Light- and heat-triggered reversible luminescent materials based on polysiloxanes with anthracene groups

Article abstract of DOI:10.1039/c7ra12201b

In this study, reversible silicone elastomers were successfully developed by light-triggered dimerization and heat depolymerization of the anthryl groups. Polysiloxanes with anthryl groups were prepared from poly(aminopropylmethylsiloxane) (PAPMS) with electron-donating (9-anthracenylmethyl acrylate) and electron-withdrawing (anthracene-9-carboxylic acid) units. The cross-linking networks were formed with the via 4π-4π photo-cycloadditions of the anthryl groups upon the UV light excitation (365 nm). 9-Anthracenylmethyl acrylate or anthracene-9-carboxylic acid efficiently dimerized through the photodimerization of the anthryl groups in the organic solvents, which was proven by UV-vis spectra, NMR spectra, and LC/MS. The covalent bonds between pendant anthryl groups were cleaved after heating at 120 °C. Furthermore, repeatable dimerization-depolymerization conversion was confirmed. In addition, for the first time, we found that the sunlight can also initiate the cycloaddition, which was "greener" and more environment-friendly. The green luminescence was observed from the PAPMS-1 film instead of the quenching effect caused by aggregation after the cycloaddition. Thus, a colorful UV-light emitting diode (LED) cell was obtained by coating the films on the commercially available LED cell.

Full text of DOI:10.1039/c7ra12201b

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PAPER
Light- and heat-triggered reversible luminescent
materials based on polysiloxanes with anthracene
groups†
Cite this: RSC Adv., 2017, 7, 56489
*
Dongdong Han, Hang Lu, Wensi Li, Yonghao Li and Shengyu Feng
In this study, reversible silicone elastomers were successfully developed by light-triggered dimerization and
heat depolymerization of the anthryl groups. Polysiloxanes with anthryl groups were prepared from
poly(aminopropylmethylsiloxane) (PAPMS) with electron-donating (9-anthracenylmethyl acrylate) and
electron-withdrawing (anthracene-9-carboxylic acid) units. The cross-linking networks were formed
with the via 4p–4p photo-cycloadditions of the anthryl groups upon the UV light excitation (365 nm). 9-
Anthracenylmethyl acrylate or anthracene-9-carboxylic acid efficiently dimerized through the
photodimerization of the anthryl groups in the organic solvents, which was proven by UV-vis spectra,
NMR spectra, and LC/MS. The covalent bonds between pendant anthryl groups were cleaved after
heating at 120 ꢀC. Furthermore, repeatable dimerization–depolymerization conversion was confirmed. In
addition, for the first time, we found that the sunlight can also initiate the cycloaddition, which was
“greener” and more environment-friendly. The green luminescence was observed from the PAPMS-1 film
instead of the quenching effect caused by aggregation after the cycloaddition. Thus, a colorful UV-light
emitting diode (LED) cell was obtained by coating the films on the commercially available LED cell.
Received 7th November 2017
Accepted 4th December 2017
DOI: 10.1039/c7ra12201b
rsc.li/rsc-advances
poly(aminopropylmethylsiloxane)
(PAPMS)
with
N-iso-
Introduction
propylacrylamide.11 However, these methods have some draw-
backs, including potential explosiveness of the azide
monomers, the unpleasant smell of thiol–enes, the high costs of
the catalysts, and the inherent difficulties of the [2 + 2] photo-
cross-linking reactions. Thus, it is desirable to develop
a simple, efficient, and catalyst-free system to prepare func-
tional and cross-linked silicones.
Silicone elastomers are widely used as safe and durable mate-
rials for domestic and industrial applications because of their
resistance to weathering, high and low temperatures, radiation,
and re.1–5 However, traditional silicones are normally ther-
moset materials, which are prepared by three main methods: (1)
Pt-catalyzed hydrosilylation between Si–H and vinyl groups; (2)
condensation reaction between Si-OR and –OH; (3) free-radical
cross-linking.6 Recently, many classical reactions such as thiol–
ene “click” reactions,7 Michael addition,8 and [2 + 2] photo-
cycloaddition9 were utilized to develop self-healing, thermo-
sensitive, uorescent, or reversible silicone elastomers. For
example, Brook et al. prepared a thermo-sensitive silicone
In spite of many synthetic methods used to produce silicone
materials, there are few reports about photo-cross-linking
reactions without a photo-initiator or catalysts. Anthracene is
commonly used in photo-responsive compounds, which can
form covalent bonds through photo-cycloaddition.12–22 Anthra-
cene groups undergo [4 + 4] photo-dimerization when irradiated
by UV light (l > 300 nm) and can be reversed to the original
monomers via exposure to a higher energy UV light (l < 300 nm)
(Scheme 1)23 or heating to 100–130 ꢀC.24,25 Xie et al. successfully
obtained shape-memory materials by dangling photoresponsive
anthracene groups from different polymer chains.23,26,27
elastomer via [2
+ 2] photo-cycloaddition of coumarin.
Coumarin is UV-sensitive and can undergo a reversible reaction
upon UV irradiation.9 In addition, Brook et al. prepared a self-
associating silicone elastomer based on azide/alkyne cycload-
dition reactions.10 In our group, highly sensitive stimuli-
responsive polysiloxanes (SPSis) were prepared via a facile,
efficient, and catalyst-free aza-Michael addition of
Key Laboratory of Special Functional Aggregated Materials, Key Laboratory of Colloid
and Interface Chemistry (Shandong University), Ministry of Education, School of
Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R.
China. E-mail: fsy@sdu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra12201b
Scheme
groups.23
1
Reversible photo-dimerization reaction of anthracene
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However, there are few reports on reversible silicone elastomers
based on the anthracene groups.
Based on the above discussion, we supposed that the stimuli
or environmentally responsive silicone elastomers could be
obtained from the reaction of polysiloxanes and the anthracene
groups.
Synthesis
Synthesis of 9-anthracenemethanol. Sodium borohydride
(5.0 g) was dissolved in ethanol (50 mL) and added dropwise to
the reaction mixture of 9-anthraldehyde (10.0 g) in ethanol (50
mL). The resulting mixture was sealed and then stirred for 2 h in
an ice bath. Then, water (25 mL) was poured into the mixture. A
yellow solid (8.1 g) was obtained aer ltration and vacuum
drying (yield 80%).
In this study, we appended the anthryl groups to poly-
siloxanes with different anthracene derivatives and conducted
a comprehensive examination on the reversible conversion
process with heat. It is well-known that the dimerization or
dissociation rate of anthryl dimers depends on the substituents
on the anthracene ring.28 Thus, we synthesized silicone elasto-
mers by combining poly(aminopropylmethylsiloxane) (PAPMS)
with two derivatives of 9-substituted anthracene, in which one
was linked with an electron-donating group (9-anthrace-
nylmethyl acrylate, PAPMS-1) and the other was linked with an
electron-withdrawing substituent (anthracene-9-carboxylic
acid, PAPMS-2). The prepolymer of PAPMS-1 was successfully
prepared from 9-anthracenylmethyl acrylate and PAPMS
through an aza-Michael reaction, which is operated at mild
reaction conditions, has a high efficiency, does not require
a catalyst, and produces no by-products. However, the prepol-
ymer of PAPMS-2 was prepared via “salt-forming vulcanization”6
by neutralizing PAPMS with anthracene-9-carboxylic acids to
yield an ion-association complex. Then, light- and heat-
triggered reversible silicone elastomers were prepared via [4 +
4] photo-dimerization of the pendant anthryl groups of the
prepolymers. The cross-linked network was formed among the
polymer chains aer the polymer solution was treated with UV
light (365 nm).
1H NMR (400 MHz, CDCl3): d 8.48 (s, 1H), 8.43 (d, J ¼ 9.0 Hz,
2H), 8.04 (d, J ¼ 8.4 Hz, 2H), 7.64–7.53 (m, 2H), 7.49 (dd, J ¼ 7.9,
7.0 Hz, 2H), 5.69 (s, 2H).
Synthesis of 9-anthracenylmethyl acrylate. 9-anthraceneme-
thanol (4.50 g) and trimethylamine (3.0 g) were dissolved in
25 mL dichloromethane (DCM). Then, acryloyl chloride (2.5 g)
was added dropwise to the reaction mixture. The resulting
mixture was sealed and stirred for 2 h in an ice bath. The solvent
and the unreacted acryloyl chloride were removed under
vacuum. Next, triethylamine salt was removed by column
chromatography using petroleum ether–dichloromethane
(1 : 1) as an eluent. A yellow solid (4.6 g) was obtained aer
vacuum drying (yield 81%).
1H-NMR (400 MHz, CDCl3): d 8.55 (s, 1H), 8.39 (d, J ¼ 9.0 Hz,
2H), 8.07 (d, J ¼ 8.4 Hz, 2H), 7.66–7.57 (m, 2H), 7.57–7.46 (m,
2H), 6.45 (dd, J ¼ 17.3, 1.4 Hz, 1H), 6.27 (s, 2H), 6.16 (dd, J ¼
17.3, 10.4 Hz, 1H), 5.83 (dd, J ¼ 10.4, 1.4 Hz, 1H).
Synthesis of PAPMS. PAPMS was synthesized by hydrolysis
and ring opening polymerization. Typically, 100 g D4 and 10 g 3-
aminopropyl-methyl-diethoxysilane were added to a 250 mL
three-neck ask equipped with a mechanical stirrer. Then,
0.053 g (0.05 wt%) KOH, 1.88 g water, and 1.1 g (1.0 wt%) DMSO
were added to the mixture. The mixture was reuxed and stirred
for 4 h at 100 ꢀC and then ethanol and water were removed
under vacuum. The resultant mixture was stirred slowly at
120 ꢀC for 15 h and cooled to room temperature. KOH was
neutralized by acetic acid. The mixture was heated to 180 ꢀC
under vacuum to remove the low-boiling components and
a white viscous liquid was obtained.
Sunlight-induced photo-dimerization was applied, for the
rst time, in the anthracene system, thus making it a “greener”
and more environment-friendly process. As expected, dimer-
ization efficiency of anthryl groups was remarkable when the
sunlight was applied in the PAPMS-1 system. The covalent
bonds formed via dimerization of the pendant anthryl groups
ꢀ
could be reversed when the polymer was heated at 120 C. We
found that the PAPMS-1 lm could emit green light under UV
irradiation, which made the composites suitable for optoelec-
tronic devices applications, such as light emitting diodes
(LEDs). When the polymer solution was coated on the
commercially available UV-LED cells and cured in situ, the
colorful LED cells could be fabricated in an economical and
effective way.
1H NMR (400 MHz, CDCl3) d 2.67 (dd, J ¼ 15.1, 8.1 Hz, 2H),
1.58–1.25 (m, 4H), 0.60–0.46 (m, 2H), 0.25–0.10 (m, 73H).
Preparation of the elastomers PAPMS-1 and PAPMS-2
For preparation of PAPMS-1, as shown in Scheme 2, 9-anthra-
cenylmethyl acrylate and PAPMS, with a certain mole ratio, were
added to a ask. The mixture was dissolved in the mixed
Experimental
Materials
Octamethylcyclotetrasiloxane (D4) and 3-aminopropyl-methyl-
diethoxysilane were obtained as the commercial products and
used directly. Dichloromethane, acetic acid, petroleum,
ethanol, toluene, trichloromethane, and methanol were ob-
tained from Tianjin Fuyu Chemical Co., Ltd. 9-Anthraldehyde,
acryloyl chloride and anthracene-9-carboxylic acid were
supplied by Energy Chemical Technology Co., Ltd. Sodium
borohydride was provided by the Tianjin Kermel Chemical
Reagent Co., Ltd.
Scheme 2 Synthesis route of PAPMS-1.
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solvents of methanol (1 mL) and toluene (10 mL). Then, the SDTQ600 over a temperature range from room temperature to
resultant mixture was reuxed and stirred at 45 C. The FT-IR 800 ꢀC with a heating rate of 10 ꢀC minꢁ1. The scanning electron
ꢀ
spectra of the mixture are presented in the ESI (Fig. S1†). As microscopy (SEM) images were obtained using a Hitachi S-4800
shown in Fig. S1,† the double bond disappeared aer 8 h, which (7 kV). The samples were cut and coated with a thin layer of
indicated that the aza-Michael reaction was completed. The platinum before the investigation.
mixture (PAPMS containing anthryl groups) was stirred in an ice
bath under 365 nm UV-irradiation for 1 h. Then, most of the
solvent was removed through distillation under reduced pres-
sure till the resultant mixture could still be uid. The thick
Results and discussion
Anthracene groups undergo dimerization ([4 + 4] cycloaddition
reaction) when irradiated by UV (l > 300 nm).26 In this study, the
photo-cycloaddition between the anthracene groups was
accomplished through UV-irradiation at 365 nm. In order to
conrm the effectiveness of dimerization under the UV light,
the control experiments were performed using 9-anthrace-
nylmethyl acrylate or anthracene-9-carboxylic acid under the
same conditions. The nal products were characterized by UV-
vis spectrometry, 1H-NMR, and mass spectrometry. The 1H-
NMR spectra of the dimers are presented in Fig. 1. Compared
to the spectrum of the monomer of before irradiation, few new
peaks are observed in the spectrum of the resultant dimer (aer
the UV irradiation). For 9-anthracenylmethyl acrylate, the
peaks k and l are observed at 7.0 and 6.8 ppm, respectively, aer
the UV-irradiation (Fig. 1a), which matched with the structure of
a dimer. This result conrmed the occurrence of the dimer-
ization reaction. For anthracene-9-carboxylic acid, the peaks k
and l are located at 7.0 and 6.9 ppm, respectively, aer the UV-
irradiation (Fig. 1c), which showed the abundant changes in the
anthracene unit signals compared with 9-anthracenylmethyl
acrylate aer the photo-irradiation. The results are primarily
related to the substituents on the anthracene rings. However,
the proton peaks of the monomer still existed aer the
precursor was placed in a Teon mold and then the remaining
solvent was slowly volatilized at room temperature.
In addition, for the rst time, sunlight-induced photo-
dimerization was attempted for anthryl silicone elastomers. A
mixture of 9-anthracenylmethyl acrylate and PAPMS was irra-
diated by sunlight for 3 days instead of UV light. Then, most of
the solvent was removed through distillation under reduced
pressure till the resulting mixture could still be uid. The thick
precursor was placed into a Teon mold, and then the
remaining solvent was slowly volatilized at room temperature.
PAPMS-2 was prepared via “salt-forming vulcanization” by
neutralizing PAPMS with anthracene-9-carboxylic acids to yield
an ion-association complex. Typically, anthracene-9-carboxylic
acids and PAPMS, in a certain mole ratio, were added to
a ask. The mixture was dissolved in the mixed solvents of
methanol (1 mL) and toluene (10 mL). The mixture was stirred
in an ice bath upon UV-irradiation (365 nm). Fig. S2† depicts the
IR spectra of anthracene-9-carboxylic acids, PAPMS, and the
PAPMS-2 elastomer. The results conrmed the formation of an
ion-association complex. In PAPMS-2, –COOH and –NH2 groups
disappeared completely. Therefore, the new absorption band
was observed at 1570 cmꢁ1, which was caused by the asym-
metric deformation of –COOꢁ. This result inferred the complex
+
formation of –COOꢁ with –NH3 . The mixture was stirred in an
ice bath under UV-irradiation at 365 nm. Then, most of the
mixed-solvent was removed under vacuum till the resulting
mixture can still be uid. The thick precursor was poured into
a Teon mold and the rest of the solvent evaporated naturally.
Characterization and measurements
The dimerization of the anthracene groups was accomplished
by irradiating with UV light from a Spectroline Model SB-100P/
FA lamp (365 nm, 100 W). The UV intensity was set at 4500 mW
cmꢁ2 at a distance of 38 cm. The proton nuclear magnetic
resonance (1H-NMR) spectra were recorded on a Bruker
AVANCE 400 spectrometer at 25 ꢀC using CDCl3 (or CD3OD) as
the solvent and without tetramethylsilane as an interior label.
The molecular weights were determined by an Agilent HP1100
LC-Applied Biosystems API 4000 TQ mass spectrometer (LC/
MS). The luminescence (excitation and emission) spectra of
the samples were determined with a Hitachi F-7000 uores-
cence spectrophotometer using a monochromated Xe lamp as
an excitation source. Both excitation and emission slits were set
at 5 nm. The ultraviolet absorption (UV) spectra in dichloro-
methane (or toluene) solution were detected using a Beijing TU-
1901 double beam UV-vis spectrophotometer. Thermogravi-
Fig. 1 1H-NMR spectra of the anthracene derivatives after ((a) and (c))
and before ((b) and (d)) the [4 + 4] photo-cycloaddition of 9-anthra-
cenylmethyl acrylate and anthracene-9-carboxylic acid, respectively,
metric analysis (TGA) was performed under N2 using a TA upon a UV irradiation at 365 nm for 1 h.
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irradiation, which indicated that the dimerization reaction was
incomplete. The mass spectra also showed a dimer peak and
a monomer peak in the nal product aer the irradiation as
shown in Fig. S3 and S4.† The results obtained from the 1H-
NMR and mass spectra demonstrated that only small amount
of the monomer can undergo a cycloaddition reaction upon UV
irradiation at 365 nm.
The main absorption peak of anthracene in the UV-vis
spectra is between 300 and 400 nm. Upon dimerization at the
9 and 10 positions of the anthracene moieties, the conjugated
p-system was broken, which led to a depletion of the charac-
teristic UV absorption bands of the anthracene groups. As
shown in Fig. 2, the absorption intensity decreased upon the
UV-irradiation, which indicated that the cycloaddition had
occurred. In addition, the progress of the dimerization reaction
Fig. 3 UV-vis spectra and dimer ration (insets) of the PAPMS-1 solu-
tion (a) and PAPMS-2 solution (b) with different exposure times
(365 nm).
was estimated by the proportion of the decrease in the absor- efficiency of the photo-dimerization increased with the increase
bance compared to its original value before the irradiation, in the exposure time (insets in Fig. 3). For the same irradiation
using the equation (A0 ꢁ A)/A0, where A0 and A represent the time, the absorption intensity of PAPMS-2 decreased more than
absorption intensity at 368 nm before and aer the irradiation, that of the PAPMS-1. Our data have further conrmed that the
respectively.29 It is well-known that the rate of this dimerization efficiency of photo-dimerization is higher if the substituent on
depends on the substituents of the anthracene ring and the rate the anthracene ring is an electron-withdrawing group.
of reaction with the electron-withdrawing substituents is higher
Polysiloxanes containing pendant anthryl groups are ex-
than that with the electron-donating substituents. As shown in pected to show an ascendancy of the controllable reversibility
Fig. 2, the efficiency of dimerization of the monomers was 25% and reactivity as well as avoid the formation of by-products. The
and 35%, respectively. The results achieved by UV-vis spec- reversibility of dimerization is favorable for the degradation and
trometry, 1H-NMR, and mass spectrometry proved that the the recovery of the materials. In the previous reports, the
dimerization reaction easily occurs when the substituent on the cleavage of the photo-dimers can be either optically triggered by
anthracene ring is an electron-withdrawing group.
The photo-dimerization reaction ([4
short-wavelength UV irradiation (254 nm) or thermally-induced
4] photo- bond scission.30,31 However, compared with the thermal degra-
+
cycloaddition) in the polymer solution can also be determined dation, the UV irradiation may cause some side reactions, which
by UV-vis spectroscopy. Fig. S5† shows the UV-vis spectra of the limited the use of this reversible system. In order to investigate
PAPMS-1 solution and the PAPMS-2 solution before and aer the reversible formation of the photo-cross-linking network,
the UV-irradiation. Although the absorption intensity of both a heat-triggered de-cross-linking route with a thermal annealing
polymer solutions decreased aer the UV-irradiation, the step at 120 ꢀC was designed. UV-vis spectroscopy was utilized to
absorption of the PAPMS-2 solution decreased more than that monitor the reversible dimerization of the anthracene groups.
of the PAPMS-1 solution, which was primarily attributed to the The light- (l ¼ 365 nm, 60 min) and heat-triggered (120 ꢀC)
substituents of the anthracene rings. The absorption intensity reaction, denoted as a reversible cycle, was operated. As illus-
of the polymer solution was related to the exposure time as trated in Fig. 4a and b, the absorption intensity of the PAPMS-1
illustrated in Fig. 3. The polymer solution before the irradiation solution and the PAPMS-2 solution decreased aer the irradia-
has the strongest absorption intensity in the range of 300– tion, but almost recovered to the original value aer the thermal
400 nm. The intensity of the absorption peak decreased with the process. These results indicated that some of the anthryl groups
increase in the time of irradiation, which indicated that the could form dimers aer the UV irradiation and the dimers were
cleaved to form the anthryl groups again aer the thermal
treatment. Nevertheless, more time was needed for the PAPMS-
ꢀ
1 solution for de-cross-linking (120 C, 240 min) compared to
that for the PAPMS-2 solution (120 ꢀC, 180 min). In addition, we
detected the reversible property of the lms as shown in
Fig. S6.† The results also revealed the dimerization upon UV-
irradiation at 365 nm and the bond cleavage of the anthra-
cene dimers by heat treatment (120 ꢀC). Furthermore, the model
experiment was performed. As shown in Fig. 4d, the prepolymer
PAPMS-2 was a viscous liquid at room temperature, whereas it
was solidied when exposed to a 365 nm light. It is surprising
that PAPMS-2 appeared to be a liquid when heated at 120 ꢀC and
changed into the solid sample when irradiated at 365 nm. In
addition, the reversible cycle (Fig. S7†) of PAPMS-1 was longer
than that of PAPMS-2. It is reported that the rate of this
Fig. 2 (a) UV-vis spectra of 9-anthracenylmethyl acrylate before (top)
and after (bottom) irradiation (365 nm); (b) UV-vis spectra of anthra-
cene-9-carboxylic acid before (top) and after (bottom) the irradiation
(365 nm).
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light under the UV light irradiation. Simultaneously, anthracene
is the exact fetch of most of the uorescent materials that exhibit
aggregation uorescence quenching (ACQ) effect. They can emit
a strong blue light in the solution, but their uorescence intensity
would weaken or even quench in the high-concentration solution
or solid. We were surprised to nd that PAPMS-1 emitted green
uorescence under a UV light. Thus, we investigated the uo-
rescence properties of PAPMS-1. It could be observed from Fig. 5a
that the uorescence behavior of PAPMS-1 was similar to that of
anthracene. PAPMS-1 can emit a strong blue light in the solution,
while the uorescence intensity weakened or even quenched in
the high-concentration solution or the polymer lm. Though
PAPMS-1 appears like a solid solution, the anthryl group was
aggregated due to p–p stacking. In such case, the uorescence
intensity of the lm was much lower than that of the solution.
The maximum emission wavelength of the PAPMS-1 lm was red
shied to 520 nm compared with that in the solution (450 nm).
Simultaneously, the cured lm was green under a UV light as
observed by naked eyes. In PAPMS-1, the anthryl groups were in
a dispersed state in the solution. However, they were aggregated
owing to p–p stacking in the lms, which led to the red shi of
the maximum emission wavelength and exhibited different
colors of the uorescence between the polymer solution and the
lms. Using the CIE 1931 RGB colorimetric system, the bright
emissions of the cured lm and the polymer solution can be
easily observed by the naked eyes upon excitation with a wave-
length of 398 nm as shown in Fig. 5b.
Fig.
4 (a) UV-vis spectra of the PAPMS-1 solution in different
processes; (b) UV-vis spectra of PAPMS-2 solution in different
processes; (c) UV-vis spectra of the anthracene modified polysiloxane
solution before and after the irradiation (sunlight); (d) illustrations of
PAPMS-2 at 365 nm upon repeating irradiation and heating cycles.
dissociation, which is caused by anthracene dimers that
dissociate cleanly at the elevated temperatures through a dir-
adical transition state, depends on the substituents on the
anthracene ring.32 As expected, PAPMS-2 could be easily depo-
lymerized than PAPMS-1 because the anthracene in PAPMS-2
was linked to an electron-withdrawing group.
Interestingly, the uorescence intensity of the lm changed
aer the irradiation. As shown in Fig. 6, the uorescence
intensity of the PAPMS-1 lm increased aer the UV-irradiation,
which was different from the ACQ effect. The uorescence
emission reduced due to p–p stacking in the PAPMS-1 lm. The
UV-triggered dimerization occurred between the anthracene
groups aer the irradiation. The p–p stacking was broken and
the aggregated state of the anthracene groups was changed.
Thus, the ACQ effect was weakened and the emission increased.
Sunlight is a green energy source, which includes light of
various wavelengths; when it comes to the application in
chemistry, sunlight is primarily used in the eld of photo-
catalysis. Moreover, some studies have indicated that some
reactions can be triggered by the ultraviolet ray of sunlight. For
example, silicone elastomer could be prepared via thiol–ene
click reaction induced by sunlight.33 It is an advantage that some
reactions can be induced by sunlight because they are “greener”
and do not need excessive power as compared to the conven-
tional reactions triggered by a UV light. In this study, the
sunlight was applied, for the rst time, to trigger the dimeriza-
tion reaction of the anthryl group. PAPMS-1 was chosen as an
example to design the experiment. The prepolymer PAPMS-1 was
prepared and the same amount of the PAPMS-1 solution was
diluted to 10 mL in two colorimetric tubes. One tube was placed
under the sunlight and the other was placed indoors. Aer three
days, the samples were tested by UV-vis spectroscopy as illus-
trated in Fig. 4c. The absorption intensity of the anthracene
groups decreased aer being irradiated by sunlight, which
proved that the dimerization reaction had occurred. Based on
these data, it can be concluded that our study can produce the
potential self-repair materials by exposure to sunlight.
Coating on a commercial available UV-LED light
The potential application on the commercially available LED cell
was determined because of the unique luminescent perfor-
mance of the PAPMS-1 lm. When the PAPMS-1 lm was coated
Fig. 5 (a) Fluorescence spectra of the PAPMS-1 film and its solution
under UV-irradiation at a 365 nm wavelength (excitation at 398 nm);
(b) emission colors of the films in the CIE 1931 chromaticity diagram;
CIE color coordinates: polymer solution (0.1588, 0.0478) and cured
Luminescent properties of the lms
Anthracene is commonly used in uorophores that are present in
the uorescent materials and its solution can emit a strong blue film (0.3133, 0.421).
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a UV light is described in this paper. It is evident that the photo-
cross-linking process is almost reversible, which is proven by
the data from the absorption intensity of the UV spectra ob-
tained when the target products were heated. In particular,
sunlight can be used instead of UV light to trigger the reaction
and it was applied for the rst time here in the anthracene
dimerization. Moreover, a sunlight-induced reaction is bene-
cial for the environment. The green luminescent lms based on
polysiloxane and the anthracene group were developed and
possessed the excellent lm properties. These transparent lms
could be coated on a UV-LED cell, which makes them ideal
candidates, particularly for fabricating devices used in dis-
playing and lighting in an environment-friendly way.
Fig. 6 Fluorescence spectra of the untreated and treated PAPMS-1
film upon UV-irradiation.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (No. 21774070), the National
Science Foundation of Shandong Province (ZR2015BQ008), and
Shandong Special Fund for Independent Innovation and
Achievements transformation (No. 2014ZZCX01101).
Fig. 7 (a) Images of the flat substrate coated with the luminescent film
under UV irradiation (365 nm); (b) images of a commercially available
UV-LED cell (left) and a UV-LED coated with the luminescent film
(right).
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