Macrocyclic Effects in the Mesomorphic Properties of Liquid-Crystalline Pillar[5]- and Pillar[6]arenes

Whereas the reaction of 1,4‐bis(2‐bromoethyloxy)benzene (4) with paraformaldehyde in the presence of BF3·Et2O afforded exclusively the cyclopentameric pillar[5]arene derivative (5), both cyclopenta‐ and cyclohexameric macrocycles 5 and 6 were obtained when the reaction of 4 with paraformaldehyde was performed at 45 °C in CHCl3 with FeCl3 as the catalyst. Treatment of compounds 4–6 with sodium azide provided the corresponding polyazides, to which a cyanobiphenyl building block was subsequently grafted to generate model compound 1, pillar[5]arene 2, and pillar[6]arene 3, bearing two, ten and twelve mesomorphic subunits, respectively. The liquid‐crystalline and thermal properties of the compounds were investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X‐ray diffraction (XRD). Comparison of the liquid‐crystalline properties of macrocycles 2 and 3 with those of 1 revealed the strong influence of the macrocyclic pillar[n]arene core on the mesomorphic properties. Whereas only a monotropic mesophase was observed for 1, a broad enantiotropic mesophase was evidenced for both pillar[n]arene derivatives. Pillar[n]arene (n = 5 or 6) derivatives substituted with peripheral mesogenic moieties gave rise to an enantiotropic smectic A phase over a very broad temperature range, whereas only a monotropic mesophase was observed for the corresponding constitutive acyclic monomeric subunit.