Synthesis, characterization and study the mesogenic properties of a new class of asymmetrical calamitic mesogenic materials based on imidazo[2,1-b][1,3,4]thiadiazole

Abstract

A new series of asymmetrical calamitic mesogens, 6-(4-methoxyphenyl)-2-(4-alkoxyphenyl)-imidazo[2,1-b][1,3,4]thiadiazole derivatives (M_n), has been successfully synthesized. This series is characterized by a coplanar imidazo[2,1-b][1,3,4]thiadiazole central core, featuring a methoxy‑terminal group on the imidazo side and alkoxy-terminated chains (OR = OC_nH_2n+1) on the thiadiazole side. The synthesis of the (M_n) series was achieved through a cyclization reaction between 2-amino-1,3,4-thiadiazole derivatives (N_n) and 4-methoxyphenacyl bromide. The chemical structures of the compounds were confirmed using various analytical techniques, including FT-IR, NMR (¹H & ¹³C) spectroscopy, and mass spectrometry (EI-MS). The mesomorphic properties of these calamitic mesophases were investigated using polarizing optical microscopy (POM), differential scanning calorimetry (DSC) during both heating and cooling cycles and X-ray powder diffraction (XRPD) analyses. All mesogens (M_b–M_j, M_k) exhibited enantiotropic mesomorphic behavior. Notably, the second compound in the series, M_b (n = 2), displayed a nematic (N) mesophase during heating, while both nematic (N) and smectic A (SmA) phases appeared upon cooling. In contrast, other derivatives (M_c–M_g, M_i) (n = 3, 4, 5, 6, 7, 9) exhibited both nematic (N) and smectic A (SmA) mesophases during heating and cooling. The final members of the series, M_h, M_j, and M_k (n = 8, 10, 12), exclusively exhibited the smectic A (SmA) mesophase in both thermal cycles. The observed mesophase behavior, including the presence of smectogenic and nematogenic ranges, is attributed to the coplanarity and conjugation of the central fused imidazo[2,1-b][1,3,4]thiadiazole core. Furthermore, the methoxy (-OCH₃) terminal group and the gradual elongation of the flexible alkoxy tail play a crucial role in influencing the mesomorphic properties of these compounds.