Lanthanide luminescence efficiency in eight- and nine-coordinate complexes: Role of the radiative lifetime

The photophysical parameters for the sensitization of metal-centred luminescence are analyzed in two series of complexes with tridentate and hexadentate ligands having NxOy chelating units. In particular, the radiative lifetime tau(rad) is experimentally estimated for 29 nine-coordinate Eu-III complexes and 10 eight- and nine-coordinate Yb-III complexes. The known dependence of tau(rad) on refractive index is sub-stantiated by comparing data for solid-state samples and solutions. Moreover, a clear dependence of tau(rad) with the coordination environment is evidenced and in the case of Eu-III, a comparison between tau(rad) and the nephelauxetic effect generated by the ligands is attempted. Altogether, this extensive analysis points to the importance of having a handle on tau(rad) when designing ligands for highly luminescent lanthanide-containing molecular edifices. This, in turn, should stimulate initiating theoretical considerations to unravel a reliable relationship between tau(rad) and the electronic structure of the ligands. (C) 2010 Elsevier B.V. All rights reserved.