Luminescence and Single-Molecule Magnet Behavior in Lanthanide Complexes Involving a Tetrathiafulvalene-Fused Dipyridophenazine Ligand

The reaction between
the TTF-fused dipyrido­[3,2-a:2′,3′-c]­phenazine (dppz) ligand (L) and 1 equiv of Ln­(hfac)3·2H2O (hfac– = 1,1,1,5,5,5-hexafluoroacetyacetonate)
or 1 equiv of Ln­(tta)3·2H2O (tta– = 2-thenoyltrifluoroacetonate) (LnIII = DyIII or YbIII) metallic precursors leads to four mononuclear
complexes of formula [Ln­(hfac)3(L)]·C6H14 (LnIII = DyIII (1), YbIII (2)) and [Ln­(tta)3(L)]·C6H14 (LnIII = DyIII (3), YbIII (4)), respectively. Their X-ray structures reveal that the LnIII ion is coordinated to the bischelating nitrogenated coordination
site and adopts a D4d coordination environment. The dynamic magnetic measurements show
a slow relaxation of the DyIII magnetization for 1 and 3 with parameters highlighting a slower
relaxation for 3 than for 1 (τ0 = 4.14(±1.36) × 10–6 and 1.32(±0.07)
× 10–6 s with Δ = 39(±3) and 63.7(±0.7)
K). This behavior as well as the orientation of the associated magnetic
anisotropy axes have been rationalized on the basis of both crystal
field splitting parameters and ab initio SA-CASSCF/RASSI-SO calculations.
Irradiation of the lowest-energy HOMO → LUMO ILCT absorption
band induces a 2F5/2 → 2F7/2 Yb-centered emission for 2 and 4. For these YbIII compounds, Stevens operators method
has been used to fit the thermal variation of the magnetic susceptibilities,
and the resulting MJ splittings
have been correlated with the emission lines.