Tuning mesophase topology in hydrogen-bonded liquid crystals via halogen and alkyl chain engineering

This study explores the influence of halogen substitution and alkyl chain length on the liquid crystalline properties of hydrogen-bonded supramolecules. Three series of hydrogen-bonded liquid crystals (HBLCs) were synthesized by combining 4-alkoxyphenylazopyridines as proton acceptors with varying alkyl chain lengths and ortho-halogenated (F, Cl, or Br) 4-dodecyloxybenzoic acids as proton donors. The formation of hydrogen-bonding interactions between the individual components was confirmed using FTIR spectroscopy. The self-assembly behavior of these HBLCs was characterized using differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD). Our findings demonstrate that systematic variation of the halogen atom and alkyl chain length profoundly impacts mesophase stability and type. Specifically, fluorinated HBLCs exhibit elevated melting and clearing temperatures, whereas their chlorinated and brominated counterparts show lower melting points and broader mesophase ranges. The choice of halogen also determines the type of liquid crystalline phases, resulting in the formation of tilted smectic C (SmC), orthogonal smectic A (SmA), and nematic phases. Furthermore, these materials exhibit rapid and reversible trans–cis photoisomerization upon light exposure. This work elucidates design principles for tuning the properties of HBLCs through synergistic halogen and chain-length engineering.