Thermodynamic phase transition through crystal-to-crystal process of photochromic 1,2-bis(5-phenyl-2-propyl-3-thienyl)perfluorocyclopentene

Article abstract of DOI:10.1002/asia.201301020

A photochromic diarylethene, 1,2-bis(5-phenyl-2-propyl-3-thienyl) perfluorocyclopentene (1 a), was found to have two polymorphic crystal forms, α- and β-crystals. From X-ray crystallographic analysis, the space groups of α- and β-crystals were determined to be P2₁/c and C2/c, respectively. The difference between two crystal forms is ascribed to the orientation of two of four molecules in the unit cell. The thermodynamic phase transition from α- to β-forms occurred via a crystal-to-crystal process, as confirmed by differential scanning calorimetry measurements, optical microscopic observations in the reflection mode and under crossed Nicols, and powder X-ray diffraction measurements. The movement of the molecules in the crystal was evaluated by analyzing the change of face indices before and after the phase transition. Copyright

Full text of DOI:10.1002/asia.201301020

FULL PAPER
DOI: 10.1002/asia.201301020
Thermodynamic Phase Transition Through Crystal-to-Crystal Process of
Photochromic 1,2-Bis(5-phenyl-2-propyl-3-thienyl)perfluorocyclopentene
Daichi Kitagawa and Seiya Kobatake*[a]
Abstract:
ethene,
thienyl)perfluorocyclopentene
was found to have two polymorphic
crystal forms, a- and b-crystals. From
X-ray crystallographic analysis, the
space groups of a- and b-crystals were
determined to be P21/c and C2/c, re-
spectively. The difference between two
crystal forms is ascribed to the orienta-
A
photochromic diaryl-
1,2-bis(5-phenyl-2-propyl-3-
(1a),
tion of two of four molecules in the
unit cell. The thermodynamic phase
transition from a- to b-forms occurred
via a crystal-to-crystal process, as con-
firmed by differential scanning calorim-
etry measurements, optical microscopic
observations in the reflection mode
and under crossed Nicols, and powder
X-ray diffraction measurements. The
movement of the molecules in the crys-
tal was evaluated by analyzing the
change of face indices before and after
the phase transition.
Keywords: crystal-to-crystal
pro-
cess · diarylethene · phase transi-
tion · photochromism · polymorph-
ism
Introduction
tochromic compounds reported, diarylethene derivatives are
the most promising compounds with regard to applications
in optical memory,[7] optical switches,[8] optoelectrical devi-
ces,[9] and photomechanical devices[10] because of the ther-
mal stability of both isomers, their fatigue-resistant property,
and their capability of photochromism in the solid state.
There are at least two reports on the polymorphism of di-
arylethene derivatives. 1,2-Bis(2-methyl-5-p-methoxyphenyl-
3-thienyl)perfluorocyclopentene can form four polymorphic
crystals by recrystallization from n-hexane, and the photo-
cycloreversion quantum yields of the photogenerated
closed-ring isomer in these four crystals are different from
each other because of a difference in the conformation of
the closed-ring isomers in the crystals.[5a] The platelet diaryl-
ethene crystal of 1,2-bis(2-methyl-6-nitro-1-benzothiophen-
Since Mitscherlich used the word ꢀpolymorphismꢁ for the
first time in the context of crystallography in 1822, poly-
morphism has attracted the attention of many scientists be-
cause of its importance in many areas of chemistry.[1] Poly-
morphs provide different properties, such as conductivity,[2]
magnetic properties,[3] fluorescence,[4] and photochromism,[5]
on the basis of molecular packing in each crystal. Phase
transition between polymorphic crystals has also been stud-
ied with developing the research on polymorphism.[1] The
phase transition can be divided into two types: the crystal-
to-amorphous-to-crystal process and the crystal-to-crystal
process. The former phase transition is accompanied with
the melting of the crystal. The crystal shape and the molecu-
lar packing before and after the phase transition are quite
different. By contrast, the latter phase transition takes place
while maintaining the shape of the crystal. The movement
of molecules in the crystal is very small; thus, the molecular
packing before and after the phase transition is very similar.
Photochromic compounds can reversibly change their
chemical and physical properties, such as absorption, fluo-
rescence, refractive indices, dielectric constants, and oxida-
tion-reduction potentials upon alternating irradiation with
ultraviolet (UV) and visible light.[6] Among the various pho-
3-yl)perfluorocyclopentene undergoes
a thermodynamic
phase transition to form a needle-like crystal upon melt-
ing.[11] The phase-transition temperature can be controlled
by the presence of the photogenerated closed-ring isomer in
the crystal. The surface roughness can be changed by crystal
growth of the needle-like crystals accompanying the phase
transition.
We have recently found that 1,2-bis(5-phenyl-2-propyl-3-
thienyl)perfluorocyclopentene (1a, Scheme 1) has two poly-
morphs, of which one undergoes a thermodynamic phase
transition to the other without any melting. Here we report
on the polymorphism, photochromism, and thermodynamic
phase transition of diarylethene 1a via a crystal-to-crystal
process. The motion of the molecules in the phase transition
is proposed based on single-crystal X-ray crystallographic
analysis of the two polymorphs.
[a] D. Kitagawa, Prof. Dr. S. Kobatake
Department of Applied Chemistry
Graduate School of Engineering
Osaka City University
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)
Fax : (+81)6-6605-2797
E-mail: kobatake@a-chem.eng.osaka-cu.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301020.
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Daichi Kitagawa and Seiya Kobatake
Table 1. X-ray crystallographic data for 1a-a, 1a-b, and 1a-a after heat-
ing for 0.5 h at 1008C.
1a-a
1a-b
After heating
of 1a-a[a]
Formula
Formula weight
T [K]
Crystal system
Space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
a [8]
b [8]
C31H26F6S2
576.66
303(2)
Monoclinic
P21/c
11.765(5)
19.985(7)
13.298(6)
90
111.99(3)
90
2899(2)
4
C31H26F6S2
576.66
303(2)
Monoclinic
C2/c
21.000(4)
11.980(2)
12.943(3)
90
115.058(4)
90
2949.7(10)
4
C31H26F6S2
576.66
303(2)
–
Scheme 1. Molecular structure of the diarylethene used in this work.
–
20.720
11.810
12.720
89.940
115.080
89.860
2819.1
–
Results and Discussion
Polymorphism
When diarylethene 1a was recrystallized from acetone or n-
hexane, two types of crystals with different shapes, denoted
as 1a-a and 1a-b, were obtained from each solution.
Figure 1 shows the shapes of a- and b-crystals. The a-crystal
g [8]
Volume [ꢃ3]
Z
Density [gcmÀ3
]
1.321
1.063
1.298
0.942
–
–
Goodness-of-fit on F2
R
G
0.0402
0.1122
0.0560
0.1599
–
–
[a] Although X-ray crystallographic analysis could not be established, the
cell dimensions could be determined. The cell dimensions were used for
determining face indices.
cules in the unit cell. The distances between the reactive car-
bons in the a- and b-crystals were 3.73 and 3.77 ꢃ, respec-
tively, which are sufficiently short for photocyclization to
take place in the crystalline phase.[13] Indeed, both crystals
underwent a photochromic reaction in the crystalline phase
upon alternating irradiation with UV and visible light. Upon
UV irradiation, the initially colorless single crystals of a-
and b-crystals turned blue. Both colored crystals had absorp-
tion maxima at 600 nm. The blue-colored crystals turned
colorless again upon irradiation with visible light, as shown
in Figure 1.
The molecular packing diagrams of 1a in the a- and b-
crystals are shown in Figure 2. The molecular packings in
both crystals are quite similar to each other. The difference
between the molecular packings is ascribed to the orienta-
tion of only one molecule in the unit cell. In the a-crystal,
CH–p intermolecular interactions exist between propyl and
phenyl groups. By contrast, no such interaction is observed
in the b-crystal.
Figure 1. Photochromism of 1a-a (a) and 1a-b (b) in the single-crystalline
phase. Both 1a-a and 1a-b turned blue upon irradiation with UV light.
has a block shape, which is obtained from acetone in many
cases. By contrast, the b-crystal has a plate shape with a par-
allelogram, which is obtained from n-hexane in many cases.
X-ray crystallographic analysis of these crystals was per-
formed to examine which of those are polymorphic forms.
The crystallographic data are summarized in Table 1.[12] The
crystal system of both crystals is monoclinic, but the space
groups of 1a-a and 1a-b are P21/c and C2/c, respectively.
The crystal of 1a-a has one molecule in the asymmetric
unit, while the crystal of 1a-b has half of a molecule in the
asymmetric unit. In both crystal forms, there are four mole-
Thermodynamic Phase Transition
Abstract in Japanese:
The thermodynamic stability of the polymorphic crystals
was examined by differential scanning calorimetry (DSC).
Figure 3 shows the DSC traces of the a- and b-crystals.
When the powder crystal of 1a-a was heated at a rate of
108C minÀ1, the crystal exhibited a small endothermic be-
havior due to a phase transition at 1008C and a large endo-
thermic behavior due to the crystal melting at 1608C. The b-
crystal showed only a large endothermic behavior corre-
sponding to the crystal melting at 1608C. The melting point
after the phase transition is the same for the a-crystal as
that for the b-crystal, which suggests that the phase transi-
tion occurred from the a-form to the b-form.
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Daichi Kitagawa and Seiya Kobatake
Video S1. This opaque appearance is ascribed to a slight de-
crease in the crystallinity due to a crystal-to-crystal phase
transition. Figure 5 and Video S2 also show the phase-transi-
tion behavior observed under crossed Nicols at a rate of
Figure 5. Optical microscopic photographs of an a-crystal observed under
crossed Nicols at increasing temperature: (a) 110.08C, (b) 112.58C,
(c) 113.38C, (d) 114.28C, (e) 115.08C, and (f) 120.08C.
Figure 2. Molecular packing diagrams of 1a-a (a) and 1a-b (b). The
propyl groups and perfluorocyclopentene rings in 1a-a and 1a-b are dis-
ordered.
108C minÀ1. When the temperature reached 1108C, a ther-
modynamic phase transition was initiated from the right
bottom, and the phase-transition behavior continued until
1208C. The crystal was bright from the beginning to the end
of the transtions, which means that the crystallinity was
maintained during the phase transition.
To examine the motion of the molecules during the phase
transition, a photoirradiated colored crystal was observed
under polarized light. At a certain angle (q=08), the color
became more intense. When the crystal was rotated as much
as 908, the color became weak. The polarized absorption
spectra of the colored crystals were compared before and
after the phase transition. Figure 6a shows the polarized ab-
sorption spectra of the photogenerated closed-ring isomer
ꢀ
on the (0 2 1) face of the a-crystal. The angle of 08 was set
Figure 3. DSC traces of 1a-a (a) and 1a-b (b) at a heating rate of
to a direction of the maximum absorbance. The absorption
spectra had a maximum at 600 nm. The polar plots of the
absorbance at 600 nm are shown in Figure 6a. After bleach-
ing the crystal by irradiation with visible light, the crystal
underwent a phase transition upon heating for 0.5 hour at
1008C. Figure 6b shows the polarized absorption spectra of
the photogenerated closed-ring isomer and the polar plots
after the phase transition. The shapes of the polar plots are
similar to each other before and after the phase transition.
This indicates that the molecules do not move largely, when
the molecular packing is viewed on the face. The order pa-
rameter [(A08ÀA908)/(A08+2A908)] was determined to be 0.65
and 0.61 before and after the phase transition, respective-
ly.[14] The similar order parameter indicates that the crystal-
linity is maintained during the phase transition.
108CminÀ1
.
To examine the phase-transition behavior of the single
crystal of 1a-a, the crystal was observed by optical micros-
copy in the reflection mode. The initial a-crystal is colorless
transparent. When the a-crystal was heated at a rate of
108CminÀ1, the colorless crystal turned gradually opaque
without melting of the crystal, as shown in Figure 4 and
The thermodynamic phase transition from the a- to the b-
form was analyzed by powder X-ray diffraction. Figure 7
shows the profiles at 30 and 1308C and the calculated pat-
terns of a- and b-forms obtained from single-crystal X-ray
crystallographic analysis. The diffraction profile after heat-
ing above 1308C was identical to that of the b-form. Thus,
a polymorphic phase transition was found to occur from the
a- to the b-form upon heating. The phase transition was
Figure 4. Optical microscopic photographs of an a-crystal observed in the
reflection mode at increasing temperature: (a) 1008C, (b) 1108C,
(c) 1208C, and (d) 1308C.
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Daichi Kitagawa and Seiya Kobatake
indices from the analysis of the
crystal after the phase transi-
tion. Therefore, we investigated
the change of the face indices
before and after the phase tran-
sition. Figure 8 shows the crys-
tal shape and face indices
before and after the phase tran-
sition viewed from the cross-
section. Before the phase tran-
sition, the face indices of well-
ꢀ
developed surfaces are (0 2 1),
ꢀ
(0 2 1), (0 2 1), and (0 2 1).
When the a-crystal was heated
for 0.5 h at 1008C to undergo
the phase transition, the cell di-
mensions changed to those of
the b-form, as shown in Table 1.
The face indices also changed
in two types, patterns A and B,
as shown in Figure 8: patterns
A and B were present 5 times
each in the measurement of the
Figure 6. Polarized absorption spectra of the photoirradiated colored crystals and the polar plots of absorbance
at 600 nm before (a) and after (b) the phase transition. The measurement was carried out on the (0 2 1) face
of a-crystal. The angle of 08 was set to a direction of the maximum absorbance before the phase transition.
The same face and angle was used for the measurement after the phase transition.
ꢀ
Figure 7. Powder X-ray diffraction patterns measured at 30 and 1308C.
The calculated patterns were obtained using parameters determined from
single-crystal X-ray crystallographic analysis of a- and b-forms at 308C.
Figure 8. The shape of the crystal (a) and the face indices before and
after the phase transition viewed from the cross-section (b).
thermally irreversible, and the b-form cannot return to the
a-form even at À1508C.
face indices of 10 crystals analyzed. This result means that
the phase transition takes place like a domino reaction
taking advantage of the molecule that moved first. In patter-
Movement of Molecules in the Crystal
ꢀ
ꢀ
n A, the face indices of (0 2 1), (0 2 1), (0 2 1), and (0 2 1)
ꢀ
ꢀ
ꢀ ꢀ
ꢀ
ꢀ
To investigate the movement of the molecules during the
phase transition, in situ X-ray crystallographic analysis of
a single crystal before and after the phase transition was
performed. However, X-ray crystallographic analysis of the
single crystal after the phase transition could not be per-
formed because the crystal often became opaque after the
phase transition due to a slight decrease in the crystallinity.
Nevertheless, we could determine cell parameters and face
changed to (1 1 1), (1 1 1), (1 1 1), and (1 1 1), respectively.
ꢀ ꢀ
ꢀ
By contrast, in pattern B, they changed to (1 1 1), (1 1 1), (1
ꢀ ꢀ
ꢀ
1 1), and (1 1 1), respectively. From these face indices, the
molecular packing before and after the phase transition in
both patterns has been clarified, as shown in Figure 9. We
numbered the molecules from 1 to 4 to distinguish the mole-
cules. The result clearly shows that when the phase transi-
tion occurs in pattern A, the molecules of 1 and 4 move and
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Daichi Kitagawa and Seiya Kobatake
eter with MoKa radiation (l=0.71073 ꢃ) monochromated by graphite.
The crystal structures were solved by the direct method using SIR92 and
refined by the full-matrix least-squares method on F2 with anisotropic
displacement parameters for non-hydrogen atoms using SHELXL-97.[15]
UV irradiation was carried out using
a Keyence UV-400 UV-LED
(365 nm light), and a super high pressure mercury lamp (100 W; UV-1A
filter (365 nm light excitation)) attached with the polarizing optical mi-
croscope. Visible light irradiation was carried out using a halogen lamp
(100 W).
Materials
Diarylethene 1a was synthesized according to the literature.[16] The single
crystals of 1a-a and 1a-b were obtained by recrystallization from acetone
and n-hexane, respectively.
Figure 9. The molecular packing before and after the phase transition
viewed from the cross-section. The molecules are numbered from 1 to 4,
respectively. In pattern A, the molecules of 1 and 4 move to 1’ and 4’, re-
spectively. In pattern B, the molecules of 2 and 3 move to 2’ and 3’, re-
spectively.
Acknowledgements
This work was partly supported by a Grant-in-Aid for Scientific Research
(C) (No. 24550161) from the Japan Society for the Promotion of Science
(JSPS). D.K. appreciates Research Fellowships of JSPS for Young Scien-
tists.
those of 2 and 3 do not move. By contrast, in pattern B, the
molecules of 2 and 3 moved and those of 1 and 4 do not
move. The motion of each molecule is relatively small so
that the crystal can maintain its shape after the phase transi-
tion.
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Conclusions
The polymorphism of 1,2-bis(5-phenyl-2-propyl-3-thienyl)-
perfluorocyclopentene (1a) was discovered. Both the block
a-crystal and the platelet b-crystal exhibited photochromism
in the crystalline phase. The thermodynamic phase transi-
tion from a- to b-crystals was investigated using optical mi-
croscopy, polarized optical microscopy, and powder X-ray
diffraction measurements. The results showed that the phase
transition from a- to b-crystals occurs via a crystal-to-crystal
process. Moreover, the movement of the molecules in the
crystal occurs like a domino reaction, as confirmed by meas-
uring the face indices before and after the phase transition.
This is the first example of a thermodynamic phase transi-
tion via a crystal-to-crystal process of diarylethene crystals.
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[12] CCDC 950555 (1a-a), and CCDC 950556 (1a-b) contain the supple-
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Experimental Section
General
The phase transition of crystals was observed using a Nikon ECLIPSE
E600POL polarizing optical microscope or a Keyence VHX-500 digital
microscope, equipped with a Mettler–Toledo FP82HT hot stage and an
FP90 central processor. Differential scanning calorimetry (DSC) was per-
formed using a Seiko DSC-6200 calorimeter. Polarized absorption spectra
were measured using the polarizing optical microscope connected with
a Hamamatsu PMA-11 photodetector. Powder X-ray diffraction profiles
were recorded on a Rigaku RINT-2100 diffractometer employing CuKa
radiation (l=1.54184 ꢃ). Single-crystal X-ray crystallographic analysis
was carried out using a Rigaku RAXIS RAPID imaging plate diffractom-
[13] S. Kobatake, K. Uchida, E. Tsuchida, M. Irie, Chem. Commun.
2002, 2804–2805.
[14] The small order parameter is due to two electronic transition mo-
ments of short and long axes of the molecule 1b.
[15] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refine-
ment, University of Gçttingen, Germany, 1997.
[16] D. Kitagawa, K. Sasaki, S. Kobatake, Bull. Chem. Soc. Jpn. 2011, 84,
141–147.
Received: July 31, 2013
Published online: September 23, 2013
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