Syntheses, Crystal Structures, and Luminescence of Hexafluoroacetylacetone Dinuclear Lanthanide Complexes Bridged by “Back-to-Back” Bisterpyridine

Six dinuclear lanthanide complexes [(hfac) 3 Ln(μ-tppz)Ln(hfac) 3 ] [Ln(III) = 1 -Eu, 2 -Gd, 3 -Tb, hfac = 1,1,1,5,5,5-hexafluoroacetylacetone, tppz = 2,3,5,6-tetra(pyridin-2-yl)pyrazine], [(hfac) 3 Ln(μ-tppz)Ln(hfac) 3 ]·CH 2 Cl 2 [Ln(III) = 4 -Dy, 5 -Yb, 6 -Er], and seven dinuclear lanthanide complexes [(hfac) 3 Ln(μ-1,4-bteb)Nd(hfac) 3 ] [Ln(III) = 7 -Nd, 1,4-bteb = 1,4-bis(2,2′:6′,2″-terpyridin-4′-yl)benzene] and [(hfac) 3 Ln(μ-1,4-bteb)Ln(hfac) 3 ]·2CH 2 Cl 2 [Ln(III) = 8 -Sm, 9 -Eu, 10 -Gd, 11 -Td, 12 -Dy, 13 -Yb], were synthesized and characterized by elemental analyses, infrared spectroscopy, thermogravimetric analyses, single-crystal X-ray diffraction, and powder X-ray diffraction. Solid-state luminescence investigations suggest that an efficient energy transfer process is taking place in 1 -Eu and 9 -Eu. The dominant intersystem crossing (ISC) and the ligand-to-metal energy transfer (ET) processes in the dinuclear Eu(III) complex bridged by tppz are more efficient, making 1 -Eu possess longer τ and higher quantum yield (QY) (0.687 ms and 53.47%) than 9 -Eu (0.670 ms and 14.22%). Additionally, the mononuclear complex [Eu(NO 3 ) 3 (ptpy)(H 2 O)] shows longer τ and higher QY (0.545 ms and 16.80%) than the dinuclear complex [Eu(NO 3 ) 3 (H 2 O)] 2 (μ-tppz) (0.035 ms and 1.31%) and [Eu(hfac) 3 (ptpy)] has longer τ and higher QY (0.819 ms and 59.95%) than 9 -Eu. The results imply that increasing the length of the “back-to-back” bisterpyridine ligands or the number of the Eu(III) centers could make the luminescence performances such as τ and QY decrease. There is an inefficient energy transfer existing in the Tb(III) and Dy(III) complexes because of the back energy transfer (BET) processes, which leads to an unfavorable condition in observing significantly the characteristic Tb(III)- and Dy(III)-based emissions. However, the synergistic effect of hfac and tppz (or hfac and 1,4-bteb) could sensitize the visible and near-infrared (NIR) luminescence of Sm(III) ion and the NIR luminescence of Nd(III), Er(III), and Yb(III) ions. The τ value and the calculated intrinsic quantum yield (ϕ Ln ) of 7 -Nd are 1.13 μs and 0.13%, respectively. This work unveils the synergistic sensitization effect of primary hfac ligands and the ancillary tppz and 1,4-bteb ligands and the influences of the length of the bridging bisterpyridine ligands and the numbers of the Ln(III) centers on the luminescence properties. It is anticipated that the synthesized dual-sensitized ternary lanthanide complexes might be excellent candidates for some optoelectronic applications.