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

Abstract

Synthetic access to the discotic liquid crystalline derivatives of a planar-Blatter radical has been developed and the properties of several such derivatives and their direct precursors were investigated using optical (POM), thermal (DSC), structural (powder XRD) and magnetic (VT-EPR) methods. The only effective, albeit low-yielding, 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 (Col_h), while their precursors exhibit Col_h, Col_h and 3D ordered Col_x phases or only the Col_x phase. Results suggest that planarization of the central core in mesogenic Blatter radical derivatives and cyclization of the precursors improve molecular packing and increase the phase stability. VT-EPR results show a significant increase of antiferromagnetic interactions at melting to a Col_h phase and then at the clearing temperature for one of the discotic radicals.