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Issue 13, 2015
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Photoresponsive ionic liquid crystals based on azobenzene guanidinium salts

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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.

Graphical abstract: Photoresponsive ionic liquid crystals based on azobenzene guanidinium salts

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Supplementary files

Article information

20 Oct 2014
17 Dec 2014
First published
24 Dec 2014

Phys. Chem. Chem. Phys., 2015,17, 8382-8392
Article type

Photoresponsive ionic liquid crystals based on azobenzene guanidinium salts

E. Wuckert, M. D. Harjung, N. Kapernaum, C. Mueller, W. Frey, A. Baro, F. Giesselmann and S. Laschat, Phys. Chem. Chem. Phys., 2015, 17, 8382
DOI: 10.1039/C4CP04783D

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