Diels–Alder Polymer Networks with Temperature-Reversible Cross-Linking-Induced Emission

Article abstract of DOI:10.1002/anie.202013183

A novel synthetic strategy gives reversible cross-linked polymeric materials with tunable fluorescence properties. Dimaleimide-substituted tetraphenylethene (TPE-2MI), which is non-emissive owing to the photo-induced electron transfer (PET) between maleimide (MI) and tetraphenylethene (TPE) groups, was used to cross-link random copolymers of methyl (MM), decyl (DM) or lauryl (LM) methacrylate with furfuryl methacrylate (FM). The mixture of copolymer and TPE-2MI in DMF showed reversible fluorescence with “on/off” behavior depending on the Diels–Alder (DA)/retro-DA process, which is easily adjusted by temperature. At high temperatures, the retro-DA reaction is dominant, and the fluorescence is quenched by the photo-induced electron transfer (PET) mechanism. In contrast, at low temperatures, the emission recovers as the DA reaction takes over. A transparent PMFM/TPE-2MI polymer film was prepared which shows an accurate response to the external temperature and exhibited tunable fluorescent “turn on/off” behavior. These results suggest the possible application in areas including information security and transmission. An example of invisible/visible writing is given.

Full text of DOI:10.1002/anie.202013183

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Diels-Alder Polymer Networks with Temperature-Reversible
Cross-Linking-Induced Emission Yu Jiang*, and Nikos
Hadjichristidis*
Authors: Nikos Hadjichristidis and Yu Jiang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202013183
Link to VoR: https://doi.org/10.1002/anie.202013183

10.1002/anie.202013183
Angewandte Chemie International Edition
RESEARCH ARTICLE
Diels-Alder Polymer Networks with Temperature-Reversible
Cross-Linking-Induced Emission
Yu Jiang*, and Nikos Hadjichristidis*
Abstract: A novel synthetic strategy toward reversible cross-linked
polymeric materials with tunable fluorescence properties is presented.
Dimaleimide-substituted tetraphenylethene (TPE-2MI), which is non-
emissive due to the photo-induced electron transfer (PET) between
maleimide (MI) and tetraphenylethene (TPE) groups, was used to
cross-link random copolymers of methyl (MM), decyl (DM) or lauryl
(LM) with furfuryl methacrylate (FM). The mixture of copolymer and
TPE-2MI in DMF showed reversible fluorescence with “on/off”
behavior depending on the Diels-Alder (DA)/retro-DA process, which
is easily adjusted by temperature. At high temperatures, the retro-DA
reaction of polymer networks is dominant, and the fluorescence is
quenched by the PET mechanism. In contrast, at low temperatures,
the emission recovers as the DA reaction takes over. Based on these
results, a transparent PMFM/TPE-2MI film was prepared, and the
emission behavior was investigated. It was found that the polymer film
shows an accurate response to the external temperature and
exhibited tunable fluorescent “turn on/off” behavior. These exciting
results suggest the possible application of this type of reversible
cross-linked materials in many areas, including information security
and transmission. An example of invisible/visible writing is given.
formation or rupture of cross-link bonds. This approach ensures the
tailorable properties of reversibly cross-linked materials, which
behave as thermosets or thermoplastic materials depending on the
formation or fracture of the cross-links.[9] A plethora of efforts has been
made to expand the application of reversibly cross-linked materials in
[10]
many areas,
while there are very few in the field of
reversible/tunable fluorescence.
Luminescent materials have attracted considerable attention in recent
decades, as they have promising applications in luminescent sensing,
information encoding, and biological imaging.[10] The application of
conventional organic luminogens was limited by the aggregation-
caused quenching (ACQ) effect, which means that the fluorescence
of luminogens undergoes complete quenching in the aggregation
state.[11] Recently, Tang’s group has developed a novel aggregation-
induced emission (AIE)-type luminogens, which show the opposite
phenomenon of ACQ emitting strongly in the aggregation state.[12] By
introducing the AIE-gens into the polymer structures, the resulted
polymer possesses the polymer properties as well as the AIE
characteristics.[13] These polymers show many advantages over the
small molecules, such as processability, good thermal stability,
structural diversity, and regulation of AIE behavior.[14] Actually, there
are many studies that have been reported to regulate the fluorescence
Introduction
of AIEgen-containing polymers by changing the aggregation state[15]
,
Covalently cross-linked materials (thermosets) have played an
important role in many areas such as coatings, adhesives, composite
materials, and biomaterials. They have shown outstanding properties
(e.g., modulus, thermostability, etc.) compared to the linear
thermoplastics.[1] However, permanently covalent cross-links also
bring disadvantages for the processing, recycling, and reuse of these
materials.[2] To overcome these barriers, reversible covalent bonds
(dynamic or non-dynamic) were introduced to construct the reversible
cross-linked materials by using a variety of chemistries, such as
disulfide metathesis[3], transesterification[4], transamination[5], Diels-
Alder (DA) reaction[6], and many more.[7] Among these chemistries,
thermal-induced DA reaction is one of the best options due to its
catalyst-free procedure, broad temperature range, and high
tolerance.[8] As a result, the chain topology can be adjusted by the
self-assemble behavior[16], topology[17], etc.[18] However, for cross-
linked polymeric materials, it is difficult to regulate their emission
behavior due to the insoluble and infusible nature. The development
of cross-linked materials with a reversibly covalent bond is helpful to
solve this problem. Ji’s group recently reported that AIE luminogens-
doped vitrimers could exhibit tunable fluorescence depending on the
movement of the network at different temperature.[19] Herein, we wish
to report the preparation of reversibly DA polymer networks in which
the AIEgens are incorporated into the network. The fluorescence
property can be regulated by the cross-linking/decross-linking
process through DA/retro-DA chemistry. Most importantly, the
emission behavior of the resulted materials exhibits an alterable “turn-
on/off” mode.
Photo-induced electron transfer (PET) has been widely utilized as a
mechanism for the development of fluorescent “turn-on” materials.[20]
“Fluorophore-spacer-receptor” format was usually adopted as the
fluorescent PET-based system. For example, the maleimide (MI)
group was introduced into the tetraphenylethene (TPE) molecule, and
the resulting TPE-MI showed no emission in both solution and solid-
state, because the intramolecular PET process between fluorophore
(TPE) and receptor (MI) quench the fluorescence. When the MI group
is bonded by a thiol-ene reaction, the fluorophore (TPE) lights up
because the electron transfer process is over (Scheme 1a).[21]
Compared to the irreversible thio-ene reaction, the reversible DA
reaction between MI and the furan group should result in the tunable
fluorescence of the adduct (Scheme 1b).[22] In the present work, we
Prof. Dr. Y. Jiang,
Key Laboratory of Catalysis and Energy Materials Chemistry of
Ministry of Education & Hubei Key Laboratory of Catalysis and
Materials Science, Hubei R&D Center of Hyperbranched Polymers
Synthesis and Applications, South-Central University for Nationalities,
Wuhan 430074, China.
Prof. Dr. N. Hadjichristidis
Polymer Synthesis Laboratory, KAUST Catalysis Center, Physical
Sciences and Engineering Division, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955, Saudi Arabia.
E-mail: nikolaos.hadjichristidis@kaust.edu.sa
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.202013183
Angewandte Chemie International Edition
RESEARCH ARTICLE
incorporate, for the first time, MI-substituted AIE-gens into the polymer
networks by using dimaleimide-substituted tetraphenylethene (TPE-
2MI) as a cross-linker (Scheme 2). The breakage of PET process by
reaction of TPE-2MI with furan moieties releases the AIE behavior,
and the cross-linked networks restricts the mobility of TPE molecules
to enhance their emission. This phenomenon, of the reversible cross-
linking-induced-emission, has not been reported so far. By using this
method, it is possible to regulate the emission of cross-linked
materials by reversible covalent chemistry. Besides, the relationship
(Figure S1, SI). Then the model reaction between TPE-2MI and
o
furfuryl methacrylate (FM) was performed in DMF at 80 C to check
the reactivity of the reversible DA reaction and the AIE property of DA
adduct TPE-2AFM (Scheme 4). The TPE-2AFM adduct was
characterized by 1H NMR, as shown in Figure S2a (SI). It is well
known that the DA reaction leads to a mixture of two diastereomers
(endo/exo adducts), and both of them are found in our 1H NMR
spectrum. Comparison with that of TPE-2MI (Figure S1a, SI), reveals
that the resonance signal at δ 6.83 belonging to maleimide group (d
and e) disappeared and new signals d’ and e’ (endo adduct), d’’ and
e’’ (exo adduct) between 3.00-3.80 ppm appeared, thus proving the
success of DA reaction. Moreover, the TPE-2AFM undergoes a retro-
DA reaction entirely within 5 min at a high temperature (Scheme 4).
The entire disappearance of signals of TPE-2AFM from 1H NMR
spectrum proved the success of the retro-DA reaction. The obtained
TPE-2MI and furfuryl methacrylate were also confirmed by the 1H
NMR spectrum (Figure S2b, SI). These results show efficient DA and
retro-DA reaction between TPE-2MI and furfuryl methacrylate under
different temperatures.
between emission and cross-linking also indicates
a potential
application for the visualization of the dynamic cross-linking process.
The resulted networks exhibit reversible emission behavior thanks to
the temperature-responsive DA/retro-DA reaction.
(a)
PET
PET
O
O
ν
ν
h
h
SR
N
N
O
O
RSH
irreversible
thiol-ene reaction
O
NH2
N
O
Fluroscence "OFF"
Fluroscence "ON"
O
O
O
1)
O
Zn, TiCl4
O
2) NaOAc, Ac2
O
NH2
(b)
PET
PET
O
N
NH2
O
R
O
TPE-2NH2
TPE-2MI
ν
ν
h
O
O
h
N
N
R
O
O
Scheme 3. Synthesis of TPE-2MI.
reversible
Diels-Alder reaction
O
O
N
O
N
Fluroscence "OFF"
Fluroscence "ON"
O
O
O
O
DMF, 80 o
C
O
+
O
O
DMF, 130 o
C
O
O
O
O
N
N
Scheme 1. (a) Fluorescent “turn on” strategy with irreversible thio-ene reaction
based on photo-induced electron transfer (PET) process; (b) Fluorescent “turn
on/off” strategy with reversible DA reaction in the present work.
O
O
O
TPE-2MI
TPE-2AFM
Scheme 4. The model reaction of TPE-2MI with furfuryl methacrylate.
low temperature
high temperature
TPE-2MI
TPE-2AFM
(b)
(c)
(a)
O
O
O
N
N
TPE-2AFM
after 130 oC
TPE-2MI TPE-2AFM
O
O
O
:
:
:
O
O
N
N
O
O
O
Scheme 2. Synthesis of reversibly cross-linked polymer networks from MI-
functionalized TPE derivative using the DA strategy. Temperature-reversible
cross-linking-induced emission with DA/retro-DA strategy.
Figure 1. (a) Photoluminescence (PL) spectra of TPE-2MI, TPE-2AFM and
retro-DA product in THF/H2O mixtures (80% H2O) at a concentration of 0.1 g/L
(Excitation: 342 nm, Temperature: 25 oC); (b) Photos of TPE-2MI and TPE-
2AFM solid powders taken under 365 nm UV irradiation; (c) Photos of TPE-2MI,
Results and Discussion
TPE-2AFM and retro-DA product in THF/H2O mixtures (80% H2O) at a
As illustrated in Scheme 3, TPE-2MI was synthesized from 4-
aminobenzophenone by the McMurry coupling reaction in the first
step, followed by transformation of the -NH2 to the maleimide group.
The chemical structure of TPE-2MI was confirmed by 1H and 13C NMR
concentration of 0.1 g/L under 365 nm UV irradiation.
This article is protected by copyright. All rights reserved.

10.1002/anie.202013183
Angewandte Chemie International Edition
RESEARCH ARTICLE
a
Mn, NMR, and degree of polymerization (DP) were calculated from 1H NMR
The emission characteristics of TPE-2MI and TPE-2AFM were then
investigated. TPE-2MI was non-emissive in either solution or solid-
state, while the TPE-2AFM exhibited strong emission under UV light
(Figure 1b-c and Figure S5). Furthermore, the AIE behavior of TPE-
2AFM was confirmed by the investigation of its THF/H2O solutions,
and the results were shown in Figure S6 (SI). The addition of H2O into
the THF solution induced the molecular aggregation (confirmed by
dynamic light scattering measurements) and gradually enhanced its
photoluminescence (PL) intensity. The maleimide groups, which act
as potent quenchers of TPE fluorogens via a PET mechanism in TPE-
2MI, are bonded in TPE-2AFM through the DA reaction. This results
in the strong emission of TPE-2AFM. After retro-DA reaction, the
maleimide groups are released, and PET works again to quench
significantly the fluorescence (Figure 1a and 1c). This model study
indicates that the fluorescent “turn on/off” behavior is regulated by the
temperature-responsive DA/retro-DA reaction.
spectra (400 MHz, CDCl3, 25 oC); b Đ (Mw/Mn) was determined by GPC (THF,
25 oC, PS standards).
The Diels-Alder polymer networks were prepared by dissolving the
linear copolymer and cross-linker TPE-2MI (maleimide unit/furan unit
ratio, Imal/fur = 1) in DMF. In principle, the furan and maleimide groups
undergo DA reaction below a specific temperature (usually 80 oC). At
o
higher temperatures (≥130 C), furan-maleimide linkages will cleave
through retro-DA reaction to form the initial linear copolymer chain
materials. To gain an in-depth insight into the cross-linking/uncross-
1
linking process, a series of experiments were performed. Firstly, H
NMR spectroscopy was used to check the DA adduct formation. A
mixture of PM60F40M and TPE-2MI (maleimide unit/furan unit ratio,
Imal/fur = 1) in DMF-d7 was allowed to react at 80 oC, and the 1H NMR
spectra were collected at different time intervals (Figure S4, SI). After
1.5 h, new signals around δ 4.65, 4.95, 5.51 ppm appeared, which are
similar to the chemical shift of signals of the TPE-2AFM spectrum
(Figure 1a). The increase of integration of these peaks indicated the
successful DA reaction. The solution can not be monitored after 5 h
due to its high viscosity. Then the mixture was heated at 140 oC for 5
min, the viscosity decreased, and all the aforementioned signals
disappeared, suggesting the progress of retro-DA reaction. Heat-
induced chemical cross-linking/uncross-linking process was further
confirmed by using differential scanning calorimetry (DSC). DMF
With the successful results of the model study in hand, the Diels-Alder
reversible polymer networks were prepared from TPE-2MI and a
random copolymer of methyl/decyl/lauryl methacrylate and furfuryl
methacrylate (PMFM/PDFM/PLFM). Four random copolymers with
different composition and molecular weight were synthesized by the
reversible addition-fragmentation chain transfer polymerization
(RAFT) with AIBN as initiator and 2-cyano-2-propyl dodecyl
trithiocarbonate as a chain-transfer agent (Scheme 5). The molecular
weight, determined by 1H NMR and the polydispersity index Đ by gel
permeation chromatography (GPC) with polystyrene standards, are
given in Table 1. The calculated composition of the random
copolymers is consistent with the feed ratios of the two monomers
(Figure S3, SI). All GPC traces are monomodal with narrow molecular
weight distributions (Figure S11-12, SI).
o
solution of PM60F40M/TPE-2MI (Imal/fur = 1) was heated to 80 C, and
small volumes of the DMF solution were taken out at different time
intervals for characterization. The cross-linked networks were
obtained after removing the DMF and unreacted small molecular. As
showing in Figure S13 (SI), PM60F40M has a distinct glass transition
temperature (Tg) at 83.5 oC. During the cross-linking process, the Tg
of the cross-linked networks increased over time, which reached its
o
plateau value at 109.5 C after 24 h. This could be ascribed to the
co
x
(
)
(
)
y
O
cross-linking induced constraints of polymer chains. Comparison to
that of linear PM60F40M, the Tg of the formed networks become broad
due to the heterogeneous dynamics. Moreover, the DSC of networks
also showed two endothermic peaks at the temperature around 140
and 160 oC, indicating the retro-DA reaction of endo and exo adduct
(Figure S14, red line, SI). All of these results prove the formation of
cross-linked polymer networks with DA reaction at lower temperatures
and its uncross-linking process at higher temperatures.
O
RAFT agent, AIBN
DMF, 80oC
O
O
O
+
O
O
O
R
R
O
O
MM: R = methyl
DM: R = decyl
LM: R = lauryl
FM
PMFM, PDFM or PLFM
Scheme 5. Synthesis of a random copolymer of methyl methacrylate (MM),
decyl methacrylate (DM) or lauryl methacrylate (LM) with furfuryl methacrylate
(FM) by the reversible addition-fragmentation chain transfer polymerization.
The fluorescence of the polymer networks was then studied. As the
DA reaction proceeds, the viscosity of the DMF solution increased.
The cross-linked polymer was swelling, and the DMF solution was
taken out and diluted to monitored the PL intensity. The unreacted
mixture PM60F40M/TPE-2MI showed no emission due to the PET
effect between maleimide and TPE groups, but as the DA reaction
goes on, it becomes fluorescent. As shown in Figure 2a, the PL
intensity of the solution is gradually enhanced with increasing reaction
time and reaches its highest value after approximately 20 h. It shows
a similar tendency with Tg value, which rises before 20 h and reaches
its highest value after that (Figure 2b). These results indicate that the
PL intensity increases along with the degree of cross-linking. The
PM60F40M/TPE-2MI mixture achieves an equilibrium state around 20
h at 80 oC, and the Tg and PL intensity doesn’t increase anymore after
that. This phenomenon indicates that the dynamic cross-linking
process can be visualized by monitoring the PL intensity.
Table 1. Molecular characteristics of the random copolymers PMFM, PDFM and
PLFM.
DPa
a
Entry
Sample
Mn, NMR
Đb
Co-monomer
FM
24
40
20
10
1
2
3
4
PM62F24
M
10500
13000
18500
27000
62
60
1.09
1.13
1.25
1.45
PM60F40
M
PD60F20
M
60
PL100F10
M
100
This article is protected by copyright. All rights reserved.

10.1002/anie.202013183
Angewandte Chemie International Edition
RESEARCH ARTICLE
weakened rapidly with the rise of temperature due to the change of
PET effect in the TPE-2MI unit. The results of the reversible cycle
experiment are shown in Figure 4. The cross-linked networks
exhibited reversible fluorescent “turn on/off” behavior, which was
easily regulated by adjusting temperature between 80 and 150 °C.
The broad peak around 360-440 nm in the spectrum of cycle 8 is due
to impurities in DMF; see Figure S7 in SI.
(a)
(b)
Figure 3. PL spectra of DMF solution of PMFM/TPE-2MI or PDFM/TPE-2MI
after heating at 80 oC for 20 h (Excitation: 342 nm, Temperature: 25 oC).
(a)
Figure 2. (a) PL spectra of mixture of PM60F40M/TPE-2MI in DMF
(concentration: 0.8 g/L) at different time intervals (Excitation: 342 nm,
Temperature: 25 oC); (b) Tg (blue) of cross-linked networks and PL intensity
(red) at 483 nm versus reaction time at 80 oC.
The effects of the composition of random copolymers and the FM
comonomer (MM or DM) on PL intensity were also investigated. For
this purpose, the other two random copolymers PM62F24M and
PD60F20M were used to form networks with TPE-2MI in DMF at 80 oC
for 24 h. The influence of the furan density along the chain on the PL
intensity can be elucidated by comparing samples 1 and 2 (Table 1).
On one hand, the influence of the presence of FM comonomers on
the PL can be found by comparing samples 1 and 3 (Table 1). In all
cases, the molecular ratio was maintained at Imal/fur = 1. Besides, all
results were normalized to correspond to the same DP. It is clear that
by increasing the furan group density, the cross-linking density is
increased and, consequently, the PL intensity (Figure 3). On the other
hand, the PD60F20M/TPE-2MI reaches its equilibrium faster than
PM62F24M/TPE-2MI (Figure S8, SI), which indicates a lower cross-
linking density due to the long pendant decyl group. As a result, it
shows lower PL intensity compared to that of the PM62F24M/TPE-2MI
(Figure 3).
(b)
The temperature-responsive DA/retro-DA reaction has played a
crucial role in the emission of cross-linked networks. For the cross-
linked polymers, at a lower temperature (≤ 80 oC), the maleimide
group of TPE-2MI reacts with the furan group, and the PET effect
disappeared. Once the temperature is above 130 oC, the retro-DA
reaction is dominant. The released maleimide groups make
fluorescence to be quenched again. For instance, the emission of the
DMF solution of PM60F40M/TPE-2MI reached its highest intensity after
20 h at 80 oC, and then disappeared in 10 min when the temperature
was further increased to 150 oC. The fluorescent intensity was
Figure 4. PL spectra (a) and peak intensity at 483 nm (b) of PM60F40M/TPE-2MI
networks in DMF solution (concentration: 0.8 g/L) for several cycles by adjusting
temperature between 80 oC (4 h) and 150 oC (10 min). (Excitation: 342 nm,
Temperature: 25 oC)
To further expand the potential application of this strategy, the
construction of reversibly cross-linked networks in solvent-free
This article is protected by copyright. All rights reserved.

10.1002/anie.202013183
Angewandte Chemie International Edition
RESEARCH ARTICLE
conditions was investigated. PM60F40M and TPE-2MI (Imal/fur = 1) were
dissolved in dichloromethane, followed by immediate removal of the
solvent under vacuum and curing at room temperature. The Tg of the
bulk PM60F40M/TPE-2MI increased over time and reached a plateau
value of around 95.2 oC after 1 week (Figure S14, blue line, SI).
Comparing to that of the cross-linking process in DMF at 80 oC, the Tg
value was lower, indicating the lower degree of cross-linking. And only
incorporated in the network. In other words, the formed networks can
sufficiently restrict the rotation of the TPE molecules, no matter
whether the cross-linked polymers are vitrified or not. As
a
comparison, the cross-linking density, which can directly influence the
stiffness of the network, has more significant influence on the rotation
of TPE molecules, thus influence the fluorescence property (as the
comparison between PD60F20M/TPE-2MI and PM62F24M/TPE-2MI). It
can be concluded that the increase of the PL intensity during the
cross-linking process depends on the following two factors: a) the
o
one endothermic peak at a temperature around 140 C was found,
indicating that only endo adduct was formed at room temperature. The
Diels-Alder reaction rate can also be accelerated by increasing the
temperature in solid-state, and the ratio of the exo adduct will increase
along with the increasing temperature. The fluorescence of bulk
PM60F40M/TPE-2MI mixture also showed reversible responsiveness
to different temperatures. As shown in Figure S19 (SI), the
PM60F40M/TPE-2MI mixture in a flask shows no emission under UV
light at 0 h (immediately after solvent removal). It emits a strong
fluorescence after few hours illustrating the formation of polymer
networks. Then the lower half of the flask was put into an oil bath at
continuous DA reaction, which breaks the PET process between TPE
and MI groups, and b) the restricted mobility of the chains due to the
formation of the network, which greatly restricts the intramolecular
rotation of TPE.
(a)
140 o
rt
C
o
140 C for 10 mins, and an interesting phenomenon was observed.
PM60F
40M/TPE-2MI film
The sample at the lower half area of the flask showed non-emission,
while the sample at the upper half area was strongly emitting. This
phenomenon indicates that the emission of cross-linked polymeric
materials has a fast and accurate response to high temperatures.
(b)
rt
140 o
C
Due to their high Tg, both cross-linked polymers PMFM/TPE-2MI (Tg
= 109 oC) and PDFM/TPE-2MI (Tg= 69 oC) are vitrified at room
temperature. Therefore, it is necessary to investigate if their emission
is caused by vitrification, as in some cases[23]. For this purpose, a
network with low Tg (-55 oC, Figure S16, SI) and low cross-linking
density was prepared from a flexible linear copolymer PL100F10M
(Table 1) and TPE-2MI. The bulk PL100F10M/TPE-2MI mixtures (Imal/fur
= 1 or 0.5) become insoluble in DMF at room temperature, which
indicates the successful cross-linking process (Figure S18, SI). The
PM60F40M film with
TPE-2MI ink
(c)
o
endothermic peak at the temperature around 110 C in DSC traces
also suggested the retro-DA reaction of the endo adduct (Figure S16,
SI). As shown in Figure S16, the Tg of the bulk PL100F10M/TPE-2MI
has no obvious change during the cross-linking process, probably due
to the long lauryl chain and low cross-linking density (Figure S16).
These low Tg values indicate that the obtained polymer networks are
not vitrified at room temperature. The fluorescence property of the
bulk PL100F10M/TPE-2MI was also monitored. As shown in Figure S9,
the PL intensity increased gradually as the cross-linking proceeded,
despite the networks were not vitrified. These results indicate that the
emission of the networks is caused by cross-linking instead of
vitrification. In order to further support this conclusion, another TPE-
doped cross-linked polymer was prepared by mixing PL100F10M, 1,1‘-
(methylenedi-4,1-phenylene)bismaleimide (BMI) and TPE in
dichloromethane (Imal/fur = 0.5), followed by immediate removal of the
solvent under vacuum and curing at room temperature. BMI, one of
the most commonly used linker in DA cross-linked polymers, was
used to cross-link linear copolymer PL100F10M to form the polymer
networks with low Tg (Figure S17, SI). However, this bulk
PL100F10M/BMI/TPE mixture was non-emissive either before or after
cross-linking (Figure S10, SI). The movement of the polymer chains
greatly increases the freedom for TPE and therefore its rotation
intramolecularly. While for TPE-2MI cross-linked polymers, the TPE
molecules are integrated into the rigid network, which restrict the
intramolecular rotation of TPE molecules. Although the networks are
not vitrified at room temperature, the movement of the long lauryl
chain can rarely influence the mobility of TPE molecules which are
Figure 5. (a) Photos show the fluorescent “turn on/off” cycle under 365 nm UV
light; (b) Photos show the “writing-erasing” cycle under 365 nm UV light; (c) PL
spectra and peak intensity at 483 nm (inset) of PM60F40M/TPE-2MI mixture film
for several cycles by adjusting temperature between rt (24 h) and 140 oC (10
min).
We further performed fluorescence turn “on/off” experiments to show
the potential applications of this strategy, such as temporary
information transmission. A transparent PM60F40M/TPE-2MI film was
then prepared by the spin coating process on a glass substrate, and
its reversible emission performance was studied (Figure 5c). The
emission of polymer film exhibited “turn on/off” behavior depending on
the environment temperature (Figure 5a), and the process can be
repeated a few times. Furthermore, the TPE-2MI solution was further
used as an ink to write on the PM60F40M coated surface. As illustrated
in Figure 5b, the “AIE” letters could be written on the PM60F40M film,
which is invisible under UV light at the beginning, but they appear after
few hours as the networks formed. The emission intensity became
This article is protected by copyright. All rights reserved.

10.1002/anie.202013183
Angewandte Chemie International Edition
RESEARCH ARTICLE
[5]
a) Y. Spiesschaert, M. Guerre, L. Imbernon, J. M. Winne, F. Du Prez,
Polymer 2019, 172, 239-246; b) H. Liu, H. Zhang, H. Wang, X.
Huang, G. Huang, J. Wu, Chem. Eng. J. 2019, 368, 61-70; c) W.
Denissen, M. Droesbeke, R. Nicolaÿ, L. Leibler, J. M. Winne, F. E.
Du Prez, Nat. Commun. 2017, 8, 1-7.
a) S. S. Patil, A. Torris, P. P. Wadgaonkar, J. Polym. Sci. Part A:
Polym. Chem. 2017, 55, 2700-2712; b) K. Jin, S.-s. Kim, J. Xu, F. S.
Bates, C. J. Ellison, ACS Macro Lett. 2018, 7, 1339-1345.
a) C. A. Tretbar, J. A. Neal, Z. Guan, J. Am. Chem. Soc. 2019, 141,
16595-16599; b) R. L. Snyder, D. J. Fortman, G. X. De Hoe, M. A.
Hillmyer, W. R. Dichtel, Macromolecules 2018, 51, 389-397; c) M.
M. Obadia, A. Jourdain, P. Cassagnau, D. Montarnal, E.
Drockenmuller, Adv. Funct. Mater. 2017, 27, 1703258; d) M. Röttger,
T. Domenech, R. van der Weegen, A. Breuillac, R. Nicolaÿ, L.
Leibler, Science 2017, 356, 62-65.
a) K. K. Oehlenschlaeger, J. O. Mueller, J. Brandt, S. Hilf, A. Lederer,
M. Wilhelm, R. Graf, M. L. Coote, F. G. Schmidt, C. Barner‐
Kowollik, Adv. Mater. 2014, 26, 3561-3566; b) C. García-Astrain, A.
Gandini, C. Peña, I. Algar, A. Eceiza, M. Corcuera, N. Gabilondo,
RSC Adv. 2014, 4, 35578-35587.
C. J. Kloxin, C. N. Bowman, Chem. Soc. Rev. 2013, 42, 7161-7173.
a) A. Schmidt, M. Anderson, N. R. Armstrong, J. Appl. Phys. 1995,
78, 5619-5625; b) T. Jüstel, H. Nikol, C. Ronda, Angew. Chem.-Int.
Ed. 1998, 37, 3084-3103; c) J.-C. G. Bünzli, Chem. Rev. 2010, 110,
2729-2755.
stronger over time. Moreover, the “AIE” letters could be rapidly erased
in 10 mins by increasing the environment temperature to 140 oC, and
it can be recovered in a few hours if the high-temperature environment
was removed.
[6]
[7]
Conclusion
In summary, we have synthesized a non-emitting TPE-2MI that can
be used as a cross-linker for furan-based random copolymers to
construct polymer networks through Diels-Alder strategy. The PL
intensity of the polymer networks increases with the cross-linking
density, which indicates that this strategy can be used to visualize the
dynamic cross-linking process. The resulted networks show strong
emission due to the break of the PET process between TPE and MI
groups, as well as the restricted intramolecular rotation of the TPE
molecules. The PL intensity of the networks can be enhanced by
increasing the density of the furan unit and shortening the pendant
chain of the linear copolymer because both parameters are increasing
the cross-linking density. Moreover, a novel reversible covalently
cross-linked polymeric material with tunable fluorescence was
obtained. The polymer networks in DMF shows tunable fluorescence
property depending on the cross-linking/uncross-linking process. With
the increase of temperature (≥ 130 oC), the formed polymer networks
[8]
[9]
[10]
[11]
[12]
T. Förster, K. Kasper, Z. Phys. Chem. (Muenchen, Ger.) 1955, 59,
976-980.
a) J. Luo, Z. Xie, J. W. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok,
X. Zhan, Y. Liu, D. Zhu, Chem. Commun. 2001, 1740-1741; b) Y.
Hong, J. W. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40, 5361-
5388; c) J. Mei, N. L. Leung, R. T. Kwok, J. W. Lam, B. Z. Tang,
Chem. Rev. 2015, 115, 11718-11940.
[13]
R. Hu, N. L. Leung, B. Z. Tang, Chem. Soc. Rev. 2014, 43, 4494-
4562.
[14]
[15]
R. Hu, Y. Kang, B. Z. Tang, Polym. J. 2016, 48, 359-370.
S. K. Saha, K. R. Ghosh, J. P. Gao, Z. Y. Wang, Macromol. Rapid
Commun. 2014, 35, 1592-1597.
undergo retro-DA reaction, and the fluorescence is quenched. On the
contrary, the emission is recovered at low temperature (≤ 80 oC) due
[16]
a) Y. Jiang, N. Hadjichristidis, Macromolecules 2019, 52, 1955-
1964; b) C.-T. Lai, R.-H. Chien, S.-W. Kuo, J.-L. Hong,
Macromolecules 2011, 44, 6546-6556.
L. Fang, C. Huang, G. Shabir, J. Liang, Z. Liu, H. Zhang, ACS Macro
Lett. 2019, 8, 1605-1610.
a) P. Li, D. Zhang, Y. Zhang, W. Lu, J. Zhang, W. Wang, Q. He, P.
Theato, T. Chen, ACS Macro Lett. 2019, 8, 937-942; b) C.-A. Chou,
R.-H. Chien, C.-T. Lai, J.-L. Hong, Chem. Phys. Lett. 2010, 501, 80-
86.
Y. Yang, S. Zhang, X. Zhang, L. Gao, Y. Wei, Y. Ji, Nat. Commun.
2019, 10, 3165.
a) D. Wu, A. C. Sedgwick, T. Gunnlaugsson, E. U. Akkaya, J. Yoon,
T. D. James, Chem. Soc. Rev. 2017, 46, 7105-7123; b) B. Daly, J.
Ling, A. P. De Silva, Chem. Soc. Rev. 2015, 44, 4203-4211.
Y. Liu, Y. Yu, J. W. Lam, Y. Hong, M. Faisal, W. Z. Yuan, B. Z. Tang,
Chem.-Eur. J. 2010, 16, 8433-8438.
F. Li, X. Li, X. Zhang, Org. Biomol. Chem 2018, 16, 7871-7877.
S. Bao, Q. Wu, W. Qin, Q. Yu, J. Wang, G. Liang, B. Z. Tang, Polym.
Chem. 2015, 6, 3537-3542.
to the DA reaction and suspension of the PET mechanism. This
behavior with “turn on/off” mode can be easily regulated by
temperature. Furthermore, a transparent PMFM/TPE-2MI film was
prepared, and its emission also is shown “turn on/off” behavior, with a
fast and accurate response to the environment temperature. As an
application example, the successful “writing-erasing” on a polymer
surface indicates the potential application of this material in many
areas, such as temporary information transmission.
[17]
[18]
[19]
[20]
[21]
Acknowledgments
[22]
[23]
The research reported in this publication was supported by King
Abdullah University of Science and Technology (KAUST).
Conflict of interest
The authors declare no conflict of interest.
Keywords: cross-link • Diels-Alder reaction • temperature-response •
cross-linking induced emission * reversible photoluminescence • AIE
[1]
X. Chen, M. A. Dam, K. Ono, A. Mal, H. Shen, S. R. Nutt, K. Sheran,
F. Wudl, Science 2002, 295, 1698-1702.
[2]
[3]
A. Rahimi, J. M. García, Nat. Rev. Chem. 2017, 1, 1-11.
a) C. Lv, K. Zhao, J. Zheng, Macromol. Rapid Commun. 2018, 39,
1700686; b) L. Imbernon, E. Oikonomou, S. Norvez, L. Leibler,
Polym. Chem. 2015, 6, 4271-4278.
[4]
a) D. Montarnal, M. Capelot, F. Tournilhac, L. Leibler, Science 2011,
334, 965-968; b) T. Liu, C. Hao, S. Zhang, X. Yang, L. Wang, J. Han,
Y. Li, J. Xin, J. Zhang, Macromolecules 2018, 51, 5577-5585.
This article is protected by copyright. All rights reserved.

10.1002/anie.202013183
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
RESEARCH ARTICLE
Novel Diels-Alder polymer
networks with tunable AIE
behavior was constructed
low temperature
high temperature
Yu Jiang*, Nikos
Hadjichristidis*
from
non-emission
Page No. 1 – Page No. 6
Fluorescence "off"
Fluorescence "on"
dimaleimide-substituted
Diels-Alder Polymer
Networks with
Temperature-Reversible
Cross-linking-Induced
Emission
tetraphenylethene (TPE-2MI)
O
:
rt
O
and
linear
furan-based
O
O
N
N
O
copolymers. The resulted
140 o
C
O
:
:
O
O
polymer
tunable
networks
have
“turn
N
Fluorescence "off"
Fluorescence "on"
N
O
O
O
fluorescent
on/off” property, which shows
an accurate response to
temperature.
This article is protected by copyright. All rights reserved.