Mechanism of the Transformation of a Stiff Polymer Lyotropic Nematic Liquid Crystal to the Cholesteric State by Dopant-Mediated Chiral Information Transfer
In this work the lyotropic liquid crystal nematic state of poly(n-hexyl isocyanate) (PHIC) was converted to the cholesteric state by doping with a variety of chiral small molecules and with optically active polyisocyanates. Circular dichroism experiments in dilute isotropic PHIC solutions show that several of the small molecules used as dopants in the liquid crystal work cause an excess of one helical sense in the otherwise helically dynamically racemic PHIC. Both the helical sense and excess of this sense in dilute solution correlate to the cholesteric sense and twisting power of the same dopants in the concentrated PHIC liquid crystal solutions. The experimental results are consistent with a mechanism of cholesteric formation in which the chiral dopants displace slightly the otherwise equal population of the dynamically interconvertable mirror helical senses of the PHIC, which is then further amplified by the liquid crystal matrix. Direct evidence for the amplification of the intrinsic helical sense excess of the polyisocyanate by the liquid crystal state could be demonstrated by comparing the cholesteric properties of systems doped with polyisocyanates in which the ratio of the mirror helical senses of the dopants in two compared experiments are identical but in one experiment the helical senses could interconvert and in a separate experiment could not interconvert. The amplification mechanism is driven by the reduction in population of the mobile kinked helical reversals which likely act as “bad neighbors” to the local liquid crystal organization.
