Scientific report 2014

Scientific Report  

January – December 2014

The main objective of the fourth stage of the project was the synthesis and characterization of several liquid crystals metallomesogens based on ionic self-assembly approach, that spans over two years period (2014 and 2015), as indicated in the working plan.

Research activities were focused on the synthetic approach of such materials as well as the investigation of their mesomorphic behavior. Preliminary results showed that it is possible to, by employing the strategy developed by us, obtain nematic ionic liquid crystals (there are only few reports concerning nematic ILC, mainly based on imidazolium and ammonium salts and none on pyridinium salts [1-5]). Therefore, in this stage we continued to investigate a series of nematic ILC with different spacer length (6, 9, 10) and various counterion  (X = NO3-, PF6-, BF4-, OTf-) in order to establish a structure-properties relationship.

Nematic ionic liquid crystals (ILC)

Pyridinium salts as nematic ILC were synthesized starting from 4-hydroxypyridine, by using tetrabutylammonium bromide as phase transfer catalyst. The intermediates were the corresponding 4-pyridones derivatives.  The products were characterized by mean of IR spectroscopy (in order to identify the counterion and its exchange by metathesis reactions) and by 1H and 13C NMR spectroscopy to confirm their chemical structure (Fig. 1). 

spectru rmn

Fig. 1. The 1H-RMN spectrum of a nematic ILC.

The LC properties were investigated by using a combination of techniques: differential scanning calorimetry (DSC), polarized optical microscopy (POM) and variable-temperature powder X-ray diffraction (XRD). The observations made confirmed that the unique mesophase seen is a nematic phase, based on the Schlieren texture detected by POM (Fig. 2b), as well as by DSC data (Fig. 2a) and XRD data. The XRD pattern shows two broad signals, one in the small-angle region and the second one around 2θ=20o, assigned to the molten-like alkyl chains, confirming once again the nematic phase.

curba dsctextura MOP

 

Fig. 2 The DSC curve of 6 (left) POM picture of the nematic phase (200X) at 80oC (right).

All these ILC showed an intense blue-light emission, both in solid state and in solution, at room temperature with high quantum yields values up to 70% making them attractive to be employed in OLED devices. Moreover, this emission is preserved over the entire range of the nematic phase, up to 100oC (Fig. 3).

spectre emisie

Fig. 3 The solid-state emission spectra corresponding to different counterions.

Columnar ILC based on pyridinium salts

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 56oC 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 (NO3-, BF4- and PF6-) resulted in mesophase suppression (Fig. 4). 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.

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Fig. 4.  Synthesis of pyridinium salts with different counterions.

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Fig. 5. Optical texture showing a spherulitic texture at 25oC (200x).

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Fig. 6. Powder X ray diffraction pattern for 12a (n=12)  (left) emission spectra recorded at various temperatures (right).

The luminescence properties were also studied in dichloromethane solution when only one emission band was observed with its maximum located about 320 nm assigned to an intraligand transition which are significantly blue-shifted by comparison to solid-state emission spectra recorded in solid, suggesting an aggregate-type emission in solid state [6,7].

Ionic metallomesogens through self-assembly of [MX4]n- or [M(CN)2]- units.

A series of bis(imidazolium) and pyridinium derivatives previously prepared were used for coupling with [M(CN)2]- units, where M=Au(I) or Ag(I) in order to yield luminescent ionic liquid crystals. Generally, the new materials didn’t show the expected liquid crystalline properties. Perhaps the [M(CN)2]- features, in particular the single charge unit, didn’t allow the coupling of more than one functionalized organic cation unit. It is necessary the increase of mesogenic group density on the organic cation in order to generate desired LC properties. Therefore we turned our attention towards polyoxometalates (in particular based on Eu - [EuW10O36]9-) as they are highly charged and multiple mesogenic units can be assembled around, in order to finely tune the LC properties. Moreover, the excellent emission properties (high emission lifetime and quantum yields) make them very appealing materials to be employed in various electrooptical applications. Indeed, we proved that this strategy works very well both for imidazolium and pyridinium cations.

 textura MOP textura MOP iradiere

spectre emisie

 

Cat9[EuW10O36]

Fig. 7. POM pictures recorded at 95oC in normal light; at 95oC with UV light irradiation; the emission spectra recorded at various temperature.

Dimeric ILC based on pyridinium salts

Based on the previous reports that the dimeric ILC shows a higher thermal stability than their monomeric counterpart, as well as an extended mesophase range, we were interested in developing a new synthetic strategy to prepare dicationic imidazolium and pyridinium salts. The objectives were followed here: the design of higher thermally stable LC materials and incorporation of a high number of mesogenic units with the aim that at a later stage to realize the coupling with different emissive units.

Luminescent hybrid materials 

We were interested in the preparation of hybrid materials containing Ln-based polyoxometalates units. Thus, we prepared flexible and transparent PMMA films doped up to 10% without phase separation and transparency loss. A bis(imidazolium) salt was used as plasticizer (up to 40% without phase separation) in order to get flexible films The glass transition temperature drops significantly down to 40oC for a 40% weight plasticizer. To insure the miscibility of the [EuW10O36]9- unit, this was previously modified with a new type of bis(imidazolium) derivative to enhance the full dissolution in the polymer matrix. 

 film PMMAfilm PMMA iradiat

Fig. 8. Flexible and transparent PMMA film (left) the PMMA film under UV irradiation (right).

 

Incorporation of covalent metallomesogenic units by using specific ortho-metallation reactions followed by bridge – cleavage reactions to prepare several different ionic mononuclear species.

A series of orthometallation reactions with the imine ligans functionalized with quaternary ammonium groups were performed in order to prepare cyclometalated Pt(II) complexes. Various acac and N-benzoyl thiourea (BTU) derivatives have been employed as auxiliary ligands for these complexes. The preliminary assessment of the thermal bahavior showed that these complexes have no LC properties. In this way, we intend to modify the acac as well as the BTU ligands with different alkoxy groups in order to generate LC properties. This step will be continued and developed in 2015.

People

Some of these results were included in a Master thesis (Isabela Dumitru – Master Program « Chemistry of Advanced Materials » at University of Bucharest). Laura Badea, former Master student and team member, was registered as PhD student at the University of Bucharest, Faculty of Chemistry. Part of the results shown in here were included in her first intermediate research report.   

Conclusions

A series of new luminescent LC materials were prepared by incorporation of new emissive entities (such as polyoxometalates, etc.). These luminescent properties were kept unaffected both in crystalline state and liquid crystalline phase. Several studies regarding the metallomesogens with covalent units and laterally attached mesogenic units were undertaken. Thus, a series of cyclometalated Pt(II) compounds with different auxiliary ligands acac and BTU were prepared and characterized. Remarkable results concerning the stability of a nematic phase for ILC based on pyridinium salts were obtained. The effect of the alkyl chain length and counterion type was evidenced for the nematic phase.

By changing the type and number of mesogenic groups we were able to prepare a wide range of LC materials and, importantly, to tailor the LC properties, varying from calamitic to discotic behavior.

Two papers were published and a third one was submitted for publication.

References

1.   K. Goossens, P. Nockermann, K. Driesen, B. Goderis, C. Gorller-Walrand, K. Van Hecke, L. Van Meervelt, E. Pouzet, K. Binnemans, T. Cardinaels, Chem. Matter., 2008, 20, 157-168

2.   X. Lan, L. Bai, X. Li, S. Ma, X. He, F. Meng, J. Mol. Struct., 2014, 1075, 515-524.

3.   X. Li, X. Lan, S. Ma, L. Bai, F. Meng, M. Tian, Liq. Cryst., 2014, doi : 10.1080/02678292.2014.951884.

4.   P.L. Chiu, C.Y. Chen, J.Y. Zeng, C.Y. Lu, H.M. Lee, J. Organomet. Chem., 2005, 690, 1682-1687.

5.   W. Li, J. Zhang, B. Li, M. Zhang, L. Wu, Chem. Commun., 2009, 5269-5271. 

6. X.-H. Jin, C. Chen, C.-X. Ren, L.-X. Cai, J. Zhang, Chem. Commun., 2014, 50, 15878-15881.

7. P.S. Campbell, M. Yang, D. Pitz, J. Cybinska, A.-V. Mudring, Chem. Eur. J., 2014, 20, 4704-4712.

 

 

 

 

 

Pagină actualizată la 09 Februarie 2015.