Synthesis and optical properties of poly[4-methacryloxy-(4′-carboxy)-azobenzene]

Article abstract of DOI:10.1080/15421406.2018.1551321

In the present work, the homopolymer built from the free radical polymerization of methacrylic monomer incorporating an azobenzene side-group has been synthesized and structurally characterized. The optical properties such as refractive index, extinction

Full text of DOI:10.1080/15421406.2018.1551321

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Synthesis and optical properties of poly[4-
methacryloxy-(4′-carboxy)-azobenzene]
M. Sypniewska, L. Skowronski, P. Popielarski, I. Savchenko, O. Krupka, V.
Smokal & B. Derkowska-Zielinska
To cite this article: M. Sypniewska, L. Skowronski, P. Popielarski, I. Savchenko, O. Krupka, V.
Smokal & B. Derkowska-Zielinska (2018) Synthesis and optical properties of poly[4-methacryloxy-
(4′-carboxy)-azobenzene], Molecular Crystals and Liquid Crystals, 672:1, 178-185, DOI:
10.1080/15421406.2018.1551321
To link to this article: https://doi.org/10.1080/15421406.2018.1551321
Published online: 13 Jun 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2018, VOL. 672, NO. 1, 178–185
https://doi.org/10.1080/15421406.2018.1551321
Synthesis and optical properties of poly[4-methacryloxy-(40-
carboxy)-azobenzene]
M. Sypniewskaa, L. Skowronskib, P. Popielarskic, I. Savchenkod, O. Krupkad,
V. Smokald, and B. Derkowska-Zielinskaa
aInstitute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University,
b
Torun, Poland; Institute of Mathematics and Physics, UTP University of Science and Technology,
c
d
Bydgoszcz, Poland; Institute of Physics, Kazimierz Wielki University, Bydgoszcz, Poland; Faculty of
Chemistry, Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
KEYWORDS
ABSTRACT
azo dyes; thin film;
absorption coefficient;
refractive index; extinction
coefficient
In the present work, the homopolymer built from the free radical
polymerization of methacrylic monomer incorporating an azoben-
zene side-group has been synthesized and structurally characterized.
The optical properties such as refractive index, extinction coefficient,
absorption coefficient and optical energy band gap of poly[4-metha-
cryloxy-(40-carboxy)-azobenzene] thin film prepared by high vacuum
sublimation method were determined using spectroscopic ellipsome-
try combined with transmittance measurements.
We found that in the spectrum of extinction coefficient there are
ꢀ
ꢀ
absorption bands, which are assigned as the n-p and p-p elec-
tronic transitions of the azo compound.
1. Introduction
Azo dyes are one of the most important class of chemical compounds. They consist of
two phenyl rings connected by an –N=N– azo group. This configuration causes a strong
absorption in the ultraviolet and visible regions. In addition, the spectral and photo-
physical properties of azobenzene are very sensitive to their substitution pattern and
various polymer systems [1–5]. Azo polymers can generally be classified as azo polymers
of the main chain and side chain. Depending on other functionalities of polymer back-
bones, azo aromatic groups may be present in polymers such as polyesters, polycarbon-
ates, polysilanes, polyamides, polycarbonates and polymethylmethacrylate [6].
In general, azobenzenes can be divided into three classes: the azobenzene-type mole-
cules, the aminoazobenzene-type and the pseudo-stilbenes. They are distinguished by a
spectral and photophysical response that is closely related to their substitution patterns.
The azobenzene-type molecules have the most prominent absorption features in the
UV. Aminoazobenzenes are substituted in the ortho- or para- position with the electron
donating group. They also exhibit strong absorption properties in the visible-blue
region. The pseudo-stilbenes have a strong and broad absorption feature throughout the
visible. They also have nonlinear optical properties, and often have the best photo-
switching response. Pseudo-stilbenes are characterized by a strongly asymmetric electron
CONTACT B. Derkowska-Zielinska
ß 2018 Taylor & Francis Group, LLC
beata@fizyka.umk.pl

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
179
distribution. It results from being substituted at the 4 and 40 positions with electron-
donating and electron-withdrawing groups [7,8].
One of the most important properties of azobenzene is its ability to exist in two
forms of isomeric: trans and cis. This compound also has the ability to reverse photoiso-
merization between thermally stable trans-state and metastable cis-state after photon
absorption [1–6,9]. The role of the excited singlet and triplet states in trans-cis isomer-
ization was studied extensively [10–12].
Azobenzene can be used in various optical applications such as liquid crystal displays
(LC), nonlinear optical devices, optical switching and information storage devi-
ces [9,13,14].
In this work, the optical properties of thin layer of azobenzene with a carboxyl group
in the para-position were determined using spectroscopic ellipsometry combined with
transmittance measurements.
2. Experimental
2.1. Material
Synthesis of 4-hydroxy-(40-carboxy)-azobenzene: 4-aminobenzoic acid (0.1 mol) was
dissolved in a solution of concentrated hydrochloric acid:water (50:50) (100 mL). The
mixture was cooled to 0 ꢁC in an ice-water bath, and then sodium nitrite (0.1 mol) dis-
solved in a small amount of water added dropwise. The reaction mixture was stirred for
2 h, and then phenol (0.1 mol) in NaOH solution (100 mL) was added slowly. The
resultant solution was stirred for 4 h at 0-5 ꢁC and for an additional 10 h at room tem-
perature. The precipitate formed was collected by vacuum filtration and finally dried.
1
The product was purified by recrystallization from ethanol, yield: 87–92%. H NMR
(400 MHz, DMSO, d, ppm): 7.93-6.89 (3d, s, 7H, Ph-H), 11.4 (s, 1H, COOH), 10.19 (s,
1H, OH).
Synthesis of 4-methacryloxy-(40-carboxy)-azobenzene: 4-hydroxy-(40-carboxy)-azo-
benzene (0.01 mol) was dissolved in 25 mL anhydrous THF, and freshly distilled trie-
thylamine (1.53 mL, 0.011 mol) was added to the solution. The reaction mixture was
then cooled to 0-5 ꢁC. Methacryloyl chloride (1.07 mL, 0.011 mol) in THF (5 mL) was
injected gradually to the above solution via a glass syringe while the solution tempera-
ture was kept below 5 ꢁC. The reaction mixture was stirred overnight at room tempera-
ture. The resulting precipitate was filtered off. Purification was followed by
1
recrystallization from methanol, yield: 63-76%. H NMR (400 MHz, DMSO, d, ppm):
8.12-6.92 (3d, s, 7H, Ph-H), 11.3 (s, 1H, COOH), 6.28 (s, 1H C=CH2, cis), 5.82 (s, 1H
C ¼ CH2, trans), 2.06 (s, 3H, =C-CH3).
Polymerization. Poly[4-methacryloxy-(40-carboxy)-azobenzene] was synthesized by
free-radical polymerization in DMF. The polymerization was carried out in 10 wt.%
DMF solution of monomer with AIBN as free radical initiator (10 wt.% with respect of
monomer mass) at 80 ꢁC for more than 30 hours in thermostat. The polymerization
mixture was poured into methanol. The precipitate solid was filtered, dissolved in DMF,
and reprecipitated into methanol and then dried at 20 ꢁC overnight.

180
M. SYPNIEWSKA ET AL.
Figure 1. Schematic representation of the apparatus for the evaporation of thin layers: 1- substrate,
2- deposition thin film, 3- vaporized material, 4- target material, 5- evaporator, 6- heater, 7- vac-
uum pump.
2.2. Thin film
The polymer azobenzene layers have been prepared by high vacuum sublimation at
pressure about 2 ꢂ 10ꢃ6 Torr and temperature about 150 ꢁC. Poly[4-methacryloxy-(40-
carboxy)-azobenzene] powder was loaded into a quartz effusion cell with a nozzle of
10 mm in diameter. The n-type (001)Si and/or quartz substrates were located above
15 cm from the quartz effusion cell. The deposition rate was 0.2 nm/s [15–18]. Before
evaporation, the 1 cm ꢂ1 cm silicon and/or quartz substrates were degreased in acetone,
in a second step ethanol, and finally in deionized water using ultrasonic bath
for 30 minutes.
Thermal evaporation technique was used to produce a thin film of poly[4-methacry-
loxy-(40-carboxy)-azobenzene]. This technique is based on particle sources in the form
of Knudsen cells. Knudsen cells, generally utilize a containment crucible, which is sur-
rounded or exposed to a controllable heat source. The crucible is heated in a controlled
fashion utilizing a thermocouple sensor. The source material is thereby heated, melted
and evaporates from the open end of the crucible. The vapor deposition process consists
of three stages: obtaining pairs of the applied material, transport of vapors on the
source-substrate road, increase of the layer of adsorbed particles [17,19]. Figure 1
presents a schematic diagram of an apparatus for vapor deposition of thin films.
2.3. Measurements
Transmission measurements (T) were performed using spectrophotometer Cary 5000
(Agilent). Using the V-VASE device (J.A.Woollam Co., Inc.) the ellipsometric azimuths
(W and D) were measured for selected angles of incidence (i.e. 65ꢁ, 70ꢁ and 75ꢁ). All
optical measurements were performed in the range 1-5 eV at room temperature.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
181
Figure 2. Confocal microscopy image of poly[4-methacryloxy-(4’-carboxy)-azobenzene] thin film.
Using the built four-medium optical model (substrate/azo layer/rough layer/ambient),
the optical properties such as absorption coefficient (a), refractive index (n) and extinc-
tion coefficient (k) of poly[4-methacryloxy-(40-carboxy)-azobenzene] thin film were
determined. These optical parameters were defined using Gauss-shape and Sellmeier-
type dispersion relations in the absorption and transparent regions, respectively
[9,13,18–23]. The optical constants of the rough layer and its thickness were described
according to the Bruggeman Effective Medium Approximation (BEMA) model [18–21].
Taking into account the ellipsometric azimuths and transmission spectra, the model fits
of studied azobenzene were carried out [9,13,20,22].
The surface of poly[4-methacryloxy-(40-carboxy)-azobenzene] thin film was examined
using the confocal optical microscope (Lext OLS 4000, Olympus). The size of imaging
scan was 37 ꢂ 37 lm2.
3. Results and discussion
Figure 2 presents the surface topography of poly[4-methacryloxy-(40-carboxy)-azoben-
zene] thin film. One can see that this area is not very smooth. Therefore, the rough
layer cannot be neglected and should be taken into account in determining the optical
parameters measured by spectroscopic ellipsometry (SE) method. The thickness of
poly[4-methacryloxy-(40-carboxy)-azobenzene] dense layer is estimated at 133 nm 1 nm
and rough layer is 94 2 nm.
Figure 3 presents the transmission spectrum of poly[4-methacryloxy-(40-carboxy)-azo-
benzene] thin film deposited on quartz substrate using thermal evaporation technique.
For studied poly[4-methacryloxy-(40-carboxy)-azobenzene] thin film we can see one
ꢀ
absorption band in the range of 3-4.5 eV. This band is defined as p-p electronic transi-
tions due to the strong internal charge transfer (ICT) character of azo dyes in
side chain.
The inset of Figure 3 presents molecular structures of poly[4-methacryloxy-(40-car-
boxy)-azobenzene]. This structure consists of two benzene rings connected by an N=N

182
M. SYPNIEWSKA ET AL.
Figure 3. Transmission spectrum of poly[4-methacryloxy-(4’-carboxy)-azobenzene] thin film. Inset:
Molecular structures of poly[4-methacryloxy-(4’-carboxy)-azobenzene].
Figure 4. Refractive index and extinction coefficient of poly[4-methacryloxy-(4’-carboxy)-azobenzene]
thin film.
azo group. On one side, polymethacrylate polymer was attached, on the other a carboxyl
group (COOH) was placed in the para- position.
Figure 4 presents refractive index (n) and extinction coefficient (k) of poly[4-metha-
cryloxy-(40-carboxy)-azobenzene] thin film, which were deposited on Si substrate using
thermal evaporation technique, determined from spectroscopic ellipsometry (SE)
method. It is well known that refractive index is the basic property of the material used
in optoelectronic devices. The anomalous dispersion behavior of refractive index is vis-
ible in the range of 3-5 eV. This dependence is directly associated with shape of the k
spectra by the Kramers-Kronig relation. Whereas, the normal dispersion of the n is
observed from 1 eV to 3 eV. In comparison with well-known Disperse Red 1 (DR1)
compound, refractive index of poly[4-methacryloxy-(40-carboxy)-azobenzene] thin film
is high and varies from 1.84 to 1.62 in the normal dispersion regime, whereas for DR1
it changes from 1.58 to 1.50. One can also notice that the dispersion regime for poly[4-

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
183
Figure 5. Absorption coefficient of poly[4-methacryloxy-(4’-carboxy)-azobenzene] thin film.
methacryloxy-(40-carboxy)-azobenzene] thin film is shifted to blue compared to
DR1 [20,22].
The spectrum of extinction coefficient (k) presents a broad absorption band in the
range of 2-5 eV. This characteristic is consistent with the data obtained from transmis-
sion measurement (see Figure 3). However, in the k spectrum there are additional
absorption bands at energies of about 2.4 eV = 517 nm (E1), 3.5 eV =354 nm (E2) and
4.1 eV = 302 nm (E3). These additional bands are visible in the k spectrum because of
ꢀ
ꢀ
the measurement method (i.e. SE) and they are assigned as the n-p (E1) and p-p
(E2þE3) electronic transitions of the azo compound [13]. In comparison with DR1,
poly[4-methacryloxy-(40-carboxy)-azobenzene] has wider absorption band, which is
shifted more in the blue direction [20,22].
Figure 5 presents the absorption coefficient of poly[4-methacryloxy-(40-carboxy)-azo-
benzene] thin film, which is related to the extinction coefficient (k) by the following for-
mula: a ¼ 4pk=k, where k is the wavelength. Therefore, the similar shape of additional
bands is visible in both the k and a spectra.
Optical energy band gap (Eogpt) is one of the most important factors determining
the use of material in optoelectronic applications. We can determine Eogpt by plotting
1=m
the function ðahꢀÞ
versus photon energy (hꢀ) (see Figure 6) using the following
Tauc [9,13,21–24] equation:
ꢀ
ꢁ
m
opt
ð
Þ
ahꢀ ¼ B hꢀ ꢃ Eg
where a is the absorption coefficient (cmꢃ1), hꢀ is the photon energy (eV), B is the
band tailing parameter, and m ¼ 1/2 for a direct allowed transition, m ¼ 3/2 for a direct
forbidden transition, m ¼ 2 for an indirect allowed transition and m ¼ 3 for an indirect
forbidden transition.
This relationship assumes that the densities of the electron states in the valence and
conduction bands, near the band gap, have a parabolic distribution and, also, that the
matrix elements for the interband transitions associated with the photon absorption are
equal for all the transitions [23,25].

184
M. SYPNIEWSKA ET AL.
Figure 6. Tauc plot of poly[4-methacryloxy-(4’-carboxy)-azobenzene] thin film.
Figure 6 shows Tauc plot used to determine the optical energy band gap of poly[4-
methacryloxy-(40-carboxy)-azobenzene]. By fitting a straight line to the obtained data,
the Egopt was determined and is equal 2.17 0.02eV.
Conclusions
The homopolymer poly[4-methacryloxy-(40-carboxy)-azobenzene] containing azoester in
the side chain was synthesized by thermoinitiated free-radical polymerization process
and characterized. Refractive index, extinction and absorption coefficients, as well as
optical energy band gap of azo dye thin film prepared by high vacuum sublimation
method were determined using spectroscopic ellipsometry combined with transmittance
measurements.
We found that transmission spectrum shows wide absorption band in the range of 3-
ꢀ
4.5 eV, which is defined as p-p electronic transitions due to the strong internal charge
transfer (ICT) character of azo dyes in side chain. Whereas, the extinction coefficient
spectrum presents three absorption bands (due to different measurement method i.e.
ꢀ
SE) at energies of about 2.4 eV, 3.5 eV and 4.1 eV, which are assigned as the n-p and
p-p electronic transitions of the azo compound, respectively. In the normal dispersion
ꢀ
regime refractive index varies from 1.84 to 1.62. Optical energy band gap was deter-
mined using the Tauc method and estimated as 2.17 0.02eV. Therefore, poly[4-metha-
cryloxy-(40-carboxy)-azobenzene] can be proposed as a material for optical data storage,
holography and photo-switching of optical elements.
Acknowledgements
Financial support to instrumentation was obtained from European Regional Development Fund
(Stage 2 of the Regional Centre of Innovativeness) and Polish Ministry of Science and
Higher Education.
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