Experimental and theoretical study of ligand field, 4f-4f intensities and emission quantum yield in the compound Eu(bpyO2)4(ClO4)3

The high resolution emission spectrum of the Eu3+ ion in the compound Eu(bpyO(2)),(ClO4)(3), where bpyO(2) = 2,2 ' -bipyridine-1,1 ' -dioxide, has been previously studied in solid state and frozen solution. The compound crystallizes in the monoclinic P2, space group with the cell parameters a = 14.730(1) Angstrom, b = 13.585(1) Angstrom, c = 22.967(2) Angstrom and beta = 91.46(1)degrees. The coordination polyhedron can be described as a distorted cube into a square antiprism with symmetry close to D-2, The experimental emission quantum yield (q) was measured according to a method previously described and a q-value of 15% was obtained. By using the structural crystallographic data a theoretical ligand field and intensity analysis was carried out, and the sparkle model was applied to obtain the electronic structure of the organic part of the compound. From these results, intramolecular energy transfer rates were evaluated according to a recently developed model. An appropriate set of rate equations for the normalized populations of the levels involved was solved numerically, by using the 4th order Runge-Kutta method, and a theoretical q-value could be obtained (19.6%), which is in good agreement with experiment. A relevant aspect is that the reason for this rather low q-value could be explained in terms of the relative position of the lowest ligand triplet energy level with respect to the D-5(1) and D-5(0) levels of the Eu3+ ion. The theoretical analysis has also shown that, in this compound, a slight decrease in the energy of the ligand triplet level is sufficient to quench almost completely the Eu3+ luminescence. (C) 2001 Elsevier Science B.V. All rights reserved.