Surface Alignment, Anchoring Transitions, Optical Properties and Topological Defects in Nematic Bent-Core Materials C7 and C12

Abstract

We address the status of oxadiazole mesogens, C7 and C12, reported to show the biaxial nematic phase, by exploring material aspects (chemical stability, surface anchoring, optical and dielectric properties, topological defects) linked to the type of nematic order. We demonstrate that the isogyres splitting in conoscopic patterns of homeotropic state depends on sample thickness and is associated with variations of molecular tilt along the normal to substrates. We observe isolated topological point defects (boojums and hedgehogs), as well as nonsingular “escaped” disclinations pertinent only to the uniaxial nematic order. Our conclusion is that C7 and C12 feature only a uniaxial nematic phase and the apparent biaxiality is caused by surface effects.

Keywords:

• Anchoring transition
• bent-core nematics
• biaxial phase
• conoscopy
• topological defects

Acknowledgment

The research was supported by Department of Energy, grant DOE-FG02-06ER 46331. We thank Samsung Electronics Corporation for the provided materials. We thank V. M. Pergamenshchik, D. W. Allender, S. Kumar, Yu. A. Nastishin, and M. Mathews for discussions and suggestions.

We wish to thank the Kresge Foundation and donors to the Kresge Challenge Program at The University of Akron, The Ohio Board of Regents and The National Science Foundation (CHE-0341701 and DMR-0414599) for funds used to purchase the NMR instruments used in this work. We thank C. Wesdemiotis and V. Scionti for the help with HPLC/MS analysis.