Ion conformation and orientational order in a dicationic ionic liquid crystal studied by solid-state nuclear magnetic resonance spectroscopy

Abstract

Ionic liquids crystals belong to a special class of ionic liquids that exhibit thermotropic liquid-crystalline behavior. Recently, dicationic ionic liquid crystals have been reported with a cation containing two single-charged ions covalently linked by a spacer. In ionic liquid crystals, electrostatic and hydrogen bonding interactions in ionic sublayer and van der Waals interaction in hydrophobic domains are the main forces contributing to the mesophase stabilization and determining the molecular orientational order and conformation. How these properties in dicationic materials are compared to those in conventional monocationic analogs? We address this question using a combination of advanced NMR methods and DFT analysis. Dicationic salt 3,3′-(1,6-hexanediyl)bis(1-dodecylimidazolium)dibromide was studied. Local bond order parameters of flexible alkyl side chains, linker chain, and alignment of rigid polar groups were analyzed. The dynamic spacer effectively “decouples” the motion of two ionic moieties. Hence, local order and alignment in dicationic mesophase were similar to those in analogous single-chain monocationic salts. Bond order parameters in the side chains in the dicationic smectic phase were found consistently lower compared to double-chain monocationic analogs, suggesting decreasing contribution of van der Waals forces. Overall dication reorientation in the smectic phase was characterized by low values of orientational order parameter S. With increased interaction energy in the polar domain the layered structure is stabilized despite less ordered dications. The results emphasized the trends in the orientational order in ionic liquid crystals and contributed to a better understanding of interparticle interactions driving smectic assembly in this and analogous ionic mesogens.

Wetting Behavior and Support Interactions in Imidazolium-Based Supported Ionic Liquid Phase Materials─A Systematic Study by Solid-State Nuclear Magnetic Resonance Spectroscopy

“…16,17 Majhi et al used solid-state NMR spectroscopy under static conditions to investigate the order parameter of IL smectic phases and were able to trace the influence of water hydrogen bonding on these phases. 18,19 Using MAS NMR, the same group recently investigated hydrogen bonding and phase transition in a series of aprotic, long-chain (C 12 ) mesogenic ILs with halide and BF 4 − counter anions impregnated in KIT-6 and MCM-41. 20 In 2012, the groups of Haumann and Buntkowsky have investigated the IL 1-ethyl-3-methyl-1Himidazolium bis(trifluoromethylsulfonyl)imide [C 2 C 1 im]-[NTf 2 ] on a silica gel with different degrees of pore filling by MAS NMR.…”

Complex-tensor theory of simple smectics

“…Here, we exclusively consider the simplest smectics, with only lamellar broken translational symmetry and layer-normal broken rotational symmetry. Such simple lamellar ordering is observed in systems with little-to-no nematic ordering, such as the striped phases of short-range attraction and long-range repulsion colloidal systems 44 , 60 , 61 , ionic liquid crystals (which commonly transition directly from isotropic to smectic phases) 62 and the flat plate ("lasagna”) phase of nuclear pasta 63 . We focus solely on this idealistic premise for lamellae to demonstrate the fundamental suitability of E for avoiding the phase ambiguity and to show its utility in simulating confining geometries.…”

Insights into cation–anion hydrogen bonding in mesogenic ionic liquids: an NMR study

“…40,41 On the other hand, in the smectic phase discussed below, the layered assembly of cations may impose constraints on cation conformational dynamics and alignment of the imidazolium group and the side chains. 10,36,37 In protic ILCs based on the cation C 12 im, the primary centre for HB is shifted to NH protons and thus HB to CH protons weakens. 6 A significant decrease of CS of CH protons in the ring is observed for the salt with Cl anion compared to that in its aprotic counterpart (Fig.…”

“…40,41 On the other hand, in the smectic phase discussed below, the layered assembly of cations may impose constraints on cation conformational dynamics and alignment of the imidazolium group and the side chains. 10,36,37…”

“…Orientational order and cation conformation in these materials have been characterized in our previous studies using dipolar NMR methods. 10,36,37 The phase transition temperatures and residual water contents in the investigated samples are compiled in Table S1 in the ESI. † IL/porous solid composites were prepared either by the postimpregnation method or the template synthesis method.…”