage, and the alkyl tail part. These mono-imine derivatives have the following features. (1) The extension of the aromatic part drastically increases the HTP and (2) the attachment of the alkoxy tail part is also effective as HTP inducers, indicating the importance of structural similarity between the liquid crystal host molecule and the chiral dopant. HTPs were also examined for di-imine derivatives with two chiral parts at both edges of the mesogenic part. In this case, HTPs are variable from 65 to 140 μm-1, depending on the conformation of the central mesogenic group that produces the delicate coordination of the two chiral parts. The maximum value of 140 μm-1 may be one of the largest values among the chiral dopants that have thus far been prepared on a basis of the asymmetric carbon. By mixing a low percentage of this dopant, the resulting chiral nematic liquid crystals can possess pitches comparable to the wavelength of visible light.
2007-08-15