Abstract
Unique combination of ionic conductivity and anisotropic physical properties in ionic liquid crystals leads to new dynamic properties exploited in modern technological applications. Structural and dynamics information at atomic level for molecules and ions in mesophases can be obtained by nuclear magnetic resonance (NMR) spectroscopy through the measurements of dipole–dipole spin couplings. While 13C–1H and 15N–1H dipolar NMR spectra can be routinely acquired in samples with natural isotopic abundance, recording 15N–13C dipolar NMR spectra is challenging because of the unfavourable combination of two rare isotopes. In the present study, an approach to measure 15N–13C dipole-dipole NMR spectra in static liquid crystalline samples with natural abundance is introduced. We demonstrate that well-resolved spectra can be recorded within 10 h of experimental time using a conventional NMR probe and a moderately strong magnetic field. The technique is applied to a thermotropic smectic mesophase formed by an ionic liquid with imidazolium-based organic cation.
Highlights
Ionic liquid crystals (ILC) belong to a new class of materials that are ionic liquids capable of forming liquid crystalline phases on cooling from isotropic state [1,2,3].Russia 4 Department of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm, SwedenUnique feature of a material in the liquid crystalline state is that a high degree of molecular translational and rotational mobility is combined with partial orientational and positional order
Deuterium (2H) Nuclear magnetic resonance (NMR) can be employed in isotopically labelled liquid crystals to gather orientational and dynamic information [8]; this approach requires chemically demanding and expensive synthesis of deuterated LC samples [12]. 2H NMR spectra in samples with natural isotopic abundance (NAD NMR [11]) can be recorded; resonance assignment of NAD spectra is often ambiguous [13]
Representative 15N NMR spectra in the smectic phase of the ionic liquid C12mimBF4 are shown in Fig. 1 along with the spectrum obtained in the isotropic phase
Summary
Ionic liquid crystals (ILC) belong to a new class of materials that are ionic liquids capable of forming liquid crystalline phases on cooling from isotropic state [1,2,3]. 13C NMR applied to samples with the natural isotopic abundance allows obtaining site-specific structural and dynamic information by achieving a high spectral resolution and by selective suppression/introduction of anisotropic spin interactions [9, 10]. Few examples of 14N NMR studies of liquid crystals in terms of ordering and dynamic properties were possible in relation with particular mesophase orientations with respect to the magnetic field and/or molecular geometries [16,17,18,19]. It has been recently demonstrated that the nitrogen-15 NMR in bulk LC materials at the natural abundance level (NAN15 NMR) can be routinely recorded in non-spinning samples with a high molecular orientational order and strong anisotropic spin interactions [20]. We focus on a thermotropic smectic mesophase formed by an ionic liquid with imidazolium-based organic cation [23]
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