Photochromic LC-polymer composites containing azobenzene chromophores with thermally stable Z-isomers

Article abstract of DOI:10.1039/c4tc00015c

Novel types of photochromic liquid crystalline (LC) polymer-based composites containing chiral photochromic azobenzene moieties were elaborated. For this purpose, a new chiral azobenzene-containing methacrylic monomer was synthesized. On one of the benzen

Full text of DOI:10.1039/c4tc00015c

Journal of
Materials Chemistry C
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PAPER
Photochromic LC–polymer composites containing
azobenzene chromophores with thermally stable
Z-isomers†
Cite this: J. Mater. Chem. C, 2014, 2,
4482
b
c
Alexey Bobrovsky, a Valery Shibaev,a Martin Cigl,bc Vera Hamplova, Frantisek Hampl
*
ꢀ
´
ˇ
and Galina Elyashevitchd
Novel types of photochromic liquid crystalline (LC) polymer-based composites containing chiral
photochromic azobenzene moieties were elaborated. For this purpose, a new chiral azobenzene-
containing methacrylic monomer was synthesized. On one of the benzene rings of the azobenzene
chromophore, there are methyl substituents in both positions ortho to the azo group. This substitution
significantly increases the thermal stability of the photoinduced Z-form. The first type of novel
photosensitive composite is a glass cell filled with cholesteric polymer-stabilized layers produced by
thermal polymerization of
a mixture containing a synthesized chiral photochromic azobenzene
monomer, a nematic mixture of cyclohexane derivatives, mesogenic diacrylate and a thermal initiator.
UV-irradiation of such samples leads to E–Z isomerization of the photochromic groups of the
azobenzene monomer units, accompanied by a shift of the selective light reflection peak to a long
wavelength spectral region. This process is thermally and photochemically reversible; the kinetics of the
selective light reflection shift was studied. The second type of novel LC composite was obtained by the
introduction of the same cholesteric photochromic mixture into porous stretched polyethylene (PE)
films. Due to the highly anisotropic porous structure of PE, a uniaxially aligned nematic phase was
obtained inside the pores. Irradiation with UV and visible light provides the possibility of dichroism and
birefringence photocontrol in the obtained LC composite films. The possibility of photo-optical image
recording on the prepared composite was demonstrated. It is noteworthy that the images recorded by
the novel composite types have extremely high thermal stability (weeks) in comparison with those based
on other azobenzene derivatives.
Received 3rd January 2014
Accepted 5th March 2014
DOI: 10.1039/c4tc00015c
www.rsc.org/MaterialsC
systems may either be formed as mixtures of LCs with azo-
benzene derivatives, or an azobenzene moiety may be covalently
bound in mesogens, thus forming photochromic materials.1–16
The combination of LC order with photochromism provides the
possibility of obtaining unique materials that are highly sensi-
tive to light action, which induces reversible E–Z isomerization
and signicant structural changes in these systems.
One of the interesting and promising phenomena observed
in such systems is so-called photoinduced isothermal phase
transition, resulting from E–Z photoisomerization of the azo-
benzene groups and formation of bent Z-isomers.1–16 The low
anisometry of the Z-form induces a phase transition from an
ordered to a less ordered mesophase, or even complete dis-
ordering (isotropization) of the sample. As a result of light
action, signicant and reversible changes in the optical prop-
erties, such as birefringence, dichroism and diffraction effi-
ciency (for photosensitive diffractive gratings), occur.12
1 Introduction
The design and investigation of “smart” photosensitive and
eld-responsive materials is a rapidly growing eld in modern
materials science. Among organic photosensitive materials,
photoisomerizable azobenzene-based substances possess
remarkable photochromic properties, providing a large variety
of photoinduced phenomena, including photo-optical activity,
photomechanical actuation, photoregulated wetting–dewetting
effects, etc.1–4 Liquid crystalline (LC) systems possessing
photochromic azobenzene moieties are of special interest. Such
aFaculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119991,
Russia. E-mail: bbrvsky@yahoo.com
bInstitute of Physics, Academy of Sciences of the Czech Republic, 182 21 Prague 8,
Czech Republic
cDepartment of Organic Chemistry, Institute of Chemical Technology, 166 28 Prague 6,
Czech Republic
dInstitute of Macromolecular Compounds, St. Petersburg, 199004, Russia
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc00015c
Numerous research groups have focused much attention on
the design of new chiral photochromic dopants, including
those containing azobenzene groups capable of E–Z photo-
isomerization followed by changes in the helical twisting
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Table 1 Components of the polymerizable photochromic LC mixture
Name
Chemical structure
Content (wt%)
55.0
Nematic mixture
MLC6816
(Merck)
n ¼ 2, 3, 4, 7
Nematic
diacrylate
RM257
4.2
Azobenzene-
containing
monomer
MDATL
39.2
1.6
Thermoinitiator
AIBN
power.2–26 Due to the fact that such changes are usually associ- porous PE lms of low molar mass components from the irra-
ated with the decrease in the anisometry of azobenzene, the diated zone to the non-irradiated zones and vice versa, which
introduction of these substances into nematic matrices allows also leads to instability and fast erasing of the photorecorded
the possibility of the creation of cholesteric LC materials with images.
phototunable selective light reection band positions.
In the present paper, successful ways of overcoming of the
In our previous papers, we described new types of photo- above-mentioned drawbacks are presented. Firstly, we have
chromic and uorescent materials based on stretched aligned synthesized a new chiral azobenzene-containing compound
porous polyethylene (PE) lms.27–32 Our approach for the crea- that exhibits an extremely thermally stable Z-form (monomer
tion of these LC composites is based on embedding photo- MDATL). Secondly, we have introduced the polymerizable dia-
chromic or uorescent LC mixtures in a highly porous matrix of crylate RM257 into the LC mixture. Copolymerization of RM257
PE. The highly anisotropic oriented structure of the porous PE– with monomer MDATL allows one to create a polymer network,
LC molecules, together with the fact that functional dopant stabilizing the LC composite and preventing component diffu-
molecules are well-aligned along the stretching direction of the sion. The components of the prepared photochromic mixture
PE lms, provides the possibility of creating exible, birefrin- are listed in Table 1.
gent, dichroic, and uorescent materials. Addition of photo-
For the nematic matrix, we selected the mixture of cyclo-
chromic moieties, such as diarylethenes, spiropyranes, and hexane derivatives MLC6816 (Merck) due to their high trans-
especially azobenzenes allowed us to obtain photosensitive and parency in the UV range,33 diminishing light absorbance and
photomechanical-mobile lms based on PE.27–32
increasing the depth of light penetration through the photo-
In our previous paper,29 we demonstrated the possibility of chromic LC samples. Moreover, this mixture has a refractive
photomanipulation of the dichroism and birefringence of LC– index similar to that of PE, which reduces light scattering in the
PE composites, owing to E–Z isomerization of the azobenzene LC–PE composites, as was shown by us previously.30
dopant, embedded in the pores of PE together with the LC
Monomer MDATL contains two methyl substituents on one
mixture. UV light irradiation leads to conversion of the E-form of the benzene rings of the azobenzene fragment (in positions
of the azobenzene moiety, which has a rigid rod-like shape, to ortho to the azo group), providing high thermostability of the
the bent Z-isomer, disrupting the LC order. Despite signicant photoinduced Z-form, and a chiral terminal group that induces
photoinduced changes in the dichroism and birefringence, the formation of the cholesteric phase, which is of special interest
obtained photochromic LC composites have noticeable draw- due to the possibility of helix pitch tuning and selective light
backs associated with the thermal reversibility of the E–Z reection on UV irradiation.
isomerization process, which results in fast recovery of the
In order to avoid competing photoinduced isomerization
initial birefringence and, thus, causes the photo-optically processes during polymerization, we used the thermoinitiator
recorded images to be rapidly erased.
AIBN instead of other widely used photointiators.9,34
Another important disadvantage of the LC composites
The rst part of the paper describes the photo-optical
described previously27,29,30 is the interdiffusion inside the properties of the planar-oriented polymer-stabilized cholesteric
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mixture in glass cells and focuses on studying the photoinduced formylbenzoic acid, 6. In the rst step, acid 6 was esteried with
changes in the helix pitch values and the position of the selec- (S)-hexyl lactate using a mild method (DCC-coupling) to avoid
tive light reection band. The second part is devoted to the racemization. In the second step, the formyl group of ester 7 was
study of PE-based composites and isothermal photoinduced oxidized to a carboxylic group by the action of potassium
phase transitions, leading to decreases in the birefringence and permanganate in pyridine. In the nal step of the synthesis, acid
dichroism.
8 and azo-phenol 5 were reacted via DCC-coupling to yield
monomer MDATL. A detailed description of the synthesis is pre-
sented in the ESI.† Phase transitions of monomer: N* 36–37 C
I (LC phase at room temperature; crystallization does not take
place).
ꢀ
2 Experimental
Materials
All substrates and reagents for the synthesis of monomer MDATL
were purchased from either Sigma-Aldrich or Acros Organics. The
nematic mixture MLC6816 and diacrylate RM257 (Merck), as well
as initiator AIBN (Aldrich), were used as received.
LC composite lm preparation
The rst type of composite lms was preparedꢀ by thermal
polymerization of the mixture (see Table 1) at 60 C in a glass
cell with a 10 mm gap, whereas the second type was based on
porous PE lms with thicknesses of ꢁ14 mm.
Synthesis of monomer MDATL
MDATL monomer was synthesized according to the synthetic
In the case of glass cells for the preparation of uniaxially
route shown in Scheme 1. The synthesis of the photosensitive aligned LC layers, a photoalignment method was used.35,36 For
part of the monomer started from p-acetamidophenol (1), which this purpose, glass substrates spin-coated with a solution of the
was alkylated with 1,10-dibromodecane, and subsequently, the sodium salt of poly[1-[4-(3-carboxy-4-hydroxy-phenylazo)benze-
N-acetyl group was cle by acid hydrolysis in dilute sulfuric nesulfonamido]-1,2-ethanediyl] (PAzo) (Aldrich) in chloroform
acid. The thus obtained hydrogensulfate, 3, was diazotized in (2 mg mLꢂ1) were used.36 Before the cell preparation, the
acetic acid and reacted with 3,5-dimethylphenol, following the substrates were irradiated with polarized polychromatic light
standard azo-coupling procedure to yield azocompound 4. A from a mercury lamp (DRSh-350, 20 min, ꢁ15 mW cmꢂ2). The
methacrylate moiety was introduced to the side-chain of the thickness of the PAzo layers was ca. 4 nm to ensure their low
molecule by treating phenol 4 with an excess of potassium absorbance. Cells were lled with the polymerizable mixture at
methacrylate in DMSO to obtain azo-phenol 5. Acid 8, with a room temperature, followed by annealing at 60 ꢀC for 3 days (in
chiral side chain, was prepared in two steps starting from 4- order to ensure completeness of polymerization).
Scheme 1 Synthesis of monomer MDATL.
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The second type of LC composite was prepared using
Journal of Materials Chemistry C
stretched porous PE as a polymer matrix. Microporous PE lms
were obtained from commercially available low density PE (Mw
ꢁ 1.4 ꢃ 105, Mw/Mn ꢁ 6–8, Tm ꢁ 132 C), as described previ-
ꢀ
ously.37 During the extrusion and stretching processes, the
polymer lms were deformed and oriented, stimulating the
formation of a porous structure with pore sizes of about 50–
500 nm. Pore size distributions were measured by the ltration
porometry method described elsewhere.37
Phase behaviour and selective light reection study
The polarizing optical microscopy investigations were per-
formed using a LOMO P-112 polarizing microscope equipped
with a Mettler TA-400 heating stage. Differential scanning
calorimetry (DSC) was performed on a Perkin Elmer DSC-7
thermal analyzer (at a scanning rate of 10 K minꢂ1).
Fig. 1 Absorbance spectra of the mixture in planar-aligned glass cells
before and after thermal polymerization (3 days at 60 ꢀC).
For the selective light reection study, absorbance spectra
were recorded using a Unicam UV-500 UV-vis spectrophotometer.
Photo-optical investigations
Photochemical investigations were performed using an optical
setup equipped with a DRSh-350 ultra-high pressure mercury
lamp and an MBL-N-457 diode laser (457 nm, CNI Laser). To
prevent heating of the samples due to the IR radiation of the
mercury lamp, a water lter was introduced into the optical
scheme. A quartz lens was applied to ensure a plane-parallel
light beam. Using the lters, light with wavelengths of 365 nm
and 436 nm was selected. The intensity of the light was
measured using a LaserMate-Q (Coherent) intensity meter.
Non-polarized absorbance spectra were measured using a
Fig. 2 Shift of the selective light reflection peak during UV irradiation
Unicam UV-500 UV-vis spectrophotometer. For polarized UV- (365 nm, ꢁ2 mW cmꢂ2) of the planar-oriented polymer-stabilized
mixture in a glass cell. Spectra were recorded every 20 s during irra-
diation. The dashed line shows the selective light reflection peak after
20 min of UV irradiation.
visible spectroscopy, the angular dependence (with a step-width
of 10ꢀ) of the polarized light absorbance was measured using a
TIDAS photodiode array UV-visible spectrometer (J & M)
equipped with
a rotating polarizer (Glan–Taylor prism
controlled by a computer).
selective light reection peak to shorter wavelength (lmax ꢁ 610
nm, Fig. 1). Most probably, formation of a polymer network
induces shrinking during the course of polymerization,
implying a decrease in the period of the helical supramolecular
structure.
3 Results and discussion
Photo-optical properties of the polymer-stabilized cholesteric
mixture in glass cells
First of all, let us consider the photo-optical properties of the
Irradiation of the cells by UV light (365 nm) leads to E–Z
planar-oriented polymer-stabilized cholesteric mixture in glass isomerization of the chiral azobenzene chromophores, which
cells. The prepared photochromic mixture does not show any decreases their anisometry (Fig. 3) and, consequently, also their
phase separation and forms a cholesteric mesophase at room helical twisting power (bE [ bZ). As a result, a fast shi of the
temperature; the clearing temperature for the mixture (without selective light reection peak to a long wavelength spectral
AIBN) is 63–65 ꢀC. The mixture exhibits selective light reection range (near-IR) occurs (Fig. 2). It is important to stress that the
in the visible spectral range, with a maximum at ꢁ650 nm photoinduced Z-form of the chiral-photochromic units is rela-
(Fig. 1). Using these data, the helical twisting power of the new tively stable at room temperature in the dark, i.e. thermal back
chiral photochromic monomer MDATL was calculated, and it is conversion into the E-form and helix twisting occurs rather
equal to ca. 6 mmꢂ1
.
slowly (Fig. 4). Moreover, our investigation of the thermal Z–E
Thermal polymerization of the mixture in glass cells was relaxation revealed rather unusual phenomena: instead of a
carried out in order to obtain planar anchoring (see Experi- gradual shi of the selective light reection peak back to the
mental section for details), affording stable planar-oriented short wavelength spectral region, we observed the appearance
lms with selective light reection in the orange–red spectral and growth of the short wavelength peak during annealing of
range (Fig. 2). The polymerization process leads to a shi of the the samples at 30 ꢀC (Fig. 4 and S1†). At the same time, a
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demonstrating that the rate of the photoinduced changes is
fast, even at moderate light intensities (ꢁ2 mW cmꢂ2).
Photo-optical properties of the LC–PE composite
The second type of LC composite was obtained by thermal
polymerization of the above-mentioned mixture (Table 1) inside
pores of the stretched PE lms (see Experimental part for the
details). Using this approach, stable, exible, and highly bire-
fringent lms were obtained. The LC composite lms display
noticeable linear dichroism in the visible spectral range (Fig. 6):
the absorbance of light polarized along the stretching direction
of the lms is higher than that of light polarized along the
perpendicular direction. Fig. 6b shows polar diagrams of the
polarized absorbance at 480 nm that corresponds to the n–p*
electronic transition range of azobenzene chromophores
(absorbance at l < 400 nm is too high to be adequately
measured). These results indicate a signicant degree of coop-
erative uniaxial orientation of chromophores and mesogens
that could be explained by complete untwisting of the chole-
steric helix due to the intrinsic anisometry of the porous PE
structure (Fig. 7). We observed a similar phenomenon in PE-
based composites in cholesteric LC mixtures with helix pitch,
Fig. 3 Molecular model of the MDATL side chains showing the
decrease in their anisometry due to E–Z photoisomerization.
Fig. 4 Changes in absorbance spectra of the UV-irradiated mixture
film (in glass cell) during thermal Z–E isomerization at 30 ꢀC. Spectra
were recorded every 20 min during relaxation.
decrease in the intensity of the long wavelength peak is
observed. The origin of this unexpected phenomenon is still
unclear, but, most likely, it is associated with the “memory
effect” provided by the polymer network.
On the other hand, action of visible blue light (436 nm) on
the UV-irradiated cells induces a fast gradual shi of the
selective light reection band back towards its initial position
(Fig. S2†). Fig. 5 shows the kinetic curves of the photoinduced
spectral shi of the selective light reection peak,
Fig. 6 (a) Polarized absorbance spectra of LC-PE composite film
before and after UV irradiation (40 min). (b) Corresponding polar plots
Fig. 5 Kinetics of the selective light reflection peak shift during UV and of polarized absorbance before, after UV irradiation and after subse-
visible light irradiation of the polymer-stabilized mixture.
quent visible light action (436 nm, 40 min).
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Fig. 7 SEM microphotograph showing the microporous structure of
the porous PE film (a), and schematic representation of the orientation
of the LC molecules along the PE film stretching direction and the
photoinduced phase transition initiated by E–Z isomerization of the
azobenzene fragments (b).
Fig. 9 Polarizing optical microscopy photos of the LC–PE composite
film after 20 min of UV irradiation through the mask (a), after 14 days at
room temperature (b), and after 15 min of visible light irradiation (436
nm) (c). The bar corresponds to 100 mm. The temperature of the
sample is ca. 22 ꢀC.
Fig. 8 Changes in the dichroism of the PE composite film: (a) under
UV and visible light irradiation; (b) under polarized visible light irradi-
ation (457 nm laser, ꢁ0.5 W cmꢂ2); laser polarization was parallel and
perpendicular to the LC director of the PE composite film.
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corresponding to selective light reection in the visible or IR reection for the rst type of composite, whereas, for the PE-
spectral ranges.27
based composite, photovariation of dichroism and birefrin-
UV irradiation of the composite lms signicantly decreases gence was demonstrated. It was also shown that the presence of
the birefringence and dichroism values (Fig. 6). As seen from two methyl substituents on the azobenzene core signicantly
Fig. 8a, the dichroism values drop almost to zero, whereas improves the thermal stability of the photoinduced Z-form,
subsequent visible light action partially recovers the initial which opens up new opportunities for the use of such materials
state. This effect is most likely associated with the photoin- in optics and photonics.
duced isothermal phase transition from the nematic to the
isotropic phase of the LC material embedded in the porous PE
structure (Fig. 7). This effect is explained by formation of the
Acknowledgements
bent Z-isomer of the azobenzene chromophore (Fig. 3 and 7) This research was supported by the Russian Foundation of
and disruption of the mesophase order. It is noteworthy that we Fundamental Research (12-03-00553-a, 13-03-00648, 13-03-
did not observe the same isothermal phase transition in the 12071, 13-03-12456 and 13-03-00219), the Czech Science Foun-
glass cells described in the previous section, even aer 3 hours dation (project no. P204/11/0723 and 13-14133S) and specic
of UV irradiation. Most probably, the porous structure of PE university research (MSMT no. 20/2013).
plays a crucial role, and not only induces untwisting of the
cholesteric helix but also partially decreases the degree of
mesophase order. Unfortunately, in the case of the LC
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