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

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.

S2. 2D spectra in the isotropic phase Figure S2. COSY (a) and HMQC (b) spectra of C 6 (C 12 im) 2 Br 2 sample in bulk isotropic phase at 150 C. In the HMQC spectrum, only the aliphatic region is shown.
S3. HETCOR spectra in the smectic phase Figure S3. HECTOR (a) and HETCOR-PDLF (b) spectra of C 6 (C 12 im) 2 Br 2 sample in the aligned smectic phase at 122 C. Figure S4. NAD NMR spectrum of C 6 (C 12 im) 2 Br 2 sample in smectic phase at 122 C. The assignment of the side chain methylenes in this spectrum is based on the comparison to the dipolar splittings in Fig. 2 (in the main text) which exhibit a monotonous decrease along the side chains. Signals from the imidazolium rings and first methylene in the side chains are not observed due to broader lines with lower signal-to-noise ratios and overlap with other lines. b) Calculated from equation S1, assuming negative sign of the dipolar coupling. c) The splitting was unresolved in the PDLF spectrum.

S4. Natural abundance deuterium NMR
The procedure to infer the signs and magnitudes of the dipolar couplings of the aliphatic sites is based on the comparison of the dipolar coupling constants CH d , estimated from the to the corresponding quadrupolar splittings Q   measured in NAD spectrum. In a sample with the director perpendicular to the external magnetic field, the experimental dipolar coupling CH d is related to the local bond order parameter where CH b is a rigid lattice dipolar coupling constant. Corresponding quadrupolar splittings for the deuterated site is given by

S5. Dipolar-recoupled magic-angle-spinning spectra
The dipolar spectra of the C-H pairs in the imidazolium moiety were recorded under the magic angle spinning (MAS) condition using the amplitude-and phase-modulated crosspolarization (APM-CP) dipolar recoupling scheme 4 . The splittings in the APM-CP spectra are given by /2 , where   1 / 2 is the scaling factor of the APM-CP sequence. Thus, comparing the spectral splittings obtained by the PDLF and APM-CP techniques (Table  S2), the magnitudes and signs of the constants CH d for the imidazolium C-H pairs were determined. Figure S5. Cross-sections for the imidazolium carbons along the dipolar dimension from 2D APM-CP spectrum in C 6 (C 12 im) 2 Br 2 smectic phase at 88 C. , b) Hz a) Calculated from APM-CP spectra. b) Signs and magnitudes are inferred from Eq. (S1) by comparing d CH (PDLF) to the values obtained from APM-CP spectra. c) The value in the parenthesis is obtained assuming (incorrectly) the opposite sign of the coupling constant. Figure S6. 1 H NMR spectra of the sample C 6 (C 12 im) 2 Br 2 in different phases. The spectrum in the smectic phase was recorded in the aligned sample. The water peak is shown on a larger vertical scale in the insert in the spectrum (a).

S6. 1 H NMR spectra
The spectrum in the isotropic phase indicated the presence of 5 mol% water in the sample (Fig. S6a). This small amount of water was expected to have a negligible effect on the order and dynamics of ions [5][6][7] .