TY - JOUR

T1 - NMR spectroscopic studies of cation dynamics in symmetrically-substituted imidazolium-based ionic liquid crystals

AU - Majhi, Debashis

AU - Komolkin , Andrei V.

AU - Dvinskikh , Sergey V.

N1 - Publisher Copyright: © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

PY - 2020/7/2

Y1 - 2020/7/2

N2 - Ionic liquid crystals (ILCs) present a new class of non-molecular soft materials with a unique combination of high ionic conductivity and anisotropy of physicochemical properties. Symmetrically-substituted long-chain imidazolium-based mesogenic ionic liquids exhibiting a smectic liquid crystalline phase were investigated by solid state NMR spectroscopy and computational methods. The aim of the study was to reveal the correlation between cation size and structure, local dynamics, and orientational order in the layered mesophase. The obtained experimental data are consistent with the model of a rod-shaped cation with the two chains aligned in opposite directions outward from the imidazolium core. The alignment of the core plane to the phase director and the restricted conformations of the chain segments were determined and compared to those in single-chain counterparts. The orientational order parameter S~0.5–0.6 of double-chain ionic liquid crystals is higher than that of corresponding single-chain analogues. This is compatible with the enhanced contribution of van der Waals forces to the stabilization of smectic layers. Increased orientational order for the material with Br− counterions, which exhibit a smaller ionic radius and higher ability to form hydrogen bonds as compared to that of BF4−, also indicated a non-negligible influence of electrostatic and hydrogen bonding interactions. The enhanced rod-shape character and higher orientational order of symmetrically-substituted ILCs can offer additional opportunities in the design of self-assembling non-molecular materials.

AB - Ionic liquid crystals (ILCs) present a new class of non-molecular soft materials with a unique combination of high ionic conductivity and anisotropy of physicochemical properties. Symmetrically-substituted long-chain imidazolium-based mesogenic ionic liquids exhibiting a smectic liquid crystalline phase were investigated by solid state NMR spectroscopy and computational methods. The aim of the study was to reveal the correlation between cation size and structure, local dynamics, and orientational order in the layered mesophase. The obtained experimental data are consistent with the model of a rod-shaped cation with the two chains aligned in opposite directions outward from the imidazolium core. The alignment of the core plane to the phase director and the restricted conformations of the chain segments were determined and compared to those in single-chain counterparts. The orientational order parameter S~0.5–0.6 of double-chain ionic liquid crystals is higher than that of corresponding single-chain analogues. This is compatible with the enhanced contribution of van der Waals forces to the stabilization of smectic layers. Increased orientational order for the material with Br− counterions, which exhibit a smaller ionic radius and higher ability to form hydrogen bonds as compared to that of BF4−, also indicated a non-negligible influence of electrostatic and hydrogen bonding interactions. The enhanced rod-shape character and higher orientational order of symmetrically-substituted ILCs can offer additional opportunities in the design of self-assembling non-molecular materials.

KW - ionic liquids

KW - liquid crystals

KW - Ionic liquid crystals

KW - molecular orientational order

KW - NMR spectroscopy

KW - Ionic liquids

KW - Liquid crystals

KW - Molecular orientational order

UR - https://www.mdpi.com/1422-0067/21/14/5024

UR - http://www.scopus.com/inward/record.url?scp=85088012942&partnerID=8YFLogxK

U2 - 10.3390/ijms21145024

DO - 10.3390/ijms21145024

M3 - Article

VL - 21

SP - 1

EP - 14

JO - International Journal of Molecular Sciences

JF - International Journal of Molecular Sciences

SN - 1422-0067

IS - 14

M1 - 5024

ER -