Elastic and electro-optical properties of flexible fluorinated dimers with negative dielectric anisotropy

ABSTRACT

We report the synthesis and the temperature dependencies of optical, dielectric, and elastic properties of four homologous dimeric mesogens in the uniaxial nematic (N) phase. The studied dimers are derived from 2ʹ,3ʹ-difluoroterphenyl units connected via flexible alkyl chains. The compounds differ in the length of either the flexible alkyl chain or the terminal groups. These achiral bimesogens exhibit the twist-bend nematic (N_TB) phase with a nanoscale pitch of the modulated nematic director, as established by the freeze-fracture transmission electron microscopy. All four homologs have negative dielectric anisotropy owing to the permanent dipole oriented normal to the long molecular axes. A longer flexible bridge length yields an expanded temperature range of the N phase, stronger dielectric anisotropy, larger birefringence and bend elastic constant $K_{33}$. Longer alkyl terminal chains increase birefringence but decrease the absolute value of dielectric anisotropy and $K_{33}$. All four bimesogens show a dramatic decrease of $K_{33}$ down to ~0.5 pN on approaching the N-N_TB transition. A longer bridge yields a longer period of twist-bend modulation in the N_TB phase.

Graphical abstract

KEYWORDS:

• Twist-bend nematic
• liquid crystal
• fluorinated flexible dimers
• elastic moduli
• birefringence
• dielectric anisotropy

Acknowledgments

The freeze-fracture transmission electron microscopy studies were performed at the (cryo) TEM facility at the Advanced Materials and Liquid Crystal Institute, Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surfaces in Advanced Materials.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed here.

Additional information

Funding

The work was supported by the NSF grant [ECCS-1906104].