“Cymothoe sangaris”:  An Extremely Stable and Highly Luminescent 1,2-Hydroxypyridinonate Chelate of Eu(III)

Abstract

The synthesis, structure, and characterization of a new class of luminescent agents based on the 1,2-hydroxypyridinone chelator are reported. The prototype complex, [Eu(5LIO-1,2-HOPO)2]-, demonstrates superb aqueous stability [pEu = 18.64(10)] and is highly emissive [Phit = 0.21(3)], emitting almost pure "red" light (lambdaem = 612 nm). The crystal structure reveals a coordination geometry reminiscent of a "butterfly", hence the soubriquet of "Cymothoe sangaris" to identify with the unique species of red butterfly found only in Central Africa.

Aryl-Bridged 1-Hydroxypyridin-2-one: Sensitizer Ligands for Eu(III)

“…The maximum absorption, at ca. 342 nm, is significantly red shifted compared to aliphatic bridged analogs 8,9 and is in excellent agreement with the TD-DFT calculation. This is consistent with the expectation that the aryl bridged ligand, compared with the previously reported aliphatic bridged complexes, has a more delocalized electronic excited state.…”

“…Despite this obvious structural difference, the coordination polyhedron, as determined by shape analysis, 8 remains closest to the bicapped trigonal prismatic (C 2v ) geometry, and the observed coordinate bondlengths (Eu-O = 2.393 Å) are similar to previous reports. 8,9 Solution thermodynamic experiments assessed the stability of the (5) 9 ), these data do not follow the usual trend that more acidic ligands (of the same chelate group) will form weaker metal complexes. 12 Instead, while the most acidic ligand is o-Phen-1,2-HOPO, it is evident that the 5LI-1,2-HOPO ligand forms the weakest ML 2 chelate.…”

“…This ligand bridges two 1,2-HOPO units via an aryl group, yielding a higher degree of conjugation when compared to previously reported alkyl linkages. [8,9] The resultant stability constants and optical properties of this new ligand architecture are compared to the parent 5LI-8 and 5LIO-1,2-HOPO 9 compounds.* To whom correspondence should be addressed. raymond@socrates.berkeley.edu.…”

“…Despite this obvious structural difference, the coordination polyhedron, as determined by shape analysis, 8 remains closest to the bicapped trigonal prismatic (C 2v ) geometry, and the observed coordinate bondlengths (Eu-O = 2.393 Å) are similar to previous reports. 8,9 Solution thermodynamic experiments assessed the stability of the [Eu(o-Phen-1,2-HOPO) 2 2 ] − complex, as determined by its pEu, can be partially attributed to the 1,2-HOPO chelate which forms stable complexes with lanthanide ions. 10 However, when compared to the complexes linked by an aliphatic linear spacer (5LIO-1,2-HOPO, pEu ~ 18.64 (10) 8 and 5LI-1,2-HOPO, pEũ 18.35 (5) 9 ), these data do not follow the usual trend that more acidic ligands (of the same chelate group) will form weaker metal complexes.…”

“…12 Instead, while the most acidic ligand is o-Phen-1,2-HOPO, it is evident that the 5LI-1,2-HOPO ligand forms the weakest ML 2 chelate. For the 5LIO-1,2-HOPO derivative, it was suggested [9] the improved stability was a result of intermolecular hydrogen bonding involving the amide protons and the central oxygen of the bridge, which rigidifies the ligand providing an improved chelate effect. Clearly, a similar argument applies in the present case, explaining the improved complex stability over 5LI-1,2-HOPO when using the o-Phen scaffold.…”