Self-assembly of 4-aryl-1H-pyrazoles as a novel platform for luminescent supramolecular columnar liquid crystals

Abstract

Supramolecular liquid crystals containing the unprecedented 4-aryl-1H-pyrazole unit are reported. This moiety is able to self-assemble by H-bonding to give columnar mesophases and also display luminescent properties in the visible region. The molecular structures consist of a polyalkoxybenzamide group substituted at the nitrogen atom by the 4-(3,5-dimethyl-1H-pyrazol-4-yl)phenyl unit. A family of compounds was synthesized to evaluate the supramolecular structures and the mesomorphic properties.The 3,4,5-tri-n-decyloxybenzamide derivative is a non-discoid molecule that displays a hexagonal columnar mesophase and a rectangular columnar mesophase. Powder X-ray diffraction studies show that these molecules are able to self-assemble in columns, and that a columnar stratum of the hexagonal columnar mesophase contains five molecules on average. A model for the columnar organization is proposed in which a self-assembled aggregate is formed by an antiparallel arrangement of the molecules partially interdigitated and interacts by hydrogen bonds. The amide group was changed by an ester group, in order to asses the role of the amide and pyrazole groups in the self-assembly. The ester compound also exhibited a columnar mesophase with similar cell parameters. This means that, together with the tapered shape of the molecule, the 1H-pyrazole moiety, and not the amide group, is essential for the aggregation leading to the mesomorphism. In addition, the single crystal structure of a model compound N-(4-(3,5-dimethyl-1H-pyrazol-4-yl)phenyl)benzamide was solved. The structure was a methanol solvate in which each pyrazole is engaged in hydrogen bonding to form a dimer through methanol bridges. The pyrazole molecules stack along the a axis and interact by hydrogen bonding through the amide groups to yield parallel infinite chains. Benzamide compounds are luminescent in the solid state and in the liquid crystalline state, in the blue region of the visible spectrum.

Supramolecular Columnar Liquid Crystals with Tapered‐Shape Simple Pyrazoles Obtained by Efficient Henry/Michael Reactions

“…The results show that the molecule is not flat and the pyrazole and the phenyl rings are not co-planar but are twisted by 46.78.T his structure is consistentw ith reported single-crystal structures of 4-aryl-3,5-dimethylpyrazoles. [5,27] The methyl groups of the methoxy substituent in the meta-position lie closet ot he plane of the phenylr ing (9.48 and 48), whereas that in the para-position is almostperpendicular to the phenyl ring (78.48).…”

“…[13] In this way, and after optimizing the reaction temperature and time in order to avoid the deacylations ider eaction that leads to benzylm ethyl ketones, [14] the synthesis of 3-aryl-2,4-pentanodiones with one electron-withdrawing group (NO 2 ), or with one electron-donating group (OH or OCH 3 )inthe para-position was achieved with yields of 74 %a nd 44 %, respectively in our previouswork. [5] This route could be applied to the required 1,3-diketones by using the commerciallya vailable 4-iodo-1,2-dimethoxybenzene and 5-iodo-1,2,3-trimethoxybenzene. In subsequents teps, the diketone would be demethylated and alkylated with the long chains, that is, ad eprotection step would be necessary.H owever,a ll attempts to obtain the diketones gave only very low yields (less than 20 %).…”

“…[3] In particular, the 1H-pyrazole heterocycle is able to form intermolecularN H···N interactions and this ability has been successfully exploited to yield columnar mesophases with luminescentp roperties with either disc-like supramolecular dimeric entities [4] or aggregates formed by taperedm olecules. [5] In terms of molecular structure, ac ommon molecular design for columnar liquid crystalsi st oh avea romatic rings substituted with two or three long alkoxy tails (C 6 or longer). To achieve this goal, it is necessary to adapt previously described synthetic routes for non-substituted analogs, or methoxy-substituted analogs, and this task is not always straightforward.…”

Symmetric 3,5-pyrazole and isoxazole heterocycles comprising a bent core unit: synthesis and mesomorphic characterisation

“…The moiety is a non-discoid molecule that displays a hexagonal columnar mesophase. [22,23] Significant efforts have been devoted to find new synthetic methods for preparation of pyrazole derivatives. [24] Heller et al worked out an efficient, simple and easy method for the synthesis of 3,5-disubstituted pyrazoles.…”

Highly efficient blueish-green fluorescent OLEDs based on AIE liquid crystal molecules: from ingenious molecular design to multifunction materials

“…[9][10][11][12][13][14][15][16][17][18] Kato and co-workers have developed many luminescent mesogens formed by a rigid, π-conjugated core (such as pyrene and anthracene) functionalised with peripheral alkoxy chains. [19][20][21][22] Although this strategy can effectively generate liquid crystallinity, the emission quantum yield (Φ PL ) is often rather low, probably due not only to non-radiative transitions via intramolecular vibration caused by the periphery flexible chains, but also the aggregation-induced quenching of the rigid π-conjugated core.…”