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

ORIGINAL PAPER

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

Alexey Bobrovsky Æ Valery Shibaev ÆVera Hamplova Æ Miroslav Kaspar ÆMilada Glogarova

Received: 16 November 2008 / Accepted: 24 November 2008 / Published online: 20 February 2009

� 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].

NN R'

R

N

R

N

R'

E-isomer Z-isomer

λ1

λ2, Τ

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].

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

A. Bobrovsky (&) � V. Shibaev

Faculty of Chemistry, Moscow State University, Leninskie gory,

119992 Moscow, Russia

e-mail: [email protected]

V. Hamplova � M. Kaspar � M. Glogarova

Institute of Physics, Academy of Sciences of the Czech

Republic, 182 21 Prague 8, Czech Republic

123

Monatsh Chem (2009) 140:789–799

DOI 10.1007/s00706-009-0108-8

works demonstrated the unique possibilities of photoregu-

lation of supramolecular structure and optical properties,

self-organization, modification of surface tension, etc.

[15–18].

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.

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.

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.

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

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.

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

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.

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.

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’’.

790 A. Bobrovsky et al.

123

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.

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

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.

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).

NHCOCH3HO1. Br(CH2)10Br

2. H2SO4NH2Br(CH2)10O

NaNO2

phenol

Br(CH2)10O N N OH

1

2

K I

acetoneI(CH2)10O N N OH

3

CH2=CH-COOKCH2=CHCOO(CH2)10O N N OH

4

HO COCH3

1. methylbutyl bromide

2. NaBrOCH3CH2CHCH2O

CH3

*COOH

5

4 + 5DCC

DMAPCH2=CHCOO(CH2)10O N=N OCO OCH2CH*CH2CH3

CH3

6

Polymerization

CH2 CH

COO (CH2)10 O N

N O

O

OCH2CH*CH2CH3

CH3n

Photochromic group

Chiral group Mesogenic group Spacer

Scheme 1

Chirooptical and photooptical properties of a novel side-chain azobenzene-containing LC polymer 791

123

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).

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

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

200 300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

0.6

Abs

orba

nce

λ / nm

λ / nm

0

1

2

4

6

8

10 min

Φ-Φ*

π-π*

n-π*

200 300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

0.6

10 min

6

2

1

0

Abs

orba

nce

a

b

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

120 140 160 180

1100

1200

1300

1400

1500

1600

1700

λ / n

m

T / °C

Fig. 1 Temperature dependence of selective light reflection wave-

length detected in planarly oriented polymer films


123

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

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.

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.

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

0 10 20 30 40 50

0.10

0.15

0.20

0.25

0.30

0.35

0.40

436 nm365 nm

Abs

orba

nce

Time / min

321 nm

337 nm

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

250 300 350 400 450-6

-5

-4

-3

-2

-1

0

1

2

After UV irradiation (365 nm)

Before

CD

/ m

degr

.

λ / nm

Fig. 4 CD spectra of fresh polymer film before and after UV

irradiation

UV-irradiation

Annealing

Fresh spin-coated film

Isotropic state

Coexistence of homeotropic orientation and helical structure

Quartz substrate

Fig. 5 Idealized scheme of photoinduced and thermoinduced struc-

tural changes in the spin-coated film of the polymer

200 300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

0.6

Abs

orba

nce

λ / nm

before

30 min

1 day

Fig. 6 An effect of annealing at 120 �C for 30 min and 24 h on the

absorbance spectra of the polymer film


123

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.

250 300 350 400 450 5000.0

0.1

0.2

0.3

0.4

0.5 D321

=0.10

⊥

||

Abs

orba

nce

λ / nm

250 300 350 400 450 500

λ / nm

45o

Light Source

Spectrometer

0.0

0.1

0.2

0.3

0.4

0.5

80 oCD

321=0.26

⊥

||

Abs

orba

nce

20 40 60 80 100 1200.05

0.10

0.15

0.20

0.25

0.30

(Fresh film)

Dic

hroi

sm a

t 321

nm

Temperature / °C

a

b

c

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

300 400 500 600

-30

-25

-20

-15

-10

-5

0

5

10

CD

/ m

degr

.

λ / nm

Before

120 oC for 30 min

120 oC for 2 days

Fig. 8 CD spectra of polymer film before and after annealing for

30 min and two days

0 5 10 15 20 25 30

0.12

0.14

0.16

0.18

0.20

0.22

436 nm365 nm

337 nm

321 nm

Abs

orba

nce

Time / min

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

0 5 10 15 200.10

0.15

0.20

0.25

0.30

0.35

0.40

Abs

orba

nce

Time / min

isotropic film

smectic (annealed) film

Fig. 10 Kinetics of absorbance changes at 321 nm during UV

(365 nm, 0.9 mW/cm2) irradiation for isotropic and annealed smectic

films


123

Another interesting difference between the fresh and

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.

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.

Photoorientation phenomena in amorphous and smectic

spin-coated films

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

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).

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.

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

300 400 500 600-30

-25

-20

-15

-10

-5

0

5

CD

/ m

degr

.

λ / nm

before 365 nm 436 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)

250 300 350 400 450 5000.0

0.1

0.2

0.3

0.4

0.5

⊥||

||

⊥

Abs

orba

nce

λ / nm

Before:||⊥

After:||⊥

0.0

0.1

0.2

0.3

0.4

0

30

60

90

120

150

180

210

240270

300

330

0.0

0.1

0.2

0.3

0.4

Abs

orba

nce

at 3

21 n

m

before 100 min

a

b

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)


123

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

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.

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.

Conclusion

The novel synthesized polymer, consisting of a mesogenic

azobenzene chromophore and a chiral fragment, forms a

Fig. 15 Polarizing optical microphotograph showing spin-coated

film of polymer irradiated with white light through a mask for 40 min

0 20 40 60 80 100

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Dic

hroi

sm a

t 321

nm

Time / min

fresh annealed

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

0.0

0.2

0.4

0.6

Irradiation at 0o,second step

Annealing

Irradiation at 90o

Annealing

Irradiation at 0o

Annealing

Fresh film 100 min

Fresh film

Dic

hroi

sm a

t 321

nm

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


123

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.

Experimental

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-

pling with phenol using a standard procedure. 1H 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

hours. 1H 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), 3.20 (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

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

two days. 1H 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)

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

be 99.5%. 1H NMR (CDCl3, 300 MHz): d = 8.18 (d, 2H,

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.

Polymerization

Liquid crystalline chiral–photochromic polymer was syn-

thesized by a radical polymerization of the corresponding


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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.

Study of phase behaviour and selective light reflection

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.

Photooptical investigations

Thin films of the polymer for photooptical experiments

were obtained by the spin-coating technique using solu-

tions of different concentration in chloroform. For drying

the spin-coated films were kept at room temperature for

one day. The thickness of the films was 100–200 nm as

estimated from UV–visible spectral data.

Photochemical investigations were performed using a

special optical set up equipped with a DRSh-250 ultra-

high-pressure mercury lamp. To prevent heating of the

samples because of the IR irradiation of the lamp, a water

filter was introduced in the optical scheme. To obtain the

plane-parallel light beam, a quartz lens was applied. During

the irradiation, a constant temperature of the test samples

was maintained by using a Mettler FP-80 heating unit. Use

of filtered light with the wavelengths 365 and 436 nm was

selected. Photoorientation in films was induced by using

non-filtered light from the same lamp. A Glan–Taylor

prism was used as a polarizer. The intensity of light was

measured by LaserMate-Q (Coherent) intensity meter.

Spectral measurements were performed using a Unicam

UV-500 UV–visible spectrophotometer and a Jasco J-500C

spectropolarimeter. The linearly polarized spectra of the

film samples were studied with a Tidas spectrometer

(J&M) equipped with a rotating polarizer (Glan–Taylor

prism controlled by computer software). The dichroism

values, D, of the polymer films were calculated from the

spectra by use of Eq. 1

D ¼ Ak � A?� �

= Ak þ A?� �

ð1Þ

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.

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.

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

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