A single sensitizer for the excitation of visible and NIR lanthanide emitters in water with high quantum yields

Article abstract of DOI:10.1002/anie.201106748

Eight makes a happy Ho(l)me: The versatile octadentate TIAM ligand forms lanthanide complexes (Ln=Sm, Eu, Tb, Dy, Ho) with high quantum yields in water. This ligand is an efficient sensitizer, and also shields the metal center from solvent quenching, as shown by an X-ray diffraction study of the Ho complex. Copyright

Full text of DOI:10.1002/anie.201106748

Angewandte
Chemie
DOI: 10.1002/anie.201106748
Lanthanide Complexes
A Single Sensitizer for the Excitation of Visible and NIR Lanthanide
Emitters in Water with High Quantum Yields**
Ga-Lai Law, Tiffany A. Pham, Jide Xu, and Kenneth N. Raymond*
In recent years the use of lanthanide luminescence for
biological applications has been of increasing importance.[1]
The development of such luminescent compounds has been
due to the burgeoning demand for multifunctional and
efficient luminescent markers to probe signal transduction,
neurobiology, cancer, stem cell biology, and infectious dis-
eases.[2] This has initiated a focus on multiplexing assays, for
which the multifunctional design is still problematic.
Approaches using nanoparticles and quantum dots,[3] have
been somewhat successful, but the limitations of these
materials are still significant.[4]
Herein we present a versatile, multidentate ligand that has
been found to sensitize both visible and near-infrared (NIR)
Figure 1. a) Chemical structure of the H(2,2) scaffold (above) and the
emitters by using the same excitation wavelength, with
significantly high quantum yields. This has been a persistent
challenge, as different lanthanides have different emitting
states that are easily quenched by nonradiative decay
processes.[5] We have previously described the properties of
several emissive terbium complexes with high quantum yields
that feature the 2-hydroxyisophthalamide binding unit
(IAM).[6,7] Past studies have shown that similar tetradentate
and octadentate ligands[7] form TbIII complexes with compa-
rable photophysical properties. We ascribe this to the fact that
modifying the functional groups of the sensitizer does not play
a major role in changing the electron distribution of the ligand
chromophore and its chelating oxygen atoms, based on
investigation of different electron-withdrawing and electron-
donating groups on the para position of the aromatic ring on
the chromophore. Using TD-DFT calculations and screening
studies with tetradentate ligands,[8] the inclusion of an addi-
tional amide group on the para position of the ring was
thought to be favorable to the electronics of the system. It is
further hypothesized that large molecular appendages will
result in better shielding of the metal center. The TIAM
binding moiety was introduced in the ligand design to
investigate both of these hypotheses.
TIAM binding moiety (below) in H4L. b) View of the X-ray crystal
structure of [HoL]ꢀ (see also Figure 4a).
been found to sensitize a range of visible emitters, namely Sm,
Eu, Tb, Dy, and Ho, which emit in both the visible and NIR
regions. The sensitization of the metal occurs through the
ligand, which is simultaneously the chromophore and chela-
tor.[9] These emitters display uncommonly high luminescent
properties in water, given that they are extremely sensitive to
O-H vibronic quenching.
The [LnIIIL]ꢀ complexes (Ln = Sm, Eu, Tb, Dy, Ho) were
characterized by the crystal structure of the holmium complex
(Figure 1b) and by HT-ESI mass spectrometry. The synthesis
and characterization data is shown in the Supporting Infor-
mation and the Experimental Section.
In general, emission from holmium complexes is rare,
especially in the solution state, as their electronic structure is
susceptible to non-radiative deactivation. There have been
fewer than ten reports on holmium complexes with docu-
mented solution state luminescence and photophysics to
date.[11] We show herein the emission spectra of holmium in
water with emission in both the visible and NIR regions
(Figure 2). The observed peaks are assigned at about 640 nm
and 990 nm with a slight shoulder at 1010 nm (the most
prominent band), and the weaker transitions are observed at
1210 and 1450 nm, which correspond to the transition bands
The octadentate ligand (H4L) has four TIAM chromo-
phores attached to an H(2,2) backbone (Figure 1a), and has
[*] Dr. G.-L. Law, T. A. Pham, Dr. J. Xu, Prof. K. N. Raymond
Department of Chemistry, University of California Berkeley
Berkeley, CA 94720-1460 (USA)
5
5
5
5
of 5F5! I8, 5F5! I7, 5I6! I8, and 5F5! I6, respectively. These
bands are observed owing to the relaxation of the photons
from the multiple upper 4f levels to the 5F5 and 5I6 first excited
states of the Ho3+ ion before decaying to the 5I6, 5I7, 5I8
(Figure 2). This is the first time that the weak transition
bands at 1210 and 1450 nm have been reported in aqueous
solution. In most cases, these bands are not observed due to
strong reabsorption of the weakly emitted NIR radiation by
the solvent, which has an absorption coefficient of nearly two
orders of magnitude higher than the holmium transitions.[12]
E-mail: raymond@socrates.berkeley.edu
[**] The lanthanide luminescence research is supported by the Director,
Office of Science, Office of Basic Energy Sciences, Division of
Chemical Sciences, Geosciences, and Biosciences of the U.S.
Department of Energy at LBNL under Contract No. DE-AC02-
05CH11231, and related support is from NIH Grant HL069832 for
other aspects of this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106748.
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Figure 2. HoL emission spectra in water and less than 5%DMSO at
lexc =330 nm. The strongest emission in the NIR is at 940/1010 nm.
5
Upper inset: 5F5! I8, very weak emission in the visible region at
5
5
5
5
640 nm; lower inset: the I6! I8 and F5! I8 transitions bands at 1210
Figure 3. Room-temperature emission spectra of a) TbL b) DyL,
c) EuL, and d) SmL at lexc =350 nm in water with less than 5%
DMSO.
and 1455 nm in the NIR region.
The only measurable lifetime was at 1010 nm and gave an
monoexponential curve (tobs = 11 ns; Table 1). In aqueous
solution the other transition bands were too weak to give
conclusive values; however, aqueous emission spectra, quan-
the 6HJ (J = 5/2, 7/2, 9/2, and 11/2, respectively). For DyL, the
characteristic bands associated with the emitting 4F9/2 state of
DyIII were observed at 481.0, 575.6, 664.1, and 752.5 nm,
5
6
where the hypersensitive F9/2! H13/2 is the most dominant.
For EuL and TbL, these were the two most emissive
complexes as expected as they have fewer excited states and
ground states in contrast to Sm and Dy. TbL was the most
luminescent, giving the highest quantum yield (F ꢁ 47%) For
EuL, it should be noted that from the spectra, the ratio of the
7F4 transition of the europium is unusually high relative to the
7F2 transition. This is uncommonly observed for europium but
has been previously reported,[13] and can be an attributing
factor for the low quantum yield obtained as it indicates
a different coordination geometry. Another possible cause of
the quenching could be by electron transfer processes.
The triplet state of the gadolinium complex was observed
from studies at 77 K at 23148 cmꢀ1, which is masked at room
temperature. This is shown by the predominant ligand
fluorescence of the complexes, centered around 432 nm in
water (Supporting Information, Figure S3), which also indi-
cates inefficient energy transfer. Microsecond lifetimes
(58 ms) confirm the phosphorescence observed which is
promoted by the heavy metal effect of the Gd center.[14]
The high emitting states of Gd
Table 1: Photophysical data of HoL in water.
HoL
Transitions
lem(max) [nm]
tH
2 O
5
Ho (f–f)
5F5! I8
ca. 645
990/1010
1210
1450
407
–
5
5F5! I7
11 ns
–
–
48 ns
58 ms
5
5I6! I8
5
5F5! I8
L (fluorescence)
L (phosphorescence)
S1!S0
T1!S0
432
tum yields, and lifetimes have been collected for the other
four visible emitters, namely SmL, EuL, TbL, and DyL
(Table 2).The emission spectra with their assigned f–f tran-
sitions are shown in Figure 3.
The SmIII complex produced four characteristic bands,
peaking at 562.5, 605.0, 650.0, and 710.0 nm, which are
responsible for the transitions from the emitting 4G5/2 state to
(6P7/2) prevent energy transfer to
the metal ion. The general energy
Table 2: Photophysical data of LnL complexes in aqueous media.
transfer mechanism of these com-
[f]
[f]
LnL
DE[a]
lem(max)
[nm]
tH
tD
q[b,d]
q[c,e]
FH
2O
FHEPES
[%]
2O
2 O
[cmꢀ1
]
[ms]
[ms]
[%]
plexes involves population of the
ligand singlet state followed by
intersystem crossing to the triplet
state and finally to the nearest
excited states of the metal.
These complexes all show rela-
tively high quantum yields in
water, with extinction coefficients
of about 23000 Lmolꢀ1 cmꢀ1. The
T1!Ln(f–f)
Tb
Eu
Dy
Sm
2659
5909
2469
2659
545
614
574
605
2050
840
25.0
21.0
2610
1320
–
0.2
0.4
47.2
11.6
48.0
10.8
0.07[c]
0.36
ꢀ0.1
0.1
ꢀ0.5[e]
0.3[e]
–
0.09[c]
0.35
226.0
ꢀ0.4[d]
[a] Tb (f–f=5D5), Eu (f–f=5D1), Dy (f–f=6H1), Sm (f–f=6H5/2). [b] Parker’s equation. [c] Horrocks’
equation. [d] Hakala. [e] Kimura. [f] Approximately F for visible region of the f–f emission shown in
Figure 3; integration extrapolated to baseline.
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quantum yield of terbium is around 47%, while europium was
poorly sensitized relative to higher values cited in the
literature; however, its quantum yield is still higher than
that of the commercially used europium cryptand assay of
about 2%.[15] Thus, all these complexes show potential for
multiplexing applications, as they can all be excited with the
same wavelength. The lifetimes are relatively long, and from
both Horrocksꢀ and Parkerꢀs corrected q equations,[16] the
calculated q value is approximately zero, which suggests that
there are no coordinating water molecules in the inner sphere.
This is also supported by the crystal structure of HoL.
From the photophysical measurements it can be con-
cluded that the triplet state of the ligand alone cannot account
for the remarkable luminescence properties, as all five
lanthanide emitters have different excited states. The energy
differences between the triplet level of the ligand and the
lowest emitting states of the five different lanthanides account
for the different sensitization efficiencies. For example, with
terbium complexes, an energy gap of 3500 cmꢀ1 or higher is
necessary to facilitate efficient and irreversible energy trans-
fer.[17] Here the energy gap is much smaller (ca. 2659 cmꢀ1),
whereas for EuL the gap of about 5909 cmꢀ1 is much greater
than the ideal of 1800 cmꢀ1, a prerequisite to prevent back
energy transfer. For the europium system, the high triplet of
5
the ligand can facilitate relaxation to the D2 transition (ca.
21500 cmꢀ1) instead of to the 5D1 state.
We believe that along with the well-matched triplet state,
the strong luminescent properties were attributed to the
coordination geometry and the protruding amide groups on
the exterior of the ligand that limit solvent access to the metal
center. These factors contribute to the reduction in non-
radiative decay processes, which is especially important for
lanthanides with numerous closely emitting states and hyper-
sensitive transitions. The crystal structure of the holmium
complex supports this hypothesis, as the metal center is well
encapsulated by the ligand, while the para amide groups
extend from the complex acting like a secondary shield from
solvent molecules.
Rhomboid single crystals suitable for X-ray diffraction of
HoL as the pyridinium salt were grown by layering diethyl
ether onto a 5% aqueous DMF/MeOH 1:1 (v/v) solution of
the complex. The HoL complex spontaneously resolves to
crystallize in the chiral orthorhombic space group C2221
(Figure 1b).[10] The crystallographic data (Table S1) and
selected bond distances and angles (Table S2) are in the
Supporting Information. Interestingly, the holmium atoms are
not equivalent and do not lie on the eight general positions of
this space group, but rather on two different special positions
with C2 symmetry axes. The unit cell is comprised of two
[HoL]ꢀ complexes (with metal centers denoted as Ho1 and
Ho2) with a coordination number of eight (Figure 4a) and
significantly different coordination environments (Fig-
ure 4b,c).
Figure 4. a) ORTEP diagram of the X-ray crystal structure of the Ho1
anionic complex. Ellipsoids are set at 50% probability; hydrogen
atoms are omitted for clarity. Ho orange, C black, O red, N blue.
b),c) Inner coordination environments of the Ho1 (b) and Ho2 (c)
cations. The bold edges of the corresponding idealized polyhedra are
those spanned by the binding moieties.
analysis of the coordination environment around Ho1 reveals
that it is best described as a dodecahedron (SM = 7.04 (D2d),
11.96 (C2v), and 15.64 (D4d)), while Ho2 is closest to a square
antiprism (SM = 7.47 (D4d), 8.34 (D2d), and 10.19 (C2v);
Figure 4b,c), similar to several other eight-coordinate hol-
mium complexes.[11c,19] Both structures are intermediate
between the ideal D2d and D4d geometries, with similarly
seen D2 symmetry in each case. For Ho1, the bidentate ligands
are twisted off the D2d mirror planes, while for Ho2 the top
and bottom faces of the antiprism are rectangular rather than
square.
The TIAM ligands alternate in binding through the amide
carbonyl connected to the backbone and the pendant one. As
ꢀ
expected, the average length of the (hydroxy) O Ho bond is
ꢀ
significantly smaller than that of the (keto) O Ho bond
(2.30 ꢁ vs. 2.38 ꢁ). The aromatic rings pendant from the same
side of the bridge engage in notable p stacking, while
hydrogen bonding is observed between the amide protons
and the coordinating hydroxy oxygen atoms. These interac-
tions most likely contribute to the stability of the complex.
In conclusion, we have reported a new holmium lumines-
cent complex and its emission spectra in both the visible and
The three most common polyhedra describing the coor-
dination geometry for an eight-coordinate complex are the
bicapped trigonal prism (C2v), square antiprism (D4d), and
trigonal dodecahedron (D2d). The shape-measure (SM)
parameter is a reference to the agreement between these
idealized polyhedra and the observed structure.[18] Shape
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external groups on the ligand can form a secondary shield to
reduce external quenching effects and thus is a significant
factor to consider in ligand design. An unmet need in the field
of biological imaging applications is for multiplex assays using
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Experimental Section
[10] Crystal data: C54H64N14O16Ho (Mr = 1330.12), light-pink rhom-
boid, 1.04 ꢃ 0.52 ꢃ 0.27 mm3, orthorhombic, space group C2221
(No. 20), a = 17.455(5), b = 47.542(13), c = 27.414(8) ꢁ, V=
22750(11) ꢁ3, Z = 8, 1calcd = 0.777 Mgmꢀ3, m = 0.737 mmꢀ1, Mo
X-ray radiation source l = 0.71073 ꢁ, T= 163(2) K, 293944
measured reflections, 20707 independent reflections, R1 (I >
2s(I)) = 0.0367, wR2 (all data) = 0.0930, GOF = 1.079.
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All chemicals and solvents were used without further purification
unless otherwise stated. The LnIII salts utilized were of the highest
available purity (> 99.99% or (> 99.999%). The synthesis of H4L is
shown in the Supporting Information. Mass spectra were obtained by
the QB3/Chemistry Mass Spectrometry Facility at the University of
California, Berkeley, CA.
X-ray diffraction data collection was performed at the X-Ray
Facility in the College of Chemistry at the University of California,
Berkeley, using procedures detailed in the Supporting Information.
Resulting drawings of molecules were produced with ORTEP-3 for
Windows.[20] CCDC 797480 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
Received: September 22, 2011
Revised: November 2, 2011
Published online: January 24, 2012
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Keywords: holmium · lanthanide sensitization ·
octadentate ligands · Vis/NIR emitters · X-ray diffraction
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