Effect of counterion on the mesomorphic behavior and optical properties of columnar pyridinium ionic liquid crystals derived from 4-hydroxypyridine

doi:10.1016/j.molstruc.2014.11.059

Journal of Molecular Structure

Volume 1083, 5 March 2015, Pages 245-251

https://doi.org/10.1016/j.molstruc.2014.11.059 Get rights and content

Highlights

•
ILC pyridinium salts derived from 4-hydroxypyridine were prepared and studied.
•
N-3,4,5-tridodecyloxybenzyl-4-pyridone shows a monotropic Col_h phase.
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Bulkier counterions ( ${BF}_{4}^{-}$ or ${PF}_{6}^{-}$ ) destabilize the LC phase.
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3,4,5-Tridodecyloxybenzyl pyridinium bromide salt shows two LC columnar phases.

Abstract

A series of 3,4,5-tridodecyloxybenzyl pyridinium salts derived from 4-hydroxypyridine has been designed and prepared. The liquid crystalline properties of these compounds were investigated by polarized optical microscopy, differential scanning calorimetry and powder X-ray diffraction while their thermal stability was studied by thermogravimetric analysis. The N-3,4,5-tridodecyloxybenzyl-4-pyridone intermediate shows a monotropic columnar hexagonal mesophase ranging from 56 °C down to room temperature while the corresponding bromide dodecyl O-alkylated pyridinium salt shows one enantiotropic columnar mesophase and one additional monotropic columnar phase at lower temperatures. Replacing bromide ion (Br⁻) with other counterions ( ${NO}_{3}^{-}, {BF}_{4}^{-}$ and ${PF}_{6}^{-}$ ) resulted in mesophase suppression. These luminescent pyridinium salts show weak emission in dichloromethane solutions at room temperature and a pronounced red-shifted emission in solid state. Photoluminescent properties of the pyridinium salts do not depend significantly on the nature of counterion employed.

Graphical abstract

Influence of counterion on the mesomorphic behavior and photophysical properties of columnar pyridinium ionic liquid crystals derived from 4-hydroxypyridine

Introduction

Ionic liquid crystals (ILC) are a special class of materials that combine in a unique manner the liquid crystal (LC) and ionic liquid (IL) properties. They are closely related to ionic liquids (IL) that show very interesting properties such as low volatility, nonflammability and high-ionic conductivity. In this respect, they are regarded as good candidates to replace conventional organic solvents in organic reactions. The most attractive and investigated ILC systems include ammonium, phosphonium, imidazolium, pyridinium and pyrrolidinium salts. The anisotropic ionic conductivity is one of the most interesting features of ILC, due to the presence of ionic units in the liquid crystal phase. Several reviews covering this topic were published in the recent years [1], [2]. Amongst them, pyridinium-based ILC, whose mesomorphism has been known since long time, show similar properties with the related imidazolium based ILC [3].

In this work we report preliminary studies regarding the synthesis and LC investigation of a series of pyridinium ionic liquid crystals derived from 4-hydroxypyridine with different counterions such as bromide (Br⁻), nitrate ( ${NO}_{3}^{-}$ ), tetrafluoroborate ( ${BF}_{4}^{-}$ ) and hexafluorophosphate ( ${PF}_{6}^{-}$ ). It is worth mentioning that 4-hydroxypyridine is a very appealing starting material as it can be either N- or O-alkylated in the first step [4], [5], [6], yielding either 4-pyridones or O-substituted pyridines respectively, followed by a second alkylation step to yield desired ILC materials. This represents a big advantage as different mesogenic groups can be introduced separately in the molecule with the aim of finely tune the liquid crystalline properties. For instance, Lin et al. [7] reported ILC based on 4-hydroxypyridine motif with simple alkyl chains, both O- and N-alkylated pyridinium salts. Generally, all these ILC show a SmA phase whose stability depends on the alkyl chain length and the counterion employed. On the other hand, there are only several examples of pyridinium based ILC that display columnar phases. One representative example has a classical hexaalkyloxytriphenylene moiety connected via a flexible alkyl spacer to the pyridinium ring [8]. There is a growing interest in the discovery of new materials showing columnar liquid crystal phases as they found their applications in electronic or optoelectronic devices, in particular due to their ability to display improved anisotropic charge transportation [9]. 3,4,5-trialkyloxybenzyl bromide is a well-known alkylation reagent for imidazole or pyridinium units that can lead to LC materials able to display columnar phases depending on alkyl chains length employed [10], [11], [12]. We were interested to investigate the structure – mesomorphic properties relationship of a series of pyridinium salts with different counterions, having on one side the 3,4,5-trialkyloxybenzyl moiety and an alkyl chain group at the other side of the molecule. The emission properties of these salts were also studied in solution and solid state.

Section snippets

Experimental

All the chemicals were used as supplied. 3,4,5-tridodecyloxybenzyl bromide was synthesized as reported elsewhere [12]. C, H, N analyses were carried out with a Perkin Elmer instrument. IR spectra were recorded on a Bruker spectrophotometer using ATR technique. UV–Vis absorption spectra were recorded by using a Jasco V-660 spectrophotometer. ¹H and ¹³C NMR spectra were recorded on a Varian Gemini 300 BB spectrometer operating at 300 MHz, using CDCl₃ as solvent. ¹H chemical shifts were referenced

Results and discussions

The reaction of 4-hydroxypyridine with 3,4,5-tridodecyloxybenzyl bromide 1 in THF, in the presence of NaOH and tetrabutylammonium bromide (TBABr) as phase transfer catalyst, gave the 4-pyridone derivative 2, as the main product in relatively high yield 67% (Scheme 1). The 4-pyridone derivative 2 was then further reacted with a large excess of dodecylbromide to give the quaternized pyridinium bromide salt 3. This second alkylation step proceeds in relatively high yield, 70%, but an excess of

UV–Vis and emission properties

Recent reports showed that this type of pyridinium-based ionic liquid crystals [14], [15] could be good candidates for emission-based applications. For this reason, the optical properties of pyridinium salts have been investigated and the results are summarized in Table 2. The electronic absorption spectra of all pyridinium salts 3–6, measured in dichloromethane solutions, exhibit one band around 250 nm and a shoulder near 280 nm (Fig. 6). These pyridinium salts can be regarded as donor–acceptor

Conclusions

We have demonstrated that it is possible to introduce different mesogenic groups in two alkylation steps starting from 4-hydroxypyridine in order to prepare ILC pyridinium salts. In this way, a series of N-3,4,5-trialkyloxybenzyl pyridinium salts with different counterions has been prepared and their mesomorphic properties have been investigated by a combination of POM, DSC and X-ray powder diffraction. Two columnar mesophases were identified for bromide pyridinium salt while replacing the

Acknowledgments

This work was supported by a grant of the Romanian Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-ID-PCE-2011-3-0384.

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Effect of counterion on the mesomorphic behavior and optical properties of columnar pyridinium ionic liquid crystals derived from 4-hydroxypyridine

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Results and discussions

UV–Vis and emission properties

Conclusions

Acknowledgments

Tetrahedron Lett.

Tetrahedron Lett.

Tetrahedron Lett.

Tetrahedron

Materials

Chem. Rev.

Eur. J. Org. Chem.

J. Heterocyclic Chem.

Soft Matter

Angew. Chem. Int. Ed.