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-hexy l isocyanate) (PHIC)was converted to the cholesteric state by doping w ith a variety of chiral small molecules and with optically active poly isocyanates. Circular dichroism experiments in dilute isotropic PHIC s olutions show that several of the small molecules used as dopants in t he Liquid crystal work cause an excess of one helical sense in the oth erwise helically dynamically racemic PHIC. Both the helical sense and excess of this sense in dilute solution correlate to the cholesteric s ense and twisting power of the same dopants in the concentrated PHIC L iquid crystal solutions. The experimental results are consistent, with a mechanism of cholesteric formation in which the chiral dopants disp lace slightly the otherwise equal population of the dynamically interc onvertable mirror helical senses of the PHIC, which is then further am plified by the Liquid crystal matrix. Direct evidence for the amplific ation of the intrinsic helical sense excess of the polyisocyanate by t he Liquid crystal state could be demonstrated by comparing the cholest eric properties of systems doped with polyisocyanates in which the rat io of the mirror helical senses of the dopants in two compared experim ents are identical but in one experiment the helical senses could inte rconvert and in a separate experiment could not interconvert. The ampl ification mechanism is driven by the reduction in population of the mo bile kinked helical reversals which likely act as ''bad neighbors'' to the local liquid crystal organization.