Control of Photomechanical Crystal Twisting by Illumination Direction

Article abstract of DOI:10.1021/jacs.7b13605

Photomechanical molecular crystals have been investigated as mesoscopic photoactuators. Here, we report how the photomechanical twisting of 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) crystals depends on illumination direction. The ribbon-like crystal of 1a could be successfully prepared by a sublimation method. The ribbon crystal exhibited reversible photomechanical crystal twisting upon alternating irradiation with ultraviolet (UV) and visible light. Moreover, changing the UV illumination direction with respect to the crystal resulted in different twisting modes, ranging from helicoid to cylindrical. Control of photomechanical crystal deformation by illumination direction provides a convenient and useful way to generate a variety of photomechanical motions from a single crystal.

Full text of DOI:10.1021/jacs.7b13605

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Communication
Control of photomechanical crystal twisting by illumination direction
Daichi Kitagawa, Hajime Tsujioka, Fei Tong, Xinning Dong, Christopher J. Bardeen, and Seiya Kobatake
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Feb 2018
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Control of photomechanical crystal twisting by illumination direc-
tion
Daichi Kitagawa,† Hajime Tsujioka,† Fei Tong,‡ Xinning Dong,‡ Christopher J. Bardeen,*,‡ and Seiya
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Kobatake*,†
†Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, 3ꢀ3ꢀ138 Sugimoto, Sumiyoshiꢀ
ku, Osaka 558ꢀ8585, Japan.
‡Department of Chemistry, University of California, 501 Big Springs Road, Riverside, CA 92521, USA
Supporting Information Placeholder
development of these materials. We recently found that a diaꢀ
rylethene derivative, 1,2ꢀbis(2ꢀmethylꢀ5ꢀphenylꢀ3ꢀ
ABSTRACT: Photomechanical molecular crystals have been
investigated as mesoscopic photoactuators. Here, we report how
the photomechanical twisting of 1,2ꢀbis(2ꢀmethylꢀ5ꢀphenylꢀ3ꢀ
thienyl)perfluorocyclopentene (1a) crystals depends on illuminaꢀ
tion direction. The ribbonꢀlike crystal of 1a could be successfully
prepared by a sublimation method. The ribbon crystal exhibited
reversible photomechanical crystal twisting upon alternating irraꢀ
diation with ultraviolet (UV) and visible light. Moreover, changꢀ
ing the UV illumination direction with respect to the crystal reꢀ
sulted in different twisting modes, ranging from helicoid to cylinꢀ
drical. Control of photomechanical crystal deformation by illumiꢀ
nation direction provides a convenient and useful way to generate
a variety of photomechanical motions from a single crystal.
thienyl)perfluorocyclopentene (1a, Scheme 1) can be crystallized
in different shapes depending on crystal growth method: slow
evaporation of organic solvent yields large blockꢀlike crystals,
while sublimation yields ribbonꢀlike crystals. Moreover, the ribꢀ
bon shaped crystals exhibit photomechanical twisting. In this
paper, we have investigated the effects of illumination direction
on the photomechanical twisting. Control of the illumination anꢀ
gle of the incident light can tune the twisting mode between a
helicoid type and a cylindrical helix type. Our results may lead to
new and possibly useful modes of photomechanical motion of
molecular crystals.
F
F
F F
F
F
F
F
F
F
F
F
UV
Photomechanical materials convert photon energy into mechanꢀ
ical motion. The use of light can achieve precise spatial control
without the need for direct physical contact. They are promising
candidates as actuators in the future.1 Especially, photomechanical
molecular crystals show some advantages over polymerꢀbased
materials, including faster response time,2,3 higher Young's moduꢀ
lus,4 and an ordered crystal structure which can be easily characꢀ
terized by Xꢀray diffraction techniques.5ꢀ7 Multiple researchers
have prepared various photomechanical crystals that show differꢀ
ent motions such as contraction,2,8,9 expansion,10,11 bending,11ꢀ34
fragmentation,8,35ꢀ38 and twisting.19,39,40 One challenge is to harꢀ
ness these crystals to do more complex work via controlled shape
changes. Examples of using crystal shape to control motion inꢀ
clude recent work by Lu et al., on the shapeꢀdependent photoinꢀ
duced bending, curling, rolling, and salient behaviors of
styrylbenzoxazole derivatives.41 Another example by Bardeen et
al. demonstrated that branched microcrystals of one anthracene
derivative could show light driven ratchetꢀlike rotation.42 The
crystal structures prepared by slow pHꢀreprecipitation method
generate a "defective" crystal which results in a chiral shape that
can lead to the unidirectional rotation by twisting of its branch
arms. In this previous work, light illumination was carried out
evenly and no effort was made to investigate the effect of illumiꢀ
nation conditions in detail.
CH3
H3C
1a
CH3
H3C
1b
Vis.
S
S
S
S
Scheme 1. Molecular structure change of diarylethene used in this work
Figure 1. Optical microphotographs of (a) block shape crystals prepared
by slow evaporation from nꢀhexane and acetone mixed solution of 1a and
(b) ribbon shape crystals prepared by sublimation of 1a. Scale bar is 1
mm.
When diarylethene 1a was recrystallized from an organic
mixed solvent of nꢀhexane and acetone using a slow evaporation
method, blockꢀshaped crystals could be obtained as shown in
Figure 1a.43,44 On the other hand, we found that the ribbon crystals
appeared after sublimation as shown in Figure 1b. The details of
the sublimation method are described in the Supporting Inforꢀ
mation. To determine the molecular orientation in the crystal,
powder Xꢀray diffraction (PXRD) measurements were performed.
Our group has reported the effect of illumination conditions
such as wavelength and power on photomechanical crystal deforꢀ
mations like bending.22,24 Controlling more complex shape changꢀ
es using light exposure conditions is important for the further
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Molecular orientation of 1a in the crystal viewed from (001) face. (e)
Illustration of crystal shape and corner angles viewed from (001) face.
Figures 2a and b show PXRD pattern of the ribbon crystal and the
pattern calculated from singleꢀcrystal Xꢀray crystallographic data
of the block crystal of 1a. Singleꢀcrystal Xꢀray crystallography of
the block shape crystal of 1a showed a monoclinic crystal system
and a space group of P21/c (Table S1).43 As shown in Figure 2a,
only diffraction peaks corresponding to the (002) and (006) Miller
planes were observed for the ribbon. This result means that the
flat surface of the ribbon is the (001) face. Figure 2c shows optical
microphotographs of the ribbon crystal. The corner angles of the
crystal were 122°, 116°, and 122°. Figure 2d shows the molecular
packing in the unit cell viewed from the (001) face, and Figure 2e
illustrates the crystal shape viewed from the (001) face. Calculatꢀ
ed corner angles in the illustration are 122°, 116°, and 122°, reꢀ
spectively. These angles are consistent with the single crystal
structure, which means that the block crystal and the ribbon crysꢀ
tal are the same polymorph. The long axis and short axis of the
ribbon crystal correspond to the bꢀaxis and aꢀaxis in the unit cell,
respectively.
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We then investigated the photomechanical behavior of crystalꢀ
line 1a. Large block shaped crystals usually have widths on the
order of 0.1 mm. Although they undergo photochromic reaction
from the openꢀring isomer 1a to the closedꢀring isomer 1b,43,44
they do not exhibit any photomechanical motion due to their large
bending inertia. On the other hand, when UV irradiation (365 nm
light) was carried out on the ribbon crystal, it exhibited photomeꢀ
chanical twisting accompanying the color change from colorless
to blue as shown in Figure 3, Figure S1, and Video S1. Moreover,
upon irradiation with visible light, the blue color disappeared and
the crystal returned to its initial straight shape. This photomechanꢀ
ical behavior is ascribed to the reversible photochromic reaction
of diarylethene molecules in the crystalline phase. In fact, the
photomechanical crystal twisting could be repeated more than 10
cycles.
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Figure 3. Photoreversible crystal twisting of the ribbon crystal of 1a. UV
irradiation was carried out from left side on the image. The ribbon crystal
was fixed to the glass capillary by glue. Scale bar is 300 ꢀm.
Twisting is a commonly observed photomechanical response of
the ribbon morphology.19,39,40 It is usually assumed that the twist
period is determined solely by internal strain generated at the
interface of the reactantꢀproduct bilayer. If generating photoprodꢀ
uct on one side of the ribbon is the only requirement for twisting,
then one would naively expect that the final twist shape should be
the same for different illumination conditions. To see if this was
true, we first investigated whether the twist direction could be
controlled by varying the illumination conditions. Figure 4 shows
the photomechanical twisting behavior of the ribbon crystal when
UV irradiation was carried out from four different directions (5
sec and 26.3 mW cm−2 for each panel). Magnified images are also
shown in Figure S2. When the ribbon crystal was irradiated with
UV light from upper back to front as shown in Figure 4a, it twistꢀ
ed into a leftꢀhanded helix. On the other hand, the ribbon crystal
twisted into a rightꢀhanded helix when it was irradiated with UV
light from lower back to front as shown in Figure 4b. Moreover,
when the UV irradiation was carried out from front to back as
shown in Figures 4c and d, the ribbon crystal twisted in the rightꢀ
handed helix and the leftꢀhanded helix, respectively, which means
that the ribbon crystal tends to twist toward the incident light.
These results suggest that changing the direction of the incident
light induces extra photoproduct on one side or the other of the
crystal ribbon which would bias the twist. Once the twist starts in
one direction, it will continue in that direction. It is kind of like
symmetry breaking in physics: a small bias will send the system
to one or the other outcomes.
Figure 2. (a) PXRD pattern of a ribbon crystal of 1a and (b) the pattern
calculated from single crystal Xꢀray crystallographic data of 1a. (c) Optiꢀ
cal microphotograph of a ribbon crystal of 1a. Scale bar is 100 ꢀm. (d)
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Figure 4. Photoinduced twisting behavior of the ribbon crystal of 1a in different direction depending on the angle of the incident light. Scale bar is
300 ꢀm.
The detailed shape of the twist also depends on illumination
conditions. We investigated the effect of the angle of the incident
light on the photomechanical crystal twisting as shown in Figure
5. UV irradiation was carried out as shown in the image (30 sec
and 26.3 mW/cm2 for each panel). Magnified images are also
shown in Figure S3. When the ribbon crystal was irradiated with
UV light from tip of the crystal (incident light angle = 0°), the
ribbon crystal twisted into a helicoid shape (Figure 5a). On the
other hand, when UV irradiation was incident on the ribbon at
larger angles, it gradually transformed into a cylindrical helix
shape (Figure 5bꢀg). The incident light angle determines the angle
of twist. Eventually, when the ribbon crystal was irradiated with
an angle of 90° as shown in Figure 5h, the crystal exhibited bendꢀ
ing rather than twisting. In order to parameterize the variable
twisting behavior, the crystal profile viewed from side (L) and
height of the first helix from the glass capillary (H) were plotted
against the incident light angle as shown in Figure 6. The actual
length of the straight crystal is constant (ca. 1600 ꢀm). When the
As the light is rotated, the two molecular orientations are no longꢀ
er equally excited and a diagonal stress component leads to heliciꢀ
ty. At very shallow angles (~0°), only one orientation of moleꢀ
cules is excited, because the other population has its transition
dipole moment aligned close to parallel to the direction of light
propagation. In this case, the stress tensor arises from only the
subpopulation rotated at +20°, with both positive (expansive) and
negative (compressive) components. These are close to the condiꢀ
tions for generating a pure twist.46 In other words, by exciting
differently oriented molecules, the photoinduced strain tensor in
the crystal (and thus the mode of photomechanical deformation)
can be controlled by the direction of UV light irradiation. This
model requires additional work to confirm it but does provide a
framework for future experiments.
incident light angle (θ) becomes larger, L and H became smaller
and larger, respectively, due to the formation of a cylindrical heꢀ
lix.
Katonis et al. demonstrated the ribbons that consist of liquid
crystalline polymer can show curling, leftꢀhanded and rightꢀ
handed helical twisting depending on the direction in which they
are cut.45 In that case, they had to make a new material every time
they wanted a different helicity. Using molecular crystals, we can
tune the helicity simply by changing the light direction. We hyꢀ
pothesize that the sensitivity of the crystal shape change to light
direction results from preferential excitation of differently orientꢀ
ed molecules within the crystal, generating an effective bilayer
structure whose internal stress leads to helical ribbon deforꢀ
mations.6,19,39,40,46 A qualitative model that considers the two limꢀ
iting cases is described below and in the Supporting Information.
Briefly, a single crystal of 1a consists of two subpopulations with
molecules oriented at angles of +20° and −20° with respect to the
bꢀaxis. Each molecule can generate stress both parallel and perꢀ
pendicular to its long axis. The ability of a light beam to interact
with each subpopulation depends on the direction of incidence
relative to their transition dipole alignments. For example, if the
light propagates parallel to the dipole, the molecule cannot be
excited since the polarization will always be perpendicular to the
dipole moment. If the light is incident perpendicular to the b-c
edge (90°), it excites all molecules equally and they will generate
stress both parallel and perpendicular to the ribbon long axis. The
resulting stress tensor, aligned with the ribbon shape, can generate
bending but has no diagonal component to initiate helical motion.
Figure 5. Different twisting motion, a helicoid and a cylindrical helix,
depending on the angle of the incident light. Scale bar is 300 ꢀm.
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Figure 6. Plot of the crystal length viewed from side (L) and height of the
first helix from the glass capillary (H) against the incident light angle ( ).
θ
In conclusion, we have fabricated ribbonꢀshaped crystals of 1a
by a sublimation method and investigated their photomechanical
motion. The ribbon shape crystal exhibited photoreversible twistꢀ
ing upon alternating irradiation with UV and visible light. Both
the twisting direction and mode (helicoid versus cylindrical) could
be controlled by the direction of the incident light. These results
indicate that the stress tensor on the crystal surface induced by
photochromic reaction could be controlled by the illumination
direction. While the detailed mechanism needs to be investigated
further, the use of illumination angle to tune the mechanical reꢀ
sponse illustrates that photomechanical molecular crystals provide
unique opportunities for the control of their motion.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website. Magnified images of photomechanical
crystal twisting, qualitative model for twisting, Illustration of two
types of twisting: a helicoid and a cylindrical helix, strain tensor
on the crystal surface, and preparation of the ribbon crystals of 1a
by sublimation method (PDF). Movie of photomechanical crystal
twisting: a helicoid and a cylindrical helix (Video S1ꢀS2).
AUTHOR INFORMATION
Corresponding Author
* Eꢀmail: kobatake@aꢀchem.eng.osakaꢀcu.ac.jp
* Eꢀmail: christopher.bardeen@ucr.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was partly supported by JSPS KAKENHI Grant Numꢀ
ber JP26107013, JP15K21725 in Scientific Research on Innovaꢀ
tive Areas “Photosynergetics” (S.K.) and JSPS KAKENHI Grant
Number JP16K17896 in Scientific Research for Young Scientists
(B) (D.K.), and the National Science Foundation grant DMRꢀ
1508099 (C.J.B).
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44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
6
ACS Paragon Plus Environment