Photoinduced reversible formation of microfibrils on a photochromic diarylethene microcrystalline surface

Article abstract of DOI:10.1002/anie.200602126

(Chemical Equation Presented) In a different light: A photoinduced change in surface morphology provides reversible changes in the superhydrophobic properties of a photochromic diarylethene film. The effect is achieved by the reversible formation of fine fibril structures on the coated microcrystalline surface upon irradiation by UV or visible light (see picture).

Full text of DOI:10.1002/anie.200602126

Communications
Superhydrophobic Effect
solution of 1 in chloroform onto a substrate, which gave a
contact angle of the water droplet of around 1208. Upon
irradiation with UV light, the film surface became super-
hydrophobic (contact angle 1638). SEM images showed that
the surface was covered with microfibrils of diameter around
1 mm. Upon irradiation with visible light, the surface again
became flat and recovered the initial contact angle of 1208.
Diarylethene derivatives are promising artificial photo-
responsive molecules that show reversible transformation
between open- and closed-ring isomers with different absorp-
tion spectra.[3] As they also show thermally irreversible and
fatigue-resistant photochromic performance,[4] intensive stud-
ies have aimed at applications such as molecular memories
and switches.[5–13] Herein, we report a new function for the
cast film of a derivative.
There are two approaches to controlling surface wett-
ability. One is to change the surface morphology, because it is
known that increasing surface roughness results in a super-
hydrophobic surface, an effect that is widely observed on lotus
leaves.[14,15] Such micrometer-scale rugged or fractal structures
were also artificially prepared to make superhydrophobic
surfaces.[1b,16,17] The other approach is a change of polar-
ity.[18–22] Several reports have attempted to control surface
wettability by using photochromic compounds that change
the surface polarity.[21] We report changes in photoinduced
wettability based on the changes in morphology of a photo-
chromic diarylethene crystal and thin film.
DOI: 10.1002/anie.200602126
Photoinduced Reversible Formation of
Microfibrils on a Photochromic Diarylethene
Microcrystalline Surface**
Kingo Uchida,* Norikazu Izumi, Shinichiro Sukata,
Yuko Kojima, Shinichiro Nakamura,* and Masahiro Irie
Lotus and taro leaves have hydrophobic surfaces, which in
nature consist of micrometer-scale rods or projections that
exhibit a super water-repellent property called the lotus
effect. The contact angle of a water droplet on such surfaces is
161.0 Æ 2.78. Recently, many reports concerning changes in
water droplet contact angle as a result of changes in surface
morphology and surface polarity have been published.[1]
Additionally, changes in photoresponsive wettability have
also been reported.[2]
Herein, we report a new photoinduced change in surface
morphology that provides superhydrophobic properties,
obtained by the photoinduced reversible formation of fine
fibril structures on coated microcrystalline surfaces. The
origin of the reversible formation of fibrils is the photo-
isomerization of a photochromic diarylethene molecule 1 (see
Scheme 1) of a thin film. The reversible changes in surface
morphology of a thin film made from photochromic diary-
lethene 1 were followed by scanning electron microscopy
(SEM), optical microscopy, and contact angle measurements
of a water droplet. The film was prepared by coating a
The photochromism of a diarylethene, 1,2-bis(2-methoxy-
5-trimethylsilylthien-3-yl)perfluorocyclopentene
(1o;
Scheme 1), in a hexane solution is shown in the Supporting
[*]Prof. K. Uchida, N. Izumi, S. Sukata
Department of Materials Chemistry
Faculty of Science and Technology, Ryukoku University and
CREST, Japan Science and Technology Corporation
Seta, Otsu 520-2194 (Japan)
Fax: (+81)77-543-7483
Scheme 1. Reversible formation of open-ring isomer 1o and closed-
ring isomer 1c.
E-mail: uchida@chem.ryukoku.ac.jp
Homepage: http://www.chem.ryukoku.ac.jp/~uchida/
UchidaLab5a.htm
Dr. Y. Kojima, Dr. S. Nakamura
Mitsubishi Chemical Group
Science and Technology Research Center, Inc. and
CREST, Japan Science and Technology Corporation
1000 Kamoshida, Yokohama 227–8502 (Japan)
E-mail: shin@cc.m-kagaku.co.jp
Information. The open-ring isomer is colorless, and absorp-
tion maxima of the spectrum were observed at l = 255 (e =
2.8 ” 104 mÀ1 cmÀ1) and 325 nm (e = 7.7 ” 103 mÀ1 cmÀ1). Upon
UV irradiation (254 nm), the solution turned blue and a new
band was observed at 599 nm (e = 5.2” 10 3 mÀ1 cmÀ1). Upon
irradiation with visible light, the color disappeared and the
bands of the open-ring isomer were regenerated (see
Scheme 1). The coloration and discoloration quantum yields
were 0.37 and 0.0015, respectively. The reversible formation
of the two isomers was not only observed in solution but also
in the crystalline phase.
An SEM image of the crystalline surface is shown in
Figure 1a. Upon UV irradiation (254 nm), the color became
deep blue within 5 min, yet no change in surface morphology
was observed at this moment at room temperature. After
irradiation for 10 min the crystal was stored in the dark, but
5 min later small microfibrils had already started to grow (see
Prof. M. Irie
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu University
6-10-1 Hakozaki, Fukuoka 812-8581 (Japan)
[**]This work was partly supported by Grants-in-Aid for Scientific
Research (B) (18350101) from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT) of the Japanese Govern-
ment. We thank Prof. Takahiro Seki and Mitsuo Hara for the powder
X-ray analysis. We also express thanks to Prof. Hiroshi Komatsu,
Prof. Tsuyoshi Tsujioka, Dr. Takao Matsuzaki, Prof. Nobuo Niimura,
Dr. Seiya Kobatake, and S. Matsumoto for their helpful discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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The phase diagram of
photoirradiated crystals
was measured to investi-
gate why microfibrils grew
after
UV
irradiation
(Figure 4). The melting
points of the open- and
closed-ring isomers are
99.8–100.3
and
143.2–
144.18C,
respectively,
whereas the melting point
of the mixture of open- and
closed-ring
isomers
decreased to around 308C
at a certain ratio (24:76
open-ring/closed-ring iso-
mers). The result suggests
that a homocrystal of the
Figure 1. Reversible changes in surface morphology of a single crystal of 1 (scale bar: 10 mm). a) SEM image
of the crystal surface of 1o from a side view (”1000) before UV irradiation (254 nm). b) SEM image from a
side view (”1000) of the surface after UV irradiation (254 nm, 12 W, 10 min) and storage in the dark for 24 h.
c) SEM image from a side view (”1000) of the surface after irradiation with visible light (l>500 nm, 500 W,
20 min) and storage in the dark for 24 h. d–h) Optical microscopic images (”500) of the appearance and
disappearance of fibrils upon irradiation with UV and visible light (500-W Xe lamp, l>500 nm). d) Before
irradiation with UV light, the surface was flat. e) After UV irradiation (254 nm, 10 min) and storage in the dark
for 5 h at room temperature, microfibrils were generated. Irradiation with visible light for f) 5, g) 10, and
h) 60 min.
closed-ring
isomer
is
formed from the melting
of the crystal at this ratio.
Supporting Information). After storage for 24 h, the surface
was covered with many microfibrils, as shown in Figure 1b.
Upon irradiation of the surface with visible light (l > 500 nm),
the blue color and the microfibrils disappeared (see Fig-
ure 1c). The surface changes at the same point were also
monitored from a side view by using an optical microscope
(see Figure 1d–h). The disappearance process (Figure 1e–h)
of the fibrils was well monitored.
Measurement of the contact angle of a water droplet is a
simple and direct method for evaluating wettability. The
contact angle of the film surface before UV irradiation was
1208 (Figure 2a). Upon irradiation of the surface with UV
light (254 nm), the color became blue and the angle was
reduced to around 908 (Figure 2b) after a short irradiation
time (1–2min). After 24 h the angle had gradually increased
to 1638 (Figure 2c and d).
The colored surface was irradiated with visible light to
regenerate the open-ring isomer. After 20 min the blue color
disappeared. The contact angle was also monitored during the
fibril disappearing process. It gradually decreased and finally
recovered to the initial 1208 (Figure 2e).
Powder X-ray diffraction measurements were carried out
to elucidate the composition of the microfibrils. Figure 3
shows a comparison of the measured powder diffraction
patterns and estimated patterns from single-crystal structure
analysis of the open- (1o) and closed-ring (1c) isomers. The
fibril pattern was in good agreement with that of the closed-
ring isomer. Diffraction peaks at 5.1, 7.7, and 15.08 are
Figure 2. Contact angles of a water droplet on a microcrystalline film
of diarylethene 1. a) Water droplet (1.2 mm 1) on the microcrystalline
surface before irradiation with UV light (254 nm, 12 W). The contact
angle was 120Æ28. b) Water droplet after UV irradiation for a short
time (1–2 min). The contact angle changed to 92Æ28. c) Water droplet
after irradiation (10 min) with light and storage in the dark at room
temperature for 24 h. The contact angle increased to 163Æ28. d) Time
profile of the contact angle during the fibril generation process after
UV irradiation. e) Profile during fibril disappearance upon irradiation
of the colored fibril structured surface with visible light.
¯ ¯
appropriate for the (001), (010), and (111) lattice planes of the
closed-ring isomer crystal, respectively. Ordinarily, closed-
ring isomers formed in the crystal of open-ring isomers are
strained by the lattice and have different structures from
closed-ring isomer crystals obtained by recrystallization in
solution.[23,24] The above results indicate that the fibril crystal
formed on the film grew freely from the crystal lattice of the
open-ring isomer.
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imately 20–30 mm and was measured by observing the scratched
surface by SEM.
A scanning electron microscope (JEOL JSM-5200) and an optical
microscope (Leica DMLP) were used to study the surface micro-
structure. Photoirradiation (visible light: l > 500 nm) was carried out
using a Ushio 500-W xenon lamp with a cutoff filter (Toshiba color
filter Y-50), and UV irradiation was carried out by using a Topcon UV
lamp PU-2(12W). X-ray crystallographic analysis was performed
using a Bruker SMART CCD X-ray diffractometer. Suitable colorless
prism-shaped (1o) and dark blue needle crystals (1c) were obtained
by recrystallization from ethanol. Crystals with dimensions of 0.5 ”
0.4 ” 0.15 mm3 (1o) and 0.5 ” 0.1 ” 0.05 mm3 (1c) were mounted on a
glass fiber and aligned on a Bruker SMART 1000 CCD diffractom-
eter (platform with full three-circle goniometer) at room temper-
ature. The diffractometer was equipped with a 1K CCD detector set
50.0 mm from each crystal. Intensity measurements were performed
using graphite-monochromated MoKa radiation from a sealed ceramic
diffraction tube (Siemens). Generator settings were 50 kV/40 mA.
SMART was used for preliminary determination of the unit cell
constants and data collection control. The intensities of the reflections
of the hemisphere were collected by a combination of three sets of
exposures (frames). Each set had a different angle f for the crystal
and each exposure covered a range of 0.38 in w; 1800 frames were
collected for each crystal with an exposure time of 5.0 s per frame for
1o and 10.0 s per frame for 1c. Data integration and global cell
refinement was performed with SAINT. The final unit cells were
obtained after integration from the xyz centroids of 2912 reflections
for 1o and 1352reflections for 1c. Intensity data were corrected for
Lorentz and polarization effects, scale variation, and decay and
absorption; based on the intensities of symmetry-related reflections
measured at different angular settings (SADABS), multiscan absorp-
tion correction was applied and reduced to F2o.
Figure 3. X-ray diffraction patterns of needles obtained from the sur-
face by scratching (blue line), and of 1o (green line) and 1c (pink line)
derived from single-crystal analysis.
Crystal data and structural refinement parameters for 1o:
¯
C21H26F6O2S2Si2, Mr = 544.72, triclinic, space group P1, a =
10.122(6), b = 11.268(6), c = 12.814(7) Š, a = 103.355(11), b =
94.298(13), g = 74.981(12)8, V= 1373.3(13) Š3, Z = 2, 1calcd
=
Figure 4. Phase diagram of mixtures of open- (1o) and closed-ring
isomers (1c) formed by irradiation of diarylethene crystals with UV
light (313 nm).
1.317 gcmÀ3
, , T= 298(2) K, F(000) = 564,
m(MoKa) = 0.337 mmÀ1
R1 = 0.0685 (wR2 = 0.2056) for 5717 unique reflections (Rint
0.0172) with a GOF of 0.927. Crystal data and structural refinement
=
parameters for 1c: C21H26F6O2S2Si2, Mr = 544.72, triclinic, space group
¯
P1, a = 6.697(2), b = 11.777(4), c = 17.470(6) Š, a = 96.899(6), b =
96.993(6),
g = 98.472(6)8,
V= 1339.2(8) Š3,
T= 298(2) K, F(000) = 564,
Z = 2,
1calcd =
The shape of the crystals of the closed-ring isomer was a
needle or fibril when obtained by recrystallization from a
solution containing this isomer. The crystal shape tends to
grow in one direction along the a axis. Upon UV irradiation,
the open-ring isomer is converted to the closed-ring isomer.
When conversion exceeds 24%, crystal growth of the closed-
ring isomer starts to form fibril structures on the film surface.
To confirm this mechanism, the sample was kept in air and
water at different temperatures. Below around 08C the fibril
did not grow, but we found fibril formation in water and in air
around 308C. The results also support the melt mechanism as
depicted in the phase diagram. Fibril growth was not only
observed on glass and metal substrates but also on plastics
and paper. The film will be applicable to materials with
photoinduced reversible surface water repellency.
1.351 gcmÀ3
,
m(MoKa) = 0.346 mmÀ1
,
R1 = 0.0791, (wR2 = 0.1898) for 4496 unique reflections (Rint
=
0.0359) with a GOF of 0.883. Data were collected on a Bruker
SMART 1000 CCD diffractometer with graphite-monochromated
MoKa radiation (l = 0.71073 Š). The structure was solved by direct
methods using SHELXS97 and refined by full-matrix least-squares
against F2 with SHELXL97.[25] CCDC 293161 (1o) and 293162 (1c)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: May 28, 2006
Published online: August 22, 2006
Keywords: crystal growth · hydrophobic effect · photochromism ·
.
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