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Goossens, Karel,Rakers, Lena,Heinrich, Benoî,t,Ahumada, Guillermo,Ichikawa, Takahiro,Donnio, Bertrand,Shin, Tae Joo,Bielawski, Christopher W.,Glorius, Frank American Chemical Society 2019 Chemistry of materials Vol.31 No.23
<P>We describe the synthesis, supramolecular organization, and thermal characteristics of an unprecedented family of symmetric 1,2,3,4,5-pentaalkylimidazolium ([(C<SUB><I>n</I></SUB>)<SUB>5</SUB>im]<SUP>+</SUP>) salts equipped with halide, nitrate, or pentacyanocyclopentadienide ([Cp(CN)<SUB>5</SUB>]<SUP>−</SUP>) counterions. Salts containing relatively small anions were obtained as low-melting solids, whereas those with [Cp(CN)<SUB>5</SUB>]<SUP>−</SUP> anions were found to be ionic liquids even below room temperature. A permethylated derivative, [(C<SUB>1</SUB>)<SUB>5</SUB>im][Cp(CN)<SUB>5</SUB>], proved to be exceptional. Upon heating, the salt self-organized into a new type of organic ionic plastic crystal (OIPC) mesophase, which was termed M<SUB>hex</SUB> and whose anisotropic structure featured hexagonally ordered, rotating anionic stacks positioned within a continuum composed of disordered cations. The structure of the mesophase resembles that of classical columnar liquid-crystalline phases, despite the absence of long, flexible chains. In the M<SUB>hex</SUB> phase, the cations surrounding the anionic columns effectively fulfill the role of “softening” structural constituents, much in the same way as flexible chains. The discovery of the novel mesophase, which displays a two-dimensional, and thus intrinsically anisotropic, lattice resulting from the rotation of entire ionic assemblies around a columnar axis, represents a new paradigm in the field of OIPCs. Relatively high ionic conductivities were measured in the M<SUB>hex</SUB> phase, particularly after doping with the corresponding sodium salt, Na[Cp(CN)<SUB>5</SUB>], demonstrating the materials’ potential for use in electrochemical applications such as sodium-ion batteries.</P> [FIG OMISSION]</BR>