Photochromism and mechanism of pyrazolones in crystals: Structural variations directly observed by X-ray diffraction

Article abstract of DOI:10.1039/c1jm12267c

Structural changes of photochromic pyrazolones were first verified by an X-ray crystallographic method directly. A tautomeric transformation between enol and keto form isomers occurs, accompanied by proton transfer during the photochromic processes.

Full text of DOI:10.1039/c1jm12267c

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Photochromism and mechanism of pyrazolones in crystals: structural
variations directly observed by X-ray diffraction†
a
a
Jixi Guo, Dianzeng Jia, Lang Liu,a Hui Yuan,a Mingxi Guo,a Dongling Wua and Feng Libc
*
Received 21st May 2011, Accepted 27th June 2011
DOI: 10.1039/c1jm12267c
the mechanism of photochromic reactions in crystals is still open. In
Structural changes of photochromic pyrazolones were first verified
by an X-ray crystallographic method directly. A tautomeric trans-
formation between enol and keto form isomers occurs, accompanied
by proton transfer during the photochromic processes.
the case of organic photochromic crystals, however, it is usually hard
to obtain sufficient colored species detectable with the X-ray crys-
tallographic method,9 because the photo-reaction takes place on the
surfaces of macroscopic single crystals.10 It is still a challenge to
directly probe the structure changes brought about by photochro-
mism of pyrazolones, while it is critical for understanding the
photochromic mechanism and designing new photochromic systems.
In this work, the crystal structural changes related to the
photochromism of pyrazolones were first observed by using X-ray
crystallographic analysis and their photochromic mechanism was
demonstrated. A new photochromic compound: 1-phenyl-3-(4-
fluorophenyl)-4-(2-chlorobenzal)-5-hydroxypyrazole 4-methyl-
The reversible change of color as the result of structural, environ-
mental, and/or molecular orientation changes of materials, which
have distinguishably different absorption properties by electromag-
netic stimulation, is well-known as photochromism,1 and it has been
investigated extensively for its diverse potential uses such as optical
data storage2 and switches.3 Although a variety of organic photo-
chromic compounds in solutions4 or in dispersed systems5 have been
investigated, it is rare to find photochromic organic compounds in the
crystalline state in literature. Therefore, it is of technical as well as
fundamental interest to seek photochromic materials in the crystalline
state.6
thiosemicarbazone (1), which performs
a
reversible
photoisomerization reaction under alternating UV light irradia-
tion/heating and visible light in the solid state (Scheme 1), was
successfully synthesized by condensing 4-acylpyrazolone with
methylthiosemicarbazone. The same yellow colored materials can
be yielded by UV light irradiation or heating the white sample of
1a. In order to increase the populations of colored species in single
crystals suitable for X-ray crystallographic analysis, we found that
Toward this end, we have devoted our effort to develop a photo-
chromic system derived from pyrazolones, which exhibit reversible
photochromism in the crystalline state.7 It is found that the materials
can change their color from white to yellow upon irradiation with UV
light, and then return to white upon storage in visible light or heating.
The photochromic system shows advantages including high sensi-
tivity, excellent fatigue resistance and reversible fluorescent switching
properties with high contrast signals.7b,c Based on spectroscopic
measurements together with theoretical calculations, our studies
indicate that the photochromism of pyrazolones could originate from
the tautomerism between the enol form (E-form) and keto form
(K-form) by photo-chemically triggered proton transfer.8 However,
ꢀ
it was effective to treat the materials by heating at 180 C. The
yellow crystals of 1b, which were very similar to those irradiated by
UV light (Fig. S1, ESI†), were obtained by heating the single
crystal of 1a for X-ray diffraction. The crystal structure analysis of
the materials reveals the photochromic mechanism of the
aKey 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
bCBEN and Department of Chemistry, Rice University, Houston, TX,
77081, USA
cState Laboratory of Surface and Interface Science and Technology,
School of Materials and Chemical Engineering, Zhengzhou University of
Light Industry, Zhengzhou, 450002, Henan, P. R. China
† Electronic supplementary information (ESI) available: Experimental
details, IR and XPS spectra, Tables of crystal data, structure
refinement, selected bond lengths, bond angles data, hydrogen bonds.
CCDC reference numbers 797540 and 797541. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1jm12267c
Scheme 1 Photochromic reactions of 1.
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pyrazolone family for the first time. It was found that the photo-
chemically or thermally triggered intra- andintermolecular proton
transfers have resulted in tautomerism in single crystals.
Compound 1 has an absorption band at 443 nm after irradiation
with light of 365 nm (Fig. 1A). Subsequently, the white materials
changed to yellow, and the yellowed 1b can keep their color for more
than one year when stored in the dark at room temperature. The
colored reactions can take place not only under UV light irradiation,
but also by heating. The chromic behaviors occurred after heating,
and the spectra of the yielded products were exactly the same as those
obtained under UV light irradiation (Fig. S2, ESI†). In other words,
the structure of the materials could convert from the E-form of 1a
into the K-form of 1b both photo-chemically and thermally. The
yellow colored materials can be bleached back to white under visible
light irradiation ($420 nm). A characteristic fluorescence band of 1
was observed at 443 nm after excitation at 320 nm as shown in
Fig. 1B. Upon irradiation with 365 nm light, the intensity of the
fluorescence band decreased gradually with increasing the irradiation
time, and the emission reached a photostationary state after irradi-
ation for 160 min. The switchable fluorescent property of 1 may also
arise from converting the E-form of 1a to the K-form of 1b. The
fluorescent contrast between the E-form and K-form isomers of 1 was
32 : 1. Compared to other pyrazolones reported previously, this is the
highest photo-induced fluorescent modulation observed, which is
essential for designing optical devices. Fluorescent switches could be
realized by alternating light irradiation based on the photochromic
reaction of 1 in the crystalline state.
Fig. 2 Molecular structures of (A) 1a and (B) 1b with atom numbering.
The disordered part in 1a are omitted for clarity and the ellipsoid
represents 10% and 30% displacement of atoms, respectively. Difference
ꢁ3
ꢀ
Fourier maps of (C) 1a and (D) 1b. The contour lines are at 0.02 e A
intervals. The peak corresponding to the changes is indicated by an
arrow.
longer than that of 1a, N2–C9 is shorter than that of 1a and the C8–
C9 bond of 1b is also shorter than that of 1a. The length of the O–C7
bond of 1a is considerably shorter than the standard length of the
To obtain direct evidence for verifying the photoisomerization
mechanism of pyrazolones, crystal structures of 1a11 and 1b12 were
identified using single-crystal X-ray crystallography (Fig. 2A and 2B).
In order to increase the population of the K-form in the single crystal,
1a was treated at 180 ꢀC to obtain the thermal products of 1b. The IR
ꢀ
O–C bond [1.350 A] and longer than the length of the O]C bond
13
ꢀ
[1.222 A]. These results suggest that the two isomers (E-form and
K-form) might coexist in the crystal of 1a. The length of the O–C7
bond of 1b becoming shorter than that of 1a after heating indicates
that the amount of the K-form isomer increases in the crystal
produced by heating. It is safe to conclude that there is equilibrium
between the E-form and K-form isomers in the crystals.
ꢀ
and XPS spectra of the sample heated at 180 C showed that the
thermal products obtained by heating were identical with the
photoproducts obtained by irradiation with 365 nm light (Fig. S3,
ESI†). Selected bond lengths obtained from the X-ray crystallo-
graphic analysis of 1 are listed in Table 1. After comparing the data
displayed in Table 1, we can find that the length of the O–C bond
varies significantly between 1a and 1b. The O–C7 bond of 1b is
The H atoms on O and N2 atoms were treated as disordered in the
refinement of structure 1a, and the population of the E-form was
determined to be ca. 34.6% based on the occupancy of hydrogen
atom connected to oxygen. The structure 1b was refined as the pure
K-form due to no significant residual peak located around the oxygen
atom. The conclusion becomes much clearer based on the difference
of Fourier synthesis using the refined structure from which only the
tautomeric hydrogen atom is removed. The difference synthesis for
structure 1a locates two peaks assigned to two hydrogen atoms, one
connected to O and the other to N2 (Fig. 2C). In contrast, the
difference synthesis for the structure 1b locates no distinguished
residual peak that can be assigned to the hydrogen atom connected to
O (Fig. 2D). Hence, the X-ray diffraction can unambiguously and
directly display the occurrence of tautomeric transformation of 1
between E-form and K-form isomers in the crystal. This is the first
and direct observation of molecule structural changes accompanying
ꢀ
shorter than that of 1a by 0.14 A. Thus, the C7–C8 bond of 1b is
ꢀ
Table 1 Selected bond lengths of 1 (A)
Compd
O–C7
C7–C8
C8–C9
C9–N2
Fig. 1 (A) Absorption spectra changes of 1 in crystalline powders:
before irradiation or heating (solid line) and after irradiation or heating
(dotted line). (B) Fluorescence changes of 1 irradiated under UV light
with an interval time of 20 min (lex ¼ 320 nm).
1a
1b
1.249(5)
1.235(4)
1.436(5)
1.440(5)
1.381(5)
1.382(5)
1.360(5)
1.357(4)
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the photochromism of pyrazolones. The X-ray diffraction data have
proved that the E-form of 1a coverts into the K-form of 1b after
photochromic reaction.
of C]O groups. The relative amounts of the tautomers can be
estimated from the E-form/K-form isomer ratio of 62 : 38 for 1a
before irradiation. The E-form/K-form isomer ratio changed to
40 : 60 for 1b after UV irradiation. These results are consistent with
X-ray crystallographic analysis. The population of C]O increases
along with a decreasing of the C–O population after illumination
with UV light. The data indicate that the white E-form of 1a trans-
forms to the yellow K-form of 1b during the photochromic process.
The K-form exists appreciably in the crystalline state and increases in
population after UV light irradiation/heating.
ꢀ
Based on the existence of intramolecular [N4–H/O] (2.695(4) A,
ꢀ
0
ꢀ
ꢀ
156(3) ) and intermolecular [S/H–N2 ] (3.240(3) A, 172(4) )
hydrogen bonds (Fig. S4B, ESI†), we can find convenient channels
for transferring protons exist in the crystalline state of pyrazolones.
Firstly, an intermediate state thiol form 1a0 is established under UV
light irradiation (as shown in Scheme 1). The H on N4 atom transfers
to S atom by means of intramolecular rearrangement, forming
intermolecular [S–H/N20]. One proton subsequently transfers from
O atom to N4 atom by the channel of intramolecular [N4/H–O],
forming another intramolecular hydrogen bond [N4–H/O].
Another proton transfers from S atom to N20 atom by the channel of
intermolecular [S–H/N20] at the same time, and forms another
intermolecular hydrogen bond [S/H–N20]. The processes of intra-
and intermolecular proton transfer lead to enol–keto
photoisomerization.
In conclusion, direct observation of structure changes has strongly
proved the mechanism of photochromic reaction of pyrazolones in
single crystals. The length of the O–C7 bond of white products
ꢀ
becomes shorter than that of the yellow products by 0.014 A. Hence,
the X-ray diffraction can unambiguously and directly display the
occurrence of the tautomeric transformation between the E-form and
K-form isomers in single crystals of 1. The IR and XPS results of 1
before and after photo-irradiation are well consistent with those from
X-ray crystallographic analyses and further verify the mechanism of
photochromic pyrazolones. The photochromic reactions of pyr-
azolones are due to tautomerization between their E-form and
K-forms accompanied by proton transfer.
Photochromic reactions of pyrazolones have been further investi-
gated with IR and XPS spectra, which strongly support the mecha-
nism suggested: transformation from the E-form to the K-form
occurs in the materials after UV irradiation. The structure differences
between the E-form and the K-form of 1 result in IR and XPS spectra
changing clearly (Fig. 3). Under irradiation of 365 nm light, a new
sharp band attributed to C]O stretching vibration appears at 1671
cmꢁ1 in the IR spectrum of 1 for the formation of the K-form of 1b
along with a relative intensity increase of the band at 3220 cmꢁ1
(Fig. 3A). The band of 3220 cmꢁ1 can be ascribed to N2–H vibration
in the solid state. Ultraviolet irradiation can activate the trans-
formation of 1 from the E-form to the K-form. While IR spectros-
copy is a useful method to study the hydrogen bonded structure,14 it
cannot give definite structure change corresponding to the O–H
stretching band of 1. Because the binding energies in the XPS spectra
of materials are sensitive to the chemical environment perturbation,
the materials were also characterized with XPS for distinguishing the
tautormers.15 We have focused our efforts on examining the O1s
spectra changes for verifying the photochromic mechanism. The O1s
XPS spectra of 1 are shown in Fig. 3B, which can be decomposed into
two peaks by curve fitting. The O1s peaks at 531.8 and 532.3 eV can
be assigned to oxygen atoms of C–O groups, and the peaks corre-
sponding to binding energies of 529.6 and 530.3 eV to oxygen atoms
The authors thank the National Natural Science Foundation of
China (20866009, 21062020 and 21071130), and the Natural Science
Foundation of Xinjiang Uygur Autonomous Region of China
(2009211B02).
References
1 E. Hadjoudis and I. M. Mavridis, Chem. Soc. Rev., 2004, 33, 579–588;
K. Amimoto and T. Kawato, J. Photochem. Photobiol., C, 2005, 6,
207–226.
€
2 H. Durr, H. Bouas-Laurent, Photochromism: Molecules and Systems;
Elsevier: Amsterdam, The Netherlands, 1990; S. Kawata and
Y. Kawata, Chem. Rev., 2000, 100, 1777–1788; C. C. Corredor,
Z. L. Huang and K. D. Belfield, Adv. Mater., 2006, 18, 2910–2914;
G. Y. Jiang, S. Wang, W. F. Yuan, L. Jiang, Y. L. Song, H. Tian
and D. B. Zhu, Chem. Mater., 2006, 18(2), 235–237.
3 Y. Yokoyama, Chem. Rev., 2000, 100, 1717–1739; G. Berkovic, Chem.
€
Rev., 2000, 100, 1741–1753; C. Weber, F. Rustemeyer and H. Durr,
Adv. Mater., 1998, 10, 1348–1351; J. Areephong, T. Kudernac,
J. J. D. D. Jong, G. T. Carroll, D. Pantorott, J. Hjelm,
W. R. Browne and B. L. Feringa, J. Am. Chem. Soc., 2008, 130,
12850–12851; L. Y. Zhu, W. W. Wu, M. Q. Zhu, J. J. Han and
J. K. Hurst, J. Am. Chem. Soc., 2007, 129, 3524–3526.
4 Y. Kishimoto and J. Abe, J. Am. Chem. Soc., 2009, 131, 4227–4229;
Y. Chen and D. X. Zeng, J. Org. Chem., 2004, 69, 5037–5040;
R. Siewertsen, H. Neumann, B. Buchheim-Stehn, R. Herges,
C. Nather, F. Renth and F. Temps, J. Am. Chem. Soc., 2009, 131,
15594–15595.
5 K. Uchida, M. Saito, A. Murakami, S. Nakamura and M. Irie, Adv.
Mater., 2003, 15, 121–125; T. Tsujioka, M. Kume and M. Irie, J.
Photochem. Photobiol., A, 1997, 104, 203–206; R. Pardo, M. Zayat
ꢁ
and D. Levy, J. Mater. Chem., 2009, 19, 6756–6760; R. Metivier,
ꢁ
ꢁ
S. Badre, R. Meallet-Renault, P. Yu, R. B. Pansu and K. Nakatani,
J. Phys. Chem. C, 2009, 113, 11916–11926; S. Z. Xiao, Y. Zou,
M. X. Yu, T. Yi, Y. F. Zhou, F. Y. Li and C. H. Huang, Chem.
Commun., 2007, 4758–4760; D. Levy, Chem. Mater., 1997, 9, 2666–
2670; J. Whelan, J. T. C. Wojtyk and E. Buncel, Chem. Mater.,
2008, 20, 3797–3799.
6 M. Irie, Chem. Rev., 2000, 100, 1685–1716; E. Hadjoudis and
I. Mavridis, Chem. Soc. Rev., 2004, 33, 579–588; K. Amimoto and
T. Kawato, J. Photochem. Photobiol., C, 2005, 6, 207–226;
T. Yamada, S. Kobatake, K. Muto and M. Irie, J. Am. Chem. Soc.,
2000, 122, 1589–1592; J. Harada, R. Nakajima and K. Ogawa, J.
Am. Chem. Soc., 2008, 130, 7085–7091; I. Yildiz, E. Deniz and
F. M. Raymo, Chem. Soc. Rev., 2009, 38, 1859–1867.
Fig. 3 (A) FT-IR spectra and (B) O1s spectra (XPS) of 1 before and
after UV irradiation. The colored lines in (B) show the results of curve
fitting.
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7 J. X. Guo, D. Z. Jia, L. Liu, H. Yuan and F. Li, J. Mater. Chem.,
2011, 21, 3210–3215; J. X. Guo, L. Liu, G. F. Liu, D. Z. Jia and
X. L. Xie, Org. Lett., 2007, 9, 3989–3992; L. Liu, X. Y. Xie,
D. Z. Jia, J. X. Guo and X. L. Xie, J. Org. Chem., 2010, 75, 4742–
4747; J. X. Guo, L. Liu, D. Z. Jia, J. H. Wang and X. L. Xie, J.
Phys. Chem. A, 2009, 113(7), 1255–1258.
8 H. Lin, D. L. Wu, L. Liu and D. Z. Jia, THEOCHEM, 2008, 850, 32–
37; Y. T. Zhong, L. Liu, G. F. Liu, D. L. Wu, J. X. Guo and D. Z. Jia,
J. Mol. Struct., 2008, 889, 259–264.
17794/4207 [R(int) ¼ 0.0436]. The refinement converged to R1
(observed reflections with I > 2s(I)) ¼ 0.0706, wR2 ¼ 0.1866 and
S ¼ 1.097. CCDC 797540.
479.95,
12 Crystal data for 1b: yellow, C24H19N5OFSCl,
M
¼
monoclinic, P21/c, a ¼ 12.4127(16), b ¼ 14.3970(18), c ¼ 13.3934
ꢀ
3
ꢀ
ꢀ
(14) A, b ¼ 108.020(2) , V ¼ 2276.1(5) A , Z ¼ 4, Dc ¼ 1.401 g
cmꢁ3, m ¼ 0.295 mmꢁ1, and T ¼ 293(2) K. Reflections collected/
unique 17943/4200 [R(int) ¼ 0.0477]. The refinement converged to
R1 (observed reflections with I > 2s(I)) ¼ 0.0666, wR2 ¼ 0.1951 and
S ¼ 1.076. CCDC 797541.
9 T. Yamada, S. Kobatake, K. Muto and M. Irie, J. Am. Chem. Soc.,
2000, 122, 1589–1592; T. Yamada, K. Muto, S. Kobatake and
M. Irie, J. Org. Chem., 2001, 66, 6164–6168.
13 K. Ogawa, Y. Kasahara, Y. Ohtani and J. Harada, J. Am. Chem. Soc.,
1998, 120, 7107–7108.
10 P. Naumov, A. Sekine, H. Uekusa and Y. Ohashi, J. Am. Chem. Soc.,
2002, 124, 8540–8541; J. Harada, H. Uekusa and Y. Ohashi, J. Am.
Chem. Soc., 1999, 121, 5809–5810.
14 J. N. Moorthy, P. Mal, R. Natarajan and P. Venugopalan, Org. Lett.,
2001, 3, 1579–1582; E. Hadjoudis, A. Rontoyianni, K. Ambroziak,
T. Dziembowska and I. M. Mavridis, J. Photochem. Photobiol., A,
2004, 162, 521–530.
15 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, 6336–6344.
11 Crystal data for 1a: colorless, C24H19N5OFSCl, M ¼ 479.95,
monoclinic, P21/c, a ¼ 12.4255(8), b ¼ 14.4075(8), c ¼ 13.4165(7)
ꢀ
3
ꢁ3
ꢀ
ꢀ
A, b ¼ 107.9890(10) , V ¼ 2284.4(2) A , Z ¼ 4, Dc ¼ 1.396 g cm
,
m ¼ 0.294 mmꢁ1, and T ¼ 293(2) K. Reflections collected/unique
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 12202–12205 | 12205