Investigating subtle 4f vs. 5f coordination differences using kinetically inert Eu(iii), Tb(iii), and Cm(iii) complexes of a coumarin-appended 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate (DO3A) ligand

Abstract

TitleInvestigating subtle 4f vs. 5f coordination differences using kinetically inert Eu(iii), Tb(iii), and Cm(iii) complexes of a coumarin-appended 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate (DO3A) ligand In order to reveal subtle differences between the solution chemistries of trivalent 4f and 5f elements, the physicochemical and photophysical properties of europium(III), terbium(III) and curium(III) complexes formed with a 7-methoxy-coumarin appended 1,4,7,10-tetraazadodecane-1,4,7-triacid (DO3A) ligand were studied. All three complexes were found to be kinetically inert and exhibit stability constants similar to their 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) equivalents. The Cm(III) and Eu(III) complexes feature strong sensitised emission, while the triplet energy of the coumarin prohibits efficient sensitisation of the Tb(III) analogue. The data presented here indicate significant differences in perturbation of the sensitising chromophore photophysics between the 4f and 5f elements. In contrast, the size of the metal center appears to not be a determining factor for the physicochemical properties of these kinetically inert Eu(III), Tb(III,) and Cm(III) complexes.

Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions

“…The relevant part of the molecule is the coumarin chromophore that is covalently attached to a 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetate (DO3A) binding pocket with an inert yttrium(III) ion, a terbium(III) ion that act as an excited state energy transfer acceptor, or a europium(III) ion that can act as an energy acceptor and as a PeT quencher. The complexes are kinetically inert,[1d], [9a], and by scrutinising the coumarin fluorescence we aim to understand the excited energy transfer cascade within the complexes and differentiate between energy transfer and PeT quenching in the europium(III) complex.…”

“…Eu(OTf) 3 (98 %), Tb(OTf) 3 (98 %), were obtained from Strem Chemicals, KI (99 %) were from Sigma‐Aldrich and used as received. The synthesis protocols for ligands and complexes have been described elsewhere . Eu.DOTA was prepared as described in the literature by Desreux …”

“…We have been focused on fully unravelling the physicochemical properties of a series of complexes between actinide(III) and lanthanide(III) ions and a coumarin‐appended 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetate ligand ( Ln . L , Figure ) . As we believe that we understand the photophysics of these complexes, we decided to investigate intermolecular vs. intramolecular collisional quenching in Y .…”

“…The energy migration processes are: i , diffusion to form an encounter complex; ii , association to form a species that allow for electron exchange; iii , energy transfer EnT; iv , dissociation and diffusion. Insert depict the equilibrium governing the solution structure of the complexes that are present in two forms in solution …”

“…We have studied the Ln . L complexes in great detail, and understand the solution structure and the excited state energy transfer processes of these complexes in HEPES buffer . Here, we have performed collision quenching experiments that reveal that:…”