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

The reaction between
(4,5-bis­(2-pyridyl-N-oxidemethylthio)-4′,5′)-ethylenedithiotetrathiafulvene
(L1) or -methyldithiotetrathiafulvene
(L2) ligands and Ln­(hfac)3·nH2O precursors (LnIII = Pr, Tb, Dy, Er, and Yb) leads to the formation of seven
dinuclear complexes of formula [Ln2(hfac)6(H2O)x(Ly)2] (x = 2 and y = 1 for LnIII = Pr (1); x = 0 and y = 1 for LnIII = Tb (2), Dy (3), Er (4) and Yb (5); x = 0 and y = 2 for
LnIII = Tb (6) and Dy (7)). Their
X-ray structures reveal that the coordination environment of each
LnIII center is filled by two N-oxide
groups coming from two different ligands Ly. 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 |MJ = ±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 4I13/2 → 4I15/2 (6660 cm–1) and 2F5/2 → 2F7/2 (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 (MJ = ±3/2) fits exactly the second emission line (234 cm–1). While no out-phase-signal is detected for 3, the change of ligand L1 → L2 induces a change
of coordination sphere symmetry around the DyIII increasing
the energy splitting between the ground and first excited states,
and 7 displays a single molecule magnet behavior.