Photoluminescence and white-light emission in two series of heteronuclear Pb(ii)–Ln(iii) complexes

Abstract

Two series of Pb(II)–Ln(III) heteronuclear coordination complexes are assembled from a tripodal ligand triCB-NTB ((4,4′,4′′-(2,2′,2′′-nitrilotris(methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzoic acid)). In the Pb₂LnL₂ series, the Ln³⁺ ion is encapsulated in a highly symmetrical {LnO₆} octahedron with an inversion center, and Pb–Ln–Pb clusters are formed by the linkage of carboxyl groups on triCB-NTB ligands to Pb²⁺ and Ln³⁺ simultaneously. However, in the PbLn₂L₂ series, the Ln³⁺ ion is encapsulated in a distorted {LnO₉} polyhedron without an inversion center. Pb²⁺ ions are coordinated with benzimidazole and apical N atoms on the ligand in isolation, and the carboxyl groups link two Ln³⁺ ions into a Ln–Ln cluster. This structural variation leads to different photoluminescence behaviour in these two series of complexes. Most importantly, the linkage of the Pb–Ln–Pb clusters causes more perturbation to the excited states of the ligand. Therefore, a more obvious ligand-to-metal charge transfer (LMCT) process is observed in the Pb₂LnL₂ series, and the energy transfer to the accepting levels of Ln³⁺ ions becomes more efficient. Furthermore, the combination of LC (ligand-centered) + LMCT + MC (metal-centered) emissions in the Pb₂EuL₂ complex results in single component white light emission.