Ionic Liquid Crystals: Self-assembly of Imidazolium Salts Containing an l-Glutamic Acid Moiety

Abstract

Liquid-crystalline (LC) behavior and ionic conductivities of imidazolium salts containing an L-glutamic acid moiety have been studied. The ionic conductivities of the (CF 3 SO 2 ) 2 N À salt forming the columnar LC phase are higher than those of the Br À salt in the columnar LC state. The ionic conductivity shows a sudden decrease at the columnar-micellar cubic LC transition of the Br À salt.Self-assembly of liquid-crystalline (LC) molecules with functional moieties is one of the versatile approaches to the preparation of functional soft materials with dynamic and anisotropic nature.1-4 Ionic liquid crystals 5-8 have attracted attention because they have potentials for applications as ion conductors, 6 redox-active materials, 7 and ordered solvents. 8 They form a variety of LC nanostructures 1,2,9 such as micelles, cylinders, layers, and bicontinuous cubic structures through ionic interactions and nanosegregation between ionic and non-ionic moieties.Recently, we have reported on one-dimensional ion conduction in ionic columnar liquid crystals, 6a-6d 1-methyl-3-[3,4,5-tris(alkyloxy)benzyl]imidazolium salts containing BF 4 À , PF 6 À , CF 3 SO 3 À , and (CF 3 SO 2 ) 2 N À (TFSI À ). The LC properties and ionic conductivities were strongly dependent on the properties of counter anions. To obtain highly ion-conductive materials, the use of TFSI À is considered to be desirable because of its negative charge delocalization and weak electrostatic interactions with cations.10 However, the preparation of LC imidazolium TFSI À salts that form columnar phases in wide temperature ranges has not yet been achieved. 6a-6c For the preparation of self-assembled materials that exhibit stable mesophases, it should be important to control intermolecular interactions and the balance of volume fraction of ionic and non-ionic parts of the molecules. 2,9,11 Our molecular design here is to attach a fan-shaped L-glutamic acid derivative bearing bis(alkyloxy)phenyl moieties 12 to imidazolium salts (Figure 1). The glutamic acid derivative can be used as a molecular building block to produce stable columnar and micellar cubic LC materials upon hydrogen bonding.12 In addition, the incorporation of a more bulky lipophilic part than those of previously reported ionic molecules 6a-6c into imidazolium salts may induce a structural change from columnar to micellar cubic phases, leading to the on-off switch 3d of ion conduction (Figure 2). Herein, we report on self-assembly of imidazolium-based ionic liquid crystals 1 and 2 exhibiting stable columnar and cubic LC phases and their ionic conductivities.The LC properties of 1 and 2 are summarized in Table 1. Compound 1 exhibits columnar and cubic phases, while compound 2 shows only a columnar phase in the temperature range that is wider than those of previously reported imidazoliumbased LC salts with TFSI À . 6b The X-ray diffraction (XRD) patterns confirm that the LC phases of 1 and 2 at 80 C are hexagonal columnar phases. The intercolumnar distances for both of 1 and 2 are 50 Å . The XRD pattern o...

Spontaneous bulk organization of molecular assemblers based on aliphatic polyether and/or poly(benzyl ether) dendrons

“…The Kato group reported diverse assembling dendritic systems based on Percec-type monodendron units. [32][33][34] Their self-assembled ionic materials have mostly involved low-dimensional columnar structures as a supramolecular motif. For example, ionic monodendrons 15 and 16 bearing imidazolium salt at the focal group self-organized into a hexagonal columnar structure where the one-dimensional cylindrical core domain mediated ionic transportation (Figure 9a).…”

Supramolecular Structures in the Presence of Ionic Liquids

“…The structure of imidazolium ionic LCs is much simplex, which makes its application in many fields confined. In order to resolve this problem, much effective effort has been made (Goossens et al, 2008;Kumar & Pal, 2005;Pal & Kumar, 2006;Seo et al, 2007;Yasuda et al, 2010;Yazaki et al, 2008b;Yazaki et al, 2010;Yoshio et al, 2004;Yoshio et al, 2006;Yoshio et al, 2007;Yoshizawa et al, 2004;Zhang et al, 2008d), especially the work of Kato and collaborators (Yasuda et al, 2010;Yazaki et al, 2008b;Yazaki et al, 2010;Yoshio et al, 2004;Yoshio et al, 2006;Yoshio et al, 2007). They observed the appearance of a hexagonal columnar LC structure in 1-methylimidazolium ILs with tri-alkoxy substituted phenyl groups, a class of LC mesogers, attached to the nitrogen atom Yoshio et al, 2006;Yoshio et al, 2007).…”

“…Besides a wide temperature range of LC phase Yoshio et al, 2006;Yoshio et al, 2007) and tunable feasible adjustment of LC properties by altering counter anions (Yoshio et al, 2007), the synthesized compounds in columnar LC state were highly anisotropic ion-conductive in one-dimension Yoshio et al, 2006;Yoshio et al, 2007), which would be useful for transportation of ions, energy, and information at the nanometer level. Recently, Kato et al (Yazaki et al, 2008b) prepared two new 1-methylimidazolium-based ionic LCs, containing an L-glutamic acid moiety and bis(alkyloxy)phenyl groups ( Figure 17). The ionic conductivities and LC structures of the obtained 1-methylimidazolium ILs could be conveniently adjusted by anions, which would potentially be used as an on-off switch of ions ( Figure 17) (Yazaki et al, 2008b).…”

“…Recently, Kato et al (Yazaki et al, 2008b) prepared two new 1-methylimidazolium-based ionic LCs, containing an L-glutamic acid moiety and bis(alkyloxy)phenyl groups ( Figure 17). The ionic conductivities and LC structures of the obtained 1-methylimidazolium ILs could be conveniently adjusted by anions, which would potentially be used as an on-off switch of ions ( Figure 17) (Yazaki et al, 2008b). Besides imidazolium ionic LCs, other thermotropic ionic LCs based on pyridinium (Bowlas et al, 1996;Cruz et al, 2000;Cui et al, 2002;Gordon et al, 1998;Neve et al, 2000;Neve et al, 2001;Taubert, 2004), quaternary ammonium (Alami et al, 1993;Arkas et al, 1999;Lu et al, 1997;Pott & Meleard, 2009;Tittarelli et al, 1997), quaternary phosphonium (Abdallah et al, 2000;Chen et al, 2002;Gowda et al, 2004), pyrrolidinium (Goossens et al, 2009), dithiolium (Artzner et al, 1997) and guanidinium ILs (Kim et al, 2005;Mathevet et al, 2005;Sauer et al, 2008) have also been explored.…”

Functional liquid-crystalline polymers and supramolecular liquid crystals

“…[161][162][163][164][165][166][167] Some of folic acid derivatives show ion-responsive phase transitions from smectic to columnar phases. 161 The design of these supramolecular assemblies has been extended to ion-conductive 164,166 and luminescent 167 materials.…”