Chapter 11
Columnar Liquid Crystalline Imidazolium Salts: Self-Organized One-Dimensional Ion Conductors 1
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Masafumi Yoshio , Tomohiro Mukai , Hiroyuki Ohno , and Takashi Kato * Downloaded by UNIV OF MELBOURNE on October 13, 2014 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0975.ch011
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Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Department of Biotechnology, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo 184-8588, Japan 2
A series of columnar liquid crystalline (LC) imidazolium tetrafluoroborate salts, 1-methyl-3-{3,4,5-tris(alkyloxy) -benzyl}imidazolium tetrafluoroborate (Im-n; η = 6-18), where η is the carbon number of the aliphatic alkyl chains, have been prepared and studied as anisotropic ion-conductors. Their self-assembled structures in the fluid columnar L C states have been examined by X-ray diffraction measurements. In the columnar phases, the imidazolium portions self-assemble into the inner part of the columns through nano-segregation between the ionic and non-ionic parts driven by electrostatic interactions. The columns can be oriented macroscopically in two directions by different methods: the direction parallel to a glass surface by mechanical shearing; the direction perpendicular to the modified surface of a glass substrate with 3-(aminopropyl)triethoxysilane. Anisotropic ionic conduc -tivities have been measured for columnar ionic liquids Im-8 and Im-12 that are aligned parallel to the surface of the glass substrate. These columnar ionic liquids have been found to function as one-dimensional ion-conductors.
© 2007 American Chemical Society
In Ionic Liquids IV; Brennecke, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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Introduction Ionic liquids are functional liquids that are non-volatile and highly ion conductive. They have advantages for applications as reaction solvents for organic syntheses (7-7) and as electrolytes for energy devices such as batteries and capacitors (8-13). For further functionalization of ionic liquids, introduction of self-organized structures such as liquid crystalline (LC) ordering would be important (14-16). Two types of L C materials based on ionic liquids have been reported (7735). The first example is the organic salts chemically modified with appropriate aliphatic and/or aromatic groups. A large variety of thermotropic ionic liquid crystals have been known (36-39). For thermotropic ionic liquid crystals exhibiting low melting temperatures, Seddon et al. reported on long-chain alkyl modified imidazolium and pyridinium salts containing perfluorinated anions such as tetrafluoroborate and hexafluorophosphate anions. They show smectic L C phases at room temperature range (19-21). These materials are called liquid crystalline ionic liquids. The second type is self-assembly of non-mesogenic ionic liquids with mesogenic molecules partially miscible with ionic liquids (5555). We previously described that the mixing of a conventional ionic liquid, 1ethyl-3-methylimidazolium tetrafluoroborate, and hydroxy-functionalized mesogenic molecules leads to the formation of nano-layered ionic liquids exhibiting two-dimensional ionic conduction (34,35). For these materials, liquid crystalline phases are induced by nano-segregation of block molecules (40,41). Intermolecular interactions such as hydrogen binding (42,43) and ion-dipolar interactions (44-48) play key roles in the formation of nanosegregated L C structures (14). Here we report on self-assembled structures of columnar ionic liquids, a series of fan-shaped imidazolium tetrafluoroborate salts containing 3,4,5tris(alkyloxy)benzyl groups (Im-«; η = 6-18). In our previous communication (57), we reported on the L C and anisotropic ion-conductive properties of Im-8 and Im-12. No columnar L C ionic liquids based on imidazolium salts had been reported until we developed columnar ionic liquids (57) and a polymerizable columnar ionic liquid (32) as a new family of one-dimensional ion-conductors. In addition, one-dimensional ion transport was achieved for the columnar ionic liquids forming fluid columnar L C states. Such function may be useful for the development of energy devices with nanoscale order. For the preparation of these columnar materials showing anisotropic functions, molecular design (shape, interactions, and phase-segregation) and control of macroscopic columnar orientations is essential (14,15).
In Ionic Liquids IV; Brennecke, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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Results and Discussion
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Synthesis and Liquid Crystalline Properties Fan-shaped imidazolium salts Im-#i (n = 6-18) shown in Figure 1 were prepared by a quaternization reaction of TV-methyl imidazole and 3,4,5tris(alkyloxy)benzyl chloride, followed by anion exchange with silver tetrafluoroborate (31). A l l of Im-w show hexagonal columnar liquid crystalline phases (Colh) both on heating and cooling. The phase transition behavior of Imw is summarized in Table I. Figure 2 shows transition temperatures as a function of the aliphatic chain length (w) of Im-w on heating. Only Im-6 and Im-8 form glassy phases forming C o l order. On the increase of the chain length (w = Ι Ο Ι 8) the melting points increase from -10 to 88 °C. The clearing points show a maximum point for w = 14. It is found that the thermal température range of the C o l phase is tunable by changing the alkyl chain length for the salts. h
h
CH (CH ) _ 0 3
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M
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CH (CH )„_ 0.^ 3
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CH (CH ) . 0 3
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Im-#i (#i = 6-18) Figure 1. Molecular structure of l-methyl-3-{3,4,5tris(alkyloxy)}benzylimidazolium tetrafluoroborate salts Im-n, where η is the alkyl chain length (n = 6, 8, 10, 12, 14, 16, 18).
Self-assembled columnar structures were examined by X-ray diffraction measurements. The results are summarized in Table II. Three peaks with the ratio of 1: 1Λ/3: 1/2 and a broad halo were obtained for all of Im-w, which indicate the formation of a disordered hexagonal columnar packing. The intercolumnar distance (a) was calculated with the following equation: a = 2/V3,
