Design of Ionic Liquid Crystals Forming Normal-Type Bicontinuous Cubic Phases with a 3D Continuous Ion Conductive Pathway

Abstract

We have prepared a series of pyridinium-based gemini amphiphiles. They exhibit thermotropic liquid–crystalline behavior depending on their alkyl chain lengths and anion species. By adjusting the alkyl chain lengths and selecting suitable anions, we have obtained an ionic amphiphile that exhibits a normal-type bicontinuous cubic phase from 38 °C to 12 °C on cooling from an isotropic phase. In the bicontinuous cubic liquid–crystalline assembly, the pyridinium-based ionic parts align along a gyroid minimal surface forming a 3D continuous ionic domain while their ionophobic alkyl chains form 3D branched nanochannel networks. This ionic compound can form homogeneous mixtures with a lithium salt and the resultant mixtures keep the ability to form normal-type bicontinuous cubic phases. Ion conduction measurements have been performed for the mixtures on cooling. It has been revealed that the formation of the 3D branched ionophobic nanochannels does not disturb the ion conduction behavior in the ionic domain while it results in the conversion of the state of the mixtures from fluidic liquids to quasi-solids, namely highly viscous liquid crystals. Although the ionic conductivity of the mixtures is in the order of 10–7 S cm–1 at 40 °C, which is far lower than the values for practical use, the present material design has a potential to pave the way for developing advanced solid electrolytes consisting of two task-specific nanosegregated domains: One is an ionic liquid nano-domain with a 3D continuity for high ionic conductivity and the other is ionophobic nanochannel network domains for high mechanical strength.

Functional Ionic Liquid Crystals

“…85 pyridinium-based gemini amphiphiles containing various counteranions such as BF 4 − , TfO − , and Tf 2 N − . 86 Unfortunately, the ionic conductivities exhibited by these Cub bi LCs were 3 orders of magnitude lower (10 −7 S cm −1 ) than those of earlier reported analogous materials. Although the reasons for the lower ionic conductivities of these Cub bi LCs (10 −7 S cm −1 ) were not discussed, they might be explained as follows.…”

“…Cano and co-workers reported Sm A LCs with ion-conductive 2D nanochannels formed from the salts of pyrazolium cations bearing long alkyl chains with various counteranions such as BF 4 – , Cl – , ReO 4 – , TfO – , and TsO – . Ichikawa et al prepared Cub bi LCs with interconnected 3D nanochannels from the salts of pyridinium-based gemini amphiphiles containing various counteranions such as BF 4 – , TfO – , and Tf 2 N – . Unfortunately, the ionic conductivities exhibited by these Cub bi LCs were 3 orders of magnitude lower (10 –7 S cm –1 ) than those of earlier reported analogous materials.…”

Current Topics in Ionic Liquid Crystals

“…Moreover, cubic ILCs have emerged as promising candidates as well. Cubic phases can exhibit high ionic conductivities even at rather low temperatures and at rather low fractions of liquid electrolyte [264–267] …”

Discotic Star Mesogen with Thymine Nucleobases Exhibiting a Rare Gyroid Cubic Mesophase with 3D Conductivity

“…[19] They are of high interest because monodomains of such assemblies do not need alignment for charge or ion transport owing to their 3D channel network. [20][21][22][23][24][25][26] Bicontinuous cubic Ia � 3d phases of discotic mesogens are extremely rare and were first discovered by Aida et al in a series of imidazolium salt decorated triphenylenes. [21] Their formation is rationalized by the general phase diagram for two segregating building blocks.…”