A comparative study of helical sense and twisting power in low-molar-mass and polymeric chiral nematics

Abstract

The issues of helical sense and helical twisting power (HTP) have been addressed using low molar mass analogues of chiral nematic copolymers containing (S)-(+)-2-methoxy-2-phenylethanol, (S)-(-)-1-phenylethanol, and (R)-(-)-methyl mandelate. As reported by other workers, it is found that the empirical rules proposed by Gray et al. for interpreting helical sense are limited in scope. However, the handedness observed in the new copolymeric system and that of the copolymers reported earlier are consistent with the low molar mass counterparts. The HTP's of chiral nematic mixtures based on (S)-(-)-1 -phenylethanol and (/Z)-(-)-methyl mandelate are much lower than those of the corresponding copolymeric structures reported earlier, which is attributed to hindered rotation imposed by polymerization. In contrast, chiral nematic mixtures containing (S)-(+)-2-methoxy-2-phenylethanol show an HTP value only slightly lower than that of the copolymer. In addition, for all three systems, the selective reflection wavelength is observed to increase with decreasing temperature, consistent with Keating's model and with the molecular statistical theory of Kimura et al.

Detecting, Visualizing, and Measuring Gold Nanoparticle Chirality Using Helical Pitch Measurements in Nematic Liquid Crystal Phases

“…Chen and co-workers reported a related comparison of HTP values between chiral LC polymers and their low molar mass counterparts. 59 By comparing the amount of chiral ligand present (i.e., standardizing the HTP values based on the amount of chiral molecules in each mixture), we found that the NP1ÀLC1 and DS1ÀLC1 mixtures had similar helical pitch values, despite the NP1ÀLC1 containing two to 3 orders of magnitude fewer chiral ligand molecules than the DS1ÀLC1 mixture at the same wt % (e.g., 0.5 or 5 wt %, entry 1 and 2 in Supporting Information Table S1). This indicates that for a meaningful comparison on the efficiency to induce an N* phase in achiral N-LC hosts we need to compare mixtures with the same amount of chiral ligand molecules present at a given concentration in LC1.…”

“…For the HTP values of NP1 and NP2 , we calculated 1522.1 and 91801.1 μm –1 , respectively. Chen and co-workers reported a related comparison of HTP values between chiral LC polymers and their low molar mass counterparts . By comparing the amount of chiral ligand present ( i.e.…”