Influence of helical twisting power on the photoswitching behavior of chiral azobenzene compounds: Applications to high-performance switching devices

Article abstract of DOI:10.1002/chem.200600954

Five photochromic chiral azobenzene compounds and one nonphotochromic chiral compound were synthesized and characterized by IR, ¹H NMR spectroscopy, and elemental analysis. Cholesteric liquid crystalline phases were induced by mixing of the non

Full text of DOI:10.1002/chem.200600954

FULL PAPER
DOI: 10.1002/chem.200600954
Influence of Helical Twisting Poweron the Photoswitching Behaviorof
Chiral Azobenzene Compounds: Applications to High-Performance
Switching Devices
Md. ZahangirAlam, Teppei Yoshioka, Tomonari Ogata, Takamasa Nonaka, and
Seiji Kurihara*[a]
Abstract: Five photochromic chiral
azobenzene compounds and one non-
photochromic chiral compound were
synthesized and characterized by IR,
1H NMR spectroscopy, and elemental
analysis. Cholesteric liquid crystalline
phases were induced by mixing of the
nonphotochromic chiral compound and
one of the photochromic chiral azoben-
zene compounds in a host nematic
liquid crystal (E44). The helical pitch
of the induced cholesteric phase was
determined by Canoꢀs wedge method
and the helical twisting power (HTP)
of each sample was thus determined.
The helical twisting powers of azoben-
zene compounds were decreased upon
UV irradiation, due to trans!cis pho-
toisomerization of azobenzene mole-
cules. Among the azobenzene com-
pounds synthesized in our study, Azo-
5, with isomannide (radical) as chiral
photochromic dopant, showed the
highest HTP and contrast ratio (Tmax/
Tmin). Photoswitching between compen-
sated nematic phase and cholesteric
phase was achieved through reversible
transQcis photoisomerization of the
chiral azobenzene molecules through
irradiation with UV and visible light,
respectively. Transmission rates (con-
trast ratios) increased with decreasing
helical pitch length in the induced cho-
lesteric phase. The influence of helical
twisting power on the photoswitching
behavior of chiral azobenzene com-
pounds is discussed in detail.
Keywords:
chirality
·
helical
structures
·
liquid crystals
·
photophysical properties
Introduction
the director of the individual mesogens to rotate through a
full 3608—is a measure of the chirality of the system. The
optical properties of ChLCs depend, in part, on their pitch-
es, and these can be altered either by changing the struc-
tures of the liquid crystals themselves or by admixing auxil-
iaries that change the helical alignments of the molecules of
the ChLCs.[5–7] When illuminated with white light, ChLCs re-
flect light of particular wavelengths dependent on the helical
pitch of the LC phase.[8] According to well established theo-
ries, the selective reflection wavelength (l) of a ChLC is
governed by Equation (1):
Cholesteric liquid crystals (ChLCs) have attracted the keen-
est interest of scientists because of their unique helical
supramolecular structures, which are capable of selective
light reflection in particular spectral regions.[1–3] Chiral pho-
tochemical switches are of interest because they can induce
the formation of cholesteric phases in nematic hosts, and
their helical pitches can be controlled by their photoisomeri-
zation.[4] A ChLC can be induced in a nematic liquid crystal
(NLC) system through doping with a chiral compound. Cho-
lesteric phases are characterized by the helical packing of
the mesogens with a certain sign and a certain pitch. The
helical pitch—defined as the distance in a LC needed for
l ¼ nP
ð1Þ
where P is the helical pitch length and n is the refractive
index of the liquid crystal.
Another important parameter of molecular design is the
so-called helical twisting power (HTP), defined as the ability
of a chiral group to induce cholesteric mesomorphism in a
nematic host. HTP depends on the dipole–quadrupole inter-
actions of the chiral molecule with its nematogenic neigh-
bors, the anisotropy of the nematic host phase, and on the
[a] Dr. M. Z. Alam, T. Yoshioka, Dr. T. Ogata, Prof. T. Nonaka,
Prof. S. Kurihara
Department of Applied Chemistry and Biochemistry
Faculty of Engineering, Kumamoto University
Kurokami 2-39-1, Kumamoto 860-8555 (Japan)
Fax : (+81)96-342-3679
E-mail: kurihara@gpo.kumamoto-u.ac.jp
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order parameter. HTP is related to the helical pitch by
Equation (2):[9]
decomposition, isomerization, and racemization—of the
molecules incorporated in the system,[25] ChLCs are signifi-
cant for optical applications such as Ch displays, optical data
recording, photo-optical triggers, polarizers, and reflec-
tors.[26]
1
ð2Þ
HTP ¼
P ꢀ c
In this paper we report the influence of the HTPs of
chiral azobenzene compounds on their reversible photo-
switching behavior in terms of changes in phase structure
and the influence of the orientation state.
where c is the concentration of the dopant.
When photochromic dopants such as spiropyrans, azoben-
zenes, and so on are dissolved in a Ch phase, photomodula-
tion of the macroscopic chirality can be induced by the use
of suitable light. As the molecular structure is changed by
light irradiation, different HTPs are created. Azobenzene
molecules undergo reversible transQcis photoisomerization
on irradiation with light of appropriate wavelength, and
when an azobenzene is doped into a ChLC, the HTP can be
changed by reversible photoisomerization. Thus, as outlined
in Equation (2), we can control the helical pitch through re-
versible photoisomerization.[10]
Many studies on photocontrol of selective reflection of
ChLCs have been carried out in recent years. Sackmann
showed that selective photoswitching of light reflection, re-
sulting in color changes, can be obtained with a ChLC
doped with azobenzene,[11] Witte et al. used a menthone de-
rivative,[12–14] while Shibaev reported a photochromic copoly-
mer of both azobenzene and menthone derivatives.[15–17] In
addition, a relationship between molecular structure and
HTP of a chiral compound and control of helical structure
was reported by Ichimura et al.[18–20] Azobenzene systems
represent very attractive phototriggers in Ch media, thanks
to their resistance to photofatigue, the simplicity of the mol-
ecules, and the ease of modification of their molecular struc-
tures.
Results and Discussion
Properties of compensated nematic LC mixtures: The pho-
tochromic chiral azobenzene compounds (Azo-n) and the
nonphotochromic chiral compound (Menth) used in our ex-
periments are shown in Figure 1, while Figure 2 presents the
changes observed in the absorption spectra of Azo-5 in THF
during UV irradiation. The UV spectra of Azo-5 showed a
strong absorption at around 352 nm, due to a p–p* transi-
When a ChLC is placed between two pieces of glass, it
forms a parallel texture or a focal conic texture, producing
random polydomain structures through surface orientation.
LC molecules form homeotropic textures upon application
of voltage, turning to a vertical direction adopting an
aligned orientation. Our research is aimed at the construc-
tion of various light-controllable devices, and we have re-
cently reported the first reversible full-color display device
making use of selective reflection by ChLCs.[21]
When a suitable ratio of chiral compounds possessing the
ability to produce opposite right and left helical structures
and to cancel one another is used, the result is known as a
compensated NLC, and does not produce a helical struc-
ture.[22] When one component of a chiral system constituting
a compensated NLC is a photochromic compound such as
an azobenzene, it may be possible to change the state of
compensation through a change in the HTP effected by pho-
toisomerization, so that a ChLC with a helical structure may
be developed. Previously we have also reported on reversi-
ble photoswitching between compensated NLC (transparent
state) and ChLC (light-scattering) states of chiral azoben-
zene molecules.[23]
Since the chirality of a ChLC is sensitive to physical stim-
uli such as temperature, pressure, and electric field,[24] as
well as to chemical modification—including light-induced
Figure 1. Structures of photochromic and nonphotochromic chiral com-
pounds used in this study.
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Chem. Eur. J. 2007, 13, 2641– 2647

Chiral Azobenzene Compounds
FULL PAPER
and after irradiation depend on the molecular structure of
each chiral substituent.
Compensated NLC samples were prepared on the basis of
the calculated HTP values, by mixing the chiral azobenzene
compounds and the nonphotochromic chiral compound in
E44. The ratios (E44/Menth/Azo-n) and phase transition
temperatures of those samples are shown in Table 2, the
Table 2. Phase transition temperatures of (E44/Chiral/Azo-n) mixtures
observed during cooling.
Azo-n (E44/Chiral/Azo-n) [wt.%] Phase transition temperature [8C]
Figure 2. Changes in the absorption spectrum of the chiral photochromic
compound Azo-5 in THF on UV irradiation.
Azo-1
Azo-2
Azo-3
Azo-4
Azo-5
80:1.5:18.5
80:4.4:15.6
80:6.3:13.7
80:7.2:12.8
80:9.8:10.2
N 85 I
N 82 I
N 82 I
N 79 I
N 84 I
tion in the trans-azobenzene moiety, with a weak absorption
at around 435 nm originating from the n–p* transition.
Upon UV irradiation the absorbance at 352 nm is decreased
due to trans!cis photoisomerization while the absorbance
at 435 nm is increased a little. A photostationary state is
reached after irradiation with UV light for 60 s, while on ir-
radiation with visible light reversible photoisomerization be-
havior is observed. All the photochromic azo compounds
displayed similar reversible photoisomerization behavior on
irradiation with UV and visible light.
A Ch phase was induced by mixing each chiral azoben-
zene compound and the nonphotochromic chiral compound
in the host nematic liquid crystal (E44). The helical sense
(helical rotatory direction) of the induced cholesteric liquid
crystal was affected by the chirality of each chiral substitu-
ent: all the chiral azobenzene compounds produced left-
handed helixes, while a right-handed helix was obtained
from chiral Menth. The helical pitch of each chiral azoben-
zene compound (Azo-n) and Menth compound were deter-
mined by Canoꢀs wedge method[27] and the HTP was calcu-
lated in each case by use of Equation (2). The HTPs of the
chiral azobenzene compounds (Azo-n) are summarized in
Table 1, while the HTP of the Menth compound was 128”
108 mꢁ1 molꢁ1 per g E44. The HTPs of the azobenzene com-
pounds after UV irradiation are also shown in Table 1: all
were decreased upon UV irradiation, as a result of photoiso-
merization of the azobenzene compounds from their trans
to their cis forms on UV irradiation. These results suggest
that the strengths of the HTPs and the changes in HTPs
(DHTP in Table 1) of the azobenzene compounds before
N = Nematic phase. I = Isotropic phase.
proportion of E44 as host NLC in this study being 80
weight-percent. A Schlieren texture corresponding to a
NLC was confirmed in all samples by polarized optical mi-
croscopic observations, and upon UV irradiation these com-
pensated NLC samples were converted into Ch phases with
helical structures. Fingerprint textures observed by polarized
optical microscopy confirmed the formation of ChLC state.
Figure 3 shows polarizing microscope photographs of an
induction process with the sample (E44/Chiral/Azo-2
80:9.8:10.2 wt%) changing to a Ch phase from a compensat-
ed N phase on UV irradiation. In the initial state the oppo-
site helical senses of the Azo-2 and the Menth each compen-
sated the otherꢀs HTP. Consequently, a Schlieren texture in-
dicating the N phase was observed (left-hand photograph).
Figure 3. Reversible photochemical phase transition between a compen-
sated nematic phase and
80:9.8:10.2 wt%) on UV and visible light irradiation.
a cholesteric phase (E44/Chiral/Azo-2
The compensated N state is de-
stroyed upon UV irradiation
because of the photochemical
decrease in the HTP of Azo-2,
with a right-handed Ch phase
based on Menth developing and
the fingerprint texture confirm-
ing the formation of a Ch phase
(right-hand photograph). In ad-
dition, Figure 3 also indicates
Table 1. Helical twisting powers and helical pitch lengths of cholesteric LCs induced by UV irradiation, to-
gether with contrast ratios (Tmax/Tmin) calculated from the differences in transmittance by UV and visible light
irradiation at 258C.
Spectrum Azo-n
HTP [”108 mꢁ1 molꢁ1 per g E44]
DHTP [%] Helical pitch [mm] Tmin [%] Tmax/Tmin
Before irradiation After irradiation
1
2
3
4
5
Azo-1
Azo-2
Azo-3
Azo-4
Azo-5
10.7
32.0
36.6
49.0
129
10.5
20.9
19.9
10.8
20.5
ꢁ2
30.1.089
4.82
2.23
1.23
0.69
1
ꢁ35
ꢁ46
ꢁ78
ꢁ84
45
27
21
16
2.0
3.3
4.2
5.6
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S. Kurihara, M. Z. Alam, et al.
that the helical pitch decreased with UV irradiation. Rever-
sible photoisomerization from the cis to the trans form was
observed on visible light irradiation, and the HTP was now
compensated once more, so the compensated N phase was
recovered again on visible light irradiation.
Photoswitching behavior: Each sample was injected into a
5 mm homeotropic and homogeneous glass cell by capillary
force. Figure 4 shows a schematic diagram illustrating the
Figure 4. Experimental setup for measuring photochemical switching be-
havior.
measurement of the photochemical switching behavior of
chiral azobenzene compounds, determined under irradiation
with UV and visible light. To investigate the influence of the
orientation of the cell on photoswitching behavior, two
types of cell—homeotropic and homogeneous glass cells—
were used. Figure 5a shows the results obtained with a ho-
meotropic glass cell. In the initial state (before UV irradia-
tion), all samples were in a compensated nematic phase and
showed about a 90% transmission rate, forming a homeo-
tropic structure. On UV and visible light irradiation, howev-
er, all samples except for Azo-1(Azo-1did not show photo-
switching behavior at all, and the transmission rate was con-
stant in the vicinity of 90%) showed photoswitching behav-
ior. The photoswitching rates also differed for different sam-
Figure 5. Changes in the intensity of light transmitted from a diode laser
(670 nm, 3.8 mWcmꢁ2) through mixtures of E44, chiral, and Azo-n (80/x/
20ꢁx wt%) in a 5 mm homeotropic glass cell (A) and a homogeneous
glass cell (B) on UV (3.2 mWcmꢁ2) and visible (8.3 mWcmꢁ2) light irra-
diation at 258C. 1: Azo-1. 2: Azo-2. 3: Azo-3. 4: Azo-4. 5: Azo-5.
Figure 6 shows transmission spectra of the sample (E44/
Chiral/Azo-5 80:9.8:10.2 wt%) in the transparent state and
in the light-scattering state. In the initial state the transmis-
sion rate was as high as about 80%, and from Figure 6 it
can also be seen that there was no wavelength dependence
in the region above 500 nm, but that the transmission rates
ples. Table 1shows HTP and
DHTP values for each
azobenzene compound, helical pitches (determined by
Canoꢀs wedge method) of the Ch phases induced by UV
light irradiation, and contrast ratios (Tmax/Tmin) calculated
from the maxima and the minima of the transmission rates
in Figure 5. The results shown in Table 1revealed that sam-
ples with lower helical pitches showed higher contrast ratios
or, in other words, that a shorter helical pitch produces a
focal conic texture with high density and indicates a strong
light-scattering state. Therefore, it is not only the strength of
HTP but also the quantity of change in HTP (DHTP) that is
important for photoswitching behavior, due to the difference
in phase structures. It is believed that the DHTP values had
a large influence on photoswitching speed and the transmis-
sion rate; that is, the contrast ratios of the chiral azobenzene
compounds.
Figure 6. Transmittance spectra of a mixture of E44, chiral, and Azo-5
(80/9.8/10.2 wt%) before (dotted line) and after (solid line) UV light ir-
radiation for 30 s at 258C.
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Chiral Azobenzene Compounds
FULL PAPER
at 500 nm or less were decreased both in the initial (trans-
parent) state and in the light-scattering (opaque) state be-
cause of the accumulation of absorption wavelengths of the
azobenzene chromophores in that region. From the above, it
is believed that there is potential to intercept light of a wide
wavelength range, including in the visible domain, in the
light-scattering state of the sample in this study. The results
of photoswitching in a homogeneous glass cell were similar
to those obtained with the homeotropic glass cell, as shown
in Figure 5b. In the homogeneous glass cell, compensated
NLCs formed a parallel texture and the sample consequent-
ly became transparent. Upon UV irradiation the transmis-
sion rate was decreased and the transparent state was con-
verted into to an opaque one (light-scattering state).
We investigated the effect of UV light intensity on trans-
mittance of the sample (E44/Chiral/Azo-5 80:9.8:10.2 wt%)
in the light-scattering state in homeotropic and homogene-
ous glass cells to elucidate the outbreak mechanism of the
light-scattering state. Figure 7 shows the transmission rate of
the (E44/Chiral/Azo-5) sample as a function of irradiation
time with different power intensities of UV light. The
strength of the UV irradiation was 0.4, 1.0, 1.5, and
3.2 mWcmꢁ2. In the case of the homeotropic cell the photoi-
somerization rate became slower with decreasing intensity
of UV irradiation, as shown in Figure 7a, with the transmis-
sion of the light-scattering state finally reaching a constant
value regardless of the intensity of UV irradiation. Because
the Ch phase always forms a focal conic texture in the ho-
meotropic glass cell, this means that the Ch phase can form
the same light-scattering state regardless of the progress of
photoisomerization (Figure 8). On the other hand, in the
case of the homogeneous glass cell, the transmission rate
was found to be strongly dependent on the intensity of the
irradiating UV light. It is assumed that this is due to the in-
fluence of the orientation power of the homogeneous glass
cell. The Ch phase tends to produce a planar structure in
the homogeneous glass cell, and consequently shows selec-
tive reflection properties. However, it is thought that the in-
fluence of orientation power of a cell becomes low when the
phase structure changes instantly (Figure 8). Therefore, the
transmittance obtained under illumination with high-intensi-
ty light was extremely low and was found to be dependent
on the light intensity.
Figure 8. Schematic representation of reversible photoswitching between
transparent and opaque state.
The contrast ratios of the chiral azobenzene compounds
calculated from the differences in transmittance caused by
UV and visible light irradiation with an intensity of
3.2 mWcmꢁ2 at 258C in homeotropic and homogeneous
glass cells are summarized in Table 3. These values do not
show any significant differences. From Table 1it is evident
that the helical pitch length of the Ch phase influences the
contrast ratio calculated from the difference in transmit-
tance caused by UV and visible light irradiation, the contrast
ratio increasing with decreasing helical pitch length. Both
helical pitch length and helical twisting power thus influ-
enced the photoswitching behavior of chiral azobenzene
compounds.
Figure 7. Changes in the intensity of transmitted light from a diode laser
(670 nm, 3.8 mWcmꢁ2) through an (E44/Chiral/Azo-5 80:9.8:10.2 wt%)
mixture in a 5 mm homeotropic glass cell (A) and in a homogeneous glass
cell (B) under UV light irradiation of various intensities at 258C.
a) 0.4 mWcmꢁ2. b) 1.0 mWcmꢁ2. c) 1.5 mWcmꢁ2. d) 3.2 mWcmꢁ2
.
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S. Kurihara, M. Z. Alam, et al.
Table 3. Contrast ratios of chiral azobenzene compounds calculated from
the differences in transmittance induced by UV and by visible light
(3.2 mWcmꢁ2) irradiation in a homeotropic or a homogeneous glass cell
at 258C.
Spectrum No. Azo-n Homeotropic glass cell Homogeneous glass cell
Tmin [%]
Tmax/Tmin
Tmin [%]
Tmax/Tmin
1
2
3
4
5
Azo-1
Azo-2
Azo-3
Azo-4
Azo-5
89
45
27
214.2
16
1.0
2.0
3.3
24
89
42
27
3.8
17
1.0
2.1
3.4
5.6
5.4
Application of these chiral azobenzene compounds to high-
efficiency photoswitching devices: The objective of our re-
search is to improve the performance of photoswitching, en-
abling application to various optical devices. To investigate
the photoswitching behavior of the chiral azobenzene com-
pounds, the influence of the compositions of the ChLC mix-
tures on photoisomerization rates—that is, transmission
rates—was investigated. The data given in Table 4 show that
Figure 9. Changes in diode laser (670 nm) transmittance through (E44/
Chiral/Azo-5) mixtures in
a
homogeneous glass cell on UV
(3.2 mWcmꢁ2) and visible light (8.3 mWcmꢁ2) irradiation at 258C. Com-
positions of (E44/Chiral/Azo-5) mixtures are: 80:9.8:10.2 wt% (c),
75:12.3:12.7 wt% (a), and 70:14.8:15.2 wt% (b).
Conclusion
Compensated N phases were induced by mixing of a non-
photochromic chiral compound (Menth) and photochromic
chiral azobenzene compounds (Azo-n) with a low molecular
weight host LC (E44) in the ratios that compensated one
anotherꢀs HTP. Reversible phase transformation between
the compensated N phase (transparent state) and the Ch
phase (opaque state) was achieved by means of reversible
photoisomerization under UV and visible light irradiation,
respectively. The photoswitching times and transmission
rates (contrast ratios) of the chiral azobenzene compounds
under UV and visible light irradiation conditions were found
to be related to the quantity of change in HTP on photoiso-
merization. Contrast ratios (Tmax/Tmin) increased with de-
creasing helical pitch length of the induced cholesteric
phases. In the case of the homogeneous glass cell, LC mole-
cules were easily influenced by the orientation of the cell.
The light-scattering state of a sample had the potential to in-
tercept light of all of the visible domain, and consequently a
high efficiency of photoswitching was possible, together with
a high contrast ratio. The results of this study are very signif-
icant in terms of application of the photoswitching behavior
of the chiral azobenzene compounds to various photodevi-
ces.
Table 4. Phase transition temperatures during cooling, helical pitch
lengths induced by UV irradiation, and contrast ratios (Tmax/Tmin) of
(E44/Chiral/Azo-5) mixtures.
E44/Chiral/Azo-5
[wt.%]
Phase
transition
temperature
[8C]
Helical pitch
Tmin
[%]
Tmax/
Tmin
[mm]
80:9.8:10.2
75:12.3:12.7
70:14.8:15.2
N 84 I
N 76 I
N 70 I
0.69
0.47
0.33
3.0
2.4
2.3
30.3
37.9
39.6
N = Nematic phase. I = Isotropic phase
the helical pitches of ChLCs could be changed by varying
their compositions: the helical pitches of the ChLCs de-
creased with increases in the concentrations of the chiral
dopant and the photochromic chiral azobenzene compounds.
Table 4 shows the changes in the helical pitches and contrast
ratios of (E44/Chiral/Azo-5) ChLC systems with changes in
composition. Azo-5 was chosen for these experiments be-
cause of its high DHTP value. The changes in the transmit-
ted light intensity of a diode laser (670 nm) through (E44/
Chiral/Azo-5) mixtures in a 25 mm homogeneous glass on
UV and visible light irradiation at 258C are presented in
Figure 9. The contrast ratios of the ChLC systems increased
with increasing concentration of the chiral dopant and pho-
tochromic chiral azobenzene compounds, while the trans-
mission rates of the light-scattering states of the ChLCs with
lower helical pitch decreased to 2% and the contrast ratios
increased to about 40%, the focal conic texture becoming
denser and the transmission rate decreasing as a conse-
quence. Photoswitching with high contrast ratios is thus pos-
sible with these photochromic chiral azobenzene com-
pounds.
Experimental Section
Synthesis of photochromic and nonphotochromic chiral compounds: Pho-
tochromic chiral azobenzene compounds Azo-1to Azo-4 were synthe-
sized by diazo coupling between 4-aminobenzoic acid and phenol to give
4-(4-hydroxyphenylazo)benzoic acid. Azo-5 was synthesized by diazo
coupling between 3-aminobenzoic acid and phenol to give 3-(4-hydroxy-
phenylazo)benzoic acid. After etherification with hexyl bromide, esterifi-
cation with suitable chiral alcohols was carried out in the presence of di-
cyclohexylcarbodiimide (DCC) in dichloromethane and the product was
purified by column chromatography (silica gel, CHCl3 as eluent) and re-
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Chem. Eur. J. 2007, 13, 2641– 2647

Chiral Azobenzene Compounds
FULL PAPER
crystallization from ethanol. The following alcohols were used to synthe-
size the corresponding chiral azobenzene compounds: cholesterol for
Azo-1, (S)-(+)-octanol for Azo-2, ethyl (S)-(ꢁ)-lactate for Azo-3,
(1R,2S,5R)-(ꢁ)-menthol for Azo-4, and isomannide for Azo-5.
(Sigma Koki Co.) for ultraviolet (UV) irradiation (366 nm) or SCF-42L
(Sigma Koki Co.) for visible light irradiation (436 nm).
Helical pitch measurement: The helical pitch of each induced cholesteric
phase was determined by the Cano wedge method.[27] Experiments for
determining helical pitch were performed with injected samples in a
wedge cell (EHC, Japan).
Menth: The nonphotochromic chiral compound (Menth) was synthesized
as follows. 4-Hydroxybenzoic acid was treated with 3,4-dihydro-2H-pyran
for protection of the hydroxy group and then with (1S,2R,5S)-(+)-men-
thol in the presence of DCC in dichloromethane to give 1-menthyl 4-(tet-
rahydropyran-2-yloxy)benzoate. This benzoate derivative was heated at
608C in ethanol for 4 h in the presence of pyridinium p-toluenesulfonate
to yield 2-menthyl 4-hydroxybenzoate, and this benzoate derivative was
condensed with terephthalic acid in dichloromethane in the presence of
DCC to yield the crude chiral compound, which was purified by column
chromatography (silica gel, CHCl3 as eluent) and recrystallized from eth-
anol.
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University Press, Cambridge, UK 1977.
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E44: Low molecular weight nonchiral nematic LC, purchased from
Merck Co. Ltd., used as received.
The synthesized compounds were characterized by elemental analysis
and by IR and 1H NMR spectroscopy. The characterization data of the
compounds are as follows.
Azo-1: 1H NMR (CDCl3): d = 0.8–2.2 (m, 29H; methylene), 4.1(t, 2H;
ArOCH2-), 5.0 (m, 1H; COOCH-), 7.0–8.2 ppm (m, 8H; aromatic); IR:
n˜ = 1604 (aromatic), 1710 cmꢁ1 (C=O); elemental analysis calcd (%) for
C29H40N2O3: C 75.0, H 8.62, N 6.03; found: C 74.7, H 8.58, N 6.15.
Azo-2: 1H NMR (CDCl3): d = 0.7–1.8 (m, 27H; methylene), 4.0 (t, 2H;
ArOCH2-), 5.0 (m, 1H; COOCH-), 7.0–8.2 ppm (m, 8H; aromatic); IR:
n˜ = 1600 (aromatic), 1709 cmꢁ1 (C=O); elemental analysis calcd (%) for
C27H38N2O3: C 73.9, H 8.73, N 6.39; found: C 73.8, H 8.63, N 6.63.
[11] E. Sackmann, J. Am. Chem. Soc. 1971, 93, 25.
Azo-3: 1H NMR (CDCl3): d = 0.9, 1.6 (m, 6H; methyl), 1.2–1.8 (m,
11H; methylene), 4.0 (t, 2H; ArOCH2-), 4.2 (t, 2H; COOCH2-), 5.3 (m,
[12] P. van de Witte, M. Brehmer, J. Lub, Adv. Mater. 1998, 10, 1 7.
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2000, 12, 1 6.
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hara, Adv. Mater. 2005, 17.
1H; COOCH-), 7.0–8.2 ppm (m, 8H; aromatic); IR: n˜
= 1601 (aro-
matic), 1716, 1725 cmꢁ1 (C=O); elemental analysis calcd (%) for
C24H30N2O3: C 67.6, H 7.09, N 6.57; found: C 67.5, H 7.00, N 6.68.
Azo-4: 1H NMR (CDCl3): d = 0.8–2.2 (m, 29H; methylene), 4.1(t, 2H;
ArOCH2-), 5.0 (m, 1H; COOCH-), 7.0–8.2 ppm (m, 8H; aromatic); IR:
n˜
=
1604 (aromatic), 1710 cmꢁ1 (C=O); elemental analysis calcd for
C29H40N2O3: C 75.0, H 8.62, N 6.03; found: C 74.7, H 8.58, N 6.15.
Azo-5: 1H NMR (CDCl3): d = 1.3–1.8 (m, 16H; methylene), 4.0 (t, 4H;
ArOCH2-), 5.4 (m, 2H; COOCH-), 7.0–8.6 ppm (m, 16H; aromatic); IR:
n˜ = 1598 (aromatic), 1721 cmꢁ1 (C=O); elemental analysis calcd (%) for
C44H50N4O8: C 69.3, H 6.61, N 7.34; found: C 69.3, H 6.59, N 7.34.
[22] E. Sackmann, J. Am. Chem. Soc. 1968, 90, 1 3.
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J. Goodby, G. W. Gray, W.-H. Spiess, V. Vill), Wiley-VCH, Wein-
heim, 1998, Chapter 4, p. 382.
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2501; d) A. Y. Bobrovsky, N. I. Boiko, V. P. Shibaev, Liq. Cryst. 1998,
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H. Matsuda, Adv. Mater. 2000, 12, 94; d) R. A. M. Hikmet, H. Kem-
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Angew. Chem. Int. Ed. Engl. 1984, 23, 348.
Nonphotochromic chiral compound (Menth): 1H NMR (CDCl3): d
=
0.8–2.2 (m, 36H; methylene), 5.0 (m, 2H; COOCH-), 7.3–8.4 ppm (m,
12H; aromatic); IR: n˜ = 1064 (aromatic), 1708 cmꢁ1 (C=O); elemental
analysis calcd (%) for C42H50O8: C 73.9, H 7.38; found: C 73.8, H 7.39.
Characterization of azobenzene compounds and liquid crystal mixtures:
LC mixtures were prepared by addition of one of the azobenzene com-
pounds (Azo-n) and/or a nonphotochromic chiral compound (Menth) to
the host nematic LC (E44). The thermal phase transition behavior of the
synthesized compounds and their LC mixtures was examined by polariz-
ing optical microscopic observation (POM, Olympus BHSP polarizing
optical microscope; Mettler FP80 and FP82 hot stage and controller).
Preparation of compensated nematic LC mixtures: Compensated nematic
LC mixtures were prepared by mixing the nonphotochromic chiral com-
pound and a photochromic chiral azobenzene compound in E44 in the
ratio that compensated the otherꢀs HTP. The LC mixtures were injected
into a 5 mm glass cell with homogeneous or homeotropic alignment (EHC
Co., Ltd.) by capillary force.
Photoresponsive properties: While the sample was irradiated the change
in the helical structure was explored by monitoring of transmitted light
intensity through the samples with a diode laser (Suruga Seiki Co.;
670 nm; 3 mWcmꢁ2). The photoirradiation was carried out with a 500 W
high-pressure Hg lamp (Ushio) fitted with a glass filter: UTVAF-35
Received: July 4, 2006
Revised: October 11, 2006
Published online: January 3, 2007
Chem. Eur. J. 2007, 13, 2641– 2647
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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