Jump to main content
Jump to site search

Issue 12, 2019
Previous Article Next Article

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

Author affiliations

Abstract

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 5D07F5,6 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.

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

Back to tab navigation

Supplementary files

Publication details

The article was received on 19 Dec 2018, accepted on 27 Feb 2019 and first published on 27 Feb 2019


Article type: Paper
DOI: 10.1039/C8DT05005H
Dalton Trans., 2019,48, 4035-4045

  •   Request permissions

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

    A. Nonat, D. Esteban-Gómez, L. Valencia, P. Pérez-Lourido, J. L. Barriada, L. J. Charbonnière and C. Platas-Iglesias, Dalton Trans., 2019, 48, 4035
    DOI: 10.1039/C8DT05005H

Search articles by author

Spotlight

Advertisements