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Using Polarized Spectroscopy to Investigate Order in Thin-Films of Ionic Self-Assembled Materials Based on Azo-Dyes

Nano-Science Center & Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 København Ø, Denmark
Authors to whom correspondence should be addressed.
Nanomaterials 2018, 8(2), 109;
Received: 13 December 2017 / Revised: 8 February 2018 / Accepted: 12 February 2018 / Published: 15 February 2018
(This article belongs to the Special Issue Mechanics, Electrical and Optical Properties of Nano-Thin Films)
Three series of ionic self-assembled materials based on anionic azo-dyes and cationic benzalkonium surfactants were synthesized and thin films were prepared by spin-casting. These thin films appear isotropic when investigated with polarized optical microscopy, although they are highly anisotropic. Here, three series of homologous materials were studied to rationalize this observation. Investigating thin films of ordered molecular materials relies to a large extent on advanced experimental methods and large research infrastructure. A statement that in particular is true for thin films with nanoscopic order, where X-ray reflectometry, X-ray and neutron scattering, electron microscopy and atom force microscopy (AFM) has to be used to elucidate film morphology and the underlying molecular structure. Here, the thin films were investigated using AFM, optical microscopy and polarized absorption spectroscopy. It was shown that by using numerical method for treating the polarized absorption spectroscopy data, the molecular structure can be elucidated. Further, it was shown that polarized optical spectroscopy is a general tool that allows determination of the molecular order in thin films. Finally, it was found that full control of thermal history and rigorous control of the ionic self-assembly conditions are required to reproducibly make these materials of high nanoscopic order. Similarly, the conditions for spin-casting are shown to be determining for the overall thin film morphology, while molecular order is maintained. View Full-Text
Keywords: ionic self-assembly; thin films; molecular orientation; non-covalent forces; soft materials ionic self-assembly; thin films; molecular orientation; non-covalent forces; soft materials
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Kühnel, M.R.-T.; Ahmad, M.; Andersen, F.; Bech, Á.B.; Bendixen, H.K.L.; Nawrocki, P.R.; Bloch, A.J.; Bora, I.; Bukhari, T.A.; Bærentsen, N.V.; Carstensen, J.; Chima, S.; Colberg, H.; Dahm, R.T.; Daniels, J.A.; Dinckan, N.; El Idrissi, M.; Erlandsen, R.; Førster, M.; Ghauri, Y.; Gold, M.; Hansen, A.; Hansen, K.; Helmsøe-Zinck, M.; Henriksen, M.; Hoffmann, S.V.; Hyllested, L.O.H.; Jensen, C.; Kallenbach, A.S.; Kaur, K.; Khan, S.R.; Kjær, E.T.S.; Kristiansen, B.; Langvad, S.; Lund, P.M.; Munk, C.F.; Møller, T.; Nehme, O.M.Z.; Nejrup, M.R.; Nexø, L.; Nielsen, S.S.H.; Niemeier, N.; Nikolajsen, L.V.; Nøhr, P.C.T.; Orlowski, D.B.; Overgaard, M.; Ovesen, J.S.; Paustian, L.; Pedersen, A.S.; Petersen, M.K.; Poulsen, C.M.; Praeger-Jahnsen, L.; Qureshi, L.S.; Ree, N.; Schiermacher, L.S.; Simris, M.B.; Smith, G.; Smith, H.N.; Sonne, A.K.; Zenulovic, M.R.; Sørensen, A.W.; Sørensen, K.; Vogt, E.; Væring, A.; Westermann, J.; Özcan, S.B.; Sørensen, T.J. Using Polarized Spectroscopy to Investigate Order in Thin-Films of Ionic Self-Assembled Materials Based on Azo-Dyes. Nanomaterials 2018, 8, 109.

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