Liquid Crystals

Overview of liquid crystal topics covered by the Laschat Research Group

Discotic Liquid Crystals

The discovery of stable liquid crystalline phases formed by disk-shaped molecules with long alkyl chains in their periphery is generally dated to the seminal work of Chandrasekar published in 1977.[1] Since then discotic liquid crystals have attracted the attention of many research groups worldwide. Due to the one dimensional charge and ion transport in the columnar mesophase, the ability of liquid crystals (LCs) to self-heal structural defects by thermal annealing and the ease of processing via spin coating, drop casting and other solution processing methods highly useful applications[2–4] could be realized such as organic solar cells,[5] organic field effect transistors[6] and organic light emitting diodes.[7] For the application possibilities the electronic structure of discotics is of great importance.

Boroxines

Introducing heteroatoms to the core unit can be used to tune the electronic features of the liquid crystals. Boroxines as a core unit for discotic compounds provide a quick access to tripodal molecular architecture. These boroxines are the condensation product of three boronic acids and therefore consist of a 6-membered (-B-O-)3 ring system. The published synthesis towards these mesogens provides the first target-oriented approach to liquid crystalline boroxines. With simple threefold alkylated benzene side-groups they display broad, highly ordered Colh mesophases.[8]

Crown Ethers

Another possibility in creating molecular electronics give the crown-ether based liquid crystals. These consist of three main parts the central crown-ether, varying number of triphenylenes and the peripheral side chains.[9] Previous work showed the application potential, e.g. photo conductivity or carrier mobility.[10]These properties can be tuned by varying the components of the molecular construction kit. Through changes of the molecular structure important features like the geometry of the mesophase[11] or the mesophase range towards room temperature can be altered.

[1] Chandrasekhar, S.; Sadashiva, B.K.; Suresh, K.A. Pramana, 1977, 9, 471–480.
[2] Sergeyev, S.; Pisula, W.; Geerts, Y.H. Chem. Soc. Rev. 2007, 36, 1902–1929. 
[3] Woltman, S.J.; Jay, G.D.; Crawford, G.P. Nat. Mater. 2007, 6, 929–938. 
[4] Kaafarani, B.R. Chem. Mater. 2011, 23, 378–396. 
[5] Kumar, S. Curr. Sci. 2002, 82, 256–257. 
[6] Bushby, R.J.; Kawata, K. Liq. Cryst. 2011, 38, 1415–1426. 
[7] Kopitzke, J.; Wendorff, J.H. Chemie Unserer Zeit 2000, 34, 4–16. 
[8] Wöhrle, T.; Baro, A.; Laschat, S. Materials 2014, 7, 4045–4056. 
[9] Kaller, M.; Tussetschläger, S.; Fischer, P.; Deck, C.; Baro, A.; Giesselmann, F.; Laschat, S.; Chem. Eur. J. 2009, 15, 9530–9542. 
[10] Staffeld, P.; Kaller, M.; Beardsworth, S. J.; Tremel, K.; Ludwigs, S.; Laschat, S.; Giesselmann, F.: J. Mater. Chem. C 2013, 1, 892–901. 
[11] Wöhrle, T.; Kirres, J.; Kaller, M.; Mansueto, M.; Tussetschlager, S.; Lascha, S.; J. Org. Chem. 2014, 79, 10143−10152.

Calamitic Ionic Liquid Crystals

Ionic liquid crystals are now an established field of research in organic synthesis as they combine the advantages of ionic liquids (ILs) and liquid crystals (LCs) creating a class of substances with very promising industrial applications for example in displays. Calamitic liquid crystals were the first type of LCs investigated. For some time the calamitic shape was considered necessary for liquid crystalline behaviour.[1] While many other shapes of liquid crystals are known for ionic liquid crystals (ILCs) calamitic shapes offer great potential due to the formation of bilayer structures formed by a charged LC and it’s counter ion, which offer one-dimensional ion conductivity.[2] The ILCs we focus on typically consist of a central mesogenic unit that has a flexible alkyl chain on one end and the ionic head group attached either directly or via an alkyl chain on the other.[3,4] 

Guanidinium Ion Pairs

Guanidine, with its ability to form stable cations has been used in ionic liquids for quite some time now. To create a class of ILCs with low melting points we try to create ion pairs where both the cation and anion contain a mesogenic group such as p-alkoxybiphenyl or p-alkoxybenzene with a charged head group. Guanidine suits our purpose very well with its stability, while sulfonates give non-nucleophilic anions.[3,5]

Phenylpyrimidines

The phenylpyrimidine structure has created considerable interest over time as a mesogen as it allows to compare a flat core (2-phenylpyrimidine 1) versus a twisted one (5-phenyl-pyrimidine 2).[4] 

[1] D. Vorländer, Z. Phys. Chem., Stoechiom. Verwandtschaftsl. 1923, 105, 211-254.
[2] T. Kato, Science, 2002, 295, 2414-2418. 
[3] M. Butschies, W. Frey, S. Laschat, Chem. Eur. J. 2012, 18, 3014-3022. 
[4] G. F. Starkulla, S. Klenk, M. Butschies, S. Tussetschläger, S. Laschat, J. Mater. Chem. 2012, 22, 21987-21997. 
[5] M. Butschies, M. M. Neidhardt, M. Mansueto, S. Laschat, S. Tussetschläger, Beilstein J. Org. Chem. 2013, 9, 1093-1101.

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