Stepwise enhancement of fluorescence induced by anion coordination and non-covalent interactions

Article abstract of DOI:10.1039/d0dt03788e

A multi-level regulation of fluorescence enhancement upon anion coordination and subsequent binding of a guest (methyl viologen) was presented by a bis-bis(urea)-decorated tetraphenylethene (TPE) ligand with an assembly-enhanced emission characteristic.

Full text of DOI:10.1039/d0dt03788e

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Stepwise enhancement of fluorescence induced
by anion coordination and non-covalent
interactions†
Cite this: DOI: 10.1039/d0dt03788e
Received 3rd November 2020,
Accepted 1st December 2020
Dan Zhang, ‡ Jie Zhao,‡ Liping Cao, * Dong Yang,
Xiao-Juan Yang and Biao Wu
Bozhong Chen, Le Yu,
DOI: 10.1039/d0dt03788e
*
rsc.li/dalton
A multi-level regulation of fluorescence enhancement upon anion strating that anions could also act as coordination centers to
coordination and subsequent binding of a guest (methyl viologen) construct diverse supramolecular architectures like metal
1
8
was presented by a bis–bis(urea)-decorated tetraphenylethene ions.
The anion-coordination-driven cages and helicates
(
TPE) ligand with an assembly-enhanced emission characteristic. possess the capacity for encapsulation of guest molecules, sta-
1
9
bilizing active molecules and controlling reactivity.
Fluorescence enhancement has attracted considerable atten-
tion owing to both easily detected sensing event and its poten-
tial applications as diagnostic, monitoring and analytical tools
in medical, biochemical, and environmental science.
However, conventional luminophores used in the real world
tend to exhibit fluorescence quenching because of their aggre-
gation. Luminogens with the feature of aggregation-induced
emission (AIE), especially tetraphenylethene (TPE), have been
extensively studied in the past twenty years due to the brigh-
tened emission by aggregate formation and wide range of
applications such as fluorescence “turn-on” or “light-up” bio-
sensors, biological/chemical probes, biological imaging,
light-harvesting, circularly polarized luminescence materials,
and so on. In previous research, the TPE group has been incor-
porated to metal–organic frameworks (MOFs), covalent
organic frameworks (COFs), supramolecular organic frame-
works (SOFs), donor–acceptor (D–A) complexes, porous
materials, polymers, macrocycles, and cages.
Nevertheless, the exploration of the applications in more
aspects, such as catalysis, drug delivery, gas absorption and
storage, as well as sensing, still needs more efforts.
1
In order to explore ACDA and its potential applications in
fluorescent materials and biosensors, we previously reported
the tetrakis(bisurea)-decorated tetraphenylethene ligand,
showing fluorescence “turn-on” properties upon coordination
2
3
20
with phosphate ions in a wide range of concentrations.
Inspired by this work, we expect to utilize ligand L, in which
two bis(urea) moieties are attached to a tetraphenylethene
(
TPE) core (Scheme 1), to achieve multi-level regulation of fluo-
4
5
6
rescence enhancement by anion coordination and guest
binding. According to our previous work, such bis–bis(urea)
7
8
21
ligands have the potential to form a discrete motif through
coordination to phosphate anions. Interestingly, L assembled
complexes, exhibiting obvious “turn-on” fluorescence.
9
1
0
1
1
12
1
3
14
15
16
In contrast to the well-studied supramolecular architectures
containing transition metal ions, the anion-coordination-
driven assembly (ACDA) strategy has been overlooked for a
long time due to the intrinsic properties and lack of reliable
1
7
coordination geometry of anions. In recent years, a series of
2 3
and A (A = anion) tetrahedral anion cages and A L
A
4
L
4
4
L
6
triple anion helicates have been successfully achieved, demon-
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry
of the Ministry of Education, College of Chemistry and Materials Science,
Northwest University, Xi’an 710127, China. E-mail: chcaoliping@nwu.edu.cn,
wubiao@nwu.edu.cn
†
Electronic supplementary information (ESI) available. CCDC 2025855–2025857.
For ESI and crystallographic data in CIF or other electronic format see DOI:
Scheme 1 (a) Chemical structure of ligand L; (b) first-level “turn-on”
fluorescence in a TPE rotor by ACDA and aggregation; (c) second-level
fluorescence enhancement by adding guest MV .
10.1039/d0dt03788e
2+
‡
These authors contributed equally to this work.
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Moreover, the fluorescence is further enhanced by introducing
a guest cation (methyl viologen, MV ), which is bound by similar to 1 despite the different counter-cation. The SO4
non-covalent interactions between the two anion complexes. anions are bound through eight N–H⋯O hydrogen bonds,
Herein the multi-level regulation of the fluorescence of TPE is respectively, ranging from 2.773 Å to 3.023 Å. Four TBA
reported (Scheme 1).
The complex [TBA]
4
[(SO
4
)
2
(L)
2
] (2) also shows a 2 : 2 structure
2
+
2−
+
4−
counter-cations are located around the [(SO ) (L) ] moiety and
4 2 2
The bis–bis(urea) ligand L was synthesized by a four-step two of them interact with phenyl rings of TPE cores through
strategy and the product (A) of the first step was purified from cation–π interactions (Fig. 1b). A comparison of the two anion
2
2
a mixture of (Z) and (E) isomers (for synthetic details see the complexes demonstrates that the shortest phenyl⋯phenyl sep-
ESI†). First of all, single crystals of the monohydrogen phos- aration between the two TPE cores within one A complex is
phate complex, [K([18]crown-6)] [(HPO (L) ] (1) (the phos- 4.105 Å (C18⋯C41; Fig. S6a†) for complex 1, and 3.562 Å
phate anion was generated in situ from K PO and [18]crown-6, (C19⋯C38) and 3.585 Å (C64⋯C69; Fig. S10†) for 2. The
2 2
L
4
4
)
2
2
1
H
3
4
and [18]crown-6 is used to dissolve K
solvent) were obtained. Complex 1 shows a 2 : 2 anion-to- Δδ = 1.08 ppm) and the upfield shift of –CH
3
PO
4
in an organic NMR spectra showed the obvious downfield shifts of –NH (avg
(Δδ = −0.09 ppm,
ligand ratio, in which two ligands stack to embrace two anions Fig. 1d) compared with ligand L. From H NMR titration (0–2.0
Fig. 1a), similar to the monohydrogen phosphate complex of equiv.), a two-step binding process was observed that changed
the tetrakis(bisurea)-decorated TPE ligand in our previous from 1 : 1 to 1 : 2 (L : A, Fig. S13†) with more added anions. Due
3
1
(
2
1
+
work. Four [K([18]crown-6)] counter cations situate around to there are two types of assembly in the binding of L with
2−
complex 1 and form weak C–H⋯π interactions with the dan- SO
4
, the details such as the binding ratio cannot be obtained.
gling phenyl rings of TPE (3.556–3.776 Å, Fig. S9b†), contribut- However, by comparison, it can be found that titration of the
ing to the restriction of the intramolecular rotation of TPE and 1 : 1 mode is consistent with complex 2. Namely, in solution,
2−
1
fluorescence enhancement. Two of them coordinate to the O the binding of L with SO
4
is 2 : 2. H NMR variable tempera-
atoms of urea groups and the other two interact with the O ture and concentration experiments indicated that the coordi-
2
−
2−
atoms of HPO4 . Both HPO4 anions are coordinated by four nation of L with anions is stable in complexes 1 and 2 as the
urea groups from two bis(urea) moieties through eight N– 2 : 2 mode (Fig. S16, S17 and S19, S20†), and the DOSY spec-
H⋯O hydrogen bonds (N⋯O distances in the range of trum revealed that complex 2 is a single assembly (Fig. S23†).
1
3−
2
.748–2.974 Å and N–H⋯O angles from 144° to 162°). The H The ESI-MS study confirmed its composition [L (SO ) (TBA)]
2
4 2
2
−
NMR spectra showed the distinct downfield shifts of –NH (obsd 742.62 vs. calcd 742.62) and [L
2 4 2 2
(SO ) (TBA) ]
(obsd
signals (avg Δδ = 2.26 ppm) and the upfield shift of –CH
3
of L 1235.11 vs. calcd 1235.11) (Fig. S27†).
Δδ = −0.11 ppm, Fig. 1c) compared with ligand L. H NMR Subsequently, the binding behaviors of L with different
(0–2.0 equiv., generated in situ from anions were examined by employing fluorescence experiments
1
(
2
−
titration of L with HPO
4
3
−
2−
K
2
HPO
4
and [18]crown-6) showed that the peaks remain (Fig. 2). With the addition of 1.0 equiv. of PO
4
or HPO
4
unchanged after 1.0 equiv. (Fig. S12†). Combined with the ions, the emission intensity increased by about 14-fold and
2
−
solid-state structure, it appears that L and HPO
in the 2 : 2 mode. The DOSY spectrum showed that complex 1 equiv. of SO
is a single assembly (Fig. S22†). The ESI-MS study confirmed ment, while other anions induced no obvious change (e.g.,
4
assembled 12-fold, respectively, along with 30 nm red shifts. Adding 1.0
2
−
4
ions caused about 6-fold fluorescence enhance-
3
−
−
−
−
−
−
−
−
−
−
the 2 : 2 stoichiometry [L
62.93 vs. calcd 762.93) and [L
obsd 1295.96 vs. calcd 1295.96) (Fig. S26†).
2
(HPO
4
)
2
([18]crown-6)K]
(HPO ([18]crown-6)
3 4 4 2 4
(obsd NO , Br , Cl , ClO , I , HSO , H PO , AcO , and F , as
2
−
+
7
2
4
)
2
2
K
2
]
TBA salts; 1.0 equiv.).
(
The most interesting aspect of the assembly of L with phos-
phate may be fluorescence enhancement. However, in the solu-
3
−
2−
tion state, the anion exists as a mixture of PO
(
4
and HPO
4
1
H NMR titration, see Fig. S14†), and the binding depends on
the concentration. As the concentration increases, the PO
3
−
4
assembly tends to form (L = 40 mM, Fig. S15†). The ESI-MS
4
−
Fig. 1 (a) Crystal structure of [(HPO
4
)
2
(L)
2
]
(1) showing the coordi-
2
−
nation of two HPO
4
anions by two ligands; (b) crystal structure of
(2) with the coordination of two SO
ligands and cation–π interactions between TBA and phenyl; and
NMR spectra (400 MHz, 298 K, [D ]DMSO) of complex 1 (c) and complex
(d).
4
−
2−
[
(SO
4
)
2
(L)
2
]
4
anions by two
+
1
H
Fig. 2 Fluorescence emission spectra (λex = 415 nm) of L (1.0 × 10−
4
M
6
2
in DMSO) upon the addition of 1.0 equiv. of different anions.
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study demonstrated the formation of [L
2
(PO
1598.07)
4
)
2
([18]crown-
UV-vis studies on the binding property of L to phosphate
2
−
−4
6
) K ]
(obsd
1598.07
vs.
calcd
and anions have also been carried out in DMSO (1.0 × 10 M).
4
4
2
−
[L
2
(PO
4
)
1
(HPO ([18]crown-6)
4
)
1
3
K
3
]
(obsd 1447.52 vs. calcd Addition of 1.0 equiv. of phosphate anions (as the [18]crown-6-
1
447.52) (Fig. S28†), the latter indicating the coexistence of K salt) to the DMSO solution of L induced a bathochromic
two anions in the system. Unfortunately, we cannot obtain shift of the spectrum. A new intense broad absorption band at
single crystals, but PM6 calculation on L with phosphate (2 : 2) 467 nm was formed when 2.0 equiv. of MVCl was added to
2
supports the rationality of this structure (Fig. S34†). In the solution of L with 1.0 equiv. of phosphate anions (Fig. 4a),
addition, the DOSY spectrum also showed the generation of a which is consistent with the optimum excitation wavelength of
2
+
solely 2 : 2 complex in a concentrated solution (Fig. S24†).
the phosphate complex containing MV (complex 3) and
2
+
2+
As we know, methyl viologen (MV ) possesses excellent elec- favours the interaction between MV and complex 1 in the
tron-accepting properties and is an efficient electron-transfer newly formed complex 3 (Fig. 3b). The UV-vis spectrum of
medium, which is likely to interact with electron-rich systems phosphate anions (1.0 × 10 M) with 2.0 equiv. of MVCl
2
3
−4
2
in
such as anion complexes 1 and 2. Hence, it is expected that DMSO shows a broad absorption band at 410 nm. The UV-vis
2
+
MV can be utilized as a guest to further change the fluo- titration experiments revealed the binding constants, K = 4.04
1
5
−1
4
−1
2
rescence of an anion assembly. When 2.0 equiv. of methyl violo- × 10 M and K = 1.05 × 10 M (see the ESI, Fig. S31†).
gen dichloride (MVCl
solution of L (1.0 × 10 M) with 1.0 equiv. of phosphate anions M in DMSO) decreased at 500 nm along with the increase of a
, dissolved in water) was added to the Meanwhile, the fluorescence emission of complex 1 (1.0 × 10−
4
2
−4
(
generated from K PO
3 4
and [18]crown-6) in DMSO, the fluo- new emission at 544 nm upon the addition of 0–4.0 equiv. of
(Fig. 4b), which suggests that the emission wavelength
4 nm) (Fig. 3a). The H NMR spectra demonstrated that the has changed with the formation of the new complex 3. It was
assembly transformed from mixed anions to a single complex found that complex 3 and the MVCl /phosphate system
in solution after the addition of MVCl (Fig. S25†). Notably, the possess the same emission wavelength, which indicates that
addition of 2.0 equiv. of MVCl to a solution containing only the fluorescence of TPE gradually decreased due to a photo-
rescence intensity further increased with a red shift (Δλ = MVCl
4
2
1
2
2
2
−4
phosphate anions (1.0 × 10 M) also induced fluorescence induced electron transfer (PET) process between the anion
2
+
emission, which may be due to the electrostatic interaction complex and MV . The fluorescence enhancement at 544 nm,
2
4
2+
and alkaline environment. In order to study the mechanism which belongs to MV , is caused by electrostatic and other
of the fluorescence enhancement, both excitation and emission non-covalent interactions between the anion complex and
2
+
spectra were collected. The emission spectrum of L with MV .
.0 equiv. of PO4 (λex = 415 nm), and the excitation spectra
3−
2+
1
The interaction between the anion complex and MV was
proved by the crystal structure of the sulfate complex [(MV)
3
−
of L with 1.0 equiv. of PO
4
and 2.0 equiv. of MVCl
2
(λem
=
5
(
4 2
44 nm), as well as the excitation spectra of phosphate anions LSO ] (4) from the mixture of sulfate ions, ligand L, and
1.0 × 10− M) and 2.0 equiv. of MVCl (λ = 544 nm) were MV . Compared with complex 2, SO
4
2+
2−
anions exhibit one
2
em
4
obtained (Fig. 3b). From Fig. 3b we can find the partial overlap less hydrogen bond in complex 4. Only seven N–H⋯O hydro-
3
−
between the emission spectrum (L with PO
4
) and excited spec- gen bonds were formed (N⋯O distances in the range of
3−
3−
trum (L with PO4 and MVCl ; PO
with MVCl ), which indi- 2.79–3.03 Å, and N–H⋯O angles from 145° and 173°, Fig. 5a).
2
4
2
cated that the second-step fluorescence enhancement may arise This may be due to the existence of electrostatic interaction
2
+
2+
2−
2−
from MV . Meanwhile, compared with the fluorescence of L, between MV and SO
4
, leading to the SO
4
anion deviating
there is no change after adding MVCl , indicating that MVCl
alone has no interaction with L (Fig. S29†).
from the optimal position to form hydrogen bonds. The 2 : 2
anion-to-ligand coordination of 4 is similar to that of 2, but
the counter-cations are bound in a different way. A 32.2°
2
2
Fig. 3 (a) Fluorescence spectra (λex = 415 nm) of L (1.0 × 10− M in
4
DMSO), L with 1.0 equiv. of phosphate anions, L with 1.0 equiv. of phos-
phate anions and 2.0 equiv. of MVCl
M in DMSO) with 2.0 equiv. of MVCl
equiv. of phosphate anions (λex = 415 nm) and excitation spectra of L
with 1.0 equiv. of phosphate anions and 2.0 equiv. of MVCl (λem
47 nm) and only phosphate anions (1.0 × 10 M in DMSO) with 2.0
equiv. of MVCl (λem = 547 nm).
2
, and phosphate anions (1.0 × 10−
4
Fig. 4 (a) Absorption spectra of L upon the addition of 1.0 equiv. of
2
; (b) emission spectra of L with 1.0
phosphate anions and both 1.0 equiv. of phosphate anions and 2.0
2 2
equiv. of MVCl , and only phosphate anions and 2.0 equiv. of MVCl . (1.0
−4
2
=
× 10 M in DMSO); (b) fluorescence titrations of phosphate complex 1
−4
−4
5
(1.0 × 10 M in DMSO) upon the addition of 0–4.0 equiv. of MVCl
Inset: The increase of the fluorescence intensity at λ = 544 nm.
2
.
2
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exist in complex 3. PM6 calculations of 3 showed the ration-
ality of this structure (Fig. S35†).
We have designed a bis–bis(urea) ligand (L) by incorporat-
ing the tetraphenylethene (TPE) fluorophore. The weakly emis-
sive L in a dilute solution displays a large fluorescence
enhancement with the addition of phosphate or sulfate
anions, which is due to the restriction of TPE cores through
anion coordination. More interestingly, the fluorescence of the
anion complex shows a second enhancement with an accom-
panying red shift upon adding methyl viologen dichloride
(
MVCl ). Crystal structures, fluorescence, UV-vis spectra, and
2
NMR and ESI-MS results demonstrate the stable existence of
and ligands; (b) CH⋯π interactions the phosphate/sulfate complexes containing MV and mul-
Fig. 5 Crystal structure of [(MV)LSO
4
−
]
2
(4). (a) View along the a axis with
2
2+
the hydrogen bonds between SO
4
2
+
2−
between the MV guest and L of the SO
4
complex (the anion and part tiple interactions between them. The above multi-level assem-
2
+
symmetric L and MV were omitted for clarity); (c) the side view and
stacking mode of complex 4 (part non-acidic hydrogen atoms and sol-
vents were omitted for clarity).
bly processes provide a supramolecular strategy for stepwise
enhancement of fluorescence.
Conflicts of interest
2
+
torsion angle between two pyridine rings of MV can be
observed instead of a coplanar configuration, which should be There are no conflicts to declare.
2
+
due to the formation of the CH⋯π interaction between MV
and the dangling phenyl rings of TPE (2.837 Å of C65–
H65⋯phenyl, Fig. 5b), as well as the CH⋯π interaction Acknowledgements
2
+
between another MV and the terminal aryls of L (3.236 Å of
C62–H62⋯phenyl, Fig. 5b). It can be seen from the packing
We are grateful for the financial support from the National
Natural Science Foundation of China (21772154).
2
+
mode of the crystal structure that two MV cations insert
4
−
between two [(SO ) (L) ] dimers and are close to the two dan-
4
2
2
gling phenyl rings of TPE (Fig. 5c).
1
Notes and references
Moreover, the H NMR spectra of complex 4 and guest
2
+
MVCl show that MV in 4 exhibits a slight upfield shift, indi-
2
1
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3
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+
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4
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