Synthesis and Investigation of Nematic Liquid Crystals with Flexoelectric Properties

Abstract

We describe the synthesis and characterization of a series of model nematic liquid crystals with transverse dipole moments used to study the flexoelectric effect in guest−host mixtures with commercial liquid crystal host. The flexoelectric coefficicent of the mixtures, containing only 10% by weight of the dopant, are up to 6 times higher than those of the pure hosts. The length, bend angle, and dipole moments of the molecules are systematically varied to investigate any correlations with the flexoelectric effect. We find that the flexoelectric coefficients increase with molecular length, are inversely correlated with the bend angle, and are independent of the dipole moment of the dopant. Although these findings seem to contradict predictions from dipolar flexoelectric theories, they can be reconciled by considering the properties of both the guest and host in the mixture. Thiophenes and dimesogens show particularly large flexoelectric effects. This work should inform the molecular design of new materials with enhanced flexoelectric properties.

Rod-shaped dopants for flexoelectric nematic mixtures

“…However, the induced values of the flexoelectric coefficients were small, perhaps due to the weak flexoelectric properties of the nematic host. Nematic liquid crystals with a banana shape, such as the 2,5-disubstituted thiophenes (9) and (10), were reported to exhibit small-to-relatively-large flexolelectric coefficients in either the pure state or as dopants in E7 at room temperature (21)(22)(23). The requirement for molecules with a banana shape appears to be confirmed by studies of the photo-induced cis-and trans-isomers of azo compounds (24).…”

“…The requirement for molecules with a banana shape appears to be confirmed by studies of the photo-induced cis-and trans-isomers of azo compounds (24). However, there are indications that linear molecules with a rigid rod-like aromatic core of limited shape anisotropy may also be useful dopants to induce large values of the flexoelectric coefficients (22). However, the liquid crystalline dopants studied were exclusively almost completely apolar or possessed a significant dipole moment across the long molecular axis.…”

“…Several bistable nematic prototypes (8,12) were demonstrated using homologues of the calamitic 4-nalkyl-4¢-cyanobiphenyl nematic liquid crystals, such as 4-cyano-4¢-pentylbiphenyl (K15) and 4-cyano-4¢-heptyloxybiphenyl (2), synthesised over thirty years ago. Surprisingly only a few nematic and chiral nematic liquid crystals, such as the compounds (3)(4)(5)(6)(7)(8)(9)(10) shown in Table 1, have been synthesised specifically to study the flexoelectric properties of their nematic phases (14)(15)(16)(17)(18)(19)(20)(21)(22)(23). The values of the flexoelectric coefficient of the nematic phase required for use in individual types of LCD is either not known or is commercially sensitive information.…”

“…A measured using the nematic mixture E7 at room temperature using the HAN method (22). B measured using pure MBBA (1) at room temperature (13).…”

“…J measured using pure two-ring 2,5-disubstituted thiophene (9) at room temperature using the HAN method (21). K measured for a nematic mixture of the three-ring 2,5-disubstituted thiophene (10) (10 wt%) in E7 at room temperature using the HAN method (using e = 0.76 C N -1 m -1 for E7) (22).…”