Paramagnetic discotic liquid crystals based on planar benzo[e][1,2,4]triazin-4-yls: Synthesis and properties†

Abstract

Synthetic access to discotic liquid crystalline derivatives of the planar Blatter radical has been developed and properties of several such derivatives and their direct precursors were investigated with optical (POM), thermal (DSC), structural (powder XRD) and magnetic (VT-EPR) methods. The only effective, albeit low yield, method for the synthesis of the final radicals was photocyclization of appropriate precursors. These precursors were obtained from a common intermediate already containing two wedge-shaped groups by nucleophilic substitution of the fluorine atom with an aryloxy anion. In an alternative strategy, the photocyclization precursors were obtained by substitution followed by introduction of one or two wedge-shaped groups via Suzuki coupling reactions. Experimental (XRD) and computational (DFT) mechanistic investigation of a model phenoxy derivative demonstrated that the cyclization takes place without Smiles rearrangement, observed in naphthyloxy analogues. Powder XRD analysis revealed that only two radicals form mesophases (Colh), while their precursors exhibit Colh, Colh and 3D ColFddd or only 3D ColFddd phase. Results suggest that planarization of the central core in mesogenic Blatter radical derivatives and cyclization of the precursors improves molecular packing and increases the phase stability. VT-EPR results show a significant increase of antiferromagnetic interactions at melting to a Colh phase and then at the clearing temperature for one of the discotic radical.