Effect of molecular structure on chiro-optical and photo-optical properties of smart liquid crystalline polyacrylates

Article abstract of DOI:10.1021/ma401010t

Comparative study of chiro- and photo-optical properties of three novel chiral photochromic liquid crystalline azobenzene-containing comb-shaped polyacrylates was performed. The synthesized polymers contain the same polyacrylate backbones and uniform spac

Full text of DOI:10.1021/ma401010t

Article
pubs.acs.org/Macromolecules
Effect of Molecular Structure on Chiro-Optical and Photo-Optical
Properties of Smart Liquid Crystalline Polyacrylates
Alexey Bobrovsky,†,* Valery Shibaev,† Alexej Bubnov,‡ Vera Hamplova,‡ Miroslav Kaspar,‡
̆
́
̌
and Milada Glogarova‡
́
†Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119992 Russia
‡Institute of Physics, Academy of Sciences of the Czech Republic, 182 21 Prague, Czech Republic
S
* Supporting Information
ABSTRACT: Comparative study of chiro- and photo-optical properties of three novel
chiral photochromic liquid crystalline azobenzene-containing comb-shaped polyacry-
lates was performed. The synthesized polymers contain the same polyacrylate
backbones and uniform spacer size (10 methylene units), but have different structure
of azobenzene-containing mesogenic groups with different chiral tail. The polymers are
characterized by crystalline, SmA*, SmC*, and N* phases formation depending on
chemical structure of the variable fragments. Presence of the lateral chlorine substituent
in the mesogen fragment leads to an extreme decrease of isotropization temperature
and causes a disappearance of cholesteric mesophase. Circular dichroism (CD) spectra
demonstrate formation of helical order elements even in the amorphousized spin-
coated polymer films. Possibility of manipulation of CD values and degree of the helical
order by light and annealing was established and studied. UV-irradiation of all polymers induces E−Z isomerization of
azobenzene groups and thus disrupting the LC-order. Besides, irradiation of films by polarized light leads to photoorientation
process due to cycles of E−Z−E isomerization. As a result, appearance of significant linear dichroism dependent on the relative
position of azobenzene moiety in aromatic core was evidenced.
chiral-photochromic azobenzene-containing LC polymers
having different structure of chiral terminal group.23
INTRODUCTION
■
Azobenzene-containing polymer systems are very promising
materials from both academical and practical points. Their
application in photonics and optoelectronics is very promising
owing to their high photosensitivity combined with strong
fatigue resistance.1 They are very interesting because position of
absorbance band and photooptical features are predetermined
not only by the molecular chromophore structure (presence of
different substituents)1−3 but also by the supramolecular
structure, for example, by self-assembled liquid crystalline
(LC) media forming nematic, smectic and cholesteric phases.4
Moreover, E−Z isomerization of azobenzene groups results in
changes of their molecular shape and polarity, which usually
causes significant structural transformations having impact on
phase transitions,5−7 wettability and surface tension modifica-
tion,8−10 and even photoactuation phenomena.11−15
In most cases the action of polarized light on azobenzene-
containing systems induces photoorientation of chromophores
perpendicular to the polarization plane of excitation light that
can be used for of LC-director reorientation,16 induction of
birefringence and dichroism,1 preparation of “command
surfaces”,17 etc.
Let us point out that similar investigations of photo-chiro-
optical properties of LC polymers were performed by us
earlier.29−33 We have also demonstrated a complicated photo-
and thermoinduced behavior of the azobenzene-containing
cholesteric copolymers and found elements of chiral organ-
ization even in amorphousized films of cholesteric polymers
prepared by spin-coating technique. The E−Z isomerization
induced by the UV-irradiation reversibly disrupts the helical
superstructure, whereas an annealing of the films leads to more
pronounced helical structure formation.30,33 In the case of
smectogenic copolymers, homeotropic alignment of chromo-
phores and mesogens (with the molecular axis normal to the
film plane) is observed.31,33
However, despite a large number of papers devoted to the
azobenzene-containing LC systems, there are still a number of
questions related to the structure-properties relationship
regarding the chiro-optical behavior, mechanism of photo-
orientation phenomena, etc. Moreover, up to now it is almost
impossible to predict the polymorphism type for LC polymer
just considering only their chemical structure.
Taking into account the above-mentioned problems in the
present paper we continued the study of chiral-photochromic
In recent years remarkable attention is focused on the study
of photoswitchable chiral self-organized systems (i.e., possess-
ing the liquid crystalline behavior see, e.g., refs 18 and 19) or
photoinduced chirality induction.20−31 For example, chiro-
optical properties and photoinduced changes in supramolecular
organization under light action were studied for a number of
Received: May 16, 2013
Revised: May 20, 2013
Published: May 29, 2013
© 2013 American Chemical Society
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LC polymers focusing on their phase behavior, chiro-optical
properties, and effect of photoinduced E−Z isomerization on
mesomorphism, supramolecular chirality, and photoorientation
processes. We have synthesized three polymers (P1, P2, P3)
having the same polyacrylate backbone and spacer consisting of
10 methylene units.
These polymers have the same azobenzene-containing
chromophore fragments, but different relative position of
−NN− and −COO− linkers in respect of the spacer and the
chiral terminal group. In polymer P1 −NN− group is
located closer to the spacer, whereas in polymers P2 and P3
azobenzene chromophores are placed near the chiral fragment.
Methylbutyl chiral chain was used in polymers P1 and P2 while
the lactate based moiety was used in P3 to induce stronger
chirality. In addition, polymer P3 has lateral chlorine
substituent in ortho position to the spacer. The presence of
the lateral substituent and lactate based chiral part allows one to
expect the decrease of phase transition temperatures similarly as
observed earlier.34−36 Also changes in rotational mobility of the
side groups in the comb-shaped liquid crystalline polymers can
be expected.
In this work we have focused our attention on the study of
chiro- and photooptical properties, as well as photoorientation
processes in thin spin-coated films of synthesized polymers in
order to elucidate an influence of the chromophore chemical
structures, polymorphism and film thermal prehistory on their
photooptical behavior.
Scheme 1. General Procedure for Synthesis of Monomer M3
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Table 1. Phase Transition Temperatures of Monomers (M1, M2, M3), Respective Polymers (P1, P2, P3) (in °C) and Their
Transition Enthalpies (in Parentheses, J/g) Measured on Cooling (10 K/min)
monomer/polymer
phase transitions
M1
M2
M3
P1
Cr 61 (51.0) SmC* 87 (1.6) N* 152 (2.1) I
Cr 91 (73.0) SmC* 110 (0.1) TGBA* 112 (0.01) N* 153 (1.6) I
Cr −23 (1.2) TGBA* 62 (0.4) N* 79 (0.8) I
Cr 127 (5.1) N* 194 (1.8) I
P2
Cr 118 (5.8) SmC* 145 (a) SmA* 154 (0.6) N* 194 (2.3) I
g 15 SmA 70−80 (a) I
P3
a
Enthalpy of phase transition was not detected by DSC; temperature of the phase transition has been determined by polarizing optical microscopy.
dd (1H, CH−); 5.80 and 6.40 d + d (2H, CH2); 5.35 q (1H,
EXPERIMENTAL PART
■
*CH); 4.15 m (6H, 2 × COOCH2, CH2OAr); 1.68d (3H, CH3C*);
1.20−1.80 m (22H, CH2); 0.9 t (3H, CH3).
Synthesis. Synthesis of monomers M1 and M2 was published in
our previous papers.30,33 Monomer M3 was synthesized according to
the procedure presented in Scheme 1.
Polymerization. The polymers were synthesized by a radical
polymerization of corresponding acrylic monomers (M1, M2, M3) in
benzene solution in the presence of 2 wt % (with respect to monomer)
of AIBN. After 3 day storage at 65 °C the solvent was evaporated and
solid product was washed several times by boiling ethanol. Yield of
polymerization was about 60−70%. Such relatively low yield is
explained by competing radical transfer reaction promoted by
azobenzene fragment. During polymer synthesis a number of low-
molar-mass products (dimers and oligomers) are formed.
Molecular masses (Mw) and polydispersity of polymers (Mw/Mn),
determined by GPC chromatography using instrument “Knauer” had
values in the following ranges: Mw ∼ 10000−15000, Mw/Mn ∼ 1.5−
1.7. Obtained values of molar masses correspond to degree of
polymerization of about 15−20. Such low degree of polymerization is
explained by the high chain transfer constant to NN double bond of
photochromic monomers. This effect is associated with formation of a
stable hydrazyl radical.38,39
Phase Behavior and Selective Light Reflection. The phase
transition temperatures of monomers (M1-M3) and polymers (P1−
P3) were detected by differential scanning calorimetry (DSC) using a
Perkin-Elmer DSC-7 thermal analyzer (scanning rate of 10 K/min).
The polarizing optical microscope investigations were performed
using LOMO P-112 polarizing microscope equipped by Mettler TA-
400 heating stage.
For selective light reflection study films of polyacrylates were
prepared between two glass plates coated with polyvinyl alcohol and
rubbed in one direction in order to achieve a good planar alignment.
Thickness of the films was fixed by 20 μm Teflon spacers. Before
investigation the films were annealed during 30 min at temperature 10
K below the clearing point followed by slow cooling down (1 K/min).
Transmittance spectra were recorded on a Hitachi U3400 UV−vis−
NIR spectrophotometer.
Photo-Optical Investigations. Thin films of the polymers for
photo-optical experiments were obtained by spin-coating technique
using polymer solutions in chlorophorm (c ∼ 20 mg/mL). In order to
completely remove any traces of chlorophorm the spin-coated films
were kept at room temperature during one day. Thickness of the films
was estimated from the UV−vis spectral data as 100−200 nm.
Photochemical investigations were performed using an optical set
up equipped with a DRSh-350 ultrahigh pressure mercury lamp and
MBL-N-457 diode laser (457 nm, CNI laser). To prevent heating of
the samples due to the IR irradiation of the mercury lamp, a water
filter was introduced in the optical setup. To ensure the plane-parallel
light beam, a quartz lens was applied. Using the filters a light with the
wavelengths 365 and 436 nm was selected. The intensities of light
were equal to ∼2.0 mW/cm2 (365 nm), ∼1.0 mW/cm2 (436 nm) for
lamp, and ∼0.5 W/cm2 for laser.
Spectral measurements were performed using Unicam UV-500
UV−vis spectrophotometer. CD spectra were measured by JASCO J-
500C spectropolarimeter.
The studies of photoorientation process were performed using
polarized UV/visible spectroscopy. For this purpose the angular
dependence (with a step-width of 10°) of the polarized light
absorbance was measured using a photodiode array UV/visible
Preparation of 4-(Acryloyloxydecyloxy)-3-chlorobenzoic acid (3).
3-Chloro-4-hydroxyacetophenone (1) (0.1M; 17.0 g) obtained from o-
chlorophenol by Friedel−Crafts acylation and subsequent hydrolysis
was alkylated by 0.3 M (90 g) of 1,10-dibromodecane in a solution of
6 g of potassium hydroxide in water/dioxane mixture (1:1). The
solution was then refluxed for 10 days. After that 500 mL of water was
added to the reaction mixture and the mixture was evaporated to half
volume to remove dioxane. The cold residue was extracted twice by n-
hexane to remove unreacted dibromodecane.
Acid (2) was obtained by oxidation reaction using NaBrO in
dioxane solution at 0 °C by usual method. The crude product was
crystallized from ethanol, dried and converted into iodide by boiling in
sodium iodide/acetone mixture for 6 h. The reaction mixture was
poured into water, cooled and separated by suction. Finally, the
product was dried in vacuo at 60 °C. The yield was 20 g (45%).
Dry potassium acrylate (0.1 M) was dissolved in hot dimethyl
sulfoxide and, after cooling down, 19 g of 4-(iododecyloxy)-3-
chlorobenzoic acid was added and the mixture was stirred for several
days at room temperature. Then the reaction mixture was poured into
water, the precipitate was filtered off by suction and washed with dilute
hydrochloric acid and water. Crude acid (3) was finally crystallized
from acetone and undesirable byproduct (4) was discharged by
column chromatography on silica gel. Total yield was 6.5 g (15%).
1H NMR of 3 (300 MHz, CDCl3). 8.12 s (1H, ortho to −Cl); 7.98 dd
(1H, para to −Cl); 6.95 d (1H, meta to −Cl); 6.12 dd (1H, CH−);
5.81 and 6.40 d + d (2H, CH2); 4.18 t (2H, COOCH2); 4.08 t (2H,
CH2OAr); 1.40−1.90 m (16H, CH2).
Preparation of Mesogenic Phenol 5. The mesogenic chiral phenol
5 was prepared from (S)-amyllactate using a similar method described
elsewhere.37
1H NMR of 5 (300 MHz, CDCl3). 8.17 d (2H, ortho to −COO); 7.80
dd (4H, ortho to −NN−); 6.90 d (2H, ortho to −OH); 5.32 q (1H,
C*H); 4.18 t (2H, COOCH2); 1.68 d (3H, CH3C*); 1.20−1.60 m
(6H, CH2); 0.90 m (3H,CH3).
Preparation of Monomer M3. Final product M3 was obtained by
condensation of acid 3 and mesogenic phenol 5 in dichloromethane/
tetrahydrofuran solution in presence of dicyclohexylcarbodiimide as a
condensation agent and dimethylaminopyridine as a catalyst. The
crude product was purified by column chromatography under day light
elimination. Silica gel (0.063−0.100 mm, Merck) was used as a
stationary phase using a mixture (99.8: 0.2) of dichloromethane and
acetone as an eluent. Product was crystallized twice from methanol.
1
Structure of final product was confirmed by H NMR (300 MHz,
Varian). The chemical purity of materials was checked by high
pressure liquid chromatography (HPLC) using a silica gel column
(Biosphere Si 100−5 μm, 4 × 250, Watrex) with a mixture of 99.9% of
toluene and 0.1% of methanol as an eluent and detection of the eluting
products by a UV−vis detector (λ = 290 nm). The chemical purity was
found better than 99% under these conditions.
1H NMR of M3 (300 MHz, CDCl3). 8.27 m (3H, ortho to −COOC*,
ortho to −Cl); 8.10 d (1H, para to −Cl); 8.00 dd (4H, ortho to −N
N−); 7.41 d (2H, ortho to −OCO); 7.00 d (1H, meta to −Cl); 6.10
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spectrometer TIDAS (J&M) equipped with rotating polarizer (Glan−
Taylor prism controlled by a computer).
RESULTS AND DISCUSSION
■
Phase Behavior and Optical Properties of Planarly
Oriented Polymers Films. Mesomorphic properties of the
monomers (M1−M3) and related polyacrylates (P1−P3) are
shown in Table 1. Three synthesized polymers demonstrate
very different phase behavior. Polymer P1 forms only the
cholesteric phase and a low temperature crystalline phase,
whereas structurally similar polymer P2 exhibits the cholesteric
phase and two smectic phases (Table 1). Despite the different
polymorphism, clearing temperatures for the both polymers are
completely the same (∼194 °C). Some details of mesophases
structure are considered in our previous paper.37
Figure 2. Polarizing optical microscopy photo of focal conic texture of
polymer P3 after annealing at 63 °C for several days. Scale bar
corresponds to 100 μm.
Cholesteric mesophase of the polymers P1 and P2 possesses
the selective light reflection in near IR spectral region (Figure
S1 in Supporting Information). Wavelength of the selective
light reflection maximum λmax extremely increases under
cooling (Figure 1), which is associated with helix unwinding
due to increase of smectic order fluctuations under temperature
decrease.
core in polymer P3 (see the models of the polymers side-chain
in Figure S2, Supporting Information).
Considering the phase behavior of synthesized polymers, one
may conclude that the structure of the mesogenic fragment,
namely, the relative position of −NN− and −COO− linkers
and, in particular, the introduction of lateral chlorine
substituent plays a crucial role in polymorphism and mesophase
thermostability. For the polymer P2 position, the −NN−
bond closer to the chiral terminal group favors smectic phase
formation. Introduction of lateral chlorine substituent to the
mesogenic group of polymer P3 prevents cholesteric phase
formation and strongly decreases its clearing temperature.
Thermo-Optical Properties of Spin-Coated Polymers
Films. Absorbance spectra of three polymers were studied on
thin amorphousized films prepared by spin-coating technique.
As seen in Figure 3 the normalized absorbance spectra are quite
Figure 1. Temperature dependencies of selective light reflection
wavelength for the polymers and corresponding monomers (M1, M2).
For corresponding monomers (M1 and M2) we have
analyzed properties of the selective light reflection and found
that they have a similar spectral position of the selective light
reflection bands but curves of the temperature dependencies
are strongly shifted to the shorter wavelengths (see Figure 1)
due to the lower phase transitions temperatures.37 This result
allows one to conclude that helical twisting power of chiral
fragment is the same for monomer and corresponding polymer,
i.e. polymer backbone has no influence on pitch of cholesteric
helix but strongly increases the phase transition temperatures.
The shift in the phase transition temperatures between
polymers and their monomers is about 60 K. For our
knowledge, in spite of a large number of papers concerning
cholesteric side-chain polymers (see, for review, refs 40 and
41), this is the first direct experimental evidence comparing the
selective light reflection properties of homopolymers with
corresponding monomers.
Polarizing optical microscopy (Figure 2) and DSC data have
revealed that polymer P3 forms only a smectic mesophase. On
cooling polymer P3 sample from isotropic melt well-defined
“batonettes” are formed typical for smectic phases.42 The
clearing temperature of this polymer is more than 100 °C lower
than that for polymers P1 and P2. Such significant disruption of
mesophase thermostability is caused by the presence of
chlorine atom laterally substituted on aromatic mesogenic
Figure 3. Normalized absorbance spectra of the amorphousized
polymer films obtained by spin-coating. Wavelengths of maximum of
π−π* electronic transition are shown.
similar for all polymers due to similarity of azobenzene-
containing chromophore structures. The absorbance bands at
320−330 nm correspond to π−π* electronic transition of
azobenzene group. Peaks at ∼250 nm are the superposition of
Φ−Φ* electronic transition of aromatic cores of azobenzene
moieties and π−π*, n−π* electronic transitions of phenyl-
benzoate chromophores in the aromatic mesogenic cores of all
polymers. Note that latter peak for polymer P3 has lower
intensity than for polymers P1 and P2 probably due to the
presence of the chlorine substituent bringing about decrease of
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Figure 4. Changes of absorbance spectra of the polymers P1 (a), P2 (b), and P3 (c) films under annealing for 30 min at 130 °C (polymers P1, P2)
and 70 °C (polymer P3). (d) Polar plot of polarized absorbance measured at 45° to films normal (see Figure S3 in Supporting Information for
experimental details).
Figure 5. (a) CD spectra of amorphousized polymers films. (b) CD spectra of P3 film before and after annealing at 70 °C for 30 min followed by
fast cooling to room temperature.
extinction coefficients of π−π*, n−π* electronic transitions of
the phenylbenzoate chromophores.43
orientation of mesogens at the air−LC interface and addition-
ally provoked by growth of the smectic (layered) mesophase
starting from the surface to films bulk.44,45
It is noteworthy that degree of homeotropic orientation is
maximal for polymer P2 forming smectic mesophase with high
transition temperature to the cholesteric phase on heating
(Figure 4d, Table 1). Low degree of out-of-plane orientation in
smectic polymer P3 probably can be explained by a lower order
parameter of mesogen fragments due to the effect of the lateral
substituent.
Summarizing the spectral properties of the polymers one can
conclude that phase behavior and thermal treatment of polymer
films play a crucial role for optical properties of polymer films
reflected in the shape of UV−vis absorbance spectra.
Chiro-Optical Properties of Spin-Coated Polymer
Films. Although the just-prepared amorphousized spin-coated
films of polymers have no birefringence and do not exhibit light
scattering, circular dichroism (CD) spectra show noticeable
peaks in the spectral range corresponding to π−π* and n-π*
A slight difference in absorbance of polymers P1 and P2
having completely the same chromophores can be explained by
different degree of aggregation which becomes much stronger
after annealing these polymers as demonstrated in Figure 4.
Absorbance spectra of diluted solutions of polymers P1 and P2
completely coincide (see Figure S3 in Supporting Information).
Annealing of spin-coated films at temperatures above the
glass transition49 decreases absorbance corresponding to π−π*
electronic transition of azobenzene group (Figure 4a−c). This
effect is related to homeotropic (out-of-plane) orientation of
chromophores due to annealing and smectic or crystalline
phase formation. Such chromophore alignment was proved by
measurements of the polarized absorbance spectra at angle of
45° to film normal (see Figure S4, Supporting Information, for
experimental details).
As was shown earlier homeotropic alignment in spin-coated
films under annealing is caused by a tendency to normal
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Figure 6. Temperature dependences of absolute CD values for annealed films of polymers P1 (a), P2 (b), and P3 (c).
Figure 7. Changes of CD spectra of amorphousized films of polymers P1 (a), P2 (b), and P3 (c) under UV-light (365 nm, 10 min) and subsequent
visible light irradiation (436 nm, 20 min).
electronic transitions of azobenzene chromophores (Figure 5).
It is noteworthy, that all three polymers in good solvents do not
exhibit the Cotton effect and CD signals in the spectral range
above 210 nm. Thus, CD peaks obtained for spin-coated films
are definitely related to the supramolecular chirality and reflects
an appearance of elements of cholesteric order of chromo-
phores even in amorphousized spin-coated films. Presence of
elements of cholesteric order in its turn leads to asymmetrical
exciton coupling of azobenzene chromophores.46
explained by absence of ordered crystalline phases in polymer
P3 due to the presence of the lateral substituent which
increases mobility of side groups providing easier possibility to
form chiral superstructures. An annealing of polymer P3 films,
as well as polymers P1 and P2 strongly decreases CD values,
which is related to the formation of nonhelical phases,
crystalline or SmA* structures (Figure 5b). Nevertheless, it is
important to stress that coexistence of at least two phases takes
place in the annealed films (crystalline and N* for polymer P1,
crystalline and SmC* for P2, and, probably N* and SmA* for
P3). This assumption comes up because CD values do not
become zero even after long annealing time (Figures 5b,6).
The highest CD values, of both positive and negative signs,
are found for polymer P3, whereas for polymers P1 and P2 the
CD signal is weaker (Figure 5a). Such difference could be
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Figure 8. (a) Polarized absorbance spectra before and after 220 s of polarized light irradiation (457 nm, ∼0.5 W/cm2) of amorphousized film of the
polymer P1. Spectra were measured for direction of polarized light along and perpendicular to preferred chromophores orientation. (b) Polar
diagram of polarized absorbance before and after 220 s of irradiation.
Figure 6 shows temperature dependencies of maximal values
of CD (Figure 6a−c) for the studied polymers. Polymers P1
and P3 demonstrate an increase of CD values under heating
followed by decrease at approaching to the isotropic phase
(Figure 6a,c). For polymer P2 temperature dependence of CD
value is more complicated (Figure 6b). In all the cases increase
of CD maximum could be related to elements of cholesteric
order formation of helically twisted mesophases, N* for
polymer P1, and SmC*, and N* for polymer P2. Surprisingly,
the same feature is found for polymer P3 exhibiting only the
SmA* phase. Probably, some elements of helical order are
formed at temperatures between ∼55 and 75 °C which cannot
be detected by conventional polarizing optical microscopy
Figure 9. Kinetic curves of dichroism growth during polarized light
irradiation of polymers P1, P2, and P3 (laser 457 nm, ∼0.5 W/cm2).
method (Figure 2).
The complicated character of the temperature dependencies
of CD values for polymers P1 and P2 can be explained by
nonlinear relation between induced CD values and helix pitch
as was shown earlier experimentally and theoretically for
cholesteric low-molar-mass and polymer systems.47,48
UV irradiation leads to E−Z isomerization of chromophores
(see absorbance spectra changes in Supporting Information,
Figure S5). This process is accompanied by a significant
decrease in CD values for all polymers (Figure 7), which is
explained by bent-shaped form of Z-isomer disrupting elements
of helical order. For polymer P2 the CD peak almost disappears
(Figure 7b), whereas for polymer P3 the CD values even
change their sign in some wavelength regions (Figure 7c).
Subsequent visible light irradiation changes the ratio between
E and Z isomers and increases the concentration of the E-form,
inducing partial recovery of CD values (Figure 7).
higher for the polarization direction along chromophore
orientation (perpendicular to the polarization plane of the
excitation laser light). This process is caused by cycles of E−Z−
E isomerization, rotational chromophores diffusion eventually
leading to their alignment in direction perpendicular to the
polarization plane of the excitation light.1,17
Analyzing the polarized absorbance spectra the dichroism
values, D, of the polymers films were calculated from the
spectra by use of eq 1.
D = (A − A⊥)/(A + A⊥)
(1)
where A|| is the absorbance of light polarized along the
preferred chromophore orientation direction and A⊥ is the
absorbance perpendicular to this direction.
Thus, the amorphousized spin-coated films of the synthe-
sized polymers are characterized by the Cotton effect with high
values of induced CD values, which can be manipulated by the
thermal treatment and UV and visible light irradiation. The
observed peculiarities allow one to conclude that CD
spectroscopy is a powerful tool for detection the chiral
supramolecular structure in such polymer systems providing
the possibility to detect even traces of helical superstructure and
two phase coexistence.
Comparison of kinetics of dichroism growth revealed that
process rate and values of maximal achievable dichroism are
much higher for polymer P1 (Figure 9). For polymer P2 having
the same chromophore but a different position of the −NN−
bond in the aromatic core, the dichroism growth occurs more
slowly and its values is about twice lower than that for polymer
P1. Such an extreme difference could be explained only by
variation in the position of the −NN− bond in the
photochromic fragment.
Photoorientation Processes in Spin-Coated Polymers
Films. Irradiation of the studied polymers with polarized light
of blue laser (457 nm) induces photoorientation process and
appearance of a large linear dichroism in spin-coated polymer
films (Figures 8 and 9). Polarized light absorbance is much
The scheme in Figure 10 illustrates a possible reason in the
difference of the photoorientation process efficiency. As
mentioned above photoorientation takes place due to the
cycles of E−Z−E isomerization and therefore the shape of the
Z-isomer plays a key role in this process. As clearly seen in
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Figure 11. Polarizing optical microphoto showing spin-coated film of
the polymer P1 irradiated through the mask during 10 s (457 nm);
scale bar corresponds to 100 μm.
liquid-crystalline azobenzene-containing side-chain polyacry-
lates was performed. Considering the obtained experimental
results several important findings can be formulated. (i)
Presence of the lateral chlorine substituent in mesogen
fragment of the side group leads to an extreme decrease of
LC-transition temperatures and complete disappearance of
cholesteric mesophase. (ii) Even the amorphousized spin-
coated polymer films possess the noticeable values of circular
dichroism (in the spectral range corresponding to the
azobenzene chromophores absorbance) caused by a formation
of helical order elements. (iii) The possibilities of manipulation
of CD values and degree of helical order by light action or
thermal treatment were demonstrated. (iv) It was shown that
irradiation of polymer films by polarized light causes photo-
orientation process accompanied by an appearance of
significant linear dichroism.
Figure 10. Comparison of polymers P1, P2, and P3 side-chain
idealized models (in Z-form). Length and “width” of side groups as
well as anisometry ratio is also depicted.
It should be pointed out that the observed photo-optical
phenomena are of considerable interest for possible application
of chiral-photochromic polyacrylates in photonics and opto-
electronics. In particular, the synthesized polymers could be
used as photoactive media for optical data storage with the
possibility of nondestructive data reading. Optical recording
could be realized by photoinduced local change of birefringence
(in the case of polarized light action, see Figure 10) or optical
activity modulation (by UV-light action). In the both cases,
reading of recorded information could be performed by light
with wavelength outside the azobenzene chromophores
absorbance band ensuring the absence of erasing during
reading.
Figure 10, anisometry of Z-form of side group in polymer P1 is
lower than that of polymers P2 and P3. The spacers consisting
of methylene units have higher flexibility in comparison with
the mesogenic core, but connection to the polymer backbone
significantly diminishes their mobility. Thus, Z-isomeric side
groups in polymer P1 having lower anisometry have stronger
influence on their polymer environment providing more free
volume for the rotational diffusion after back Z−E isomer-
ization and resulting in more effective reorientation in
comparison with polymers P2 and P3.
Introduction of lateral substituent results in further decrease
of rate of dichroism growth (polymer P3) due to disruption of
photoinduced orientational order by decrease of the
chromophores anisometry (in E-form).
Annealing at temperatures higher than the glass transition
temperature of all polymers leads to a complete dichroism
disappearance due to homeotropic alignment and formation of
polydomain structures of ordered phases as discussed in
previous sections.
It is noteworthy that induction of dichroism is accompanied
by the appearance of photoinduced birefringence, which can be
used for the photopatterning (Figure 11). The recorded image
is stable during at least several months at room temperature.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures showing transmittance and absorbance spectra of
polymers and molecular models of polymers side chains. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: (A.B.) bbrvsky@yahoo.com.
CONCLUSIONS
The study of phase behavior, chiro-optical properties, and
photoorientation processes in three novel chiral-photochromic
■
Notes
The authors declare no competing financial interest.
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Macromolecules
Article
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ACKNOWLEDGMENTS
■
This research was supported by the Russian Foundation of
Fundamental Research (11-03-01046a, 12-03-00553-a, 12-03-
31115, and 13-03-00648) and by a grant from the Czech
Science Foundation CSF 13-14133S and RMES 8815.
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