Odd-Even Effect on Luminescence Properties of Europium Aliphatic Dicarboxylate Complexes

Abstract

The odd-even effect in luminescent [Eu₂ (L)₃ (H₂ O)_x ]⋅y(H₂ O) complexes with aliphatic dicarboxylate ligands (L: OXA, MAL, SUC, GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x=2-6 and y=0-4), prepared by the precipitation method, was observed for the first time in lanthanide compounds. The final dehydration temperatures of the Eu³⁺ complexes show a zigzag pattern as a function of the carbon chain length of the dicarboxylate ligands, leading to the so-called odd-even effect. The FTIR data confirm the ligand-metal coordination via the mixed mode of bridge-chelate coordination, except for the Eu³⁺ -oxalate complex. XRD results indicate that the highly crystalline materials belong to the monoclinic system. The odd-even effect on the 4 f-4 f luminescence intensity parameters (Ω₂ and Ω₄ ) is explained by using an extension of the dynamic coupling mechanism, herein named the ghost-atom model. In this method, the long-range polarizabilities ( ${\alpha }^{*}$ ) were simulated by a ghost atom located at the middle of each ligand chain. The values of ${\alpha }^{*}$ were estimated using the localized molecular orbital approach. The emission intrinsic quantum yield ( $Q_{Ln}^{Ln}$ ) of the Eu³⁺ complexes also presented an the odd-even effect, successfully explained in terms of the zigzag behavior shown by the Ω₂ and Ω₄ intensity parameters. Luminescence quenching due to water molecules in the first coordination sphere is also discussed and rationalized.

Keywords: dicarboxylate complexes; europium; luminescence; odd-even effect; photoluminescence intensity parameters.