Ionic liquid crystals derived from guanidinium salts: induction of columnar mesophases by bending of the cationic core

Antill, Lewis M.; Neidhardt, Manuel M.; Kirres, Jochen; Beardsworth, Stuart J.; Mansueto, Markus; Baro, Angelika

doi:10.1080/02678292.2014.896052

Cited by 23 publications

(16 citation statements)

References 31 publications

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“…[1][2][3][4][5] With high thermal stability, the ease of charge delocalization and coordination properties as well as the possibility to attach up to six different substituents on the guanidine moiety has driven research activities in diverse directions, resulting in their use as superbases, [6,7] ligands for coordination complexes, [8][9][10][11] organocatalysts, [12][13][14][15][16] stimuli-responsive materials, [17] hydrogels, [18] anion exchange polymer electrolytes for fuel cells, [19] and biologically active compounds [20][21][22][23][24][25][26][27][28][29] for drug development. Furthermore, guanidinium salts have also entered the field of ionic liquid crystals (ILCs) [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] as an alternative cationic head group to the imidazolium-derived ILCs, which have dominated this research area so far. …”

Section: Introductionmentioning

confidence: 99%

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

et al. 2014

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A series of phenylguanidinium salts 3 . X, which are linked via an alkoxy spacer either to a 4-decyloxy-or 4-cyanosubstituted biphenyl mesogen, was prepared and the mesomorphism studied. A decyloxybiphenyl core and a spacer of at least C6 chain length were required for mesophase formation. Replacement of the chloride counterion by other anions like bromide or tetrafluoroborate improved the thermal stability of the mesophase. A comparison of substitution pattern (meta v. para) on the phenyl ring revealed decreased melting and clearing points for the bent cationic head group. All guanidinium ionic liquid crystals 3 displayed only smectic A (SmA) phases. A packing model is assumed where the molecules in a bilayer stack over each other in opposite direction with interdigitated terminal decyloxy groups and spacers.bromides 9 giving the tethered compounds p-10a,b and m-10b-e in 78-88 % yield. N-Boc-deprotection to derivatives p-11a,b and m-11b-e was achieved very cleanly with methane sulfonic acid in CHCl 3 . Derivative m-11a was obtained in 94 % yield by acidic hydrolysis [62] of acetamide m-7a. The aniline derivatives 11 were finally treated with chloro-N,N,N 0 ,N 0tetramethylformamidinium chloride [63] and either NEt 3 or NaHCO 3 . Workup strictly required an inert gas atmosphere to provide the neat target guanidinium salts 3 . Cl in 70-98 % yield. To further study the anion effect, decyloxybiphenyl guanidinium chloride m-3a . Cl was submitted to salt metathesis with NaBr, KI, NaOTf, NaBF 4 , KPF 6 , KOAc, and KSCN yielding the corresponding guanidinium ILCs m-3a . X (Scheme 2). Anion Effect in Guanidinium ILCsComparison of the 1 H NMR spectra of phenylguanidinium salts m-3a . X revealed a significant downfield shift of the N-H signal from ,7.5 to 12.0 ppm in the following order: PF 6 , BF 4 , OTf , I , SCN , Br ECl (Fig. 1).This correlation between the anion and the N-H proton shift indicates the presence of contact between the ion pairs that also led to small chemical shifts of the aromatic protons of the phenylguanidinium moiety (Fig. 1, grey). To estimate the effect of concentration on the N-H signal, 1 H NMR measurements of m-3b . Cl with concentrations varying from 0.83 to 2.50 mg mL À1 were carried out. The N-H signal shift of 0.1 ppm turned out to be negligible with respect to the strong dependence of the NH signal on the anion (see Supplementary Material for details). This effect rather might be due to either the interaction of the anion with the anisotropic cone of the aryl ring than due to the conjugation between the guanidinium cation and the aryl ring. Previous DFT calculations of several guanidinium salts revealed no or only very little conjugation, i.e. the guanidinium moiety behaves like an isolated cation. [39] In agreement with these studies, the N-H signal shifted upfield with increasing anion radii [64,65] with an almost linear correlation (Fig. 2).

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Section: Introductionmentioning

confidence: 99%

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

et al. 2014

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“…The mesogen ILC A on the other hand shows a phase transition from an isotropic to a smectic A phase at 82 °C and also a crystalline phase below 42 °C. They were synthesized according to the published procedure and showed the expected spectral and physical properties. For the Kerr effect measurements the ILC samples were recrystallized after the synthesis until they were free from water and small ions like chloride or sodium.…”

Section: Resultsmentioning

confidence: 99%

Large Electro‐Optic Kerr Effect in Ionic Liquid Crystals: Connecting Features of Liquid Crystals and Polyelectrolytes

et al. 2018

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The electro-optic Kerr effect in simple dipolar fluids such as nitrobenzene has been widely applied in electro-optical phase modulators and light shutters. In 2005, the discovery of the large Kerr effect in liquid-crystalline blue phases (Y. Hisakado et al., Adv. Mater. 2005, 17, 96-98.) gave new directions to the search for advanced Kerr effect materials. Even though the Kerr effect is present in all transparent and optically isotropic media, it is well known that the effect can be anomalously large in complex fluids, namely in the isotropic phase of liquid crystals or in polyelectrolyte solutions. Herein, it is shown that the Kerr effect in the isotropic phase of ionic liquid crystals combines the effective counterion polarization mechanism found in polyelectrolytes and the unique pretransitional growth of the Kerr constant found in the isotropic phase of nematic liquid crystals. Maximum Kerr constants in the order of several 10 m V (ten times higher than the Kerr constant of the toxic nitrobenzene and less temperature sensitive than Kerr constants of nematic liquid crystals) make ionic liquid crystals attractive as new class of functional materials in low-speed Kerr effect applications.

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“…The variation of the counterion is reported to have a profound effect on the mesomorphism of ILCs . We thus investigated the symmetrical acyclic guanidinium salts 3(14 , 14)X with different counterions by DSC in comparison with the chloride 3(14 , 14)Cl (Table , entries 5, 13–17, Figure ).…”

Section: Resultsmentioning

confidence: 99%

“…For example,M altese crosses were observed for 4 (18,18)Cl at 90 8C The variation of the counterion is reported to have ap rofound effect on the mesomorphism of ILCs. [47,[59][60][61][62] We thus investigated the symmetrical acyclicg uanidinium salts 3(14,14)X with different counterions by DSC in comparison with the chloride 3 (14,14)Cl (Table 1, entries 5, 13-17, Figure 5). For all counterions (X = Cl, Br,I ,B F 4 ,P F 6 ,O Tf) enantiotropic mesomorphism was detected.…”

Section: Synthesis Of the Guanidinium Tyrosinatesmentioning

confidence: 99%

Tyrosine‐Based Ionic Liquid Crystals: Switching from a Smectic A to a Columnar Mesophase by Exchange of the Spherical Counterion

Neidhardt

Wolfrum

Beardsworth

et al. 2016

Chemistry A European J

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Synthetic strategies were developed to prepare l-tyrosine-based ionic liquid crystals with structural variations at the carboxylic and phenolic OH groups as well as the amino functionality. Salt metathesis additionally led to counterion variation. The liquid-crystalline properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS). The symmetrical ILC chlorides bearing the same alkyl chain at both the ester and ether but either an acyclic or cyclic guanidinium group displayed enantiotropic SmA mesophases with phase widths of 31-88 K irrespective of the head group. It was particularly the replacement of chloride in the acyclic guanidinium ILC by hexafluorophosphate that induced a phase change from SmA to Col . This phase change was attributed to a higher curvature of the interface due to the larger anion, which increased the effective head group cross-sectional area of the amphiphilic ILC. The unsymmetrical acyclic guanidinium chlorides, bearing a constant C ester and variable alkyl chains on the phenolic position, formed enantiotropic SmA phases. The derivative with the largest difference in chain lengths, however, displayed a Col phase, resulting from discoid aggregates of the cone-shaped guanidinium chloride. The results are discussed in terms of the packing parameters, which indicate that the phase behaviour of the thermotropic tyrosine-based ILCs shows analogies to those of lyotropic liquid crystals.

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Ionic liquid crystals derived from guanidinium salts: induction of columnar mesophases by bending of the cationic core

Cited by 23 publications

References 31 publications

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

Large Electro‐Optic Kerr Effect in Ionic Liquid Crystals: Connecting Features of Liquid Crystals and Polyelectrolytes

Tyrosine‐Based Ionic Liquid Crystals: Switching from a Smectic A to a Columnar Mesophase by Exchange of the Spherical Counterion

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