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

The bis(β-diketone) ligands 1,3-bis(3-phenyl-3-oxopropanoyl)benzene, H2L1 and 1,3-bis(3-phenyl-3-oxopropanoyl) 5-ethoxy-benzene, H2L2, 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 [M2L13] where
M = Eu, Nd, Sm, Y, Gd and [M2L23], where M = Eu, Nd or anionic quadruple-stranded dinuclear lanthanide
units, [Eu2L14]2- . The crystal structure of the free ligand H2L1 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
[Eu2L13], 0.16% and 13 μs for [Sm2L13], and 0.6% and 1.5 μs for [Nd2L13]. The energy level of the ligand
triplet state was determined from the 77 K spectrum of [Gd2L13]. 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 [Eu2L14]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.