Heterocyclic 3,5-disubstituted phenylpyrazoles and isoxazoles: synthesis and mesomorphic behavior

Three new series of heterocyclic pyrazoles Mpz-Cn and Epz-Cn and isoxazoles Iz-Cn were prepared, characterized, and their mesomorphic properties investigated. These heterocyclic derivatives were obtained by condensation–cyclization of β-diketones with methylhydrazine, 2-hydradinoethanol or hydroxylamine in refluxing THF/ethanol. One single crystallographic structure of nonmesogenic Epz-C4 was determined by X-ray analysis. It crystallizes in a triclinic space group P−1. An H-bond-induced structure was formed, giving a better aspect ratio required for the formation of mesophases. Both series of pyrazoles Mpz-Cn and EpZ-Cn exhibited monotropic smectic A phases, in contrast, isoxazoles Iz-Cn formed enantiotropic smectic C phases. A lack of H-bond due to a removal of hydrogen atom from pyrazolyl rings (–NH) significantly lowered the clearing temperatures of both Mpz-Cn and EpZ-Cn. These compounds have good thermal stabilities.