Synthesis, properties, and mechanism of two novel photoisomerization pyrazolones in the solid state

Article abstract of DOI:10.1016/j.molstruc.2012.06.001

Two novel photoisomerization compounds derived from pyrazolones have been synthesized successfully. The photoisomerization properties of materials have been carefully investigated by measuring the UV-vis absorption and fluorescence spectra of the material

Full text of DOI:10.1016/j.molstruc.2012.06.001

Journal of Molecular Structure 1035 (2013) 271–276
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, properties, and mechanism of two novel photoisomerization
pyrazolones in the solid state
a
a
a
b,c
Mingxi Guo a, Jixi Guo a, Dianzeng Jia a, , Hu Liu , Lang Liu , Anjie Liu , Feng Li
⇑
a 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, PR China
b State Laboratory of Surface and Interface Science and Technology, School of Materials and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, Henan,
PR China
c University of Texas, M.D. Anderson Cancer Center, Houston, TX 77002, USA
h i g h l i g h t s
"
"
"
Two novel photoisomerization compounds have been synthesized and demonstrated.
Their color changed from yellow to brown by visible light irradiation which observed for the first time in this system.
The high changes of fluorescence intensity between the ‘‘on’’ and ‘‘off’’ states are significant in the solid state.
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel photoisomerization compounds derived from pyrazolones have been synthesized successfully.
The photoisomerization properties of materials have been carefully investigated by measuring the
UV–vis absorption and fluorescence spectra of the materials. It was found for the first time among the
derivatives of pyrazolones that the compounds change their color from yellow to brown with an
efficiency of more than 95% of modulating the fluorescence behaviors, after irradiated with visible light.
The unique fluorescent changes of the two compounds between their ‘‘on’’ and ‘‘off’’ states in the solid
state indicate that the materials could be used as building blocks to construct novel optical switches.
The mechanism of the photoisomerization was further studied with FT-IR, XPS and X-ray single crystal
diffraction. The photoreaction resulted from isomerization between the enol and keto forms of the
materials is accompanied by intra- and inter-molecular proton transfer.
Received 23 March 2012
Received in revised form 16 May 2012
Accepted 4 June 2012
Available online 11 June 2012
Keywords:
Pyrazolone
Photoisomerization
Fluorescence modulation
Optical switch
Proton transfer
Hydrogen bond
Ó 2012 Published by Elsevier B.V.
1. Introduction
successfully in our labs to modulate the photoreaction property
in the solid state [17–19]. Upon irradiated with UV light of
The development of organic photoreaction molecules has at-
tracted considerable interests, because of their diverse potentials
in the fields of optical memory [1–4], molecular switches [5–7],
molecular sensors [8,9] and information processors [10,11]. How-
ever, most of organic photoreaction compounds only show photo-
reaction properties in solution [12,13], which dramatically limits
their applications in optical solid state devices. Pyrazolones, which
can exhibit excellent photoreaction properties in the solid state,
will be the most potential materials for actual applications
[14–16]. As promising candidates for designing and fabricating
photonic switches, a series of photoisomerization compounds
derived from pyrazolones have been designed and synthesized
365 nm, the as-prepared materials change their color from white
to yellow, and then return to white again after kept in visible light
or heated. Their photoisomerization properties, however, are also
limited in both responding to ultraviolet and changing their color
from white to yellow only. Toward the end of designing and
fabricating optic switches, it is desirable to further modulate their
photoreaction property through tailoring the structures of
pyrazolones.
Herein, two novel photoisomerization compounds, 1,3-diphenyl-
4-(3-chlorobenzal)-5-hydroxypyrazole4,4-Dimethyl-3-thiosemicar-
bazide (1a)and1,3-diphenyl-4-(3-bromobenzal)-5-hydroxypyrazole
4,4-Dimethyl-3-thiosemicarbazide (2a) were synthesized by intro-
ducing dimethylthiosemicarbazide (Scheme 1) into pyrazolones
for modulating their optical properties. The materials were
characterized with 1H NMR, MS and elemental analysis. The
⇑
Corresponding author. Tel.: +86 991 8583083; fax: +86 991 8588883.
E-mail address: jdz0991@gmail.com (D. Jia).
0022-2860/$ - see front matter Ó 2012 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.molstruc.2012.06.001

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M. Guo et al. / Journal of Molecular Structure 1035 (2013) 271–276
and all of the other hydrogen atoms were placed in geometrically
idealized positions and refined using riding model.
2.2. Synthesis
1.3-Diphenyl-5-pyrazolone was synthesized with the method
described in literatures [20]. 4,4-Dimethyl-3-thiosemicarbazide,
3-chlorobenzoylchloride, 3-bromobenzoylchloride and other
materials were purchased from commercial suppliers. The solvents
were purified with standard procedures.
2.2.1. 1,3-Diphenyl-4-(3-chlorobenzal)-5-hydroxypyrazole 4,4-Dimethyl-
3-thiosemicarbazide (1a)
1,3-Diphenyl-4-(3-chlorobenzal)-5-pyrazolone were synthesized
with the method reported [21]. Yield: 68%; M.p. 153.5–154.9 °C;
Anal.Calcd for C22H15N2O2Cl (%): C, 70.50; H, 4.03; N, 7.47. Found
C, 71.39; H, 4.83; N, 8.02.
Compound 1a was prepared by refluxing 1,3-diphenyl-
4-(3-chlorobenzal)-5-pyrazolone (1.1245 g, 3 mmol) and 4,4-Di-
methyl-3-thiosemicarbazide (0.3575 g, 3 mmol) were dissolved in
EtOH (15 mL) together with a few drops of glacial acetic acid,
and the mixture was stirred and refluxed for 1 h at 80 °C (Scheme
2). After cooling down to room temperature in the dark, the solid
was separated by filtration and washed with ethanol to afford
the yellow powders. Yield: 81%; M.p. 175.4–176.9 °C; Anal.Calcd
for C25H22N5OSCl (%): C, 63.08; H, 4.65; N, 14.71. Found C, 62.98;
H, 4.54; N, 14.63; MS (m/z): 476.1 [M]+; 1H NMR (400 MHz,
Scheme 1. Photochemical reactions of 1 and 2.
photoreactive properties of the materials were further investigated
by measuring their UV–vis absorption and fluorescence spectra.
FT-IR, XPS and crystallographic structure analysis were studied
carefully for a deeper insight into the photoreactions of the mate-
rials and understanding their photoreactive mechanism. It was
found that the materials undergo a photoisomerization from the
enol form to the keto form through an intra- and inter-molecular
proton transfer upon visible light irradiation. The new compounds
change their color from yellow to brown upon irradiation only
with visible light, and their fluorescent modulation efficiency is
higher than 95%. The materials exhibit novel photoisomerization
for the difference of their structure, compared to those reported
by this group previously.
DMSO-d6):
8.083–6.963 (m, 14H, phenyl-ring), 3.365–3.326 (m, 6H,
CH3 + CH3); FT-IR (
, cmꢀ1): (before irradiation): 3123, 3063
(NAH), 1626 (C@O), 1548, 1596 (C@N), 1494
(phenyl), 1532, 1455 (pyrazole-ring); (after irradiation): 3122,
(NAH), 1627 (C@O), 1547, 1596 (C@N),
(phenyl), 1530, 1455 (pyrazole-ring).
d 12.713 (s, 1H, Pz-OH), 12.017 (s, 1H, N4AH),
m
t(NAH), 2204
t
t
t
t
t
3062
1502
t
t
(NAH), 2201
t
t
t
t
2.2.2. 1,3-Diphenyl-4-(3-bromobenzal)-5-hydroxypyrazole 4,4-
Dimethyl-3-thiosemicarbazide (2a)
2. Experimental
1,3-Diphenyl-4-(3-bromobenzal)-5-pyrazolone were synthe-
sized with the method reported [21]. Yield: 84.9% M.p. 104.8–
106.7 °C. Anal.Calcd for C22H15N2O2Br (%): C, 63.02; H, 3.61; N,
6.68. Found C, 62.83; H, 3.68; N, 6.92.
2.1. General
1H NMR spectra were recorded using an INOVA-400 NMR Spec-
trometer with DMSO-d6 as the solvent. Melting points were taken
on a TECH XT-5 melting point apparatus. Mass spectrum were
determined with HP1100 LC-MS using electrospray ionization
mass spectrometry. The elemental analyses were made on FLASH
EA 1112 Series NCHS-O analyzer. UV–visible absorption spectra
were studied using a Hitachi UV-3010 spectrometer equipped with
an integrating sphere accessory. Fluorescence spectra were
measured on a fluorescence spectrophotometer (Hitachi F-4500).
FT-IR spectra were recorded by using infrared diffuse reflectance
spectroscopy in the range of 400–4000 cmꢀ1 on a BRUKER EQUI-
NOX-55 spectrometer. X-ray photoelectron spectra (XPS) were
measured with a Perkin–Elmer PHI 5300 System. A tungsten lamp
(k > 400 nm, 40 W) with a color filter was used as visible light
source.
Compound 2a was prepared by an analogous method to 1 and
obtained as a yellow solid in a yield: 83%; M.p. 172.2–174.5 °C;
Anal.Calcd for C25H22N5OSBr (%): C, 57.70; H, 4.26; N, 13.45. Found
C, 57.61; H, 4.15; N, 13.31; MS (m/z): 522.0 [M+]; 1H NMR
(400 MHz, DMSO-d6): d 12.692 (s, 1H, Pz-OH), 12.003 (s, 1H,
N4AH), 8.085–6.962 (m, 14H, phenyl-ring), 3.325–2.778 (m, 6H,
CH3 + CH3); FT-IR (
m
, cmꢀ1): (before vis irradiation): 3123, 3062
(NAH), 1626 (C@O), 1547, 1596 (C@N), 1496
(phenyl), 1530, 1455 (pyrazole-ring); (after vis irradiation):
(NAH), 2202 (NAH), 1628 (C@O), 1547, 1596
(phenyl), 1530, 1454 (pyrazole-ring).
t(NAH), 2202
t
t
t
t
t
3122, 3061
t
t
t
t(C@N), 1502
t
t
Single crystals of 2 suitable for X-ray crystallographic work
were grown by slow evaporation of its ethanol solution at room
temperature. The crystallographic data were collected on an imag-
ing plate system (Rigaku R-AXIS SPIDER) with a graphite mono-
chromatized Mo K
a radiation (k = 0.71073 Å, x-scans). Crystal
structures was solved by direct methods and refined on F2 by
full-matrix least-squares methods with the SHELXTL-97 program.
4363 unique measured reflections were used in the refinement.
All non-hydrogen atoms were refined anisotropically. The hydro-
gen atoms on nitrogen atoms were located from the Fourier maps,
Scheme 2. Synthesis routine for 1 and 2.

M. Guo et al. / Journal of Molecular Structure 1035 (2013) 271–276
273
3. Results and discussion
3.1. UV–vis absorption spectra
The UV–vis absorption spectra change of two compounds in so-
lid state at room temperature for different irradiation time inter-
vals are recorded. As shown in Fig. 1, it can be observed that a
new band appears around 500–700 nm for 1 and 2, and their inten-
sities increase with irradiation time. However, the intensities in-
crease of 2 between 500 nm and 700 nm are little stronger than
those of 1, which could result from the substituent on phenyl of
the 3-position of pyrazolone-ring changing from Cl to Br. The re-
sults demonstrate that the photoisomerization from the yellow
enol form I to the brown keto form II (Scheme 1). Compared to
other pyrazolones reported previously, this is a new phenomenon.
After photochemical reaction, the colors of 1b and 2b are stable at
room temperature for more than one year, which indicates that the
materials are very stable in air and they can retain their coloration
memory for a very long time.
3.2. Fluorescence spectra and properties
The fluorescence spectroscopy changes of compound 1 and 2
driven by visible light were investigated at room temperature.
The emission peak of the yellow enol form of 1a is observed at
538 nm when excited at 420 nm (Fig. 2A). The intensity of the
emission gradually decreases in accompanying with prolonging
the time of irradiation. After 280 min, the yellow enol form of 1a
changes into the photostationary state of the brown keto form of
Fig. 2. Fluorescent emission spectra of powders 1 (A) and 2 (B) by visible light
(k > 400 nm) irradiation at room temperature (kex = 420 nm).
1b. For compound 1, the ratios of fluorescence contrast between
the enol form and keto form isomers is about 9:1, and the fluores-
cent modulation efficiency is 96.02%. In comparison, compound 2a
also shows similar emission behaviors after irradiation with visible
light (k > 400 nm) and excited at 420 nm, but it reaches the photo-
stationary state of keto form of 2b in only 105 min. For compound
2, however, it is noteworthy that the ratio of fluorescence contrast
between its enol form and keto form isomers is 19:1, and its effi-
ciency of fluorescent modulation is 97.71%. Compared to other
pyrazolones reported previously, the fluorescent modulation effi-
ciencies of compound 1 and 2 are much higher. The highly en-
hanced contrast fluorescent switching properties are promising
for designing optical switches.
3.3. IR and XPS spectra
To further confirm the above photochemical reaction mecha-
nism, FT-IR and XPS spectra are employed to investigate the config-
urational change in solid state. The structure differences between
enol form and keto form of 1 and 2 resulted in FT-IR and XPS spec-
tra changes clearly (Fig. 3). As shown in FT-IR, broad absorption
bands in the range of 2210–3500 cmꢀ1 are observed, characteristic
of Schiff bases with a medium strength intramolecular hydrogen
bond [22]. After visible light (k > 400 nm) irradiation at room tem-
perature, the strong sharp band of 2201 cmꢀ1 attributed to the
N3AH stretching vibration for the formation of keto form of 1b
along with relative intensity decrease of the band at 1547 cmꢀ1
(Fig. 3A). The band of 1547 cmꢀ1 can be ascribed to C@N vibration.
With visible light (k > 400 nm) irradiation, the intensity of a
Fig. 1. Absorption spectra of 1 (A) and 2 (B) powders upon the irradiation of visible
light (k > 400 nm) at room temperature.

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M. Guo et al. / Journal of Molecular Structure 1035 (2013) 271–276
Fig. 3. FT-IR spectra of 1 (A) and 2 (B) before and after visible irradiation (k > 400 nm); XPS spectra of O1s core level region of 1 (C) and 2 (D) before and after visible
irradiation (k > 400 nm).
smooth band at 1627 cmꢀ1 increases dramatically, which indicates
that its structure gradually change from CAO to C@O. Compound 2
shows similar spectra change after irradiated under visible light
(k > 400 nm) (Fig. 3B). After irradiated with visible light (k > 400
nm) at room temperature, the strong and sharp band at
2202 cmꢀ1 attributed to the N3AH stretching vibration following
atoms of keto form for 1a and 1b, respectively. We found that pop-
ulations of keto form of 1b increased along with decreasing of enol
form of 1a after illuminated by visible light. Compound 2 shows
similar spectra change under visible light (k > 400 nm) (Fig. 3D).
The first O1s peak, which occurs at 532.94 eV (53.72%) and
532.87 eV (33.39%), is assigned to oxygen atoms of enol form,
and the second peak corresponding to binding energy values of
530.95 eV (46.28%) and 530.98 eV (66.61%) is ascribed to oxygen
atoms of keto form for 2a and 2b, respectively. The result confirms
that the populations of brown keto form increases along with
decreasing of yellow enol form after illuminated by visible light.
with decreased relative intensity of a smooth band at 1547 cmꢀ1
.
The intensity of a smooth band at 1628 cmꢀ1 increases dramati-
cally, which also indicates that the structure of compound 2 grad-
ually changes from CAO to C@O. The results reveal that the two
compounds convert from enol to keto form in accompanying with
proton transfer after irradiation with visible light (k > 400 nm).
The IR spectroscopy is a very important tool to study the hydro-
gen bonded structure [23,24], but it cannot give definite structure
change corresponding to the OAH stretching band. It is well known
that the binding energies of materials are sensitive to the chemical
environmental perturbation [25], so it is possible to distinguish the
‘‘enol’’ and ‘‘keto’’ forms of pyrazolones with XPS (see Table 1).
Fig. 3C and D shows the XPS signals of the oxygen 1s core levels
for 1 and 2 before and after irradiation. The peak of O1s in XPS
spectra of powders 1 (Fig. 3C) can be decomposed into two peaks
by curve fitting. The first O1s peak at 532.37 eV (60.69%) and
532.95 eV (37.01%) is assigned to oxygen atoms of enol form, and
the second peak corresponding to binding energy values
530.36 eV (39.31%) and 530.98 eV (62.99%) is ascribed to oxygen
3.4. Crystal structure characterization and mechanism of
photoisomerism
Single crystals of 2 were obtained by slow evaporation of etha-
nol solutions at room temperature. In order to further understand
the relation between the conformation and the photoisomerization
behaviors of the pyrazolone derivatives in the crystalline phase,
the final structural confirmations of 2 were determined by X-ray
crystallographic analysis. The molecular structure of 2 is shown
in Fig. 4A. The crystal data and structure refinement details are gi-
ven in Table 2. For compound 2, the bond length of C(7)AO is
1.268(6) (Å), which is consistent with the length of C@O double
bond. It can be thus deduced that the crystal is in the keto form.
Table 1
X-ray photoelectron spectroscopy of compound 1 and 2.
Before irradiation
After irradiation
O1s binding energy (eV)
Peak assignment
O1s binding energy (eV)
Peak assignment
1a
2a
532.37 (60.69%)
530.36 (39.31%)
532.94 (53.72%)
530.95 (46.28%)
Enol form
Keto form
Enol form
Keto form
1b
2b
532.95 (37.01%)
530.98 (62.99%)
532.87 (33.39%)
530.98 (66.61%)
Enol form
Keto form
Enol form
Keto form

M. Guo et al. / Journal of Molecular Structure 1035 (2013) 271–276
275
Table 3
Hydrogen bonds for 2.
DAHꢁ ꢁ ꢁA
d(DAH)
(Å)
d(Hꢁ ꢁ ꢁA)
d(Dꢁ ꢁ ꢁA)
(DAHꢁ ꢁ ꢁA)
(°)
(Å)
(Å)
N(4)AH(4 N)ꢁ ꢁ ꢁO
0.861(10)
1.73(2)
2.50(3)
2.557(6)
3.268(5)
161(6)
149(5)
N(3)AH(3 N)ꢁ ꢁ ꢁS#1 0.859(10)
Symmetry codes: #1 ꢀx + 1, ꢀy + 2.
convenient channel to transfer protons. Under visible light irradia-
tion, the intramolecular proton transfers from the O atom to the
N(4) atom by the channel of [N(4)ꢁ ꢁ ꢁHAO]. This results in the for-
mation of another intramolecular hydrogen bond [N(4)AHꢁ ꢁ ꢁO]. At
the same time, the intermolecular proton transfer from the S0 atom
to the N(3) atom through the channel of [N(3)ꢁ ꢁ ꢁHAS0] yields an-
other intermolecular hydrogen bond [N(3)AHꢁ ꢁ ꢁS0]. Therefore, it
is safe for us to conclude that the photoisomerization from enol
(I) to keto form (II) isomers is due to proton transfers via the intra-
and inter-molecular hydrogen bonds via the intermediate state (I0)
as shown in Scheme 1. The photoreaction process of the title
compounds is much different from other analogous compounds
reported by our groups previously, which only exhibit an intermo-
lecular proton transfer from the O atom of the hydroxyl group to
the nitrogen atom of the adjoining pyrazole ring [26]. These results
verify our idea of introducing dimethylthiosemicarbazide group
into pyrazolones for modulating their photoreaction properties.
The structure of pyrazolone derivatives dramatically affects their
photoreaction properties.
4. Conclusions
Fig. 4. Molecular structure (A) and hydrogen bond connection diagrams (B) of
compound 2.
Two novel compounds of pyrazolones with dimethylthiosemic-
arbazide group (1 and 2) have been synthesized successfully. It was
found that the materials exhibit complete different photoreaction
properties, compared to other pyrazolone derivatives reported pre-
viously in our groups. The new products change their color from
yellow to brown after only irradiated with visible light. The materi-
als also have high fluorescent modulation efficiency. The unique
photoreaction property is resulted from the photoisomerization be-
tween the enol and keto forms of the materials in accompanying
with intra- and inter-molecular hydrogen bonds proton transfer.
The approach may open a new pathway for us to tune the photore-
action properties of pyrazolones through tailoring their structures.
Table 2
Crystal data and structure refinement for 2.
2
Empirical formula
Formula weight
C25H22N5OSBr
520.45
Temperature
Wavelength
Crystal system
293(2) K
0.71073 Å
Triclinic
Space group
Unit cell dimensions
Volume
P ꢀ 1
a = 9.9764(9) Å
1178.75(18) Å3
2
a = 86.820(2
Z
Calculated density
Absorption coefficient
F(000)
1.466 mg/m3
1.860 mmꢀ1
532
Supplementary data
Crystal size
0.29 ꢂ 0.18 ꢂ 0.12 mm
Crystallographic data for the structure in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplemental publication CCDC-832379. These data can be
obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 0 1223 336033 or email:
deposit@ccdc.cam.ac.uk).
Theta range for data collection
Limiting indices
Reflections collected/unique
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
3.38–25.49°
ꢀ12 ꢃ h ꢃ 12, ꢀ13 ꢃ k ꢃ 13, ꢀ14 ꢃ l ꢃ 14
9788/4363 (Rint = 0.0382)
Empirical
0.8076 and 0.6145
Full-matrix least-squares on F2
4363/2/309
1.089
Final R indices [I > 2
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
r
(I)]
R1 = 0.0565, wR2 = 0.1459
R1 = 0.1122, wR2 = 0.2226
0.024(4)
Acknowledgments
0.767 and ꢀ1.124 eÅꢀ3
This work was supported by Program for Changjiang Scholars
and Innovative Research Team in University of Ministry of
Education of China (IRT1081), the National Natural Science
Foundation of China (21061013, 21062020 and 21071130), special-
ized Research Fund for the Doctoral Program of Higher Education
(2011650113001), the Natural Science Foundation of Xinjiang
Uygur Autonomous Region of China (2009211B02).
Hydrogen bond connection diagrams of 2 are shown in Fig. 4B. The
hydrogen bonds are listed in Table 3. There are intermolecular
[N(3)AHꢁ ꢁ ꢁS0] (3.268 Å) and intramolecular [N(4)AHꢁ ꢁ ꢁO]
(2.557 Å) hydrogen bonds existing in 2, which provide
a

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M. Guo et al. / Journal of Molecular Structure 1035 (2013) 271–276
[16] H. Yuan, J.X. Guo, D.Z. Jia, M.X. Guo, L. Liu, D.L. Wu, F. Li, Photochem. Photobiol.
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