Solid-state photochromic behavior and thermal bleaching kinetics of two novel pyrazolone phenylsemicarbazones

Article abstract of DOI:10.1002/cphc.201001070

Two novel photochromic compounds, 1,3-diphenyl-4-benzal-5-hydroxypyrazole 4-phenylsemicarbazone (1a) and 1,3-diphenyl-4-(4-nitrobenzal)-5-hydroxypyrazole 4-phenylsemicarbazone (2a), are synthesized and characterized by elemental analysis, mass spectrometr

Full text of DOI:10.1002/cphc.201001070

DOI: 10.1002/cphc.201001070
Solid-State Photochromic Behavior and Thermal Bleaching
Kinetics of Two Novel Pyrazolone Phenylsemicarbazones
Samat Abdurehman,[a] Lang Liu,[a] Dianzeng Jia,*[a] Jianping Hu,[a] Jixi Guo,[a] and
Xiaolin Xie[b]
Two novel photochromic compounds, 1,3-diphenyl-4-benzal-5-
hydroxypyrazole 4-phenylsemicarbazone (1a) and 1,3-diphen-
yl-4-(4-nitrobenzal)-5-hydroxypyrazole 4-phenylsemicarbazone
(2a), are synthesized and characterized by elemental analysis,
ance, and reversible fluorescence switching properties in the
solid state in comparison to reported pyrazolone thiosemicar-
bazones. The thermal bleaching process obeys first-order kinet-
ics. Bleaching of powders at 1308C is completed within 90 s
for 1b (the colored isomer of 1a) and 150 s for 2b (the col-
ored isomer of 2a). The activation energy for the thermal
bleaching process is determined to be 69 and 95 kJmolꢀ1, with
frequency factors of 9.5ꢀ107 and 9.4ꢀ1010 sꢀ1 for 1b and 2b,
respectively.
1
mass spectrometry, FTIR spectroscopy, and H NMR spectrosco-
py. Their properties, including photochromic behavior, fluores-
cence properties, and thermal bleaching kinetics, are investi-
gated. The results show that the two compounds exhibit im-
proved photochromic performance in coloration and thermal
bleaching rates, excellent photostability, high fatigue resist-
1. Introduction
Photochromic materials are a well-known class of molecules
that change color upon irradiation with light; the photogener-
ated species can be converted back to the initial species either
by heat or by subsequent irradiation with a specific wave-
length of light. The main interest in photochromic materials fo-
cuses on their applications, including optical waveguides and
shutters, optical data storage, and ophthalmic plastic lenses.[1]
Solid-state photochromism, which is rarely encountered, en-
ables prospective potential applications in information stor-
age,[2,3] molecular machines,[4] optical switches, displays, sen-
sors,[5] and nonlinear optics.[6] Appropriate switching rates, par-
ticularly the thermal bleaching rate, are indispensable for cer-
tain applications, such as optical data processing and light
modulators.[7] High fatigue resistivity and fast fading kinetics
are currently attracting much interest for use in vehicles.[8] Due
to the wide diversity of the technical applications of photo-
chromic compounds, requirements with respect to their spec-
tral, thermodynamic, and kinetic characteristics are manifold
and the synthesis of compounds with desirable properties
must be based on the results of investigations into the general
relationship between their molecular structures and spectral
and kinetic properties. Extensive investigations have been car-
ried on spiropyrans, spiroxazine, dithienylethenes, fulgides, and
Schiff bases.[9] However, most of them show reversible photo-
chromic behavior only in solution. It is so far rather rare to find
single-component solid-state photochromism, whereas solid-
state photochromic materials for constructing optical switches
are indispensable for practical applications of optical switches.
Thus, the development of new compounds with photochromic
behavior in the solid state is very important for their applica-
tions.
We have investigated a series of photochromic compounds
derived from pyrazolones condensed with thiosemicarbazide
derivatives,[10] but the majority show irreversible photoisomeri-
zation behavior. Thus, many efforts have been dedicated to de-
signing the structures by changing the substituent groups at
the 3- or 4-position on the pyrazolone ring and using other
thiosemicarbazone derivatives; for example, 1-phenyl-3-
methyl-4-(2-fluorobenzal)-5-hydroxypyrazole 4-methylthiosemi-
carbazone/4-ethylthiosemicarbazone[10a] and 1,3-diphenyl-4-(2-
chlorobenzal)-5-hydroxypyrazole
4-methylthiosemicarbazo-
ne,[10b] which exhibit reversible photoisomerization in the solid
state due to the transformation between an enol form and a
keto form through hydrogen bonds upon irradiation with light
or by heating. Unfortunately, their performances are unsatisfac-
tory, so their derivatives need to be further developed by
structural modification to improve the photochromic proper-
ties. Lately, a kind of semicarbazone derivative, 1,3-diphenyl-4-
(3-bromobenzal)-5-hydroxypyrazole 4-phenylsemicarbazone,[11]
has been synthesized successfully by introducing the phenylse-
micarbazide unit, which exhibits excellent photostability, high
[a] S. Abdurehman, Dr. L. Liu, Prof. D. Z. Jia, J. P. Hu, Dr. J. X. Guo
Key Laboratory of Material and Technology for Clean Energy
Ministry of Education, Key Laboratory of Advanced
Functional Materials, Autonomous Region
Institute of Applied Chemistry, Xinjiang University
Urumqi 830046, Xinjiang (China)
Fax: (+86)991-8588883
E-mail: jdz0991@gmail.com
[b] Prof. X. L. Xie
National Anti-counterfeit Engineering Research Center
School of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
Wuhan 430074 (China)
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Photochromic Behavior and Thermal Bleaching of Pyrazolone Phenylsemicarbazones
fatigue resistance, and good reversible photochromic and fluo-
rescence properties in the solid state in comparison to those
of pyrazolone thiosemicarbazones reported previously.[10] All
these results gave us encouragement to make novel photo-
chromic compounds. In addition, we previously studied the
steady-state photochromic properties of pyrazolone thiosemi-
carbazones in the solid state, but rarely paid attention to kinet-
ic studies on the thermal bleaching reactions (rate constants
and activation energies) and the activation parameters
¼
¼
¼
(DH ,DS , and DG ), which may open a new pathway for the
design of compounds with outstanding photochromic proper-
ties.
Herein, we describe the synthesis and photochromic behav-
ior of two newly designed pyrazolone 4-phenylsemicarbazone
derivatives (Scheme 1). Compared with previously reported
pyrazolone thiosemicarbazones, the two compounds exhibit
an improved photochromic performance in coloration and de-
coloration rates as well as high fatigue resistivity, in particular
rapid thermal bleaching kinetics in the solid state. Moreover,
the effect of substituents on the phenyl group of the 4-posi-
tion of the pyrazolone ring on the thermal bleaching kinetics is
discussed.
2. Results and Discussion
2.1. Photochromic and Fluorescence Properties in the Solid
State
The absorption spectra of 1a and 2a induced by photoirradia-
tion at room temperature in the solid state are shown in
Figure 1. Upon irradiation by 365 nm light, the white powders
1a change to yellow due to photoinduced proton transfer
from one moiety to another. At the same time, a new broad
band in the range of 370–450 nm appears and its intensity in-
creases with irradiation time (Figure 1A). After 40 min of irradi-
ation, no more evolution in the spectra is observed, which in-
dicates that the keto form (1b) is obtained.[12,13] The proton
transfer and configuration rearrangement of the p electrons
lead to significant absorption spectral changes in the solid
state.[14] However, the yellow keto form (1b) can revert slowly
to the white enol form (1a) in the dark at room temperature.
Upon heating at 1208C, the yellow powders rapidly turn to
white, which is accompanied by the disappearance of the ab-
sorption band in the range of 370–450 nm. Obviously, the
Figure 1. Absorption spectra of 1a and 2a under irradiation by 365 nm
light, and thermal bleaching spectra of 1b and 2b by heating at 1208C in
the solid state. A) Absorption spectra of 1a (0, 10, 20, 40 min). B) Absorption
spectra of 2a (0, 10, 36, 55, 113, 158, 208, 223 min). Insets: first-order kinetic
plots for the photocoloration reaction of 1a and 2a under 365 nm light.
sample undergoes a complete reverse transformation from the
keto form (1b) to the enol form (1a). When the bleached pow-
ders are re-exposed to UV light, they turn yellow again; the ab-
sorption intensity is approximately that of the first colored
state. The photochemical process exhibits good reversibility.
Similarly, upon irradiation with 365 nm light, pale yellow
powders 2a turned yellow and the maximum absorption ap-
peared at 460 nm as the keto
form (2b) was generated (Fig-
ure 1B). Additionally, when the
sample was put in the dark or
even exposed to light for more
than half a year at room temper-
ature, no changes in its UV spec-
tra were observed, which indi-
cates that the keto form is very
stable and retains its coloration
memory for a rather long time.
Scheme 1. Synthesis route and photoisomerization process of 1a and 2a. DPBP-PSC=1,3-diphenyl-4-benzal-5-hy-
droxypyrazole 4-phenylsemicarbazone; DP4NO2BP-PSC=1,3-diphenyl-4-(4-nitrobenzal)-5-hydroxypyrazole 4-phe-
nylsemicarbazone.
However, the yellow material
(keto form, 2b) reverts to pale
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2339

D. Z. Jia et al.
yellow (enol form, 2a) by heating. Compared with that of 1b,
the absorption maximum of 2b has a red-shift phenomenon
due to the introduction of the nitro substituent on the phenyl
group of the 4-position of the pyrazolone ring, which indicates
that substituents on this phenyl group have a significant effect
on the photochromic properties.
The corresponding first-order rate constant of the photoiso-
merization was determined by fitting the experimental data to
Equation (1):[15]
kt ¼ ln½ðA1 ꢀ A0Þ=ðA1 ꢀ AtÞꢁ
ð1Þ
where A0, At, and A are the observed data corresponding to
1
400 and 460 nm at time zero, time t, and infinite time of the
reaction, respectively; k represents the photocoloration rate
constant. The photocoloring process is fitted to a first-order re-
action, and the kinetic constant k is 7.15ꢀ10ꢀ4 sꢀ1 for 1a and
1.45ꢀ10ꢀ4 sꢀ1 for 2a from the kinetic plot in the inset of Fig-
ure 1A and B, respectively. Obviously, the photocoloring rate
of 1a is five times faster than that of 2a.
For the previously reported compound 1,3-diphenyl-4-(2-
chlorobenzal)-5-hydroxypyrazole
4-methylthiosemicarbazo-
ne,[10b] the kinetic constant of the enol-to-keto photoisomeriza-
tion is 1.72ꢀ10ꢀ5 sꢀ1, which indicates that its photoisomeriza-
tion reaction is slower than that of 1a. Furthermore, under irra-
diation by 365 nm light, a new absorption band appears in the
range of 400–500 nm, which has a red-shift phenomenon in
comparison with that of 1b. The results indicate that the
oxygen atom on the side chain and the phenyl group at the
terminal have a significant effect on the photochromic proper-
ties. The incorporation of the phenylsemicarbazide leads to the
formation of a novel system of pyrazolone phenylsemicarba-
zone, which enhances the molecular electronic conjugation,
thereby resulting in an increase of the photocoloring rate.
However, for 1,3-diphenyl-4-(3-bromobenzal)-5-hydroxypyra-
zole 4-phenylsemicarbazone (DP3BrBP-PSC),[11] the kinetic con-
Figure 2. Photochromic cycles under irradiation by 365 nm light and heating
at 1208C of 1 and 2 in the solid state.
stant of the enol-to-keto photoisomerization is 1.13ꢀ10ꢀ3 sꢀ1
,
We further investigated the fluorescence properties of the
two compounds during their photochromic reaction in the
solid state, as shown in Figure 3. A characteristic fluorescence
band for the enol form 1a was observed at 360 nm by 250 nm
excitation. The intensity decreased gradually with continuous
irradiation by 365 nm light, and after about 30 min it stayed
constant, which indicates that the white enol form (1a) had
changed to the yellow keto form (1b). The fluorescence inten-
sity could be returned fully to the initial state after heating at
1208C. A reversible fluorescence switch was realized along
with the photochromic reaction. Compound 2a showed a simi-
lar emission spectrum under the irradiation by 365 nm light
when excited at 320 nm; a fluorescence band was observed at
460 nm, its intensity decreased gradually with irradiation time,
and the keto form 2b was obtained after irradiation for
66 min. Furthermore, it was found that the fluorescence peak
of 1a exhibits an obvious blue shift by as much as 100 nm in
comparison to 2a, which shows clearly that the substituent on
the phenyl group of the 4-position of the pyrazolone ring has
a significant effect on the position of the emission peak.
which indicates that its photoisomerization reaction is faster
than that of 1a and 2a. These results further confirm that the
substituents on the phenyl group of the 4-position of the pyra-
zolone ring have a significant effect on the photochromic
properties. The substituent Br increases the rate of photoiso-
merization, but the NO2 group decreases the rate in compari-
son to 1a.
For practical applications as photochromic materials, it is in-
teresting to investigate the reversible photochromic behavior
in the solid state. As shown in Figure 2, photoisomerization re-
versible cycle experiments of 1 and 2 were performed. There
was no significant degradation after 15 or 10 cycles for 1 and
2, respectively. However, the fatigue resistance of 1 is better
than that of 2, which may be attributed to the substituent
effect. These results indicate that the photochromism of pyra-
zolones 1 and 2 exhibits good reversibility, excellent photosta-
bility, and high fatigue resistance. Thus, they may be promising
candidates for optoelectronic applications, such as high-desti-
ny three-dimensional optical recording media, optical switches,
and color displays.[16]
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Photochromic Behavior and Thermal Bleaching of Pyrazolone Phenylsemicarbazones
Figure 3. Fluorescence changes of 1a (excited at 250 nm) and 2a (excited at
320 nm) under irradiation by 365 nm light.
Figure 4. Decay profiles of 1b and 2b, monitored at 400 and 460 nm in the
solid state at 90, 100, 110, and 1308C.
2.2. Thermal Bleaching Reactivity
Table 1. First-order rate constants for 1b!1a (k) and 2b!2a (k) esti-
mated from the decay curves.
Before performing the measurements, the powders 1a/2a
were irradiated with 365 nm light for a sufficient amount of
time so that they changed to 1b/2b. Then we investigated
the thermal bleaching of 1b and 2b at various temperatures.
Figure 4 shows the decay curves of the absorbance at 400 (1b)
and 460 nm (2b) in the temperature range from 90 to 1308C.
It takes nearly 600 and 1500 s to completely decolor 1b and
2b at 908C, respectively, whereas the decoloring was accelerat-
ed with an increase of temperature. Complete decay took
nearly 90 and 150 s at 1308C for 1b and 2b, respectively. Thus
the temperature has a direct effect on the bleaching process.
To gain more insight into the corresponding photochrom-
ism,[15] thermal bleaching kinetic rate constants can be deter-
mined from the plot of ln[(A ꢀA0)/(A ꢀAt)] against time (t).
1b!1a
k [sꢀ1
2b!2a
k [sꢀ1
T [K]
]
]
363
373
383
403
1.03ꢀ10ꢀ2
2.39ꢀ10ꢀ2
4.78ꢀ10ꢀ2
1.02ꢀ10ꢀ1
1.94ꢀ10ꢀ3
3.83ꢀ10ꢀ3
1.01ꢀ10ꢀ2
4.24ꢀ10ꢀ2
seen that the substituent may play an important role in deter-
mining the thermal bleaching rates.
We then determined the activation energy of the bleaching
process by applying the Arrhenius equation in the 363–403 K
temperature intervals [Eq. (2)]:
1
1
The first-order kinetic curves are shown in Figure 5. The first-
order rate constants (k) of 1b!1a and 2b!2a are summar-
ized in Table 1. The rate constant k was evaluated to be 1.02ꢀ
10ꢀ1 and 4.24ꢀ10ꢀ2 sꢀ1 at 1308C for 1b and 2b, respectively.
The results indicate that the photochromic compound with a
nitro substituent (2b) shows a slower bleaching rate than that
of the unsubstituted one (1b). Thus, rate constants are strong-
ly dependent on the structure of the compounds. It can be
Ea
RT
ð2Þ
ln k ¼ ln A
Figure 6 shows the temperature dependence of the rate
constant for the thermal bleaching reaction of 1b and 2b.
Taking k as the rate constant for the reverse reaction, an Arrhe-
nius plot of lnk as a function of 1/T reveals a straight line. Con-
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2341

D. Z. Jia et al.
Figure 5. First-order kinetic plots for the thermal bleaching reaction of 1b!
1a and 2b!2a at various temperatures in the solid state.
Figure 6. Arrhenius plot of the thermal bleaching rate constant of 1b!1a
and 2b!2a.
sequently, the activation energy (Ea) and frequency factor (A)
were determined: Ea =69 kJmolꢀ1 and A=9.5ꢀ107 sꢀ1 for 1b;
Ea =95 kJmolꢀ1 and A=9.4ꢀ1010 sꢀ1 for 2b. Note that the fre-
quency factor of 1b is smaller than that of 2b by more than
three orders of magnitude. The relatively small frequency
factor implies that the reverse reaction involves specific reor-
ientation and a large reduction of the entropy factor.[17] The
decrease in Ea values enhanced the thermal back reaction, in
other words, the slower bleaching rate is due to the slightly
higher activation energy.
Table 2. Thermodynamic parameters for the thermal bleaching process
in the solid state.
¼
¼
¼
Compound
DH [kJmolꢀ1
]
DS403K [JKꢀ1 molꢀ1
]
DG403K [kJmolꢀ1
]
1b!1a
2b!2a
66
92
ꢀ102
ꢀ45
107
110
To further analyze the reactions, by using the Eyring equa-
tion the activation parameters of the thermal back reactions
1b is lower than that for 2b, whereas there is a very significant
decrease of DS for 1b relative to 2b.
¼
¼
¼
and the enthalpies and entropies of activation (DH and DS ,
respectively) were evaluated in the temperature range from 90
to 1308C [Eq. (3)]:
The kinetic parameters of the thermal bleaching, determined
in the appropriate temperature range for each compound,
were satisfactorily fitted to Arrhenius and Eyring relationships,
which allowed kinetic (rate constants and activation energies)
¼
¼
k
T
DH 1
kB DS
ð3Þ
ln ¼ ꢀ
þ ln
þ
¼
¼
¼
R
T
h
R
and activation parameters (DH , DS , and DG ) of the thermal
reactions to be determined. These parameters are significantly
different for the two compounds. The nitro substituent on the
phenyl group of the 4-position of the pyrazolone ring increas-
es both the activation energy and the frequency factors of the
thermal bleaching reaction due to its strong electron-with-
drawing properties.
As shown in Figure 7, both Eyring plots show an excellent
¼
¼
straight line; the DH and DS values from standard least-
squares analysis of the Eyring plots are summarized in Table 2.
The activation free energy barrier is calculated from
¼
¼
¼
¼
DG ¼ DH ꢀ TDS . From Table 2, it can be seen that DH for
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Photochromic Behavior and Thermal Bleaching of Pyrazolone Phenylsemicarbazones
(365 nm, 1579 mWcmꢀ2 on the sample) was applied with a ZF-8 ul-
traviolet analysis instrument; the reflective filter size was 200ꢀ
80 mm2 and the distance between sample and light source was
15 cm. Fluorescence spectra were obtained using a Hitachi F-4500
fluorescence spectrophotometer. FTIR spectra were obtained on a
Bruker VERTEX-70 spectrometer with the samples as KBr pellets.
1H NMR spectra were recorded on an Inova-400 NMR spectrometer
with [D6]DMSO as the solvent and TMS as internal standard. The
mass spectra were determined with an HP 1100 liquid chromato-
graph–mass spectrometer. Elemental analysis was performed with
a FlashEA 1112 elemental analyzer. Melting points (uncorrected)
were determined with a TECH X-6 melting point apparatus. Pow-
ders for testing photochromic properties were directly obtained
from the synthesis without any further processing.
Materials: 4-Phenylsemicarbazide (PSC), 4-nitrobenzoyl chloride,
and benzoyl chloride were purchased from Aldrich Company, USA.
All other chemicals and solvents were purchased from commercial
suppliers and used as received without additional purification. 1,3-
Diphenyl-5-pyrazolone (DPP),[18] 1,3-diphenyl-4-benzoyl-5-hydroxy-
pyrazole (DPBP), and 1,3-diphenyl-4-(4-nitrobenzoyl)-5- hydroxypyr-
azole (DP4NO2BP)[19] were synthesized according to the literature.
Synthesis of 1,3-Diphenyl-4-benzal-5-hydroxypyrazole 4-Phenylse-
micarbazone (1): A mixture of DPBP (3 mmol) and PSC (3 mmol) in
EtOH (20 mL) solution, containing a few drops of glacial acetic
acid, was stirred for 6 h at 808C in an oil bath. After cooling to
room temperature, white powders were obtained. The crude prod-
uct was filtered and purified by recrystallization using EtOH. Yield:
1
84%; m.p. 196.7–198.58C; H NMR ([D6]DMSO): d=11.900 (1H, Pz-
OH), 9.737 (1H, N-H), 9.102 (1H, N-H), 7.950–6.977 ppm (20H,
phenyl ring); IR (KBr): n˜ =3343, 3331 (NꢀH), 1655 (C=O), 1599 (C=
N), 1556, 1450 (phenyl ring), 1509, 1394 cmꢀ1 (pyrazole ring); MS:
[M+H]+: calcd: 474.19; found: 474.2; elemental analysis calcd (%)
for C29H23N5O2: C 73.37, H 4.79, N 14.81; found C 73.59, H 4.90, N
14.79.
Figure 7. Eyring plots for the thermal bleaching reaction over a temperature
ranging from 363 to 403 K.
Synthesis of 1,3-Diphenyl-4-(4-nitrobenzal)-5-hydroxypyrazole 4-
Phenylsemicarbazone (2): Compound 2 was synthesized by the
same method as 1. Yield: 78%; m.p. 215.4–217.08C; 1H NMR
([D6]DMSO): d=12.134 (1H, Pz-OH), 10.119 (1H, N-H), 9.217 (1H, N-
H), 8.203–7.004 ppm (19H, phenyl ring); IR (KBr): n˜ =3364, 3342
(NꢀH), 1649 (C=O), 1600 (C=N), 1556, 1454 (phenyl ring), 1523,
1348 cmꢀ1 (pyrazole ring); MS: [M+H]+: calcd: 519.17; found:
519.0; elemental analysis calcd (%) for C29H22N6O4: C 67.17, H 4.28,
N 16.21; found: C 67.28, H 4.34, N 16.16.
3. Conclusions
Two novel pyrazolones (1 and 2) have been synthesized suc-
cessfully by introducing a phenylsemicarbazide unit. They
show reversible photoisomerization upon irradiation by UV
light and heating, excellent photostability, high fatigue resist-
ance, and reversible fluorescence properties in the solid state.
Additionally, based on the kinetics of the thermal bleaching re-
actions, the rate constants, activation energies, and activation
¼
¼
¼
parameters (DH , DS , and DG ) are obtained. The results indi-
cate that the nitro substituent on the phenyl group of the 4-
position of the pyrazolone ring increases both the activation
energy and the frequency factors of the thermal bleaching re-
action, thereby leading to a decrease in the photocoloring
rate, especially in the thermal bleaching rates, and the red-shift
maximum absorption band due to a strong electron-withdraw-
ing effect. The findings may open a new pathway for the
design of compounds with outstanding photochromic proper-
ties.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation China (Nos.20762010, 21061014), the Program for New
Century Excellent Talents in Universities of China (NCET-09–0904),
the Key Project of the Ministry of Education, China (209138), and
the Science Foundation of Xinjiang (BS090116).
Keywords: fluorescence
pyrazolones · semicarbazides
·
kinetics
·
photochromism
·
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