1,4-Bis(Acylhydrazone)-Based Polycatenar Liquid Crystals: Self-Assembly, Molecular Switching, and Gelation Properties

Photochromic materials allow their properties to be reversibly modulated by using light, offering broad applicability and significant scientific interest. In the case of liquid crystals (LCs), this opens up possibilities for controlling the self-organization, its properties, and even extinguishing mesomorphism. Among the diverse liquid crystalline functional groups, acylhydrazones remain the underexplored photochromic group despite their advantageous characteristics, such as ease of synthesis, versatility, reversible <i>E-Z</i> photoisomerization, and gelation. Building on efforts to demonstrate the versatility of this group, this study introduces, for the first time, a second acylhydrazone unit into linear polycatenar molecules, enabling a systematic investigation of the effects of structural variations, including the number of peripheral alkoxy chains, the orientation of the acylhydrazone group, and expansion of the rigid core. These modifications led to the synthesis of six new molecules, whose thermal, mesomorphic, switching, and gelation properties were systematically investigated. The results demonstrated that the introduction of a second acylhydrazone unit significantly enhanced these properties, with 5 molecules showing LC properties. Depending on the molecular design, mesomorphism was stabilized, and self-organization varied between hexagonal columnar (Col_h) and bicontinuous cubic (Cub_bi) phases. The orientation of the acylhydrazone units influenced mesomorphism, drastically altered the photoisomerization rate, and induced mild luminescence. These properties were correlated with intermolecular interactions and the way they promoted self-organization in the respective mesophases. Additionally, the two molecules formed stable, photoisomerizable gels, highlighting the versatility of acylhydrazone units in the development of multifunctional materials.