H–bonded mesogenic unsymmetric bis-heterocycles

Five new series of bis−heterocycles containing isoxazole and pyrazole were reported, and their mesomorphic properties were investigated. A polar phenolic−OH group incorporated in compounds 1b–c and 1e is to probe the possible H−bonds induced. Two single crystals of mesogenic compounds 1a and 1b (all n = 8) were obtained, and their single crystal and molecular structure were resolved. Intra− and intermolecular H−bonds observed in both crystal lattices were probably attributed to the formation of mesophases in these bis−heterocyclic structures. The CH−π interaction was also found to be important. All the precursors 2a−d were mesogenic, giving N, SmA, or/and SmC phases, however, other compounds 2e were not mesogenic. On the other hand, bis−heterocyclic compounds 1a−c were mesogenic, exhibiting SmC or SmA phases, and other compounds 1d−e were not mesogenic. The shape effect and H−bonds induced inherently from the core might be attributed to the formation of the mesophases in these bis−heterocycles. All derivatives showed good stabilities at temperature below T = 299−327 °C on TGA. The PL spectra of all compounds 1a−e showed one intense peak at λ_max = 317−386 nm, and these photoluminescent emissions originated from heterocyclic core.