Photoluminescence and white-light emission in two series of heteronuclear Pb(II)-Ln(III) complexes

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 Pb(2)LnL(2) series, the Ln(3+) ion is encapsulated in a highly symmetrical {LnO(6)} 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 Ln(3+) simultaneously. However, in the PbLn(2)L(2) series, the Ln(3+) ion is encapsulated in a distorted {LnO(9)} 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 Ln(3+) 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 Pb(2)LnL(2) series, and the energy transfer to the accepting levels of Ln(3+) 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.