Azobenzene-Based Organic Salts with Ionic Liquid and Liquid Crystalline Properties

Two sets of new azobenzene-based bromide salts are synthesized, and their thermal photochromic properties are studied. Both sets are based on the imidazolium cation. The first set (1) features a symmetric biscation where two imidazolium head groups (Im) with different alkyl chains (Cn) are connected to a central azobenzene unit (Azo): [Azo(C1-Im-Cn)2]; n = 6, 8, 10, 12, 14. The other one contains an n-alkyl-imidazolium cation (Cn-Im) bearing a terminal azobenzene unit (C1-Azo) substituted with an alkoxy chain (O-Cm) of either two (2) or six (3) carbon atoms: [C1-Azo-O-Cm-Im-Cn]; m = 2, n = 8, 10, 12 and m = 6, n = 8, 10, 12, 14, 16. For both cation classes, the influence of alkyl chains of varying length on the thermal phase behavior was investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). For five compounds (Azo(-C1-Im-C12)2 (1d), Azo(-C1-Im-C12)2 (1e), C1-Azo-O-C2-Im-C10 (2b), C1-Azo-O-C2-Im-C12 (2c), and C1-Azo-O-C6-Im-C16 (3e)), the formation of a liquid crystalline phase was observed. The biscationic salts (1) are all comparatively high melting organic salts (180–240 °C), and only the two representatives with long alkylchains (C12 and C14) exhibit liquid crystallinity. The monocationic salts with an O–C2 bridge (2) melt between 140 and 170 °C depending on the alkyl chain length, but from an alkyl chain of 10 and more carbon atoms on they form a smectic A liquid crystalline phase. The representatives of the third set with a O–C6 bridge qualify as ionic liquids with melting points less than 100 °C. However, only the representative with a hexadecyl chain forms a liquid crystalline phase. Representative single crystals for all sets of cations could be grown that allowed for single crystal structure analysis. Together with small-angle X-ray scattering experiments they allow for a more detailed understanding of the thermal properties. Through irradiation with UV-light (320–366 nm) all compounds undergo trans–cis isomerization, which reverses under visible light (440 nm).Reprinted (adapted) with permission from Cryst. Growth Des., 2015, 15 (9), pp 4701–4712. Copyright 2015 American Chemical Society.</p

Structure-property relationship of ionic liquids

Das Ziel dieser Arbeit ist es Struktur- und Eigenschaftsbeziehungen ionischer Flüssigkeiten aufzuklären. Der Einfluss struktureller Veränderungen des Kations und die Auswirkungen von Anionen-Variationen werden in dieser Arbeit untersucht. Verschiedene Serien von Triazolium-basierenden Kationen mit unterschiedlichen Alkylkettenlängen und Bromid-Gegenionen wurden synthetisiert und charakterisiert. Des Weiteren wurden Anionen verschiedener Größe und Stärke eingesetzt um stabilisierende Wirkungen auf die Mesophasen zu untersuchen. Zusätzlich wurden neue Imidazolium-basierende Kationen mit Azobenzene-Einheiten als mesogene Gruppe untersucht um photoschaltbare ionische Flüssigkristalle zu entwickeln. Aufgrund ihrer guten Leitfähigkeit und der gleichzeitig hoch geordneten Struktur wurden ILCs als Elektrolyte in Farbstoffsolarzellen eingesetzt. Der Effekt der Kettenlängenänderung und der Symmetrie wurden hier ebenfalls untersucht.The aim of this work is to unravel relationships between structural and thermal properties of low melting salts in order to understand what favours structural organization in ionic liquids. The influences of structural modifications of the cation and variation of anion on the structural organization are studied in this thesis. To this avail, series of triazolium based cations with varying alkyl chain lengths and different counter anions have been synthesized and characterized by differential scanning calorimentry, optical polarizing microscopy and small angle X-ray scattering as well as single crystal X-ray structure analysis. Additionally, new imidazolium based cations with azobenzene as mesogenic group were developed in order to create novel photoswitchable ionic liquid crystals. The use of various triazolium based ILs and ILCs as electrolytes in DSSCs has been tested

Influence of the Counteranion on the Ability of 1-Dodecyl-3-methyltriazolium Ionic Liquids to Form Mesophases

The influence of the counteranion on the ability of the mesogenic cation 1-methyl-3-dodecyl-triazolium to form mesophases is explored. To that avail, salts of the cation with anions of different size, shape, and hydrogen bonding capability such as Cl–, Br–, I–, I3–, PF6–, and Tf2N– [bis(trifluorosulfonyl)amide] were synthesized and characterized. The crystal structures of the bromide, the iodide, and the triiodide reveal that the cations form bilayers with cations oriented in opposite directions featuring interdigitated alkyl tails. Within the layers, the cations are separated by anions. The rod-shaped triiodide anion forces the triazolium cation to align with it in this crystal structure but due to its space requirement reduces the alkyl chain interdigitation which prevents the formation of a mesophase. Rather the compound transforms directly from a crystalline solid to an (ionic) liquid like the analogous bis(trifluorosulfonyl)amide. In contrast, the simple halides and the hexafluorophosphate form liquid crystalline phases. Their clearing points shift with increasing anion radius to lower temperatures.Reprinted with permission from Cryst. Growth Des., 2015, 15 (2), pp 752–758. Copyright 2014 American Chemical Society.</p

Azobenzene-Based Organic Salts with Ionic Liquid and Liquid Crystalline Properties

Two sets of new azobenzene-based bromide salts are synthesized, and their thermal photochromic properties are studied. Both sets are based on the imidazolium cation. The first set (1) features a symmetric biscation where two imidazolium head groups (Im) with different alkyl chains (Cn) are connected to a central azobenzene unit (Azo): [Azo(C1-Im-Cn)2]; n = 6, 8, 10, 12, 14. The other one contains an n-alkyl-imidazolium cation (Cn-Im) bearing a terminal azobenzene unit (C1-Azo) substituted with an alkoxy chain (O-Cm) of either two (2) or six (3) carbon atoms: [C1-Azo-O-Cm-Im-Cn]; m = 2, n = 8, 10, 12 and m = 6, n = 8, 10, 12, 14, 16. For both cation classes, the influence of alkyl chains of varying length on the thermal phase behavior was investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). For five compounds (Azo(-C1-Im-C12)2 (1d), Azo(-C1-Im-C12)2 (1e), C1-Azo-O-C2-Im-C10 (2b), C1-Azo-O-C2-Im-C12 (2c), and C1-Azo-O-C6-Im-C16 (3e)), the formation of a liquid crystalline phase was observed. The biscationic salts (1) are all comparatively high melting organic salts (180–240 °C), and only the two representatives with long alkylchains (C12 and C14) exhibit liquid crystallinity. The monocationic salts with an O–C2 bridge (2) melt between 140 and 170 °C depending on the alkyl chain length, but from an alkyl chain of 10 and more carbon atoms on they form a smectic A liquid crystalline phase. The representatives of the third set with a O–C6 bridge qualify as ionic liquids with melting points less than 100 °C. However, only the representative with a hexadecyl chain forms a liquid crystalline phase. Representative single crystals for all sets of cations could be grown that allowed for single crystal structure analysis. Together with small-angle X-ray scattering experiments they allow for a more detailed understanding of the thermal properties. Through irradiation with UV-light (320–366 nm) all compounds undergo trans–cis isomerization, which reverses under visible light (440 nm).Reprinted (adapted) with permission from Cryst. Growth Des., 2015, 15 (9), pp 4701–4712. Copyright 2015 American Chemical Society.</p

Azobenzene-Based Organic Salts with Ionic Liquid and Liquid Crystalline Properties

Two sets of new azobenzene-based bromide salts are synthesized, and their thermal photochromic properties are studied. Both sets are based on the imidazolium cation. The first set (<b>1</b>) features a symmetric biscation where two imidazolium head groups (Im) with different alkyl chains (Cn) are connected to a central azobenzene unit (Azo): [Azo­(C1-Im-Cn)₂]; <i>n</i> = 6, 8, 10, 12, 14. The other one contains an <i>n</i>-alkyl-imidazolium cation (Cn-Im) bearing a terminal azobenzene unit (C1-Azo) substituted with an alkoxy chain (O-<i>Cm</i>) of either two (<b>2</b>) or six (<b>3</b>) carbon atoms: [C1-Azo-O-<i>Cm</i>-Im-Cn]; <i>m</i> = 2, <i>n</i> = 8, 10, 12 and <i>m</i> = 6, <i>n</i> = 8, 10, 12, 14, 16. For both cation classes, the influence of alkyl chains of varying length on the thermal phase behavior was investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). For five compounds (Azo­(-C1-Im-C12)₂ (<b>1d</b>), Azo­(-C1-Im-C12)₂ (<b>1e</b>), C1-Azo-O-C2-Im-C10 (<b>2b</b>), C1-Azo-O-C2-Im-C12 (<b>2c</b>), and C1-Azo-O-C6-Im-C16 (<b>3e</b>)), the formation of a liquid crystalline phase was observed. The biscationic salts (<b>1</b>) are all comparatively high melting organic salts (180–240 °C), and only the two representatives with long alkylchains (C12 and C14) exhibit liquid crystallinity. The monocationic salts with an O–C2 bridge (<b>2</b>) melt between 140 and 170 °C depending on the alkyl chain length, but from an alkyl chain of 10 and more carbon atoms on they form a smectic A liquid crystalline phase. The representatives of the third set with a O–C6 bridge qualify as ionic liquids with melting points less than 100 °C. However, only the representative with a hexadecyl chain forms a liquid crystalline phase. Representative single crystals for all sets of cations could be grown that allowed for single crystal structure analysis. Together with small-angle X-ray scattering experiments they allow for a more detailed understanding of the thermal properties. Through irradiation with UV-light (320–366 nm) all compounds undergo <i>trans–cis</i> isomerization, which reverses under visible light (440 nm)

Mesophase Stabilization in Ionic Liquid Crystals through Pairing Equally Shaped Mesogenic Cations and Anions

The synthesis and properties of a set of novel ionic liquid crystals with congruently shaped cations and anions are reported to check whether pairing mesogenic cations with mesogenic anions leads to a stabilization of a liquid crystalline phase. To that avail 1-alkyl-3-methyl-triazolium cations with an alkyl chain length of 10, 12, and 14 carbon atoms have been combined with <i>p</i>-alkyloxy-benzenesulfonate anions with different alkyl chain lengths (<i>n</i> = 10, 12, and 14). The corresponding triazolium iodides have been synthesized as reference compounds where the cation and anion have strong size and shape mismatch. The mesomorphic behavior of all compounds is studied by differential scanning calorimetry and polarizing optical microscopy. All compounds except 1-methyl-3-decyltriazolium iodide, which qualifies as an ionic liquid, are thermotropic ionic liquid crystals. All other compounds adopt smectic A phases. A comparison of the thermal phase behavior of the 1-methyl-3-decyltriazolium bromides to the corresponding <i>p</i>-alkoxy-benzensulfonates reveals that definitely the mesophase is stabilized by pairing the rod-shaped 1-alkyl-3-methyltriazolium cation with a rod-like anion of similar size