Chirooptical and photooptical properties of a novel side-chain azobenzene-containing LC polymer

Article abstract of DOI:10.1007/s00706-009-0108-8

A novel chiral-photochromic side-chain polyacrylate with azobenzene fragments in the side groups has been synthesised. It was shown that the polymer forms a smectic phase and a cholesteric supramolecular helical structure with selective light reflection in IR spectral range. Thin spin-coated films of the polymer were prepared and their photooptical and chirooptical properties were studied in detail. It was found that UV irradiation of the films led to E-Z isomerization of the azobenzene moieties with high conversion, which is dependent on thermal prehistory of the films. Subsequent action of visible light results in partial recovery of the E-isomer content, whereas annealing leads to the full back conversion. Circular dichroism (CD) measurements revealed formation of the helical supramolecular structure even in the initial spin-coated polymer films. The E-Z isomerization induces complete disruption of helical order in non-annealed films of the polymer, whereas in the smectic phase of the annealed film only a significant decrease in CD values was found. In addition, the photoorientation phenomena induced by polarized light were studied. It was shown that polarized light induces linear dichroism in the films provided by azobenzene group orientation and the dichroism is stable at room temperature for a prolonged time. These combined chirooptical and photooptical features of this novel polymer enable one to consider this multifunctional compound as a promising material for photonics and for optical applications.

Full text of DOI:10.1007/s00706-009-0108-8

Monatsh Chem (2009) 140:789–799
DOI 10.1007/s00706-009-0108-8
ORIGINAL PAPER
Chirooptical and photooptical properties of a novel side-chain
azobenzene-containing LC polymer
Alexey Bobrovsky Æ Valery Shibaev Æ
Vera Hamplova Æ Miroslav Kaspar Æ
Milada Glogarova
Received: 16 November 2008 / Accepted: 24 November 2008 / Published online: 20 February 2009
Ó Springer-Verlag 2009
Abstract A novel chiral–photochromic side-chain poly-
acrylate with azobenzene fragments in the side groups has
been synthesised. It was shown that the polymer forms a
smectic phase and a cholesteric supramolecular helical
structure with selective light reflection in IR spectral range.
Thin spin-coated films of the polymer were prepared and
their photooptical and chirooptical properties were studied
in detail. It was found that UV irradiation of the films led to
E–Z isomerization of the azobenzene moieties with high
conversion, which is dependent on thermal prehistory of
the films. Subsequent action of visible light results in
partial recovery of the E-isomer content, whereas annealing
leads to the full back conversion. Circular dichroism (CD)
measurements revealed formation of the helical supramo-
lecular structure even in the initial spin-coated polymer
films. The E–Z isomerization induces complete disruption
of helical order in non-annealed films of the polymer,
whereas in the smectic phase of the annealed film only a
significant decrease in CD values was found. In addition,
the photoorientation phenomena induced by polarized light
were studied. It was shown that polarized light induces
linear dichroism in the films provided by azobenzene group
orientation and the dichroism is stable at room temperature
for a prolonged time. These combined chirooptical and
photooptical features of this novel polymer enable one to
consider this multifunctional compound as a promising
material for photonics and for optical applications.
Keywords Liquid crystals ꢀ Photochemistry ꢀ
Spectroscopy ꢀ Chirality
Introduction
Azobenzene-containing polymers are an interesting and
promising class of macromolecular compounds which can
be used for a variety of applications—in optics, optoelec-
tronics, photonics, holography, data storage, etc. [1–3]. The
reasons for such interest in azobenzene derivatives are
quite clear. On irradiation with light azobenzene moieties
of such polymers undergo reversible E–Z isomerization
with high quantum yield and the polymers have rather high
fatigue resistance [1, 2].
N
N
λ1
N
R'
R
N
λ2, Τ
R
R'
E-isomer
Z-isomer
It should be emphasized that isomerization of the azo-
benzene derivatives is accompanied by a large change in
the anisometry and dipole moment. Both effects affect the
phase behaviour and optical properties of low-molar-mass
and polymer systems [3].
A. Bobrovsky (&) ꢀ V. Shibaev
Nowadays the attention of many research groups
working with azobenzene-containing polymers is focused
on several topics including study of surface relief grating
formation [4–8], holography [9–11], and chirality induction
by means of the action of circularly or elliptically polarized
light [12–14]. The most interesting outcome of the above
Faculty of Chemistry, Moscow State University, Leninskie gory,
119992 Moscow, Russia
e-mail: bbrvsky@yahoo.com
V. Hamplova ꢀ M. Kaspar ꢀ M. Glogarova
Institute of Physics, Academy of Sciences of the Czech
Republic, 182 21 Prague 8, Czech Republic
123

790
A. Bobrovsky et al.
works demonstrated the unique possibilities of photoregu-
lation of supramolecular structure and optical properties,
self-organization, modification of surface tension, etc.
[15–18].
temperature of the polymer is about 38 °C. Enthalpy of
isotropization is 1.3 J/g, whereas transition SmX*–N* is
characterized by a large enthalpy value (9.1 J/g), which
indicates formation of an ordered smectic phase. It is
noteworthy that the corresponding monomer A10AZOF*
exhibits only the cholesteric mesophase with phase tran-
sition temperatures Cr 87 N* 152 I.
In our recent papers we have studied chirooptical and
photooptical properties of thin films of liquid crystalline
(LC) cholesteric azobenzene-containing copolymers and
mixtures [19–24]. Peculiarities of the helical structure
formation were found and the effect of light on the cho-
lesteric polymer films with thickness comparable with or
even less than the helix pitch dimension was investigated.
In most cases, the polymer systems studied consisted of
two different types of side fragment, photochromic and
chiral, or employed a mixture of the nematic azobenzene-
containing copolymer with the chiral dopant.
Planarly oriented films of the polymer have selective
light reflection in near infrared spectral region (Fig. 1).
Under cooling, significant helix unwinding takes place,
which is related to the formation of smectic order elements
[25–27].
Optical properties of amorphous spin-coated films
of polymer
Continuing the above-mentioned works we have synthe-
sized a novel complex liquid crystalline chiral azobenzene-
containing polymer PA10AZOF*1 (Scheme 1) containing a
photochromic azobenzene moiety, which is chemically linked
with a benzoyl group, forming a three-benzene-ring rigid
mesogen.
Using the spin-coating technique thin amorphous films of
the polymer were prepared from chloroform solution. Such
films are characterized by three peaks of absorbance
(Fig. 2). A weak broad peak in the visible region (at
*450 nm) corresponds to the n–p* electronic transition of
the azobenzene chromophore, and a strong peak with
maximum at 321 nm is attributed to the p–p* transition of
the same moiety. The absorbance with a maximum at
250 nm is due to the p–p* and the n–p* transitions of the
phenylbenzoate fragments and the aromatic U–U* transi-
tions of azobenzenes. The position of the n–p* electronic
transition of the azobenzene fragment is shifted to shorter
wavelength in comparison with solution (*360 nm),
because of the strong tendency to H-aggregates [28]
(azobenzene chromophores with antiparallel packing).
Formation of the aggregates even proceeds for film for-
mation during the spin-coating procedure; subsequent
storage of the films at room temperature does not lead to
any spectral changes.
This rigid-rod fragment has a chiral isoamyl terminal
group at one end responsible for cholesteric phase forma-
tion. These combined chiral–photochromic fragments are
chemically linked to the polymer backbone through a long
aliphatic spacer consisting of ten methylene groups pro-
viding autonomy to side groups for LC phase formation.
The paper focuses on the study of the phase behaviour and
the photooptical and chirooptical properties of thin films of
this novel liquid crystalline homopolymer containing chiral
and photochromic azobenzene groups in the same mono-
meric unit. The study of helical structure formation in thin
spin-coated films of this polymer and the possibility of
structure manipulation by the action of light represent a
special interest. Elucidation of the mutual influence and
interplay between chirality, photochromism, and self-
organization is the main objective of our investigations.
During UV irradiation (365 nm) the absorbance corre-
sponding to the p–p* transition of the azobenzene groups
strongly decreases, whereas intensity of the n–p* electronic
transition slightly increases as is shown by arrows in
Figs. 2a, 3. Subsequent visible light irradiation leads
to recovery of the shape of the absorbance spectra, but
only partially (Figs. 2b, 3). Such spectral changes are
attributed to E–Z and partial back Z–E photoisomerization
processes.
Results and discussion
Phase behaviour and optical properties of planarly
oriented films of the polymer
According to polarizing optical microscopy investigations
the polymer is characterized by the phase transitions SmX*
125–128 N* 187–190 I. The cholesteric (chiral nematic)
mesophase of the polymer forms typical planar texture with
oily streaks, whereas the smectic phase forms only non-
typical texture. According to DSC data, the glass transition
It is noteworthy that the maximum of the p–p* transition
recovered by visible light irradiation is shifted to a longer
wavelength (338 nm) compared with the initial film. This
effect is associated with disruption of the H-aggregates
because of the E–Z and Z–E isomerization cycles. For-
mation of these aggregates and complete recovery of the
spectra takes a prolonged time (several days) at room
temperature but can be accelerated by heating the films at
temperatures above 60 °C.
1
In the subsequent text this chiral azobenzene-containing polymer
PA10AZOF* will be simply represented by ‘‘polymer’’.
123

Chirooptical and photooptical properties of a novel side-chain azobenzene-containing LC polymer
791
Scheme 1
1. Br(CH2)10Br
2. H2SO4
NaNO2
phenol
HO
NHCOCH3
Br(CH2)10
O
NH2
1
K I
Br(CH2)10
O
N N
OH
I(CH2)10
O
N N
OH
acetone
2
3
CH2=CH-COOK
N N
OH
CH2=CHCOO(CH2)10
O
4
1. methylbutyl bromide
2. NaBrO
*
CH3CH2CHCH2O
CH3
COOH
HO
COCH3
5
DCC
4 + 5
CH2=CHCOO(CH2)10
O
N=N
OCO
OCH2CH*CH2CH3
CH3
DMAP
6
Polymerization
Photochromic group
O
O
OCH2CH*CH2CH3
CH3
CH2 CH
N
N
n
COO (CH2)10
O
Chiral group
Spacer
Mesogenic group
Summarizing results of photochemistry investigations
one can conclude that the photoisomerization behaviour of
the polymer films is similar to that of azobenzene deriva-
tives with similar substituents in solution [2] but
aggregation phenomena lead to more complicated spectral
changes and kinetics of the processes.
azobenzene, the action of visible light cannot lead to
complete conversion (to 100%) of the E-isomer [2] and
thus to regeneration of the helicity. The presence, even at
small concentrations, of the Z-isomer (*10%) in amor-
phous films prevents helical organization.
Spin-coated amorphous films of the polymer exhibit
significant induced circular dichroism in the spectral range
corresponding to absorbance of the azobenzene fragments
(Fig. 4). As was demonstrated and discussed above,
appearance of induced circular dichroism is associated with
the formation of elements of the helical structure which
leads to asymmetrical exciton coupling of azobenzene
chromophores [29, 30] (Fig. 5). Interestingly, the UV
irradiation leads to almost complete disappearance of the
CD signal (Fig. 4) that is associated with photoinduced
transition to the isotropic state as shown in Fig. 5. More-
over, subsequent action of visible light does not result in
any back increase in CD values. Similarly as for
Optical properties of spin-coated polymer films
in the smectic phase
Let us consider the photooptical properties of the polymer
films in the liquid crystalline state which is produced by
annealing of the films at temperatures above the glass
transition.
Annealing of spin-coated films at temperatures higher
than 60 °C results in significant spectral changes in
absorbance and in circular dichroism that can be explained
by partial homeotropic orientation of the azobenzene
groups (for a model see Fig. 5), their smectic ordering, and
further aggregation (Figs. 6, 7).
123

792
A. Bobrovsky et al.
0.6
0.5
0.4
0.3
0.2
0.1
0.0
a
1700
1600
1500
1400
1300
1200
1100
Φ-Φ*
0
π-π*
1
2
4
6
8
10 min
n-π*
200
300
400
500
600
λ / nm
0.6
0.5
0.4
0.3
0.2
0.1
0.0
b
120
140
160
T / °C
180
Fig. 1 Temperature dependence of selective light reflection wave-
length detected in planarly oriented polymer films
10 min
6
2
1
0
After 30 min of annealing at temperatures higher than
60 °C absorbance of the p–p* electronic transition of
azobenzene groups strongly decreases remaining stable
after prolonged annealing time (Fig. 6).
200
300
400
500
600
For elucidation of the tendency to form the homeotropic
orientation (azobenzene groups oriented along the film
plane normal) we measured absorbance of polarized light
for films tilted by 45° to the probe beam of spectrometer
(inset in Fig. 7a). As seen in Fig. 6a, out-of-plane (or
homeotropic) alignment of chromophores even takes place
in fresh spin-coated films. Nevertheless, the values of
corresponding dichroism calculated using Eq. 1 are rela-
tively small (*0.10 at 321 nm).
Annealing of the film at temperatures of 70–120 °C
increases values of out-of-plane dichroism up to 0.26 at
321 nm (Fig. 7), whereas thermal treatment below 60 °C
does not induce out-of-plane order even after several days,
because of the higher viscosity of the polymer films at
temperatures closer to the glass transition. Polarizing
optical microscopy observations do not reveal any texture
at all stages of annealing at different temperatures; these
films remain optically isotropic, or birefringent domains
are too small to be clearly visible. It should be pointed out
that before annealing the maxima of absorbance for the
light polarized in parallel and perpendicular directions are
situated almost at the same wavelengths of light (321 nm
for A|| and 319 nm for A\; Fig. 7a). On the other hand,
annealing leads to a shift of absorbance in the preferred
direction to shorter wavelength for A|| (315 nm), and to
longer wavelength for A\ (330 nm) (Fig. 7b). Such
λ / nm
Fig. 2 Spectral changes in polymer film during a UV (365 nm,
0.9 mW/cm2) and b visible light (436 nm, 0.6 mW/cm2) irradiation
for the time indicated
behaviour is explained by the orientational redistribution of
chromophore aggregates in such a way that H-aggregated
azobenzene fragments become oriented homeotropically
whereas non-aggregated chromophores are distributed in
space more or less randomly.
Annealing has a large effect on both CD values and
spectral shape of the CD curve (Fig. 8). The negative peak
strongly increases its absolute value and a significant
positive peak at the wavelength corresponding to the n-p*
transition (*420 nm) appears. Moreover, nonzero values
of CD have been found also in the spectral range outside
the azobenzene moieties’ absorbance (up to 600 nm).
These observations allow one to confirm the development
of the helical structure in thin spin-coated films of polymer
during annealing.
On the other hand, coexistence of the homeotropic ori-
entation of the side groups and their helical structure
arrangement is impossible because the uniaxial orientation
excludes helical supramolecular organization. Neverthe-
less, there are two possible explanations of the behaviour
123

Chirooptical and photooptical properties of a novel side-chain azobenzene-containing LC polymer
793
Isotropic state
0.40
0.35
0.30
0.25
0.20
0.15
0.10
321 nm
337 nm
UV-irradiation
Fresh spin-coated film
Quartz substrate
436 nm
365 nm
Annealing
Coexistence of homeotropic
orientation and helical structure
Fig. 5 Idealized scheme of photoinduced and thermoinduced struc-
tural changes in the spin-coated film of the polymer
0.6
0.5
0
10
20
30
40
50
0.4
Time / min
before
30 min
Fig. 3 Kinetics of absorbance changes during UV (365 nm, 0.9 mW/
cm2) and subsequent visible light (436 nm, 0.6 mW/cm2) irradiation
at two wavelengths. Absorbance at 321 nm corresponds to azoben-
zene groups in H-aggregated state, whereas that at 337 nm
corresponds to the monomeric form
0.3
0.2
0.1
0.0
1 day
discussed above allowing one to remove this contradiction.
One is the mesophase coexistence in the sample. In our
films two mesophase structures are possible, the SmX* and
cholesteric phases, which could coexist in the temperature
range below the Sm-N* phase transition detected by POM
200
300
400
500
600
λ
/ nm
Fig. 6 An effect of annealing at 120 °C for 30 min and 24 h on the
absorbance spectra of the polymer film
2
and DSC in thick films of the polymer (see scheme in
Fig. 5). In polymer liquid crystals coexistence of two or
more phases at the same temperature in a wide temperature
range is not an uncommon phenomenon [31–34], which is
explained by the presence of defects and high viscosity
because of the polymer backbone.
1
0
After UV irradiation (365 nm)
-1
-2
-3
-4
-5
-6
Another possible hypothesis explaining nonzero CD
values in combination with the homeotropic alignment is
the formation of a tilted chiral smectic phase (SmC*,
SmF*, or SmI*), which is characterized by intrinsic helical
structure [35]. The director of such smectic mesophases is
tilted to the layer normal, and rotates from layer to layer
forming a helical structure. Unfortunately, POM observa-
tion cannot prove this hypothesis because of the small
thickness of the films or negligible birefringence in the
homeotropically oriented SmC* phase.
Before
Let us consider the effect of light on annealed films of
polymer. UV and subsequent visible light induce qualita-
tively the same spectral changes in the annealed films as
was observed for the freshly prepared films (cf. Figs. 3, 9).
However, conversion of photoisomerization is noticeably
250
300
350
400
450
λ
/ nm
Fig. 4 CD spectra of fresh polymer film before and after UV
irradiation
123

794
A. Bobrovsky et al.
Spectrometer
45o
0.22
0.20
0.18
0.16
0.14
0.12
365 nm
436 nm
a
0.5
0.4
0.3
0.2
0.1
0.0
D321=0.10
Light Source
||
321 nm
337 nm
⊥
250 300 350 400 450 500
λ / nm
0.5
0.4
0.3
0.2
0.1
0.0
b
80 oC
D321=0.26
||
⊥
0
5
10
15
20
25
30
250 300 350 400 450 500
Time / min
λ / nm
Fig. 9 Kinetics of absorbance changes of annealed film during UV
(365 nm, 0.9 mW/cm2) and visible light (436 nm, 0.6 mW/cm2)
irradiation at two wavelengths. Absorbance at 321 nm corresponds to
azobenzene groups in H-aggregated state, whereas the that at 337 nm
corresponds to the monomeric form
c
0.30
0.25
0.20
0.15
0.10
0.05
(Fresh film)
0.40
isotropic film
smectic (annealed) film
20 40 60 80 100 120
0.35
Temperature / °C
0.30
0.25
0.20
0.15
0.10
Fig. 7 Polarized absorbance spectra measured at 45° to film normal
for (a) fresh films and (b) film after annealing at 80 °C. (c)
Temperature dependence of dichroism
10
5
0
-5
0
5
10
15
20
-10
Time / min
Before
120 oC for 30 min
-15
Fig. 10 Kinetics of absorbance changes at 321 nm during UV
(365 nm, 0.9 mW/cm2) irradiation for isotropic and annealed smectic
films
120 oC for 2 days
-20
-25
-30
lower, as is clearly seen in Fig. 10. Smectic order prevents
the E–Z isomerization process, because the azobenzene
fragments in the Z isomer are less stable in a more ordered
smectic polymer matrix. Annealing and subsequent order-
ing causes a decrease in free volume, which is necessary
for the E–Z isomerization process.
300
400
500
600
λ
/ nm
Fig. 8 CD spectra of polymer film before and after annealing for
30 min and two days
123

Chirooptical and photooptical properties of a novel side-chain azobenzene-containing LC polymer
795
Another interesting difference between the fresh and
a
annealed films appeared in relative changes in absorbance
corresponding to chromophores in monomeric and aggre-
gated states (compare Figs. 3, 9). For as-prepared spin-
coated films difference between the values and kinetics of
absorbance decreases at both wavelengths (321 and
337 nm) are small, whereas for annealed film absorbance
change at 321 nm is a factor of *1.4 lower. In other
words, isomerization is more strongly suppressed in
aggregated chromophores presented in the smectic struc-
ture of the annealed films.
0.5
0.4
0.3
0.2
0.1
0.0
||
⊥
Before:
||
⊥
||
After:
||
⊥
⊥
UV irradiation of annealed films results in drastic
changes in CD spectra (Fig. 11). Both, positive and nega-
tive peaks become lower after irradiation. However, in
comparison with the fresh film (Fig. 4) irradiation does not
cause vanishing of the CD signal. Irradiation with visible
light leads to only partial recovery of CD peaks, but we
have found that annealing at 120 °C could be used for fast
back increase of the initial CD spectra.
250
300
350
400
450
500
λ
/ nm
90
b
before
100 min
120
60
0.4
0.3
0.2
0.1
30
150
Photoorientation phenomena in amorphous and smectic
spin-coated films
0.0 180
0.1
0
Irradiation of fresh or annealed spin-coated films of poly-
mer by polarized UV or visible light (in the spectral range
of azobenzenene absorbance) leads to the appearance of
significant linear dichroism (Fig. 12). This effect is asso-
ciated with well-known photoorientation phenomena often
taking place in azobenzene-containing systems under the
action of polarized light. In this paper we present data
obtained using as irradiation source the nonfiltered light of
a mercury lamp containing the most effective azobenzene
photoorientation emission lines, such as 365, 405, and
436 nm covering both the p–p* and n–p* electronic
0.2
210
330
0.3
0.4
240
300
270
Fig. 12 a Spectra of polarized light absorbance of fresh polymer film
before and after polarized light irradiation (100 min of nonfiltered
light of mercury lamp (*5 mW/cm2)); b polar plot of absorbance at
321 nm for fresh film and film irradiated with nonfiltered light of
mercury lamp (*5 mW/cm2)
transition regions. In addition, we have analyzed the effect
of filtered light of the same lamp selecting lines 365 or
436 nm and found the same values of dichroism but,
because of the lower intensity, the time to reach saturation
of dichroism is much longer (several hours).
5
0
-5
It should be emphasized that annealing of the film
reduces its capacity to be oriented by polarized light
(Fig. 13). Smectic ordering reduces the mobility of azo-
benzene fragments and prevents, to some extent, not only
isomerization, as was shown above, but also rotational
movement of the side groups.
-10
before
365 nm
436 nm
-15
-20
-25
-30
We have performed a study of reorientation possibility in
polymer films by use of irradiation–annealing cycles with
changing of the direction of the light polarization (Fig. 14).
The first cycle of irradiation was done for the fresh films at
definite polarization direction (0°) and resulted in high value
of dichroism. After that the film was annealed for 30 min at
120 °C and irradiated again with the same polarization
300
400
500
600
λ / nm
Fig. 11 CD spectra of annealed polymer film before and after UV
irradiation (365 nm, 15 min) followed by 436 nm irradiation
(20 min)
123

796
A. Bobrovsky et al.
first steps of irradiation, but only small increase in dichroism
takes place. The next step of annealing again results in
dichroism decreasing to small but measurable values.
Eventually, the last cycle of irradiation was performed with
the polarization direction coinciding with the first steps of
irradiation. A larger value of dichroism was obtained. The
experiment described above showed the existence of some
‘‘memory’’ effect, i.e. the polymer film ‘‘remembers’’ the
first direction of the photoinduced orientation and chro-
mophores have a tendency to be oriented in this direction
more easily than in the perpendicular one. The reasons for
such behaviour are not completely clear and can be associ-
ated with polymer backbone orientation which is fixed in a
definite direction after the first step of irradiation. Another
possible explanation of the ‘‘memory’’ effect is possible side
photochemical reactions (photo-Fries rearrangement, for
example) leading to crosslinking of the film and fixation of
the preferred orientation.
0.6
0.5
0.4
0.3
0.2
0.1
0.0
fresh
annealed
0
20
40
60
80
100
Finally, study of photooptical recording using thin spin-
coated films of the new polymer was performed. Figure 15
is a polarizing optical microphotograph of the sample
irradiated by polarized light through a mask. Bright areas
correspond to the irradiated zones with noticeable bire-
fringence. Horizontal stripes, clearly visible in the
irradiated zones, are because of thickness variation which
appeared during the spin-coating process. It is noteworthy
that the recorded image is quite stable at room temperature,
for months at least.
Time / min
Fig. 13 Dichroism growth during irradiation with nonfiltered light of
a mercury lamp (*5 mW/cm2) for fresh and annealed (one day at
120 °C) polymer film
direction of acting light. The dichroism value becomes lower
than in the first cycle, because of reduced azobenzene groups
mobility in the smectic phase. After that the film was
annealed again. It is remarkable that annealing in this case
does not completely remove the initial orientation and the
dichroism value is still noticeable, ca. 0.09. Subsequent
irradiation with perpendicular direction of light polarization
induces reorientation in the direction perpendicular to the
Conclusion
The novel synthesized polymer, consisting of a mesogenic
azobenzene chromophore and a chiral fragment, forms a
Fresh film
100 min
0.6
0.4
0.2
0.0
Irradiation
at 0o
Irradiation
at 0o,
second step
Irradiation
at 90o
Annealing
Annealing
Annealing
Fresh film
Fig. 14 Plot demonstrating dichroism changes under repetitive
irradiation-annealing cycles (nonfiltered light of a mercury lamp
(*5 mW/cm2)). First cycle of irradiation was done for fresh films at
polarization direction 0° (relative to probe beam), after that film was
annealed for 30 min at 120 °C and irradiated again with the same
polarization direction of acting light. After that the film was annealed
and irradiated with the perpendicular direction of light polarization.
The film was then annealed again and a second step of irradiation at
0° was performed
Fig. 15 Polarizing optical microphotograph showing spin-coated
film of polymer irradiated with white light through a mask for 40 min
123

Chirooptical and photooptical properties of a novel side-chain azobenzene-containing LC polymer
797
smectic and a cholesteric phase exhibiting selective light
reflection in the IR spectral range. Photooptical and chi-
rooptical properties of the thin spin-coated films of the
polymer can be controlled by the action of heat and light.
UV irradiation leads to E–Z isomerization with high con-
version, which depends on the thermal prehistory of the
films. Subsequent visible light action results in partial
recovery of the E-isomer content, whereas annealing
results in full back conversion. E–Z isomerization induces
complete disruption of the helical elements in the nonan-
nealed films of polymer whereas for the annealed film in
the smectic phase only a significant decrease of CD values
was found. Polarized light action on amorphous (nonan-
nealed) or smectic (annealed) films of the polymer induces
linear dichroism provided by oriented azobenzene groups
and dichroism values that are stable at room temperature
for long time. Peculiarities of photoorientation behaviour
were investigated in polymer films with amorphous and
smectic states. It was shown that annealing of the films
results in decrease of values of photoinduced anisotropy.
solution was poured into 1 dm3 water, filtered by suction,
washed twice with water, and crystallized from ethanol.
The red product was dried in vacuo at room temperature for
1
two days. H NMR (DMSO-d6, 300 MHz): d = 7.80 (m,
4H, ortho to –N=N–), 7.02 (d, 2H, ortho to OR), 6.90 (d,
2H, ortho to –OH), 6.30 and 5.90 (2d, 2H, CH2=), 6.1 (m,
1H, =CH–COO), 4.08 (m, 4H, CH2O), 1.2–1.8 (m, 16H,
CH2) ppm.
(S)-(-)-4-(2-Methylbutoxy)benzoic acid (5)
The 4-(2-methylbutoxy)benzoic acid (5) was obtained by
alkylation of 4-hydroxyacetophenone by (S)-(-)-1-bromo-
2-methylbutane in ethanol–water solution in the presence
of NaOH. The next step of preparation was a common
haloform reaction with NaOBr in dioxane solution fol-
lowed by crystallization of the acid from ethanol. 1H NMR
(CDCl3, 300 MHz): d = 8.10 (d, 2H, ortho to –COOH),
6.95 (d, 2H, meta to –COOH), 3.80 (m, 2H, CH2OAr), 1.90
(m, 1H, *CH), 1.30 and 1.60 (2 m, 2H, CH2CH3), 1.00 (d,
3H, CH3–C*), 0.95 (t, 3H, CH3) ppm.
Synthesis of combined chiral–photochromic mesogenic
monomer A10AZOF* (6)
Experimental
The mesogenic photochromic phenol 4 was reacted with
acid 5 in dichloromethane with dicyclohexylcarbodiimide
and dimethylaminopyridine as a condensation agent. When
the reaction was complete, the dicyclohexylurea was
removed by filtration, solvent was evaporated, and crude
product was purified by column chromatography on silica
gel (Kieselgel 60, Merck) using a mixture of dichloro-
methane and acetone (99.8:0.2) as eluent. It was then
crystallized twice from ethanol. The structure of the pre-
pared product was confirmed by 1H and 13C NMR
spectroscopy. The chemical purity of the compound was
checked by high-pressure liquid chromatography (HPLC),
which was carried out with an Ecom HPLC chromatograph
using a silica gel column (Separon 7 lm, 3 9 150 mm;
Tessek) with a mixture of 99.9% toluene and 0.1% meth-
anol as eluent, and UV–visible detection of the eluting
products (k = 290 nm). The chemical purity was found to
4-x-Bromodecyloxy-40-hydroxyazobenzene (2)
4-x-Bromodecyloxy-40-hydroxyazobenzene
(2)
was
obtained from 4-acetamidophenol by alkylation with
1,10-dibromodecane in dioxan–water solution in the pres-
ence of NaOH. The crude product was hydrolysed with
dilute (20%) H2SO4 by boiling for 3 h. After standing at
room temperature for 12 h, the solid sulfuric salt of amine
1 was isolated by filtration, by suction, dried, and washed
twice with n-hexane followed by diazotisation and cou-
1
pling with phenol using a standard procedure. H NMR
(CDCl3, 300 MHz): d = 7.95 (dd, 4H, ortho to –N=N–),
6.96 and 7.02 (2d, 2*2H, meta to –N=N–), 4.02 (t, 2H,
CH2OAr), 3.40 (t, 2H, CH2Br), 1.9 m (4H, CH2), 1.2–1.5
(m, 12H, CH2) ppm.
4-x-Iododecyloxy-40-hydroxyazobenzene (3)
The bromo derivative 2 was converted into iodide 3 by
boiling with excess sodium iodide in acetone for several
1
be 99.5%. H NMR (CDCl3, 300 MHz): d = 8.18 (d, 2H,
1
hours. H NMR (CDCl3, 300 MHz): d = 7.95 (dd, 4H,
ortho to –COO), 7.95 (dd, 4H, ortho to –N=N–), 7.35 (d,
2H, ortho to –OCO), 7.00 (dd, 4H, ortho to –OR), 6.40 and
5.80 (2d, 2H, CH2=), 6.10 (m, 1H, =CH–COO), 4.18 (t, 2H,
COOCH2), 4.05 (t, 2H, CH2CH2OAr), 3.90 (m, 2H, *CH–
CH2OAr), 1.30 and 1.80 (2 m, 19H, CH2, CH*), 1.02 (d,
3H, CH3–C*), 0.98 (t, 3H, CH3CH2) ppm.
ortho to –N=N–), 6.96 and 7.02 (2d, 2*2H, meta to –N=N–),
4.02 (t, 2H, CH2OAr), 3.20 (t, 2H, CH2I), 1.9 m (4H, CH2),
1.2–1.5 (m, 12H, CH2) ppm.
Photochromic acrylic monomer 4
Iodide 3 (0.1 mol) was dissolved in 150 cm3 dry dimethyl
sulfoxide and then 0.2 mol dry potassium acrylate (dried
24 h under vacuum at 30 °C) and 0.1 g hydroquinone was
added. After reacting at room temperature for five days the
Polymerization
Liquid crystalline chiral–photochromic polymer was syn-
thesized by a radical polymerization of the corresponding
123

798
A. Bobrovsky et al.
acrylic monomer 6 in benzene solution in the presence of
2% (to monomer) AIBN. After three days storage at 65 °C
the solvent was evaporated and the solid product was
washed several times with boiling ethanol. Yield was 60%.
Such relatively low yield is explained by competing radical
transfer reaction promoted by the azobenzene fragment.
Molecular mass of polymer Mw *15,000 and polydisper-
sity Mw/Mn *1.7 were determined by GPC using a Knauer
instrument.
prism controlled by computer software). The dichroism
values, D, of the polymer films were calculated from the
spectra by use of Eq. 1
À
Á À
Á
D ¼ A ꢁ A = A þ A
ð1Þ
?
?
k
k
where A|| is the absorbance at the preferred chromophore
orientation direction and A\ is the absorbance perpendicular
to this direction. To study the out-of plane photoorientation
phenomena we measured the angular distribution of the
polarized absorbance spectra at an angle of approximately 45°
to film normal.
Study of phase behaviour and selective light reflection
Acknowledgments This research was supported by the Russian
Foundation of Fundamental Research (08-03-00481), Program
COST-D35, WG 13-05, and project No. OC 175 of the Ministry of
Education Youth and Sports of the Czech Republic.
The phase transition temperatures of the polymer were
detected by differential scanning calorimetry (DSC) with a
Perkin–Elmer DSC-7 thermal analyzer at a scanning rate of
10 K/min. The polarizing microscope investigations were
performed using a Mettler TA-400 thermal analyzer and a
LOMO P-112 polarizing microscope. For selective light
reflection study thin films were prepared between two glass
plates by melting of a small amount of the polymer. The
thickness of the films was predetermined by 11 lm glass
beads. Before investigation all films were annealed for
30 min at a temperature 10 ° below the clearing point
followed by slow cooling (1 °/min). Transmittance spectra
were recorded by a Hitachi U3400 UV–visible–NIR
spectrophotometer.
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