Mass migration on a polymer surface caused by photoinduced molecular rotation

Article abstract of DOI:10.1039/c1cc14375a

We demonstrated the formation of a photoinduced surface relief grating using thin films comprising a photochromic molecular motor, 9-(2-phenyl-2,3- dihydro-cyclopenta[a]naphthalen-1-ylidene)-9H-fluorene. Results show that mass migration occurred by patter

Full text of DOI:10.1039/c1cc14375a

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Mass migration on a polymer surface caused by photoinduced molecular
rotationw
Kunihiko Okano,*a Shohei Ogino,a Masuki Kawamotob and Takashi Yamashita*a
Received 20th July 2011, Accepted 23rd September 2011
DOI: 10.1039/c1cc14375a
We demonstrated the formation of a photoinduced surface relief
grating using thin films comprising a photochromic molecular
motor, 9-(2-phenyl-2,3-dihydro-cyclopenta[a]naphthalen-1-ylidene)-
9H-fluorene. Results show that mass migration occurred by
patterned light irradiation prepared from interfered laser beams
and a photomask.
studies, the mechanism of SRG formation is not yet fully
understood. We also consider the reported system to be a new
model that will aid in understanding the mechanism of SRG
formation.
The photoresponsive molecular motor was firstly designed
and developed in the 1999,12a in which monodirectional rotation
was around a central carbon–carbon double bond in a chiral,
helical alkene, with each 3601 rotation involving four discrete
isomerization steps activated by UV light or a change in
system temperature (Scheme 1). So far, several types of
molecular motors have been developed based on distinct
concepts. Based on the previous study, we have chosen and
synthesized 9-(2-phenyl-2,3-dihydro-cyclopenta[a]naphthalen-
1-ylidene)-9H-fluorene ( 1), and prepared poly(methyl methacrylate)
(PMMA) film added with each isomer for SRG formation.
The sample films (ca. 200 nm thickness) were fabricated by
casting on a clean glass substrate from toluene solution
containing 3 wt% 1/PMMA (weight ratio 50 : 50). Since the
glass transition temperature affects the mass migration
phenomena in SRG, we checked the thermal property of the
sample by differential scanning calorimetry (DSC) before
attempting SRG formation of the film. The second DSC
heating curve indicated a Tg value of 70 1C, which is lower
than that of PMMA due to the plasticizing effect.
Photochromism is a phenomenon in which exposure of a
compound to light induces a change in its colour by photo-
isomerization. In recently developed advanced materials,
photoisomerization changes the shapes of photochromic
molecules and this can enhance to mechanical deformation in a
broad size range from nm to cm.1 For example, the designed
photochromic liquid-crystalline elastomers show macroscopic
deformation upon exposure to light.2 In addition, nanostructures
formed from liquid-crystalline block copolymers containing
an azobenzene moiety can be controlled during the process of
photoisomerization.3 In the case of micrometre-scale mani-
pulation of photochromic materials, the surface relief grating
(SRG) that results from photoinduced mass migration has
attracted considerable attention since the discovery in 1995.4
In particular, azobenzene polymer is mostly used as a material
for SRG formation. Previous studies show that SRG is formed
in a broad category of material systems such as amorphous
polymer,5 amorphous low-molecular-weight compounds,6
crystals,7 and liquid-crystalline polymers.8 Moreover, photo-
chromic moieties other than azobenzene, such as spiropyran,9
diarylethene,10 and hexaarylbiimidazole,11 have also been
applied for the mass transfer of the film.
We reported the formation of SRG by using a photochromic
molecular motor, which shows anomalous isomerization
behavior: monodirectional rotation via photo and thermal
isomerization processes.12 The photochromic molecular motor
has four structures in the isomerization process. Therefore, the
trial performed in this study would increase the possibility of
applying the mass migration process in SRG formation.
Moreover, even after many experimental and theoretical
a Department of Pure and Applied Chemistry, Tokyo University of
Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan.
E-mail: kokano@rs.noda.tus.ac.jp, yama@rs.noda.tus.ac.jp;
Fax: +81 471241501; Tel: +81 471241501
b RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c1cc14375a
Scheme 1 Photochemical cis/trans isomerization and thermal helix
inversion steps of 1.12d
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Fig. 1 (a) Changes in the UV-vis absorption spectra of the sample film
during irradiation with UV light (220 mW cmÀ2) at room temperature.
(b) Changes in the CD spectra of the sample film containing an
(S)-isomer (initial state) of 1 by the photoisomerization process.
Photochemical cis/trans isomerization and the thermal
isomerization processes both resulting in a helix inversion
were examined in the sample films. The resolution of the
enantiomer of 1 was obtained by chiral HPLC (Chiralpak
IA column). Fig. 1a shows representative UV-vis spectra of the
film as measured for the initial states at 25 1C. Upon irradiation
of the film, cis/trans isomerisation afforded a new isomer,
which was monitored by UV-vis spectroscopy as a shift of the
absorption around 390 nm to a broad band at higher wave-
length. This bathochromic shift is attributed to enhanced twist
of the central olefinic bond.12b,d The irradiation of the film
containing the enantiopure compound 1 at room temperature
showed inversion of the major absorption bands in the CD
spectrum (Fig. 1b), indicating cis/trans photoisomerization
and simultaneous helix inversion. The photoinduced isomerized
CD spectrum was restored to the initial state by the thermal
effect, and this tendency was enhanced by an increase in the
temperature as shown in Fig. 2. Noticeably, the thermal
isomerisation process was much slower in the PMMA than
in solution at 25 1C, which means that thermal isomerisation
behaviour is restricted in the polymer matrix (see ESIw).
Fig. 3a illustrates the optical setup used for recording
holographic gratings in the sample films. Two s-polarized
beams at 364 nm with an equal intensity of 130 mW cmÀ2
from an Ar+ laser were used as writing beams. The inter-
ference beams were symmetrically crossed on the film surface
at a crossing angle of 141, leading to an interference pattern with
Fig. 3 (a) Schematic illustration of the optical setup for grating
formation. BS, M, and P represent the beam splitter, mirror, and
polarizer, respectively. The film containing the (S)-isomer (initial state)
was irradiated with interfered UV beams (364 nm, 130 mW cmÀ2). (b)
Time course profiles of the first-order diffraction of He–Ne beam to
evaluate the SRG growth. (c) Topographical AFM image and height
profile of the film irradiated for 30 min.
a fringe spacing of 1.5 mm according to L = l/(2siny), where
l and y are the wavelength and the incident angle of the writing
laser beams, respectively. The two beams were collimated to a
diameter of about 2.0 mm on the film surface. A He–Ne laser
beam at 633 nm was used as a probe beam. The grating
formation was confirmed in real time by monitoring the intensity
of the first-order diffraction beam with a photodiode.
As shown in Fig. 3b, the intensity of diffracted light
gradually increased for 10 min irradiation at room temperature,
which means that the grating formation occurred due to the
difference in the refractive indexes of the bright and dark areas
of the interference patterns. Photoisomerization in the sample
film was induced immediately after irradiation (Fig. 1). There-
fore the timescale between the grating formation and photo-
isomerization is different. This is probably because the
irradiation energy first softens the film surface, and then
induces mass migration to form SRG. The result also implies
that the gratings comprise mainly SRG rather than refractive
index modulation inside the film because modulation of the
refractive index occurred immediately for the isomerisation
process.8b Next, using atomic force microscopy (AFM), we
examined the surface morphology of the film after irradiation
for 30 min (Fig. 3c). We observed that the irradiation of
interference beams results in the formation of SRG on the
film surface. Regular surface modulation was achieved with a
spatial period of 1.5 mm, which was in accordance with the
calculated value. In addition, we observed a peak-to-trough
modulation depth of ca. 10 nm. Moreover, a similar SRG
structure was observed even in the case where (R)-isomer in
the initial state was used as a dye (see ESIw).
Fig. 2 Thermal relaxation process of the sample films containing an
(S)-isomer (initial state) of 1 from the photostationary state. These
profiles were recorded by CD spectroscopy at 291 nm.
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11892 Chem. Commun., 2011, 47, 11891–11893
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In summary, the phototriggered mass migration behavior was
examined using a PMMA film consisting of a photochromic
molecular motor. Upon irradiation with interfered beams of an
Ar+ laser at 364 nm, SRG is formed on the sample film by
photoinduced rotation. In the photo patterning procedure with a
photomask, the film also forms SRG. The knowledge obtained in
this work will greatly enhance the applicability of materials to the
exertion of phototriggered mass transfer systems based on the
specific photochromism of molecular rotation.
The authors wish to express their gratitude to Dr Atsushi
Shishido for permitting the use of the optical setup required
for formation of gratings. This work was partly supported by a
CLUSTER (second stage) of the Japan Ministry of Education,
Culture, Sports, Science.
Notes and references
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Fig. 4 Top view topographical AFM images and height profiles
along the line shown in the film after UV irradiation (365 nm;
300 mW cmÀ2) with a photomask at room temperature (a) and at 90 1C
(b). (c) Possible process of the SRG formation.
An SRG structure of azo-polymer is dependent on polarization
of writing beams,13 because an azobenzene chromophore is
aligned by polarization of light (Weigert effect).14 Therefore,
firstly, we measured polarized absorption spectra of a PMMA/
molecular motor film after irradiation of polarized UV light at
365 nm (see ESIw). This result shows no anisotropic property of
the molecular motor. Next, linearly polarized UV light was used
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temperature, whereas an efficient and large mass transfer
(B150 nm) was observed at 90 1C (Fig. 4). These results
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c
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