Temperature dependence of the photoinduced micro-crystalline surface topography of a diarylethene

Article abstract of DOI:10.1039/c3ce41336e

By alternate irradiation with UV and visible light, reversible topographical changes were observed. Step and valley formation while keeping the crystal confinement was observed below the glass transition temperature (T _g), while crystal growth via a softened state was observed above the T_g on the same diarylethene crystalline surface.

Full text of DOI:10.1039/c3ce41336e

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Temperature dependence of the photoinduced micro-
crystalline surface topography of a diarylethene†
Cite this: CrystEngComm, 2013, 15,
400
8
a
a
a
ab
Noriko Fujinaga, Naoki Nishikawa, Shingo Sakiyama, Seiji Yamazoe,
c
d
e
f
Yuko Kojima, Tsuyoshi Tsujioka, Satoshi Yokojima, Shinichiro Nakamura
and Kingo Uchida*a
Received 8th July 2013,
Accepted 23rd August 2013
DOI: 10.1039/c3ce41336e
www.rsc.org/crystengcomm
By alternate irradiation with UV and visible light, reversible
topographical changes were observed. Step and valley
formation while keeping the crystal confinement was observed
1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene
(1o) turned blue without a change in the shape of the crystal
upon irradiation with UV light at room temperature. The col-
our change is a manifestation of the formation of the closed-
g
below the glass transition temperature (T ), while crystal growth
1
6
via a softened state was observed above the T
g
on the same
ring isomer (1c). Upon irradiation with UV (366 nm) light,
new steps appeared on the (100) surface of the crystal which
disappeared again upon irradiation with visible light
(λ > 500 nm). The step height of ca. 1 nm corresponds to
one molecular layer. Irradiation with UV light formed
valleys on the (010) surface that also disappear upon irradi-
ation with visible light (λ > 500 nm). The changes to the
surface morphology can be rationalised by the changes to
the molecular structure of diarylethenes in the single crys-
tal (Fig. S1, ESI†).
diarylethene crystalline surface.
Photochromic compounds undergo reversible colour changes
1
,2
upon alternating irradiation with UV and visible light. Such
photochromic compounds have been applied for photoinduced
3–11
surface shape changes.
Diarylethenes, which are amongst
the most studied photochromic compounds, are thermally
stable in both coloured and colourless states and show high
1
2,13
fatigue resistance.
Due to the relatively minor extent of
structural change that accompanies the photochromism of the
diarylethene derivatives, they can undergo photoisomerization
even in the crystalline state, where the distance between the two
reactive carbon atoms of the open-ring isomer in the crystalline
Recently, we have shown that topographical changes of the
surface can be achieved by the growth and regrowth of crystals
of the open-ring or closed-ring isomers upon alternate irradia-
tion with UV and visible light at the eutectic temperature for
14
17–19
state is less than 4 Å. Irie et al reported the photoinduced for-
both isomers.
However, such crystal growth was observed
mation and deformation of steps and valleys on the surface of a
diarylethene single crystal. The colourless single crystals of
previously below the eutectic temperature in a particular dia-
rylethene derivative. This observation raises an important
15
19
question as to the factors that determine the transition tem-
perature for the two photo-induced changes in the surface
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: +8177 543 7483; Tel: +81 77 543 7462
(
step and valley formation reported by Irie and crystal growth
b
via the eutectic state reported by our group). In the present
contribution, we report the temperature dependence of the
topographical changes observed on single crystals of 1o.
The melting points of mixtures of 1o and 1c were deter-
mined by DSC and the phase diagram obtained is shown in
Fig. 1. The melting points of 1o and 1c are 130 and 189 °C,
respectively. The eutectic temperature is 121 °C, where the
ratio of 1o to 1c is 86 : 14.
Department of Chemistry, School of Science, the University of Tokyo, Hongo 7-3-1,
Bunkyo-ku, Tokyo, 221-0033, Japan
c
Mitsubishi Chemical Group Science and Technology Research Center, INC., 1000
Kamoshida, Yokohama 227-8502, Japan
d
Department of Arts and Sciences, Faculty of Education, Osaka Kyoiku University,
4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582, Japan
e
f
School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1
Horinouchi, Hachioji, Tokyo 192-0392, Japan
Nakamura Laboratory, RIKEN Research Cluster for Innovation, 2-1 Hirosawa,
Wako, Saitama 351-0198, Japan
1
7,18
Our previous studies
indicated that reversible photo-
†
Electronic supplementary information (ESI) available: estimated crystal
induced topographical changes of 1 were expected to occur
above 121 °C. Prior to irradiation with UV light, the surface
coated with the film of 1o was characterized by SEM. Upon
irradiation with UV light, a relatively rough surface structure
st
nd
shapes from the crystal units of 1o and 1c, DSC curves of the 1 and 2
−1
heating of 1o with heating rate of 10
K
min
,
and XRD spectra of
microcrystalline surface of 1o before and after heating processes. See DOI:
0.1039/c3ce41336e
1
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eutectic temperature of 121 °C for 5 h in the dark. The surface
turned blue and micron sized structures, formed from crystals
of 1c, were observed on the surface by SEM (Fig. 2b). Pro-
longed heating (total 24 h) of the surface at the eutectic
temperature resulted in further growth of the micro-crystals
(
5
Fig. 2c). Irradiation with visible light (λ > 480 nm) for
min at 121 °C resulted in complete decolouration and a pro-
nounced change in the shape of the crystals (Fig. 2d). How-
ever, the growth of crystals of 1c was observed even below the
eutectic temperature. Indeed, at 100 °C, albeit more slowly
than at the eutectic temperature, crystal growth was observed
(
Fig. 2e, f). At 50 °C, crystallization was not observed within
2
4 h (Fig. 2g). However, rapid crystallization like “flowering
2
0
on chocolate” was observed after 48 h (Fig. 2h).
Fig. 1 The phase diagram of mixtures of open (1o)- and closed (1c)-ring isomers
The Bravais, Friedel, Donnay and Harker (BFDH) crystal
morphology of 1o and 1c was estimated using Mercury CSD
.0, to ascertain whether or not the crystals formed upon irra-
2
1
formed by irradiation at 313 nm.
2
diation with visible and UV light were due to the formation
was observed (Fig. 2a) that is ascribed to the presence of
microcrystals of 1o, generated upon solvent evaporation. The
microcrystalline surface was irradiated with UV light (313 nm)
for 5 min at room temperature followed by heating at the
of the crystals of 1c and 1o, respectively, and to understand
2
2–24
the crystal shapes.
rectangular parallelepiped and that of 1c is a hexagonal pillar
Fig. S2, ESI†). These morphologies are clearly found in
The estimated crystal shape of 1o is a
(
Fig. 2b (cubic shape of 1o crystals) and Fig. 2c (hexagonal
shape of 1c crystals).
Since the mobility of molecules on the crystal surface is
significantly different from that of the same species in the
bulk or within the film because of their high surface energy,
we used the atomic force microscopy (AFM) force curve
2
5–28
(
FC)
to understand the state of the surface. The results
are summarized in Fig. 3. The FC for the surfaces before and
after UV irradiation at 22 °C did not present a hysteresis
(
Fig. 3a and b). These data indicate that the surface was still
hard even after irradiation with UV light.
At 30 °C, the FC showed a slight hysteresis without UV irra-
diation, indicating that the crystal surface softens at that tem-
perature. The FCs for the surfaces before and after UV
irradiation at 22 °C are indicative of a relatively hard surface.
−
1
The slopes (A–B) of the FCs were 36 and 37 nN nm , respec-
tively. On the other hand, at 30 °C, the surface prior to irradia-
−
1
tion with UV light shows a slope of A–B of only 6.2 nN nm .
These data indicate that the hard surface softens dramatically
at around 30 °C. Above 50°C, the surfaces were too soft to
measure the slopes. These data are in agreement with the low-
est temperature at which photoinduced topographical
changes were observed, with the glass transition temperature
of 1o determined to be 27.9 °C by DSC measurements (Fig. 4).
At higher temperatures (50, 100 °C), the hysteresis became
larger, as shown in Fig. 3d, e. In particular, the FC at 100 °C is
not stable and it changed every second. The surface was soft
and could not support the cantilever. In contrast, the FC
became unstable after UV irradiation even at 30 °C, as shown
in Fig. 3f. The FC resembles that of Fig. 3e. Generation of 1c
by irradiation with UV light results in softening of the surface.
Irie and his co-workers have noted that the crystal surface was
Fig. 2 Surface topographical changes of a film of diarylethene 1o. (a–g: ×3000,
h: ×500) SEM images: (a) before UV irradiation, (b) after UV irradiation for 5 min
followed by storage at 121 °C for 5 h in the dark, (c) after UV irradiation and stored at
1
21 °C for 24 h in the dark, (d) after irradiation with visible light (λ > 480 nm) for 5 min
at 121 °C, (e) after irradiation with UV light for 5 min followed by storage at 100 °C for
h in the dark, (f) after irradiation with UV light followed by storing at 100 °C for 24 h
in the dark, (g) after irradiation with UV light and storage at 50 °C for 5 h in the dark,
h) after irradiation with UV light and storage at 50 °C for 48 h in the dark.
5
1
5
considered to be hard, even when irradiated with UV light.
The glass transition temperature (T ) of 1o was determined by
(
g
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followed by evaporation in vacuo in the dark. Additionally,
mixtures with other ratios (1o : 1c = 71 : 29 and 1o : 1c = 55 : 45)
were prepared. The first heating and cooling processes were
performed in the same way as for 1o. The second heating
cycle was performed up to 60 °C. The obtained T values were
g
similar to those of 1o (Fig.S3, ESI†).
The microcrystalline surface of 1o was characterized by
XRD. Prior to heating, several diffraction peaks were observed,
which were not observed after the second heating cycle (up to
50 °C), confirming the change of the crystalline film to an
amorphous state during the first heating cycle (Fig. S4, ESI†).
These data indicate that the molecules are strongly con-
fined in the lattice in the crystalline state and reversible step
and valley formation was observed upon irradiation with UV
g
light below T . In contrast, the liquidity of the film increased
above the T and the molecules are able to move and photo-
g
generated 1c self-aggregates to form lumpy crystals on the
surface. This is a common feature in the photoinduced topo-
graphical changes that we observe.
Fig. 3 Force curves obtained by AFM on single crystalline surfaces of 1o ((a) before
and (b) after irradiation with UV light at 22 °C. (c) Before irradiation with UV light
at 30 °C. (d) Before irradiation with UV light at 50 °C. (e) Before irradiation with UV
light at 100 °C. (f) After irradiation with UV light at 30 °C.) Approach and release
processes are depicted as red and blue lines, respectively. The speed of the
Conclusions
In conclusion, reversible photo-induced topographical
changes by crystal growth of 1c and 1o occurs via softening
of the surfaces of single crystals and microcrystalline film is
−
1
approach and the release of the cantilever were both 100 nm s . The spring
−
1
constant was 42 N m
.
observed above the T
and valley formation on the crystalline surface was observed
below the T , these phenomena are switched at the T where
g
molecular confinement is drastically changed. Below the T ,
the confinement within the crystalline lattice is effective even
with the formation of 1c in the crystals upon irradiation with
UV light. Therefore, formation of step and valley microstruc-
tures was observed on single crystalline surfaces. In contrast,
g
of the open-ring isomer. While step
DSC to understand the reason why the surface switches from
g
g
hard at 22 °C to soft at 28 °C upon irradiation with UV light.
st
nd
DSC curves of the 1 and 2 heating cycle of 1o are shown
in Fig. 4.
When heated for the first time, the crystals melted at
1
33 °C and became amorphous (solidification without crys-
tallization) upon cooling. The Tg was observed at 27.86 °C
in the second cycle.
Samples of 1o and 1c were dissolved in chloroform in vari-
ous mole ratios and the mixed samples were prepared by sol-
the confinement is relaxed above the T upon irradiation with
g
UV light and photogenerated 1c self-aggregates to form the
microcrystals of 1c on the surface via the softened state.
These observations of the intricate relation between the T
g
vent evaporation to allow for the determination of T . For
example, for the measurement of a 1o : 1c = 85 : 15 mixture,
and surface mobility hold broader significance for surface
topographical changes in photo-control of self-aggregation
and surface modification and solid state photo-control in
supramolecular systems.
g
8
1
.6 mg of 1o and 1.4 mg of 1c (the sum of 1o and 1c is
0 mg) were dissolved into ca. 4 mL of dichloromethane,
Acknowledgements
This work was supported by the Sekisui Chemical Founda-
tion “Innovations Inspired by Nature” Research Support
Program, Ryukoku University Science and Technology Fund,
and A-STEP (Adaptable & Seamless Technology Transfer
Program through Target-driven R&D) FS stage: Exploratory
Research from the Japan Science and Technology Agency
(
JST). The authors are also grateful to Zeon Co., Ltd., for
providing the octafluorocyclopentene to synthesize 1o. We
express gratitude to Ms. Chie Kawai and Ms. Shiho Moriguchi
of Shimadzu Techno-Research Inc. for the measurement of
the force curves of UV-irradiated DAE crystalline surfaces at
st
nd
Fig. 4 DSC curves of the 1 and 2 heating cycle of 1o with a heating rate of
−1
10 K min .
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