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Polar Order,Mirror Symmetry Breaking,and Photoswitching of Chirality and Polarity in Functional Bent-Core Mesogens
Authors:Dr. Mohamed Alaasar  Dr. Marko Prehm  Dr. Sebastian Belau  Dr. Nerea Sebastián  Marharyta Kurachkina  Prof. Dr. Alexey Eremin  Changlong Chen  Prof. Dr. Feng Liu  Prof. Dr. Carsten Tschierske
Affiliation:1. Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt Mothes Str. 2, 06120 Halle (Saale), Germany;2. Department of Nonlinear Phenomena, Institute of Physics, Otto von Guericke University Magdeburg, Magdeburg, Germany;3. State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049 P. R. China
Abstract:In recent years, liquid crystals (LCs) responding to light or electrical fields have gained significant importance as multifunctional materials. Herein, two new series of photoswitchable bent-core liquid crystals (BCLCs) derived from 4-cyanoresorcinol as the central core connected to an azobenzene based wing and a phenyl benzoate wing are reported. The self-assembly of these molecules was characterized by differential scanning calorimetry (DSC), polarizing light microscopy (POM), electro-optical, dielectric, second harmonic generation (SHG) studies, and XRD. Depending on the direction of the COO group in the phenyl benzoate wing, core-fluorination, temperature, and the terminal alkyl chain length, cybotactic nematic and lamellar (smectic) LC phases were observed. The coherence length of the ferroelectric fluctuations increases continuously with decreasing temperature and adopts antipolar correlation upon the condensation into superparaelectric states of the paraelectric smectic phases. Finally, long-range polar order develops at distinct phase transitions; first leading to polarization modulated and then to nonmodulated antiferroelectric smectic phases. Conglomerates of chiral domains were observed in the high permittivity ranges of the synclinic tilted paraelectric smectic phases of these achiral molecules, indicating mirror symmetry breaking. Fine-tuning of the molecular structure leads to photoresponsive bent-core (BC)LCs exhibiting a fast and reversible photoinduced change of the mode of the switching between ferroelectric- and antiferroelectric-like as well as a light-induced switching between an achiral and a spontaneous mirror-symmetry-broken LC phase.
Keywords:azobenzenes  ferroelectricity  liquid crystals  mirror symmetry breaking  photoswitching
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