Polarization effects in mesogenic isoxazoles and 1,3,4-oxadiazoles

Four new series of unsymmetric isoxazoles and 1,3,4-oxadiazoles were prepared, characterized and their mesomorphic properties investigated. Isoxazoles were obtained by condensation–cyclization of β-diketones with hydroxylamine hydrochloride in refluxing THF, while 1,3,4-oxadiazoles were obtained from hydrazine-carboxylate in refluxing POCl3. Two single crystallographic structures were determined by X-ray crystallographic analysis. A correlated dimeric structure was formed by H-bonds in isoxazoles 1a (n=6), leading to a more elongated structure required for the formation of mesophases. All compounds 1a–c formed N, SmA or/and SmC phases. In contrast, compounds 1d exhibited columnar phases, and an Ncell=14.6 obtained from powder XRD data indicated that a correlated structure formed by four molecules was probably induced in Colh phases. The better mesomorphic behavior formed in 1a than 1b might be attributed to stronger intermolecular interactions and higher polarization induced in isoxazoles 1a.

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