Photoinduced twisting of a photochromic diarylethene crystal

Article abstract of DOI:10.1002/anie.201304670

Let's do the twist! Molecular crystals of a diarylethene derivative exhibit photoreversible twisting upon irradiation with UV and visible light. The left-handed and right-handed helix formation is ascribed to the contraction of the crystal in a diagonal direction, depending on which face is irradiated with UV light. Copyright

Full text of DOI:10.1002/anie.201304670

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Angewandte
Communications
DOI: 10.1002/anie.201304670
Photochromic Actuators
Photoinduced Twisting of a Photochromic Diarylethene Crystal**
Daichi Kitagawa, Hiroyasu Nishi, and Seiya Kobatake*
Photoresponsive materials have been a topic of much
attention in both industrial and academic research fields
because photoirradiation offers a non-contact, non-destruc-
tive means of inducing a response.[1–3] In particular, recent
reports have described the development of photomechanical
devices with photoresponsive organic crystals, such as diaryl-
ethene, anthracene, azobenzene, and furylfulgide.[4–13]
Diarylethenes undergo thermally irreversible and fatigue-
resistant photochromic reactions, even in the single-crystal-
line phase.[14] The shape of diarylethene crystals can be
modulated solely by photoirradiation. A photoreversible
step-and-valley formation can occur on diarylethene single-
crystal surfaces upon alternating irradiation with ultraviolet
(UV) and visible light, because of a slight change in molecular
volume that occurs between the open- and closed-ring
isomers during the photochromic reaction.[15] This change in
molecular volume plays an important role in other processes,
such as bulk crystal deformation. Single microcrystals of
diarylethenes have been found to exhibit rapid and reversible
macroscopic changes in shape and size upon irradiation with
UV and visible light.[4–8]
seen from a relationship between the face irradiated with UV
light and the direction of the crystal twisting. Such photo-
induced crystal twisting provides a new type of photome-
chanical actuator.
Figure 1 illustrates the structural transformation and the
photoreversible single crystal twisting of 1-(2-methyl-5-(4-(1-
naphthoyloxymethyl)phenyl)-3-thienyl)-2-(2-methyl-5-
phenyl-3-thienyl)perfluorocyclopentene (1a) upon alternat-
Recently, 9-anthracenecarboxylic acid (9AC) crystals
have been shown to twist upon UV light irradiation.[11] 9AC
can undergo a reversible [4+4] photocyclodimerization in the
crystalline phase. Bardeen and co-workers have suggested
that the crystal twisting is induced by generating interfacial
strain within the microcrystal between unreacted monomer
and photoreacted dimer regions. The crystal relaxes back to
the original shape over the course of minutes, as thermal
dissociation of the dimers returns them to their monomeric
forms.
Herein, we report the photoinduced crystal twisting of
a novel photochromic diarylethene crystal. The colorless
needle-like crystals of diarylethene rapidly twisted, accom-
panied by a color change to blue upon irradiation with UV
light. The twisted crystal relaxed back to its original shape in
a few seconds under visible light irradiation. The photo-
reversible twisting of the crystal could be repeated over 30
cycles. The photoinduced crystal twisting was caused by
contraction of the crystal in a diagonal direction, as can be
Figure 1. Chemical structures and photoreversible crystal twisting of
diarylethene 1a upon irradiation with UV (l=365 nm) and visible light
(l>500 nm). A movie of the photoreversible twisting in real time is
available in the Supporting Information (Video S1).
ing irradiation with UV (l = 365 nm) and visible light (l >
500 nm). The single crystal was prepared by recrystallization
from an n-hexane/ether solution. The thickness, width, and
length of the crystal are ca. 1.5 mm, 11 mm, and 320 mm,
respectively. Upon UV light irradiation over the whole
crystal, the molecules in the crystal underwent a photocycli-
zation reaction from the open-ring isomer to the closed-ring
isomer, and the crystal of 1a rapidly twisted, accompanied by
a color change of the crystal from colorless to blue. The
twisting and color of the crystal was maintained in the dark.
By irradiation with visible light, the blue color disappeared
through a back reaction to regenerate the open-ring isomer
and the twisted crystal relaxed back to its original shape in
a few seconds. Such reversible twisting upon alternating
irradiation with UV and visible light could be repeated over
30 cycles (Figure S1). These results revealed that the crystal
twisting is induced by the photochromic reactions of diaryl-
ethene molecules in the crystal.
[*] D. Kitagawa, Dr. H. Nishi, Prof. Dr. S. Kobatake
Department of Applied Chemistry, Graduate School of Engineering,
Osaka City University, Sugimoto 3-3-138
Sumiyoshi-ku, Osaka 558-8585 (Japan)
E-mail: kobatake@a-chem.eng.osaka-cu.ac.jp
There are two directions of twisting, left-handed helix and
right-handed helix. The helix that twists counterclockwise to
the direction of the movement is referred to as a left-handed
helix. The reverse is called a right-handed helix (Figure S2).
To determine whether both helix types exist in this case, the
crystals on the petri dish were irradiated with UV light, and
the number of resulting left- and right-handed helical crystals
[**] This work was partly supported by a Grant-in-Aid for Scientific
Research (C) (24550161) from the Japan Society for the Promotion
of Science (JSPS). D.K. appreciates Research Fellowships from JSPS
for Young Scientists. The authors gratefully acknowledge the Rigaku
Corporation for X-ray crystallographic analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304670.
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The difference of the crystal surface on (0 1 0) and (0 À1
0) is distinguished by the shape of the crystal surface and the
absorption anisotropy of the closed-ring isomer (Figure S5).
When the (0 À1 0) face was irradiated with UV light, the
crystal twisted into a right-handed helix. In contrast, the
crystal twisted into a left-handed helix when the (0 1 0) face
was irradiated with UV light, as shown in Figure S6 and
Videos S1 and S2. When UV light irradiation was performed
from both sides on (0 À1 0) and (0 1 0) faces on the whole
crystal, it contracted (Figure 4). This result indicates that the
Figure 2. Photoinduced crystal twisting into left-handed and right-
handed helices.
was determined. Both helix types were found, and in roughly
equal amounts (Figure 2 and S3). It was noticed that the
direction of twisting depends upon the face irradiated with
UV light. However, half of all the crystals were not twisted. It
is due to the fact that the twisting, or lack thereof, depends
upon the crystal thickness.
Single-crystal X-ray crystallography of a crystal of 1a
showed a monoclinic crystal system and a space group of P21
(Table S1).[16] The crystal is chiral, but the origin is due to
a disorder of perfluorocyclopentene. Therefore, if the disor-
der is absent, the space group should be P21/c. Figure 3 shows
the shape and the molecular packing of crystal 1a before UV
Figure 4. Shape of the contracted crystal. The colorless crystal (a) was
irradiated with UV light to form the colored crystal (b). The colored
crystal was produced with UV light irradiation from both sides on the
crystal surfaces for a few minutes. The photoisomerization took place
homogeneously in the crystal. c) The crystal shapes before (a) and
after (c) UV light irradiation. The right edges of the colorless and
colored crystals are superimposed.
crystal twisting was caused by the heterogeneity of the
photochromic reaction in the thickness direction of the crystal
because of the high absorbance of the crystal. Some diaryl-
ethene crystals thus far have been reported to cause the
photoinduced reversible bending.[4–8] In the present case, the
contraction is considered to occur in the diagonal direction.
When irradiated with UV light, the photoisomerization of the
diarylethene molecules from the open-ring isomer 1a to the
closed-ring isomer 1b occurs in the crystalline phase. The
twisted thiophene rings become coplanar, and the thickness of
each molecule is reduced. The closed-ring isomers in the
photoirradiated crystal can stack with each other, resulting in
changes in the cell dimensions. This change in cell dimensions
causes the contraction or twisting of the crystal.
The change in cell dimensions upon UV light irradiation is
generally confirmed by X-ray crystallographic analysis. We
here determined the changes in the cell dimensions from the
shape change of the contracted crystal after UV light
irradiation (Figure 4). The crystal was irradiated with UV
light from all of directions because of homogeneous photo-
isomerization. The coordinates on the four corners of the
crystal before and after UV light irradiation was plotted in
a rectangular coordinate system. When transforming from
a rectangular coordinate system to an oblique coordinate
system, the cell dimensions changed so that the coordinates
might become the same in two different oblique coordinate
systems. The cell parameters correspond to a change from a =
6.10 ꢀ, c = 11.36 ꢀ, and b = 93.48 to a’ = 5.98 ꢀ, c’ = 10.08 ꢀ,
Figure 3. Molecular packing of crystal 1a. The crystal shape (a) and
molecular packing (b) of 1a in the crystal viewed from the (0 À1 0)
face are depicted. The crystal shape in (a) is shown by a bold line.
irradiation, as viewed from the (0 À1 0) face. The packing
diagrams viewed from other surfaces are shown in Figure S4.
The diarylethene molecules are regularly oriented to the
direction of the a axis with a tilt angle of ca. 508, and aligned
perpendicularly to the (0 À1 0) face. All of the diarylethene
molecules are fixed in a photoreactive antiparallel conforma-
tion, and the distance between the reactive carbon atoms in
the thiophene rings is 3.50 ꢀ. It is short enough for the
diarylethene to undergo photocyclization in the crystalline
phase.[17]
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Angewandte
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and b’ = 96.58, respectively. The a and c axes contracted as
much as 2.0% and 11%, respectively, and the b angle
enlarged as much as 3.3%. The shape of the unit cell viewed
from the (0 À1 0) face before and after UV light irradiation is
shown in Figure S7. The contraction of the unit cell takes
place in the direction of top-left to bottom-right when the unit
cell was viewed from the (0 À1 0) face. On the other hand, the
contraction of the unit cell takes place in the direction of top-
right to bottom-left when the unit cell was viewed from the
(010) face. The photochromic reaction proceeds in a gradient
to the thickness direction because of high absorption in the
crystal. Therefore, the crystal can twist into a right-handed or
left-handed helix depending on the contraction direction, as
shown in Figure 5. There is an induction period of ca. 2 s until
the crystal begins to twist (Figure S8). The strain required to
handed helix. The direction of the twisting depends upon the
face irradiated with UV light. Furthermore, the direction of
contraction in the unit cell was determined using the
contracted crystal, which underwent a photocyclization reac-
tion over the whole crystal by irradiation from both sides of
the crystal surfaces. From these results, the twisting of the
crystal requires both a gradient of photocyclization conver-
sion in the thickness direction and the contraction of the
crystal in the diagonal direction.
Received: May 30, 2013
Revised: June 26, 2013
Published online: July 19, 2013
Keywords: actuators · crystal engineering · diarylethenes ·
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mechanical properties · photochromism
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perform the twist arises in this period. The photoirradiation
time to the reversed side during the twisting is shorter than
the period. The direction of twisting can be determined by the
starting irradiation face.
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of novel diarylethene crystals. The photoreversible twisting of
the diarylethene crystal could be repeated over 30 cycles by
alternating irradiation with UV and visible light. The crystal
twisting takes place in both a left-handed helix and a right-
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