Lanthanide clusters of phenanthroline containing a pyridine-pyrazole based ligand: magnetism and cell imaging

Article abstract of DOI:10.1039/d0dt04122j

In this contribution, we report the synthesis, characterization and luminescence-magnetic properties of Ln-clusters (Ln = Gd³⁺, Eu³⁺and Tb³⁺) using a new pyridine-pyrazole functionalized ligand fitted with a chromophoric phenanthroline backbone. The unorthodox N-rich ligand forms isostructural trinuclear lanthanide complexes with a topology that closely resembles two interdigitating hairpins. The clusters crystallize in chiral space groups and also exhibit chirality for bulk samples, which were further confirmed using solid state CD spectra. Magnetic studies on the complexes reveal their interesting features while the Gd cluster shows a significant cryogenic magnetic cooling behaviour with a moderately high magnetic entropy change of ?23.42 J kg^?1K^?1at 7 T and 2 K. On the other hand, Eu and Tb complexes exhibit interesting fluorescence properties. The compounds were subsequently used as fluorescent probes for the imaging of human breast adenocarcinoma (MCF7) cells. Live cell confocal microscopy images show that the complexes penetrate beyond the usual cytoplasm region and can be useful in imaging the nucleus region of MCF7 cells.

Full text of DOI:10.1039/d0dt04122j

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PAPER
Lanthanide clusters of phenanthroline containing
a pyridine–pyrazole based ligand: magnetism and
cell imaging†
Cite this: Dalton Trans., 2021, 50,
3593
a
Avik De, a Sukhen Bala,b Sayan Saha,a Krishna Sundar Das,a Sohel Akhtar,
Amit Adhikary, a Arijit Ghosh,c Guo-Zhang Huang, b Srijita Paul Chowdhuri,c
Benu Brata Das,c Ming-Liang Tong b and Raju Mondal
*
a
In this contribution, we report the synthesis, characterization and luminescence–magnetic properties of
Ln-clusters (Ln = Gd3+, Eu3+ and Tb3+) using a new pyridine–pyrazole functionalized ligand fitted with a
chromophoric phenanthroline backbone. The unorthodox N-rich ligand forms isostructural trinuclear
lanthanide complexes with a topology that closely resembles two interdigitating hairpins. The clusters
crystallize in chiral space groups and also exhibit chirality for bulk samples, which were further confirmed
using solid state CD spectra. Magnetic studies on the complexes reveal their interesting features while the
Gd cluster shows a significant cryogenic magnetic cooling behaviour with a moderately high magnetic
entropy change of −23.42 J kg−1 K−1 at 7 T and 2 K. On the other hand, Eu and Tb complexes exhibit
interesting fluorescence properties. The compounds were subsequently used as fluorescent probes for
the imaging of human breast adenocarcinoma (MCF7) cells. Live cell confocal microscopy images show
that the complexes penetrate beyond the usual cytoplasm region and can be useful in imaging the
nucleus region of MCF7 cells.
Received 3rd December 2020,
Accepted 4th February 2021
DOI: 10.1039/d0dt04122j
rsc.li/dalton
sensing, bioimaging and scanning.22–36 Interestingly, while at
a first glance the underlying materials properties may look
Introduction
There is practically no substitution for lanthanide (Ln) based widely different from each other but at the core there exists
functional materials in sight for generating sophisticated high- unity amid diversity. All these properties primarily arise from
end appliances.1–5 Lanthanide ions can simultaneously act as the encrypted chemical features of either a specific lanthanide
magnetic moment carriers, intrinsic Lewis acidic sites and ion/cluster or a particular functional group of the organic
photoluminescent centres.6–13 Owing to their unique mutually ligand through a metal–ligand coordination mode.
inclusive magnetic, catalytic and optical features, Ln based
There has been persistent research into the development of
materials have naturally attained an indispensable position in multifunctional materials through cumulating various pro-
our modern society.14–21 Their presence encompasses numer- perties of lanthanides or organic functionalities in a single
ous applications in a wide range of fields including cutting- system.37–44 The idea works on the utilization of the pre-
edge defense technologies, high-tech computer or mobile acquired inorganic chemistry knowledge of a particular Ln
hardware, hybrid vehicles, wind turbines, optical display ion/cluster, a particular organic functional group or metal–
devices, and life-saving biomedical applications such as bio- ligand coordination mode by grafting it to a functional
material for multipurpose use.45–49 Clearly, the optimization of
the system by choosing a suitable Ln centre or modulating the
aSchool of Chemical Sciences, Indian Association for the Cultivation of Science, 2A &
organic part of the ligands or imparting a specific metal–
2B, Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India.
ligand environment holds the key to success.50–54 One such
E-mail: icrm@iacs.res.in
popular target is luminescent–magnetic materials, resulting
from the culmination of two distinctly different yet Ln-specific
properties: luminescence and magnetism.55–60
Among the lanthanide-magnetism topics, an important
aspect would be magnetic refrigeration based on the magneto-
caloric effect (MCE).61–63 The Gd3+ ion, with its large spin-only
magnetic moment (S = 7/2), weak superexchange interactions
bKey Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education,
School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China
cLaboratory of Molecular Biology, School of Biological Sciences, Indian Association
for the Cultivation of Science, 2A & 2B, Raja S. C. Mullick Road, Jadavpur, Kolkata
700032, India
†Electronic supplementary information (ESI) available. CCDC 2047785–2047787.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0dt04122j
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and quenched orbital momentum driven reversible change in nating functional groups that provide the all-important coordi-
temperature with changing magnetic field, is considered as nation geometry along with the bridging modes between
the most promising metal ion for ultralow cryogenic paramagnetic Ln centres and
technology.64–66 Indeed, the majority of the complexes chromophore.102–106 Previous results from our group show that
reported for MCE studies are composed of Gd3+ ions.67–71
pyridine–pyrazole based ligands are highly effective at forming
a
grafted organic
The optical features of Ln are however completely different cages and clusters with both transition metals and
from those of other metals. Because of the origin of the parity- lanthanides.107–110 The ligand systems are non-conventionally
forbidden f–f transition, the energy absorption of the Ln ion is fitted with soft N-rich functional groups but are highly
low which results in very weak luminescence.72 However, the effective at generating Ln-based cages and clusters with
desired luminescence effect from Ln-complexes can be various sizes, shapes and nuclearity. Most of these clusters
extracted using the so-called process “luminescence sensitiz- exhibited the potential of good magnetic materials but showed
ation” or the “antenna effect”, where a highly absorbing chro- weak fluorescence properties due to the absence of effective
mophoric part attached to the ligand plays the pivotal chromophores. Naturally, a robust strategy would be to modify
role.73–80 The three-step processes essentially use ligand the ligand system with the introduction of a chromophoric
chromophores for light absorption followed by transferring group, while keeping the design fundamentals of magnetic
the energy to the lanthanide ion which subsequently emits the materials intact.
light in the form of luminescence with all its advantageous
Herein, we report three luminescent-magnetic Ln-clusters
characteristic features, while bypassing the shortcomings such (Ln = Gd3+, Eu3+ and Tb3+) using a new pyridine–pyrazole func-
as a low quantum yield of individual Ln ions. Another interest- tionalized ligand fitted with a chromophoric phenanthroline
ing aspect of sensitized luminescence is the precise emission backbone,3-(2-pyridyl)-N-[(E)-[9-[(E)-[[3-(2-pyridyl)-1H-pyrazole-
with a specific colour.81,82 For example, irrespective of the exci- 5-carbonyl]hydrazono]methyl]-1,10-phenanthrolin-2-yl]methyl-
tation energy, Eu3+ and Tb3+ always fluoresce from the reso- eneamino]-1H-pyrazole-5-carboxamide (Phen). Interestingly, all
nance levels 5D0 (17 250 cm−1
)
and 5D4 (20 430 cm−1), three complexes crystallize with a chiral space group, and they
respectively.83,84 Such unique luminescence behaviour has were spectroscopically further studied using solid state circular
enabled the use of Ln-complexes as useful probes for cellular dichroism (CD). The magnetic properties of the Tb-complex
sensing and imaging.85–92 Of course, to be an effective probe and Gd-complex were investigated which reveals an interesting
for sensing internalization into cells is an important issue. magnetocaloric effect for the latter. The fluorescence pro-
This is exactly where the Ln-complexes of low nuclearity with perties of the Eu and Tb clusters were highly encouraging and
nanoscale dimensions have shown great promise for a number further used for the imaging of human breast adenocarcinoma
of biomedical applications.93–95
(MCF7) cells with no apparent cytotoxicity.
Designing of a ligand is another important aspect of mag-
netism and luminescence from a lanthanide cluster.96–98 Both
the properties are extremely sensitive to even tiny distortions
of the local coordination geometry of the individual Ln centres
as well as their ligand mediated assembly and the final struc-
Results and discussions
Ligand design and synthesis
tural topology.99–101 In the context of luminescent–magnetic In a nutshell, the current ligand designing principle demands
material design, ligand design should aim at including coordi- a fusion of two compatible functional moieties capable of
Scheme 1 Structure of the ligand (Phen).
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Fig. 1 Crystal structures (ellipsoid with 50% probability) of (a) Phen-Gd, (b) Phen-Tb, and (c) Phen-Eu (grey: C, red: O, cyan: N, magenta: Gd, light
green: Tb, and green: Eu).
inducing mutually inclusive magnetic and luminescence fea- Descriptions of molecular structures
tures simultaneously in the system. Our previous ligand
The Phen-Ln complexes are isostructural in nature (Fig. 1).
systems, fitted with a prototype pyridyl–pyrazolyl carbohydra-
Therefore, only the Phen-Gd complex is used for the structural
zide functionality containing multiple coordinating sites, have
description in this work. The molecular structures along with
been extremely beneficial for bridging the Ln centres and
the coordination environment of the individual lanthanide
thereby facilitating the ferromagnetic or antiferromagnetic
ions, crystallographic table and selected bond lengths and
interactions. In the present work, we have modified the ligand
angles are separately reported in the ESI.†
by introducing a 1,10-phenanthroline unit as a dual-purpose
chelating backbone-cum-chromophore (Scheme 1). The 1,10-
phenanthroline moiety with an extended delocalized rigid
structure will not only enhance the fluorescence intensity by
an antenna effect but also contain a chelating unit which will
ultimately help bring additional coordinating sites near the Ln
ions.
The ligand (Phen) can be prepared in good yield following a
typical Schiff base condensation reaction between 3-(pyridin-2-
yl)-1H-pyrazole-5-carbohydrazide and 1,10-phenanthroline-2,9-
dicarbaldehyde (Scheme S1†). All the lanthanide complexes
with a general formula of [LnIII3(Phen)2(NO3)5] (Phen-Ln,
where Ln = Gd3+, Tb3+ and Eu3+) were synthesized in excellent
crystalline form simply by refluxing Phen with the nitrate salts
of the individual lanthanides. Reasonably good diffraction
quality single crystals of the complexes were obtained after a
few days from the undisturbed cooled solution and
structural studies were carried out using X-ray single-crystal
diffraction data. All the complexes were also routinely charac-
Fig. 2 Crystal structure of Phen-Gd (nitrate, solvent molecules and
hydrogen atoms are omitted for clarity); the inset shows the picture of
terized by FT-IR spectroscopy, elemental analysis and mass
spectroscopy.
the crystal of Phen-Gd. All the three complexes show the same type of
crystal & crystal structure (magenta: Gd, red: O, blue: N, and grey: C).
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The single crystal X-ray structure of Phen-Gd reveals that it µ-amide oxygen atoms bridging the third one (Gd1), which
crystallizes in the orthorhombic Fdd2 space group (Table S6†). resides in the intersecting region. The Gd ions are arranged in
The asymmetric unit of the structure consists of half of the a linear fashion in the resultant trinuclear unit with an inter-
[GdIII3(Phen)2(NO3)5] unit and a DMF molecule as the solvent metallic distance of 4.184(5) Å. Finally, all five nitrate ions
of crystallization. The ligand (Phen) adopts a two-prong adopt a bidentate chelating mode. One such nitrate group
pendent orientation where the phenanthroline part acts as the binds to the middle Gd atom, whereas two nitrate groups bind
pendent and the pyridine–pyrazole part constitutes each prong to each of the other two Gd atoms from top and bottom
(Fig. 2). The ligand adopts some interesting hairpin like orien- (Fig. 3a and b).
tation wherein two inner walls house the array of the coordi-
For the trinuclear core, comprising the Gd–O bond lengths,
nating sites of the pyridyl–pyrazolyl units facing each other. Gd–O–Gd angles, and Gd⋯Gd separations, the geometric para-
Subsequently, two Phen molecules undergo interdigitation meters with Gd–O bonds vary from 2.402(5) to 2.511(6) Å, the
engulfing three Gd atoms, two of them (Gd2) being buried Gd–O–Gd angles span from 116.08(17)° to 118.79(19)°, and the
deep inside the trough regions and with the help of four Gd⋯Gd separation is 4.184(5) Å (Fig. 3c). The peripheral Gd–N
Fig. 3 Coordination environments around the (a) Gd1 and (b) Gd2 centers, (c) trinuclear Gd3 core with coordination motifs; coordination geometry
observed around the metal centres in Phen-Gd, (d) bicapped square antiprism (for Gd1) and (e) tetradecahedron (for Gd2).
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bond lengths are found to lie in the range from 2.550(6) to thesis of a chiral complex molecule is the use of a chiral ligand
2.650(6) Å and are in agreement with the literature. The selec- as a precursor. Recent reports show that spontaneous resolu-
tive bond lengths and angles are listed in Table S3.†
tion can also be a key technique to impart chirality in a metal
For Phen-Tb, the Tb–O bond lengths vary from 2.384(9) to complex or cluster.117–121 The complexes of the present study
2.493(12) Å, the Tb–N bond lengths are found to lie in a range crystallize in a chiral Fdd2 space group with corroborating
from 2.525(10) to 2.630(11) Å and the Tb–O–Tb angles span Flack parameter values of 0.082(9) and 0.144(16) for Phen-Eu
from 116.5(3)° to 119.4(4)° (Table S4†). For Phen-Eu and Phen-Tb, respectively. This prompts us to confirm the
(Fig. S16†), the Eu–O bond lengths vary from 2.413(5) to 2.527 presence of chirality in bulk samples with the help of solid-
(6) Å, the Eu–N bond lengths are found to lie in a range from state circular dichroism (CD) spectroscopy using diffused
2.557(6) to 2.653(6) Å and the Eu–O–Eu angles span from reflectance techniques on KBr pellets of the crystalline powder
115.75(17)° to 118.62(19)° (Table S5†). The value of the dis- samples. As illustrated in Fig. 4, the signal obtained in the CD
tance between two adjacent lanthanide units (Eu–Eu is 4.189 spectra for Phen-Eu is better pronounced than that of Phen-
(5) Å; Tb–Tb is 4.182(5) Å) is also close to the value for Gd.
Tb. The signals for Phen-Eu appear at 240, 397, and 295 nm,
whereas for Phen-Tb the signals appear at 247, 402, 515, 299,
and 471 nm. CD measurements were performed with different
SHAPE analysis
The crystal structures exhibit two distinctly different local batches of the samples. In all the cases, more or less similar
coordination environments around the Ln centers. While both spectra were obtained, which confirms the enantiomeric purity
the metal centres adopt the GdN4O6 coordination motif their of the samples.
coordinating sites and local geometries are different. From the
continuous shape measure (CShM) values111,112 for the Gd1 &
Gd2 centers of Phen-Gd, we found that the central Gd ion
Magnetic studies
Considering the fact that the primary aim of this study was to
generate magnetic–luminescent Ln complexes, we carried out
magnetic studies using the resultant cluster complexes.
Accordingly, the solid state direct current magnetic suscepti-
bility data of the polycrystalline samples of Phen-Gd and Phen-
Tb were collected in a temperature range of 2–300 K under a
magnetic field of 0.1 T and are shown in the χmT (χm = molar
magnetic susceptibility) vs. T (temperature) plot. The room
temperature (300 K) χmT values of Phen-Gd and Phen-Tb are
23.39 and 35.26 cm3 K mol−1, respectively, which are very close
to the calculated values for the three isolated Gd3+ and Tb3+
ions. The observed values of the three non-interacting Gd3+
(8S7/2, g = 2) and Tb3+ ions (7F6, g = 3/2) are 23.62 and
35.43 cm3 K mol−1, respectively. With decreasing temperature,
the χmT value for Phen-Gd decreases slowly up to 30 K
(22.79 cm3 K mol−1) and below this temperature the χmT value
increases rapidly at low temperature which is due to the ferro-
(GD1) has a geometry closer to a bicapped square antiprism
(JBCSAPR-10). On the other hand, the terminal Gd ions (Gd2)
adopt a nearly tetradecahedral (2 : 6 : 2) (TD-10) geometry
(Fig. 3d and e). The list of potential coordination polyhedra of
the 10-coordinate metal centers and Continuous Shape
Measure (CShM) values for each Ln ion are given in Table S7.†
The CShM value of Gd1 for the bicapped square antiprism is
3.45375, which is the lowest among all the possible geometries
of a 10-coordinate metal center. Similarly, the CShM value of
Gd2 for the tetradecahedron is 1.50029, which is the lowest
among all the possible geometries. The Gd1 and Gd2 centers
of the complex show D4d and C2v symmetry, respectively. The
bicapped square antiprism has 16 triangular faces and 24
edges, whereas the tetradecahedron possesses 22 edges and
layers in a ratio of 2 : 6 : 2. This geometry has 12 triangular and
2 square faces. For both the centers the geometries are slightly
distorted.
As all the three Phen-Ln complexes are isostructural in
nature, the geometries of the metal centers of the other two
complexes are also similar to that of Phen-Gd. For Phen-Tb,
the central Tb (Tb1) and terminal Tb ions (Tb2) favor the
bicapped square antiprism geometry (the CShM value is
3.45684) and tetradecahedral (2 : 6 : 2) (TD-10) geometry (the
CShM value is 1.45976), respectively (Fig. S14†).
Similarly, for Phen-Eu the central Eu (Eu1) and terminal Eu
ions (Eu2) favor the bicapped square antiprism geometry (the
CShM value is 3.52737) and tetradecahedral (2 : 6 : 2) (TD-10)
geometry (the CShM value is 1.42902), respectively (Fig. S15†).
Circular dichroism (CD) spectra
The crystal structures or more specifically the space groups of
the Phen-Ln crystals invoke an interesting topic of inorganic
chemistry, chirality. Although often overlooked, chirality
remains one of the most important properties of traditional in-
organic complexes.113–116 The popular technique for the syn- Fig. 4 CD spectra of Phen-Eu and Phen-Tb.
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magnetic interaction between the neighboring Gd3+ ions 23.64 cm3 mol−1 K and θ = −3.47 K and this C value is close to
(Fig. 5a) and reaches the value of 25.49 at 2 K. But for Phen- the room temperature χMT value of 23.39 cm3 mol−1 K. The
Tb, with decreasing temperature, the χmT value decreases negative θ value further confirms the antiferromagnetic inter-
slowly and below 50 K the χmT value rapidly decreases which is actions between the adjacent metal centers at higher tempera-
due to the antiferromagnetic interaction between the neigh- ture, whereas at low temperature Phen-Gd showed ferro-
boring Tb3+ ions (Fig. 5c).
magnetic interactions among the neighboring Gd centers. But
Moreover, the data were fitted using the Curie–Weiss in the case of Phen-Tb, fitting of the Curie–Weiss equation
equation, χM = C/(T − θ), where C is the Curie constant and θ is gives us C = 15.28 cm3 mol−1 K and θ = −0.431 K and this
the Curie–Weiss constant (Fig. 5d and e). The fitting of the negative and low θ value suggests weak antiferromagnetic
1/χM vs. T (T more than 100 K) plot gives us the value of C = interactions between adjacent Tb centers.
Fig. 5 χMT vs. T plot for (a) Phen-Gd, (b) model used for the fitting of the plot, χMT vs. T plot for (c) Phen-Tb, and 1/χM vs. T plot for (d) Phen-Gd and
(e) Phen-Tb (stright line for fitting using the Curie–Weiss equation).
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For further confirmation of the magneto-structural corre- where H is the Hamiltonian, J signifies the magnetic coupling
lation, the data were fitted (Fig. 5a and b) using the constant and S signifies the spin of the metal center. From the
Hamiltonian given below:
best fitting data, we calculated J = +3.82(2) cm−1 and g = 2.0.
The low positive value of J represents the weak ferromagnetic
interaction among the neighbouring Gd centres.
Gadolinium based coordination compounds are one of the
key compounds that are used as molecular magnetic refriger-
3
X
H ¼ ꢀJðS1S2 þ S2S3Þ ꢀ gμBH
Si
i¼1
Fig. 6 (a) Field dependent isothermal magnetization and (b) temperature dependencies (2 to 10 K) of the magnetic entropy change (−ΔSm) for
Phen-Gd as obtained from the magnetization data.
Fig. 7 Fluorescence spectra of (a) Phen-Tb and (b) Phen-Eu, and fluorescence titration spectra for (c) Tb and (d) Eu (the excitation wavelength is
350 nm for every case).
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ants and exhibit the magnetocaloric effect (MCE) because of increase in the magnetic field and a decrease in temperature.
their marginal magnetic anisotropy and large spin (S = 7/2). In The maximum value of the entropy change (−ΔSm) is 23.42 J
addition to that, because of the effective shielding of the 4f orbi- kg−1 K−1 at 7 T and 2 K (Fig. 6b) which is comparable with those
tals of GdIII, the metal–metal magnetic exchange interactions of some reported GdIII complexes.122–136 The theoretical entropy
are very weak, which can also increase the MCE value. The mag- change value is calculated from the equation:
netic entropy change (ΔSm) is a vital parameter which reveals
the magnitude of the MCE. Considering the abovementioned
fact, we have explored our Phen-Gd compound as a molecular
magnetic refrigerant which exhibits the magnetocaloric effect.
ꢀ ΔSm ¼ nR lnð2S þ 1Þ
¼ 51:865 J molꢀ1 Kꢀ1
¼ 24:22 J kgꢀ1 Kꢀ1
The isothermal field-dependent magnetization data of Phen-Gd
the value is slighltly higher than the experimental value as there
were obtained and at a higher magnetic field, the magnetization
is a weak anti-ferromagnetic interaction present among the
value tends to saturate. At a 7 T magnetic field and 2 K tempera-
nearest Gd3+ centers. This result suggests that our Phen-Gd
ture the magnetization value reaches 20.96 NµB which is close to
compound can act as a promising molecular magnetic refriger-
the saturation value of 21 NµB for the three isolated Gd3+ ions (g =
ant in low temperature applications.
2) (Fig. 6a). From the magnetization data the magnetic entropy
change values are calculated using the Maxwell relationship:
Photoluminescence properties
ꢀ
ꢁ
ð
δMðT; HÞ
δT
ΔSm
¼
dH
The magnetic properties of Phen-Lns are highly encouraging
and naturally prompt us to explore our second target feature,
H
The obtained ΔSm values at different temperatures and mag- luminescence. Accordingly, the optical properties of the Tb
netic fields were plotted. The values of ΔSm increase with an and Eu clusters are characterized by photoluminescence spec-
Fig. 8 TEM images of the nanoparticles of (a) Phen-Tb and (c) Phen-Eu, and SAED pattern data of (b) Phen-Tb and (d) Phen-Eu.
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troscopy. The requisite solutions were prepared by mixing the phase purity of the samples was confirmed using the
1 mg of the powder sample of the respective Phen-Ln (Ln = powder X-ray diffraction data which show excellent matching
Eu3+ and Tb3+) with 1 mL of DMSO solvent and then sonicated with the simulated pattern generated from the corresponding
for a few minutes. The resultant yellow-colored solutions were single crystal X-ray diffraction structure (Fig. S6 and S7†). On the
irradiated with a light of 350 nm. As illustrated in Fig. 7a and other hand, considering the fact that the size of a common
b, the emission spectra show the characteristic peaks for the human cell is much smaller than that of our bulk crystals of
Eu and Tb complexes. For Phen-Tb two peaks are visible at lanthanide clusters, we had to make crystalline nanoparticles
around 490 nm and 544 nm resulting from the electronic tran- using a step-down procedure. We made powders of Phen-Eu
5
5
sition of D4–7F6 and D4–7F5, respectively. On the other hand, and Phen-Tb by manually grinding their crystals using a mortar
the Phen-Eu spectrum reveals four peaks at 578 nm, 589 nm, and pestle. Then a solution was prepared by mixing 1 mg of
615 nm and 639 nm, which can be assigned to the electronic powders with
1 mL of DMSO and then sonicated for
transitions of D0–7F0, D0–7F1, D0–7F2, and D0–7F3, respect- 10 minutes. A yellow-colored clear solution was obtained for
ively. The observed distinctive photoluminescence features can both the samples. Then the solutions were filtered using
be attributed to the presence of chromophores such as phe- 0.22 µm syringe filter paper to remove all the unwanted bigger
nanthroline. The ligand (without any metals) shows a hump sized particles. DLS, TEM & ESI-MS were performed with the fil-
like emission signal at around 400 nm which is totally trate. The DLS results (Fig. S10 and S11†) and TEM image
different in nature from the emission signals responsible for (Fig. 8a and c) show that the particle size of Phen-Eu is around
the lanthanide complex of the ligand. We have also performed 200 nm, whereas the particle size of Phen-Tb is much smaller
the titration of the ligand solution with lanthanide salt solu- (<5 nm). Furthermore, the SAED data from TEM show the crys-
tions which shows that with an increase in the salt concen- tallinity of the nanoparticles in both the cases (Fig. 8b and c).
5
5
5
5
tration the peaks for the lanthanide increase (Fig. 7c and d).
Another important issue is the stability of the complexes in solu-
tion that would be used for cell imaging. The ESI-MS data
(Fig. S9†) reveal the fragmented mass of the complexes which
confirms the stability of the complex in solution, which has
been subsequently used for cell imaging studies (Fig. 9).
Bio-imaging applications
The luminescent nature and low-nuclearity crystal structures led
us to believe that Phen-Tb and Phen-Eu would be ideal candi-
dates for bio-imaging applications. However, in order to have
any meaningful bio-imaging applications, the materials have to
Cellular imaging
fulfill certain preconditions, such as dimensions, particle size, Having confirmed the fulfillment of the preconditions of crys-
and purity. For example, the probe needs to be of sufficiently talline nanoscale sized photoluminescent materials, the poten-
smaller size for internalization into cells. First and foremost, tial of the Phen-Eu and Phen-Tb solutions for biomedical
Fig. 9 Live cell confocal microscopy showing the delivery of compounds Phen-Eu (red) and Phen-Tb (green) in the nucleus and cytoplasm.
Representative confocal images showing the localization of Phen-Eu (30 µM) and Phen-Tb (30 µM) in human breast adenocarcinoma (MCF7) cells as
monitored by the intrinsic fluorescence of the drugs (Phen-Eu: both green and red channels, middle panel, and Phen-Tb: both green and red chan-
nels, middle panel) and with Hoechst 33342 (the blue channel, left panel) used as a nuclear marker. The right panel shows merged images.
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Fig. 10 Cell survival assay with compounds Phen-Eu and Phen-Tb reveals no apparent cytotoxicity. (a) Survival curves of the MCF7 cells treated
with Phen-Eu for 72 h. Cell viability was determined by MTT assays. Error bars represent the standard deviation (n ≥ 3). (b) Survival curves of the
MCF7 cells treated with Phen-Tb for 72 h. Cell viability was determined by MTT assays. Error bars represent the standard deviation (n ≥ 3).
applications was explored. One such important and topical {TbIII3} and {EuIII3} cores. The complexes are isostructural and
biomedical application of stable crystalline fluorescent par- exhibit a topology that closely resembles two interdigitating
ticles would be the labeling of live cells. In the present context, hairpins. The clusters crystallize in chiral space groups and
we use Phen-Eu and Phen-Tb for the live cell labeling of also exhibit chirality for bulk samples, which were further con-
human breast adenocarcinoma (MCF7) cells. First and fore- firmed using the solid state CD spectra. The magnetic
most, any possible cytotoxic effects of our complexes were measurement of the Gd cluster reveals a significant cryogenic
assessed with the help of standard cell cytotoxicity (MTT) magnetic refrigeration with
a moderately high magnetic
assay experiments. None of the complexes exert any adverse entropy change of −23.42 J kg−1 K−1 at 7 T and 2 K. The intro-
effects on the cells and enhance the prospect of Phen-Eu and duction of a 1,10-phenanthroline moiety, on the other hand,
Phen-Tb as effective probes for the labelling of cancer cells. made a huge impact on the fluorescence properties of the Eu
Further confirmation comes from corroborating survival rates and Tb complexes. With no apparent cytotoxic adverse effects,
of ca. 90%, determined from the survival curves of the MCF7 the complexes were subsequently used for the imaging of
cells after treatment with Phen-Eu and Phen-Tb for 72 h human breast adenocarcinoma (MCF7) cells. The live cell con-
(Fig. 10). The optimal results were obtained when the experi- focal microscopy images show that the complexes penetrate
ments were carried out with a diluted 30 µM concentration of beyond the usual cytoplasm region and can be useful in
the Phen-Eu and Phen-Tb solutions. The experimental imaging the nucleus region of MCF7 cells.
microscopy images collected and analysed using a live cell con-
focal microscope were beyond our expectation. In excellent
agreement with the HRTEM and DLS results, the sizes of the
Phen-Eu and Phen-Tb complexes were indeed of sufficiently
nanoscale dimensions to achieve cell-permeability and as such
they easily pass through the cell membrane during the incu-
bation period of 4 hours. Usually the conventional cellular
spectra were obtained on a Nicolet MAGNA-IR 750 spectro-
sensing, internalization of probes into cells, proceeds only up
to cytoplasm. In the present case, the images clearly show that
analysis was performed with a PerkinElmer 2400 Series-II CHN
both the probes go beyond the conventional limit to penetrate
through the nuclear membrane and enter into the nucleus of
the cells. Localization into the nucleus can be confirmed by
the crystalline samples were collected using Quantum Design
using nuclear marker Hoechst 33342 (blue channel, left
panel), simultaneously. Furthermore, the enlarged images of
the cells unequivocally confirm the localization of our com-
dimethyl sulfoxide solvent. The photoluminescence measure-
plexes in the nucleolus, and hence prove the importance of
Experimental
General materials and methods
Materials. All reagents and solvents were purchased from
commercial sources & used without further purification. FTIR
meter with the samples prepared as KBr pellets. Elemental
analyzer. UV–visible spectroscopy was performed using a
PerkinElmer Lambda 950 UV–vis instrument. Magnetic data of
1
MPMS (Evercool, 7 T). H NMR analysis was performed with a
Bruker spectrometer operating at 400 MHz using a deuterated
ments were performed with a Horiba Jobin Yvon Fluoromax-3
spectrometer using steady-state 450 W Xe lamps as the exci-
tation source, and the excitation of the samples was performed
using these complexes for biomedical applications.
in the UV region at 350 nm. Mass spectra were recorded on a
Conclusions
Q-ToF Micro YA263 high resolution (Waters Corporation) mass
The aim of the work presented herein was to generate multi- spectrometer by positive ion mode electrospray ionization and
functional lanthanide complexes using a newly synthesized the spectra were collected in methanol. Powder X-ray diffrac-
pyridyl–pyrazolyl-based ligand containing a chromophoric tion data at room temperature were collected using a Rigaku
1,10-phenanthroline backbone. Accordingly, we have success- Smart lab X-ray diffractometer. Dynamic light scattering
fully synthesized three lanthanide complexes with {GdIII3}, experiments were performed using a Malvern particle size ana-
3602 | Dalton Trans., 2021, 50, 3593–3609
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lyzer (Model No. ZEN 3690 ZETASIZER NANO ZS 90 version carbonyl]hydrazono]methyl]-1,10-phenanthrolin-2-yl]methylene-
7.03), and transmission electron microscopy was performed amino]-1H-pyrazole-5-carboxamide (Phen) was synthesized by
using a Jeol JEM 2010 HRTEM.
the condensation reaction between 3-(pyridin-2-yl)-1H-pyrazole-
Single crystal X-ray diffraction. Crystal data were collected at 5-carbohydrazide and 1,10-phenanthroline-2,9-dicarbaldehyde.
100 K using a Bruker D8 VENTURE Microfocus diffractometer To a mixture of 1.18 g of 1,10-phenanthroline-2,9-dicarbalde-
equipped with a PHOTON II detector, with Mo Kα radiation hyde and 2.03 g of 3-(pyridin-2-yl)-1H-pyrazole-5-carbohydra-
(λ = 0.710 73 Å), controlled by the APEX3 (v2017.3-0) software zide, 40 mL of ethanol was added and then refluxed at 80 °C for
package and processed using SAINT. Raw data were integrated 8 hours (yield: 67%). Anal. calcd (found) for C32H22N12O2 (%): C,
1
and corrected for Lorentz and polarization effects using the 63.44 (64.85); H, 3.76 (3.24); N, 27.73 (28.62), H NMR spectra/δ
Bruker APEX III program suite. The structures were solved by (ppm) (400 MHz, DMSO-d6): 14.268 (s, 2H, pz NH), 12.421 (s,
direct methods in SHELXS and refined by full-matrix least 2H, amide NH), 9.7 (d, 2H, amide NH), 8.944 (dd, 2H, pyH),
squares on F2 in SHELXL. Crystallographic data are summar- 8.634 (m, 4H, aromatic H & non-aromatic CH), 8.413 (d, 2H, aro-
ized in Table S6.†
matic H), 8.158 (m, 4H, aromatic H), 7.947 (td, 2H, aromatic H),
Live-cell confocal microscopy. Live-cell imaging was carried 7.524 (s, 2H, pz CH), IR (400–4000 cm−1): 3379w, 3230m, 3049m,
out using a confocal laser-scanning microscope (Leica TCS 1670vs, 1618s, 1597s, 1543s, 1497s, 1396s, 1310s, 1234m, 1159s,
SP8) with a UV-laser and 63×/1.4 NA oil objective equipped 1067s, 999s, 854s, 787s. ESI-MS m/z (M + H): 608.22.
with a heated environmental chamber set to 37 °C with
Synthesis of complex Phen-Eu ([EuIII3(Phen)2(NO3)5]). Phen
optimal CO2 facility. Briefly, MCF7 cells seeded on a cover (0.121 g, 0.2 mmol) and Eu (NO3)3·6H2O (0.134 g, 0.3 mmol)
glass bottom dish (Genetix, Biotech Asia Pvt. Ltd) were incu- were dissolved in a solvent mixture containing 15 mL of DMF
bated with the compounds Phen-Eu and Phen-Tb separately and 10 mL of methanol. The mixture was stirred for half an
for 6 h. The nuclei were subsequently stained with Hoechst hour and then refluxed in an oil bath at 80 °C for 8 hours.
33342 (Blue) (Sigma). This was followed by live cell confocal After cooling the mixture to room temperature, the greenish
microscopy. Images were collected and processed using the yellow solution was filtered and kept undisturbed for crystalli-
Leica software and sized in Adobe Photoshop 7.0.137
zation. After 7 days, light greenish yellow colored octahedron
Cell survival assay. MCF7 cells (6 × 103) were seeded in shaped X-ray diffraction quality single crystals were obtained.
96-well plates (BD Biosciences, USA) and treated with the com- The crystals were washed with methanol and dried. (Yield-
pounds Phen-Eu and Phen-Tb at the indicated concentrations. 86%) Anal. calcd (found) for C70H58Eu3N31O21 (%): C, 39.55
After 72 h of treatment, cell survival was assessed by the 3-(4,5- (39.28); H, 2.73 (2.25); N, 20.43 (19.80). IR (400–4000 cm−1):
dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) 3431w, 3227m, 2924s, 1664vs 1610s, 1526s, 1448s, 1384s,
assay. Briefly, the cells were washed with 1× PBS and treated 1302s, 1194m, 1014s, 903s, 791s.
with the MTT reagent (Sigma) for 3 h at 37 °C and the result-
ing formazan was dissolved in 100 µl of dimethyl sulfoxide. Tb was synthesized using a similar procedure to that of Phen-
The plates were analyzed using Molecular Devices Eu. The only difference is that here Tb(NO3)3·5H2O (0.131 g,
Synthesis of complex Phen-Tb ([TbIII3(Phen)2(NO3)5]). Phen-
a
SpectraMax M2 microplate reader at 570 nm. The percent inhi- 0.3 mmol) was used instead of Eu(NO3)3·6H2O. (Yield-75%)
bition of viability for each concentration of the compounds Anal. calcd (found) for C70H56Tb3N31O21 (%): C, 39.19 (39.12);
was calculated with respect to the control. Data represent the H, 2.61 (2.29); N, 20.25 (19.65). IR (400–4000 cm−1): 3421w,
mean values s.e.m. for three independent experiments.137
3224m, 2924s, 1664vs, 1612m, 1526m, 1448s, 1385s, 1304s,
Synthesis of 1,10-phenanthroline-2,9-dicarbaldehyde. 1,10- 1194s, 1014s, 903s, 791s.
Phenanthroline-2,9-dicarbaldehyde was synthesized following a
Synthesis of complex Phen-Gd ([GdIII3(Phen)2(NO3)5]). Phen-
previously reported procedure.138 A mixture of 15 mL of 1,4- Gd was synthesized using a similar procedure to that of Phen-Eu.
dioxane & 1 mL of water was added to 0.56 g (5 mmol) of selenium The only difference is that here Gd(NO3)3·6H2O (0.135g, 0.3mmol)
dioxide. Then the mixture was stirred & heated to reflux at 100 °C was used instead of Eu(NO3)3·6H2O. (Yield-71%) Anal. calcd
in a double neck round bottom flask. A separate clear solution was (found) for C70H58Gd3N31O21 (%): C, 39.27 (39.14); H, 2.71 (2.12);
prepared by adding 0.5 g (2.4 mmol) of 2,9-dimethyl-1,10-phenan- N, 20.29 (19.68). IR (400–4000 cm−1): 3431w, 3231m, 1664vs,
throline to 10 mL of 1,4-dioxane & mixed well. Then this solution 1604m, 1529s, 1447s, 1385s, 1304s, 1194m, 1014s, 905s, 791s.
was added dropwise to the previously refluxed mixture. After that
the total mixture was further refluxed for 2 more hours. Then the
mixture was filtered under hot conditions & the filtrate was left to
cool. A yellow needle shaped product was obtained after cooling
Conflicts of interest
and washing with warm 1,4-dioxane (yield-80.1%).
There are no conflicts to declare.
Synthesis of 3-(2-pyridyl)-N-[(E)-[9-[(E)-[[3-(2-pyridyl)-1H-pyr-
azole-5-carbonyl]hydrazono] methyl]-1,10-phenanthrolin-2-yl]
methyleneamino]-1H-pyrazole-5-carboxamide (Phen). The
crude yellow colored compound (1,10-phenanthroline-2,9-
Acknowledgements
dicarbaldehyde) previously synthesized was used for this A. D. and K. S. D. are thankful to the Department of Science
process. 3-(2-Pyridyl)-N-[(E)-[9-[(E)-[[3-(2-pyridyl)-1H-pyrazole-5- and Technology (DST), India for the INSPIRE fellowship. S. S.
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Dalton Transactions
and S. P. C. are supported by the UGC Senior Research
Fellowship, India. S. A. is supported by the CSIR Senior
Research Fellowship, India. R. M. gratefully acknowledges the
Department of Science and Technology (DST), India, for finan-
cial assistance under the Empowerment and Equity
Opportunities for Excellence in Science Programme (Project
No. EEQ/2018/000194).
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