Abstract
This chapter describes the liquid crystalline properties of the ionic liquid crystals (ILC) based on pyridinium salts as well as their metal-containing compounds with an emphasis on the recent systems described in literature. The main factors that influence the liquid crystalline properties of pyridinium ILC are discussed. Selected thermal data are given according to mesogenic group employed and its position (either N-substitution or pyridinium ring substitution) and the number of structural cationic units (mono-, di-, or polycationic pyridinium ILC).
Keywords
- ionic liquids
- liquid crystals
- pyridinium salts
1. Introduction
Ionic liquid crystals (ILCs) are extensively studied nowadays due to their unique properties resulting from the combination of liquid crystal (LC) and ionic liquid (IL) properties. Several reviews covering this topic were published in the recent years [1–3]. The field of ILCs is continuously growing as many recent applications were found: solar cells, membranes, battery materials, electrochemical sensors, or electroluminescent switches. Different factors are responsible for governing the nature of ILC phases, such as the molecular shape, location, and size of ionic groups, intermolecular interactions, and microphase segregation. Thus, the hydrophobic interactions between the long alkyl chain groups, ionic, dipole-dipole, anion–cation hydrogen bonding, and cation-π interactions together with the π-π stacking of the aromatic rings, all have a contribution to the stabilization of the liquid crystalline phase. For instance, there is a strong tendency to stabilize lamellar (smectic) phases, with SmA the most common phase for ILC, due to electrostatic interactions and ion-ion stacking in ILC. The combination of all these factors leads to LC behavior ranges from typical calamitic materials to discotic. The imidazolium or pyridinium derivatives, one of the most studied classes of ILC, with weakly coordinating anions, such as tetrafluoroborate (BF4−) and hexafluorophosphate (PF6−), are well known for their high thermal and electrochemical stabilities. It is worth to mention here that the pyridinium-based ILC has been known for a long time, displaying very similar properties with the related imidazolium-based ILC.
The most common types of LC phases displayed by ILCs are represented in Figure 1. Their identification relies upon three characterization techniques: polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and powder X-ray diffraction (XRD). The usual textures seen by POM for different mesophases are presented in Figure 2. It is very common to see the SmA phase of ionic mesogens as a so-called oily streak texture by POM, in particular during heating runs (Figure 2). When cooling from the isotropic state, very often the SmA phase can develop spontaneous homeotropic alignment due to interactions developed between the cationic groups and the glass substrate surface (the samples are sandwiched between two microscope slides), and further orientation of the mesogenic groups. As a result, the microscopy image contains large dark regions corresponding to these homeotropic alignments.
The POM technique is an important tool for mesophase identification, but an ultimate technique that confirms unequivocally the phase type is the XRD method. The later one can give also important information regarding the internal organization within the mesophase resulting from the different molecular packing related to the chemical structure of the ionic mesogens and the nature of interactions between them.
The thermal behavior as well as the mesophase type of pyridinium-based ILCs depends on several factors: the position and the nature of the mesogenic group attached to the pyridinium ring and the counterion employed. These structural factors will be discussed further and selected examples will be presented to illustrate their influence on the mesomorphic behavior. The pyridinium-based ILC classification was made according to mesogenic group used and its position (either N-substitution or pyridinium ring substitution) and the number of cationic units contained in their structure (mono-, di-, or polycationic pyridinium ILC).
2. Monocationic pyridinium ionic liquid crystals
2.1. Simple protonated pyridinium salts
The simplest 4-alkyl substitution protonated pyridinium salts with halide as counterions (
By changing the alkyl group with an alkoxy group, the resulting protonated chloride pyridinium salts show a SmA phase, with decomposition near the isotropization process. The melting and isotropization temperatures of these products were very insensitive to the chain lengths, suggesting that the hydrophobic interactions are the least significant factor in the thermal behavior of such protonated pyridinium salts [5].
A series of more elaborated protonated pyridinium salts have been prepared by the reaction between pyridine derivatives and phosphoric acid with the aim of studying their anhydrous proton conduction. X-ray diffraction measurements suggested that the pyridinium salts formed a bilayer structure with head-to-head configuration in the SmA phase [6].
The protonated pyridinium cation was employed to generate liquid crystalline materials having as counterions a series of wedge-shaped benzenesulfonate mesogens
When the protonated pyridinium moiety is part of a α-diketone compound, the resulting β-diketone pyridinium chloride salts are not mesomorphic. Replacement of chloride anion with the tetrachlorozincate ion produced mesomorphic ionic salts displaying a SmA phases over a broad temperature range [9]. The same authors reported the thermal behavior and photophysical properties of the metal-free β-diketone pyridinium ligands, and their allyl-palladium(II) complexes (
2.2. N-alkylated pyridinium salts
2.2.1. N-methyl pyridinium salts
A clear trend for izotropization temperatures was seen for N-methyl pyridinium salts, meaning that, based on strong dependence of these temperatures on the alkyl chain length, longer alkyl chain length in position 4 of the pyridinium ring lead to higher transition temperatures. Moreover, it was observed that the 1-methyl-4-alkoxycarbonylpyridinium iodides salts have thermochromic properties. Color changes were observed on heating at both crystal to crystal and crystal to mesophase transitions, while such an observation could not be made for the 1-methyl-4-alkylpyridinium iodides salts [4, 12, 13].
2.2.2. Other N-alkylated pyridinium salts
The simple N-alkylated chloride pyridinium salts
Importantly, these salts (
Replacement of chloride anion with bulkier hexafluorophosphate anion led to mesophase destabilization for pyridinium salts with alkyl chain shorter than 16 carbon atoms. Extensive studies were performed for N-alkyl pyridinium salts with alkylsulfates as counterion, all of them displaying a typical SmA phase, the mesophase temperature range depending on the combination of chain lengths of both N-alkyl and the aliphatic chain connected to the sulfate group [16, 17].
The tetrachlorocuprate pyridinium salts show a very interesting and rich polymorphism for alkyl chain longer than or equal to 12 carbon atoms, with hexagonal columnar, cubic, and SmA phases appearing in the order of increasing the chain length and/or temperature [18].
By exchanging the chloride anion with biologically active picrate, dodecylbenzenesulfonate, or cholate anions, different thermal behavior was found [19]. Thus, only the compound with dodecylbenzenesulfonate anion displayed a SmA phase stable up to 152°C, while the other two showed no liquid crystalline properties. The absence of mesomorphic properties in the case of picrate anion was attributed to the formation of interlayer 3-D hydrogen bond network between pyridinium and picrate ions.
2.2.3. Chiral N-alkylated pyridinium salts
Recently, a series of N-alkylated pyridinium salts containing a chiral center in the four positions with respect to nitrogen atoms have been prepared and investigated by Laschat et al. [20, 21]. Their thermal properties were compared to their imidazolium counterparts. Generally, these salts show a SmA phase, with melting and isotropization temperatures lower than the temperatures of their corresponding imidazolium counterparts. The mesophase stability range depends on the alkyl group length as well as on the counterion and it was found to decrease in the following order: Br− > OAc− > BF4− > I− > SCN−. The pyridinium salts with hexafluorophosphate anion show no liquid crystalline properties.
For chiral 1-citronellylpyridinium bromide salt
2.2.4. N-alkylated pyridinium salts derived from picoline
The pyridinium salts derived from either 4- or 3-picoline,
A series of N-alkylated pyridinium salts, having an alkoxy group in 4-position of pyridinium ring, were synthesized by the reaction of either N-alkyl-4-pyridones or 4-alkoxypyridines with corresponding alkyl bromides. Further, hexafluorophosphate and tetrafluoroborate pyridinium salts could be prepared by a metathesis reaction with the ammonium salts.
These salts exhibit smectic A phase. The transition temperatures, both melting and clearing, as well as the mesophase stability were influenced by the alkyl chain length and the counterion type. The hexafluorophosphate anion, PF6−, produced smaller mesophase ranges, followed by tetrafluoroborate anion, BF4−, when compared to bromide pyridinium salts as a consequence of a weaker cation-anion interaction for the BF4− or PF6− than for the Br− [5].
Pyridinium salts with a 1,3-dioxane ring attached in 4-position were prepared and their liquid crystalline properties were investigated [25, 26]. A very wide range for the SmA mesophase (between −24 and 150°C) was found for ionic thermotropic liquid crystal system having two rings in its central core (
First, LC phases were observed only for long alkyl chains on both sides of the molecule. Furthermore, the authors found that the enantiomeric purity influences significantly the nature of the mesophase (chiral nematic phase N* for pure single enantiomeric compounds and nematic and smectic phases for racemic mixture) and only slightly the transition temperatures between solid state and LC phase and LC phase and isotropic state. Moreover, the LC phase stability depends on the counterion size (higher stability for halide and tetrafluoroborate anions) due to the contribution of bigger anions to decreasing the packing of the mesomorphic cationic units. Yousif et al. found an enantiotropic cholesteric phase (N*) for a series of quaternized cholesteryl isonicotinates with tosylate counterions [27]. The assignment of such a mesophase was made based on polarizing optical microscopy when a planar texture with bright oily streaks was identified for such compounds. No additional XRD studies were undertaken to characterize this phase.
Several other pyridinium ILCs having a second aromatic ring connected to the 4-position of the pyridinium ring via a linking group (azo,
2.2.5. N-Alkyl-4’-substitution-stilbazolium halides
N-alkyl-stilbazolium ILC, having different groups at the 4’-position of the stilbazolium head unit, were investigated by several authors [23, 30, 31]. The 4’-substitution at the stilbazolium unit has a great influence on the LC properties. For instance, the compounds with NO2 or CN groups and bromide as counterion show a relatively narrow temperature range of the LC phase [30]. By introduction of chloride anions, the LC phase stability increases and the compounds decompose before reaching the isotropic phase. The use of dialkylamino group at the 4’-position in the stilbazolium core led to a significant decrease of the temperature range of the mesophase. For compounds
2.2.6. Monocationic oxadiazole pyridinium salts
The pyridinium bromide salts
More recently, a systematic study of liquid crystalline and photophysical properties of a series of pyridinium salts was reported [34, 35], where the 1,3,4-oxadiazole unit connects one pyridinium ring and one mono- or di-substitution phenyl ring with alkoxy groups.
The N-decyl pyridinium salts exhibit SmA mesomorphism, regardless of the counterion, while their N-methyl counterparts show decomposition and no liquid crystalline properties for iodine and nitrate anions, and, for the remaining anions (BF4−, ClO4−, and DS), the salts decompose before reaching the isotropic phase from the previous SmA phase, suggesting that the thermal properties (melting point, mesophase range, and clearing or decomposition temperature) are sensitive to counterion exchange. Moreover, the two pyridinium salts with DS as counterion are similar in terms of thermal behavior, and this was explained by XRD studies. These compounds have a monolayered SmA phase, instead of a bilayered SmA phase found for the others compounds of the series, and the same molecular lengths (due to the presence of the same counterion), and such a different mesophase structure could account for their similar behavior.
An interesting example showing how the 3’- or 4’-substitution pattern of the pyridinium ring affects the thermal behavior is represented by the iodides and trifluoromethanesulfonates salts derived from perfluoroalkylated 1,2,4-oxadiazolylpyridines 28–31 [36]. Thus, the 3’-substitution derivatives exhibited liquid crystalline properties (SmX phase) on a narrow temperature range, while the corresponding 4’-substitution derivatives were found to pass from the crystalline state straight to the isotropic phase. Such a behavior was attributed to a charge delocalization over the entire structure, including the oxadiazole ring, which makes the cation/anion electrostatic interactions weaker and, thus, leading to the mesophase detabilization for 4’-substitution compounds.
The fact that only the iodide salts showed liquid crystalline properties was explained by the greater coordinating ability of the iodide anions with respect to the trifluoromethanesulfonates anions. Moreover, all iodide salts showed thermochromism phenomena suggesting prospective applications in optoelectronics.
2.3. Monocationic pyridinium salts with mesogenic groups attached to nitrogen atom
A series of pyridinium salts with the 3,4,5-tridodecyloxybenzyl moiety attached to nitrogen atom and different counterions (bromide, nitrate, tetrafluoroborate, and hexafluorophosphate)
The pyridinium bromide salt shows one enantiotropic columnar mesophase and one additional monotropic columnar phase at lower temperatures. The size of counterion has a significant contribution to the LC phase stability. Surprisingly, when the bromide ion (Br−) was replaced with bulkier counterions (NO3−, BF4−, and PF6−), the resulting products showed no liquid crystalline behavior. The photoluminescent properties of these pyridinium salts were investigated both in solution and solid state and it has been shown that their emission is only slightly influenced by the nature of counterion employed.
The classical triphenylene unit was connected to pyridinium ring via a flexible methylene flexible chain to give discotic liquid crystals [38]. The mesophase was identified to be a columnar phase based on microscopy observations of optical textures. The stability of this columnar phase was found to depend both on the peripheral alkyl chain length as well as the methylene spacer length. Thus, the stability of columnar phase increases by increasing the number of carbon atoms on the peripheral chains of the triphenylene core, while longer spacer connecting the triphenylene unit with the pyridine ring destabilized the mesophase.
Interesting results were obtained for pyridinium bromides salts containing a biphenyl core and alkyl chains of different lengths
When an additional ring was added, the new pyridinium bromide salts
The XRD studies together with molecular modeling indicated that π-π interactions counterbalance the strong ionic forces leading to a full segregation of molecular parts in the smectic structures.
2.3.1. Dendritic pyridinium ionic liquid crystals
Pyridinium chloride salts having dendritic building blocks connected to the nitrogen atom have been investigated by Percec et al. [41]. Depending on the number of peripheral alkoxy groups, these molecules were found to form columns or spheres, leading to 2D hexagonal columnar phases or a 3D cubic phase, respectively.
2.3.2. Nematic pyridinium ionic liquid crystals
Starting from 4-hydroxypyridine, it is possible to attach two cyanobiphenyl mesogenic units, via flexible alkyl spacer, on both sides of the pyridinium ring giving rise to a series of nematic pyridinium liquid crystals [42]. The nematic phase has numerous technological applications due to its highest fluidity of all LC phases and hence the possibility to align it by applying an external electric/magnetic field. Moreover, the nematic phase is commonly used in electro-optical devices. The nematic phase is quite rare in the case of ILCs, several examples have been reported so far for ammonium [43, 44], imidazolium [45–48], pyridinium-based ILC [49], or miscellaneous type of ILC [50–54]. Generally, as the smectic phases are the most common phases for ILCs, especially due to electrostatic interactions, the nematic phase can be seen rather as an exception. It has to be reminded here the cholesteryl-containing compounds 21a-g that display a cholesteric phase (chiral nematic phase N*) at relatively high temperatures and on a narrow temperature range. Compounds
The nematic phase was identified by using the combination of the three techniques: POM, DSC, and XRD, as well as, miscibility studies with 5 CB and doping with a chiral compound. A marbled texture or thread-like texture could be seen by POM, which flashed brightly under pressure, while several samples also exhibited regions with not well developed Schlieren texture. Previous examples of pyridinium ILC showing a Schlieren texture were assigned to a SmC phase [39]. For this reason, additional XRD studies were undertaken to rule out the possibility of misinterpretation of experimental data. Indeed, the diffractograms showed no sharp peaks in the low-angle region, but just a broad signal centered at 4.5 Å assigned to the average intermolecular separation, close to the typical value for liquid crystalline phases, confirming the nematic phase nature. Additional confirmation came from miscibility studies with common nematic LC, a mixture of
The NMR data are strongly dependent on the anion-cation interaction specific to ILC, and they can be related to the presence of hydrogen-bonding interactions that cause downfield chemical shift of the related H atoms for imidazolium- and pyridinium-based ILC. Stronger interactions with anions lead to further downfield chemical shifts, in particular for C-H adjacent to nitrogen atom. The chemical shifts of these protons, from the 1H NMR spectra of pyridinium salts
3. Dicationic bis(pyridinium) and polycationic pyridinium salts
3.1. 4,4’-Bipyridinium-based ionic liquid crystals
The 4,4’-bipyridinium salts or viologens are a special class of materials that show interesting properties, such as electrochromism and electrical conductivity, which were successfully employed in producing liquid crystalline materials. The reports dealing with liquid crystals based on 4,4’-bipyridinium salts significantly grew after 2000. It is important to mention that such materials can be part of a two steps reduction process, and this process is reversible, as depicted below.
Additionally, it is of interest to note here that many dicationic and tetracationic ILC based on the 4,4’-bipyridinium rigid core (viologen-based ILC) have been investigated so far, displaying mesomorphic properties typically of calamitic and discotic materials [55–59].
The mesogenic properties of compounds based on asymmetric viologen salts of bis(trifluoromethanesulfonyl)amide ([NTf2]−)
Columnar phases could be obtained for liquid crystals bearing two 3,4,5-tris(alkoxy) benzyl units attached to the 4,4’-bipyridinium moiety [61]. The viologen product bearing six octyloxy chains shows a hexagonal columnar (Colh) phase, while the analogues with longer alkoxy chains (12 or 16) show rectangular columnar (Colr) phases. For these products, the clearing temperatures could not be measured due to thermal degradation before reaching the isotropic state. Different molecular packing (Colh or Colr) was explained by formation of more elliptical molecular structures for compounds with elongated terminal alkoxy chains that may prefer to form the Colr phases.
3.2. Dicationic bis(pyridinium) salts with flexible linker
Bis(pyridinium) salts with flexible spacers
The study of emission properties of these luminescent bis(pyridinium) salts revealed a weak emission in dichloromethane solutions at room temperature, with quantum yields up to 4.4%. Their solid-state emission is significantly red shifted by comparison to solution emission spectra recorded in dichloromethane, suggesting a more complex emission mechanism, probably an aggregate-type emission in solid state [62].
3.3. Dicationic bis(pyridinium) salts with a rigid core
Dicationic pyridinium salts with an anthracene moiety connecting two pyridine rings substitution with tris(alkoxy)benzyl groups
The anthracene moiety act as a fluorophore in the center of molecules, and the solvate of pyridiniums salts
3.4. Tripodal pyridinium ILC
Photoluminescent tripodal pyridinium-based ionic liquid crystals were reported by Kato et al. [64, 65]. These tricationic pyridinium salts show thermotropic hexagonal or rectangular columnar phases or cubic phases with a large temperature ranges. The columnar or cubic phase stability is given by the difference in the anion size. For small-size anions (bromide), the cationic pyridinium cores should be packed more closely through electrostatic interactions and hence, the molecules prefer to self-assemble in cubic structures at higher temperatures (
Acknowledgments
This work was supported by a grant of the Romanian Authority for Scientific Research, CNCS-UEFISCDI, and project number PN-II-ID-PCE-2011-3-0384.
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