Control of the orientation and photoinduced phase transitions of macrocyclic azobenzene

Article abstract of DOI:10.1002/chem.201302674

Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups (1) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal-isotropic phase-crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be -0.84. Heating enhances the thermal cis-to-trans isomerization and subsequent cooling returned crystals of the trans/trans isomer. Melt, crystallize, and align by light: A macrocyclic compound bearing two azobenzene groups exhibits crystal-liquid-crystal phase transitions on irradiation with UV light (see figure). The photochemically generated crystal underwent reversible phase transitions between the crystal and the mesophase on UV and visible-light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light; the value of the order parameter was 0.84.

Full text of DOI:10.1002/chem.201302674

FULL PAPER
DOI: 10.1002/chem.201302674
Control of the Orientation and Photoinduced Phase Transitions of
Macrocyclic Azobenzene
Emi Uchida,[a] Kouji Sakaki,[b] Yumiko Nakamura,[b] Reiko Azumi,[a] Yuki Hirai,[c]
Haruhisa Akiyama,[c] Masaru Yoshida,[c] and Yasuo Norikane*[a]
Abstract: Photoinduced phase transi-
tions caused by photochromic reactions
bring about a change in the state of
matter at constant temperature.
Herein, we report the photoinduced
phase transitions of crystals of a photo-
photoinduced phase transitions (crys-
tal–isotropic phase–crystal) on UV
light irradiation. The photochemically
generated crystal exhibited reversible
phase transitions between the crystal
and the mesophase on UV and visible
light irradiation. The molecular order
of the randomly oriented crystals could
be increased by irradiating with linear-
ly polarized visible light, and the value
of the order parameter was determined
to be ꢀ0.84. Heating enhances the
thermal cis-to-trans isomerization and
subsequent cooling returned crystals of
the trans/trans isomer.
responsive
macrocyclic
compound
Keywords: crystal growth · isomeri-
zation · phase transitions · photo-
chemistry · photochromism · self-as-
sembly
bearing two azobenzene groups (1) at
room temperature on irradiation with
UV (365 nm) and visible (436 nm)
light. The trans/trans isomer undergoes
Introduction
On the other hand, photoinduced phase transitions involv-
ing the solid phase are of potential interest because they
bring about a dramatic change in the characteristics of the
solid and make it useful for applications that involve
changes in optical properties,[6] surface wettability,[7] and ad-
hesive characteristics.[8] In particular, controlling the crystal-
lization, melting, and orientation of the crystal by light is in-
triguing but challenging. However, photochromic reactions
in the crystalline phase are limited to photoresponsive mole-
cules that undergo very little conformational change such as
diarylethene systems.[9] In the case of azobenzene, reactions
only occur near the surface of the crystals.[6,10] There are
only a few reports, including our previous study,[11,12] on the
phase transition between the molecular crystal and the iso-
tropic phase in azobenzene derivatives.
Changes in states of matter are generally caused by
a change in temperature at constant pressure. In contrast,
photochemical reactions are often associated with a change
in the state of matter, even at constant temperature. This is
possible due to changes in molecular shape of the photore-
ACHTUNGTRENNUNGsponsive molecules. Successful examples can be found in
liquid-crystal (LC) systems based on azobenzene,[1–5] which
exhibit photoinduced phase transitions between the LC and
the liquid phase or between two different LC phases. These
phase transitions are triggered by the drastic topological
change in molecular shape between trans and cis isomers of
azobenzene on irradiation with ultraviolet (UV) and visible
light.
Associated with the photochemical switching property of
azobenzene, molecular alignment of azobenzene chromo-
phores can be controlled by irradiation with linearly polar-
ized (LP) light in polymeric and LC systems.[3,5,13,14] The di-
rection of the transition moment of trans-azobenzene is ap-
proximately parallel to the molecular long axis. The mole-
cules with their transition moments parallel to the polariza-
tion direction of LP light are excited and isomerize. On the
other hand, molecules with their transition moments perpen-
dicular to the direction of polarization of light are not excit-
ed. After repetition of trans-cis-trans isomerizations, molecu-
lar orientation perpendicular to the electronic vector of po-
larized light is increased. The photoinduced orientation has
been reported in azobenzene polymers in the amor-
[a] Dr. E. Uchida, Dr. R. Azumi, Dr. Y. Norikane
Electronics and Photonics Research Institute
National Institute of Advanced Industrial Science
and Technology (AIST)
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan)
E-mail: y-norikane@aist.go.jp
[b] Dr. K. Sakaki, Dr. Y. Nakamura
Energy Technology Research Institute
National Institute of Advanced Industrial Science
and Technology (AIST)
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan)
[c] Dr. Y. Hirai, Dr. H. Akiyama, Dr. M. Yoshida
Nanosystem Research Institute
National Institute of Advanced Industrial Science
and Technology (AIST)
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan)
AHCTUNGTRENNUNG
phous,[5,14a] semicrystalline[14b,c] and liquid crystalline[14d–f]
states, however, to the best of our knowledge there have
been no examples in a crystalline system.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302674.
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The isomerization of azobenzene is affected by polarity,[15]
viscosity,[15c] and local free volume of the media[16] when azo-
benzene is dissolved or dispersed in solvent or polymers.
Dense packing and a lack of free volume inhibits isomeriza-
tion in the crystalline state.[17] The isomerization is also al-
tered by its own molecular structure when azobenzene is at-
tached to bulky substituents[18] or is connected in a cyclic
structure.[19] In particular, we have been interested in the
effect of macrocyclic structures, in which multiple azoben-
zene chromophores are tethered together by covalent
bridges. These molecules show interesting properties such as
multistate photoswitching properties,[20] molecular hinge-like
motion,[21] and self-assembly in crystal,[20i,j,22] LC,[11,23] and
gel[24] states.
Results and Discussion
Molecular design: The target compound used in this study
was a macrocyclic molecule (1), composed of two azoben-
zene moieties with long alkoxy chains connected by -CH2-
bridges (Figure 1a). We anticipated that the molecular con-
formations of this molecule would be restricted by ring
strain, and that the cavity of the cyclic structure would pro-
vide a free volume for isomerization. As expected from mo-
lecular simulations and the crystal structure of the macrocy-
clic backbone,[20j] isomerization of the azobenzene units re-
sulted in drastic conformational change of the three possible
isomers (trans/trans, trans/cis, and cis/cis, Figure 1b). This af-
fects the intermolecular interactions, including the packing
of the alkyl chains, and results in huge differences in the
melting temperatures. The trans/trans isomer is rectangular
in shape with the pair of alkoxy chains roughly parallel to
each other, whereas the macrocyclic backbone shows
a bowl-like conformation, which may weaken the intermo-
lecular p–p interactions. In the trans/cis isomer, the trans
azobenzene moiety is flat whereas the cis azobenzene
moiety is bent. As a result, the molecular symmetry of the
whole molecule decreases and self-assembly is expected to
be unfavorable in this case. In contrast, the cis/cis isomer, al-
though not flat, has a higher symmetry, with a D2 point
group, and has the alkoxy chains pointing in different direc-
tions. The molecular structures shown in Figure 1b are simu-
lated at a single molecule level, so they do not reflect inter-
molecular interactions or crystal packing. However, they
indeed show qualitatively that isomerization of azobenzene
unit(s) brings about change in molecular conformation.
In this paper, we report an azobenzene-based macrocyclic
compound (1) that exhibits phase transitions involving two
crystal phases at room temperature, as shown in Figure 1.
Thermal properties: The phase transition temperatures of
trans/trans-1 were investigated by differential scanning calo-
rimetry (DSC; Figure 2) and polarizing optical microscopy
(POM). The trans/trans isomer of 1 showed a crystalline
phase up to 1108C, an LC phase (smectic C) above 1108C,
and a clearing point at 1458C. Small transition signals, which
were not observed in the previous study,[15] appeared around
Figure 1. Macrocyclic azobenzene showing photoinduced phase transi-
tions. a) Chemical structure of the trans/trans isomer of macrocyclic azo-
benzene 1. b) Isomerization of the azobenzene moiety drastically changes
the molecular shape, as predicted by crystal structure of the backbone
and DFT calculations. c) Schematic illustration of the phase transitions.
Rational design of molecular shape allows photoisomeriza-
tion in the crystalline phase and self-assembly of each
isomer. Furthermore, we found that the molecular orienta-
tion of crystals of 1 can be controlled by irradiation with LP
light. Our results demonstrate that light not only induces
“melting” and “crystallization” but can also control the mo-
lecular orientation within the crystal.
Figure 2. Differential scanning calorimetry (DSC) thermogram of trans/
trans-1 on the first cooling (blue line) and second heating (red line) cycle
at rates of 2 Kminꢀ1
.
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Phase Transitions of Macrocyclic Azobenzene
FULL PAPER
1458C corresponding to phase transitions involving another
LC phase. This is likely a nematic phase due to its small
phase transition enthalpies. According to the above experi-
mental results of DSC, to “thermally melt” crystals of trans/
trans-1, the samples must be heated to temperatures above
1108C.
Photoinduced phase transition on UV light irradiation: Fig-
ure 3a–d shows POM photographs of crystals of the trans/
trans isomer of 1 on UV irradiation at 365 nm at room tem-
perature (see also Video 1 in the Supporting Information).
After irradiating for 0.5 s, the birefringence of the crystal
was lost and the angular outline of the crystal became
rounder. Maximum loss of the birefringence was observed
after 2.0 s irradiation (Figure 3c). However, on further irra-
diation, the birefringence of the sample again increased
(Figure 3d), indicating molecular reordering of the sample.
More evidence for the phase transition into another or-
dered phase on UV irradiation was obtained from X-ray dif-
fraction (XRD) experiments. A crystalline film of the trans/
trans isomer of 1 exhibited many diffraction peaks, with the
strongest peak being observed at 2q=3.48 (Figure 4a).
These diffraction peaks disappeared rapidly on UV irradia-
tion at 365 nm with the simultaneous appearance of a new
peak at 2q=4.58 and other small reflections at wider angles
(Figure 4b).[25]
Figure 3. Polarizing optical photomicrographs of trans/trans-1 crystals
a) before irradiation, and after irradiating for b) 0.5, c) 2.0, and d) 50 s
with 365 nm light at room temperature. The angle between the analyzer
and the polarizer of the POM was ca. 808 to enable clearer observation
of the isotropic liquid.
porting Information for experimental details). For example,
a film of trans/trans-1, when exposed to 365 nm light for
300 s (at the photostationary state (pss) as shown in Fig-
ure 5a), consisted of 13, 10, and 77% of the trans/trans,
trans/cis, and cis/cis isomer of 1, respectively (Table 1). The
isomer ratio obtained in this experiment shows that the exci-
UV/Vis spectroscopy clearly
shows that photoisomerization
of the azobenzene moiety does
occur in crystals of the trans/
trans isomer of 1. Figure 5a
shows the change in the spectra
on irradiating with light of
wavelength 365 nm. The initial
state of the trans/trans isomer
exhibits absorption bands that
are characteristic of the trans
azobenzene chromophore, al-
though the p–p* absorption
band is redshifted relative to
the absorption band observed
in solution (Figure S5) due to
packing interactions of the
chromophores in the crystalline
phase. On irradiation with UV
light (365 nm), a decrease in
the intensity of the absorption
of the trans azobenzene moiety
was observed along with an in-
crease in the absorption in the
visible region. This behavior
can be attributed to the isomer-
ization of azobenzene. Photo-
ACHTUNGTRENNUNGisomerization was also con-
firmed by measuring the UV/
Vis spectra of photoirradiated
Figure 4. Photoinduced phase transitions of 1 observed by XRD and POM a,e) before irradiation, b,f) after ir-
radiation with 365 nm light, c,g) after irradiation with 436 nm light, and d,h) after annealing the photoirradiat-
ed film at 1508C and subsequent cooling to RT. Photographs f) and g) are long-exposure shots taken for clear-
films in solution (see the Sup- er observation of the irradiated area.
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17393

Y. Norikane et al.
ure S8. Thus, molecular design is important for achieving
photoinduced phase transition between the crystal and
liquid phases. A detailed consideration of the relationship
between molecular structure (including cyclic or acyclic),
molecular packing/interaction, melting point, and photore-
AHCTUNGTREGsNNUN ponsive property is required, and we are investigating this
issue by synthesizing analogues and performing single-crys-
tal X-ray studies.
Photoinduced phase transition on visible light irradiation:
To achieve reverse cis-to-trans isomerization, visible light
(436 nm) was applied to a sample that had already been ir-
radiated with light of 365 nm until the pss was reached. The
UV/Vis spectra in Figure 5b show the recovery of the trans
chromophore. However, we did not observe complete recov-
ery of the trans/trans isomer, but, rather, the trans/cis isomer
was established as the major product instead. The ratio of
the isomers at the pss was determined to be 42, 51, and 7%
of trans/trans, trans/cis, and cis/cis isomer, respectively
(Table 1). Interestingly, more trans/trans isomer was recov-
ered in the film state than in solution (Table 1). POM
images (Figure 4g and Video 2 in the Supporting Informa-
tion) show some morphological texture, and XRD peaks at-
tributed to the crystal of the cis/cis isomer disappeared and
no significant diffraction was observed (Figure 4c). These
results indicate that crystals of the cis/cis isomer exhibit
a phase transition to form a mesophase (probably a nematic
phase) with low molecular ordering on irradiation with light
of 436 nm. On irradiating with UV light (365 nm), this
Figure 5. a) Change in the absorption spectra of the crystalline film of
1 on irradiation with 365 nm light. b) Change in the absorption spectra
on irradiating with 436 nm light, the sample that had previously been ir-
radiated with 365 nm light until the photostationary state was reached.
mesoACTHUNRTGNEUNGphase underwent a phase transition to form crystals of
the cis/cis isomer (see Video 2 in the Supporting Informa-
tion). The phase transitions can be repeated by simply
switching between the two wavelengths (365/436 nm). On
the other hand, crystals of the trans/trans isomer were ob-
tained by heating the mesophase sample to 1508C and sub-
sequent cooling to room temperature (Figure 4d and h).
Table 1. Ratio of isomers of 1 at the photostationary state.
Wavelength [nm]
Ratio [%]
trans/trans
trans/cis
cis/cis
365[a]
365[b]
436[a]
436[b]
13
1
42
15
10
9
51
61
77
90
7
Thermal cis–trans isomerization: Unlike the trans isomer of
azobenzene, which is thermally stable, the metastable cis-
isomer of azobenzene reverts back to the corresponding
trans-isomer in the dark.[26] Thus, stabilization of the cis-
isomer remains a challenge when dealing with azobenzene-
containing photoresponsive materials. In the case of com-
pound 1, thermal isomerization proceeds in a stepwise
manner (cis/cis ! trans/cis ! trans/trans) and two isomers
(trans/cis-1 and cis/cis-1) have similar thermodynamic pa-
rameters (Table 2) and the half-lives (t1/2) of both cis/cis and
24
[a] In film state. Determined by UV/Vis spectroscopy; see the Supporting
Information for details. [b] In solution (CDCl3). Determined by H NMR
spectroscopic analysis.
1
tation light penetrates well through the film sample. The cis/
cis isomer was obtained as a major product, and a similar
trend was observed in solution (Table 1). Thus, the photoin-
duced phase transition observed in POM and XRD experi-
ments is attributed to photoisomerization of azobenzene.
Control experiments were carried out by using a noncyclic
azobenzene derivative having alkoxy substituents (trans-4,4’-
di(dodecyloxy)-3,3’-dimethyl-azobenzene). In this case, al-
though this compound showed photoisomerization in solu-
tion and has a lower melting temperature (838C) compared
with the trans/trans isomer, we observed neither a significant
change in the absorption spectra nor a change in phase
when crystals were exposed to UV light, as shown in Fig-
Table 2. Thermodynamic parameters of thermal isomerization of 1.
DH°
[kJmolꢀ1
DS°
DEa
[kJmolꢀ1
]
A
N
]
[JKꢀ1 molꢀ1
]
U
[a]
[a]
k1
k2
85.4
46.8
90.2
ꢀ37.3
ꢀ197
88.0
49.4
92.7
89.7
2.00ꢁ1011
865
cis/cis ! trans/cis[b]
ꢀ24.8
8.75ꢁ1011
2.65ꢁ1011
trans/cis ! trans/trans[b] 87.2
ꢀ34.7
[a] In film state. [b] In solution (CDCl3).
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Phase Transitions of Macrocyclic Azobenzene
FULL PAPER
trans/cis are 7.2 h at 208C in solution. Notably, stabilization
of the cis isomer(s) was observed in the crystalline film of 1.
Change in the UV/Vis absorption (dark reaction) of
a sample that had previously been irradiated with 365 nm
light until the pss was reached, exhibited a dual exponential
profile with t1/2 of 9.2 h (k1, minor) and 178 h (k2, major;
Figure 6) at room temperature. The sample at the pss con-
tained the cis/cis isomer as the major component with minor
amounts of the trans/cis isomer. Thus, the slower isomerizing
component can be assigned to isomerization of the crystal
mainly containing cis/cis-1, since this process is extremely
slow compared with the kinetics in solution, indicating the
influence of steric hindrance in the crystal packing.[27] As
can be seen in the Eyring plot, the slow component showed
an abrupt increase in the reaction rate above 808C (k2’, Fig-
ure 6b). This observation indicates that the cis/cis isomer is
sterically restrained below 808C, whereas the molecule can
overcome this barrier at higher temperatures. A similar
effect has been reported in crystals of pure azobenzene be-
tween its eutectic temperature and its melting tempera-
ture.[28]
With the thermal properties detailed above in mind,
rough estimates of the melting temperature of the cis/cis-
1 crystals were obtained on the basis of POM and DSC (Fig-
ure S2). To minimize thermal isomerization, the film at the
pss at 365 nm was heated rapidly (rate of 30 Kminꢀ1). Melt-
ing of the crystals was observed around 1008C by POM. An
endothermic and a subsequent exothermic DSC signal were
observed in this temperature region, indicating melting of
the crystal and subsequent cis-to-trans thermal isomeriza-
tion. Thus, the melting point of the cis/cis-1 crystal is at least
1008C.
Control of molecular orientation: Surprisingly, the uniaxial
molecular orientation of trans/trans-1 crystals was induced
by irradiating with LP light. Figure 7a shows the polariza-
tion absorption spectra of trans/trans-1 and the photoin-
duced in-plane order parameter as a function of irradiation
time when a film sample of trans/trans-1 was sandwiched be-
Figure 7. Controlling molecular alignments in the crystalline phase.
a) UV/Vis polarization absorption spectra of a film of trans/trans-1, sand-
wiched between glass plates, before irradiation (black lines) and after ir-
radiation with LP 436 nm light at 10 mWcmꢀ2 (colored lines). Solid and
dotted lines represent Ak and A?, respectively. Inset: plot of photoin-
duced in-plane order parameter against monitoring wavelength; red and
blue lines correspond to the sample irradiated by LP light for 1000 s and
the sample annealed for 10 min after the LP irradiation, respectively.
b) Polarizing optical photomicrograph of a film being subjected to the
following protocol: the entire area of the sample was irradiated with LP
light and annealed at 808C. The sample was then covered with a mask
and then irradiated again with LP light, the polarization plane of which
was tilted at 458 with respect to the original. The sample was then an-
nealed again. Since the angle between the two photoinduced molecular
orientations are 458, the two regions are observed as bright and dark
fields by POM. Arrows at the bottom of the image represent the direc-
tions of the polarizer and the analyzer. Arrows in the photograph show
the directions of LP light.
Figure 6. a) Change in the absorption spectra of the film of 1 in the dark
at 508C, the sample that had previously been irradiated with 365 nm light
until the photostationary state was reached. Inset: the biexponential
growth of 1 (measured by absorbance at 380 nm) consists of fast (k1) and
slow (k2) first-order rate constants. b) Eyring plots of two components of
the growth observed during the dark reaction. Red and blue lines corre-
spond to k1 and k2, respectively. Although k1 lies on a straight line, the
slope of k2 changes and forms a second straight line (k2’, blue dashed
line) above 808C.
Chem. Eur. J. 2013, 19, 17391 – 17397
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17395

Y. Norikane et al.
tween glass plates and exposed to LP light of 436 nm
(10 mW/cm2). In the initial state, the orientation of the crys-
tals is random and, thus, the order parameter (S) is zero.
Conclusion
The photoinduced phase transitions involving two crystal
phases have been demonstrated in a macrocyclic azoben-
zene dimer system in which light irradiation results in
switching between three isomers with different molecular
conformations. In addition, the molecular orientation of the
crystal can be dramatically enhanced by irradiating with LP
light. This study offers a new method to control crystalliza-
tion by means of photoinduced phase transitions.
Our results show that, with rational design of molecules,
the crystalline phase of materials can be controlled by light
stimuli without changing the temperature. Studies on the in-
triguing relationship between molecular structure, crystal
structure, and phase transition behavior are underway.
The order parameter (S) is given by (AkꢀA? )/ACHTNUGTERN(NUNG A?+2Ak), in
which Ak and A? are the absorbances parallel and perpen-
dicular to the LP light, respectively. On irradiation with LP
light, the in-plane molecular orientation perpendicular to
the LP light was induced immediately and the largest value
of order parameter (ꢀ0.74) was obtained at 450 nm (Fig-
ure 7a, inset). Negative value of the order parameter means
that the direction of molecular orientation is perpendicular
to that of electronic vector of LP light. Although this kind
of photoinduced orientation has been reported in azoben-
zene polymers in the amorphous,[5,14a] semicrystalline,[14b,c]
and liquid crystalline[14d–f] states, the order parameter ob-
tained in this study is higher than those previously reported.
Furthermore, there have been no examples of photoinduced
orientation in a crystalline system. The photoinduced molec-
ular orientation is caused by absorption of light by both
trans and cis azobenzene moieties, so that the trans-cis-trans
isomerizations occur repeatedly during the irradiation and
bring about reorientation to minimize light absorption.[13] As
a result, molecular orientation perpendicular to the elec-
tronic vector of polarized light is increased. Very high order
of molecular orientation obtained in the case of trans/trans-
1 might correlate with the phase transitions involving melt-
ing and crystallization. The XRD profiles of the film before
and after irradiation with LP light showed a similar pattern,
suggesting that the crystals remained in the sample (Fig-
ure S3). On application of LP light, crystals oriented parallel
to the electronic vector of polarized light melt (or partially
melt) faster than those oriented perpendicularly. Perpendic-
ularly oriented crystals may play a key role by acting as
seeds. In addition, a further increase in the order parameter
(S=ꢀ0.84) was observed on annealing the LP (436 nm)
light irradiated sample at 808C for 10 min (Figure 7a, inset).
Annealing enhances the growth of uniaxially aligned crystals
of the trans/trans isomer from the cis/cis and the trans/cis
isomers.
Experimental Section
Materials: Compound 1 was synthesized according to the reported proce-
dure.[15] Synthesis of trans-4,4’-di(dodecyloxy)-3,3’-dimethyl-azobenzene is
described in the Supporting Information.
Sample preparation: Film samples for polarizing optical microscopy
(POM) observations and UV/Vis absorption spectroscopy were prepared
by sandwiching compound 1 with two cover glass plates (Matsunami
micro cover glass; thickness: 0.12–0.17 mm). Crystalline film samples of
trans/trans-1 were prepared by slow cooling from an isotropic melt
(1508C) to RT. The thickness of the film sample (0.7–0.8 mm) was esti-
mated by carefully peeling the glass plates and analyzing the surface of
the glass with a microfigure measuring instrument (Kosaka Laboratory
Ltd. SURFCORDER ET200). The temperature of film samples was con-
trolled with a Linkam LK-600PM stage.
X-ray diffraction: XRD were measured with a Rigaku RU-300 (CuKa
,
40 kV, 200 mA), or a Rigaku RINT-TTR (CuKa, 50 kV and 200 mA) dif-
fractometer. The diffractions were measured in the 2q–q scan mode with
0.01 or 0.028 steps.
Polarizing optical photomicrographs and irradiation experiments: Polar-
izing optical photomicrographs were recorded with an OLYMPUS BX51
microscope equipped with a high-pressure Hg lamp, optical filters, and
heat-absorbing filters so that observation on photoirradiation in situ was
possible. Other photoirradiation experiments were performed with
a high-pressure Hg lamp (Asahi Spectra Inc. REX-250 for 365, 405, and
436 nm), LEDꢂs (Asahi Spectra Inc. POT-365 LED for 365 nm, Brainvi-
sion Inc. LEX2 for 465 nm, and home built LED for 525 nm). Light in-
tensity was monitored with a Newport 1917-R optical power meter with
an 818-ST-UV photodetector.
Alternating orientations can be induced in a single sample
by using an additional irradiation. For these measurements,
the entire area of the sample was irradiated with LP light
and annealed. The sample was then covered with a mask
and then irradiated with LP light the polarization plane of
which was tilted by 458 with respect to the original. The
sample was then annealed again. Figure 7 b shows the polar-
izing optical micrograph obtained with the above mentioned
protocol. Since the two regions show up as bright and dark
fields in the micrograph, the angle between the two photoin-
duced molecular orientations is 458. As expected, informa-
tion on the molecular orientation was readily erased by irra-
diating with randomly oriented 365 nm light, or by heating
the sample to its melting point.
Acknowledgements
This work was supported in part by JSPS/MEXT KAKENHI (21750157
and 23760680 to Y. N.), the Association for the Progress of New Chemis-
try, the Iketani Science and Technology Foundation, and the Canon
Foundation. We thank Dr. Hideyuki Kihara and Dr. Takahiro Yamamoto
for helpful discussions relating to this project. We also acknowledge
Dr. Masayuki Chikamatsu, Dr. Masaru Aoyagi, and Dr. Hiroshi Takashi-
ma for performing part of the XRD, NMR, and film thickness measure-
ments, respectively.
[1] H. Yu, T. Ikeda, Adv. Mater. 2011, 23, 2149–2180.
[2] T. Ikeda, J. Mamiya, Y. Yu, Angew. Chem. 2007, 119, 512–535;
Angew. Chem. Int. Ed. 2007, 46, 506–528.
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Chem. Eur. J. 2013, 19, 17391 – 17397

Phase Transitions of Macrocyclic Azobenzene
FULL PAPER
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Received: July 10, 2013
Revised: August 12, 2013
Published online: November 8, 2013
Chem. Eur. J. 2013, 19, 17391 – 17397
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
17397