Structural investigation of anion influence on ionic liquid crystals

M. Yang, K. Stappert, B. Mallick, and A.-V. Mudring

Ionic liquid crystals (ILCs) combine the self-organization features of liquid crystals and the solvent properties of ionic liquids. Ionic liquids (ILs) are salts with melting point below 100 °C. ILCs are highly interesting because they show an unique potential for application as anisotropic ion-conductors, ordered reaction media, and templates for synthesis of zeolites, mesoporous materials, and nanomaterials. Weak hydrophobic interactions of alkyl groups, dipole–dipole, cation–π interactions as well as π -π stacking of aromatic groups and ionic interactions influence the formation of mesophases. Both the cation and anion are able to introduce mesophasic behavior, however, it is believed that mostly the cation determines the ILC properties. While many ILC cations based on various ammonium head groups have been designed, synthesized, characterized, and well reviewed, very little known is how the counter anion effect on the mesophase behavior of ILCs. In the past, we have studied the effect of adding functional groups on ILCs by investigation on bis-cationic imidazolium based ionic liquid. In this system, two imidazolium head groups are linked via different types of spacers,with Br- as the counter anion. Here, we have chosen one bisimidazolium cation, B-C12, and exchanged the Br- for several anions with different geometries. Furthermore, we have also combined the A-C12 cation with transition metal complex anions, as well as rare-earth metal anions. The thermal and structural properties have been investigated via differential scanning calorimetry (DSC), thermogravimetry (TG) and polarizing optical microscopy (POM).

N N N NRC12H25 C12H25

A-C12: R= -(CH2)6-B-C12: R= -(CH2)2-O-(CH2)2-

Judging from POM and DSC, compounds with the tetrahedral anions, BF4 and ClO4 show two different mesophases, while [B-C12][PF6] with an anion of Oh symmetry forms only one smectic LC phase over the whole mesophase temperature range. [B-C12][BTA] with the perfluorinated bis(trifluoromethane)sulfonylamide) (BTA) anion sees the formation of a smectic phase at quite low temperatures (-17.6 °C).

f 

a b  c  d e

Figure 1.Representative POM photos of a) [B-C12][BF4] at 25 °C, b) [B-C12][BF4] at 50 °C, c) [B-C12][ClO4] at 130 °C, d) [B-C12][ClO4] at 143 °C, e) [B-C12][PF6] at 45 °C, f) [B-C12][BTA] at -21 °C. Temperature dependent SAXS data obtained at beamline A2 support the assignment of lamellar phases. The primary d-spacing peak (d100) represents the average layer distance in the lamellar structure. As shown in Figure 2, two diffractions for [B-C12][BF4] emerge in the small-angle region, corresponding to a layered structure with d-spacing of 33 Å at 15 °C, and

at 40 °C the distance reduced to 30 Å. [B-C12][PF6] and [B-C12][ClO4] exhibit similar lamellar structures. The calculated layer spacing from the SAXS data are 30Å for [B-C12][PF6] and 31Å and 30 Å for [B-C12][ClO4] under different mesomorphic states. B-C12][BTA] forms mesophase below -17 °C. A measurement at this temperature was not possible.

0.00 0.05 0.10

25.85 Å

Nor

mal

ized

Int

ensi

ty u

.a.

s = 1/d [1/Å]

BC-12BF4, 15 °C, 1st Heating

Aluminium foil, RT

33.33 Å

0.00 0.05 0.10

No

rma

lized

In

tens

ity u

.a.

s = 1/d [1/Å]

BC-12BF4, 100 °C, 1st Heating

Aluminium foil, RT

30.76 Å

0.00 0.05 0.10

Nor

mal

ized

Int

ensi

ty u

.a.

s = 1/d [1/Å]

BC-12PF6, 90 °C, 1st Heating

Aluminium foil, RT

30.12 Å

0.00 0.04 0.08 0.12

BC-12ClO4, 60°C, 1st Heating

Aluminium foil, RT

Nor

mal

ized

Int

ensi

ty u

.a.

s = 1/d [1/Å]

31.65 Å30.54 Å

0.00 0.04 0.08 0.12

BC-12ClO4, 150°C, 1st Heating

Aluminium foil, RT

Nor

ma

lized

Int

ensi

ty u

.a.

s = 1/d [1/Å]

29.71 Å

Figure 2.SAXSdata from measurements at beamline A2 for compounds with [B-C12] cation. Figure 3 shows the DSC thermograms of [A-C12][MnBr4]. Remarkably, this compound shows a far more complex thermal behavior. However, the SAXS analysis shows only one layered mesophase was formed with d-spacing of 30 Å in the temperature range 100 to 130 °C. The d100 reflectionof [A-C12]3[TbBr6]2 corresponds to an interlayer distance of 28 Å.

0 50 100 150

DS

C (

mW

/mg

)

2ndHeating 2nd Cooling

Temperature /°C

Exo

Endo

0.00 0.05 0.10

30.12 Å

Nor

mal

ized

Int

ensi

ty u

.a.

s = 1/d [1/Å]

AC-12-Mn, 120 °C, 1st Heating Aluminium foil, RT

Figure 3. Thermogram of [A-C12][MnBr4] (top left) and SAXS data of [A-C12][MnBr4] (top right); POM Picture of [A-C12]3[TbBr6]2 (botton left) and SAXS data of [A-C12]3[TbBr6]2

(bottom right) .

0.00 0.05 0.10

Nor

mal

ized

Int

ensi

ty u

.a.

s = 1/d [1/Å]

AC-12-Tb, 90 °C, 1st Heating Aluminium foil, RT

28.36 Å