Many of our supramolecular assemblies are based on azobenzene derivatives, which provide an additional stimulus to gain control over the liquid crystalline and optical properties of the materials. In general, the photo-isomerization of azo-compounds yields a phase transition from the mesophase to the isotropic state. In addition, it allows us to delay the crystallization of the supramolecular assemblies. However, more interesting than the photo-controlled OFF-switching is to have a photo-trigger for introducing a new property. In a series of HBLCs based on hydroxy-benzoic acids and stilbazoles we observed a photo-activated fluorescence upon irradiation with 405 nm. The fluorescence results from the formation of a push-pull system at the stilbazole due to proton transfer. The emerging fluorescence is accompanied with a phase transition from the mesophase to the solid state and allows us to imprint patterns into the material. By heating the samples to 150°C for 1.5 h the image can be erased and after returning to the mesophase (at 90°C) a new pattern can be imprinted.
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Figure 1: Visualization of the photo-induced trans-cis isomerization of azobenzenes (A) as well as POM images showing the nematic to isotropic phase transition upon irradiation with light (405 nm). (C) shows the photo-excitation of stilbazole resulting in a proton transfer and the ON-switching of fluorescence.
"Photo-switchable Fluorescence in Hydrogen-bonded Liquid Crystals" – A. Kappelt, M. Giese,* Chem. Eur. J. 2020, accepted for publication, DOI: 10.1002/chem.202001696.
"Photo-switching and -cyclisation of hydrogen bonded liquid crystals based on resveratrol" – M. Blanke, J. Balszuweit, M. Saccone, C. Wölper, D. Doblas Jiménez, M. Mezger, J. Voskuhl,* M. Giese,* Chem. Commun. 2020, 56, 1105-1108.
"Photoresponsive Halogen-Bonded Liquid Crystals: The Role of Aromatic Fluorine Substitution" – M. Saccone, M. Spengler, M. Pfletscher, K. Kuntze, M. Virkki, C. Wölper, R. Gehrke, G. Jansen, P. Metrangolo, A. Priimagi, M. Giese*, Chem. Mater. 2019, 31, 2, 461-470.