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

A systematic study of the mesomorphic properties of three series of copper( II) complexes based on beta -diketonate ligands containing branched side ch ains is reported. These disc-like compounds have four, six and eight flexib le alkoxy side chains appended to the central core, in which two or four si de chains were substituted by bulkier secondary alkoxy groups: 1-methylbuty loxy R' = C-5(2 degrees) or 1-methylheptyloxy R' = C-8(2 degrees). The meso morphic results indicated that at least eight side chains are required to f orm stable columnar mesophases; other compounds with four or six side chain s are not mesogenic regardless of the combination of the carbon length on t he alkoxy or secondary alkoxy groups of the side chains. The compounds 3 wi th shorter R' = C-5(2 degrees) side chains were all non-mesogenic regardles s of the carbon length of three alkoxy side chains (R = C-8, C-10, C-12) us ed. However, when the longer 1-methylheptyloxy side chain R' = C-8(2 degree s) was substituted, the compounds 3b-3e with various alkoxy groups (R = C-6 , C-7, C-8, C-10, C-12) exhibited columnar phases. The mesophases were char acterized and identified as columnar hexagonal phases (Col(h)), as expected , by thermal analysis and optical polarized microscopy. The presence of the introduced secondary alkoxy groups apparently appeared to influence the fo rmation of columnar phases. The clearing points were relatively lower than other similar copper(II) compounds not substituted by secondary alkoxy side chains.