title: Photoresponsive ionic liquid crystals based on azobenzene guanidinium salts.
abstract: The use of non-ionic LC phases as anisotropic matrices for E/Z-isomerization of azo-guest molecules is often restricted due to limited solubilities and demixing effects. In this study we therefore employed an ionic liquid crystal (ILC) matrix to follow the photo-induced E/Z-isomerization of ionic mesogenic azobenzene guanidinium guests. The latter were prepared from 4-hydroxy-4'-(octyloxy)azobenzene, which was first treated with N-(bromoalkyl)phthalimides to introduce the spacer with varying chain length. Removal of phthalimide and final reaction with a formamidinium salt linked the ionic head group to the photoisomerizable azobenzene unit. Investigation of the mesomorphic behaviour revealed for all azobenzene ILCs smectic A mesophases with high translational order parameters and partial bilayers, as could be stated by layer spacing d. Similar packing behaviour was found for the solid state by X-ray crystal structure analysis. E/Z-isomerization of azobenzene ILCs which were completely miscible with the ionic LC phase of C12MIM-Br as anisotropic host was induced by irradiation with UV light and the reisomerization observed by time-resolved UV-Vis spectroscopy. For comparison, water was used as isotropic host. Z/E-reisomerization activation energies exhibited similar values of 97-100 kJ mol(-1) irrespective of spacer lengths and the type of host. The results demonstrate that a proper match of steric requirements of host and guest as well as layer spacings are needed for a decreased activation energy.
authors.affiliation: Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany. sabine.laschat@oc.uni-stuttgart.de.
authors.forename: Eugen
authors.initials: E
authors.lastname: Wuckert
authors.forename: Marc D
authors.initials: MD
authors.lastname: Harjung
authors.forename: Nadia
authors.initials: N
authors.lastname: Kapernaum
authors.forename: Carsten
authors.initials: C
authors.lastname: Mueller
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Frey
authors.forename: Angelika
authors.initials: A
authors.lastname: Baro
authors.forename: Frank
authors.initials: F
authors.lastname: Giesselmann
authors.forename: Sabine
authors.initials: S
authors.lastname: Laschat