New insights into structure and luminescence of EuIII and SmIII complexes of the 3,4,3-LI(1,2-HOPO) ligand

Article abstract of DOI:10.1021/ja5116524

We report the preparation and new insight into photophysical properties of luminescent hydroxypyridonate complexes [M^IIIL]^- (M = Eu or Sm) of the versatile 3,4,3-LI(1,2-HOPO) ligand (L). We report the crystal structure of this ligand

Full text of DOI:10.1021/ja5116524

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Communication
III
New Insights into Structure and Luminescence of Eu
III
and Sm Complexes of the 3,4,3-Li(1,2-HOPO) Ligand
Lena J. Daumann, David S. Tatum, Benjamin E. R. Snyder, Chengbao
Ni, Ga-Lai Law, Edward I. Solomon, and Kenneth N. Raymond
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5116524 • Publication Date (Web): 21 Jan 2015
Downloaded from http://pubs.acs.org on February 18, 2015
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New Insights into Structure and Luminescence of EuIII and
SmIII Complexes of the 3,4,3-Li(1,2-HOPO) Ligand
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Lena J. Daumann†, David S. Tatum†, Benjamin E. R. Snyder‡, Chengbao Ni†, Gaꢀlai Law†, Edward I.
Solomon‡ and Kenneth N. Raymond†*
†Chemical Science Division, Lawrence Berkeley National Laboratory and Department of Chemistry, University of Califorꢀ
nia, Berkeley, CA, 94720, USA
‡Stanford University, Department of Chemistry, Stanford, CA, 94305, USA
Supporting Information Placeholder
that upon mixing L with EuIII two isomers form, A and B. A is a
ABSTRACT: We report the preparation and new insight into
kinetic product that rapidly converts fully to the brighter thermoꢀ
dynamic isomer, B.
photophysical properties of luminescent hydroxypyridonate com-
plexes [MIIIL]- (M = Eu or Sm) of the versatile 3,4,3-Li(1,2-
HOPO) ligand (L). We report the crystal structure of this ligand
with EuIII as well as insights into the coordination behavior and
geometry in solution by using magnetic circular dichroism. In
addition TD-DFT calculations were used to examine the excited
states of the two different chromophores present in the 3,4,3-
LI(1,2-HOPO) ligand. We find that this complex undergoes a
transformation when made in situ to yield a complex with higher
quantum yield over time. It is proposed that the lower QY in the in
situ complex is not only due to water quenching but could also be
due to a lower degree of covalency (in the kinetic isomer) as indi-
cated by magnetic circular dichroism measurements.
O
HO
N
O
O
OH
N
H
N
N
O
O
N
N
N
H
OH
O
O
N
OH
O
Scheme 1 The spermine linked 3,4,3ꢀLi(1,2ꢀHOPO) ligand for
sensitization of EuIII and SmIII, coordinating atoms in red
The isolated complexes [ML][C5H6N] were prepared by refluxꢀ
ing a 1:1 mixture of LH4 and the corresponding MIII salt in methꢀ
anol using pyridine as base, yielding the complexes as white solꢀ
ids. Experimental details and mass spectral data can be found in
the supporting information (Figures S1ꢀS3). Xꢀray quality crystals
of [EuL]K(DMF) were obtained by slow diffusion of a mixture of
THF and cyclohexane into a DMF solution containing the comꢀ
plex and KCl. As shown in Figure 1, a ML complex is formed, in
which the EuIII ion is coordinated to eight oxygen atoms from the
Ligandꢀsensitized luminescent lanthanide(III) complexes have
unique photophysical properties, which make them exciting canꢀ
didates for a wide range of applications, including fluorescenceꢀ
based bioassays; as has been extensively reviewed.1ꢀ5 We have
shown that the 6ꢀamide derivatives of 1ꢀhydroxyꢀpyridinꢀ2ꢀone
(1,2ꢀHOPO) in tetradentate ligands form EuIII complexes with
high thermodynamic stability (pEu7.4~ 18.6) and excellent photoꢀ
physical properties (quantum yields up to 21.5%).6 Studies of the
aryl bridged tetradentate 1,2ꢀHOPO ligands have shown that the
geometries of the ligand backbones significantly affect the quanꢀ
tum yields of the corresponding europium complexes.7 We have
also recently confirmed the sensitization and emission efficiencies
of those 1,2ꢀHOPO complexes by timeꢀresolved Xꢀray absorption
near edge structure measurements at the Eu L3 edge.8 Octadentate
ligands, such as the branched H(2,2)ꢀ1,2ꢀHOPO ligand and the
linear spermine linked 3,4,3ꢀLi(1,2ꢀHOPO) ligand (LH4, Scheme
1) form significantly more stable europium complexes (pEu7.4
>
21.2) than tetradentate 1,2ꢀHOPO ligands.9,10 The 3,4,3ꢀLi(1,2ꢀ
HOPO) ligand (LH4) has been reported as an efficient actinide
sequestration agent in vivo, however, the detailed coordination
behavior towards Ln(III) and An(III) ions of this ligand are to date
unknown.10ꢀ16 This ligand has also been discussed as a chelator
for 89Zr radiopharmaceuticals.17 Abergel et al. recently demonꢀ
strated the capability of L to act as an effective chelator for CmIII
while also sensitizing CmIII photoluminescence, with a long lifeꢀ
time (383 µs) and a remarkable quantum yield in water (45ꢁ%).18
Previously, the EuIII and SmIII complexes have been prepared in
situ and QYs of 7.0 and 0.2 % have been reported.9 We propose
four 1,2ꢀHOPO units.
Figure 1 Xꢀray structure of [EuL]ꢀ (50% probability) without
hydrogen atoms, except for the amide hydrogens (NꢀH). Left: side
on view; right: top view through C2 axis.
In addition, a K+ ion and a highly ordered DMF molecule are
present in the crystal lattice, which show intermolecular interacꢀ
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Journal of the American Chemical Society
Page 2 of 4
tions with the [EuL]ꢀ units to form a oneꢀdimensional chain (Figꢀ
and D2O for the isolated complex were longer (19.4±0.1 and
ures S4 and S5). The EuꢀO distances in [EuL]ꢀ range from 2.36 to
2.44 Å and are similar to previously reported tetradentate 1,2ꢀ
HOPO complexes.6 Crystallographic parameters and selected
bond lengths can be found in Tables S1 and S2. There are two
types of 1,2ꢀHOPO units present within the ligand and LnIII comꢀ
plexes. Two of the 1,2ꢀHOPO units are connected to the backbone
by secondary amide groups (N5 and N8), which show intramolecꢀ
ular hydrogen bonding interactions (NꢀH···O = ca. 1.85 Å) with
the adjacent ONꢀoxide atoms (O2 and O6). In accordance, the secꢀ
ondary amides show coꢀplanarity with their 1,2ꢀHOPO units (torꢀ
sion angles = ca. 3.0 and 8.7º), ensuring that the π systems of the
amide functions maintain conjugation with the corresponding 1,2ꢀ
HOPO units. The tertiary amide groups, however, display large
torsion angles (ca. 62.1 and 64.0º) relative to the respective 1,2ꢀ
HOPO planes, which partially breaks the aforementioned πꢀ
conjugation. Such dramatic differences highlight the importance
of intramolecular NꢀH···O hydrogen bonding interactions for
preorganization of the ligand. Gasꢀphase DFT ground state miniꢀ
mization (B3LYP//6ꢀ31G(d,p)) of [EuL]ꢀ closely matches the
crystal structure. Only one of the four 1,2ꢀHOPO units deviates
significantly from its superposed partner, which is likely due to
crystal packing effects in the solid state, including interactions
with the K+ ion and solvent (Figure S6). The shape measureꢀ
ments19 of the calculated and crystal structure yield slightly difꢀ
ferent results. While the crystal structure shows a slight preferꢀ
ence for a C2v symmetry the calculated structure yields a dodecaꢀ
hedral (D2d) geometry for the coordinating oxygen atoms around
the EuIII ion (Table S3).
For the isolated EuIII and SmIII complexes, quantum yields of
15.6% and 0.41% were measured in aqueous solution (λex=325
nm). In addition, a new protocol reported here for the in situ prepꢀ
aration yields similar yet slightly lower QYs for [EuL]ꢀ and
[SmL]ꢀ (14.0% and 0.39%, respectively). Notably the luminesꢀ
cence QYs of the in situ EuIII and SmIII complexes increased over
time (2ꢀ4h, Figure 2) to reach the values of the isolated complexes
(Figures S9ꢀS11). We do not attribute this effect to slow complexꢀ
ation, since no luminescence from free ligand was observed. We
conclude that the lower QY for the in situ generated complex is
due to slow rearrangement of some fraction (a kinetic isomer) of
the initially formed complex.
180.9±0.6 ꢂs) than for the in situ complex (18.4±0.1 and
155.0±0.3 ꢂs), respectively. The rapid conversion of kinetic isoꢀ
mer A to the longer lived B is also provided by an experiment
which showed that τobs of in situ generated complex converged
rapidly towards the τobs of isolated complex (Figure S12).
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A close examination of the photophysical parameters was conꢀ
ducted. For the EuIII complex, the parameters in Table 1 can be
obtained from the ratio of the magnetic dipole transition
(5D0→7F1) intensity to the total emission intensity, from which the
radiative lifetime (τrad) can be estimated.7,21,22 Analysis showed
that the metal and sensitization efficiencies ηEu and ηsens are simiꢀ
lar within error for both methods of preparation (Table 1). The
most significant difference is seen for knonrad which is higher (792
sꢀ1) for the in situ complex, thus the lower QY is attributed to
increased rates of nonꢀradiative quenching. The q value is higher
(0.14) for the in situ complex but over time (~2h) reaches the
value of the isolated complex (0.04), suggesting that some portion
of the initially formed [EuL]ꢀ rearranges, and in the process disꢀ
places the quenching water molecule.
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Table 1 Photophysical parameters for [EuL]ꢀ
isolated
586 ± 60
in situ (T=0)
587 ± 63
krad [sꢀ1]
τrad [ꢂs]
ηEu
ηsens
QY [%]
q
1705 ± 177
0.469 ± 0.016
0.397 ± 0.018
15.6 ± 0.6
0.04
1721 ± 180
0.417 ± 0.045
0.356 ± 0.043
14.0 ± 0.3
0.14
τH2O [ꢂs]
τD2O [ꢂs]
knonrad [sꢀ1]
814 ± 8
1135 ± 9
623 ± 60
711 ± 7
1030 ± 6
792 ± 62
Upon comparison of the excitation and absorption spectra of
[EuL]ꢀ and with the aid of TDꢀDFT calculations we also found
that the chromophore absorbing at longer wavelength (NHꢀ1,2ꢀ
HOPO) is better at sensitizing EuIII than the one at shorter waveꢀ
length (Nꢀ1,2ꢀHOPO, Figures S7, S8 and S13, Table S4). Conꢀ
sistent with the differences between the excitation and absorption
spectrum, we observed that the quantum yield increases upon redꢀ
shifting the excitation wavelength (18.5% at 333 nm and 20.5% at
350 nm).
2.5 107
2 107
1.5 107
1 107
5 106
0
8 104
6 104
4 104
2 104
0
5
An examination of the D0→7F1 transition of isolated and in
(a)
(b)
T = 10 min
T = 60 min
T = 120 min
T = 180 min
T = 300 min
T = 1440 min
T = 10 min
T = 60 min
T = 120 min
situ [EuL]ꢀ in TRIS buffer suggests, due to the splitting into three
symmetrical peaks, a low symmetry in solution (Figure S14).23 To
further analyze potential geometric differences that might arise
from the different preparation methods, we studied the magnetic
circular dichroism spectra. A buffer/glycerol solvent system was
unsuitable due to the poor solubility of [ML]ꢀ at high concentraꢀ
tion, thus a 2:1 mixture of DMF and MeOH was used. The lumiꢀ
nescence spectra of [EuL]ꢀ and [SmL]ꢀ were also recorded under
these conditions and the absence of splitting of the 5D0→7F1 tranꢀ
sition points to a higher symmetry than in buffer (Figure S15).
Indeed, for both [Euꢀ]ꢉꢁꢂꢃꢄꢅꢆꢇꢈ and [Euꢀ]ꢁꢉꢊ ꢂꢁꢆꢋ a negative Aꢀterm
4
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104
0
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1
107
107
107
0
650
655
660
605
610
615
620
550
600
650
700
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600
650
700
750
Wavelength [nm]
Wavelength [nm]
Figure 2 Increase of luminescence intensity of in situ generated
[EuL]ꢀ (a) and [SmL]ꢀ (b) measured in TRIS (pH7.4)
First order luminescence decay measurements indicated that the
number of bound water molecules in [Euꢀ]ꢉꢁꢂꢃꢄꢅꢆꢇꢈ is essentially
zero (q=0.04).20 We have also considered the possibility of dimerꢀ
ic Eu2L2 complex formation. However, upon serial dilution of the
complexes (2 x 10ꢀ5 – 5 x 10ꢀ6 M) the luminescence lifetimes
measured remained unchanged. The lifetimes were best fit to a
mono exponential decay, consistent with but not conclusive of
5
around 465 nm, arising from the D2←7F0 transition, typical for
D2d symmetry, was found (Figure 3, Figure S16).24 Remarkably,
this is in accordance with the findings from shape analysis for the
DFT calculated structure. In addition we also observed the
5L6←7F0 transition.24,25 While both methods of preparation yield
similar MCD spectra, the relative intensities (based on the norꢀ
5
malized L6←7F0 transition) are different (for the in situ prepared
one species in solution. Intriguingly the measured lifetimes (τobs
)
5
complex the D2←7F0 MCD transition is ~16% less intense). For
in H2O and D2O for [Euꢀ]ꢉꢁꢂꢃꢄꢅꢆꢇꢈ were longer than for
[Euꢀ]ꢉꢁꢊ ꢂꢁꢆꢋ (Table 1). A slightly increased q value for the in situ
complex could indicate that a portion of the formed complex
leaves the EuIII sites open for coordination of a water molecule. A
similar observation was found for [SmL]ꢀ where lifetimes in H2O
octahedral EuIII complexes the intensity ratio between the two
5
5D2←7F0 and L6←7F0 transitions has been used to describe the
degree of fꢀorbital overlap.26,27 If this concept is transferred to the
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MCD of [EuL]ꢀ it could suggest a higher degree of orbital overlap
(2) Bünzli, J.-C. G.; Eliseeva, S. V. Chemical Science 2013, 4,
1939.
for [Euꢀ]ꢉꢁꢂꢃꢄꢅꢆꢇꢈ
.
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1.25
0.5
0.25
0
5L <ꢀ7
F
0
in situ
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in situ
5D <ꢀ7
2
F
0
1
isolated
isolated
0.75
0.5
0.25
0
ꢀ0.25
ꢀ0.25
ꢀ0.5
ꢀ0.5
9
(7) D'Aleo, A.; Moore, E. G.; Szigethy, G.; Xu, J.; Raymond, K. N.
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392 393 394 395 396 397 398
462 463 464 465 466 467 468
Wavelength [nm]
Wavelength [nm]
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Figure 3 Observed MCD transitions for [Euꢀ]ꢉꢁꢂꢃꢄꢅꢆꢇꢈ and
[Euꢀ]ꢉꢁꢊ ꢂꢁꢆꢋ measured in DMF:MeOH (2:1) at 7 T and 250K
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Zhang, X.; Stickrath, A. B.; Mortensen, D. R. J. Am. Chem. Soc.
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J.; Chang, P. Y.; Bunin, D. I.; Blakely, E. A.; Bjornstad, K. A.;
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Combining the results from magnetic circular dichroism and
the luminescence quantum yield and lifetime measurements it
appears that for maximal quantum yields and optimal water
shielding, complex preparation plays a crucial role. In order for
L4ꢀ to fully coordinate the metal ion with all four HOPO units the
complexes should be isolated prior to luminescence measureꢀ
ments. We propose that the lower QY in the in situ complex is not
only due to water quenching but could also be due to a lower deꢀ
gree of covalency (in the kinetic isomer) as indicated by magnetic
circular dichroism measurements. We are currently investigating
if this technique, combined with luminescence measurements,
could be potentially used to probe a Dexter mechanism of energy
transfer.28
ASSOCIATED CONTENT
Supporting Information
Experimental procedures, crystal structure parameters, DFT calꢀ
culation details, mass spectral and photophysical data. This mateꢀ
rial is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* Phone +1 510 642 7219, raymond@socrates.berkeley.edu
ACKNOWLEDGMENTS
We thank Benjamin Allred, Manuel SturzbecherꢀHoehne, and Jide
Xu for helpful discussions. The ligand 3,4,3ꢀLI(1,2ꢀHOPO) that
was used for growing xꢀray quality crystals of K[EuL] was a kind
donation from Dr. Rebecca Abergel at LBNL and prepared by
Ash Stevens, Inc. (Detroit, MI, USA, Lot MLꢀ11ꢀ276). This reꢀ
search is supported by U.S. Department of Energy at LBNL under
Contract No. DEꢀAC02ꢀ05CH11231 and NIH grant HL069832.
LJD is grateful for a scholarship of the Alexander von Humboldt
Foundation. ES would like to acknowledge support from the NSF
(CHEꢀ1360046) and BS would like to acknowledge support from
the Munger, Pollock, Reynolds, Robinson, Smith & Yoedicke
Stanford Graduate Fellowship. The small Molecule Xꢀray Crystalꢀ
lography Facility is supported by the NIH Shared Instrumentation
Grant S10ꢀRR027172 and the Molecular Graphics and Computaꢀ
tion Facility wishes to acknowledge the NSF CHEꢀ0233882 and
CHEꢀ0840505 grants. This technology is licensed to Lumiphore,
Inc. in which KNR has a financial interest.
(25) Binnemans, K.; Görller-Walrand, C.; Lucas, J.; Duhamel,
N.; Adam, J.-L. In Proceedings of the 3rd International Confer-
ence on Rare Earth Development and Applications 1995, p 198.
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J. C. The Journal of Chemical Physics 1982, 77, 2261.
(27) Barbanel, Y. A.; Mikhailo. Nk Zh. Neorg. Khim. 1973, 18,
699.
(28) Dexter, D. L. The Journal of Chemical Physics 1953, 21,
836.
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5D <ꢀ7F
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Wavelength [nm]
We report the preparation, crystal structure, magnetic circular dichroism and analysis of the photo-
physical properties of luminescent hydroxypyridonate EuIII (and SmIII) complexes of the versatile 3,4,3-
Li(1,2-HOPO) ligand (L). The complexes undergo a transformation when made in situ. Upon mixing L
with EuIII a kinetic isomer, A, is formed that rapidly converts to a thermodynamic isomer, B, which has
a longer luminescence lifetime and a higher quantum yield.
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