Diketonylpyridinium cations as a support of new ionic liquid crystals and ion-conductive materials: analysis of counter-ion effects

© 2016 by the authors, licensee MDPI, Basel, Switzerland. Mercedes Cano thanks the Spanish Ministerio de Economía y Competitividad (project CTQ2011-25172 and CTQ2015-63858) and Complutense University (GR3/14-910300), for funding. María Jesús Pastor acknowledges the Ministerio de Economía y Competitividad for her FPI scholarship. Cristián Cuerva is grateful to the Programa de Financiación de Universidad Complutense de Madrid-Santander Universidades (Spain), for his predoctoral contract. María Jesús Pastor, Cristián Cuerva, José A. Campo and Mercedes Cano thank to Dra. Paloma Ovejero for her help with the synthesis and characterization of the compounds. Rainer Schmidt wishes to express his gratitude to Alberto Rivera-Calzada, Carlos León and Jacobo Santamaría for allowing the use and assistance with the dielectric spectroscopy.
Ionic liquid crystals (ILCs) allow the combination of the high ionic conductivity of ionic liquids (ILs) with the supramolecular organization of liquid crystals (LCs). ILCs salts were obtained by the assembly of long-chained diketonylpyridinium cations of the type [HOO^(R(n)pyH)] + and BF_(4)^(-) , ReO_(4)^(-), NO_(3)^(-), CF_(3)SO_(3)^(-), CuCl_(4)^(2-) counter-ions. We have studied the thermal behavior of five series of compounds by differential scanning calorimetry (DSC) and hot stage polarized light optical microscopy (POM). All materials show thermotropic mesomorphism as well as crystalline polymorphism. X-ray diffraction of the [HOO^(R(12)pyH)][ReO_(4)] crystal reveals a layered structure with alternating polar and apolar sublayers. The mesophases also exhibit a lamellar arrangement detected by variable temperature powder X-ray diffraction. The CuCl_(4)^(2-) salts exhibit the best LC properties followed by the ReO_(4)^(-) ones due to low melting temperature and wide range of existence. The conductivity was probed for the mesophases in one species each from the ReO_(4)^(-) , and CuCl_(4)^(2-) families, and for the solid phase in one of the non-mesomorphic Cl^(-) salts. The highest ionic conductivity was found for the smectic mesophase of the ReO_(4)^(-) containing salt, whereas the solid phases of all salts were dominated by electronic contributions. The ionic conductivity may be favored by the mesophase lamellar structure.
Ministerio de Economía y Competitividad (MINECO)
Universidad Complutense de Madrid
Programa de Financiación de Universidad Complutense de Madrid-Santander Universidades (Spain)
Depto. de Estructura de la Materia, Física Térmica y Electrónica
Fac. de Ciencias Físicas
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