The role of ligand to metal charge-transfer states on the luminescence of Europium complexes with 18-membered macrocyclic ligands

Abstract

We report a detailed study of the photophysical properties of Eu^III and Tb^III complexes with two ligands based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing either four pyridine-2yl-methyl (L¹) or four hydroxyethyl (L²) pendant arms. The [TbL¹]³⁺ and [TbL²]³⁺ complexes present moderate luminescence quantum yields upon excitation through the ligand bands (ϕ_H2O = 7.4 and 21%, respectively). The [EuL²]³⁺ complex displays a relatively low quantum yield in H₂O (ϕ_H2O = 1.6%) that increases considerably in D₂O (ϕ_D2O = 5.3%), which highlights the strong quenching effect of the four ligand O–H oscillators. The emission spectrum of [EuL¹]³⁺ is rather unusual in that it shows a relatively high intensity of the ⁵D₀ → ⁷F_5,6 transitions, which appears to be also related to the distorted D_4d symmetry of the coordination polyhedron. Surprisingly, the quantum yield of the [EuL¹]³⁺ complex is very low (ϕ_H2O = 0.10%), considering the good protection of the Eu^III coordination environment offered by the ligand. Cyclic voltammograms recorded from aqueous solutions of [EuL¹]³⁺ display a reversible curve with a half-wave potential of −620 mV (versus Ag/AgCl), while [EuL²]³⁺ presents a reduction peak at more negative potential (−1040 mV). Thus, the L¹ ligand provides a rather good stabilisation of divalent Eu compared to the L² analogue, suggesting that the presence of a low-lying ligand-to-metal charge-transfer (LMCT) state might be responsible for the low quantum yield determined for [EuL¹]³⁺. A density functional theory (DFT) study provides very similar energies for the ligand-centered excited singlet (¹ππ*) and triplet (³ππ*) states of [EuL¹]³⁺ and [EuL²]³⁺. The energy of the ⁹LMCT state of [EuL¹]³⁺ was estimated to be 20 760 cm⁻¹ by using all-electron relativistic calculations based on the DKH2 approach, a value that decreases to 15 940 cm⁻¹ upon geometry relaxation.