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A series of new mixed-ligand lanthanide complexes [Ln(hfa)3(4-cpyNO)]2 (Ln = SmIII-HoIII and TmIII; hfa- = hexafluoroacetylacetonate; 4-cpyNO = 4-cyanopyridine N-oxide) have been synthesised by treating the corresponding lanthanide hexafluoroacetylacetonates with 4-cpyNO (molar ratio 1:1) in chloroform. Single-crystal X-ray analysis revealed that [Tb(hfa)3(4-cpyNO)]2 and [Ho(hfa)3(4-cpyNO)]2 are isostructural and are comprised of two types of symmetrically independent dimetallic molecules. Within a dimer, the eightfold-coordinated metal atoms are bridged by two 4-cpyNO ligands through the oxygen atoms of the N-oxide groups. The magnetic susceptibility data for the GdIII and TbIII complexes indicate the presence of weak antiferromagnetic interactions within the dimetallic Ln2O2 units. The thermal stability of the [Ln(hfa)3(4-cpyNO)]2 adducts was studied by thermogravimetric analysis and their volatility estimated from sublimation experiments under reduced pressure. The photoluminescent properties were measured for solid samples upon excitation at 330-360 nm. The mixed-ligand complexes of SmIII, EuIII, TbIII, DyIII and TmIII exhibit metal-centred luminescence in the entire visible spectral range with characteristic pink, red, green, yellow and blue emission, respectively. The introduction of ancillary 4-cpyNO molecules into the coordination sphere of the lanthanide(III) ions was found to significantly affect their luminescence quantum yields. [Eu(hfa)3(4-cpyNO)]2 thin films on quartz substrates were obtained by the vacuum evaporation technique and their photoluminescent properties were found to be enhanced when compared with those of the bulk samples. This suggests that these dimers could be good candidates for the fabrication of emitting layers for light-emitting diodes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)