Lanthanide-Based Dinuclear Complexes Involving an o-Quinone-Tetrathiafulvalene-o-Quinone Bridging Ligand: X-ray Structures, Magnetic and Photophysical Properties

Six dinuclear complexes, [Ln2(hfac)6(H2O)2(L)] [Ln = Y (1) and Er (2)], [Yb2(hfac)6(L)] (3), [Ln2(tta)6(L)].3CH2Cl2 [Ln = Y (4) and Er (5)] and [Yb2(tta)6(L)].2C6H14 (6), have been prepared by coordination reactions between the [Ln(beta-diketonate)3].2H2O (Ln = YIII, ErIII and YbIII; beta-diketonate = hfac or tta) building blocks and 4,4',7,7'-tetra-tert-butyl-2,2'-bi-1,3-benzodithiole-5,5',6,6'-tetrone ligand (L), which acts as a bridge between the two metal centres. Their X-ray structures reveal that the symmetries around the lanthanide atoms depend on the beta-diketonate anions and the size of the 4f ions. The dc magnetic measurements showed paramagnetic behaviour with thermal variations of the ?MT products that depend on the crystal-field symmetry around the 4f ions. The photophysical properties of the coordination complexes were studied by solid-state absorption spectroscopy, and DFT calculations were carried out on the free ligand L and the diamagnetic YIII derivatives 1 and 4. Irradiation of the charge-transfer (ILCT) band of L [?ex = 700 nm (14285 cm1)], 1 and 4 [?ex = 750 nm (13335 cm1)] induced weak fluorescence in L at 12500 cm1 (for free L) and in 1 at 12200 cm1, whereas in the case of 3, irradiation at 13335 cm1 induced weak line-shape emission assigned to the 2F5/2 ? 2F7/2 (9880 cm1) ytterbium-centred transition. No residual charge-transfer emission was observed in this last case attesting the efficient energy transfer from the excited charge-transfer state to the 2F5/2 state. The weak intensity of the emissions should mainly be attributed to energy back-transfer between the relaxed CT excited state and the CT state. Nevertheless, sensitisation of the YbIII luminescence in 3 was achieved through the lowest-energy donor excited state (13585 cm1) reported until now.