Tuning of solubility and gelation ability of oligomeric electrolyte by anion exchange

Polymer Journal (2010) 42, 759–765 & The Society of Polymer Science, Japan (SPSJ) All rights reserved 0032-3896/10 $32.00 www.nature.com/pj ORIGINAL ARTICLE Tuning of solubility and gelation ability of oligomeric electrolyte by anion exchange Nagatoshi Koumura1, Hajime Matsumoto2, Hajime Kawanami3, Nobuyuki Tamaoki4,5 and Masaru Yoshida4 To tune the miscibility of the gel-forming oligomeric electrolyte, we examined anion exchange reactions using appropriate ammonium or alkali-metal salts. Nine oligomeric electrolytes with different anions were obtained in high yields by the reactions. The solubility of these oligomeric electrolytes in organic solvents was carefully tested. Although the starting material with chloride as a counter anion was not soluble in any organic solvents, excellent miscibility and gelation ability of the oligomeric electrolytes with different anions were consequently observed with dipolar protophilic and aprotic solvents such as N,Ndimethylformamide, dimethylsulfoxide and N,N-dimethylacetamide. Furthermore, ionogels based on aliphatic ionic liquids were readily formed using the oligomer with bis(trifluoromethanesulfonyl)amide anion at a 40 g l1 concentration. It is remarkable that the ionic conductivity of the above-mentioned ionogels is almost identical to that of neat ionic liquids, despite the significant increase in the apparent viscosity. This study shows a novel and convenient approach to gelators for multiple solvents. Polymer Journal (2010) 42, 759–765; doi:10.1038/pj.2010.65; published online 4 August 2010 Keywords: anion exchange; ionic conductivity; physical gels INTRODUCTION The gel is a typical soft matter consisting of an excess of adequate liquid (for example, water or organic solvent) and a relatively small amount of gel-forming compounds. The material often shows both quasi-liquid and quasi-solid natures; thus, many potential applications have been pointed out.1–3 Regarding the physical gels, their main feature is a reversible thermal-phase transition between gel and sol states, reflecting the driving force of the gelation based on weak interactions such as hydrogen bonding, p–p stacking, van der Waals forces, charge transfer interactions and electrostatic forces among the gelator molecules.4–7 Although the gel-forming materials based on natural compounds (for example, agar, gelatin) are commercially and inexpensively available in large amounts, they cannot be used for organic solvents because of poor solubility. Difficulty in chemical modification was also observed for such common gel-forming materials. On this account, synthetic gelators promise to improve on the properties of natural gelators by chemical modification of their molecular structure. For instance, when functional groups able to respond to external stimuli are introduced into artificial gelators, the gel–sol transition can be induced by light,8–11 pH change12 or chemical triggers.13,14 Despite such an advantage of the artificial functional gels, large-scale production of synthetic gelators is often limited because of multistep syntheses and tedious purifications. Solvents also control the gel properties. Gels are generally classified into hydrogels and organogels on the basis of the used solvents, that is, water and organic solvents. Recently, ionic liquids have attracted a great deal of attention as a different type of solvent because of their distinctive properties, including chemical and thermal stability, nonvolatility, lower flammability and high ionic conductivity.15–18 Owing to these significant characteristics, there has been extensive research on ionic liquids to clarify their fundamental properties and plausible applications. Several low-molecular-weight19–21 or polymeric22,23 gelators have been developed for ionic liquids to form ionogels, and the applications of ionogel electrolytes for dye-sensitized solar cells showing long-time stability have been reported.24–27 Although there have been reports of amphiphilic gelators for multiple solvents, they are still limited in number.28–30 Recently, we reported a new oligomeric physical gelator, poly[pyridinium-1,4diyl-iminocarbonyl-1,4-phenylene-methylene chloride] 1.Cl, having pyridinium and amide moieties in its main chain (Scheme 1).31 The novel oligomeric gelator 1.Cl was synthesized by a one-pot synthesis involving condensation and subsequent intermolecular quaternization reaction. The ionic gelator 1.Cl showed many unique physical properties, especially rheological behavior, showing self-healing after mechanical collapse.31–33 In our previous report, we have preliminarily shown that the solubility and gelation ability of the oligomeric electrolyte can be tuned by an anion exchange reaction with fluorinated anions such as hexafluorophosphate (PF6) or bis(trifluoromethanesulfonyl)amide (TFSA).31 Although the initial oligomeric electrolyte 1.Cl was soluble only in hot water, the products of the 1Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan; 2Research Institute for Ubiquitous Energy Devices, AIST, Ikeda, Osaka, Japan; 3Research Center for Compact Chemical System, AIST, Sendai, Miyagi, Japan and 4Nanosystem Research Institute, AIST, Tsukuba, Ibaraki, Japan 5Current address: Research Institute for Electronic Science, Hokkaido University, Sapporo, Hokkaido, Japan. Correspondence: Dr M Yoshida, Nanosystem Research Institute, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. E-mail: Received 10 February 2010; revised 16 June 2010; accepted 18 June 2010; published online 4 August 2010 New ionic gelators by anion exchange N Koumura et al 760 O NH2 + N Cl Cl O CH2 Cl2 N O Cl Et3N N N N H H n Cl 1·Cl Scheme 1 Synthesis of oligomeric electrolyte 1.Cl. anion exchange reactions were eventually insoluble in water but readily soluble in polar organic solvents. The gelation ability of oligomeric electrolytes 1.PF6 and 1.TFSA was also briefly shown for several organic solvents and ionic liquids. The results prompted us to further research multisolvent gelators on the basis of chemical tuning by anion exchange using different types of salts. Herein, we report a number of oligomeric electrolytes with other anions, prepared from 1.Cl as a structural scaffold, to prove the usefulness of the anion exchange method. The miscibility and gelation property of the oligomeric electrolytes for several organic solvents and ionic liquids are also described. EXPERIMENTAL PROCEDURE General procedure 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance-400 (Bruker, Karlsruhe, Germany) (1H: 400 MHz, 13C: 100 MHz). Chemical shifts are denoted in d-units (p.p.m.) relative to DMSO-d6. 19F NMR was recorded on a JEOL ECA300 (JEOL, Tokyo, Japan) (19F: 300 MHz) with CFCl3 as an internal standard. The solvents were distilled and dried, if necessary, by standard methods. The reagents, including ionic liquids, (...truncated)