Luminescence modulation: Via cation-π interaction in a lanthanide assembly: Implications for potassium detection

Article abstract of DOI:10.1039/c7tc04580h

Herein luminescent lanthanide supramolecular assemblies were constructed via coordination of a crown-connected bis-terpyridine (L₁) and Ln³⁺ ions, which can self-assemble to form homogeneous spherical nanoparticles with an average di

Full text of DOI:10.1039/c7tc04580h

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Luminescence modulation by cation-π interaction in a lanthanide
assembly: implications for potassium detection
Received 00th January 20xx,
Accepted 00th January 20xx
*
Daqing Yang, Yige Wang, Dexi Liu, Zhiqiang Li and Huanrong Li
DOI: 10.1039/x0xx00000x
Herein luminescent lanthanide supramolecular assemblies were constructed via coordination of the crown-connected bis-
3
+
terpyridine (L
1
) and Ln ions, which can self-assembly to form homogeneous spherical nanoparticles with an average
www.rsc.org/
diameter of 700 nm on silicon plate surface upon slow evaporation of solvents. Potassium ions are shown to give rise to
3
+
obvious enhancement in luminescence of Ln (Ln=Eu, Tb) ions in the lanthanide assemblies. The changes are the
consequence of the capture of the cations by the crown moieties through cation-π interactions, increasing the triplet
excited energy level of L
1
and thus favoring the population of Lanthanide’s excited state from the antenna triplet state by
3
+
3+
3+
3+
means of an energy transfer. Besides, energy transfer from Tb to Eu was observed in the luminescent Eu /Tb
.
.
bimetallic assemblies (Eu/Tb L
1
) and the efficiency can be tuned by potassium concentration. Thus, Eu/Tb L
1
can serve as
+
self-calibrating sensor, which can selectively and quantitatively detect potassium by linearly correlating the K
5
7
F
3+
5
7
3+
concentration to the emission intensity ratio of D
4
→
5
transition (Tb ) to D
0
2
→ F transition (Eu ).
+
3+
sensing ability for K of Ln -based supramolecular assembly
.
Introduction
1,
(Ln L Ln=Eu, Tb or Eu/Tb with Eu to Tb molar ratio being one)
formed by self-assembly the ligand (
1
L ) that consists of an 18-
Functional supramolecular assembly triggered by metal-ligand
crown-6 (18C6) connected to two 2,2’:6’,2’’-terpyridyl (tpy)
3
moieties (Scheme 1). The tpy moieties can coordinate to Ln
ions to form luminescent supramolecular structure since they
(M-L) coordination interactions are highly important for
+
developing optical switches/devices, catalysis, sensors, drug
1
carriers, and bioapplications.
Among the various
3
+
3
+
can harvest light and sensitize the Ln emission via “antenna
effect”, while the 18C6 moiety can effectively modulate the
luminescence behavior through the binding of alkaline cations
supramolecular systems, trivalent lanthanide (Ln )-directed
M-L assemblies are being increasingly explored in the design of
advanced materials owing to their unique optical properties,
such as sharp emission lines, high luminescent efficiency and
1
0
via cation-π interactions, especially, potassium ion, which
.
enables Ln L
1
b, 2
1
to be used as potential luminescence sensor for
long-lived excited states.
These properties make them ideal
selective and quantitative detection for potassium.
5
and highly desirable candidates for the integration into
luminescent supramolecular assemblies where the emission
signals can be modulated by external stimulus, including small
7
4 5
Interestingly, the emission intensity ratio of D → F transition
3
+
5
7
3+
3+
3+
(
0 2
Tb ) to D → F transition (Eu ) of the bimetallic Eu /Tb
+
assembly can be linearly related to K concentration.
3
4
5
6
7
molecules, ions, pH, temperature, vapors, etc. As one of
the most components in constructing the assembly, the
organic linkers (ligands) are playing very important roles in
constructing supramolecular compounds flexibly by self-
Results and discussion
8
3+
The organic linker L1 was synthesized by the reaction of 4′-(4-
Bromomethyl)-phenyl)-2,2′:6′,2″-terpyridine (L ) with 4,13-diaza-
assembly coordination process and in sensitizing the Ln
emission via the so-called “antenna effect” as well as in
(
2
3
+
1
8-crown-6-ether (D-18C6) in approximately 84% yield and the
shielding the Ln
ions from deleterious quenching
9
interactions. Therefore, the design and synthesis of novel
structure is presented in Figure 1. The purity and chemical structure
3
+
of the L was verified using nuclear magnetic resonance (NMR)
organic linkers are highly significant for developing new Ln -
directed supramolecular systems having excellent luminescent
1
spectroscopy, electrospray ionization mass spectroscopy (Figure S1,
3
+
S2,
Supporting
Information)
and
elemental
analysis
properties of Ln and extra functionalities such as sensing
abilities, which still remains a great challenge.
1
characterization. The H NMR spectrum shows the presence of the
11
tpy protons as sharp singlets from δ = 7.25 ppm to δ = 8.75 ppm,
indicative of the C symmetry of L . Coordination of L to LnCl ·6H O
We report herein the formation, physical analysis and
2
1
1
3
2
(
the added molar ratio kept as 1:1) in the mixture of
.
dichloromethane and ethanol leads to Ln L . The complexation of
1
3
+
Ln to L via nitrogen atoms is obviously evidenced by the shift of
1
-1
the absorption bands at 1602, 1585, 1567, and 1543 cm assigned
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+
and the schematic illustration of self-calibrating detection for K .
Scheme 1. Graphical representation of the formation of Ln·L
1
.
Figure 1. a) Synthetic pathway for L
1
. b) Schematic representation of the formation of Ln L
1
; Photograph under near UV light illumination
.
.
.
(
365 nm) of Eu L
1
(c), Tb L
1
1
(d) as well as Eu/Tb L (e) in solid state and in MeOH solution (0.1 mM).
Furthermore, the characteristic peaks for the pyridine moieties in
1
H
NMR measurements (Figure S4) broadened and shifted
.
11
downfield, further verifying the formation of Ln L1.
To further ascertain the formation of supramolecular
assembly, scanning electron microscopy (SEM) were
.
performed. Due to the similarity in morphology of Eu L
.
Tb L
1
,
1
.
and Eu/Tb L , herein we only take Eu L as an example. The
.
1
1
SEM images shown in Figure 2a and 2b reveal that entangled
nanofibers driven by the multiple M-L interactions were
3
+
formed in MeOH solution when reacting Ln ions with ligand
, whereas homogeneous spherical nanoparticles with an
average diameter of 700 nm (Figure 2c and 2d) were observed
L
1
.
1
when the MeOH solution of Eu L was casting on silicon surface
followed by slowly evaporating the solvent. Energy-dispersive
.
Figure 2. SEM images of Eu L
plate surface (c, d).
1
in MeOH solution (a, b) and on silicon
X-ray (EDX, Figure S5) analysis reveals the formation of
3+
complex with
1
L to Eu ratio of 1:1, which can be further
confirmed by elemental analysis (experimental section) and
3+
thermogravimetric analysis (TGA, Figure S6) results. The Eu
1
2
to the pyridine ring of L
1
shown in FTIR spectra (Figure S3).
2
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ARTICLE
.
content in Eu L
1
was determined to be 13.12%, a value roughly
3
+
and Eu . In
corresponds to a molar ratio of 1:1 between
.
addition, TGA indicates that Eu L
L
1
1
shows good thermal stability.
3
+
1
The interaction of L with Ln in solution can be further
ascertained by UV/Vis absorption and luminescence spectroscopy
titration. A band centered at approximately 280 nm assigned to
π
→
π* transition of tpy moieties was observed in absorption
3
+
1
spectra of L (Figure 3a). Upon the stepwise binding with Eu , its
absorbance maximum of this band was bathochromically shifted
and its absorbance decreased gradually with the appearance of a
shoulder at 320 nm, accompanied by an isosbestic point at
3
+
approximately 300 nm, and reached the maxima with the L
1
to Eu
3
+
molar ratio being 1:1, thereby indicating the formation of Eu
complex with a 1:1 stoichiometry. The concomitant evolution in
luminescent emission spectra upon excitation at 345 nm was
presented in Figure 3b. A gradually increase in the emission
3
+
intensity was observed upon stepwise addition of Eu , and the
maximum was achieved when the molar ratio was 1:1, which slowly
3
+
leveled off when an excess amount of Eu were added. This could
be explained by a ligand quenching pathway caused the C-H
1
3
vibrations of some ligands. Similar phenomena were also
.
observed for Tb L1 in titration experiment results (Figure S7), in
.
.
.
accordance with previous reports on luminescent complexes
Figure 4. a) Digital photos of Ⅰ (Eu L
1
), Ⅱ (Eu L
2
), Ⅲ (Tb L
1
), Ⅳ
3
+
9f, 14
.
between Ln and tpy-based ligand.
(
Tb L ) in MeOH solution under near UV light illumination (365 nm).
Ln L was found highly luminescent both in solution and in solid The concentration of the solutions was 0.1 mM; b) luminescence
2
.
1
state irradiated with UV lamp as shown in Figure 1c, 1d and 1e. The emission spectra of the Ⅰ (red) and Ⅱ (black); c) luminescence
.
.
luminescence data of Eu L and Tb L are shown in Figure 4. The emission spectra of the Ⅲ (green) and Ⅳ (black) (λex = 345 nm); d)
1
1
excitation spectra (Figure S8) observed at 617 nm and 545 nm show Schematic diagram of the ligand-metal energy transfer from triplet
3
+
state of the L and L to Ln ions.
a broad band in the range 250-375 nm attributed to the absorption
1
2
.
of tpy units. The emission spectrum of Eu L
1
excited at 345 nm
shown in Figure 4b exhibit five sharp bands at 579, 593, 617,
5
7
3+
6
49and 698 nm, assigned to the D
which further confirm the coordination of tpy to Eu ions. For Tb L
excitation into the absorption of tpy at 345 nm leads to the metal-
0
→
J
F (J=0-4) transitions of Eu ,
3
+
.
1
,
3
+
centered Tb emission at 489, 545, 583 and 621 nm, assigned to
3
+
5
7
4 J
the transitions of Tb from the D level to the F levels (J= 6, 5, 4,
.
5
7
3
). The emission spectrum of Eu L
1
is dominated by the D
0
→
F
2
transition at 617 nm and the strongest emission line at 544 nm of
.
5
7
Tb L
1
is attributed to the D
4
→
5
F transition, which is responsible
for the bright red emission and green emission, respectively, shown
.
.
in Figure 1 and Figure 4. It is worthy to note that Eu L
2 2
and Tb L
(
ligand to metal ratio being 2:1) possessing the same coordination
.
.
1 1
mode and number with Eu L and Tb L while lacking the D-18C6 unit
show much weaker emission intensity (Figure 4b and 4c, black line),
which can be easily seen by naked eyes when the samples were
irradiated with UV lamp, i.e., no obvious emission can be observed
.
for the methanol solution of Tb L
2
(Figure 4a (IV)). In addition, the
.
.
decay time of Eu L
2 2
and Tb L was determined to be 0.61 ms and
0
.47 ms from the corresponding decay curves, respectively, a much
.
.
shorter value compared with that of Eu L
ms) (Figure S9). These data reveal that the D-18C6 moiety
contained in L can favor the population of Lanthanide’s excited
1 1
(0.73 ms) and Tb L (0.71
1
Figure 3. Changes in a) the absorption, b) the luminescence
emission spectra (λ =345 nm) of L (0.01 mM) upon the addition of
state from the antenna triplet state by means of an energy transfer.
This can be further verified by the energy levels of the triplet state
ex
1
EuCl
3
·6H
2
O (0
→
1.5 equivalents) in dichloro/methanol (1:1 v/v).
1 2
of L and L determined according to the method reported
5
7
3+
Inset: luminescence changes of D
0
→
F
2
at 617 nm versus Eu /L
1
.
15
previously (Figure 4d). The value is remarkably increased from
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-
1
2
0790 to 22420 cm after the integration of D-18C6 and L
S10), enabling much more suitable for sensitizing the
luminescence of both Eu and Tb with respect to L
2
(Figure
L
1
3
+
3+
2
according to
1
6
Latva’s empirical rule.
Furthermore, obvious luminescence enhancement was
.
1
observed upon adding potassium ions to Ln L in methanol, implying
.
+
that Ln L
1
can be used as luminescent sensor for K detection. This is
+
very important since K is an essential ion in biological systems and
1
7
its selective and sensitive detection is of great importance.
+
However, luminescent K sensors explored until now are based on
the luminescence intensity of single emissive transition and are
compromised to errors due to the concentration of sensor, the
1
8
drifts of the optoelectronic systems, and excitation efficiency. We
3
+
3+
observed herein that the Eu /Tb heterometallic structure
.
+
Eu/Tb L display a high selectivity toward K as shown in Figure 5. In
1
a preliminary screening experiment, different cations were added
.
to Eu/Tb L in a methanol solution. Among the cations tested, only
1
potassium ions remarkably increase the luminescence intensity of
3
+
3+
both Eu ions and Tb ions, leading to 175% and 140 % of the
3
+
3+
original value for Eu and Tb (Figure 5b), respectively, whereas
+
+
2+
2+
3+
2+
2+
3+
+
other metal such as Na , Li , Ca , Mg , Cr , Ni , Zn , Al and Ag
3
+
3+
ions can quench Eu and Tb luminescence efficiently. The shape
.
+
Figure 6. (a) Luminescent emission intensity of Eu/Tb L in MeOH
1
of the emission spectra are not perturbed by the binding of K ,
+
3
+
solution (0.1 mM) in the presence of various equivalents of K under
which means no structural rearrangement occurs at the Ln
excitation at 345 nm. Insets: the luminescence intensity at 617 nm
10b
.
+
center, indicating that Eu/Tb L is a stable luminescent K sensor.
+
1
and 545 nm with different equivalents of K ; (b) The fitted curve
.
+
The evolution of luminescent emission spectra Eu/Tb L on the K
+
1
between ITb/IEu and various equivalents of K (0.5-6 eq).
3
+
addition is presented in Figure 6. The Tb emission intensity at 545
3
+
.
nm and Eu emission intensity at 617 nm of Eu/Tb L increases
1
+
constantly when the added K is increased from 0 to 4 equivalents,
+
further increasing K concentration leads to an obvious drop in
3
+
3+
Tb emission intensity, whereas the emission intensity of Eu at
+
6
17 nm displays stable enhancement (Figure 6a). This different K -
5
7
→
3+
dependent emission pattern of D
4
F
5
transition (Tb , ITb) and
5
7
3+
D
0
→
F transition (Eu , IEu) in the bimetallic compound indicates
2
.
1
that Eu/Tb L can be used as an excellent candidate for self-
calibrating sensor, not necessarily requiring any additional
+
calibration of luminescence intensity. Remarkably, the K equivalent
+
(
eq[K ]) within the range from 0.5 to 6 can be linearly related to the
.
1
emission intensity ratio (ITb/IEu) of Eu/Tb L by Equation (1) as shown
.
1
in Figure 6b, which indicates that Eu/Tb L can also be used as an
+
excellent luminescent sensor for the quantitative analysis of K , a
detection limit down to 1 μM can be achieved, a promising value
19
compared to those previously reported in ref.
+
I
Tb/IEu=3.5179－0.2514 eq[K ]
(1)
.
The 18C6 moiety is of high importance for Eu/Tb L
1
to realize
the selective and quantitative detection of potassium ions for it can
+
be served as a lariat to selectively capture K via selective cation-π
2
0
+
interactions. The binding of K can remarkably increase the triplet
state energy level of the ligand L , e.g., it was increased from 22420
cm to 23400 cm when four equivalents of potassium ions was
added to L , and this can explain why luminescence enhancement
1
-1
-1
1
3
+
.
1
of Ln can be observed for Eu/Tb L . In addition, like other
3
+
3+
21
3+
Eu /Tb bimetallic systems, energy transfer also occurs from Tb
3
+
.
1
Figure 5. The luminescence emission spectra (a) and histogram (b) ions to Eu ions in Eu/Tb L as evidenced by the shorter lifetime of
.
3+ 5
3+ 5
of Eu/Tb L
1
in MeOH solution (0.1 mM) with various metal cations Tb D4 excited state and the longer value of Eu D0 excited state
λ =345 nm) (I, I are the intensity of Eu/Tb L in 545 nm and 617 compared with that of the corresponding monometallic sample
.
(
ex
0
1
+
.
.
nm before and after addition of K ).
Tb L
1
and Eu L
1
shown in Table 1, respectively. The energy transfer
4
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5
1
Table 1. The luminescence decay time of D state in MeOH solution obtained as brown powder in 84% yield. H NMR (400 MHz, CDCl ):
4
3
.
+
1
of Eu/Tb L with the variation equivalents of K (excited at 345 nm).
δ 8.73 (dd, 4H), 8.71 (s, 4H), 8.66 (d, 4H), 7.94-7.86 (m, 8H), 7.67 (s,
4
8
H), 7.37 (ddd, 4H), 4.13 (s, 4H), 3.80 (s, 8H), 3.63 (s, 8H), 3.17 (s,
Untreated
1
2
3
4
5
6
@
544 nm
+
1
H). ESI-MS: m/z 905.4 ([M+H] ). Elemental analysis for L , found
.
Tb L
1
.
0.71
0.70
0.74
0.68
0.76
0.66
0.78
0.64
0.82
0.62
0.82
0.59
0.81
0.56
(calcd.): C 74.27 (74.31), H 6.29 (6.24), N 12.36 (12.38).
.
.
Synthesis of Ln L : For the preparation of Eu L , EuCl ·6H O (0.1
1
1
3
2
Eu/Tb L
1
1
mmol) was dissolved in EtOH (100 mL). 0.09 g of L (0.1 mmol) in
ηTb-Eu/%
8.1
13.2
15.4
24.4
28.0
30.9
dichloromethane (100 mL) was sonicated until complete
dissolution. The resulting solution was added dropwise under
3 2
vigorous stirring to the solution of EuCl ·6H O at room temperature.
3
+
3+
After stirring at 70 °C for 8 h, the mixture was allowed to evaporate
slowly at 80 °C. The resulting powder was washed several times
efficiency from Tb ions to Eu ions (ηTb-Eu) was determined
2
2
according to previously method using Equation (2) and was listed
in Table 1.
with dichloromethane and EtOH, and dried under vacuum
.
overnight. Elemental analysis for Eu L
1
, found (calcd.): C 57.81
η
Tb-Eu=1－τ
1
/τTb
(2)
and τTb are the lifetime of the D
1
. From Table 1, we can observe that the energy transfer
.
.
5
.
(
57.79), H 4.91 (4.82), N 9.81 (9.63). In addition, Tb L
1
1
, Eu/Tb L and
where τ
.
1
4 1
state in Tb L and
.
Gd L
analysis for Tb L
.56 (9.57); Elemental analysis for Eu/Tb L
57.61), H 4.81 (4.80), N 9.54 (9.60).
1
were also easily obtained by the same method. Elemental
Eu/Tb L
efficiency changes as the K concentration changes. The changes in
.
+
1
, found (calcd.): C 57.55 (57.44), H 4.81 (4.79), N
.
9
1
, found (calcd.): C 57.65
the triplet excited energy level of L
1
and the alteration of the energy
3
+
3+
+
(
transfer efficiency from Tb to Eu caused by the presence of K
.
.
Synthesis of Ln L
2
: LnCl
3
·6H
2
O (0.1 mmol) (Ln=Eu, Tb, Gd) was
(0.2 mmol) in
are therefore responsible for the luminescent response of Eu/Tb L
1
+
dissolved in EtOH (100 mL). 0.08 g of
L
2
towards K ions.
dichloromethane (100 mL) was sonicated until complete
dissolution. The resulting solution was added dropwise under
Conclusions
3 2
vigorous stirring to the solution of LnCl ·6H O at room temperature.
After stirring at 70 °C for 8 h, the mixture was allowed to evaporate
slowly at 80 °C. The resulting powder was washed several times
with dichloromethane and EtOH, and dried under vacuum
In conclusion, this work reports a novel luminescent and self-
3
+
calibrating sensor for potassium ions constructed by Ln -
directed supramolecular self-assembly through the
3
+
3+
overnight.
coordination of Eu and Tb with a crown-connected bis-
terpyridine. The crown moiety makes the bimetallic system
highly attractive in the selective and quantitative detection of
potassium ions since the energy transfer from the ligand to
1
Characterization:
H NMR spectra was recorded at room
temperature, using perpetuated solvents as internal standards.
Elemental analysis was performed on an Elementar Vario EI system.
The TG studies were analyzed by using a SDT-TGQ 600 at a heating
3
+
3+
3+
3+
Eu and Tb as well as that from Tb to Eu can be tuned by
+
-1
+
rate of 10 °C min using dried air as purging gas. SEM images were
obtained from an FE-SEM (Hitachi S-4300) at an acceleration
voltage of 10 kV. Fourier transform infrared spectroscopy (FT-IR)
K -crown cation-π interaction. The result is that the K
concentration can be linearly correlated to the emission
5
intensity ratio of D4
7
3+
5
7
→ F5 transition (Tb ) to D0→
F2
−1
3
+
spectra were recorded in the spectral range from 4000 to 400 cm
transition (Eu ), and a detect limit down to 1 μM can be
obtained. This work highlights the importance of the design
and synthesis of organic linker for constructing luminescent
lanthanide supramolecular assembly that show promising
applications in various fields such as sensing.
with a Bruker Vector 22 spectrophotometer by using pressed KBr
pellets for solid samples. A UV-visible Agilent cray100 spectrometer
was used to determine the wavelength of the maximum UV
absorbance peak. The luminescence measurements (excitation
spectra, emission spectra, luminescence decay time) of solid state
powder and solution were measured on an Edinburgh Instruments
FS920P.
Experimental
2
Materials: 4′-(4-(bromomethyl)phenyl)-2,2′:6′,2″-terpyridine (L )
1
2a,23
was synthesized by the modified method previously reported
.
Conflicts of interest
EuCl ·6H O, TbCl ·6H O (99.99%, Beijing HWRK Chem) and 4,13-
3
2
3
2
There are no conflicts to declare.
diaza-18-crown 6-ether (D-18C6, 98%, J&K) were used as received.
Unless otherwise stated, all manipulations were conducted under
air atmosphere.
Acknowledgements
Syntesis of L
1
: D-18C6 (0.21 g, 0.8 mmol), L
2
(0.65 g, 1.6 mmol), and
CN
This work was financially supported by the National Natural Science
Foundation of China (21271060, 21502039, 21236001, 21605036),
the Hebei Natural Science Foundation (B2016202147, B2016202149,
B2017202048), and the Educational Committee of Hebei Province
Cs CO (0.65 g, 2 mmol) were refluxed at 80 °C in 120 mL of CH
2
3
3
for 12 h under nitrogen. The reaction mixture was evaporated
under reduced pressure and treated with a mixture of water and
chloroform (1:2). The organic layer was separated, evaporated
under reduced pressure. The crude product was purified by column
(
LJRC021).
chromatography over silica gel (eluent: 12:1 CH
2 2 1
Cl /MeOH), L was
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Table of Contents
3
+
A novel self-calibrating sensor for potassium ions was constructed by Ln -directed
+
supramolecular self-assembly through K -crown cation-π interaction.