Induced circular dichroism as a probe of handedness in chiral nematic polymer solutions

Achiral dyes in isotropic (acetyl)(ethyl)cellulose (AEC) solutions display no induced CD signals irrespective of acetyl content, polymer concentration or dye content. When dissolved in anisotropic AEC solutions, the dye molecules show strong induced CD bands that disappear when the phases are heated above the anisotropic/isotropic phase transition temperature. The liquid crystal induced circular dichroism (LCICD) spectra for the dyes in a series of well-characterized AEC mesophases, whose handedness depends on solvent and acetyl content, reveal that the sign of the LCICD signal is determined by the supramolecular structural features of the host matrices. Thus, acridine orange (AO) dissolved in a left-handed liquid crystalline AEC solution displays a negative LCICD whereas a positive LCICD was observed when the AO is dissolved in a right-handed AEC mesophase. The sign and intensity of the LCICD signals change with acetyl degree of substitution, solvent and temperature. In all cases, changes in sign of the CD peak correlated with changes in handedness of the chiral nematic structure, and the intensity of the CD peak increased with increasing chiral nematic pitch for a given solvent and polymer concentration. Induced circular dichroism is thus a useful probe of chiral nematic structure in these systems.