Discotic Liquid Crystals based on Tristriazolotriazines: Synthesis, Properties and the tangential-radial Isomerization

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Two isomeric [1,3,4]tristriazolo[1,3,5]triazines with C3 symmetry and pi-conjugated substituents, either in a tangential or in a radial arrangement, are investigated. Thermal treatment of the `tangential`-isomer at elevated temperatures results in a stepwise rearrangement of the heterocyclic system. The annulation of the triazoles changes and the substituents are shifted from the original `tangential`- to a ´radial`-orientation. The thus formed ´radial`-tristriazolotriazine is a new heterocyclic core for discotic liquid crystals. Independent pathways for the synthesis of both isomers include cyclocondensation and oxidation of triazinylamides and -hydrazides.Optimisation of the isomerisation procedure is combined with kinetic and mechanistic studies. For this purpose, the first tristriazolotriazines with a non-symmetric core will be prepared. Combining donor- and acceptor substitution will discriminate the triazole-triazole bonds and shed light on the selectivity and sequence of the isomerisation steps. C3-symmetric cores with an asymmetric peripherie will be prepared. míxed lengths of the alkyl chains and mixed substitution pattern are used for a tuning of the thermal properties. Unprecedented tristriazolotriazines with styryl- and diaryl-1,3,4-oxadiazole as pi-segmente will be prepared: the diaryl-1,3,4-oxadiazole will result in strongly elektron deficient discs, usefuls as n-type semiconductors. Styryl-substitution opens the door to photochemistry: [2+2]-cycloadditions in solution can lead to cyclophanes, in the LC-Phase, photochemical cross-linking can stabilise the columns. Tristriazolotriazines are strongly emissive fluorophores with the ability to form broad thermotropic mesophases. The synthesis and rearrangement of tristriazolotriazines is acompanied by extensive characterisation of the optical, electrical and thermal properties of these materials, including experiments for the induction of helical chirality in the mesophase.