Large Electro‐Optic Kerr Effect in Ionic Liquid Crystals: Connecting Features of Liquid Crystals and Polyelectrolytes

Abstract

The electro-optic Kerr effect in simple dipolar fluids such as nitrobenzene has been widely applied in electro-optical phase modulators and light shutters. In 2005, the discovery of the large Kerr effect in liquid-crystalline blue phases (Y. Hisakado et al., Adv. Mater. 2005, 17, 96-98.) gave new directions to the search for advanced Kerr effect materials. Even though the Kerr effect is present in all transparent and optically isotropic media, it is well known that the effect can be anomalously large in complex fluids, namely in the isotropic phase of liquid crystals or in polyelectrolyte solutions. Herein, it is shown that the Kerr effect in the isotropic phase of ionic liquid crystals combines the effective counterion polarization mechanism found in polyelectrolytes and the unique pretransitional growth of the Kerr constant found in the isotropic phase of nematic liquid crystals. Maximum Kerr constants in the order of several 10 m V (ten times higher than the Kerr constant of the toxic nitrobenzene and less temperature sensitive than Kerr constants of nematic liquid crystals) make ionic liquid crystals attractive as new class of functional materials in low-speed Kerr effect applications.

Dynamic Kerr and Pockels electro-optics of liquid crystals in nanopores for active photonic metamaterials

“…According to the Landau-de Gennes free energy model for LC phase transitions 67,68 the magnitude of the Kerr effect, as quantified by the Kerr constant, grows as (T − T*) −γ where T* denotes the second-order pretransitional temperature (supercooling limit of the isotropic phase) and γ is a critical exponent equal to 1 in mean-field approximation. [69][70][71][72] Also here for the confined L-C state both the Pockels and the Kerr effect exhibit an anomalous behavior in the vicinity of the paranematic-to-nematic phase transition, see Fig. 3 and 4.…”

Current Topics in Ionic Liquid Crystals

“…Pre‐transitional effects and structures are not only a phenomenon of non‐ionic liquids and LCs, but have also been observed in (ionic) solids, [87–90] ILCs [91,92] and ILs [93–96] so far. Experimental methods [97,98] and computer simulations [95,99–102] showed that also in ILs locally ordered structures can be found and thus were described as nanostructured liquids [96] .…”

“…A further method of prove for pre‐transitional effects is the electro‐optical Kerr effect [91,106–110] describing a similar phenomenon to cybotactic clusters but in the isotropic phase above the phase transitions from the liquid crystalline to the isotropic phase. These pre‐transitional effects are seen in a divergent behaviour of the electrically induced birefringence.…”

Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

“…ILCs are anisotropic fluids with long‐range orientational order caused by Coulombic interactions between cationic headgroups and counterions, nanosegregation between immiscible parts (that is, an ionic headgroup, rigid core, and lipophilic tail), minimization of free volume complemented by van der Waals interactions, π—π, and hydrogen bonding interactions . As ILCs have been reported to serve as a link between neutral liquid crystals and polyelectrolytes, insight into the structure–property relationships of aminocyclopropenium‐derived ILCs should enable better tuning of the corresponding polyelectrolytes carrying aminocyclopropenium units for both batteries and fuel cells, as well as gene‐delivery vectors (Figure ). The manipulation of the three substituents of the deltic guanidinium headgroup of aminocyclopropenium‐based LCs should not only provide a general understanding and tailoring of the bulk self‐assembly of these aromatic cations, but should also lead to ordered oligomeric aminocyclopropenium‐salt‐based catholytes in redox flow batteries .…”