Study of the mesomorphic properties and conductivity of n-alkyl-2-picolinium ionic liquid crystals

Abstract

The impact of the alkyl size moiety in the mesomorphic and conductivity properties was evaluated in 2-picolinium ([Cn-2-Pic]+, n = 6 (n-hexyl), 12 (n-dodecyl) and 16 (n-hexadecyl)) organic salts. While the lengthier chains imprint crystalline order, the shortest [C6-2-Pic][Br] easily bypasses crystallisation, vitrifying on cooling. Furthermore, both [C12-2-Pic][Br] and [C16-2-Pic][Br] were found to be ionic liquid crystals (ILCs) due to the emergence of mesophases identified by Polarised Optical Microscopy and confirmed by X-Ray Diffraction, whereas the high mobility of the shortest member impairs the manifestation of LC behaviour. The temperature dependence of conductivity, between −90 to 160 °C, disclosed multiple types of charge transport, ranging from glass transition assisted mechanism in [C6-2-Pic][Br] to thermally activated mechanisms for the two ILCs. The Arrhenian activation plots of the direct current (dc) conductivity for both [C12-2-Pic][Br] and [C16-2-Pic][Br] are sensitive to the materials’ phase transitions and the respective slopes allowed to determine the activation energies. Moreover, ionic diffusion coefficients were estimated in the T-range where dc conductivity is detected, which, for [C12-2-Pic][Br] and [C16-2-Pic][Br], includes room temperature, contrary to [C6-2-Pic][Br] that is highly affected by electrode polarisation starting at cryogenic temperatures. Nyquist and Bode plots were simulated for [C12-2-Pic][Br] and [C16-2-Pic][Br] at 24 °C by electrical RC circuits, showing a strong dependence of the resistance (R) on the chain length, while the capacitance (C) maintains almost invariant. This supports the hypothesis that charge transport is made through defects, longitudinally to the chains, in the crystalline lattice and between layers, transversally to the chain alignment, in the LC phase. The established correlation between conductivity response and material’s physical state should be considered for a rational design of electronic materials tuned by the size of the alkyl tail.