New metallomesogens derived from unsymmetric 1,3,4-thiodiazoles: synthesis, single crystal structure, mesomorphism, and optical properties

A series of copper(II) complexes 1 derived from unsymmetric 1,3,4-thiadiazoles 2 exhibiting mesogenic properties are reported. All the precursors 2 and 3 exhibited smectic A or/and smectic C phases, whereas, copper complexes formed nematic, SmA or SmC phases. The mesophases formed by derivatives 2 and 3 were probably attributed to the H-bondings induced both intramolecularly or/and intermolecularly between amide (–NH) and phenolic (–OH) groups. The crystal and molecular structures of mesogenic 2-(5-(2-(hexyloxy)naphthalene-6-yl)-1,3,4-oxadiazole-2-yl)phenol (2; n=6, m=6) were determined by means of X-ray structural analysis. It crystallizes in the monoclinic space group P-1, with a=7.4255(18) Å, b=8.209(2) Å, c=17.315(5) Å, and Z=2. An intermolecular H-bond (d=1.89 Å) between N2 and H1A atoms with an angle of 161.5° was observed. All molecules were packed as tilted layer arrangement and a π–π interaction (ca. 3.56 Å) was observed. Variable temperature FTIR and 1H NMR spectroscopies were also used to probe the possible H-bondings formed in compound 2 (m=0, n=6). The fluorescent properties of these compounds 2 were examined. All λmax peaks of the absorption and photoluminescence spectra occurred at ca. 359–363 nm and 519–537 nm, respectively. Excited state intramolecular proton transfer (ESIPT) reaction in this type of ortho-hydroxy-1,3,4-thiadiazole was also observed.