Alkylation effects in lanthanide complexes involving tetrathiafulvalene chromophores: Experimental and theoretical correlation between magnetism and near-infrared emission

Mononuclear complexes with the formula [Ln(hfac)₃(L ¹)] and [Ln(hfac)₃(L²)] with hfac^- = 1,1,1,5,5,5-hexafluoroacetylacetonate, L¹ = 2-{4,5-[4,5- bis(propylthio)tetrathiafulvalenyl]-1H-benzimidazol-2-yl}pyridine and L ² = 2-{1-methylpyridyl-4,5-[4,5-bis(propylthio)tetrathiafulvalenyl]- 1H-benzimidazol-2-yl}pyridine are reported for Ln = Y^III, Er ^III and Yb^III. The X-ray structures reveal that the Ln(hfac)₃ moieties are coordinated to the bidentate 1-(2-pyridylmethyl)benzimidazole acceptor. The coordination polyhedron is described as a more or less distorted triangular dodecahedron prism (D _2d symmetry), depending on the degree of alkylation of the ligand. The influence of this distortion on the magnetic and photophysical properties is determined by the fit of the static magnetic measurements and luminescence spectra. Irradiation of the lowest-energy intraligand charge transfer (ILCTs) bands (21740 cm^-1) induces the metal-centred ⁴I _13/2 → ⁴I_15/2 and ²F _5/2 → ²F_7/2 luminescence for the Er ^III and Yb^III complexes, respectively. The alkylation enhances both the intensity and lifetime of the Yb^III luminescence. The Er^III luminescence can be sensitised by the antenna effect, whereas the Yb^III luminescence could involve a photoinduced electron transfer (PET). Finally, the evolution of the Yb^III luminescence spectra shape due to the alkylation is directly correlated to the energy splitting of the M_J states that stem from the ²F _7/2 multiplet ground state. Ab initio calculations give evidence of the nature of the M_J ground state as well as the orientation of the associated magnetic anisotropy axis (i.e., the one that lies along the less electronegative direction). The key role of the imidazole proton of L ² is highlighted. The calculated energy splitting of the ²F_5/2 multiplet state perfectly matches the emission lines.