Abstract
The coordination compounds of the trivalent lanthanide ions (Ln(III)) have unique photophysical properties. Ln(III) excitation is usually performed through a light-harvesting antenna. To enable Ln(III)-based emitters to reach their full potential, an understanding of how complex structure affects sensitization and quenching processes is necessary. Here, the role of the linker between the antenna and the metal binding fragment was studied. Four macrocyclic ligands carrying coumarin 2 or 4-methoxymethylcarbostyril sensitizing antennae linked to an octadentate macrocyclic ligand binding site were synthesized. Complexation with Ln(III) (Ln = La, Sm, Eu, Gd, Tb, Yb and Lu) yielded species with overall −1, 0, or +2 and +3-charge. Paramagnetic 1H NMR spectroscopy indicated subtle differences between the coumarin- and carbostyril-carrying Eu(III) and Yb(III) complexes. Cyclic voltammetry showed that the effect of the linker on the Eu(III)/Eu(II) apparent reduction potential was dependent on the electronic properties of the N-substituent. The Eu(III), Tb(III) and Sm(III) complexes were all luminescent. Coumarin-sensitized complexes were poorly emissive; photoinduced electron transfer was not a major quenching pathway in these species. These results show that seemingly similar emitters can undergo very different photophysical processes, and highlight the crucial role the linker can play.
Highlights
The luminescence of the coordination compounds of lanthanide ions (Ln(III)) has found application in a variety of fields, such as in biological sensing and imaging [1], in fluorescent lamps and lasers [2], and in anti-counterfeiting [3,4,5]
Trialkylation with bromoacetamide was carried out analogous procedures, these are summarized in Scheme 1
Octadentate ligands carrying 4-methoxymethylcarbostyril or coumarin 2 sensitizing antennae mounted on a DO3A ligand binding site were prepared and characterized
Summary
The luminescence of the coordination compounds of lanthanide ions (Ln(III)) has found application in a variety of fields, such as in biological sensing and imaging [1], in fluorescent lamps and lasers [2], and in anti-counterfeiting [3,4,5]. The direct excitation of Ln(III) is inefficient due to the low absorption coefficients of the Laporte-forbidden 4f-4f transitions [6]. A common method for overcoming the challenge of low Ln(III) absorptions is to excite the metal ion through a light-harvesting chromophore, a so-called antenna [6,7]. Ln(III) luminescence properties (sharp, spiked emission peaks, long excited-state lifetimes) with the strong absorption of common organic chromophores. The presence of the antenna offers additional benefits. It can carry reactive groups for attachment to biomolecules [8], or labels (e.g., F-19) [9] for creating multimodal imaging agents. The antenna can provide a way to render the Ln(III) complex analyte-responsive [10,11,12,13,14]
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