Role of Inner- and Outer-Sphere Bonding in the Sensitization of EuIII-Luminescence Deciphered by Combined Analysis of Experimental Electron Density Distribution Function and Photophysical Data

A series of lanthanide adducts with different amounts of 1,10-phenanthroline, chloride ions, and water molecules in the inner and outer coordination spheres are investigated with the aim of relating the chemical bonding pattern in the crystals to the luminescence properties of the Eu ion: [LnCl(1)Phen(2)(H2O)(3)]Cl-2(H2O) (Ln = Eu, 1Eu; Gd, 1Gd; Tb, 1Tb), [EuCl(2)Phen(2)(H2O)(2)]Cl-1(H2O) (2), and [EuCl(2)Phen(1)(H2O)(4)]Cl-1(H2O) (3). The influence of inner-versus outer-sphere ligands on the Ln-X bond lengths and angles in the structure is examined. A detailed topological analysis of the electron density function derived from the X-ray diffraction data for 1Gd is performed within the frame of the atoms in molecule theory for the first time for a lanthanide complex.,The chemical bonding pattern is interpreted in terms of net atomic charges, bond energies, and electron transfers from the ligands to the metal ion. A noteworthy finding is that the energy of extended noncovalent interactions occurring in the second coordination sphere (H-bonding and T-stacking interactions) is comparable to that of Ln-ligand bonds. The luminescence properties of the three Eu adducts are interpreted with the results of electron density distribution function topology. An intraligand charge transfer state is identified, and its contribution in the ligand-to-europium energy transfer process is analyzed. The outcome of this study is that specific interionic interactions which are usually not considered in theoretical calculations or in the interpretation of luminescence properties play an important role in the sensitization of the Eu luminescence.