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.