Substituent effects in columnar β-diketonate metallomesogens

Abstract

A systematic study of the mesomorphic properties of two homologous series of copper(II) and palladium(II) complexes 1a–b derived from unsymmetrical β-diketonate derivatives 2 containing a variety of functional substituents (X = H, CH₃, C₂H₅, OCH₃, Cl, Br, I, CN) on the phenyl ring is reported. These disc-like molecules all contain eight n-octyloxy chains, required for the formation of columnar phases appended to the central β-diketonate core. All complexes exhibited columnar phases studied by differential scanning calorimetry, polarized optical microscopy and powder X-ray diffraction. The mesomorphic results indicated that all compounds formed stable enantiotropic columnar mesophases, and the formation of columnar phases was strongly dependent on the electronic and/or the steric factors of the substituents. For compounds containing bulky substituents (i.e. X = Me, Et) a rectangular columnar phase (Col_r) was observed, however, other compounds containing electron-withdrawing substituents (i.e. X = Cl, Br, I) exhibited a hexagonal columnar phase (Col_h). Most of the palladium(II) complexes, except for X = CH₃ and OCH₃ derivatives, have lower clearing temperatures than those of the copper(II) analogues, and this is probably due to a weaker molecular interaction between the cores in the palladium(II) complexes. Lattice constants for these two types of compound obtained by powder X-ray diffraction were found to be insensitive to the metal center incorporated. These values were all relatively close in magnitude, which indicated that the two structures were quite similar. A satisfactory linear relationship with a correlation coefficient of 0.974 between the clearing temperatures and the Hammett σ_p constants of substituents was obtained only for the copper complexes.