TY - JOUR
T1 - Study of the mesomorphic properties and conductivity of n-alkyl-2-picolinium ionic liquid crystals
AU - Santos, Andreia F. M.
AU - Figueirinhas, João L.
AU - Dias, C. J.
AU - Godinho, Maria H.
AU - Branco, Luis C.
AU - Dionísio, Madalena
N1 - Funding Information:
This work was supported by the Associate Laboratory for Green Chemistry LAQV (UID/QUI/50006/2019), i3N (UID/CTM/50025/2019) and CeFEMAS (UID/CTM/04540/2019), which are financed by national funds from FCT-MCTES and by FEDER funds through the COMPETE 2020 Program. The authors also thank the National Funds through FCT-MCTES and POR Lisboa 2020, under the project numbers PTDC/EAM-AMB/2023/2021, POCI-01-0145-FEDER-007688 and European Interdisciplinary Action (COST Action CA21159, PhoBioS). A. F. M. Santos also acknowledges FCT-MCTES for the PhD Grant (SFRH/BD/132551/2017).
Publisher Copyright:
© 2023
PY - 2023/5/1
Y1 - 2023/5/1
N2 - 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.
AB - 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.
KW - Charge migration mechanisms
KW - Diffusion coefficients
KW - Ionic conductivity
KW - Mesomorphic properties
KW - Phase transformations
KW - Picolinium ionic liquid crystals
UR - http://www.scopus.com/inward/record.url?scp=85149058034&partnerID=8YFLogxK
U2 - 10.1016/j.molliq.2023.121456
DO - 10.1016/j.molliq.2023.121456
M3 - Article
AN - SCOPUS:85149058034
SN - 0167-7322
VL - 377
JO - Journal of Molecular Liquids
JF - Journal of Molecular Liquids
M1 - 121456
ER -