A Series of Tetrathiafulvalene-Based Lanthanide Complexes Displaying Either Single Molecule Magnet or Luminescence—Direct Magnetic and Photo-Physical Correlations in the Ytterbium Analogue

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The reaction between (4,5-bis(2-pyridyl-N-oxidemethylthio)-4',5')-ethylenedithiotetrathiafulvene (L(1)) or -methyldithiotetrathiafulvene (L(2)) ligands and Ln(hfac)3·nH2O precursors (Ln(III) = Pr, Tb, Dy, Er, and Yb) leads to the formation of seven dinuclear complexes of formula [Ln2(hfac)6(H2O)x(L(y))2] (x = 2 and y = 1 for Ln(III) = Pr (1); x = 0 and y = 1 for Ln(III) = Tb (2), Dy (3), Er (4) and Yb (5); x = 0 and y = 2 for Ln(III) = Tb (6) and Dy (7)). Their X-ray structures reveal that the coordination environment of each Ln(III) center is filled by two N-oxide groups coming from two different ligands L(y). UV-visible absorption properties have been experimentally measured and rationalized by TD-DFT calculations. The temperature dependences of static magnetic measurements have been fitted. The ground state corresponds to the almost pure |M(J) = ±13/2〉 while the first excited state (±0.77|±11/2〉 ± 0.50|±3/2〉 ± 0.39|±5/2〉) is located at 19 cm(-1) and 26.9 cm(-1) respectively for 3 and 7. Upon irradiation at 77 K and at room temperature, in the range 25,000-20,835 cm(-1), both compounds 4 and 5 display a metal-centered luminescence attributed to (4)I(13/2) → (4)I(15/2) (6660 cm(-1)) and (2)F(5/2) → (2)F(7/2) (9972 cm(-1)) transitions, respectively. Emission spectroscopy provides a direct probe of the |±5/2〉 ground state multiplet splitting, which has been confronted to magnetic data. The energy separation of 225 cm(-1) between the ground state and the first excited level (M(J) = ±3/2) fits exactly the second emission line (234 cm(-1)). While no out-phase-signal is detected for 3, the change of ligand L(1) → L(2) induces a change of coordination sphere symmetry around the Dy(III) increasing the energy splitting between the ground and first excited states, and 7 displays a single molecule magnet behavior.