Construction of Highly Emissive Pt(II) Metallacycles upon Irradiation

Article abstract of DOI:10.1002/cjoc.201800577

Photoswitchable or photoactivatable fluorescent species have been found wide applications within supramolecular chemistry and materials science. In this study, we successfully constructed two highly emissive Pt(II) metallacycles from the diarylethene ligands via coordination-driven self-assembly. Different from the most known fluorescent metallacycles, the obtained metallacycles have displayed “turn-on” fluorescence switching. They are non-fluorescent in solution, but they emit highly yellow or orange fluorescence under ultraviolet irradiation. The metallacycles were well characterized by ¹H NMR, ³¹P NMR and ESI-TOF-MS. The photochromic properties of the resultant metallacycles were investigated by ¹H NMR, ³¹P NMR, UV/Vis spectrum and fluorescence spectrum. Notably, NMR studies revealed that these two metallacycles featured excellent cyclization efficiency (90% conversion efficiency). Moreover, the closed-ring isomers of the metallacycles displayed relatively high quantum yield (Φ_F = 0.5). DFT simulations demonstrated that the antiparallel configuration of the diarylethene ligand had an angle closed to 120°, which was more stable in energy compared to the parallel configuration, thus allowing for the facile construction of highly emissive metallacycles. We believe that such highly emissive metallacycles which are in-situ prepared upon irradiation can be used as new fluorescence materials for sensing and bioimaging in the future.

Full text of DOI:10.1002/cjoc.201800577

Construction of Highly Emissive Pt(II) Metallacycles upon Irradiation
Yi Qin,a Ying Zhang,b Guangqiang Yin,a Yuxuan Wang,a Changwei Zhang,a Lijun Chen,a Hongwei Tan,b Xiaopeng Li,c Lin
Xu,*,a and Haibo Yang*,a
ABSTRACT Photoswitchable or photoactivatable fluorescent species have been found wide applications within supramolecular chemistry and materials
science. In this study, we successfully constructed two highly emissive Pt(II) metallacycles from the diarylethene ligands via coordination-driven
self-assembly. Different from the most known fluorescent metallacycles, the obtained metallacycles have displayed “turn-on” fluorescence switching.
They are non-fluorescent in solution, but they emit highly yellow or orange fluorescence under ultraviolet irradiation. The metallacycles were well
characterized by 1H NMR, 31P NMR and ESI-TOF-MS. The photochromic properties of the resultant metallacycles were investigated by 1H NMR, 31P NMR,
UV/Vis spectrum and fluorescence spectrum. Notably, NMR studies revealed that these two metallacycles featured excellent cyclization efficiency (90%
conversion efficiency). Moreover, the closed-ring isomers of the metallacycles displayed relatively high quantum yield(φF = 0.5). DFT simulations
demonstrated that the antiparallel configuration of the diarylethene ligand had an angle closed to 120°, which was more stable in energy compared to
the parallel configuration, thus allowing for the facile construction of highly emissive metallacycles. We believe that such highly emissive metallacycles
which are in-situ prepared upon irradiation can be used as new fluorescence materials for sensing and bioimaging in the future.
KEYWORDS coordination-driven self-assembly, metallacycles, diarylethene, photochromism, high quantum yield
cyclopentene group, which showed “turn-on” fluorescence
Introduction
switching.[11] These photochromic molecules are initially
Coordination‐driven self‐assembly has evolved to be
a
non-fluorescent but emit brilliant fluorescence upon
photoirradiation, which thus can be potentially used in
super-resolution fluorescence imaging.[12] Therefore, we envision
that it is feasible to construct highly emissive SCCs under the
stimulus of light by using these photochromic molecules as the
building blocks. Based on our pervious investigation on functional
supramolecular self-assemblies,[13] we herein present the
successful construction of two kinds of highly fluorescent Pt(II)
metallacycles from diarylethene-containing dipyridine donor via
coordination-driven self-assembly. These two metallacycles
showed “turn-on” fluorescence switching. They are
non-fluorescent in solution but emit highly yellow or orange
fluorescence under ultraviolet irradiation. NMR and UV/Vis
spectra studies revealed that the metallacycles featured excellent
photochromic properties. Moreover, the closed-ring isomers of
the metallacycles displayed relatively high quantum yield(φF =
0.5), which might be potentially used in sensing or bioimaging in
the future.
powerful synthetic methodology providing the access to discrete
supramolecular coordination complexes (SCCs) with well‐defined
shapes and sizes including two dimensional (2‐D) polygons and
three‐dimensional (3‐D) polyhedra.[1] Such well-defined
supramolecular complexes have attracted increasing interests
because of their wide applications in catalysis, sensing, drug
delivery, etc.[2] Especially, fluorescent SCCs can be used in the field
of chemical sensors, optical devices, supramolecular biomedicines,
and bioimaging.[3] For example, in 2018, Cook et al. reported two
kinds of highly emissive bodipy-platinum supramolecular triangles,
which can be employed for cell imaging and in vitro anticancer
treatment due to their high fluorescence quantum yields and
excellent singlet oxygen generation ability.[4] Also, in 2015, our
group reported
a
supramolecular hexagon based on
tetraphenylethylene (TPE), which can be employed to detect
heparin sodium through electrostatic interactions.[5]
Although there have been some successful examples in
construction of fluorescent metallacycles or metallacages, in
general, SCCs are usually non-fluorescent or weak-fluorescent[6]
because the excitation of the fluorophore is either delocalized up
to the metal atom or transferred to the triplet state via an
ultrafast intersystem crossing (ISC) when the fluorophore is
directly coordinated to the metal.[7] Thus, the development of
highly emissive SCCs, especially in situ preparation of highly
emissive SCCs by external stimuli, is still challenging. As the
cleanest energy, light is always chosen for the development of
Results and Discussion
Synthesis and characterization of metallacycles
The 120° diarylethene-containing dipyridine donors P1o and
P2o were synthesized by employing Suzuki-Miyaura coupling
reaction
from
1,2-bis(2-ethyl-6-iodo-1-benzothiophen-1,1-dioxide-3-yl)perfluoro
cyclopentene. UV irradiation of P1o and P2o led to the formation
of the closed-ring isomers P1c and P2c, respectively. According to
the principle of “directional bonding” and “symmetry
interaction”,[14] the 120° donor ligand P1o or P2o was adopted to
interact with di-Pt acceptor ligand A to afford the hexagonal
metallacycle M1o or M2o (Scheme 1). By stirring a mixture of
acceptor A and donor P1o or P2o in an exact stoichiometric ratio
(1 : 1) in CH2Cl2 at room temperature, the hexagonal metallacycle
M1o or M2o was readily obtained without further purification.
Metallacycle M1c or M2c was prepared from the closed-ring
isomer P1c or P2c through the similar strategy.
stimuli-responsive
materials.[8]
However,
the
common
photochromic systems such as diarylethenes usually show
“turn-off” fluorescence switching.[9] It means that initially the
open-ring isomers are fluorescent, but they convert to
non-fluorescent or weak-fluorescent state under ultraviolet
irradiation. This “turn-off” fluorescence switching can’t be applied
to super-resolution fluorescence imaging such as PALM
(photoactivatable localization microscopy) or STORM (stochastic
optical reconstruction microscopy) that initially needs dark
background.[10] In 2011, Irie et al. reported a new family of
photochromic
molecules
bearing
The sharp signals of multinuclear 1H and 31P NMR analysis of
1,2-bis(2-ethyl-6-iodo-1-benzothiophen-1,1-dioxide-3-yl)perfluoro
Scheme 1 Graphical representation of self-assembled dithienylethene metallacycles and their structural transformations.
a Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
School of Chemistry and Molecular Engineering, East China Normal University,
3663 North Zhongshan Road, Shanghai 200062, China
c Department of Chemistry, University of South Florida, Tampa, Florida 33620,
United States
E-mail: hbyang@chem.ecnu.edu.cn; Tel.: 0086‐021‐62235137; Fax: 0086‐
021‐62235137
b College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
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article as doi: 10.1002/cjoc.201800577

1
Figure 1 (a) The partial H NMR spectra (500 MHz, 298 K) of ligand A, P1o, P1c and their corresponding self_assembled metallacycles M1o and M1c in
acetone-d6. (b) The 31P NMR spectra (202 MHz, 298 K) of acceptor A, and their corresponding self_assembled metallacycles M1o and M1c in acetoned-d6.
(c) ESI-TOF-MS spectrum of assembly M1o and M2o. Theoretical (top) and experimental (bottom) isotopic distribution.
the reaction products indicated the formation of discrete
metallacycles with high symmetry and ruled out the formation of
oligomers. For example, in the 1H NMR spectrum of M1o (Figure
1a), the signals corresponding to the pyridine protons of P1o
shifted towards downfield by ca. 0.40 and 0.52 (for H1 and H2,
respectively) due to the electron back‐donation from the platinum
atoms. Other signals of protons also displayed a small change. The
31P NMR spectra of M1o (Figure 1b) and M2o (Figure S1) exhibited
the similar peaks (ca. 14.32 ppm for M1o; ca. 14.36 ppm for M2o)
that were shifted upfield by ca. 5.15 and 5.11 ppm, respectively,
relative to the di-Pt acceptors A. In addition, the decrease in the
coupling of the flanking 195Pt satellites (ΔJ ≈ - 72.72 Hz for M1o
and M2o) gave the more evidence for the generation of Pt-N
bonds. The closed-ring isomers M1c and M2c exhibited the similar
characteristics with the open-ring isomers M1o and M2o.
Moreover, the formation of such metallacycles was further
confirmed by electrospray ionization time of flight mass
spectrometry (ESI-TOF-MS) spectrum. As showed in Figure 1c, the
ESI-TOF-MS spectrum showed two peaks at m/z = 1175.40 and
1182.36, corresponding to the [M1o – 5OTf]5+ and [M2o – 5OTf]5+,
respectively. Further investigations into the isotopic distributions
revealed their high resolutions, which were in good agreement
with the theoretical distributions. The ESI-TOF-MS spectrum of
the closed-ring isomers M1c and M2c (Figure S3) also showed the
similar results. All collective data in NMR and ESI-TOF-MS spectra
strongly supported the formation of multi-diarylethene hexagons.
at ca. 350 nm. An obvious color change from colorless to yellow
was also observed (Figure 3a), which meant that the open-ring
isomer P1o transformed into the closed-ring isomer P1c triggered
by the typical electrocyclic reaction. Moreover, excitation of the
band by irradiation at 450 nm exhibited the brilliant fluorescence
at ca. 537 nm (Figure S4b), whereas the starting compound P1o
showed no fluorescence because it had no absorption in the
visible region. This phenomenon of “turn-on” fluorescence is
different from the most common photochromic systems. This
process from non-fluorescent state to fluorescent state can also
be clearly seen by the naked eyes (Figure 3b). P2o exhibited the
similar photochromic properties to P1o. P2o displayed a main
absorption band at ca. 387 nm (Figure S5a), which featured a 37
nm red shift in wavelength relative to M1o due to the thiophene
groups. The irradiation at 365 nm led to the decrease in intensity
of absorption band at ca. 387 nm accompanied by the emergence
of a new absorption band at ca. 508 nm. Color change from
colorless to red was obvious (Figure 3a). Meanwhile, the
closed-ring isomer P2c exhibited bright orange fluorescence at ca.
596 nm upon excitation at 500 nm (Figure S5b). The metallacycles
M1o and M2o featured very similar photochromic properties
relative to the ligands P1o and P2o. For example, when M1o was
irradiated at 365 nm in CH2Cl2, an absorption band at ca. 460 nm
appeared accompanied by the decrease of intensity at ca. 350 nm
(Figure 2a). Excitation of the band by irradiation at 450 nm
exhibited the brilliant fluorescence at ca. 536 nm (Figure 2b). It
should be noted that the formation of Pt-N coordination bonds in
Photochromic studies
metallacycles had
a negligible influence in absorption and
With the ligands and metallacycles in hand, we first
investigated their photochromic properties. The absorption
spectrum of P1o (Figure S4a) exhibited a main absorption band at
ca. 350 nm. Upon irradiation at 365 nm in CH2Cl2, an intense band
at ca. 462 nm appeared accompanied by the decrease of intensity
emission of diarylethene moiety.
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Figure 2 (a) Absorption spectral changes of M1o (10-5 M in CH2Cl2) upon
UV irradiation at 365 nm. (b) Emission spectral changes of M1o (10-5 M in
CH2Cl2, Ex = 450 nm) upon UV irradiation at 365 nm. (c) Absorption
spectral changes of M2o (10-5 M in CH2Cl2) upon UV irradiation at 365 nm.
(d) Emission spectral changes of M2o (10-5 M in CH2Cl2, Ex = 500 nm) upon
UV irradiation at 365 nm.
Figure 3 (a) Photographs of ligands and metallacycles (10-5 M in CH2Cl2)
under room light. (b) Photographs of ligands and metallacycles (10-5 M in
CH2Cl2) under UV light.
After confirming their photochromic properties, the
photophysical properties of the closed-ring isomers of ligands and
metallacycles were then investigated (Table 1). The fluorescence
quantum yields of P1c, M1c, P2c and M2c were determined to be
0.56, 0.50, 0.46 and 0.42, respectively. The fluorescence lifetime
of P1c, M1c, P2c and M2c were determined to be 2.6 ns, 2.3 ns,
1.9 ns and 1.7 ns, respectively. Obviously, the formation of the
metallacycles had negligible effect on the fluorescence of the
diarylethene moiety and the metallacycles featured the relatively
high quantum yield. This finding was of significance because Pt-N
coordination in metallacycles usually quenched the fluorescence
of chromophore due to the formation of triplet state (owing to
that the chromophore has a strong spin-orbit coupling with the
metal atom) via an ultrafast intersystem crossing (ISC), which then
led to the nonradiative decay to the ground state. Moreover, the
“turn-on” fluorescence switching by light stimulus meant that it
could be controlled by light. Figure 3 shows the photographs of
color change and fluorescence change of various species before
and after irradiation at 365 nm. Relative to their non-fluorescent
ring-open isomer M1o or M2o, the brilliant yellow or orange
fluorescence of M1c or M2c clearly showed the successful
construction of highly emissive Pt(II) metallacycles upon
irradiation, which might be potentially applied in sensing and
bioimaging.
Moreover, we monitored such light induced transformation
process by using 1H NMR and 31P NMR spectra. Typically, the
ligand P1o or P2o had two sets of signals with a stoichiometric
ratio about 3:2 due to the existence of two different
conformations, antiparallel and parallel.[15] Upon UV irradiation,
the two sets of signals transformed gradually into one single set of
signals (Figure S6 and S7), which indicated the formation of
closed-ring isomer P1c or P2c. Similar NMR spectra changes were
found in metallacycles. For example, in the NMR spectra of M1o
(Figure S8), two sets of signals corresponding to the protons H7 on
the benzothiophene rings were located at 8.22 ppm and 8.33 ppm,
respectively. Upon irradiation at 365 nm, these peaks gradually
disappeared companied by the appearance of a new peak at 8.49
ppm due to the formation of closed-ring isomer M1c. In addition,
the signals corresponding to the protons H8 and H9 on the ethyl
groups displayed a dramatic change. All of these changes in NMR
spectra were resulted from the variation of aromaticity of
benzothiophene heterocycles due to the photocyclization
reaction.[16] Moreover, 31P NMR spectra of M1o exhibited a
negligible change upon UV irradiation (Figure S9). 1H and 31P NMR
investigations into the structural transformation process of M2o
were also performed and the similar results were obtained (Figure
S10 and S11). Notably, at the photostationary state (PSS), the
conversion yields of M1o and M2o were about 90% in spite of the
quantitative conversion yields of ligand P1o and P2o.
Table 1 Photophysical properties of the closed-ring isomers of ligands
and metallacycles.
compound
P1c
λmax/nma
λmax/nmb
537
ΦF
τF/ns
2.6
462
0.56
0.50
0.46
0.42
M1c
460
536
2.3
P2c
508
596
1.9
M2c
505
589
1.7
a Absorption maximun; b Emission maximum.
In order to obtain the further insight into structure
characteristics of metallacycles, DFT stimulations were conducted.
In general, diarylethene group has two different configurations,
antiparallel and parallel, and only the antiparallel configuration is
photo-active. As shown in Figure 4, the DFT calculations revealed
that the angle between the two pyridines of P1o-antiparallel was
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for 2.0 h to yield a homogeneous solution. The solution was then
1
transferred into the NMR tube to collect H and 31P NMR spectra.
The product was obtained by removing the solvent under vacuum.
Yield: 10.19 mg, > 99%. 1H NMR (acetone-d6, 500 MHz): δ (ppm) =
9.05 – 9.10 (m, 4H), 8.13 – 8.35 (m, 14H), 8.01 – 8.02 (m, 2H), 7.86
– 7.89 (m, 1H), 7.72 – 7.80 (m, 5H), 7.59 – 7.61 (m, 4H), 2.61 –
2.84 (m, 4H), 1.53 – 1.54 (m, 24H), 1.44 – 1.47 (m, 2.78H), 1.18 –
1.28 (m, 36H), 1.10 – 1.12 (m, 3.67H). 31P NMR (acetone-d6, 202
MHz): δ (ppm) = 14.32 (1JPt-P = 1321.08 Hz). MS (ESI-MS): m/z
calcd for [M – 5OTf]5+: 1175.30, found: 1175.39.
Preparation of hexagon M1c. The dipyridyl donor ligand P1c
(3.94 mg, 4.54 μmol) and the diplatinum 120° acceptor A (6.10
mg, 4.54 μmol) were weighed accurately into a glass vial away
from light. To the vial was added 0.5 mL acetone-d6, and the
reaction solution was then stirred in the dark place at room
temperature for 2.0 h to yield a homogeneous solution. The
solution was then transferred into the NMR tube to collect 1H and
31P NMR spectra. The product was obtained by removing the
solvent under vacuum. Yield: 10.04 mg, >99%. 1H NMR
(acetone-d6, 500 MHz): δ (ppm) = 9.13 (m, 4H), 8.54 – 8.56 (m,
4H), 8.47 (d, 2H, J = 7.6 Hz), 8.24 – 8.33 (m, 12H), 7.77 (d, 4H, J =
7.7 Hz), 7.62 (d, 4H, J = 7.6 Hz), 3.13 – 3.18 (m, 2H), 2.44 – 2.48 (m,
2H), 1.56 – 1.57 (m, 24H), 1.21 – 1.27 (m, 36H), 0.70 – 0.73 (m,
Figure 4 Simulated molecular models of parallel and anti-parallel
configurations of P1o, P2o and their corresponding closed-ring isomers
P1c, P2c obtained by DFT calculations. The values in parentheses are the
angles between the two pyridyl rings.
closed to the standard 120°, which was more beneficial to form
the molecular hexagons according to the principle of “directional
bonding” and “symmetry interaction”. Notably, due to the flexible
structure of diarylethene, the antiparallel configuration of P2o can
also form a molecular hexagon although the angle between the
two pyridines of P2o-antiparallel was smaller than 120°. The
further calculations disclosed that the angles between the two
pyridines of the closed-ring isomers P1c and P2c were also closed
to 120°. Moreover, we calculated the energies of antiparallel and
parallel configurations of P1o and P2o (Table S1), which revealed
that the antiparallel configurations were more stable than their
analogues with parallel configurations (e.g. in the case of P1o,
Eantiparellel – Eparallel = - 0.31 kJ/mol). Additionally, the optimal
molecular models of metallacycles were achieved as well by PM6
semi-empirical molecular orbital method (Figure S12 and S13). In
a word, DFT simulations demonstrated that the antiparallel
configuration of the diarylethene ligand had an angle closed to
120°, which was more stable in energy compared to the parallel
configuration, thus allowing for the facile construction of highly
emissive metallacycles in this study.
6H). 31P NMR (acetone-d6, 202 MHz): δ (ppm) = 14.32 (1JPt-P
=
1329.16 Hz). MS (ESI-MS): m/z calcd for [M – 5OTf]5+: 1175.30,
found: 1175.39.
Preparation of hexagon M2o. The dipyridyl donor ligand P2o
(3.96 mg, 4.50 μmol) and the diplatinum 120° acceptor A (6.04
mg, 4.50 μmol) were weighed accurately into a glass vial away
from light. To the vial was added 0.5 mL acetone-d6, and the
reaction solution was then stirred in the dark place at room
temperature for 2.0 h to yield a homogeneous solution. The
solution was then transferred into the NMR tube to collect 1H and
31P NMR spectra. The product was obtained by removing the
1
solvent under vacuum. Yield: 10 mg, >99%. H NMR (acetone-d6,
500 MHz): δ (ppm) = 8.98 – 9.02 (m, 4H), 8.37 (s, 1H), 8.02 – 8.24
(m, 10H), 7.59 – 7.91 (m, 11H), 2.59 – 2.63 (m, 4H), 1.54 (m, 24H),
1.43 – 1.46 (m, 2.72H), 1.21 (m, 36H), 1.10 – 1.12 (m, 3.56H). 31
P
Conclusions
NMR (acetone-d6, 202 MHz): δ (ppm) = 14.36 (1JPt-P = 1321.08 Hz).
MS (ESI-MS): m/z calcd for [M – 5OTf]5+: 1182.45, found: 1182.36.
Preparation of hexagon M2c. The dipyridyl donor ligand P2c
(4.30 mg, 4.89 μmol) and the diplatinum 120° acceptor A (6.56
mg, 4.89 μmol) were weighed accurately into a glass vial away
from light. To the vial was added 0.5 mL acetone-d6, and the
reaction solution was then stirred in the dark place at room
temperature for 2.0 h to yield a homogeneous solution. The
solution was then transferred into the NMR tube to collect 1H and
31P NMR spectra. The product was obtained by removing the
solvent under vacuum. Yield: 10.86 mg, >99%. 1H NMR
(acetone-d6, 500 MHz): δ (ppm) = 9.06 (m, 4H), 8.39 – 8.51 (m,
6H), 8.21 – 8.23 (m, 8H), 7.60 – 7.73 (m, 8H), 3.12 – 3.13 (m, 2H),
2.43 – 2.45 (m, 2H), 1.56 (m, 24H), 1.22 – 1.24 (m, 36H), 0.70 (m,
In summary, we have successfully in situ constructed two
highly emissive Pt(II) metallacycles upon irradiation. These two
supramolecular metallacycles with photochromism property were
obtained via coordination-driven self-assembly. Owing to the
photoswitchable property of diarylethene ligands, the
metallacycles can transform from the open-ring isomers to their
closed-ring isomers with high conversion yields. Moreover, the
closed-ring isomers featured
a relatively high fluorescence
quantum yield. This “turn-on” fluorescence switching is obviously
different from the common photochromic systems, in which the
fluorescence is usually quenched upon UV irradiation. Moreover,
metal coordination had no obvious influence on the emission of
ligands in this study, thus allowing for the construction of very
unusual highly emissive metallacycles. The investigations on the
future application of these metallacycles like bioimaging are
underway in our lab.
6H). 31P NMR (acetone-d6, 202 MHz): δ (ppm) = 14.36 (1JPt-P
=
1321.08 Hz). MS (ESI-MS): m/z calcd for [M – 5OTf]5+: 1182.45,
found: 1182.34.
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Experimental
All reagents were of analytical purity and used without further
purification. P1o and P2o were synthesized as described in the
supporting information. Metallacycles M1o and M2o were
dissolved in acetone-d6 for 1H and 31P NMR analysis.
Acknowledgement (optional)
Preparation of hexagon M1o. The dipyridyl donor ligand P1o
(4.0 mg, 4.61 μmol) and the diplatinum 120° acceptor A (6.19 mg,
4.61 μmol) were weighed accurately into a glass vial away from
light. To the vial was added 0.5 mL acetone-d6, and the reaction
solution was then stirred in the dark place at room temperature
H.‐B. Y. thanks the financial support provided by STCSM (No.
16XD1401000) and the Program for Changjiang Scholars and
Innovative Research Team in University.
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(The following will be filled in by the editorial staff)
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Revised manuscript received: XXXX, 2017
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Entry for the Table of Contents
Page No.
Construction of Highly Emissive Pt(II)
Metallacycles upon Irradiation
Yi Qin,a Ying Zhang,b Guangqiang Yin,a
Yuxuan Wang,a Changwei Zhang,a Lijun
Chen,a Hongwei Tan,b Xiaopeng Li,c Lin
Xu,*,a and Haibo Yang*,a
Upon UV irradiation, the metallacycles can transform from the open-ring isomers to their
closed-ring isomers. Moreover, the closed-ring isomers featured a relatively high fluorescence
quantum yield. This property of “turn-on” fluorescence switching will be very helpful in
construction of new supramolecular fluorescent materials for sensing and bioimaging.
This article is protected by copyright. All rights reserved.