Extended Rodlike Polyaromatic Receptors with Bent Tridentate Units Complexed to Lanthanide Metal Ions

A synthetic strategy is developed to attach semirigid lipophilic sidearms to the 6-positions of bent aromatic tridentate
2,6-bis(benzimidazol-2-yl)pyridine cores to produce U-shaped ligands, L6,7. Differential scanning calorimetry (DSC)
reveals that entropic contributions severely affect the isotropization processes of these flexible receptors, but no
mesomorphism is detected. The attachment of oxygen linkers to the 5- or 6-positions of the benzimidazole sidearms
lowers the ligand-centered 1ππ* and 3ππ* excited states, and the semiempirical ZINDO method assigns this
effect to a destabilization of the HOMO orbitals resulting from π-interactions. Reactions of L6 with Ln(NO3)3·xH2O provide the rodlike 1:1 complexes [Ln(L6)(NO3)3] (Ln = La−Lu), which are stable in the solid state but
partially dissociate in acetonitrile. The crystal structure of [Lu(L6)(NO3)3]·CH3CN (18a, LuC63H84N9O13, monoclinic,
P21/n, Z = 4) reveals an I-shaped arrangement of the ligand strand arising from the meridional complexation of
the bent tridentate unit to nine-coordinate Lu(III). The replacement of nitrate anions with trifluoroacetate anions
gives the centrosymmetric dimer [Lu(L6)(CF3CO2)3]2 (23, Lu2C134H162N10O20F18, triclinic, P1̄, Z = 1), in which
the symmetry-related Lu atoms are connected by two bridging carboxylates, leading to an H-shaped dimetallic
edifice. These complexes [Ln(L6)(NO3)3] and [Ln(L6)(CF3CO2)3]2 fulfill the geometrical criteria required by
precursors of calamitic metallomesogens, but no mesomorphism can be detected, while photophysical studies
indicate that the low energies of ligand-centered 3ππ* excited states drastically limit the luminescence of Eu(III)
complexes. The relationships between structural and electronic properties resulting from 5- or 6-substitutions of
the benzimidazole rings and the effects of these substitutions on photophysical and thermal properties are discussed.