Scientific report 2011-2013

Scientific Report

October 2011– October 2013

I. Preparation of dimeric ionic liquid crystals based on imidazolium salts and their use in synthesis of dinuclear metal-carbene type complexes.

The research activities of the first stage (2011-2012) covered the synthesis and characterization of a series of bromide derivatives that were used further in the preparation of ionic liquid crystals (ILC) based on imidazolium salts. The idea was to employ carefully designed derivatives that can influence significantly the liquid crystalline behavior. Firstly, the mesophase type (it is well known that ionic liquid crystals show typically lamellar phases due to electrostatic interactions; the nematic phase which is the widely used LC phase in the LCD construction was rarely seen in the case of ILC) [1]. Second, the transition temperatures and the extended range of the mesophase were targeted.

Moreover, by changing the mesogenic group type (discotic versus calamitic), new columnar phases can be induced, very useful for different applications, such as photovoltaic cells, etc. Thus, the second part of this stage included the preparation of intermediates related to bis(imidazolium) final products with different spacers between the two heterocycles.

The introduction of cholesteryl mesogenic units via an ester linking groups affords the preparation of dimeric, trimeric LC as well as the generation of chiral phases [2], in particular the chiral nematic phase N*.  This has an important relevance for different technological applications. In this respect, a series of carboxylic acids with various alkyl chain length (n = 6, 10) and bromine in terminal positions was prepared and characterized. They were further employed for preparation of corresponding acid chlorides by employing thionyl chloride in dichloromethane. The next step was the reaction between cholesterol and acid chlorides to yield cholesteryl ester derivatives (Scheme 1). Such bromide derivatives could be used later for O-alkylation Williamson type reaction to prepare new calamitic molecules with extended shape that contain additional cholesteryl groups (for example O-alkylation of 4-hydroxybenzaldehyde [3]). Another way of attaching these mesogenic groups to various units was the quaternization of bis-imidazole derivatives.

The alkyl bromide fragment can be directly attached to cholesteryl unit, in this way the use of phenyl group can be avoided. In this respect, various acid chlorides of simple bromo-alkanoic acids can be employed, some of them are commercially available (e.g. 6-bromo-hexanoyl chloride).

Bromide derivatives containing either cyanobiphenyl (CBP) or biphenyl (BP) groups were synthesized and employed as alkylating compounds (Scheme 1). All these compounds were characterized for their structure and purity by IR, 1H and 13C-NMR spectroscopy and partially by MS (mass spectrometry).

 schema 1

Scheme 1. Examples of bromide derivatives used for preparation of bis(imidazolium) salts.

According to working plan, during the January – December 2012 period, two major objectives were followed:

1. Preparation of dimeric ionic liquid crystals based on imidazolium salts and their dinuclear silver-carbene type complexes.

2. Identification and optimizing the synthetic procedures of ionic liquid crystals with laterally attached mesogenic groups for nematic phase. 

I.1. Bis(imidazolium) salts and metal-carbene

The preparation of bis(imidazolium) salts by employing the previously synthesized bromide derivatives is described in Scheme 2.  This preparation was mainly based on the alkylation reaction of the two bis imidazole compounds, with different spacer length (n=6 or 10) and different bromide derivatives either in acetonitrile or, for cholesteryl derivatives, in toluene [4, 5]. All the isolated products were purified by column chromatography before being tested for their LC properties. The yields were good to very good (50 – 70%). Most of these salts are hygroscopic compounds and show, as expected, a SmA phase stable close to ambient temperature. Importantly, the stability of SmA phase of cholesteryl containing compounds was much higher compare to other products (CBP or BP). All these products were characterized by IR, 1H and 13C NMR spectroscopy and, partially, by MS.

 

 schema 2

Scheme 2. Preparation of bis(imidazolium) salts and their silver-carbene complexes.

All these salts were employed further for the synthesis of silver-carbene complexes by reacting them with Ag2O in dichloromethane, as previously described by Arduengo [6] or Lin [7] (Scheme 2). These silver-carbene complexes can be precipitated out of the solution by adding ether and recrystallized several times to remove metallic silver side products.  The isolated products, with good yields (40 – 60%), are white or off-white products and show a very high thermal stability. In the absence of single-crystal X-ray diffraction data and based on elemental and EDX analysis indicating a Ag:Br = 1:1 ratio), we assigned a LAg2Br2 formula, where L = bis(imidazolium) ligand. Such a supposition was also based on the IR, 1H and 13C NMR spectroscopy data and does not exclude a bromide-bridged polymeric structure. By metathesis reactions, the counterion can be exchanged and the influence of counterion on the LC behavior could be studied.  The thermal behavior of these products was investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and, partially, by X-ray diffraction studies (XRD).

 textura MOP textura MOP iradiere

                (a)                                                          (b)         

Figure 1. POM pictures of SmA phase : normal light (a) and UV irradiation (b).

 

Generally, the textures seen by POM were typical for a SmA phase for CBP containing compounds. These textures could be obtained either as standalone or induced by shearing or pressing to observe the characteristic birefringence, as it is well known that well-developed SmA textures are difficult to obtain for ILC [8] But, sometimes, a well-developed focal conical textures could be seen, and such an example is shown in Fig. 1.

When such a texture was irradiated with UV light, in the 330 – 380 nm range, an intense blue emission could be detected [9]. These emissive properties will be investigated in more details in the next project stage. On the other hand, the cholesteryl-based compounds showed a well-defined polygonal or oily-streaks texture specific to SmA phase. Some of these phases were characterized and confirmed by mean of XRD. Remarkably, a nematic phase was seen for the compound containing four CBP groups at both ends. This phase was accompanied by a less kinetically stable lamellar phase, and by rapid cooling the nematic phase can be stable down to room temperature.

I.3. Bis(imidazolium) salts and their silver carbene complexes as plasticizers for PMMA

The bis(imidazolium) salts and their corresponding carbene were tested as plasticizers for PMMA in order to prepare flexible luminescent PMMA films by doping with emissive platinum(II) complexes. Several plasticizers concentrations were tested (in the 5 – 30% range). The glass transition temperatures were detected by DSC. The most promising results were obtained for bis(imidazolium) salts with PF6- anions.

Moreover, the influence of plasticizer concentration and type on the emissive properties of Pt(II) complexes was investigated. The PMMA films with bis(imidazolium) salts as plasticizers show only an intense blue emission (except cholesteryl-based imidazolium derivatives)  while the PMMA films doped with Pt(II) complexes, up to maximum 1%, showed a very bright yellow emission (Figure 3).

 filme PMMA

spectre emisie

Figure 3. (a) Morphology of PMMA film with 10% of 9b and 1% Pt(II) complex; (b) flexible PMMA film with 15% 9b and 0.2% Pt(II) complex, under UV irradiation; (c) different PMMA films with various plasticizer concentrations with and without doping with Pt(II) complex; (d) the influence of additive type on the emission properties of Pt(II) complex used as dopant.

 

We appreciate that these results can be useful further for electro-optical applications (flexible devices).

I.4. Miscibility studies with commercially available LC

In order to be used for different electro-optical applications, the LC materials with valuable properties (such emission or conductibility) have to show good miscibility with commercially available LC mixtures that are widely employed in the construction of such devices. We were interested to see that some of our products could induce chiral nematic phases in 5CB or 5OCB liquid crystals with the aim that at a later stage we could prepare emissive LC materials with chiral nematic phase.

 

II. Ionic liquid crystals with laterally attached mesogenic groups

At this stage we focused on the intermediates synthesis to be employed further in the preparation of ILC with laterally attached mesogenic groups. We chose LC systems based on ammonium groups, as they can show typical smectic phases and, in some cases, nematic phases. 

 schema 3

Scheme 3. Several examples of LC with laterally attached mesogenic groups and discotic Pd(II) complexes.

Several Schiff bases with ammonium type laterally attached mesogenic group were prepared. Some of these derivatives were employed in the cyclometalation reaction of Pt(II) and Pd(II) metal ions. Different mesogenic groups were tested.

Starting from Schiff bases containing the 2-phenylpyridine unit, several Pd(II) complexes could be successfully prepared. These complexes show columnar phases and emissive properties at room temperature. The XRD data together with DSC and POM information led to a different packing model of these complexes, depending on the number of alkoxy terminal groups. Importantly, the Pd(II) cyclometalated complexes showed room-temperature red emission, rarely seen for such Pd(II) complexes.

 

III. Preparation of ionic metallomesogens by introduction of [MX4]n- or [M(CN)2]-  units through the ionic self-assemble procedure.

Several compounds containing the [CuCl4]2- sau [ZnCl4]2- units were prepared and investigated for their LC behavior. This study will be continued in the next stage. We intend to use several polyoxometalates units, mainly based on Eu, in generating new emissive ILC. 

IV. ILC based on pyridinium salts

We started a new work regarding ILC based on pyridinium salts. These were prepared starting from 4-hydroxypyridine and using tetrabutyl ammonium bromide as phase transfer catalyst.

 structura 16structura 18 textura MOP

 

Scheme 4. Pyridone derivatives and pyridinium salts (18). Texture of columnar phase for 18.

 Some preliminary studies have indicated that the pyridinium salts with CBP groups show a nematic phase, without any additional layered phase – a very important result. The investigation was extended in order to see the effect of counterion and spacer length on the stability of such a phase. These activities will be continued in the next step. Other derivatives, with terminal alkoxy groups, n = 14 or 16, showed columnar phases on a broad range. They will be further investigated by XRD.

 

Conclusions

A series of ILC based on bis(imidazolium) salts and their silver-carbene was prepared and characterized for their LC behavior. The influence of mesogenic group type, alkyol chain length and counterion nature on the mesomorphic behavior was investigated. The IR spectroscopy was used in the first instance to detect the  Br- exchange by other anions (NO3-, BF4-, PF6-).By 1H-NMR spectroscopy, it was evidenced that the chemical shift of carbenic proton depends on the nature of the anion employed, indicating the C – H...X hydrogen bond strength, following the order: Br >NO3 >BF4 >PF6 . The mesophase stability decreases following the order: Br >NO3 >BF4 >PF6 . The PF6- derivatives are ionic liquid at room temperature and show no LC properties.  

New emissive units were included in LC matrix (metallic clusters, polyoxometalates, etc.) and the new LC materials kept unaffected the emissive properties.

Several studies were undertaken regarding new LC with laterally attached mesogenic groups. A series of double cyclopalladated LC materials were prepared and investigated. These complexes show columnar phases and emission at room temperatures.

We have prepared imidazolium and pyridinium salts that show a nematic phase, which is rarely seen for ILC.

By varying the number and the type of mesogenic groups, we were able to prepare a broad range of calamitic (displaying both lamellar and nematic phases) to discotic materials.

By judicious design of targeted molecules and materials, we were able to prepare highly emissive LC materials.

Two papers have been accepted for publication and a third one is under preparation.

 

References:

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[2]. C. V. Yelamaggad, S. A. Nagamani, U. S. Hiremath, G. G. Nair, Liq. Cryst., 2001, 28, 1009 – 1015.

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[6]. A.J. Arduengo, R.L. Harlow, M.J. Kline, J. Am. Chem. Soc., 1991, 113, 361.

[7]. C.K. Lee, C. S. Vasam, T.W. Huang, H.M.J. Wang, R.Y. Yang, C.S. Lee, I.J.B. Lin, Organometallics, 2006, 25, 3768.

[8]. W. Li, J. Zhang, B. Li, M. Zhang, L. Wu, Chem. Commun., 2009, 5269.

[9]. P. Dechambenoit, S. Ferlay, B. Donnio, D. Guillon, M. W. Hosseini, Chem. Commun., 2011, 734.

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Pagină actualizată la 09 Februarie 2015.