Luminescent Properties of Lanthanide Nitrato Complexes with Substituted Bis(benzimidazolyl)pyridines

The protonated form of the ligand 2,6-bis(1‘-methylbenzimidazol-2‘-yl)pyridine crystallizes as its perchlorate salt (HL¹)ClO₄ (1) in the orthorhombic system Pbca, with a = 13.976(3) Å, b = 14.423(3) Å, c = 19.529(4) Å, Z = 8. The proton is located on one benzimidazole N-atom, and the two N-methyl substituents lie on the same side of the pyridine N-atom (cisoid conformation). New 1:1 nitrato complexes of composition [Eu(NO₃)₃(Lⁱ)](solv‘)_y have been isolated with L⁴ (4), L⁶ (5), L⁷ (6), and L⁸ (7), and their structural and photophysical properties are compared with those of the previously reported complexes [Eu(NO₃)₃(L¹)(MeOH)] (2) and [Eu(NO₃)₃(L³)] (3). The crystal and molecular structure of [Eu(NO₃)₃(L⁷)(MeCN)]·2.5MeCN at 180 K (6a, triclinic, P1̄, a = 12.137(2) Å, b = 14.988(3) Å, c = 16.926(3) Å, α = 114.52(3)°, β = 98.28(3)°, γ = 103.99(3)°, Z = 2) shows a decacoordinated Eu^III ion to six O-atoms from the nitrates, three N-atoms from L⁷, and one N-atom from a coordinated MeCN. The metal-centered luminescence arising upon ligand excitation in the solid state is analyzed in terms of nephelauxetic effects of the ligand and crystal field splitting of the ⁷F₁ level. Quantum yields of 10^-3 and 10^-4 M solutions in MeCN are substituent dependent and may be rationalized by taking into account several factors, including the energy of the ligand singlet and triplet levels and the arrangement of the ligands in the first coordination sphere. We also show that the quantum yield of the ligand-centered luminescence decreases in the order L¹ > [La(NO₃)₃(L¹)]MeOH > [La(L¹)₃](ClO₄)₃.