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 CHCl 3 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. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.This research was supported by the University of Strasbourg, the Centre Nationale de la Recherche Scientifique (CNRS), the Spanish project CTQ2012-35692, the FEDER founds and the Swiss National Science Foundation (grant number 200020-140298). I. N. thanks the Agence National de la Recherche (ANR) and S. G. the Swiss National Science Foundation for postdoctoral fellowships.Peer Reviewe