The enhancement of the D-A effect of an asymmetric Schiff base by introducing acetyl groups into diaminomaleonitrile: Synthesis, red fluorescence and crystal structure

Article abstract of DOI:10.1039/c9ra00977a

An asymmetrical salen-type organic ligand was designed and synthesized by a new strategy developed using a precursor Ac-DMN, which is a diaminomaleonitrile (DMN) incorporated with an acetyl group. In this study, two types of asymmetrical ligands, namely Ac-DMN-salicylaldehyde (L⁰) and Ac-DMN-4-N,N-diethyl-salicylaldehyde (L¹), and their Zn(ii) coordination complexes were studied. With the electron-pushing substituent, L¹ showed interesting photoluminescence behaviour distinct from that of L⁰. In a THF solution, the maximum fluorescence emission of L¹ red-shifted to 90 nm compared with that of L⁰. Furthermore, in a THF-H₂O solution, L¹ exhibited aggregation-induced emission (AIE), but L⁰ exhibited aggregation-caused quenching (ACQ). Upon coordination with Zn(ii), the maximum emission wavelengths in THF for both the ligands were red-shifted to 100 nm and 105 nm, respectively. The crystalline solid-state photoluminescence properties were studied based on the single-crystal structural analysis.

Full text of DOI:10.1039/c9ra00977a

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The enhancement of the D–A effect of an
asymmetric Schiff base by introducing acetyl
groups into diaminomaleonitrile: synthesis, red
fluorescence and crystal structure†
Cite this: RSC Adv., 2019, 9, 14268
*
Ke Wang, Hao Su, Pengfei Wang, Wenjie Wang and Hui Li
An asymmetrical salen-type organic ligand was designed and synthesized by a new strategy developed
using a precursor Ac-DMN, which is a diaminomaleonitrile (DMN) incorporated with an acetyl group. In
this study, two types of asymmetrical ligands, namely Ac-DMN-salicylaldehyde (L0) and Ac-DMN-4-N,N-
diethyl-salicylaldehyde (L1), and their Zn(II) coordination complexes were studied. With the electron-
pushing substituent, L1 showed interesting photoluminescence behaviour distinct from that of L0. In
a THF solution, the maximum fluorescence emission of L1 red-shifted to 90 nm compared with that of
L0. Furthermore, in a THF–H2O solution, L1 exhibited aggregation-induced emission (AIE), but L0
exhibited aggregation-caused quenching (ACQ). Upon coordination with Zn(II), the maximum emission
wavelengths in THF for both the ligands were red-shifted to 100 nm and 105 nm, respectively. The
crystalline solid-state photoluminescence properties were studied based on the single-crystal structural
analysis.
Received 5th February 2019
Accepted 23rd April 2019
DOI: 10.1039/c9ra00977a
rsc.li/rsc-advances
of AIE luminescent materials does not weaken neither in solu-
tion nor in solid state via restricting the intramolecular rotation
(RIR), so it can be applied in liquid or solid phase.3 Clearly, such
Introduction
The study of asymmetric Schiff bases has been an active
research eld relative to functional materials and catalysis for
a long time since they generally exhibit unusual structures and
properties. In order to fabricate a variety of new functional
materials for their use as catalysts, exploring new asymmetric
Schiff bases is still a challenge for chemists. Asymmetric Schiff
bases with electron-donating and accepting structural units (D–
A) possess a special optical character.1 Therefore, the photo-
luminescence properties of these kinds of organic molecules as
well as their coordination compounds have drawn increasing
attention in recent years. This effect of push and pull electrons
triggers intramolecular charge transfer (ICT), which is a satis-
factory approach to increase the luminance of compounds.2 For
the aggregation of luminescent molecules, a new phenomenon
called aggregation-induced emission (AIE) has been found
based on the different molecular congurations, accumulation
and interactions, which overturn the concept of aggregation-
caused quenching (ACQ). In general, the photoluminescence
materials are crucial for chemosensors, photoelectric devices,
organic light-emitting diodes (OLEDs), photoluminescence
probes and cell imaging.4 Diaminomaleonitrile (DMN) mole-
cules have excellent rigidity and planarity.5 DMN-type Schiff
bases are oen used in AIE luminescent materials.6 However,
most of the DMN-type Schiff bases are symmetrical structures,
and the reports about asymmetric structures are rare. The main
challenge in the synthesis of an asymmetric Schiff base is how
to control target products and avoid symmetrical by-products.7
Therefore, it is very meaningful to explore a reliable asymmetric
synthesis method. In preceding studies by our group, we have
synthesized symmetric DMN-Schiff bases (2013).8 In 2017, a new
asymmetric D–A type Schiff base was obtained by a novel,
controllable method developed in our laboratory for the
synthesis of asymmetric compounds.9
In this study, we focused on the enhancement of the D–A
function of the asymmetric DMN-Schiff bases. It is the rst
example of introducing acetyl groups into the asymmetric DMN-
Schiff bases. Combining with the electron-pulling of the acetyl
group, the electron-pushing group N,N-diethyl has been incor-
porated in the salicylaldehyde side of asymmetric DMN-Schiff
bases. Two kinds of new ligands were designed and synthe-
Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and
Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China.
E-mail: lihui@bit.edu.cn
† Electronic supplementary information (ESI) available: Details about general
methods, syntheses, structure information, spectra (IR, 1H NMR, UV-vis,
uorescence), PXRD, thermogravimetric analysis. CCDC 1862896–1862898. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9ra00977a
sized,
which
are
N-((Z)-1,2-dicyano-2-((E)-(2-hydrox-
ybenzylidene)amino)vinyl)acetamide (L0) and N-((Z)-1,2-
dicyano-2-((E)-(4-(diethylamino)-2-hydroxybenzylidene)amino)
14268 | RSC Adv., 2019, 9, 14268–14275
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vinyl)acetamide (L1) (Scheme 1). Their Zn(II) coordination
compounds Zn-L0 and Zn-L1 were also obtained. The photo-
luminescence properties of L0, L1, Zn-L0 and Zn-L1 in the
solution and solid state were studied. The substituent group on
4-substitution of L1 is N,N-diethyl that has higher activity, the
introduction of which could form a D–A structure with an acetyl
group. The L0 with salicylaldehyde unit was used to compare
with L1 for studying the inuence of D–A structure to photo-
luminescence properties and compounds conformation that
could be seen from the crystal structures with the same of the
central metal. In addition, with the introduction of an electron-
pushing group, the luminescence of the complex changed from
ACQ to AIE, and the solid photoluminescence changed from
quenched to orange with red-shiing. These phenomena were
explained by enhanced ICT. The experimental results indicate
the conception of precise regulation and control, and this
design idea can be applied to further research.
Thermal properties
Thermogravimetric analysis (TGA) showed the decomposition
0
1
ꢀ
ꢀ
temperatures of L and L to be 263 C and 278 C and coor-
ꢀ
ꢀ
dination compounds to be 427 C and 310 C, respectively, in
a nitrogen atmosphere (Fig. S1 and S2†). The curves exhibit the
weight loss process. The results indicated that desirable
thermal stability of compounds were important to luminance
materials. Remarkably, L1 as an excellent photoluminescence
material showed expected stability in a solution with pH values
ranging from 3 to 8 by immersing in it for one day. The results
can be conrmed by the PXRD spectra (Fig. S3†). When the pH
value was less than 3 or greater than 8, the coordination poly-
mer was dissolved in an aqueous solution. As a result, this
material can be used in acidic and weakly alkaline environ-
ments. In addition, the material was insoluble in water that
could be applied to aqueous solution systems.
Solution uorescent properties
The UV absorption curves of L0 and L1 in THF solution are
shown in Fig. 1. The absorption peaks of L0 were observed at
325 nm, 335 nm, and 378 nm. The peak at 325 nm was attrib-
uted to the phenol group impacted by the solution resulting in
the red shi. The maximum absorption peak was located at
378 nm. L1 had two absorption peaks at 296 nm and 454 nm.
The absorption peak at 296 nm could be attributed to N,N-
diethyl group in the benzene ring. The peak at 454 nm was
attributed to ICT that resulted in the red-shi. The absorption
edges of L0 and L1 appeared at about 423 nm and 495 nm,
respectively, from which we could calculate the band gap to be
2.93 eV and 2.51 eV. It is clear that by introducing the N,N-
diethyl group, the band gap reduced observably that is good for
electron transitions and to improve photoluminescence
properties.
Result and discussion
Synthesis and characterization
The loading acetyl group of DMN (Ac-DMN) was used as
a precursor for the synthesis of asymmetric DMN-Schiff bases.
The functions of Ac-DMN are not only providing electron-
pulling but also offering coordination ability to metal ions
and inter- or intramolecular hydrogen bonding interactions.
The precursor Ac-DMN was synthesized by the reaction of acetyl
chloride with DMN in an ice-water bath. Ligands L0 and L1 were
synthesized by a reux method, in which salicylaldehyde and 4-
(diethylamino)salicylaldehyde were added to the precursor Ac-
DMN ethanol solution, respectively, to synthesize L0 and L1.
The structures of L0 and L1 were conrmed using 1H NMR, 13
C
To further understand the electron distribution in molecules
of L0 and L1, DFT calculations were operated using Gaussian 03
at the B3LYP/6-31G(d) level. By the DFT calculation, the orbital
NMR, mass spectra, FT-IR, and elemental analyses. Especially,
the single crystal structure of L1 was analyzed using X-ray single-
crystal diffraction method. Moreover, the crystals of Zn-L0 and
Zn-L1 were fostered in methanol and acetonitrile solution by
volatilizing at room temperature. The details about synthesis
and characterization of the precursor, L0, L1, Zn-L0 and Zn-L1
could be found in ESI.†
Fig. 1 UV/vis absorption spectra of L0 and L1 in 5.0 ꢁ 10ꢂ5 M THF
Scheme 1 Synthetic routes of L0, L1, Zn-L0 and Zn-L1.
solution.
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distribution could be seen more intuitively. In L0, the highest that THF had certain effect of reducing the band gap.10 In the
occupied molecular orbital (HOMO) isosurface was mainly THF solution, the molecules interacted with THF to reduce the
distributed on benzene ring and carbon–carbon double bond, band gap, resulting in red shi. Both of them indicate that the
and the lowest unoccupied molecular orbital (LUMO) isosurface band gap decreases with the introduction of the electron group,
was mainly located around the N of the DMN skeleton. When which is more conducive to the electron transition and
excited by 420 nm ultraviolet light, the electrons absorbed improves the photoluminescent properties of the compound.
energy and jumped from the ground state to the excited state,
The inuence of the electron pushing group was reected
and the electrons that absorbed energy moved from HOMO to distinctly on the photoluminescence spectra of L0, L1 and their
LUMO. It could be seen that the electron cloud in L0 was evenly coordination compounds in THF solutions, as shown in Fig. 3.
distributed on the molecule. The contribution of the excited The luminescence maximum intensities of L0 and L1 were at
electron transition energy from HOMOꢂ2 to LUMO of S3 state 430 nm and 520 nm, respectively (Fig. 3a). Clearly, the uores-
was the highest (94%), following S1 state electrons energy in cent emission of L1 red-shied signicantly by about 95 nm
HOMO-to-LUMO transition (80%). Moreover, the DFT calcula- than that of L0 because of the introduction of a donating group,
tion could explain the attribution of the peaks in the ultraviolet which resulted in ICT from N,N-diethyl to acetyl and cyan group.
spectra. The peak at 325 nm was caused by electron transition Aer coordinating with the Zn(II) ion, the luminescent
from HOMOꢂ2 to LUMO level, the peak at 335 nm was ascribed maximum intensities of Zn-L0 at 530 nm and Zn-L1 at 620 nm
to the transition from HOMOꢂ1 to LUMO, and the peak at further red-shied to about 90 nm (Fig. 3b). It should be noticed
378 nm was assigned to the HOMO-to-LUMO transition. In L1, that the maximum emission wavelengths for both ligands and
the HOMO isosurface was spread over N,N-diethyl and benzene their complexes red shi by more than 90 nm aer introducing
ring affected by the introduction of electron-pushing group, and the –N(Et)2 group. To the best of our knowledge, this is an
the LUMO isosurface was mainly located around DMN and unusual phenomenon.11 Comparing with ligands and their
around the acetyl group, which made possible for electrons coordination compounds, the emission intensities of Zn-L1 and
absorbing energy to jump from HOMO to LUMO triggered the Zn-L0 were stronger than those of L1 and L0, respectively, and
signicant transfer of electrons crossing the molecule. The the difference in values of Zn-L1 and L1 was bigger than those of
HOMO-to-LUMO electron transition of S1 absorbed the most Zn-L0 and L0, respectively. The interaction process of Zn(II) ion
energy (94%) and led to the highest intensity peak in the UV and L0 and L1 in THF could be monitored by ultraviolet titra-
spectrum at 454 nm. The distribution of the electron clouds of tion. The equivalent points were at 296 nm and 418 nm for L0
each energy level are shown in Table S1 and S2.† The band gaps (Fig. 4a) and at 408 nm and 475 nm for L1 (Fig. 4b). The binding
calculated by DFT were 3.40 eV and 3.13 eV respectively (Fig. 2), constants between the ligands L0 and L1 were 1.9333 ꢁ 104 and
which were higher than 2.93 eV and 2.51 eV calculated by 5.6988 ꢁ 103, respectively, which were calculated by Benesi–
absorption edge wavelength in THF solution, which indicated Hilderbrand curves, as shown in Fig S4 and S5.† Clearly, the
unusual uorescence properties of the ligands and their
complexes in this study are contributed by the acetyl group in L0
and L1, which is a strong electron-pulling group that enhances
the ICT effect signicantly.
The issue of different behaviors of L0 and L1 aggregations in
solution was studied in a THF/water mixture solution with
different ratios. Both L0 and L1 were soluble in THF, but had low
solubility in water. By adding water to the THF solution of the
ligands, different interesting phenomena could be noticed, as
shown in Fig. 5. When a small amount of water is present, the
photoluminescence of L0 almost remained unchanged, but
aer adding 50% of water, the photoluminescence quenched
quickly, which is a typical ACQ mechanism that happened with
the red-shi from 432 nm to 535 nm. Moreover, supplying of
water helps to form hydrogen bonds between L0 molecules,
heightening the intermolecular effect to reward the red shi.
Conversely, the luminance intensity of L1 was increased grad-
ually along with adding water. Meanwhile, the emission wave-
length was red-shi from 543 nm to 582 nm when the fraction
of water is up to 60% volume. When the volume of water
reached 90%, stratication was clearly observed, and L1 aggre-
gates from the upper layer emitted strong red uorescence,
which further indicated that photoluminescence enhancement
was caused by aggregation. This could account for the fact that
N,N-diethyl in L1 made the L1 molecule more hydrophobic than
Fig. 2 DFT calculate for molecular orbital amplitude plots of HOMO
and LUMO energy levels of L0 and L1.
14270 | RSC Adv., 2019, 9, 14268–14275
L0, which is an intrinsic quality for AIE property. It should be
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Fig. 3 PL spectra and photos of (a) L0 and L1 in THF with 5 ꢁ 10ꢂ5 M (L0: lex ¼ 420 nm; L1: lex ¼ 475 nm), (b) Zn-L0 and Zn-L1 in THF with 5 ꢁ
10ꢂ5 M (Zn-L0: lex ¼ 420 nm; Zn-L1: lex ¼ 475 nm). All photos were filmed at 365 nm in an ultraviolet light box.
noticed that the maximum emission wavelengths for both L0 X-ray single-crystal diffraction method except for L0 since its
and L1 were red-shied by more than 90 nm with the increase in good-quality single crystals were not obtained successfully even
the volume fractions of water, although the emission intensities aer many efforts. Therefore, the crystal structures of L1, Zn-L0
decreased for L0 based on ACQ and increased for L1 based on and Zn-L1 are discussed in this section. The bulky crystals of L1,
AIE mechanism. Moreover, the red shi occurred in the process Zn-L0 and Zn-L1 match up with the simulation data of their
of aggregation. Again, the acetyl-modication was responsible single crystals (Fig. 6), which are synthesized in a facile manner
for the red-shi since the function of the acetyl group to form at room temperature and by slow evaporation.
hydrogen bonds made it sensitive to the polar environment.
The molecular structure of L1 is shown in Fig. 7a. The space
Then, the acetyl group enhanced the ICT effect further. There- group was P21. The molecules were connected by strong H-
ꢀ
˚
˚
fore, the acetyl group performed dual functions of ICT and the bonding N1–H1A/O1 (1.972 A, 2.827 A, 172.15 ) into a one-
˚
red-shi uorescent emission combined with the coordination dimensional chain (Fig. 7b) with the space apart from 4.7990 A.
effects by Zn(II) ions.
Although the crystal structure of L0 has not been obtained, the
crystal structure of complex Zn-L0 has been studied which is a 1D
coordination polymer (Fig. 8a). The Zn(II) ions were ve-
coordinated, with the tridentate L0 (Zn1–O1, Zn1–N1, Zn1–N2)
and carbonyl oxygen atoms as bridge atoms (O3) and a coordi-
nating methanol group (Zn1–O2). These 1D chains are linked to
Crystal structures
In order to understand the different behaviors of photo-
luminescence properties of L0, L1 and their Zn(II) coordination
complexes, their single-crystal structures were studied using the
Fig. 4 UV-vis titration spectra of zinc and (a) L0 and (b) L1 in THF.
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Fig. 6 PXRD pattern and pictures of single crystals.
Fig. 5 PL intensity and maximum wavelength of L0 and L1 in THF/
water mixture solution with different water fractions with 5 ꢁ 10ꢂ5
M
(lex ¼ 420 and 475 nm). The pictures were unified taken at 365 nm in
an ultraviolet light box.
˚
˚
a 2D layered structure via H-bonds O2–H2B/O1 (1.806 A, 2.655 A,
153.06ꢀ). There are only van der Waals interactions between the
layers along the 3D direction (Fig. 8c). The structure of Zn-L1
comprises a trinuclear anionic cluster [Zn3(L1)3(m3-OH)]ꢂ contain-
ing m3-OH as bridging ligands and cationic [HN(Et)3]+ as coun-
terion (Fig. 9a). Three central metal atoms are bridged by m3-OH.
The structure could be described as a pseudo-cube with a vacant
position at the opposite vertex to the hydroxyl. Each coordination
Fig. 7 (a) ORTEP picture of L1, (b) 1D chain of L1 via H-bonding (N1–
H1A/O1: 1.972 A˚, 2.827 A˚, 172.15ꢀ).
Solid-state uorescent properties
For the solid state, the maximum emission wavelength for both
L0 and L1 is 596 nm with different intensities (Fig. 10a). The
uorescent emission of L0 has been completely quenched and
the L1 has a strong uorescent emission red-shied to 70 nm
relative to its THF solution. The results are consistent with the
trend of L1 and L0 in a THF/H2O solution with the different
ratios of THF/H2O for the study of ACQ and AIE. Combining
with the studies of the solution emission, there may be some
reasons to explain the novel behaviours. By the introduction of
the acetyl group, it not only provides a coordination binding site
and pushing electron density to create a more asymmetrical
electron distribution at the molecular level but also offers
a hydrogen bond function to enhance the intermolecular
interaction, which makes the two ligands more sensitive to the
polar solvents or environment from the viewpoint of supramo-
lecular chemistry. The intermolecular hydrogen bonds of L0 and
L1 are structural features for luminance with excited-state
˚
anionic cluster was linked by H-bonding O10–H10/O7 (2.204 A,
ꢀ
˚
3.013 A, 169.08 ) to form a dimer, which links to another dimer by
intermolecular forces to obtain a chain structure. The 3D struc-
tures were stacked by intermolecular forces (Fig. 9c). Coordination
modes could be seen in Scheme 1 and Fig. S4,† and the structure
from the other viewing direction of Zn-L0 and Zn-L1 are shown in
Fig. S7–S11.† The crystal data and relevant information are given
in Table S3–S6.† The structures of the two complexes are signi-
cantly different due to the introduction of N,N-diethyl groups. Zn-
L0 is an innite chain, but Zn-L1 is a trinuclear structure with
a pseudo-cube conguration. The reason can be explained as
follows: aer introducing the electron-donating group (N,N-
diethyl), the negative charge density increases, and the coordina-
tion ability of hydroxy group of benzene ring of L1 could increase
and make the hydroxy group a bridge m2-OH.
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Fig. 8 (a) Infinite ORTEP structure of Zn-L0 combined with coordi-
nation bond (Zn1–O3), (b) the coordination pattern of central atom, (c)
1D chain structure of Zn-L0, (d) 3D supramolecular structure of Zn-L0
from c axis by O2–H2B/O1 (1.806 A˚, 2.655 A˚, 153.06ꢀ).
Fig.
9
(a) Trinuclear ORTEP structure of Zn-L1 fiber-bridge by
hydroxyl (O10), (b) the coordination pattern of central atom, (c) dimer
of Zn-L1 linked by hydrogen bonding O10–H10/O7 (2.204 A˚, 3.013 A˚,
169.08ꢀ), (d) 3D supramolecular structure of Zn-L1 from b axis.
intramolecular proton transfer (ESIPT), which highly depends
on solvent environments.12 The luminescence emission spectra
are shown in Fig. S20 and S21.† It could be observed that the
emission wavelength with the increase in polarity of solvents
became larger. Meanwhile, the Stokes shi and red shi of L0
and L1 in solid state are typical luminescence properties of
ESIPT. Furthermore, the addition of water increases the polarity
of the solvent, so that it could break the equilibrium in the
original solvent, which brings intramolecular twist and
decreases coplanarity. These states were referred to twisted
intramolecular charge transfer (TICT), which makes the charge
separate between donors and acceptors, resulting in the nar-
rowing of the bond gap. The twist molecular conformation is
more stable in polar solvents, which leads to an increase in the
luminance intensity and emission efficiency.13 From the lumi-
nescence emission spectra of L1 in different solvents (Fig. S21†),
it was clear that the emission wavelength became larger with the
increase in the polarity of solvents; these phenomena illustrate
that the TICT effect is appropriate for L1. In molecular crystal
structure of L1, because of inducing the N,N-diethyl group, the
benzene ring is easily twisted around the C7–C8 bond.
Compared with the uorescence quantum yield (FF) and the
uorescence lifetimes (s) of L0 and L1, which are 2.48%, 11.31%,
0.7107 and 4.8136 ns, respectively, which proved the inuence
of TICT state for luminance of L0 and L1. These factors have
more effects on the ligand L1 than L0. Moreover, the crystal
structure study shows the L1 molecules linked with each other
in parallel and order pattern, which is responsible for the L1
molecule's AIE and photoluminescent emission to red-shi. For
L0, molecular planarity is better than L1 since it is not bulky N,N-
diethyl group, which could make L0 closer packing in crystal
lattice through p–p stacking interactions. It has typical struc-
tural characterizes for ACQ mechanism.
It is interesting to note that the crystallized solids of the
Zn(II) coordination complexes of L0 and L1 have different
performances on uorescent emissions relative to themselves.
The uorescent intensity of Zn-L0 was stronger than that of L0
and the intensity of Zn-L1 was weaker than that of L1, which can
be explained by the fact that aer coordinating, the polymetallic
clusters of Zn-L1 were formed to weaken the photo-
luminescence. Moreover, zinc had electron pulling ability so it
participated in the coordination, the distribution of electron
clouds became more balance than L1 that affected photo-
luminescent intensity.14 The uorescence quantum yields (FF)
of Zn-L0 and Zn-L1 are 12.99%, 13.87%, and the uorescent
lifetimes (s) of them are 0.7115 and 0.7285 ns, respectively.
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Fig. 10 (a) L0 and L1 on solid state (lex ¼ 475 nm), (b) Zn-L0 and Zn-L1 on solid state (lex ¼ 475 nm). All photos were filmed at 365 nm in an
ultraviolet light box.
These combinations such as coordinated bonds, hydrogen
bonds and intermolecular forces reinforce the rigidity of the
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Conclusions
The synthesis of Schiff bases with different substitution groups
at four positions has been reported, in which the introduction
of a strong electron-pushing group improves the photo-
luminescent characteristics by enhancing the ICT effect. The
acetyl group increases the possibility of hydrogen and coordi-
nation bonding in order to control the intermolecular interac-
tion and inuence the arrangement between the molecules. L1
has AIE characters, but L0 has ACQ that limits the application of
luminance materials. It is shown that the introduction of
electron-rich groups can improve the level of molecular photo-
luminescence and expand the application range. It also makes
multifarious structures of coordination compounds probable,
as proved by the crystal structures of Zn-L0 and Zn-L1. These
design ideas help synthesize better red-emitting materials in
the future. Because L1 is stable in water solution with the range
of pH values from 3 to 8, the design and synthesis of bio-uo-
rescent probes based on coordination complexes of L1 will
exhibit a bright application.
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Conflicts of interest
The authors conrm that this article content has no conict of
interest.
Acknowledgements
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This work was nancially supported by the National Natural
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