The role of ligand to metal charge-transfer states on the luminescence of Europium complexes with 18-membered macrocyclic ligands†
We report a detailed study of the photophysical properties of EuIII and TbIII 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 (ϕH2O = 7.4 and 21%, respectively). The [EuL2]3+ complex displays a relatively low quantum yield in H2O (ϕH2O = 1.6%) that increases considerably in D2O (ϕD2O = 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 → 7F5,6 transitions, which appears to be also related to the distorted D4d symmetry of the coordination polyhedron. Surprisingly, the quantum yield of the [EuL1]3+ complex is very low (ϕH2O = 0.10%), considering the good protection of the EuIII 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.