Photophysical studies of low-symmetry Sm(iii) and Tb(iii) complexes reveal remarkable quantum yields

Abstract

This paper presents the synthesis, thorough characterizations, and visible and near-infrared (NIR) photophysical properties of two original heteroleptic complexes, [Ln^III(fod)₃(bzi)] (Ln = Sm (1) and Tb (2); fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione and bzi = benzimidazole). Single-crystal X-ray diffraction (SC-XRD) was performed only on complex 2 owing to its good quality crystals. SC-XRD and NMR results indicated that the complexes are seven-coordinated in the solid and solution states. The Tb^III ion, in complex 2, is surrounded by six oxygen atoms of fod ligands and one nitrogen atom from bzi, forming a distorted capped octahedron (C_3v) geometry. This geometry was further supported by Sparkle model/PM7 and SHAPE analysis. Thermogravimetric analysis (TGA) curves showed that the complexes are stable up to ∼260–280 °C, suggesting their possible application in optoelectronic devices. Upon UV excitation, the complexes exhibited their characteristic emission transitions {Sm^III ion; ⁴G_5/2 → ⁶H_{j=5/2,7/2,9/2} (visible) and ⁶F_1/2, ⁶H_13/2, ⁶F_{3/2,5/2,7/2,9/2,11/2} (NIR region) and Tb^III ion; ⁵D₄ → ⁷F_j=6−2}. The Sm complex exhibited very intense luminescence with high quantum yield (∼7.20%), comparable to most luminescent Sm complexes reported in the literature. Photophysical parameters (photoluminescence quantum yields and lifetimes) were recorded and are presented for both complexes in the solution state, solid state, and thin films@3%PMMA. The strong luminescence of the complexes suggests that Hfod and bzi ligands act as efficient sensitizers. The CIE coordinates, obtained from emission spectra, indicated that the complexes emit in the pink (Sm^III) and green (Tb^III) regions. The band gaps determined lie in the semiconductor region, which suggests the applicability of the complexes as optoelectronic materials. Finally, CCT values were determined, which revealed that the complexes can be used as cold light sources.