Alkylation Effects in Lanthanide Complexes Involving Tetrathiafulvalene Chromophores: Experimental and Theoretical Correlation between Magnetism and Near‐Infrared Emission

Abstract

AbstractMononuclear complexes with the formula [Ln(hfac)3(L1)] 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 L2 = 2‐{1‐methylpyridyl‐4,5‐[4,5‐bis(propylthio)tetrathiafulvalenyl]‐1H‐benzimidazol‐2‐yl}pyridine are reported for Ln = YIII, ErIII and YbIII. 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 (D2d 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 4I13/2 → 4I15/2 and 2F5/2 → 2F7/2 luminescence for the ErIII and YbIII complexes, respectively. The alkylation enhances both the intensity and lifetime of the YbIII luminescence. The ErIII luminescence can be sensitised by the antenna effect, whereas the YbIII luminescence could involve a photoinduced electron transfer (PET). Finally, the evolution of the YbIII luminescence spectra shape due to the alkylation is directly correlated to the energy splitting of the MJ states that stem from the 2F7/2 multiplet ground state. Ab initio calculations give evidence of the nature of the MJ 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 L2 is highlighted. The calculated energy splitting of the 2F5/2 multiplet state perfectly matches the emission lines.