Jean-Claude Bunzli - Designing Simple Tridentate Ligands for Highly Luminescent Europium Complexes

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  Shavaleev,N.M., Eliseeva,S.V., Scopelliti,R., Bunzli,J.C.G.   Chemistry-A European Journal 2009  15 (41) 10790-10802

  Abstract: A series of tridentate   benzimidazole-substituted pyridine-2-carboxylic acids have been   prepared with a halogen, methyl or alkoxy group in the 6-position   of the benzimidazole ring, which additionally contains a   solubilising N-alkyl chain. The ligands form neutral homoleptic   nine-coordinate lanthanum, europium and terbium complexes as   established from X-ray crystallographic analysis of eight   structures. The coordination polyhedron around the lanthanide ion   is close to a tricapped trigonal prism with ligands arranged in   an up-up-down fashion. The coordinated ligands serve as   light-harvesting chromophores in the complexes with absorption   maxima in the range 321-341 nm (epsilon = (4.9-6.0) x 10(4) M(-1)   cm(-1)) and triplet-state energies between 21300 and 18800   cm(-1); the largest redshifts occur for bromine and   electron-donor alkoxy substituents. The ligands efficiently   sensitise europium luminescence with overall quantum yields   (Q(L)(Eu)) and observed lifetimes (tau(obs)) reaching 71% and   3.00 ms, respectively, in the solid state and 52% and 2.81 ms,   respectively, in CH(2)Cl(2) at room temperature. The radiative   lifetimes of the Eu((5)D(0)) level amount to tau(rad) = 3.6-4.6   ms and the sensitisation efficiency eta(sens) =   Q(L)(Eu)(tau(rad)/tau(obs)) is close to unity for most of the   complexes in the solid state and equal to approximately 80% in   solution. The photophysical parameters of the complexes correlate   with the triplet energy of the ligands, which in turn is   determined by the nature of the benzimidazole substituent. Facile   modification of the ligands makes them promising for the   development of brightly emissive europium-containing materials

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