Extended rodlike polyaromatic receptors with bent tridentate units complexed to lanthanide metal ions

A synthetic strategy is developed to attach semirigid lipophilic sidearms t o the 6-positions of bent aromatic tridentate 2,6-bis(benzimidazol-2-yl)pyr idine cores to produce U-shaped ligands, L-6,L-7. Differential scanning cal orimetry (DSC) reveals that entropic contributions severely affect the isot ropization processes of these flexible receptors, but no mesomorphism is de tected. The attachment of oxygen linkers to the 5- or 6-positions of the be nzimidazole sidearms lowers the ligand-centered (1)pi pi* and (3)pi pi* exc ited states, and the semiempirical ZINDO method assigns this effect to a de stabilization of the HOMO orbitals resulting from pi -interactions. Reactio ns of L6 With Ln(NO3)(3)(.) xH(2)O provide the rodlike 1:1 complexes [Ln(L- 6)(NO3)(3)] (Ln = La-Lu), which are stable in the solid state but partially dissociate in acetonitrile. The crystal structure of [Lu(L-6)(NO3)(3)]. CH 3CN (18a, LUC63H84N9O13, monoclinic, P2(1)/n, Z = 4) reveals an I-shaped ar rangement of the ligand strand arising from the meridional complexation of the bent tridentate unit to nine-coordinate Lu(III). The replacement of nit rate anions with trifluoroacetate anions gives the centrosymmetric dimer [L u(L-6)(CF3CO2)(3)](2) (23, Lu2C134H162N10O20F18, triclinic, P (1) over bar, Z = 1), in which the symmetry-related Lu atoms are connected by two bridgi ng carboxylates, leading to an II-shaped dimetallic edifice. These complexe s [Ln(L-6)(NO3)(3)] and [Ln(L-6)(CF3CO2)(3)](2) fulfill the geometrical cri teria required by precursors of calamitic metallomesogens, but no mesomorph ism can be detected, while photophysical studies indicate that the low ener gies of ligand-centered (3)pi pi* excited states drastically limit the lumi nescence of Eu(III) complexes. The relationships between structural and ele ctronic properties resulting from 5- or 6-substitutions of the benzimidazol e rings and the effects of these substitutions on photophysical and thermal properties are discussed.