Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films

Article abstract of DOI:10.1016/j.cclet.2016.06.037

Novel photochromic composite films have been successfully fabricated by dispersing pyrazolone derivative: 1,3-Diphenyl-4-(3-chlorobenzal)-5-hydroxypyrazole 4-phenylsemicarbazone (1a) into hydrosol of polyvinyl alcohol (PVA). The microstructure, photochromic behaviors and thermal bleaching properties were investigated by Raman spectroscopy, X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and ultraviolet–visible absorption spectroscopy (UV–vis). The results showed that 1a was not only blended but also well dispersed in the PVA polymer films with a suitable content of chromophore. Upon UV light irradiation, the composite films gradually changed from colorless to yellow and recovered fully to the initial state upon thermal bleaching. The time constants of photochromic reactions were almost the same as those of 1a observed in their crystalline state, indicating that the photochromic phenomenon is barely disturbed by the polymer matrix.

Full text of DOI:10.1016/j.cclet.2016.06.037

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Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
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Original article
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Preparation and photochromic properties of pyrazolones/polyvinyl
alcohol composite films
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Q1 Ji-Xi Guo, Ming-Xi Guo, Dian-Zeng Jia , Yin-Hua Li
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Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of Advanced Functional Materials, Autonomous Region; Institute of
Applied Chemistry, Xinjiang University, Urumqi 830046, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Novel photochromic composite films have been successfully fabricated by dispersing pyrazolone
derivative: 1,3-diphenyl-4-(3-chlorobenzal)-5-hydroxypyrazole 4-phenylsemicarbazone (1a) into
hydrosol of polyvinyl alcohol (PVA). The microstructure, photochromic behaviors and thermal bleaching
properties were investigated by Raman spectroscopy, X-ray powder diffraction (XRD), field emission
scanning electron microscopy (FE-SEM) and ultraviolet–visible absorption spectroscopy (UV–vis). The
results showed that 1a was not only blended but also well dispersed in the PVA polymer films with a
suitable content of chromophore. Upon UV light irradiation, the composite films gradually changed from
colorless to yellow and recovered fully to the initial state upon thermal bleaching. The time constants of
photochromic reactions were almost the same as those of 1a observed in their crystalline state,
indicating that the photochromic phenomenon is barely disturbed by the polymer matrix.
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Received 4 May 2016
Received in revised form 7 June 2016
Accepted 21 June 2016
Available online xxx
Keywords:
Pyrazolone
Polyvinyl alcohol
Composite film
Photochromism
Information storage
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1. Introduction
mechanism with inter- and intramolecular proton transfer [27] 28
and common methods for preparing composite films by dissolving 29
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Photochromic materials have attracted considerable attention
due to their special structures, properties and various applications
[1–5]. Pyrazolone derivatives, a new class of organic photochromic
materials, have been proven to be promising and important
photochromic compounds due to their high sensitivity and
excellent fatigue resistance in the solid state [6–9]. Photochromic
materials, which display photochromic reactions in the solid state,
are fundamental in developing their applications, such as optical
information storage [10–12], optical switches [13–16], anti-
counterfeiting [17,18] and ultraviolet protection [19]. However,
in most cases, the photochromic molecules need to be embedded
in complex environments such as bulk polymer materials or thin
films in order to translate the molecular response into the desired
macroscopic effects [20,21]. Therefore, the functional composite
films [22,23], which possess reversible photochromism with
distinguished color changes, have become a hot topic in material
research [24–26]. Pyrazolone derivatives showed no photochromic
reaction in solution and amorphous state for their special
chromophores in solvents are not suitable for this system. 30
Therefore, developing of composite films containing photochromic 31
pyrazolones will be a challenge for extending their application.
32
In previous studies, we have reported a composite film based on 33
dispersing the pyrazolone powders in hydroxypropyl methyl 34
cellulose (HPMC) [27]. The composite films showed a reversible 35
photochromic reaction under UV irradiation and thermal bleaching 36
byheating. Unfortunately, the transparencyofthe compositefilms is 37
poor. In this work, we have successfully fabricated a thin composite 38
film with 1,3-diphenyl-4-(3-chlorobenzal)-5-hydro-xypyrazole 4- 39
phenylse-micarbazone (1a) and polyvinyl alcohol (PVA) in aqueous 40
solution to avoid inactivation of pyrazolones, as shown in Scheme 41
1. The choiceofPVA isvitalforthepreparationofthecompositefilms 42
not only due to its good film-forming ability, optical transparency 43
and water solubility, but also due to its surfactant effect that enables 44
the distribution of 1a in water solutions. The fabricated composite 45
films exhibited reversible photoisomerization reactions under 46
alternating UV light irradiation and heating as the same as pure 47
1aobserved inthe crystallinestate. Theopticalcontrastcan beeasily 48
modified by changing the composite ratio of 1a and PVA. Moreover, 49
the color of 1a in film can be easier tuned by UV stimuli and fades 50
*
Corresponding author.
faster than that of 1a in the crystalline state.
51
E-mail address: jdz0991@gmail.com (D.-Z. Jia).
http://dx.doi.org/10.1016/j.cclet.2016.06.037
1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite
films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037

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J.-X. Guo et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
Scheme 1. Photochronmic reaction of pyrazolone 1, chemical structure of PVA and
photochromic composite film.
Fig. 1. Raman spectra of (a) pure PVA film, (b) pure 1a and the different contents of
1a in PVA film (c: 0.5 wt%, d: 1 wt%, e: 2 wt%).
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2. Experimental
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The powder X-ray diffraction (XRD) data of the material were
collected on a Bruker D8 Advance X-ray diffractometer. Raman
spectra were recorded on a Bruker Senterra (R200-L) raman
spectrometer. The field emission scanning electron microscopy
(FE-SEM) images were obtained on a Hitachi S-4800 scanning
electron microscope. UV–visible absorption spectra were studied
using a Hitachi U-3010 spectrometer equipped with an integrating
sphere accessory and temperature-controlled accessory. A 15 W
lamp in ZF-8 Ultraviolet Analysis Instrument with 365 nm
excitation light (1.5 W cmÀ2 on the sample) and a TECH XT-5
melting point apparatus were used as sources for photocoloration
and thermal bleaching, respectively. The distance between the
sample and light source was 15 cm.
Materials: 1,3-Diphenyl-5-pyrazolone was synthesized accord-
ing to the literature method [28], 3-chlorobenzoylchloride and
4-phenylsemicarbazide were purchased from the J&K Company.
PVA powders were purchased from the Aladdin Company. Other
materials were purchased from commercial sources, and the
solvents were purified with standard procedures.
3. Results and discussion
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3.1. Morphology and structure of 1a/PVA composite films
Raman spectroscopy is a useful tool in the characterization of
successful incorporation of 1a into the polymer matrix. The Raman
spectra of the pure PVA film, pure 1a and different contents of 1a in
PVA film are shown in Fig. 1. It can be clearly seen that the 1a/PVA
composite film exhibits the characteristic vibration peaks of 1a at
1605, 1582 and 1000 cmÀ1, and these peaks are enhanced with
increased contents of 1a in film. Meanwhile, the characteristic
bands of PVA also appeared to verify the presence of PVA in the
composite film. These results demonstrate that the 1a powders
dispersed in the polymer films without any structural alteration.
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Synthesis of photochromic pyrazolone derivative: 1,3-Diphe-
nyl-4-(3-chlorobenzal)-5-hydroxypyrazole 4-phenyl-semicarba-
zone (1a) was synthesized through reaction of 1,3-diphenyl-4-
(3-chlorobenzal)-5-hydroxypyrazole [29] and 4-phenyl-semicar-
bazone in the presence of acetic acid in ethanol at 80 8C according
to the literature [30].
Preparation of 1a/PVA composite films: The 1a/PVA composite
films were prepared by the solvent evaporation method. PVA
powders (1.5 g) were added to deionized water (10 mL) with
vigorous stirring at 80 8C, which provided a viscous mixture. In
dark room, the powder 1a (0.015 g) was slowly added to the above
solution with continuous stirring for 3 h at room temperature and
then left in the dark until the bubbles disappeared. The mixed
solution, 1a in the PVA hydrosol with content of 1 wt%, was cast on
a smooth glass groove and dried at ambient temperature in the
dark for 10 h. Then the composite film was peeled off from the glass
substrates. The fabricated composite film (1 wt%) was placed in the
dark to prevent the occurrence of photochemical reactions. For
comparison, a pure PVA film without 1a addition and 0.5 wt%,
2 wt% composite films were also fabricated following the same
procedure. Of note, powders of 1a cannot be dissolved but only
dispersed in water.
Fig. 2. XRD of (a) pure PVA film, (b) pure 1a and the different contents of 1a in PVA
film (c: 0.5 wt%, d: 1 wt%, e: 2 wt%).
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite
films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037

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Fig. 3. FE-SEM of (a) pure PVA film, insert: pure 1a, and the different contents of 1a in PVA film (b: 0.5 wt%, c: 1 wt%, d: 2 wt%).
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The morphology of materials was characterized by XRD. From
3.2. Photochromic reactions of 1a/PVA composite films
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Fig. 2, we found that the powders of 1a exhibited the characteristic
diffraction peaks at 5.9o, 15.6o, 17.2o, 19.5o and 35.3o (2
u
),
The powders of 1a can undergo a reversible photochromism 127
reaction upon the alternating treatments of irradiation with UV 128
and heating accompanying with color changes between white and 129
yellow [30]. The absorption spectra of 1a in the crystalline state 130
induced by photoirradiation at room temperature are shown in 131
Fig. 4a. However, the pure PVA film does not respond to UV light. 132
The composite films with different contents of 1a show similar 133
photochromic phenomenon compared with that of the 1a powders 134
(Fig. 4b, c, d). Upon irradiation with 365 nm light, new broad bands 135
in the range of 350–520 nm appeared for composite films with 136
different contents of 1a and their intensities increased with 137
prolonging UV light irradiation. The maximum absorption 138
wavelength of these composite films with different contents of 139
1a in the PVA matrix was observed at 410 nm, and the ratios of 140
absorbance contrast between the photo-stationary state and initial 141
state were about 3:1 (0.5 wt%), 4:1 (1 wt%) and 5:1 (2 wt%), 142
respectively. These differences are attributed to the different 143
contents of 1a embedded in PVA matrix. The photochromic 144
respectively. The lines were sharpened, which demonstrated that
the powders of 1a have highly crystalline structures. But the pure
PVA film showed an amorphous morphology and presented a
broad diffraction peak at 19.7o (2
characteristic diffraction peaks at 5.9o (2
were enhanced with increased contents of 1a in films, and the
broad diffraction peak at 19.7o (2
) of PVA still existed in the
u
). We further found that the
) of 1a in composite films
u
u
composite film. These results also provided strong evidence for
well dispersing of 1a in the PVA film.
The state of 1a in polymer film was further evidenced by FE-
SEM. Based on the above analysis, we know that 1a powders were
dispersed into the polymer matrix without any structural
alteration. As shown in Fig. 3, the surface of the pure PVA film
was very smooth. However, the surface of composite films with
different contents of 1a dispersed in PVA matrix became rougher,
which showed that rod of 1a was embedded and dispersed in the
polymer matrix.
Fig. 4. UV–vis spectra of 1a/PVA composite films (a: pure 1a, b: 0.5 wt%, c: 1 wt%, d: 2 wt%) with different irradiation times at room temperature.
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite
films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037

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Fig. 5. Fading kinetics of 1b (a) and 1b (1 wt%) in PVA film (c) at different temperatures. (b) and (d) photocoloration-thermalbleaching cycles upon alternating irradiation with
UV light and heating for 1a and 1a in PVA film, respectively.
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mechanism of 1a embedded in PVA film is the same as that of 1a in
powders.
1b and 1b/PVA composite films were 393 and 368 K, respectively.
The reversible properties of 1a and 1a in PVA films were further
investigated by alternating irradiation with 365 nm and heating
for 10 cycles (Fig. 5b,d and Fig. S2). The results indicate that the
photochromism of 1a and 1a in PVA matrix exhibit higher fatigue
resistance and sensitivity.
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We further investigated the kinetic of 1a powders and the
different contents of 1a in PVA films by monitoring their maximum
absorbance changes as a function of irradiation time (Fig. S1 in
Supporting information). The first-order kinetics was observed by
fitting the experimental data to the equation -kt = 1n[(A-A0)/(A-
At)], and the rate constants of 1a and 1a in the PVA matrix obtained
from the slope were k1a = 0.0277 minÀ1, k0.5 wt% = 0.0342 minÀ1, k1
wt% = 0.0285 minÀ1 and k2 wt% = 0.026 minÀ1, respectively. We
found that the composite films with different contents of 1a in
the PVA matrix had different coloration rates, the coloration rate
turning slower with increased content of 1a powders.
3.4. Photochromism 1a/PVA films for rewritable information storage
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The 1a/PVA composite films are suitable material for optical
information recording. As can be seen from Fig. 6, the practical
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3.3. Thermal bleaching reactivity
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In order to understand the impact of microenvironment on the
composite film, the thermal bleaching of the photochromic
compound 1b and 1b in PVA matrix were investigated at different
temperatures (Fig. 5a,c). It was found that 1b and 1b/PVA (1 wt%)
film could fade at a high speed at elevated temperatures. But the
fading speed of composite films was faster than that of 1b at the
same temperature. Sample 1b needed 900 s to be completely
bleached; however, 1b/PVA film (1 wt%) only required 150 s at
373 K. We hypothesized that this phenomenon may be due to the
interaction of PVA and 1b in the heating process. We further found
that the decline curve ascended with continuous heating at 373 K
when bleaching of composite film completely finished. The 1b/PVA
(0.5 wt%) and 1b/PVA (2 wt%) composite films showed similar
phenomenon as shown in Fig. S2 in Supporting information. This
may be due to the ascension of the kinetic curves of pure PVA film
at different temperatures, as shown in Fig. S3 in Supporting
information. Thus the best thermal bleaching temperature for pure
Fig. 6. Photographs of 1a/PVA (1 wt%) film before (a, c) and after (b, d) UV irradiation
for 2 h. The image d was covered by a mask of ‘‘V’’ during UV exposure.
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optical images of rewritable photoimaging on 1a/PVA composite
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Novel photochromic composite films were successfully pre-
pared based on the pyrazolone derivative 1a embedded in the PVA
matrix without any structural alteration by a facile and simple
casting technique. The surfactant PVA has been confirmed to be
responsible for the good distribution of 1a in composite films. The
mechanism of photochromic films resulted from the tautomeriza-
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pure 1a in crystalline state. The composite films exhibit excellent
fatigue resistance under alternating UV irradiation and heating.
PVA polymer film provides good thermal conductivity, high
mechanical strength and transparency for constructing composite
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Acknowledgments
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This work was supported by the National Natural Science
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National 973 Program on Key Basic Research Project of China
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films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037