Reactivity of mesogenic diacetylenes coupled with phase transitions between crystal and liquid crystal phases

Abstract

Among mesogenic diacetylenes, N-[4-(5-hydroxypenta-1,3-diynyl)benzylidene]-4-alkoxyaniline (DA-Cn), in which n is the number of carbon atoms in the alkoxy group, DA-C8 has been found to have four phases divided by three endothermic transition temperatures. The four phases have been assigned to crystal phase I (T < 96 °C), crystal phase II (T_CI–CII = 96 °C), crystal phase G (T_CII–CG = 115 °C) and nematic phase (T_CG–N = 123 °C), based on DCS measurements and observation under a microscope. The crystal structure of DA-C8 in phase I has been determined. Crystal data are triclinic, space group P, a = 14.580(3), b = 17.451(4), c = 10.219(3) Å, α = 93.40(2), β = 103.83(2), γ = 116.85(3)° and Z = 4. Four molecules in a unit cell are hydrogen-bonded to each other at the hydroxymethyl groups and/or the imino-nitrogens of the benzylideneaniline moieties to form a checker-like structure. The phase transitions have been monitored by X-ray powder diffraction measurements. The checker-like structure in crystal I is transformed to a layered structure in crystal phase II and in crystal phase G. The structural transformation may be coupled with a change in the hydrogen-bonding scheme. Whereas crystal phase I is inactive towards the solid state polymerization, crystal phase II polymerizes with a long induction time of 50 h at 110 °C. However, the induction time of the polymerization is shortened greatly in crystal phase G, and the polymerization in the nematic phase proceeds smoothly with almost no induction time. The results are interpreted in terms of the relative orientation of diacetylene molecules in each phase.