The shift towards higher energies is attributed to the different ligand scaffolds: the four protonated amide nitrogen atoms in the molecular structure of 2 can form hydrogen bonds with the hydroxyl groups from the pyridinone rings, thereby stabilizing the first singlet excited state of the corresponding europium complex, whereas the backbone of 1 only contains two protonated amide nitrogen atoms yielding a complex with a singlet excited state slightly higher in energy, and a shoulder at lower energy on the main absorption peak.ĪThe uncertainties were determined from the standard deviation between three independent experiments performed in aqueous buffered solutions (TRIS, pH 7.4). This spectrum is blue-shifted and has a lower molar absorption coefficient than the spectrum of 0 (λ max = 343 nm, ε = 18,200 M -1cm -1, 5 Fig. 2) shows an absorption maximum due to π− π* transitions at λ max = 315 nm (ε = 17,700 M -1cm -1). The electronic absorption spectrum of - ( Fig. Structures of the octadentate hydroxypyridonate ligands 3,4,3-LI(1,2-HOPO) ( 1, left) and H(2,2)-1,2-HOPO ( 2, right) the metal-coordinating oxygen atoms are indicated in red.Īll photophysical properties were measured in buffered aqueous solutions at pH 7.4 and relevant parameters are summarized in Table 1. In addition, the Eu luminescence sensitization properties of the antenna ligand 1 are used as a spectroscopic tool to determine the solution thermodynamic stability of the corresponding metal-complex, which provides a new method for the accurate determination of these thermodynamic parameters.
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5 The work presented herein probes the coordination chemistry and photophysical properties of the Gd(III) and Eu(III) complexes of 1, showing that the geometry of the ligand scaffold strongly affects the inner coordination sphere of the metal ion and consequently its emissive properties.
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1) has been studied for its ability to form a highly stable luminescent Eu(III) complex that contains a molecule of water in the inner coordination sphere at physiological pH. 1) is composed of 1,2-HOPO units linked to a central linear spermine scaffold and has shown potential as a therapeutic Pu(IV) and Am(III) chelator, 1 the branched tetrapodal ligand H(2,2)-1,2-HOPO ( 2, Fig. While the octadentate ligand 3,4,3-LI(1,2-HOPO) ( 1, Fig. 1, 5 The backbone geometry of the ligand must affect the thermodynamic stability and photophysical properties of the corresponding Ln- and An-complexes, but this has not been investigated in a systematic way.
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Octadentate ligands, each incorporating four 1,2-HOPO functionalities, are known to strongly bind Ln(III), An(III), and An(IV) and to act as antennae that sensitize the emission of Eu(III). 4 The coordination chemistry properties of these ligands can be fine-tuned by systematic modifications of the denticity, geometry and acidity of the backbone. The 6-amide derivative of 1-hydroxy-pyridin-2-one (1,2-HOPO) has been linked to multiple polyamine scaffolds through amide coupling, to form multidentate ligand structures used for a variety of applications such as actinide (An) and iron chelation, 1, 2 MRI contrast enhancement 3 and lanthanide (Ln) luminescence sensitization. The high-affinity bidentate hydroxypyridonate (HOPO) metal-chelating groups are related to microbial siderophores: they combine the structural features of hydroxamic acids with the electronic properties of catechols.