Radiative and nonradiative transitions in the Eu(III) hexaaza macrocyclic complex [Eu(C22H26N6)(CH3COO)](CH 3COO)Cl·2H2O

The photophysical properties of the hexaaza macrocyclic complex [Eu(C22H26N6)(CH3COO)](CH 3COO)Cl·2H2O are reported in water solution and in the solid state. The absorption spectrum and the luminescence spectrum and lifetime show that the Eu3+-macrocycle moiety remains unaltered in water solution, while the remaining coordination positions are occupied by the acetate anion or water molecules depending on the concentration of the complex. Luminescence decay measurements performed as a function of temperature (300 or 77 K) and solvent (H2O or D2O) indicate that no low-lying charge-transfer level is present and only one water molecule is coordinated to the Eu3+ ion in the presence of added acetate anions. The Eu3+ luminescence is quite efficient for f-f excitation (Φem ∼ 0.1) while it becomes very inefficient for excitation into the ligand (Φem ∼ 6 × 10-3). The efficiency of the energy transfer from the ligand to Eu3+ deduced from the emission quantum yields is compared to that calculated from the luminescence of the ligand (obtained for the Gd3+ complex at 4.2 K) and Eu3+ absorption, using the Förster-Dexter theory.