Crystal structure and photochromism of 1-phenyl-3-methyl-4-benzyl-5-one-pyrazole S-methyl thiosemicarbazone

Article abstract of DOI:10.1016/S0022-2860(03)00251-5

A new organic photochromic compound containing pyrazolone-ring photochromic functional unit: 1-phenyl-3-methyl-4-benzyl-5-one pyrazole S-methyl thiosemicarbazone (PMBP-smtsc) was synthesized. The photochromic properties and photochemical kinetics of PMBP-smtsc have been studied by UV reflectance spectra under irradiation of 365 nm light. The crystal structure analyses of photocolored product show the photochromism is due to the photoisomerization from enol form to keto form through an intermolecular proton transfer.

Full text of DOI:10.1016/S0022-2860(03)00251-5

Journal of Molecular Structure 655 (2003) 221–227
www.elsevier.com/locate/molstruc
Crystal structure and photochromism
of 1-phenyl-3-methyl-4-benzyl-5-one-pyrazole
S-methyl thiosemicarbazone
a
Lang Liu , Dian-zeng Jia , Ya-li Ji , Kai-bei Yu
a,
a
b
*
aInstitute of applied chemistry, Xinjiang University, Urumqi 830046, People’s Republic of China
bChengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, People’s Republic of China
Received 13 January 2003; revised 26 March 2003; accepted 1 April 2003
Abstract
A new organic photochromic compound containing pyrazolone-ring photochromic functional unit: 1-phenyl-3-methyl-4-
benzyl-5-one pyrazole S-methyl thiosemicarbazone (PMBP-smtsc) was synthesized. The photochromic properties and
photochemical kinetics of PMBP-smtsc have been studied by UV reflectance spectra under irradiation of 365 nm light. The
crystal structure analyses of photocolored product show the photochromism is due to the photoisomerization from enol form to
keto form through an intermolecular proton transfer.
q 2003 Elsevier Science B.V. All rights reserved.
Keywords: Pyrazolone; Photochromism; Thiosemicarbazone; Intermolecular proton transfer mechanism
1. Introduction
been much studied due to their potential importance
for the feasibility of information recording and
processing [15,16], variable transmitting materials
[17] and molecular switching devices [18], etc. Up to
now, developing new photochromic system and
determining the mechanistic aspects of the photo-
chemical process are still the power to speed the
progress of this field.
Photochromic molecular materials have been
proposed as candidates for applications in optical
recording [1–4]. Writing and erasing of data can be
achieved by photoconversion (‘switching’) between
different isomers. In recent years, photochromic
compounds have attracted numerous scientists to
study their properties, mechanisms and applications.
Especially, organic photochromic compounds such as
spiropyrans [5,6] and spiroxazine [7], fulgides [8,9],
diarylethenes [10,11] and Schiff bases [12–14] have
Thiosemicarbazones and their derivates are widely
known as having a large range of biological
applications such as antiviral, antibacterial, antima-
larial, antifungal, etc [19,20]. Now, a photochromic
functional unit is introduced into thiosemicarbazone,
we expect that products have doubling functions.
However, few photochromic compounds on thiosemi-
carbazone have been reported previously [21,22].
*
Corresponding author. Tel.: þ86-991-8580032; fax: þ86-991-
8581006.
E-mail address: jdz@xju.edu.cn (D.-z. Jia).
0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-2860(03)00251-5

222
L. Liu et al. / Journal of Molecular Structure 655 (2003) 221–227
Scheme 1.
An intermolecular H-transfer mechanism was pro-
posed on the basis of the crystal structure of PMBP
thiosemicarbazone. It is different from the photo-
chromic properties of Schiff base compounds reported
by Cohen and Schmidt [23]. Schiff bases from
salicylaldehyde are a typical class of photochromic
materials involving both excite-state intramolecular
proton transfer and cis–trans isomerization to form a
colored photoproduct from the colorless molecules
upon UV irradiation. In order to interpret further the
intermolecular H-transfer mechanism proposed
above, we cultivate single crystals of the title
compounds. It was found that the title compounds
exhibited reversible color changes induced by 365 nm
ultraviolet irradiation. And the crystal structure of the
title compound showed that the photocoloration of
A(enol-form) involved in an intermolecular proton
transfer from O-hydroxyl group to the nitrogen atom
of adjoining molecular pyrazolone-ring as show in
Scheme 1.
MeOH solution containing 5–6 drops of glacial acetic
acid at 708 c for ca 3 h by stirring. The mixture was
cooled to room temperature. The yellow crystal
suitable for X-ray analysis was obtained by allowing
MeOH to evaporate for a few days.
Table 1
Crystal data and structure refinement parameters for A*(keto-form)
Empirical formula
Formula weight
Temperature
C19H18N4 OS2
382.49
294(2) K
˚
0.71073 A
Wavelength
Crystal system
Space group
Monoclinic
P21/n
˚
Unit cell dimensions
a ¼ 11.854(2) A a ¼ 908
˚
b ¼ 14.852(4) A b ¼ 96.46(2)8
˚
c ¼ 21.930(5) A g ¼ 908
3
3836.4(15) A , 8
˚
Volume, Z
Density (calculated)
Absorption coefficient
F (000)
1.324 g/cm3
0.293 mm21
1600
0.48 £ 0.40 £ 0.12 mm3
Crystal size
u range for data collection
Limiting indices
1.668 to 24.998
0 # h14, 0 # k17, 226 # l # 25
7647
2. Experimental details
Reflections collected
Independent reflections
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices [I . 2s(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
6751 [R(int) ¼ 0.0212]
Empirical
2.1. Synthesis
0.9879 and 0.9047
Full matrix least-squares on F 2
6751/0/490
S-methyl thiosemicarbazine was synthesized by the
literature [24]. 1-phenyl-3-methyl-4-benzyl-5-one-
pyrazole S-methyl thiosemicarbazone (PMBP-smtsc)
was prepared by mixing 1-phenyl-3-methyl-4-ben-
zoyl-pyrazolone-5 (PMBP, 5 mmol) and S-methyl
thiosemicarbazide (smtsc, 5 mmol) in 50 ml of
0.844
R1 ¼ 0.0465, wR2 ¼ 0.0888
R1 ¼ 0.1085, wR2 ¼ 0.1046
0.00183(19)
3
˚
0.250 and 20.253 e/A

L. Liu et al. / Journal of Molecular Structure 655 (2003) 221–227
223
2.2. Determination of the crystal structure
of A*(keto-form)
listed in Table 3. The molecular structure and the
packing arrangement in the unit cell are shown in
Figs. 1 and 2, respectively.
Data were collected at 294 K by a Siemens P4
diffractermeter
with
MoKa
radiation
˚
3. Results and discussion
(l ¼ 0.71073 A). The structure was solved by direct
method. All calculations and drawing were performed
by the SHELXTL crystallographic software package
of molecular structure. The intensities were measured
by v-scans. Cell constants and orientation matrix for
data collection were obtained by least-squares refine-
ment of the diffraction data from reflections in the
range 1.66–24.998. Non-H atoms were located in
successive DF maps and refined anisotropically by
full-Dmatrix least-squares on F 2. Hydrogen atoms
were found from the difference Fourier map and
refined isotropically. The crystal data and structure
refinement details are given in Table 1. Non-hydrogen
atomic coordinates and thermal parameters are listed
in Table 2. The bond lengths and bond angles are
3.1. Photochromic properties
The yellow of A(enol form) changes to reddish
orange under irradiation of the 365 nm light (or
exposed in the sunlight). The color turns back to the
original yellow when it is heated. Thus, the compound
A has photochromic properties in the solid state. The
powder-UV reflectance spectra of A at 298 K for
different light irradiated times are recorded on a
HITACHI UV-3010 spectrophotometer and shown in
Fig. 3. A new absorption band appears around
480 nm, which indicates the title compound exhibits
photochromic properties in the solid state. However,
Table 2
4
2
3
˚
Atomic coordinates ( £ 10 ) and equivalent isotropic displacement parameters (A £ 10 ) for A*
Atom
x
y
z
U(eq)
Atom
x
y
z
U(eq)
S(1)
8215(1)
7320(1)
9769(2)
1425(1)
1760(1)
4096(2)
4046(2)
3561(2)
2190(2)
2030(2)
4518(3)
4927(3)
5311(3)
5285(2)
4878(2)
4495(2)
3768(2)
3102(2)
3014(2)
2419(2)
2633(2)
2692(2)
3205(2)
3249(3)
2793(3)
2289(3)
2235(2)
1744(2)
1505(3)
1779(1)
2998(1)
2672(1)
2483(1)
2724(1)
3398(1)
2772(1)
1611(2)
1041(2)
772(2)
50(1)
49(1)
53(1)
36(1)
38(1)
42(1)
43(1)
68(1)
83(2)
61(1)
57(1)
51(1)
36(1)
36(1)
33(1)
34(1)
55(1)
36(1)
36(1)
49(1)
65(1)
70(1)
67(1)
54(1)
36(1)
67(1)
S(10)
13216(1)
12440(1)
14768(2)
16689(2)
17629(2)
14517(2)
14365(3)
17641(3)
17703(3)
16864(4)
15966(3)
15904(3)
16751(2)
15743(3)
16141(2)
17300(3)
18161(3)
15389(3)
15562(3)
16260(3)
16387(3)
15832(4)
15137(3)
15003(3)
13410(3)
11263(3)
6533(1)
5385(1)
3893(2)
4064(2)
4163(2)
4572(2)
5233(2)
3915(2)
4114(3)
4606(3)
4906(3)
4728(2)
4231(2)
3995(2)
4069(2)
4148(2)
4257(2)
4054(2)
3392(2)
2654(2)
2028(3)
2142(3)
2872(3)
3495(2)
5721(2)
6099(3)
2956(1)
3965(1)
2567(1)
2539(1)
2961(1)
3979(1)
3541(1)
1623(2)
1013(2)
688(2)
71(1)
61(1)
51(1)
37(1)
36(1)
39(1)
46(1)
48(1)
70(1)
79(1)
65(1)
48(1)
34(1)
36(1)
35(1)
34(1)
52(1)
34(1)
34(1)
49(1)
62(1)
65(1)
58(1)
48(1)
45(1)
118(2)
S(2)
S(20)
O(1)
N(1)
N(2)
N(3)
N(4)
C(1)
O(10)
N(10)
N(20)
N(30)
N(40)
C(10)
C(20)
C(30)
C(40)
C(50)
C(60)
C(70)
C(80)
C(90)
C(100)
C(110)
C(120)
C(130)
C(140)
C(150)
C(160)
C(170)
C(180)
C(190)
11663(2)
12595(2)
9536(2)
9384(2)
12601(3)
12565(3)
11596(3)
10646(3)
10663(3)
11649(3)
10740(3)
11173(2)
12306(3)
13179(3)
10429(3)
10634(3)
11508(3)
11688(3)
10989(4)
10097(4)
9931(3)
C(2)
C(3)
C(4)
1065(2)
1631(2)
1908(1)
2784(1)
3211(1)
3165(1)
3488(2)
3605(1)
4284(1)
4576(2)
5213(2)
5559(2)
5279(2)
4647(2)
2510(1)
2490(2)
965(2)
C(5)
1578(2)
1902(1)
2845(1)
3481(1)
3522(1)
4066(1)
3971(1)
4479(1)
4445(2)
4915(2)
5428(2)
5468(2)
5003(2)
3469(2)
3730(2)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
C(18)
C(19)
8379(3)
6074(3)
U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.

224
L. Liu et al. / Journal of Molecular Structure 655 (2003) 221–227
Table 3
˚
Selected bond lengths (A) and angles (8) for A*
S(1)C(18)
1.661(3)
1.740(3)
1.787(3)
1.248(3)
1.374(3)
1.401(4)
1.425(4)
1.338(4)
1.285(4)
1.385(3)
1.332(4)
1.418(4)
1.364(4)
1.478(4)
122.9(3)
131.3(3)
105.8(3)
120.6(2)
113.7(2)
125.7(2)
107.5(2)
109.6(3)
109.2(3)
107.8(3)
105.8(3)
S(10)C(180)
1.648(3)
1.743(3)
1.782(4)
1.254(3)
1.374(3)
1.374(3)
1.429(4)
1.331(4)
1.290(3)
1.371(3)
1.338(4)
1.426(4)
1.372(4)
1.471(4)
121.9(3)
132.1(3)
105.9(3)
121.9(3)
112.4(3)
125.7(2)
108.8(2)
108.7(2)
109.7(3)
106.7(3)
105.9(3)
S(2)C(18)
S(20)C(180)
S(2)C(19)
S(20)C(190)
O(1)C(7)
O(10)C(70)
N(1)N(2)
N(10)N(20)
N(1)C(7)
N(10)C(70)
N(1)C(6)
N(10)C(60)
N(2)C(9)
N(20)C(90)
N(3)C(11)
N(30)C(110)
N(3)N(4)
N(30)N(40)
N(4)C(18)
N(40)C(180)
C(7)C(8)
C(70)C(80)
C(8)C(9)
C(80)C(90)
C(8)C(11)
C(80)C(110)
O(1)–C(7)–N(1)
O(1)–C(7)–C(8)
N(1)–C(7)–C(8)
N(4)–C(18)–S(1)
N(4)–C(18)–S(2)
S(1)–C(18)–S(2)
N(2)–N(1)–C(7)
C(9)–N(2)–N(1)
N(2)–C(9)–C(8)
C(9)–C(8)–C(7)
N(1)–C(7)–C(8)
O(10)–C(70)–N(10)
O(10)–C(70)–C(80)
N(10)–C(70)–C(80)
N(40)–C(180)–S(10)
N(40)–C(180)–S(20)
S(10)–C(180)–S(20)
C(70)–N(10)–N(20)
C(90)–N(20)–N(10)
N(20)–C(90)–C(80)
C(90)–C(80)–C(70)
N(10)–C(70)–C(80)
Fig. 2. Molecular packing of A*.
the title compound does not exhibit the photochro-
mism in the solution.
3.2. Kinetics of the photochromic reaction
The first-order rate constant of A(enol-form) was
determined essentially as described in the literature
[25]. The experimental data were treated with the
standard integrated expression:
A1 2 A0
A1 2 At
ln
¼ kt
ð1Þ
where A0, At, A00 are the observed absorption data
corresponding to 480 nm wavelength at the begin-
ning, at time t; and at the end of the reaction,
respectively.
Fig. 3. UV reflectance spectra of photochemical coloration of A
with 365 nm light at 298 K.
Fig. 1. Crystal structure of A*.

L. Liu et al. / Journal of Molecular Structure 655 (2003) 221–227
225
Fig. 4. kinetic plot of photoisomerization (enol ! keto) reaction of
A induced by 365 nm light.
Fig. 6. First-order kinetic plot of photoisomerization reaction of A
induced by 365 nm light after 250s.
The kinetic curve of the photochromic reaction for
the title compound is plotted according to the Eq. (1)
and shown in Fig. 4. It is observed that the
photoisomerization is not proceeded at the certain
speed during the whole reaction. For comparison
purpose, the whole plot is spited into two parts (Figs. 5
and 6). Initially, the spectral changes of A under UV
irradiation mainly occurred in the 400–550 nm region
giving rise to fast change of the color and increase of
the absorbance. Which is corresponding to the first
part of the kinetic curve in Fig. 4. A good linear fit as
found in the kinetic plot (Fig. 5) from which the first-
order or preudo-first-order rate constant was obtained
as 3.98 £ 1023 s21. For longer irradiation times of A,
the absorbance increased slowly. A good line was
found in Fig. 6, from which the first-order rate
constant was obtained as 1.26 £ 1023 s21. The results
indicate that the photochemical reaction for
A(enol-form) before 250s is faster 3 times than after
250s, and is faster 3–10 times than the analogous
compound reported previously [22], which is in
agreement with the phenomena observed. The reason
is that the photochemical reaction before 250s occurs
in the surface of the compound and the reaction after
250s occurs in the inner. So the rate constants of two
parts are different.
3.3. Analyses of crystal structure
The reliable structure data of A are not obtained
because the crystal of A is easily turned into A*
during measurement. Thus, the crystal structure data
of photocolored product are only reported in this
paper. From Fig. 1, it is observed that there are two
crystallographically independent molecules in an
asymmetric structure unit, not related one to another
by symmetry, but associated by an intermolecular
0
˚
H-bond (N2–H…O1 , 2.683 A, 1748). The H-bond
distance is slightly longer than that of PMBP
˚
thiosemicarbazone (2.632 A) and slightly shorter
than the value for PMBP methyl thiosemicarbazone
˚
(2.701 A) reported previously [22]. The correspond-
ing bond lengths and angles in the two molecules are
nearly similar. There are also intermolecular
0
0
˚
H-bonds[N4–H…S1 , 3.495 A, 1558; N4 –H…S1,
˚
3.509 A, 1578] between the neighboring molecules.
The hydrogen bonds assembled the moieties into a
supramolecule with infinite long-chain structure along
Fig. 5. First-order kinetic plot of photoisomerization reaction of A
induced by 365 nm light before 250s.

226
L. Liu et al. / Journal of Molecular Structure 655 (2003) 221–227
Scheme 2.
b axle as shown in Fig. 2. It is interaction of these
hydrogen bonds that contribute to the stabilization of
the title compound. Finally, it is important to mention
that two molecules in Fig. 1 display different
configuration. The dihedral angles between pyrazo-
lone-ring and phenyl ring of N(1), C(11) are 17.28,
61.78, respectively. In the other molecules, the
dihedral angles between pyrazolone-ring and phe-
nyl-ring of N(1)0, C(11)0 are 39.08, 64.88, respectively.
the intermolecular proton transfer mechanism of the
photochromism postulated previously [21] is further
supported by crystal structure analyses of PMBP
S-methyl thiosemicarbazone carried out in this paper.
4. Supplementary data
Supplementary data are available from the CCDC,
12 Union Road, Cambridge CB2 1EZ, UK, deposition
number 188947.
3.4. Mechanism of photochromism
The IR spectra analysis in [21] show that the
yellow compound (A) is the enol form before the
photocoloration takes place. The C7–O1 and
C70 –O10 band distances are 1.248(3) nm and
1.254(3) nm, respectively, which is indicative of
significant double character. So the reddish orange
compound is the keto-form. The structure of A*
shows that an intermolecular hydrogen bond [N2–
H…O10] exists between neighboring molecules. It
suggests that the change of color is due to the
photoisomerization from the enol form to the keto
form. It is different from the photochromic processes
of Schiff base compounds proposed by Cohen and
Schmidt. The photocoloration processes of Schiff
bases such as Salicylaldehyde and its derivatives was
suggested as the mechanism of intramolecular proton
transfer from the hydroxylic group to the imine
(–CHyN–) nitrogen atom (shown in Scheme 2).
However, it is almost impossible for A to transfer
hydroxyl proton (O–H) to N2 atom of pyrazolone-
ring in molecule itself because the proton must
surmount not only the bonding effort with N2 but
also potential barrier, especially when it pass through
pyrazolone-ring with great electron cloud density.
According to the results of structure analyses, the
photochromic process of A is considered to be
similar to that of analogous compounds with
the same environment reported previously. So
Acknowledgements
This work was supported by the National Natural
Science Forndation of China (NO: 20262005).
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