Lanthanide coordination compounds with benzimidazole-based ligands. luminescence and EPR

Abstract Ln III coordination compounds were synthesized with two benzimidazolic ligands, 2 pb = 2-(2-pyridyl)benzimidazole and 4tb  = 2-(4-thiazolyl)benzimidazole, with Eu, Gd, Tb and Dy, namely: [Ln (NO 3 ) 3 (L n ) 2 ]⋅S, where L n  =  2 pb , 4tb and S = solvent. They were characterized by IR, 1D and 2D 1 H and 13 C NMR, solid state UV–Vis–NIR absorption and emission and EPR spectroscopies, X-ray powder diffraction, elemental analyses, electrical conductivity and magnetic susceptibility. The compounds of Eu III with 2 pb and 4tb , Tb III and Dy III with 4tb showed luminescence at RT in the solid state; their emission life time ranged from 0.026 to 1.82 ms. 2 pb and 4tb sensitized lanthanides luminescence. Results of DFT theoretical calculations for the ligand ( H 2 L ) are used to assign the ligand singlet and triplet excited state energy levels. Additionally, a full analysis of the X-band EPR spectra in the solid state gave the following results, at 297 K for [Gd(NO 3 ) 3 (2 pb) 2 ]·CH 3 COCH 3 , g  = 1.990, |D| = 0.0687 cm −1 , E = 0.0165 cm −1 and λ = |E/D| = 0.240; for [Gd(NO 3 ) 3 (4tb) 2 ]⋅CH 3 COCH 3 , g  = 2.000; |D| = 0.0670 cm −1 , E = 0.000 cm −1 and λ = |E/D| = 0.000. At lower temperature, 128 K, compounds showed an increase of the rhombicity as the temperature decreased.