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 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 (L 1 ) or four hydroxyethyl (L 2 ) pendant arms. The [TbL 1 ] 3+ and [TbL 2 ] 3+ complexes present moderate luminescence quantum yields upon excitation through the ligand bands ( H 2 O = 7.4 and 21%, respectively). The [EuL 2 ] 3+ complex displays a relatively low quantum yield in H 2 O ( H 2 O = 1.6%) that increases considerably in D 2 O ( D 2 O = 5.3%), which highlights the strong quenching effect of the four ligand O-H oscillators. The emission spectrum of [EuL 1 ] 3+ is rather unusual in that it shows a relatively high intensity of the 5 D 0 → 7 F 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 [EuL 1 ] 3+ complex is very low ( H 2 O = 0.10%), considering the good protection of the Eu III coordination environment offered by the ligand. Cyclic voltammograms recorded from aqueous solutions of [EuL 1 ] 3+ display a reversible curve with a half-wave potential of −620 mV ( versus Ag/AgCl), while [EuL 2 ] 3+ presents a reduction peak at more negative potential (−1040 mV). Thus, the L 1 ligand provides a rather good stabilisation of divalent Eu compared to the L 2 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 [EuL 1 ] 3+ . A density functional theory (DFT) study provides very similar energies for the ligand-centered excited singlet ( 1 ππ*) and triplet ( 3 ππ*) states of [EuL 1 ] 3+ and [EuL 2 ] 3+ . The energy of the 9 LMCT state of [EuL 1 ] 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. The stabilization of a divalent Europium provides an efficient pathway for the quenching of the luminescence in ten-coordinate macrocyclic complexes.