Discotic liquid crystals stabilized by interionic interactions: imidazolium ion-anchored paraffinic triphenylene

Abstract

Incorporation of imidazolium ion functionalities into the paraffinic side-chain termini of a triphenylene derivative resulted in stabilization of a columnar mesophase of its liquid crystalline assembly, which was retained over a wider temperature range down to 4 °C by an externally added imidazolium-based ionic liquid.

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Discotic liquid crystalline materials¹ based on polyaromatic molecules have attracted increasing attention, because of their one-dimensional charge and energy transport capabilities in mesophases.² Along with their high processabilities, they are potential components for organic electronic and optoelectronic devices.³ From this point of view, development of discotic liquid crystals having a broad mesophase range⁴ down to room temperature^4e–g is an important subject. However, such thermotropic liquid crystals generally possess high crystal-to-mesophase transition temperatures. Furthermore, attempts to lower the crystallisation temperature usually result in lowering the clearing temperature.⁵ Here we report an interesting finding that interionic interactions among imidazolium salts can stabilize the columnar mesophase of a triphenylene (TP)-based discotic liquid crystal.

As exemplified by 1-alkyl-3-methylimidazolium tetrafluoroborates,⁶ some of these salts are liquids over a wide temperature range, even at cryogenic temperatures. Such ionic molten states are quite interesting, since interionic interactions involved usually facilitate crystallisation of electrolytes. Our molecular design is based on a TP derivative with paraffinic alkyl chains, whose termini are covalently attached to 1-methylimidazolium tetrafluoroborate (BF₄) (1a). We expected that the imidazolium ion termini could prevent crystallisation of the alkyl chains, while interionic interactions⁷ among them may stabilize the columnar assembly of the TP moiety.

Compound 1a⁸ was obtained as a colourless waxy solid by N-alkylation of 1-methylimidazole with hexakis(10-bromodecyloxy)triphenylene, followed by anion exchange from Br⁻ to BF₄⁻ using AgBF₄. We found that this new class of TP derivative, in comparison with the parent paraffinic triphenylene (2),⁵ forms an enantiotropic liquid crystal with a wide LC temperature range. As shown in Fig. 1(a), polarized optical microscopy (POM) of 1a at 103 °C under crossed polarizing conditions displayed a fan texture characteristic of a columnar mesophase. Differential scanning calorimetry (DSC) of 1a showed phase transitions, where the mesophase was observed at 111–47 °C on cooling (Fig. 2(a)). In contrast, the paraffinic TP (2) without imidazolium ion functionalities has been reported to form a columnar mesophase only in a much narrower temperature range (69–58 °C, cooling run).⁵ We also found that an imidazole-anchored TP (1b),⁸ as a non-ionic reference, does not form any thermotropic mesophases, but gives a crystalline material that melts at 34 °C.⁸ On the other hand, when an imidazolium molten salt such as 1-hexyl-3-methylimidazolium tetrafluoroborate ([C₆mim][BF₄]) was added to 2, a macroscopic phase separation resulted. Thus, the columnar LC mesophase is retained over a wide temperature range by the imidazolium ion groups covalently incorporated into the paraffinic TP (2).

Fig. 1 Optical textures of (a) 1a at 103 °C, (b) 1a at 25 °C and (c) an equimolar mixture of 1a and [C₆mim][BF₄] at 25 °C, observed under crossed polarizing conditions of a microscope on cooling from the isotropic liquids at a rate of 1 °C min⁻¹.

Fig. 2 DSC traces on second heating/cooling cycle of (a) 1a and (b) an equimolar mixture of 1a and [C₆mim][BF₄] (scan rate 10 °C min⁻¹). G, glassy state; M, mesophase; D_Col, discotic columnar mesophase; Iso, isotropic liquid.

Unexpectedly, the LC mesophase of 1a on cooling underwent a phase transition to an amorphous, viscous fluid (M in Fig. 2(a)), where the fan texture of 1a in POM disappeared completely (Fig. 1(b)). This behaviour is in contrast with those of ordinary LCs including 2, which are usually crystallised or transformed to glassy solids on cooling. We assume that this phenomenon is due to a possible competition of the π-stacking interaction among the TP units with a π-cation–π interaction⁹ between the anchored-imidazolium groups and the TP units, considering that the latter interaction is likely pronounced at lower temperatures. However, we found that 1a, upon mixing with an equimolar amount of [C₆mim][BF₄], forms a stable columnar LC phase, which is held even on cooling down to 4 °C without any phase separation. POM of the resultant mixture clearly displayed a columnar LC-originating fan texture (Fig. 1(c)). Furthermore, DSC of the mixture, on cooling from its isotropic liquid, showed the appearance of the LC phase at a higher temperature (117 °C) than in the absence of [C₆mim][BF₄] (111 °C) (Fig. 2(b)). Thus, the externally added molten salt significantly expands the mesophase temperature range of 1a from 47–111 to 4–117 °C (Fig. 2). We tentatively think that the imidazolium salt functionalities of 1a in the LC mesophase form a pseudo network by their interionic interaction, which stabilizes the columnar assembly to a certain extent. When an appropriate amount of [C₆mim][BF₄] is added to the system, it presumably participates in and reinforces the interionic network, so that the columnar mesophase is retained even at 4 °C.

In the phase diagram of 1a in the presence of [C₆mim][BF₄], the mesophase was observed at the mole fractions of [C₆mim][BF₄] ranging from 0.3 to 0.8 (Fig. 3). On the other hand, when the mole fraction of [C₆mim][BF₄] was out of the above range, dissolution of 1a (> 0.8) or no stabilization of the LC phase (< 0.3) took place. Although the mesophase of 1a, in the absence of [C₆mim][BF₄], displayed only weak X-ray diffraction (XRD) peaks, the combination of [C₆mim][BF₄] with 1a resulted in a notable enhancement of the diffraction intensity. An example is shown by XRD of a 0.2 ∶ 0.8 mixture of 1a and [C₆mim][BF₄] (Fig. 4), where diffraction peaks with d spacings of 15.4 and 7.75 Å were observed, along with a negligibly small alkyl halo at 4.5 Å. The XRD pattern is assignable to a hexagonal columnar mesophase with a lateral core-to-core separation of 17.8 Å, which is in good agreement with the diameter of 1a with tightly folded side chains (19 Å).

Fig. 3 Phase diagram of 1a in the presence of [C₆mim][BF₄], as determined by DSC on second heating runs. G, glassy state; M, mesophase; D_Col, discotic columnar mesophase; Iso, isotropic liquid.

Fig. 4 XRD pattern (scan rate of 4° min⁻¹) of a mixture of 1a and [C₆mim][BF₄] (0.2 ∶ 0.8), measured after an isotropic liquid was allowed to cool to 25 °C. Inset: magnified (× 100) diffraction pattern.

In conclusion, we have developed a new triphenylene that forms a columnar LC mesophase over a wide temperature range from 47 to 111 °C, by incorporation of imidazolium salt-anchored long C₁₀ side chains into the mesogenic core. With an externally added imidazolium molten salt, the columnar mesophase is maintained over a much wider temperature range from 4 to 117 °C. This finding is not only important for a new possibility of organic molten salts in materials science,^9,10 but may also contribute to the development of novel anisotropic soft materials for directional ion conductivity¹¹ and charge transport.

Notes and references

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8. See Supporting Information.
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Footnote

† Electronic supplementary information (ESI) available: procedures for synthesis and characterization of 1a and 1b. DSC trace of 1b. See http://www.rsc.org/suppdata/cc/b4/b414649b/

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