Dimerization of Dendrimeric Lanthanide Complexes: Thermodynamic, Thermal, and Liquid-Crystalline Properties

A series of 10 different mesomorphic semidendrimeric tridentate ligands L5-L14 grafted with terminal cyanobiphenyl groups have been synthesized. Upon reaction with Ln(NO3)(3) (Ln = trivalent lanthanide), the central 2,6-bis(N-ethylbenzimidazol-2-yl)pyridine unit is meridionally tricoordinated to the metal to give rodlike monomeric [Ln(Lk)(NO3)(3)] and H-shaped dimeric [Ln(2)(Lk)(2)(NO3)(6)] complexes. For the small Lull' cation, the monomeric complexes are quantitatively formed in a noncoordinating CD2Cl2 solution, For larger cations (Ln = Eu, Pr), the thermodynamic equilibrium 2[Ln(Lk)(NO3)(3)] [Ln(2)(Lk)(2)(NO3)(6)] can be evidenced across the complete ligand series. Detailed thermodynamic studies show that the dimeric complexes result from the formation of primary intermetallic nitrate bridges whose strength depends on the metallic size. For each complex, secondary nonspecific interstrand van der Waals interactions produce nonartifactual enthalpy/entropy compensation. In the absence of solvent, only the complexes with the most extended ligands L5 and L6 produce thermotropic mesophases. Layered organizations are dominant (smectic A) with the induction of nematogenic behavior at high temperature when interstrand interactions are modulated by methyl substitutions. Correlations between the trend of dimerization and the sequences of thermotropic mesophases are attempted.