Steric effect on the formation of columnar phases in β-diketonate copper(II) complexes

A systematic study of the mesomorphic properties of three series of copper(II) complexes based on β-diketonate ligands containing branched side chains is reported. These disc-like compounds have four, six and eight flexible alkoxy side chains appended to the central core, in which two or four side chains were substituted by bulkier secondary alkoxy groups: 1-methylbutyloxy R ' = C5(2°) or 1-methylheptyloxy R ' = C8(2°). The mesomorphic results indicated that at least eight side chains are required to form stable columnar mesophases; other compounds with four or six side chains are not mesogenic regardless of the combination of the carbon length on the alkoxy or secondary alkoxy groups of the side chains. The compounds 3 with shorter R ' = C5(2°) side chains were all non-mesogenic regardless of the carbon length of three alkoxy side chains (R = C8, C10, C12) used. However, when the longer 1-methylheptyloxy side chain R ' = C8(2°) was substituted, the compounds 3b-3e with various alkoxy groups (R = C6, C7, C8, C10, C12) exhibited columnar phases. The mesophases were characterized and identified as columnar hexagonal phases (Colh), as expected, by thermal analysis and optical polarized microscopy. The presence of the introduced secondary alkoxy groups apparently appeared to influence the formation of columnar phases. The clearing points were relatively lower than other similar copper(II) compounds not substituted by secondary alkoxy side chains.