Synthesis of main-chain hydrogen-bonded supramolecular liquid crystalline complexes: The effects of spacer on thermal behavior of mesophase

Two series of difunctional proton acceptors, stilbazole derivatives 4a-c and 6a-c with different spacers, oligo(methylene) and oligo(ethylene glycol), respectively, were synthesized. Hydrogen-bonded polymeric complexes 4/7 and 6/7 and trimeric complexes 4/82 and 6/82 (7 and 8 are aromatic dicarboxylic acids and monocarboxylic acids, respectively) were prepared, and their liquid crystallinity was examined. The effects of the spacer in 4, 6, and 7 and the terminal group in 8 on the thermal behaviors of hydrogen-bonded complexes were investigated using differential scanning calorimetry and polarizing optical microscopy. X-ray diffraction at elevated temperatures was used to verify liquid crystal phases. The study showed that the phase transition temperatures for isotropic to nematic (T_I-Ns) of polymeric complexes 4/7 and 6/7 in general decreased with the increase in length of spacer in the corresponding proton donors 7. The increase in length of the proton acceptors 6 led to a drop of T_I-Ns of the corresponding complexes 6/7; however, the T _I-Ns of complexes 4/7 did not show any correlation with the spacer length in 4. In contrast, the increase in length of the terminal group in 8 resulted in a slight decrease in T_I-Ns of trimeric complexes 4/8 ₂, but had a negligible effect on the T_I-Ns of 6/8 ₂ because of the presence of the more flexible spacer in the proton acceptors 6.