Structural, photophysical and chiro-optical properties of lanthanide complexes with a bis(benzimidazole)pyridine-based chiral ligand

The neutral Ln(III) 1:1 nitrato complexes with the chiral ligand 2,6-bis(1-S-neopentylbenzimidazol-2-yl)pyridine (L-11) have been synthesised and their stability constants measured in acetonitrile (log K-1 = 4.0-6.4). The crystal and molecular structure of [Eu(NO3)(3)(L-11)(MeCN)] shows the typical meridional planar coordination of L-11 to the metal ion and low symmetry of the coordination polyhedron. The influence of the steric hindrance generated by the substituent at R-2 on the crystal packing and bond lengths is discussed. Photophysical measurements show that ligand L-11 induces a (3)pipi*-to-Ln energy-transfer process in the Eu-III complex, while the Tb-III compound is ten times less luminescent. Addition of a second molecule of L-11 to give [Ln(ClO4)(2)(L-11)(2)](+) leads to a large quenching of the Eu-III luminescence (140-fold) due to several factors: a less efficient (1)pipi*-->(3)pipi* transfer (ca. fourfold), a smaller intrinsic quantum yield Q(Eu) (ca. threefold), and a substantially less efficient ligand-to-metal transfer (ca. 12-fold). In the case of the Tb-III complex, the decrease in the energy of the triplet state reduces further the Tb-III emission through increased back transfer. The specific rotary dispersion of the 1:1 and 1:2 complexes points to the chirality of the complexes arising mainly from the ligand, while the circularly polarized luminescence of these complexes with Eu-III and Tb-III displays a weak effect, pointing to a small diastereomeric excess in solution. Altogether, this study demonstrates that electronic, thermodynamic and photophysical properties of lanthanide complexes with aromatic terdentate ligands can be tuned by modifying the number and the arrangement of the ligands, as well as their substituents, particularly those in the R-2 and R-3 positions.