Influence of the lanthanide contraction on the transition temperatures of rare-earth containing metallomesogens with Schiff base ligands

We have synthesised liquid–crystalline rare-earth complexes with Schiff base ligands. The stoichiometry of these complexes is Ln(LH)3(NO3)3, where Ln is a trivalent rare-earth ion (Y3+, La3+–Lu3+, except Ce3+ and Pm3+), LH is the protonated form of the Schiff base 4-alkoxy-N-octadecyl-2-hydroxybenzaldimine. The mesophases were studied by optical microscopy and by differential scanning calorimetry (DSC). For the complexes with the 4-octyloxy-N-octadecyl-2-hydroxybenzaldimine ligand, a gradual increase in the melting point and a gradual decrease in the clearing point is found when going from La3+ to Lu3+. In this way, the mesophase range narrows over the lanthanide series. This is due to the lanthanide contraction. The textures observed by optical microscopy reveal the presence of a smectic A mesophase.