The Magic 4-Cyanoresocinols - Their Role in the Understanding of Phenomena at the Rod-Banana Cross-Over and Relations to Twist-Bend Phases and Other Newly Emerging LC Phase Types

ABSTRACT

In this account, bent-core molecules involving the polar cyano-group are reviewed with a focus on the intensely investigated class of 4-cyanoresorcinol bisbenzoates. Cybotaxis and phase biaxiality of their nematic phases, the transition from nematic to smectic phases and the development of tilt and polar order in their smectic phases are discussed. Of special interest are heliconical (twist–bend) smectic intermediate phases occurring at the transition from paraelectric to antiferroelectric polar smectic phases if coinciding with the transition between anticlinic and synclinic tilt correlation. For some compounds, a reentrant polar SmA phase (SmA^LTP_F) is observed below SmC and a silylated compound forms a new leaning-type phase. The formation of these achiral phases instead of the polar SmC phases involving layer chirality is discussed under the aspect of enantiophobic vs. enantiophilic organization. The effects of core structure and chain branching and, in addition, the effects of the intrinsic transient molecular, superstructural, and permanent molecular chirality on LC self-assembly are discussed.

KEYWORDS:

• Bent-core liquid crystals
• paraelectric and ferroelectric liquid crystals
• biaxiality
• synclinic and anticlinic tilt correlation
• mirror symmetry breaking
• heliconical phases
• twist–bend smectic phases
• diastereomerism
• chirality
• 4-cyanoresorcinol
• leaning phase
• magnetic field effects

Acknowledgments

The work reviewed in this manuscript is the result of several Master’s and PhD students (C. Keith, A. Lehmann, E. Westphal, M. Poppe), postdocs (S. Poppe, M. Prehm) and guest researchers (M. Alaasar, G. Shanker) over the recent decade working at Halle University and the collaboration with numerous scientists and their groups from different other places (N. Clark, Boulder; M.K. Das, Siliguri; A. Eremin, Magdeburg; J. Lagerwall, Luxemburg; F. Liu, Xi’an; H. Ocak and B. Bilgin-Eran, Istanbul and J. K. Vij, Dublin). Special thanks to M. Alaasar and H. Ocak, who designed and synthesized the azobenzene-based and chiral BCLCs, respectively, and to J. K. Vij who has supervised most of the electrooptical investigations and switching experiments. The authors work in this area has been supported by the EU with the FP7 funded collaborative project BIND (216025) and the Deutsche Forschungsgemeinschaft (Ts 39/24-1/2).

Disclosure statement

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

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Funding

This work was supported by the Deutsche Forschungsgemeinschaft [Ts 39/24]; Seventh Framework Programme [216025].