Highly Luminescent, Triple- and Quadruple-Stranded, Dinuclear Eu, Nd, and Sm(III) Lanthanide Complexes Based on Bis-Diketonate Ligands

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Abstract
The bis(β-diketone) ligands 1,3-bis(3-phenyl-3-oxopropanoyl)benzene, H2L 1 and 1,3-bis(3-phenyl-3-oxopropanoyl) 5-ethoxy-benzene, H2L 2, have been prepared for the examination of dinuclear lanthanide complex formation and investigation of their properties as sensitizers for lanthanide luminescence. The ligands bear two conjugated diketonate binding sites linked by a 1,3-phenylene spacer. The ligands bind to lanthanide(III) or yttrium(III) ions to form neutral homodimetallic triple stranded complexes [M2L 13] where M = Eu, Nd, Sm, Y, Gd and [M2L 23], where M = Eu, Nd or anionic quadruple-stranded dinuclear lanthanide units, [Eu2L 14]2- . The crystal structure of the free ligand H2L 1 has been determined and shows a twisted arrangement of the two binding sites around the 1,3-phenylene spacer. The dinuclear complexes have been isolated and fully characterized. Detailed NMR investigations of the complexes confirm the formation of a single complex species, with high symmetry; the complexes show clear proton patterns with chemical shifts of a wide range due to the lanthanide paramagnetism. Addition of Pirkle's reagent to solutions of the complexes leads to splitting of the peaks, confirming the chiral nature of the complexes. Electrospray and MALDI mass spectrometry have been used to identify complex formulation and characteristic isotope patterns for the different lanthanide complexes have been obtained. The complexes have high molar absorption coefficients (around 13 × 104 M-1cm-1) and display strong visible (red or pink) or NIR luminescence upon irradiation at the ligand band around 350 nm, depending on the choice of the lanthanide. Emission quantum yield experiments have been performed and the luminescence signals of the dinuclear complexes have been found to be up to 11 times more intense than the luminescence signals of the mononuclear analogues. The emission quantum yields and the luminescence lifetimes are determined to be 5% and 220 μs for [Eu2L 13], 0.16% and 13 μs for [Sm2L 13], and 0.6% and 1.5 μs for [Nd2L 13]. The energy level of the ligand triplet state was determined from the 77 K spectrum of [Gd2L 13]. The bis-diketonate ligand is shown to be an efficient sensitizer, particularly for Sm and Nd. Photophysical studies of the europium complexes at room temperature and 77 K show the presence of a thermally activated deactivation pathway, which we attribute to ligand-to-metal charge transfer (LMCT). Quenching of the luminescence from this level seems to be operational for the Eu(III) complex but not for complexes of Sm(III) and Nd(III), which exhibit long lifetimes. The quadruple-stranded europium complex has been isolated and characterized as the piperidinium salt of [Eu2L 14]2-. Compared with the triple-stranded Eu(III) complex in the solid state, the quadruple-stranded complex displays a more intense emission signal with a distinct emission pattern indicating the higher symmetry of the quadruple-stranded complex.

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