Columnar/smectic metallomesogens derived from heterocyclic benzoxazoles

The synthesis, mesomorphic behavior, and optical properties of two new series of metal complexes 1a,b–M (M=Pd, Cu, Zn) derived from benzoxazoles 2a,b are reported. The crystal and molecular structures of mesogenic 5-decyloxy-2-(6-decyloxybenzooxazol-2-yl)phenol and nonmesogenic bis[5-octyloxy-2-(6-octyloxybenzooxazol-2-yl) phenol]Pd(II) were determined by means of X-ray structural analysis. Two benzoxazoles 2a exhibited monotropic SmA phases, and all benzoxazoles 2b were nonmesogenic. On the other hand, metal complexes 1a–M exhibited distinctly different mesomorphism from complexes 1b–M. Complexes 1a–Pd formed SmC phases; complexes 1a–Cu and 1a–Zn formed crystal phases. In contrast, complexes 1b–Zn exhibited columnar phases, and complexes 1b–Cu and 1b–Pd were nonmesogenic. The difference of the mesomorphism in 1a–M and 1b–M was probably attributed to the geometry and/or the overall molecular shape created by 2a and 2b. The electronic configuration of metal ion might play an important role in forming the mesophases. The fluorescent properties of these compounds were also examined.