Photo-induced reversible topographical changes of photochromic dithienylethene microcrystalline surfaces

Article abstract of DOI:10.1039/b821329a

Upon alternating UV and visible light irradiation, the surface of a coated film of 1,2-bis(2-methyl-5-phenylthien-3-yl)perfluorocyclopentene showed reversible surface topographical changes between a lumpy surface with small structures (～1.0 m) and a smoot

Full text of DOI:10.1039/b821329a

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Photo-induced reversible topographical changes of photochromic
dithienylethene microcrystalline surfacesw
a
a
b
b
Norikazu Izumi, Naoki Nishikawa, Satoshi Yokojima, Yuko Kojima,
b
c
d
a
Shinichiro Nakamura, Seiya Kobatake, Masahiro Irie and Kingo Uchida*
Received (in Montpellier, France) 28th November 2008, Accepted 26th January 2009
First published as an Advance Article on the web 16th February 2009
DOI: 10.1039/b821329a
Upon alternating UV and visible light irradiation, the surface of a
coated film of 1,2-bis(2-methyl-5-phenylthien-3-yl)perfluoro-
cyclopentene showed reversible surface topographical changes
between a lumpy surface with small structures (B1.0 lm) and a
smoother surface with larger structures (5B10 lm), accompanied
by changes in the water droplet contact angle.
Diarylethenes undergo reversible photoisomerization, even
7
in their crystalline states. The photochromism of 2 in solution
and in the crystalline state has already been reported, and
its crystals maintain a clear, transparent character during
7
e,f
coloration/decoloration cycles at room temperature;
photo-induced surface changes were not observed at elevated
temperatures. The phase diagram of diarylethene 2 is shown in
Fig. 1. The melting points of 2o and 2c are 140 and 193 1C,
respectively. The eutectic point is 115 1C, where the ratio of
2o to 2c is 80 : 20.
Photochromic compounds undergo reversible color changes
1
by alternating UV and visible light irradiation. Diarylethenes,
which are some of the most well known photochromic
compounds, show thermally stable photochromic perfor-
2
mances with high fatigue resistance. Therefore, the study of
6
Based on the results of the micro-fibril formation of 1, the
photo-induced reversible topographical changes of 2 were
expected to occur above 115 1C. Before UV irradiation, the
surface of the coated film with 2o was monitored by SEM, and
the image is shown in Fig. 2a. Even before UV irradiation,
some rough structures were observed that may be attributable
to microcrystals of 2o, generated when the solvent was
evaporated. The surface was irradiated with UV light
(254 nm) for 10 min while stored at 115 1C, and then kept
in the dark for 6 h at the same temperature. The surface
was monitored by SEM and found to be covered by small
lumps with a diameter around 1 mm (Fig. 2b); some of
the surface looked hexagonal-shaped. Then, the surface
was irradiated with visible light (l 4 500 nm) while the
temperature was kept at 115 1C on the hot stage. Because of
the lower quantum yield (0.0081 (600 nm)) of the cyclorever-
sion reaction of 2c compared with the cyclization quantum
diarylethenes has especially concentrated on their properties
that are directly related to applications for molecular switches
3
and memory devices. Recently, however, we have seen
progress in using such photochromic compounds for other
applications. For example, reversible bending of rod-like or
4
thin film-like crystals has been observed. Additionally, the
selective deposition of Mg atoms and their control has been
5
shown using a diarylethene thin film substrate. The discovery
of the photo-controlling of surface wettability is one example
of this remarkable progress; i.e., we have demonstrated
the reversible formation and deformation of micrometer-
scale fibrils on microcrystalline films of diarylethenes 1,
accompanied by changes in the water droplet contact angle
(
6
CA). This finding is thus applicable for the photo-control of
surface wettability changes. The reversible changes to the CA
were attributed to reversible changes in the surface roughness,
which was estimated by fractal analysis, obtained by the box-
7f
yield (0.52 (286 nm)) of 2o, irradiation was carried out for the
initial 1 h of heating. After 90 min, the small lumps
disappeared and a different type of rough surface was
observed (Fig. 2c and d); time profiles are shown in Fig. 3.
6
b
counting method. In this Letter, we report the reversible
topographical changes of a surface of a coated film of 2o
(
Scheme 1) to show that such reversible topographical changes
are universal phenomena in the diarylethene compound
family.
a
Department of Materials Chemistry, Faculty of Science and
Technology, Ryukoku University, Seta, Otsu 520-2194, Japan.
E-mail: uchida@rins.ryukoku.ac.jp; Fax: +81 775437483;
Tel: +81 775437462
Mitsubishi Chemical Group Science and Technology Research
Center, Inc., 1000 Kamoshida, Yokohama 227-8502, Japan
b
c
Department of Applied Chemistry, Graduate School of Engineering,
Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka
558-8585, Japan
d
Department of Chemistry, Rikkyo University, Nishi-Ikebukuro
-34-1, Toshima-ku, Tokyo 171-8501, Japan
3
w Electronic supplementary information (ESI) available: Coating film
preparations, contact angle measurement details, summary crystal
data and further SEM images. See DOI: 10.1039/b821329a
Scheme 1
1
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Fig. 1 The phase diagram of mixtures of open (2o)- and closed
2c)-ring isomers formed upon UV light (313 nm) irradiation.
(
Fig. 3 Time profiles of the water droplet CA on the diarylethene
surface. UV irradiation was carried out for an initial 10 min, while
visible light irradiation was carried out for an initial 60 min. (a) The
time profile of the CA during the micro meter scale cubic structure
generation process. (b) The time profile of the CA during the cubic
structure disappearance process upon irradiation of the colored
structured surface with visible light.
6
eutectic state; a similar phenomenon was observed for 1. 6 h
later, the angle reached a constant value of 1361 (Fig. 2g). The
CA of the water droplet was enhanced due to the smaller
lumpy structures of 2c, compared with the structures of the
initial surface. The smaller angle compared with the surface of
diarylethene 1c is attributable to the crystal shape differences
between the closed-ring isomers 1c and 2c. Closed-ring isomer
1
c formed needle-like crystals, while 2c formed cubic-like ones.
Upon visible light irradiation of the surface while maintaining
the temperature, larger structures (1.0–5.0 mm) had grown on
the surface and the CA had been decreased to 1181.
The dipole moment changes of diarylethenes accompanying
the photochromism are much smaller than those of azo-
benzenes and spiropyranes. By the B3LYP/6-31G(d) method,
the dipole moment of 2o and 2c were calculated to be 3.8768 D
and 4.3181 D, respectively. Therefore the CA changes, as
shown in Fig. 3, are not attributable to the dipole changes
of the diarylethene but to surface topographical changes.
Upon alternating UV and visible light irradiation and while
maintaining the eutectic temperature, a surface with small
lumps (Fig 2b) and another with larger structures (Fig 2c,d)
was alternately observed by SEM for two cycles.
Fig. 2 Surface topographical changes of a diarylethene film of 2.
SEM images: (a) before UV irradiation (Â10 000), (b) after UV
irradiation (Â10 000), (c) after visible light irradiation (Â10 000) and
(
(
d) after visible light irradiation (Â2000). The CAs of a water droplet
e) before UV irradiation, (f) after UV irradiation for 10 min, (g) 6 h
after UV light irradiation and (h) after visible light irradiation.
At the same time, the CA of the water droplet changed during
both processes (Fig. 2e–h). The angle of a water droplet
To understand the crystal shapes, we estimated the Bravais,
Friedel, Donnay and Harker (BFDH) crystal morphology of
2o and 2c using Mercury CSD 2.0. The estimated crystal shape
of 2o is a rectangular parallelepiped and that of 2c is a
(
1.2 mm f) at the initial surface of 2o was 1201 (Fig. 2e). Upon
UV light irradiation, the CA rapidly decreased to 961 in 10 min
Fig. 2f and 3a) as the surface melted, because it was in the
(
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electron microscope (JEOL JSM-5200) and an optical micro-
scope (Leica DMLP) were used to study the surface micro-
structure. Static CAs were measured on an optical contact
angle meter (Kyowa Interface Science Co., Ltd., Drop Master
5
00) by the y/2 method at 25 1C. De-ionized water (1.5 ml) was
dropped carefully onto the surface. An average CA value was
obtained by measuring the same sample at five to ten different
positions (see ESIw). Photoirradiation (visible light: l 4
5
00 nm) was carried out using a Ushio 500 W xenon lamp with
a cut-off filter (Toshiba color filter Y-50), and UV irradiation
was carried out using a Topcon PU-2 (12 W) UV lamp.
Photoirradiation experiments at the eutectic temperature were
carried out on a Mettler Toledo FP90 instrument attached to a
FP82HT hot stage. The crystal data have already been reported
7e
in a previous paper.
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8 1C. Nucleation would be faster for 2c because of the larger
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In conclusion, photo-induced reversible topographical
changes were observed above the eutectic point of open- and
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whose crystalline photochromism has already been reported;
no surface changes were observed at room temperature.
The topographical changes seen on the surface above the
eutectic temperature should be universal phenomena in
organic photochromic systems.
1
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This work was supported by Grants-in-Aid for Scientific
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(no. 471)’’ and (B) (18350101) from the Ministry of Education,
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1
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Experimental
The film was prepared by coating a chloroform solution
À1
containing 2 (100 mg ml ) onto the substrate (glass, resin
4
871–4876; (f) S. Kobatake, K. Shibata, K. Uchida and M. Irie,
or metal), and the solvent was evaporated in vacuo. The film
thickness, which was approximately 20–30 mm, was measured
by observing the scratched surface by SEM. A scanning
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1
326 | New J. Chem., 2009, 33, 1324–1326
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009