Water-stable lanthanide-organic macrocycles from a 1,2,4-triazole-based chelate for enantiomeric excess detection and pesticide sensing

Article abstract of DOI:10.1039/d1dt00726b

Water-stable anionic Ln₂L₂-type (Ln = La^IIIand Eu^III) lanthanide-organic macrocycles have been constructed by deprotonation self-assembly of a bis-tridentate ligand consisting of two 2,6-bis-(1,2,4-triazole)-pyridine chelation arms bridged by a dibenzofuran chromophore, of which the luminescent Eu₂L₂macrocycle can be used for enantiomeric excess (ee) detection toward pybox-type chiral ligands and selective colorimetric sensing of omethoate (OMA) in water.

Full text of DOI:10.1039/d1dt00726b

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Water-stable lanthanide–organic macrocycles
from a 1,2,4-triazole-based chelate for
enantiomeric excess detection and pesticide
sensing†
Cite this: Dalton Trans., 2021, 50,
5759
Received 4th March 2021,
Accepted 26th March 2021
a
Kai Cheng,a,b Qi-Xia Bai,c Shao-Jun Hu,a,b Xiao-Qing Guo,a,b Li-Peng Zhou,
DOI: 10.1039/d1dt00726b
a,b
rsc.li/dalton
Ting-Zheng Xie c and Qing-Fu Sun
*
Water-stable anionic Ln2L2-type (Ln = LaIII and EuIII) lanthanide– “antenna effect”.6 The binding affinity of the chelating moi-
organic macrocycles have been constructed by deprotonation eties, the light absorption ability and the energy level of the
self-assembly of a bis-tridentate ligand consisting of two 2,6-bis- ligand, along with the coordination environment of the lantha-
(1,2,4-triazole)-pyridine chelation arms bridged by a dibenzofuran nide centres, determine the stability and the sensitized
chromophore, of which the luminescent Eu2L2 macrocycle can be luminescence performance of the final lanthanide complexes.
used for enantiomeric excess (ee) detection toward pybox-type Therefore, for the development of photo-functional lanthanide
chiral ligands and selective colorimetric sensing of omethoate assemblies, it is critical to design appropriate ligand skeletons,
(OMA) in water.
especially those with new chelating groups.
Previously, many lanthanide-directed coordination assem-
blies from ligands with pyridine-2,6-dicarboxamide (pcam) as
the chelation arms have been successively constructed.7
However, most of these complexes tend to dissociate in
aqueous solutions and suffer from low photoluminescence
quantum yields (Φ). From 2015, we have been focusing on the
design of bright luminescent supramolecular lanthanide com-
pounds. Apart from the pcam-based ligands, new ligands
based on (1,2,3)-triazole-pyridine-amido (trpa), (1,2,3)-triazole-
pyridine-(1,2,3)-triazole (trptr), and pyridine-2,6-bitetrazole
(pbtz) chelating moieties have been successfully developed,
and they showed much enhanced sensitization efficiency
toward various lanthanide elements, including those emitting
in the near-infrared region.8
Deprotonation self-assembly has been proved to be effective
in constructing water-stable supramolecular coordination com-
plexes.9 Very recently, we have reported a series of water-stable
anionic lanthanide organic polyhedral complexes by utilizing
the deprotonation self-assembly of pbtz-based ligands, which
show good stability under aqueous conditions and high
luminescence quantum yield (Φmax = 11% in water for the Tb
cube).8d These precedents motivate us to further design new
ligand skeletons based on the idea of deprotonation self-
assembly. While pyridine-2,6-bis-(1,2,4-triazole) (pbtr) chelates
have been known for a long time, previous studies mainly
focused on the complexes fabricated by transition metals,
such as Fe, Pd, Pt, etc.10 Lanthanide complexes with pbtr-type
ligands have been extremely rare reported. As far as we know,
the only one-type example is [LnL(NO3)n] mononuclear
complex, obtained by searching from the CCDC database.11
Coordination-directed self-assembly has become a well-estab-
lished technique for the construction of multi-functional
supramolecular structures.1 Various supramolecular com-
plexes based on transition metals have been designed and
reported.2 However, research on the controllable self-assembly
of lanthanide supramolecular complexes has clearly lagged
behind due to the difficulty in predicting the coordination
number and geometry of lanthanide elements.3 Because of the
shielded 4f electrons, lanthanides have unique photophysical
properties, including distinct well-defined emission bands,
long lifetimes, and large Stokes shifts, and thus are widely
used in phosphors, light-emitting diodes, biosensors/probes,
etc.4 Direct excitation of lanthanide ions is very difficult due to
the small molar extinction coefficient (<10−1 M−1 cm−1) that
stems from the forbidden f–f transitions.5 Nonetheless,
strongly luminescent lanthanide–organic complexes can be
obtained using suitable ligands containing suitable chromo-
phores to sensitize the lanthanide ions through the so-called
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China.
E-mail: qfsun@fjirsm.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
cInstitute of Environmental Research at Greater Bay Area; Key Laboratory for Water
Quality and Conservation of the Pearl River Delta, Ministry of Education; Guangzhou
Key Laboratory for Clean Energy and Materials; Guangzhou University, Guangzhou
510006, China
†Electronic supplementary information (ESI) available: Experimental details,
supporting figures and tables. CCDC 2065023. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/d1dt00726b
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Particularly, with regard to our previous finding that supramo-
lecular tetrahedra constructed with ligands based on trptr-type
chelation arms showed excellent luminescence properties
(Φmax = 82% in acetonitrile for the Tb tetrahedra),8c we antici-
pate that the replacement of (1,2,3)-triazole with 1,2,4-triazole
may lead to the design of new water-stable supramolecular
lanthanide complexes with bright luminescence via the depro-
tonation assembly process.
Herein, we designed a new bis-tridentate ligand (H4L) con-
sisting of two 2,6-bis-(1,2,4-triazole)-pyridine chelation arms
bridged by a dibenzofuran chromophore, and studied its
deprotonation self-assembly with LnIII (LnIII = LaIII and EuIII)
ions, by which water-stable and bright luminescent Eu2L2
macrocycles have been obtained (Scheme 1). With available
open metal sites on the LnIII centres, Eu2L2 can further carry
out post-synthetic modification with additional chiral auxiliary
ligands and becomes chiroptically-active, realizing a linear ee
detection through induced CD and CPL. Furthermore, the
selective turn-off sensing of OMA has also been demonstrated
in water, with a naked-eye recognizable color change.
As a model compound, lanthanide coordination behaviour of
the tridentate ligand 4-bromo-2,6-bis(3-(trifluoromethyl)-1H-1,2,4-
triazol-5-yl)pyridine (H2L′) had been first studied. While NMR
titration of Eu(OTf)3 (0–0.5 eq.) with H2L′ in the presence of tri-
ethylamine (2.0 eq.) in CD3CN indicated the formation of the
1 : 3 complexes (Fig. S13 and S14†), crystallization under different
M/L ratios all led to the formation of a EuL′2 complex, suggesting
that EuL′2 is a favourable thermodynamic product with this
chelate. The C2-symmetric ligand (H4L) was then synthesized
through the Sonogashira coupling reaction of H2L′ and 2,8-di-
alkynyl-dibenzofuran (see the Experimental section in the ESI† for
details). Treatment of the ligand (H4L) and trimethylamine
(4.0 eq.) with an equimolar amount of La(OTf)3 in dimethyl
sulfoxide led to the quantitative formation of a highly symmetric
product, which was confirmed by 1H, 19F, and DOSY NMR
spectra (Fig. 1B and C S16†). Compared with the free ligand,
Fig. 1 1H NMR spectra (400 MHz, DMSO-d6, 298 K) of (A) H4L with
Et3N and (B) La2L2; 1H DOSY spectrum of (C) La2L2; ESI-MS spectra of
(D) La2L2.
Ha on the pyridine shifted to the lower field, while Hb–d on the
dibenzofuran showed only very small shifts in the 1H NMR
after coordination to the metal. Similarly, the 19F signals also
showed small movements, as the CF3 groups are far from the
metal centres (Fig. S17†). DOSY also indicated the formation
of a single species in solution with D = 6.08 × 10−11 m2 s−1
.
ESI-MS unambiguously confirmed a formula of La2L2 for the
assembly with prominent peaks observed at m/z = 1044.9373
and 1119.9172, corresponding to [La2L2]2− and [La2L2(H2O)4
(DMSO)]2− (Fig. 1D). Similarly, Eu2L2 was synthesized by
simply replacing La(OTf)3 with Eu(OTf)3, and its structure was
determined by ESI-TOF-MS (Fig. S75 and Table S1†).
The single-crystal X-ray structure of the europium mono-
nuclear complex EuL′2 is shown in Fig. 2A. The crystallo-
graphic data showed that the EuIII ions are nine coordinated
by two bis(1,2,4-triazole) chelating groups and three additional
water molecules (Fig. 2B). The mean distance from the metal
centre to the nitrogen atom on pyridine is 2.69 Å, and the
Scheme 1 Self-assembly of water-stable macrocycles for ee detection and OMA sensing.
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macrocycle between 300 and 350 nm decreased and the
absorption between 250 and 275 nm increased compared to
the deprotonated ligand (Fig. S22†). Upon excitation at
333 nm, the characteristic red emission peaks of EuIII were
observed at 591, 616, 688, and 692 nm, corresponding to the
5D0
→
7FJ (J = 1–4) transitions respectively (Fig. S25†).
Surprisingly, the quantum yield of Eu2L2 is as high as 69.31%
at room temperature (Fig. S29†). The luminescence lifetime of
the 5D0 excited state was 1.9 ms (Fig. S32†). However, the
quantum yield and lifetime in CH3CN decreased to 55.66%
and 1.6 ms, respectively (Fig. S43 and S46†). Impressively,
Eu2L2 is found to be stable in water, as the characteristic red
luminescence still remained after diluting its DMSO solution
in H2O, even after 45 days. The luminescence spectrum
showed that the emission quantum yield was as high as 4.21%
in H2O (Fig. S31†). The number of solvent molecules co-
ordinated to the EuIII ions in the first coordination sphere (q)
for Eu2L2 was then determined to be 2.9 by measuring the
corresponding luminescence lifetimes in both H2O and D2O
(see ESI† for details), suggesting that the structure of Eu2L2
did not change. ESI-TOF-MS also proved that the structure of
Eu2L2 is maintained in H2O (Fig. S76†).
Fig. 2 X-ray crystal structure (A) of EuL’2 (color code for Eu: pink
sphere, C: gray, N: light blue, Br: dark red, F: cyan, H: white, and O:
orange) and (B) the local coordination environment of the Eu centre;
energy-minimized structures of a mesocate (C) and a ΛΛ-helicate (D)
for the Eu2L2 complex.
mean distance to the nitrogen atom on 1,2,4-triazole is 2.55 Å,
which are slightly shorter than those of the trptr system
(2.60 Å and 2.56 Å) and longer than those of the pbtz system
(2.56 Å and 2.43 Å).8c,d In addition, the mean distance between
the metal centre and the coordinating H2O is 2.47 Å. The di-
hedral angle of two bis (1,2,4-triazole) chelating groups is
80.16°. Potentially due to the fact that both mesocate (Λ,Δ)
and racemic mixtures of helicates (Λ,Λ and Δ,Δ) are the poss-
ible outcomes for the M2L2 complex, attempts to crystallize the
Ln2L2 complex were unsuccessful, and so their structures were
simulated by molecular mechanic modelling. In the case of a
mesocate structure (Fig. 2C), the pyridine ring and the 1,2,4-
triazole ring are distorted from the planar geometry due to
steric hindrance, which has also been found in the previously
reported crystal structures.12 The two ligands are in an antipar-
allel geometry and the whole structure is C2V symmetric. On
the other hand, in the optimized helicate structure (Fig. 2D),
the pyridine rings and the two 1,2,4-triazole rings are planar.
The two ligand chains adopted a crossover chelation geometry
toward the two lanthanide centres, where the chirality on the
two metal centresis consistent (Λ,Λ or Δ,Δ), giving rise to a
whole D2 symmetry of the helicate.
Considering that the Eu2L2 has unsaturated metal centres,
addition of chiral ancillary ligands may induce CD and CPL
responses via the post-synthetic modification mechanism.13
Indeed, titration of 2,6-bis(4-phenyl-2-oxazolinyl)pyridine (pybox,
AR and AS) with varying ee compositions into Eu2L2 led to gradu-
ally changing ICD signals in CH3CN. Perfect mirror images of
CDs in the 250–400 nm range induced by adding the opposite ee
values of pybox were observed and a linear relationship was con-
firmed between the CD responses at 310 and 345 nm and the ee
values (Fig. 3A and C). Similarly, while Eu2L2 is CPL silent,
perfect linear responses of ICPL signals against the ee values
were observed at 597, 611.5, and 695.5 nm (Fig. 3B and D) in the
presence of the pybox ligands with varying enantiomeric compo-
sitions. Chelation of the pybox onto Eu2L2 has been confirmed
by ESI-TOF-MS, with prominent peaks observed at m/z =
1428.1371 and 1535.1572, corresponding to [Eu2L2AS ]2− and
2
[Eu2L2AS (CH3CN)2(H2O)3(DMSO)]2− (Fig. S77†). Moreover, the
2
emission intensity of Eu2L2AS was obviously enhanced com-
2
pared to Eu2L2, whereas the photoluminescence quantum yields
increased from 55.66% to 74.57%, and the lifetimes also
increased from 1.6 to 3.2 ms (Fig. S45 and S47†). The emission
enhancement can be attributed to the replacement of the coordi-
nation solvent molecules with the auxiliary ligands, which effec-
tively inhibits the non-radiative inactivity of the europium ions.
The structures of Eu2L2AR and Eu2L2AS were simulated by
The photophysical parameters of L and Eu2L2 are summar-
ized in Table 1. In DMSO, the UV–vis absorption of the Eu2L2
2
2
molecular modelling, where the strong π–π stacking interactions
between the side phenyl rings of pybox and the pyridyl ring of
ligand L are indicated (Fig. 3E and F).
Table 1 Photophysical data for the ligand and macrocycles
max
Complex
λex [nm]
λ
[nm]
τobs
Φoverall
em
Omethoate (OMA) is a priority monitoring hazardous sub-
strate in pesticides due to its high toxicity to organisms and
humans.14 An effective method to monitor and detect low doses
of OMA in water is important as it is being used worldwide as
acaricide and insecticide in the agricultural field.15 Considering
that Eu2L2 is stable in water, the sensing toward common pesti-
a
Eu2L2
333
324
335
336
335
616
438
612
615
615
1.9 ms
15.2 ns
1.6 ms
320.0 µs
3.2 ms
69.31%
6.05%
55.66%
4.21%
Lb
c
Eu2L2 b
Eu2L2
Eu2L2AS
75.05%
c
2
a In DMSO. b In H2O. c In CH3CN.
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(Fig. S49–63†). Surprisingly, with the addition of 10.0 eq. of
different analytes to the aqueous solution of Eu2L2, only OMA
led to significant luminescence quenching (Fig. 4B), while no
such significant responses have been observed for all the other
pesticides. Under UV irradiation at 365 nm, the luminescence
of the solution changes from pink to yellow-green after the
addition of OMA, which is easily recognizable by the naked eye
(Fig. 4C). Titration of OMA (1–10 eq.) into Eu2L2 confirmed a
good linear relationship between the I0/I ratio at 615 nm and
the equivalents of OMA, with a detection limit as low as 9 nM
being estimated (S/N = 3) (Fig. 4D and S65†). Based on the
emergence of a broad-band emission at 500 nm and the obser-
vation of m/z signals assigned to H2L2−, [Eu@(OMA)2]3+ and
[Eu@(OMA)3]2+ in the ESI-TOF-MS spectra (Fig. S78 and S79†)
after the titration, a disassembly mechanism was accounted for
this colorimetric sensing.
Conclusions
In summary, water-stable anionic lanthanide-organic macro-
cycles have been constructed by coordination-driven self-
assembly of a new bis-tridentate 1,2,4-triazole-based ligand
with LnIII metals. Eu2L2 exhibits unparalleled photo-
luminescence quantum yields in DMSO/water and can be used
for ee detection and selective OMA sensing. Our results offer
new candidates for the ligand design toward water-stable
multi-functional lanthanide–organic architectures.
Fig. 3 (A) CD spectra of Eu2L2 (5 × 10–6 M, CH3CN) containing 2 eq. of
AR/S with various ee values, and (C) the corresponding ee calibration
plots at 310 nm and 345 nm. (B) CPL spectra of Eu2L2 (2 × 10–5 M,
CH3CN) containing 2 eq. of AR/S with various ee values, and (D) the
corresponding ee calibration plots at 597, 611.5, and 695.5 nm. Energy-
minimized structures of Eu2L2AR2 (E) and Eu2L2AS2 (F).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the NSFC (Grant No. 21825107)
and the Strategic Priority Research Program of the Chinese
Academy of Sciences (Grant No. XDB20000000).
Notes and references
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