Tetrathiafulvalene-amido-2-pyridine-N-oxide as Efficient Charge-Transfer Antenna Ligand for the Sensitization of Yb-III Luminescence in a Series of Lanthanide Paramagnetic Coordination Complexes

The tetrathiafulvalene-amido-2-pyridine-N-oxide (L) ligand has been employed to coordinate 4f elements. The architecture of the complexes mainly depends on the ionic radii of the lanthanides. Thus, the reaction of L in the same experimental protocol leads to three different molecular structure series. Binuclear [Ln(2)(hfac)(5)-(O2CPhCl)(L)(3)]center dot 2H(2)O (hfac(-)=1,1,1,5,5,5-hexafluoroacetylacetonate anion, O2CPhCl-=3-chlorobenzoate anion) and mononuclear [Ln-(hfac)(3)(L)(2)] complexes were obtained by using rare-earth ions with either large (Ln(III) = Pr, Gd) or small (Ln(III) = Y, Yb) ionic radius, respectively, whereas the use of TVIII that possesses an intermediate ionic radius led to the formation of a binuclear complex of formula [Tb-2(hfac)(4)(O2CPhCl)(2)(L)(2)]. Antiferromagnetic interactions have been observed in the three dinuclear compounds by using an extended empirical method. Photophysical properties of the coordination complexes have been studied by solid-state absorption spectroscopy, whereas time-dependent density functional theory (TD-DFT) calculations have been carried out on the diamagnetic Y-III derivative to build a molecular orbital diagram and to reproduce the absorption spectrum. For the [Yb(hfac)(3)(L)(2)] complex, the excitation at 19600 cm(-1) of the HOMO -> LUMO+1/LUMO+2 charge-transfer transition induces both line-shape emissions in the near-IR spectral range assigned to the F-2(5/2)-> F-2(7/2) (9860 cm(-1)) ytterbium-centered transition and a residual charge-transfer emission around 13150 cm(-1). An efficient antenna effect that proceeds through energy transfer from the singlet excited state of the tetrathiafulvalene-amido-2-pyridine-N-oxide chromophore is evidence of the Yb-III sensitization.