Novel metallomesogens derived from heterocyclic benzoxazoles

The synthesis, mesomorphic behavior, and optical properties of a new series of transition metal complexes 1a and 1b derived from benzoxazoles 2 are reported. The crystal and molecular structure of copper complex of 6-dodecyloxynaphthalene-2-carboxylic acid 4-(6-hexyloxybenzooxazol-2-yl)-3-hydroxyphenyl ester 1a (n=6; m=12) was determined by means of X-ray structural analysis, and it crystallizes in the triclinic space group P-1. The geometry at copper center is perfectly square-planar, and the overall molecular shape is considered as rod-shape. All precursors 2 with shorter chains (n=0, 1, 4) exhibited nematic, and all others with longer chains (n=6, 7, 8, 10, 12) formed N/SmC phases. In contrast, all copper(II) complexes 1a and palladium complexes 1b formed N phase. On the other hand, the temperature range of mesophase in compound 1a was wider than those in compound 1b. The difference of the mesomorphic properties in compounds 1a and 1b was attributed to the geometry or/and the size of the metal center. The fluorescent properties of these compounds were also examined.