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Optimization of the Sensitization Process and Stability of Octadentate Eu(III) 1,2-HOPO Complexes.

D'Aléo A, Moore EG, Xu J, Daumann LJ, Raymond KN - Inorg Chem (2015)

Bottom Line: The thermodynamic stability of the europium complexes has been studied and reveals these complexes may be effective for biological measurements.The luminescence properties for [Eu(H(14O4,2)-1,2-HOPO)](-) and [Eu(H(17O5,2)-1,2-HOPO)](-) are better than that of the model bis-tetradentate [Eu(5LIN(Me)-1,2-HOPO)2](-) complex, suggesting a different geometry around the metal center despite the geometric freedom allowed by the longer central chain in the H(mOn,2) scaffold.These differences are also evidenced by examining the luminescence spectra at room temperature and at 77 K and by calculating the luminescence kinetic parameters of the europium complexes.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of California, Berkeley, California 94720-1460, United States.

ABSTRACT
The synthesis of a series of octadentate ligands containing the 1-hydroxypyridin-2-one (1,2-HOPO) group in complex with europium(III) is reported. Within this series, the central bridge connecting two diethylenetriamine units linked to two 1,2-HOPO chromophores at the extremities (5-LIN-1,2-HOPO) is varied from a short ethylene chain (H(2,2)-1,2-HOPO) to a long pentaethylene oxide chain (H(17O5,2)-1,2-HOPO). The thermodynamic stability of the europium complexes has been studied and reveals these complexes may be effective for biological measurements. Extension of the central bridge results in exclusion of the inner-sphere water molecule observed for [Eu(H(2,2)-1,2-HOPO)](-) going from a nonacoordinated to an octacoordinated Eu(III) ion. With the longer chain length ligands, the complexes display increased luminescence properties in aqueous medium with an optimum of 20% luminescence quantum yield for the [Eu(H(17O5,2)-1,2-HOPO)](-) complex. The luminescence properties for [Eu(H(14O4,2)-1,2-HOPO)](-) and [Eu(H(17O5,2)-1,2-HOPO)](-) are better than that of the model bis-tetradentate [Eu(5LIN(Me)-1,2-HOPO)2](-) complex, suggesting a different geometry around the metal center despite the geometric freedom allowed by the longer central chain in the H(mOn,2) scaffold. These differences are also evidenced by examining the luminescence spectra at room temperature and at 77 K and by calculating the luminescence kinetic parameters of the europium complexes.

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(a) Luminescence spectrum showing thetypical decrease of luminescence intensity upon addition of increasingamounts of DTPA; (b) DTPA competition batch titration of [Eu(H(3,2)-1,2-HOPO)]− (black squares, line), [Eu(H(5O,2)-1,2-HOPO)]− (red circles, line), and [Eu(H(11O3,2)-1,2-HOPO)]− (blue triangles, line) versus DTPA. The x intercept indicates the difference in pEu between EuDTPA (pEu =19.04) and the two complexes.
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fig1: (a) Luminescence spectrum showing thetypical decrease of luminescence intensity upon addition of increasingamounts of DTPA; (b) DTPA competition batch titration of [Eu(H(3,2)-1,2-HOPO)]− (black squares, line), [Eu(H(5O,2)-1,2-HOPO)]− (red circles, line), and [Eu(H(11O3,2)-1,2-HOPO)]− (blue triangles, line) versus DTPA. The x intercept indicates the difference in pEu between EuDTPA (pEu =19.04) and the two complexes.

Mentions: The generalprocedure used to determine the pEu values of the ligands was adaptedfrom an already described study using Gd31,32 and are similar to those already reported for other complexes.29 Different volumes of a standardized DTPA stocksolution were added to solutions of constant ligand, metal, and electrolyteconcentrations. In the current work, the pH of all solutions was keptconstant at 7.4 with TRIS buffer instead of adjusting the pH to 6.0as was done in past studies,31 and thesolutions were diluted to identical volumes. After stirring the solutionsfor 24 h to ensure thermodynamic equilibrium was reached, the pH wasagain checked just before analyzing the samples spectrophotometrically.The concentrations of each ligand relative to DTPA used in the finaldata analysis ranged from 1:1 to 1:1000 (L:DTPA). Concentrations offree and complexed ligand in each solution were determined from theluminescence spectra at identical pH and concentrations. These concentrationswere used for the log/log plots (Figure 1)to give the difference in pEu between the competing DTPA and ligandof interest. In a similar way, pZn was determined by using a solutionof ZnCl2 in water as a competitor instead of DTPA.


Optimization of the Sensitization Process and Stability of Octadentate Eu(III) 1,2-HOPO Complexes.

D'Aléo A, Moore EG, Xu J, Daumann LJ, Raymond KN - Inorg Chem (2015)

(a) Luminescence spectrum showing thetypical decrease of luminescence intensity upon addition of increasingamounts of DTPA; (b) DTPA competition batch titration of [Eu(H(3,2)-1,2-HOPO)]− (black squares, line), [Eu(H(5O,2)-1,2-HOPO)]− (red circles, line), and [Eu(H(11O3,2)-1,2-HOPO)]− (blue triangles, line) versus DTPA. The x intercept indicates the difference in pEu between EuDTPA (pEu =19.04) and the two complexes.
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Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4556046&req=5

fig1: (a) Luminescence spectrum showing thetypical decrease of luminescence intensity upon addition of increasingamounts of DTPA; (b) DTPA competition batch titration of [Eu(H(3,2)-1,2-HOPO)]− (black squares, line), [Eu(H(5O,2)-1,2-HOPO)]− (red circles, line), and [Eu(H(11O3,2)-1,2-HOPO)]− (blue triangles, line) versus DTPA. The x intercept indicates the difference in pEu between EuDTPA (pEu =19.04) and the two complexes.
Mentions: The generalprocedure used to determine the pEu values of the ligands was adaptedfrom an already described study using Gd31,32 and are similar to those already reported for other complexes.29 Different volumes of a standardized DTPA stocksolution were added to solutions of constant ligand, metal, and electrolyteconcentrations. In the current work, the pH of all solutions was keptconstant at 7.4 with TRIS buffer instead of adjusting the pH to 6.0as was done in past studies,31 and thesolutions were diluted to identical volumes. After stirring the solutionsfor 24 h to ensure thermodynamic equilibrium was reached, the pH wasagain checked just before analyzing the samples spectrophotometrically.The concentrations of each ligand relative to DTPA used in the finaldata analysis ranged from 1:1 to 1:1000 (L:DTPA). Concentrations offree and complexed ligand in each solution were determined from theluminescence spectra at identical pH and concentrations. These concentrationswere used for the log/log plots (Figure 1)to give the difference in pEu between the competing DTPA and ligandof interest. In a similar way, pZn was determined by using a solutionof ZnCl2 in water as a competitor instead of DTPA.

Bottom Line: The thermodynamic stability of the europium complexes has been studied and reveals these complexes may be effective for biological measurements.The luminescence properties for [Eu(H(14O4,2)-1,2-HOPO)](-) and [Eu(H(17O5,2)-1,2-HOPO)](-) are better than that of the model bis-tetradentate [Eu(5LIN(Me)-1,2-HOPO)2](-) complex, suggesting a different geometry around the metal center despite the geometric freedom allowed by the longer central chain in the H(mOn,2) scaffold.These differences are also evidenced by examining the luminescence spectra at room temperature and at 77 K and by calculating the luminescence kinetic parameters of the europium complexes.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of California, Berkeley, California 94720-1460, United States.

ABSTRACT
The synthesis of a series of octadentate ligands containing the 1-hydroxypyridin-2-one (1,2-HOPO) group in complex with europium(III) is reported. Within this series, the central bridge connecting two diethylenetriamine units linked to two 1,2-HOPO chromophores at the extremities (5-LIN-1,2-HOPO) is varied from a short ethylene chain (H(2,2)-1,2-HOPO) to a long pentaethylene oxide chain (H(17O5,2)-1,2-HOPO). The thermodynamic stability of the europium complexes has been studied and reveals these complexes may be effective for biological measurements. Extension of the central bridge results in exclusion of the inner-sphere water molecule observed for [Eu(H(2,2)-1,2-HOPO)](-) going from a nonacoordinated to an octacoordinated Eu(III) ion. With the longer chain length ligands, the complexes display increased luminescence properties in aqueous medium with an optimum of 20% luminescence quantum yield for the [Eu(H(17O5,2)-1,2-HOPO)](-) complex. The luminescence properties for [Eu(H(14O4,2)-1,2-HOPO)](-) and [Eu(H(17O5,2)-1,2-HOPO)](-) are better than that of the model bis-tetradentate [Eu(5LIN(Me)-1,2-HOPO)2](-) complex, suggesting a different geometry around the metal center despite the geometric freedom allowed by the longer central chain in the H(mOn,2) scaffold. These differences are also evidenced by examining the luminescence spectra at room temperature and at 77 K and by calculating the luminescence kinetic parameters of the europium complexes.

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