Are chiral dopants with higher twisting power advantageous to induce wider temperature range of the blue phases?

Abstract

We studied the relationship between the stability of the blue phase (BP) and the helical twisting power (HTP) of chiral molecules doped in a non-chiral bent-core liquid crystal. Contrary to common wisdom, HTP values were found not to directly affect BP stabilisation, i.e. a wide temperature range of BPs was obtained in mixtures containing chiral dopants with relatively small HTPs, whereas the mixtures containing high-HTP chiral dopants showed narrow BP temperature ranges. Moreover, in some cases, the narrow BPs even disappeared with increasing dopant concentration. In contrast, as expected, chiral dopant with large HTPs can easily induce BPs only by small dopant ratio; the minimum concentration necessary to induce the BPs is inversely proportional to an HTP value. As a cause of such unpredictable behaviour, packing effect between a host bent-core molecule and chiral dopants was discussed on the basis of the experimental observation of cybotactic clusters. Effect of UV irradiation for the BP stability was also examined.

Keywords:

• blue phase
• chiral dopant
• helical twisting power
• photo-induced phase transition

Acknowledgement

The authors acknowledge Dr. Y. Aoki of DIC for synthesising A131. They also thank Merck for supplying chiral dopants, CB-15, S811 and ZLI-4572.