Solid-state photochromism of pyrazolones with highly improved sensitivity, fatigue resistance and reversible fluorescent switching properties

Article abstract of DOI:10.1039/c0jm03216f

Photochromic compounds derived from pyrazolones, which undergo reversible photoisomerization reactions in pure solid state, were synthesized toward the end of designing novel solid-state photonic switches. The materials exhibit high sensitivity, excellent

Full text of DOI:10.1039/c0jm03216f

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PAPER
Solid-state photochromism of pyrazolones with highly improved sensitivity,
fatigue resistance and reversible fluorescent switching properties†
a
a
a
a
b
Jixi Guo, Dianzeng Jia,* Lang Liu, Hui Yuan and Feng Li
Received 25th September 2010, Accepted 10th December 2010
DOI: 10.1039/c0jm03216f
Photochromic compounds derived from pyrazolones, which undergo reversible photoisomerization
reactions in pure solid state, were synthesized toward the end of designing novel solid-state photonic
switches. The materials exhibit high sensitivity, excellent fatigue resistance and reversible fluorescent
switching properties with distinguishable contrast signals under alternating UV irradiation and
heating. The mechanism of the photochromic reactions was verified by FT-IR and XPS. The detailed
analysis of the spectra and crystal structure of the materials revealed that their photochromism resulted
from tautomerization between their enol and keto forms accompanied by proton transfer.
Introduction
In our previous paper, we have designed and synthesized
a series of photochromic compounds derived from pyrazolone
9
condensed with thiosemicarbazide derivatives. Upon irradiation
Photochromism has attracted considerable attention since its
1
discovery in 1867 and has been investigated extensively for
with UV light of 365 nm, the materials can change their color
from white to yellow, and then partly return to white again by
keeping in visible light or heating. Unfortunately, their perfor-
mances still cannot meet the requests in optical devices concerned
with reversibility and stability for practical application. It is still
a challenge to develop new photochromic compounds with high
sensitivity and excellent fatigue resistance.
diverse applications such as optical data storage and switches
2,3
related to photochromism. Toward the end of tuning optical
properties of materials, a variety of organic photochromic
compounds have been developed successfully through molecule
4
design. However, most of the materials only exhibit a photo-
5
chromic effect in solution. It is rather rare to find single
component solid-state photochromism in reports so far, whereas
constructing optical switches based on materials in solid state is
indispensable for practical applications of optical switches.
Recently, photochromic materials were thus dispersed into
polymer matrices or sol–gel media for realizing complex solid-
state photochromism and overcoming the difficulty related to the
Herein, we successfully designed two new photochromic
compounds of 1a and 2a (Scheme 1), which exhibit highly
improved reversible photoisomerization reactions under alter-
nating UV light irradiation and heating in solid state, produced
by condensing phenylsemicarbazide with pyrazolones. The
materials exhibit much higher optical sensitivity and fatigue
6
photochromic effect in solution. The strategies, however, are
resistance in solid state, compared to those synthesized in our lab
9,10
still limited by suppressed photochromic reactivity and quantity
7
of active materials dispersed in polymer matrices. It is highly
previously.
The fluorescent emissions of the new compounds
undergo reversible ‘‘on’’ and ‘‘off’’ with high contrast signals
ꢀ
desirable to develop new photochromic systems in solid state and
even in single crystalline state for designing highly efficient
after irradiation with UV light of 365 nm and heating at 120 C,
respectively. The materials are thus promising building blocks to
8
photonic memories and photonic switches.
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, P. R. China. E-mail: jdz0991@gmail.com;
Fax: +86-991-8588883; Tel: +86-991-8583083
b
CBEN and Department of Chemistry, Rice University, Houston, TX,
77081, USA
†
Electronic supplementary information (ESI) available: Photographs of
photochromic single crystal of 1a, normalized emission spectra and XRD
spectra of powders for 1 and 2, Selected bond length, bond angles data,
hydrogen bonds and crystallographic information files (CIF) for 1a.
CCDC reference number 680802. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0jm03216f
Scheme 1 Photochromic reactions of 1 and 2
3
210 | J. Mater. Chem., 2011, 21, 3210–3215
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construct optical storage devices and switches. In order to further
understand and verify the mechanism of the photochromic
reactions of 1a and 2a, FT-IR and XPS were used to investigate
their structure changes before and after UV irradiation. In
addition, the crystallographic analysis of colorless crystal of 1a
has also been carried out for studying the photochromic process.
The results indicate that the photochromism arises from proton
transfer from the O atom of hydroxyl group to the N atom of
pyrazole-ring, accompanying their structure change from enol
form (E-form) to keto form (K-form) as shown in Scheme 1. The
fascinating systems could eventually lead to realizing novel
devices for photonic applications.
Scheme 2 Synthesis of the pyrazolones 1a and 2a
white powders were isolated from the solution and separated by
filtration. The crude product was purified by recrystallization
ꢀ
using EtOH. Yield: 84%. Mp. 204.2–205.1 C. d
ꢀ
H
(400 MHz,
Experimental
DMSO-d , 25 C, TMS): 12.016 (1H, Pz–OH), 9.842 (1H, N5–
6
H), 9.128 (1H, N4–H), 7.945–6.985 (19H, phenyl-ring). nmax
ꢁ1
/
General
cm : 3359 3340 y(N–H), 1651 y(C]O), 1600 y(C]N), 1556,
1
+
507 y(phenyl), 1520, 1454 y(pyrazole-ring). MS: m/z M : 507.80.
1
H NMR spectra were performed on an INOVA-400 NMR
Elemental analysis calcd for C H N O Cl: C, 68.57; H, 4.37; N,
29 22
spectrometer with DMSO-d6 as solvent. Mass spectrum was
determined with HP1100 LC-MS using electrospray ionization
mass spectrometry. Melting point was measured with a TECH
XT-5 melting point apparatus. The elemental analyses were
made on FLASH EA 1112 Series NCHS–O analyzer. Absorp-
tion spectra were measured on Hitachi UV-3010 spectrometer
equipped with an integrating sphere accessory. Fluorescence
spectra were studied using a Hitachi F-4500 fluorescence spec-
trophotometer. FT-IR spectra were recorded by using infrared
5
2
1
3.79%. Found C, 68.50; H, 4.42; N, 13.67%.1,3-diphenyl-4-
(
4-bromobenzal)-5-hydroxypyrazole 4-phenylsemicarbazone (2).
Compound 2 was synthesized with the method similar to the
ꢀ
preparation of compound 1. Yield: 81%. Mp. 201.1–202.3 C.
ꢀ
d
H
(400 MHz, DMSO-d
6
, 25 C, TMS): d 11.963 (1H, Pz–OH),
9
.817 (1H, N5–H), 9.115 (1H, N4–H), 7.935–6.955 (19H, phenyl-
ꢁ1
ring). nmax/cm : 3364 3333 y(N–H), 1650 y(C]O), 1600
y(C]N), 1556, 1508 y(phenyl), 1522, 1453 y(pyrazole-ring). MS:
+
m/z M : 552.10. Elemental analysis calcd for C29
ꢁ
1
H
22
N
5
O
2
Br: C,
diffuse reflectance spectroscopy in the range 400–4000 cm on
a BRUKER EQUINOX-55 spectrometer. X-Ray photoelectron
spectra (XPS) were measured with a Perkin-Elmer PHI 5300
6
3.05; H, 4.01; N, 12.68%. Found C, 63.15; H, 3.92; N, 12.66%.
ꢁ2
System. An ultraviolet lamp (1.5 W cm ) and a TECH XT-5
melting point apparatus were used as sources for photo-
coloration and thermbleaching, respectively. Powder X-ray
diffraction (XRD, MXP18AHF, MAC, Japan) using Cu-Ka
Results and discussion
The two new compounds 1 and 2 undergo reversible photo-
coloration and thermalbleaching reactions in pure solid state.
Upon irradiation with light of 365 nm, the E-form of 1a and 2a
were converted to K-form 1b and 2b, respectively, accompanied
by distinguishable changes in the absorption spectra as shown in
ꢀ
radiation (l ¼ 1.54056 A) was used to identify the as-synthesized
powders. Photochromic reaction of powders (crystalline state,
the size of crystal grain is 26 nm and 34 nm for 1a and 2a,
respectively) were all characterized at room temperature.
ꢀ
Fig. 1. The systems can then be decolored by heating at 120 C.
After white 1a, for example, changed into yellow 1b under UV
irradiation, a strong and broad absorption band centered at
Materials
4
09 nm appeared in Fig. 1A. The absorption at 409 nm dis-
ꢀ
1
.3-Diphenyl-5-pyrazolone (DPP) was synthesized with the
appeared instantaneously after heating at 120 C, and the initial
color and spectrum of the materials were completely recovered.
The photochemical process exhibits perfect reversibility.
Compound 2a also underwent excellent photochromic reaction
under UV irradiation of 365 nm light. A new absorption band
centered at 399 nm, which corresponds to the K-form of 2b,
appeared in the absorption spectra in Fig. 1B, and this process
was accompanied by a distinct color change from white to
yellow. The absorption band disappeared again after the sample
11
method described in the literature. 4-Phenylsemicarbazide
PSC), 4-chlorobenzoylchloride and 4-bromobenzoylchloride
(
were purchased from the Aldrich Company, USA. Other mate-
rials were purchased from commercial sources, and the solvents
were purified with standard procedures.
1
,3-diphenyl-4-(4-chlorobenzal)-5-pyrazolone (DP4ClBP) and
1
,3-diphenyl-4-(4-bromobenzal)-5-pyrazolone (DP4BrBP) were
12
synthesized by the method reported. For DP4ClBP. Yield:
ꢀ
ꢀ
7
3%. Mp. 155.4–156.8 C. Elemental analysis calcd for
was heated at 120 C and the original absorption spectrum was
C
22
H
15
N
2
O
2
Cl: C, 70.50; H, 4.03%; N, 7.47. Found: C, 70.48; H,
ꢀ
fully recovered. After comparing the absorption band of 2b with
that of 1b, we found that the absorption band was blue shifted by
as much as 10 nm. This could result from the reduced p-conju-
gation system for the substituent on phenyl of the 4-position of
pyrazolone-ring changing from Cl to Br. The insert charts in
Fig. 1A and 1B show that there is no significant degradation
detected by absorption for 1 and 2, respectively, after alterna-
tively photo-coloring and thermal-bleaching for 15 cycles. The
results indicate that the photochromism of pyrazolones 1 and 2
4
.42%; N, 7.50. For DP4BrBP. Yield: 69%. Mp. 147.4–148.5 C.
Elemental analysis calcd for C H N O Br: C, 63.02; H, 3.61;
2
2
15
2
2
N, 6.68%. Found C, 62.95; H, 3.57; N, 6.71%.
,3-Diphenyl-4-(4-chlorobenzal)-5-hydroxypyrazole 4-phenyl-
1
semicarbazone (1). DP4ClBP (3 mmol) and PSC (3 mmol) were
dissolved in EtOH (20 mL) together with a few drops of glacial
acetic acid, and the mixture was stirred and refluxed for 6 h at
ꢀ
8
0
C (Scheme 2). After cooling down to room temperature,
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Fig. 2 Fluorescence changes of 1 (A) and 2 (B) with UV irradiation
(lex ¼ 260 nm).
Fig. 1 Absorption spectra of 1 (A) and 2 (B): before irradiation with
3
65 nm light at room temperature (solid line), after UV irradiation
ꢀ
(dashed line), and then after thermal bleaching at 120 C (dotted line).
reaction of 1a and 2a in solid state for the first time. The highly
improved optical sensitivity and fatigue resistance of the two
compounds in solid state together with their highly enhanced
contrast fluorescent switching properties are promising for
designing novel optical devices.
The inset in (A) and (B) shows photocoloration-thermalbleaching cycles
upon alternating irradiation with UV light and heating for 1 and 2,
respectively.
exhibit highly improved fatigue resistance and sensitivity in solid
state, which is critical to achieve designing information storage
devices and optical switches, compared to photochromic mate-
With the aim of understanding the mechanism of the photo-
chromic reaction of 1a and 2a, we investigated the structural
origin of their drastic color changes in solid state with IR spectra
(Fig. 3). The structure differences between their E-form and K-
9,13
rials reported so far.
It is interesting to find that both
ꢁ1
pyrazolones 1 and 2 are photochromic in the solid state, but not
in solution.
form resulted in spectra changes in the range of 3000–3500 cm
clearly. Under irradiation of 365 nm light, a new sharp band
ꢁ1
We further investigated the fluorescent properties (Fig. 2) of
the two compounds during their photochromic reaction in solid
state. A characteristic fluorescence band was observed at 370 nm
by 260 nm excitation for the E-form 1a. Upon irradiation with
attributed to the N–H stretching vibration appears at 3387 cm
for the formation of K-form 1b along with relative intensity
ꢁ1
decrease of a smooth band at 3252 cm (Fig. 3A). The band of
ꢁ1
3252 cm can be ascribed to O–H vibration due to the red shift
3
65 nm light, the intensity of the fluorescence band decreased
dramatically, and it reached the photostationary state of K-form
b with fluorescence on/off ratio of 6 : 1 after 8 min in accom-
induced by the strong hydrogen bonds in solid state. There are no
ꢁ
1
significant changes in the region of 650–2000 cm . Due to the
1
functional group C]O in the side tail of 1 and 2 also shows
ꢁ1
panying with blue-shift of the band of 7 nm. The fluorescence
intensity of the materials returned fully to the initial state after
a strong stretching vibration at 1651 cm , which obscure the
vibration change in the characteristic region attributed to the
structure change from C–O to C]O, it is unavailable for us to
monitor the change of the C–O bond. Compound 2 shows similar
spectra changes under UV light of 365 nm. A new sharp band
ꢀ
heating at 120 C. In comparison, compound 2a also showed
similar emission behavior after irradiation with UV light and
excited at 260 nm, but with blue shift of 4 nm. It is noteworthy
that the fluorescence on/off ratio of 2a reaches 10 : 1, which
highlights the much improved high contrast ratio. The change
should be attributed to photoinduced transformation from
E-form to K-form by proton transfer, which resulted in changes
of the p conjugation system in the two compounds. These results
indicate that fluorescent switches have been realized successfully
by irradiation with UV light and heating based on photochromic
ꢁ1
attributed to N–H stretching vibration appears at 3386 cm in
accompanying with decreased relative intensity of a smooth
ꢁ1
band at 3259 cm . The results strongly support our
suggested photochromism mechanism here and in our previous
9
c
reports. Pyrazolones convert from the E-form to K-form
accompanied by proton transfer after irradiation with UV light
of 365 nm.
3
212 | J. Mater. Chem., 2011, 21, 3210–3215
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photochromic mechanism proposed. The O 1s XPS spectra of 1
and 2 are shown in Fig. 4, which can be decomposed into two
peaks by curve fitting. The first O 1s peak which occurs at 532.7
eV and 532.8 eV for our samples are assigned to oxygen atoms of
C–O group, and the second peak corresponding to binding
energy values 530.6 eV and 530.7 eV are ascribed to oxygen
atoms of C]O group for 1 and 2, respectively. The relative
amounts of the tautomers can be estimated from E-form/K-form
isomers ratio of 55 : 45 for 1a, and 45 : 55 for 2a before irradi-
ation. It is clearly observed that the E-form/K-form isomers ratio
changed to 34 : 66 for 1b and 2b after UV irradiation. The
populations of C]O increased along with decreasing of C–O
after illuminated by UV light of 365 nm, which directly indicates
that the white E-form 1a and 2a transform to K-form 1b and 2b
during the photochromic process. The XPS results are consistent
with those from FT-IR spectra, and thus further confirm the
suggested mechanism of photochromic pyrazolones. Based on
IR and XPS spectra results, we can safely conclude that the
photochromism of pyrazolones is due to tautomerization
between E-form and K-form by phototriggered proton transfer.
Compound 1 not only shows photochromism in powders, but
also in macroscopic single crystal. In order to further verify the
mechanism of photochromic pyrazolones, we tried to obtain
crystal structure information changing in accompanying with the
photochromism in single crystal using single crystal X-ray
diffraction, but failed for the low population of the K-form
species in the single crystal under ordinary UV irradiation. Only
the structure of the E-form of pyrazolone 1a before irradiation
was determined (Fig. 5). Single crystal of 1a suitable for X-ray
diffraction analysis was obtained by slow evaporation of their
ethanol solution at room temperature in the dark. The crystal of
Fig. 3 (A) FT-IR spectra of 1, before UV irradiation (black), after UV
irradiation (grey). (B) FT-IR spectra of 2, before UV irradiation (black),
after UV irradiation (grey).
While IR spectra is a useful method to study the hydrogen
bonded structure, it cannot give definite structure change cor-
responding to the O–H stretching band. However, binding
energies of the spectral features are sensitive to the chemical
environment perturbation, and it is possible to distinguish the
tautormers in pyrazolones. Hence, XPS was considered as
a powerful tool to study the tautomerism problem in hydrogen
15
1
1
a belongs to a monoclinic system, P2 /c. There are two non-
centrosymmetric molecules of 1a in the asymmetric unit, but
conformations of the two molecules differ slightly. The length of
ꢀ
C–O bond of pyrazolone-ring is 1.336 and 1.328 A in the two
molecules, respectively, which is consistent with the length of C–
O single bond. However, the length of C–O bond of phenyl-
ꢀ
semicarbazone is 1.252 and 1.244 A, respectively, which is in
14
bonded system. In our experiment we focus our efforts on
examining the O 1s spectra changes for further verifying the
accordance with the length of C]O double bond. It can be
deduced that the structure of 1a is the E-form in the colorless
Fig. 4 (A) O 1s spectra (XPS) of 1, before irradiation with UV light
(top), after irradiation (bottom). (B) O 1s spectra (XPS) of 2, before
Fig. 5 Molecular structure of 1a with the atom numbering. The ellipsoid
irradiation with UV light (top), after irradiation (bottom).
represents 50% displacement of atoms.
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Natural Science Foundation of Xinjiang Uygur Autonomous
Region of China (2009211B02) and Scientific Research Program
of the Higher Education Institution of Xinjiang (XJE-
DU2008S08) and SRFDP20070755001.
Notes and references
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ꢀ
ꢀ
ꢀ
2
.560(2) A, 173(4) and O1B–H/O2A, 2.589(2) A, 170(4) ), and
then 1D chains are formed by connecting those dimmers by other
0
ꢀ
intermolecular hydrogen bonds (N4A–H/O2A , 3.114(3) A,
1
ꢀ 0 ꢀ
ꢀ
51(2) and N4B–H/O2B , 3.050(3) A, 135(2) ). There are no
5
6
direct hydrogen bonds between the O atom and N atom of the
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8
Conclusions
Two novel pyrazolones (1 and 2), which exhibit greatly improved
photochromic properties, have been synthesized successfully by
introducing a phenylsemicarbazide unit. They show excellent
fatigue resistance and distinguished contrast fluorescence emis-
sion signals in the solid state. The mechanism of the photo-
chromic reactions was due to tautomerization between their enol
and keto forms accompanied by proton transfer, and this process
was confirmed by FT-IR and XPS. The progress provides a new
insight for the design and synthesis of perfect photochromic
compounds in solid state and even in macroscopic single crystal.
The excellent photochromic performance of the new pyrazolones
can satisfy the fundamental requirements for designing new
optoelectronic devices, such as optical memories and switches.
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This work was supported by the National Natural Science
Foundation of China (21062020 20866009 and 20762010), the
3
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J. W. Chung, Y. You, H. S. Huh, B. K. An, S. J. Yoon, S. H. Kim,
S. W. Lee and S. Y. Park, J. Am. Chem. Soc., 2009, 131,
(Rigaku R-AXIS SPIDER) with a graphite monochromatized Mo-
ꢀ
ꢀ
Ka radiation (l ¼ 0.71073 A, u-scans, 3.07 # q # 27.48 ). Crystal
2
structures was solved by direct methods and refined on F by full-
8163.
1
4 E. Ito, H. Oji, T. Araki, K. Oichi, H. Ishii, Y. Ouchi, T. Ohta,
N. Kosugi, Y. Maruyama, Y. Naito, T. Inable and K. Seki, J. Am.
Chem. Soc., 1997, 119, 6336.
matrix least-squares methods with the SHELXTL-97 program,9279
unique measured reflections were used in the refinement. All non-H
atoms were refined anisotropically. The H atoms on oxygen and
nitrogen atoms were located from the Fourier maps, and all of the
other H atoms were placed in geometrically idealized positions. The
1
5 Crystal data for 1a: colorless,
monoclinic, P2
b ¼ 106.586(1) , V ¼ 4986.39(17) A3, Z ¼ 8, D
C
29
H
22
N
5
O
2
Cl,
M
¼
507.97
ꢀ
1
ꢀ
/c, a ¼ 10.982(2), b ¼ 18.017(3), c ¼ 26.297(6) A,
ꢀ
c
¼ 1.353 g cm–3, m
refinement converged to R
0.0486, wR
2
680802†.
1
(observed reflections with I > 2s(I)) ¼
¼
0.191 mm–1, F(000) ¼ 2112 and
T
crystallographic data were collected on an imaging plate system
¼
153(2) K. The
¼ 0.1215 and S ¼ 1.112. CCDC deposition number:
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 3210–3215 | 3215