New Class of Bright and Highly Stable Chiral Cyclen Europium Complexes for Circularly Polarized Luminescence Applications

Article abstract of DOI:10.1021/acs.inorgchem.6b01546

High g_lum values of +0.30 (ΔJ = 1, 591 nm, in DMSO) and -0.23 (ΔJ = 1, 589 nm, in H₂O) were recorded in our series of newly designed macrocyclic europium(III) complexes. A sterically locking approach involving a bidentate chromophore

Full text of DOI:10.1021/acs.inorgchem.6b01546

Article
pubs.acs.org/IC
New Class of Bright and Highly Stable Chiral Cyclen Europium
Complexes for Circularly Polarized Luminescence Applications
Lixiong Dai,‡,§ Wai-Sum Lo,‡ Ian D. Coates,∥ Robert Pal,∥ and Ga-Lai Law*,‡
‡Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong SAR,
China
§Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China
∥Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
S
* Supporting Information
ABSTRACT: High glum values of +0.30 (ΔJ = 1, 591 nm, in
DMSO) and −0.23 (ΔJ = 1, 589 nm, in H2O) were recorded
in our series of newly designed macrocyclic europium(III)
complexes. A sterically locking approach involving a bidentate
chromophore is adopted to control the formation of one
stereoisomer, giving rise to extreme rigidity, high stability, and
high emission intensity. The combination of a chiral
substituent on a macrocyclic chelate for lanthanide ions
opens up new perspectives for the further development of
circulary polarized luminescent chiral tags in optical and
bioapplications.
rotation of the acetate arm (Λ or Δ), leading to four
dynamically interconvertible geometries in solution.14 The Δ
and Λ helicities interconvert rapidly in solution, while that
between δ and λ occurs at a rate of around 50 Hz for
unsubstituted DOTA26 complexes;1a,14b therefore it is vital to
control the formation of one chiral isomer in order to maximize
the CPL performance.
INTRODUCTION
■
Circularly polarized luminescence (CPL) spectroscopy has
several advantages over other chiroptical techniques in the
study of chiral molecules and chiral systems.1 This is the
emission analogy of circular dichroism (CD),2 which measures
the difference in absorption between the left- and right-handed
circularly polarized light, giving chiroptical information on the
electronic ground state of molecules.3 CPL spectroscopy
measures the differential emission of the left- and right-handed
circularly polarized light and provides information on the
excited-state properties of the compounds.4 The degree of CPL
is commonly reported as a luminescence dissymmetry factor
(glum) value, which is defined as glum = 2(IL − IR)/(IL + IR),
where IL and IR are the emission intensity of left and right
circularly polarized light.5−7 Most chiral organic fluorophores
have low |glum| values (∼10−5−10−2), despite brilliant
fluorescence quantum yields.8 Lanthanide(III) cations are
considered to be good candidates due to their spherical nature,
which avoids the problem of anisotropy, and their magnetic
dipole allowed f−f transitions could give rise to larger glum
values.9 Chiral lanthanide complexes with |glum| values within
the range of 0.05 to 0.5 are reported,9,10 but most of these
complexes have low luminescence and are formed from
multicomponent ligands, making them less stable for CPL
application in biological environments.
Back in 2002, Desreux’s group reported a synthetic strategy
to control the preference of stereoisomer formation of a chiral
DOTA complex15 by introducing four chiral substituents onto
the cyclen ring and four more on the pendant acetate arms,
forming one sterically locked isomer. Sherry subsequently
found that while chiral substituents on the acetate arms were
necessary, only one chiral benzyl group was needed on the
cyclen ring to induce single isomer formation.16 Parker’s group
designed a DOTA-based ligand with no stereogenic centers on
the cyclen ring but four chiral tetraamide ligands that formed a
series of lanthanide(III) complexes in predominantly one
isomer in solution (Figure 1).17 We sought to further develop
synthetic strategies to gain steric control of the ligands, and this
work presents our new sterically locking approach by
incorporating four substituents on the tetraaza ring and a
bidentate chromophore coordinating through a phosphate arm
and an N atom of the pyridine skeleton. Complexes synthesized
by this approach are at the same time extremely rigid.15,16,18
The rigidity of the complex directly determines the average of
local helicity of metal ion coordination and the degree of
mixing of the electric and magnetic dipole transition moments,
Tetraazacycloalkanes and especially cyclen (1,4,7,10-tetraa-
zacyclododecane) derivatives are known to form very stable
lanthanide complexes that are well-suited to a wide range of
biological applications,11,12 even for CPL studies.1a,13 However,
lanthanide complexes of cyclen derivatives may exist as two
pairs of stereoisomers due to ring inversion (λ or δ) and
Received: July 4, 2016
© XXXX American Chemical Society
A
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The luminescence quantum yields (Φ) should be high
enough for practical applications.21 We have particularly
devised the integration of an efficient chromophore (ε ≈
19 000 M−1 cm−1)22 for achieving conformational rigidity (vide
supra) as well as imparting respectable luminescence properties
to our europium(III) center, achieving good glum and Φ values.
Moreover, their stability and water solubility of the complexes
are pivotal for biological applications. Lanthanide(III) adducts,
despite their large glum values, are generally water-sensitive and
stable only in nonpolar solutions.23,24 Parker’s group reported a
racemic europium(III) complex with induced CPL properties
upon binding with α1-AGP protein, reaching a glum of −0.23
(ΔJ = 1, 598 nm),25 the largest obtained thus far for a
macrocyclic (including NOTA and DOTA derivatives)26
lanthanide complex in water.
In this work, we present a series of newly designed, sterically
locked chiral cyclen europium(III) complexes that exhibit good
luminescent quantum yields, large dissymmetry factors, water
solubility (except 4 and 5), and high stability. By modifying the
chiral substituents, we were able to observe a trend in the CPL
properties, laying down a definitive blueprint for designing and
synthesizing practical lanthanide-based CPL probes for
recognition of chiral biomolecules (Scheme 1).
Figure 1. Two strategies to rigidify lanthanide(III) cyclen chelate for
CPL study.17
which was proved to increase the magnitude of DOTA-based
lanthanide complexes’ glum values.1a,6
Europium(III) is often used for studying CPL due to its
generally decent luminescence quantum yields,19 allowing the
5
7
magnetic dipole allowed D0 → F1 transition to be observed
relatively easily.20 Muller et al. reported an exceptional glum of
+1.38 (ΔJ = 1, 595 nm, in CHCl3) for a heterobimetallic
tetrakis(diketonate) europium(III) adduct.9b Nevertheless, in
order to improve from being good to excellent candidates,
more factors have to be considered.21
Scheme 1. Structures and Synthetic Routes of Designed and Synthesised Europium Complexes for CPL Study
B
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Table 1. Spectroscopic Properties of the Complexes Measured at Room Temperature
a
b
1
2
3
4
5
6a
6b
355
7
λabs(max)
353
2.52
7.6
356
356
364
3.01
11.1
354
352
356
c
R
2.43
14.0
14.0
50.0
1.12
1.76
2.44
14.9
14.3
53.0
1.20
1.77
2.69
2.74
2.41
2.66
d
f
ΦH2O/MeOH (%)
12.7
10.2
21.9
de
,
ΦHEPES (%)
6.9
12.4
9.8
20.9
d
ΦDMSO (%)
τH2O/MeOH (ms)
47.0
1.01
1.44
45.3
1.39
1.74
0.05
36.9
30.1
40.4
46.4
44.1
49.7
1.12
1.04
1.25
f
τD2O/MeOD (ms)
1.70
1.77
1.83
g
q (Parker’s, Horrocks’)
0.06, 0.01
23.6
0.10, 0.02
26.0
0.02, 0.05
28.1
0.07, 0.01
26.6
0.11, 0.03
24.9
0.00, 0.06
30.2
ΦEu (%)
Eu
h
ηsens (%)
32.3
53.8
53.0
47.0
41.0
72.6
a
b
c
d
7
7
Measured in MeOH, insoluble in H2O. Measured in DMSO, insoluble in H2O and MeOH. I(5D0 → F2)/I(5D0 → F1). Relative to quinine
e
sulfate in 0.1 M H2SO4 (λexc = 350 nm, Φ = 0.577). Estimated errors of quantum yield or lifetime are 15% and 10%, respectively. 0.1 M HEPES
f
g
h
buffer, pH 7.4. Measuring the 5D0 → 7F2 transition. Calculated from ref 34. Errors in quantum yield or lifetime are 15%. Sensitization efficiency;
calculated according to ref 35.
bidentate chromophore are sufficient for stereocontrol, whereas
compounds 6a and 6b showed that an achiral cyclen could be
compensated with chiral acetate arms.
RESULTS AND DISCUSSION
■
In compounds 2−5, chiral substituents were introduced onto
the tetraaza macrocyclic backbone, whereas in 6a and 6b, the
stereogenic centers are on the pendant acetate arms. HPLC
analyses show that pure chiral complexes were obtained. We
believe that the compounds did not racemize because the
stereogenic centers were not involved in reactions and the
In addition to the nine coordination sites, the rigid structures
are beneficial in ensuring good protection of the lanthanide(III)
emitting center from vibrational energy loss and coordination
of solvent molecules, as evidenced by the luminescence
quantum yields brought about by the antenna effect and q
values (≈0), obtained by the luminescence lifetimes of the
complexes in H2O and D2O (MeOH and MeOD for 4).32−34
The photophysical properties are summarized in Table 1. The
increased bulkiness of the chiral substituent in 4 and 5
contributed to their poor water solubility; hence their
measurements were done in methanol and/or DMSO.
1
reaction conditions of the synthetic routes were not harsh. H
NMR spectra of the complexes were also obtained to prove the
purity of our complexes. We believe the appearance of one set
of protons beyond 10 ppm corresponding to the ring axial
protons27 on the cyclic backbone indicates that only one
isomeric coordination geometry is present (Figures S32, S33).
The stability of the complexes toward racemization over time
1
1
As all the complexes have the same chromophore, it is not
surprising to see their absorption maxima to be at a similar
position, except for 4, which showed a marked red-shift
compared to the other complexes. The shift in absorption is
caused by electronic communication between the electro-
positive lanthanide center and the chromophore. The
asymmetry ratio (R)defined as the ratio between the
was also monitored by H NMR (Figure S34). The H NMR
spectrum of 3 was consistent after 9 months of storage, but
upon heating at 90 °C, a new set of peaks appeared, believed to
be from racemization. Nevertheless, only the original set of
peaks remained as the pD was adjusted from 6.0 to 7.5 and
further heated for 12 h at 90 °C. It is also interesting to note
that while the purity of the ligands may not be highly
satisfactory, pure complexes could be obtained by HPLC after
complexation. This phenomenon is similar to Sherry’s16 and
Botta’s28 works, in which the purifications of DOTA-based
ligands were relatively problematic or impractical. In a separate
work of ours, we managed to distinguish two isomers of chiral
DOTA lanthanide complexes very easily by HPLC, with its
5
7
integrated intensities of the electric dipole D0 → F2 and
5
7
magnetic dipole D0 → F1 transitions of europium(III)is a
parameter that measures the deviation from centrosymmetric
geometry.36 A marked difference in the R of 4 is consistent with
the considerable shift in absorption maximum, suggesting a
different coordination environment of the europium(III) in 4
among the series, which is expected due to the steric bulkiness
of the isobutyl groups. These are not observed for 5, however,
as we believe the steric effect of the planar phenyl ring of the
benzyl group could not be alleviated by appropriate rotation,
whereas that of the isobutyl group could.
29
1
purity vindicated by H NMR; therefore we have confidence
that compounds 2−7 all exist as a single isomer.
We designed and synthesized these two kinds of chiral
cyclens, as we learned from the literature that introducing
chirality in these two areas could lead to steric locking.15,16
However, we also discovered that for DOTA-based complexes
it is not enough to just have one chiral structural feature.30
Summarizing the aforementioned literature, we believe that for
DOTA-based ligands with acetate arms chirality on the cyclic
backbone is required and the steric locking property is
complemented by having chiral coordinating acetate arms. In
our design, we took a leaf from Parker’s book on synthesizing
chiral NOTA-based ligands; they reported “complete stereo-
control” by a mono-C-substitution on the triazacyclononane
backbone with three bidentate chromophores and no chiral
acetate coordinating groups.31 We decided to merge the two
notions and create our own chiral DOTA-based ligands.
Compounds 2−5 proved that a chiral cyclic backbone and a
The luminescence quantum yields of the complexes are
decent in water and HEPES solution, although they are much
higher in DMSO, due to the nonradiative quenching by O−H
oscillators of water molecules in the second coordination
sphere (Table 1). The intrinsic quantum yields35a calculated
value evaluating the quantum yield of Eu(III) via direct metal
excitationof the complexes are calculated, and, as expected,
the values are very similar, except for 4. The higher value
corroborates with the shift in absorption maximum, for which
we believe the reason is due to a shorter europium−
chromophore distance and better interaction. On the other
hand, the sensitization efficiencies of the complexes vary. The
sensitization efficiency is a parameter determining the efficiency
C
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Figure 2. Emission spectra of 2 (left) and CPL spectra of 2 (right, black) and 3 (right, red).
of energy transfer from the photoexcited chromophore to the
emitting lanthanide center, obtained by comparing the intrinsic
quantum yield and overall quantum yield.35 As 2 and 3 are
enantiomers, their structural arrangement should be identical,
and hence their ηsens values are reasonably very similar; the
same applies to 6a and 6b, a pair of diastereomers. The
relatively remarkable ηsens of 7 is attributed to its structural
rigidity, as both the tetraaza backbone and the acetate arms are
modified with a chrial substituent, i.e., double chirality. The
double chirality is also attributed for the increase in
luminescence quantum yield compared to that of 6a/6b.
The opposite CPL spectra with nearly identical absolute
intensities obtained from enantiomers 2 and 3 in both H2O and
DMSO proved that the CPL properties were induced by the
chirality of the substituents on the macrocyclic ring (Figure 2).
However, there is no necessary correlation between the
bulkiness of the chiral group on the complex’s dissymmetry
factors, as comparison between 2, 4, and 5 gave a V-shaped
trend in their glum in DMSO (Figure 3), implying that the
Table 2. Summary of Luminescence Dissymmetry Values
(glum) at Specified Wavelengths
ΔJ = 1 (λ)
ΔJ = 1 (λ)
ΔJ = 3 (λ)
ΔJ = 3 (λ)
complex
H2O
DMSO
H2O
DMSO
2
−0.11 (591)
+0.12 (591)
−0.18 (591)
+0.17 (591)
−0.11 (591)
+0.30 (591)
−0.04 (591)
−0.11 (591)
−0.17 (588)
−0.28 (653)
+0.21 (653)
+0.17 (657)
+0.13 (657)
−0.12 (657)
+0.25 (657)
−0.04 (657)
−0.10 (653)
−0.29 (657)
3
4
5
6a
6b
7
−0.03 (588)
−0.04 (591)
−0.23 (589)
−0.05 (653)
−0.11 (653)
−0.33 (652)
result of small noncovalent interactions around the lanthanide
ions from the aromatic moieties of the benzyl group thus
changing the sign of the CPL signal.23
6a and 6b have chiral substituents incorporated onto the
pendent acetate arms. It is clearly shown that the CPL signal of
6a is considerably weaker than that of 6b in DMSO (Figure 4)
Figure 3. Comparison of CPL and glum spectra of 2 (black), 4 (red),
and 5 (blue).
Figure 4. Comparison of CPL and gem spectra of 6a (black) and 6b
(red). Above: CPL spectra. Below: glum spectra (in DMSO, 295 K, λexc
= 352 nm, Abs ≈ 0.3).
nature of the chiral substituentsuch as its freedom of
rotationand the changes in coordination environment all
have influential effects on the CPL properties, rather than a
straightforward relationship with bulkiness. Nevertheless, the
+0.30 (ΔJ = 1, 591 nm) value obtained for 5 in DMSO (Table
2) is highest among reported values regarding europium-based
macrocyclic complexes. It is also worth mentioning that the
more intense CPL spectrum of 5 is opposite those of 2 and 4, a
and H2O (Figure S18); so are the glum values. These results are
not satisfactory for practical use, and while we report with
confidence that 6a and 6b are optically pure, we cannot rule out
any difference in their coordination environments, which is
likely the reason behind the data mismatch.
The CPL spectra of 7 with chiral substituents on both the
macrocycle ring and acetate arms were compared with 2 (chiral
D
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substituents on macrocycle ring only) and 6 (chiral substituents
on acetate arms only) (Figure 5). The structural features of 2
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Figure 5. Comparison of CPL spectra of 2 (black), 6b (red), and 7
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and 6b were combined to give 7, and the CPL properties
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In summary, we have presented a new class of stable and
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5
7
value of 5 at the D0 → F1 transition (+0.3) is the highest
among europium(III) complexes with high luminescence
589 nm
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complexes for studying biological chiral environments.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01546.
■
S
Experimental details, photophysical spectra, high-reso-
lution mass spectra, NMR spectra, and HPLC chromato-
grams (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ga-lai.law@polyu.edu.hk.
Notes
■
The authors declare no competing financial interest.
́
(22) Soulie, M.; Latzko, F.; Bourrier, E.; Placide, V.; Butler, S. J.; Pal,
ACKNOWLEDGMENTS
R.; Walton, J. W.; Baldeck, P. L.; Le Guennic, B.; Andraud, C.; Zwier,
J. M.; Lamarque, L.; Parker, D.; Maury, O. Chem. - Eur. J. 2014, 20,
8636.
■
The authors like to thank Prof. David Parker for the use of his
CPL instrument. We gratefully acknowledge the financial
support from The Hong Kong Polytechnic University
(4BCC8), the Hong Kong Polytechnic University Central
Research Grants (PolyU 5096/13P), and the Research Grants
Council, University Grants Committee Hong Kong (PolyU
253002/14P).
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E
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