Stefan Tussetschläger

Columnar Mesophases Controlled by Counterions in Potassium Complexes of Dibenzo[18]crown-6 Derivatives

Dibenzo[18]crown-6 derivatives 1 with two lateral tetraalkyloxy o-terphenyl units were prepared and converted to the corresponding complexes KX1 (X = halide, BF(4), PF(6), SCN) and NH(4)PF(6)1. Complexation was probed by MALDI-TOF spectrometry and NMR spectroscopy. Downfield shifts of (1)H NMR signals for complexes with soft anions Br, I, SCN, and PF(6) indicated the presence of tight ion pairs, whereas complexes with hard anions F, Cl, or BF(4) showed no or little shifts. In (13)C NMR spectra, upfield shifts were detected for soft anions. The character of the anion also influenced the mesomorphic properties of complexes MX1 (M = K, NH(4)), which were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and XRD in comparison to neat 1. Hard anions slightly stabilize or even destabilize the mesophase. Soft anions, however, improve the mesomorphic properties yielding mesophases with up to 70 degrees C phase widths in the case of KI1, KPF(6)1, and NH(4)PF(6)1. For complexes KSCN1 with a soft and bridging anion, the balance between mesophase stabilization and high order is shifted in favor of the plastic crystal phase.

p-Alkoxybiphenyls with guanidinium head groups displaying smectic mesophases

Novel calamitic guanidinium salts with p-alkoxybiphenyl or p-alkoxyphenyl core unit (7, 11, 12) have been synthesised, utilising the corresponding nitro compounds as synthetic intermediates. Differential scanning calorimetry (DSC), polarising optical microscopy (POM) and temperature-dependent X-ray diffraction studies reveal smectic A (SmA) mesophases for all series. Whereas the p-alkoxybiphenyl guanidinum chlorides 7 display SmA phases with a minimum chain lengths of C8 and broad mesophase width from 80°C (for C8 derivative 7b) up to 180°C (for C16 derivative 7g), the corresponding p-alkoxyphenyl guanidinium chlorides 11 display much smaller mesophase width from 4°C (for C10 derivative 11c) up to 72°C (for C16 derivative 11e). For the series 12 a large influence of the anion on the clearing point is observed, with clearing points ranging from 209°C for the guanidinium chloride 7f down to 127°C for guanidinium hexafluorophosphate 12d.

Wedge-shaped 1,2-diamidobenzenes forming columnar mesophases via hydrogen bonding

Two series of o-diamidobenzenes, bearing long-chain alkyl substituted gallic ester groups, have been synthesised. The 3,4,5,6-unsubstituted amides 7 with a minimum chain length of C9 exhibited columnar rectangular mesophases of p2mm symmetry with Z = 2. The liquid crystalline phases spread over phase width of extremely wide temperature intervals from -50 to 118°C. However, the phase width slightly decreased with an increase in the length of alkyl substituents. Despite their very similar molecular structure, 1,2-dimethoxy-4,5-diamides 10 revealed a different packing behaviour in their mesophases, due to steric factors. The diamides 10 formed columnar rectangular mesophases with p2mm symmetry, but with Z = 3. In a comparison with unsubstituted analogues 7, compounds 10 displayed even broader mesophases (with temperature intervals up to 220°C).

Crown ethers with lateral ortho-terphenyl units: effect of ester groups and sodium salts on the mesomorphic properties

Unsymmetrical crown ether derivatives 7 and 12 with one lateral o-terphenyl unit bearing different ester substituents were synthesized starting from methoxymethyl (MOM) protected bromobenzenes 3 and 8 by conversion into the respective borolanes 4 and 9, twofold Suzuki coupling with dibromobenzo[15]crown-5, acidic deprotection and finally esterification either with various alkanoic chlorides or gallic acids. Reaction with NaI provided the complexes [NaI·(7a–g)] and [NaI·(12a–d)], respectively. Uncomplexed crown ethers as well as their complexes [NaI·7a–c] and [NaI·12a–d] are non-mesogenic. In the case of [NaI·(7d–g)], however, the complexation induced a mesophase formation. As exemplarily shown for [NaI·7d], different textures were observed upon cooling, a fan-shaped texture, which is typical for a columnar hexagonal mesophase, and a striped fan-shaped texture, indicating a second mesophase Colx at low-temperature. From the SAXS diffraction pattern this mesophase was assigned to be columnar rectangular. For [NaI·7g] a different diffraction pattern was found, from which the low-temperature mesophase might be attributed to a soft crystal or a highly ordered columnar mesophase with orthorhombic symmetry.

Towards room temperature ionic liquid crystals: linear versus bent imidazolium phenylpyrimidines

The synthesis and the mesomorphic properties of novel imidazolium salts with mesogenic 2-phenylpyrimidine or 2-alkylpyrimidinecarboxylic acid central cores are reported. The mesogenic units are connected to the imidazolium head groups via an alkoxy spacer. In order to adjust the mesomorphic properties the length of the alkoxy spacer and the terminal alkyl group, the counter ion, the substitution pattern of the imidazolium head group and the molecular geometry (linear vs. bent) are varied and the corresponding compounds were investigated in detail using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray scattering (WAXS, SAXS). Whereas SmA phases with monolayer orientation were observed for imidazolium salts with short N-substituents (R = CH3, C4H9) at the imidazolium head group, the corresponding derivatives with longer N-substituents (R = C12H25, C12H25OC6H4) displayed SmA phases with bilayer orientation irrespective of a linear or bent geometry. For two derivatives, p-5(10,8) and p-5(12,8), a SmC phase was observed. Indeed, bending of the mesogenic core led to a shift of the mesophases towards lower temperatures. Several of the meta-2-phenylpyrimidine derivatives as well as 2-pyrimidine carboxylates displayed melting points below 50 °C. For 2-pyrimidine carboxylates replacement of a bromide anion by triflate resulted in a further decrease of the melting transition close to ambient temperature.

Desmosine-Inspired Cross-Linkers for Hyaluronan Hydrogels

We designed bioinspired cross-linkers based on desmosine, the cross-linker in natural elastin, to prepare hydrogels with thiolated hyaluronic acid. These short, rigid cross-linkers are based on pyridinium salts (as in desmosine) and can connect two polymer backbones. Generally, the obtained semi-synthetic hydrogels are form-stable, can withstand repeated stress, have a large linear-elastic range, and show strain stiffening behavior typical for biopolymer networks. In addition, it is possible to introduce a positive charge to the core of the cross-linker without affecting the gelation efficiency, or consequently the network connectivity. However, the mechanical properties strongly depend on the charge of the cross-linker. The properties of the presented hydrogels can thus be tuned in a range important for engineering of soft tissues by controlling the cross-linking density and the charge of the cross-linker.

Pushing steric bias in the Scholl reaction to access liquid crystalline crown ethers.

Sterically congested o-terphenyl crown ethers with alkoxy substituents at the 2,3,4-position or 3,4,5-position were synthesized from the corresponding tetrabromodibenzo[15]crown-5 and the corresponding boronic acids or borolanes via Suzuki cross-coupling and subsequently cyclized to the corresponding triphenylenes utilizing the Scholl reaction. Both series of compounds were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction (SAXS, WAXS) regarding their mesomorphic properties. While all but one of the 3,4,5-substituted derivatives displayed liquid crystalline behavior (Col(h) and Col(r)), only the 2,3,4-substituted triphenylene with the shortest alkoxy chains was liquid crystalline (Col(r)).

Increased mesophase range in liquid crystalline crown ethers via lower molecular symmetry

Herein, we report the synthesis and characterisation of liquid crystalline crown ethers bearing central [15]crown-5, unsymmetric [18]crown-6 and [21]crown-7 with lateral o-terphenyl and triphenylene substituents. Comparison with similar derivatives of [12]crown-4, symmetric [18]crown-6 and [24]crown-8 revealed beneficial effects of symmetry reduction on the mesomorphic properties. Decreasing symmetry led to broader phases and melting points at lower temperatures.

Influence of spacer chain lengths and polar terminal groups on the mesomorphic properties of tethered 5-phenylpyrimidines

Summary Based on 5-(4-hydroxyphenyl)-2-octylpyrimidine 8, 5-phenylpyrimidine derivatives 3–7, 9 with different spacer chain lengths (C2 up to C6) and different terminal polar groups (Br, Cl, N3, OH, CN) were synthesized by etherification and nucleophilic substitution. The mesomorphic behaviour of these compounds was investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS and SAXS) and revealed smectic A mesophases for bromides, chlorides and azides 3, 4 and 6. For these compounds a maximum phase width was observed for the C5 spacer regardless of the terminal group, whereas the hydroxy- and cyano-substituted derivatives 5 and 7, respectively, were non mesomorphic and showed only melting transitions.

Headgroups versus symmetry in congruent ion pairs: which one does the job in mesomorphic aryl guanidinium and aryl imidazolium sulphonates?

Ionic liquid crystals based on congruent ion pairs composed of mesogenic cations and anions of similar shape provide an attractive tool for the tuning of mesophase properties. Here, the effect of the number and symmetry of lipophilic side chains and the type of head group on the phase type and thermal mesophase properties was probed by the synthesis and investigation of two series of novel guanidinium and imidazolium sulphonates and compared with the corresponding iodides. Their mesomorphic properties were examined via differential scanning calorimetry, optical polarising microscopy and X-ray diffraction. While derivatives bearing only one alkoxy chain in either cation or anion with up to three alkoxy chains in total within the ion pairs display smectic A mesophases, hexagonal columnar mesophases were observed for all other compounds with four or five alkoxy chains totally irrespective of the head group. However, with increasing steric bulk, i.e. with a total of six alkoxy chains, the symmetry of the aryl moiety in the anion as well as the type of head group becomes relevant, resulting in hexagonal columnar or plastic phases for guanidinium sulphonates with symmetrical anions, while those with unsymmetrical anions were non-mesomorphic. In contrast, the corresponding imidazolium sulphonates displayed cubic phases for the combinations of symmetrical cation/symmetrical anion and symmetrical cation/unsymmetrical anion. If both ions are unsymmetrically substituted, the imidazolium sulphonate displayed a hexagonal columnar phase. The results further demonstrate the utility of the congruent ion pairs for tailor-made ionic liquid crystals.