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10Жидкие кристаллы и их практическое использование. 2022. Т. 22, № 4. С. 63-72 Liquid Crystals and their Application. 2022. Vol. 22, No. 4. P. 63-72 ISSN 1991-3966 (print), 2499-9644 (online), Journal homepage: http://nano.ivanovo.ac.ru/journal/ru/
Original Article
PHOTO-INDUCED ANISOTROPY OF METHYL RED AZO DYE IN POLYMER FILM
Vladimir M. Kozenkov1, Viktor V. Belyaev12, Denis N. Chausov34*
'Moscow Region State University «MRSU», Mytishi, Russia 2RUDN University (People's Friendship University of Russia), Moscow, Russia 3Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia 4Moscow University for Industry and Finance "Synergy", Moscow,Russia
A R TIC L E IN FO :
A B S T R A CT
Article history: Received 22 August 2022 Approved 20 September 2022 Accepted 25 September 2022
Key words:
Methyl Red,
azo dye layers,
LC photo-aligning material,
photo-induced optical
anisotropy,
rotational diffusion
A photo-induced anisotropy (PIA) of Methyl Red (MR) azo dye embedded in PMMA polymer solid films was investigated by studying dichroic spectra and photo-induced birefringence. The reversible PIA of MR dye was measured in situ during photoactive irradiation process. Photoinduced order increases with the rise of exposure power. It is shown that PIA dark relaxation slows down at higher exposure energy. The MR rotational diffusion was evaluated at different exposure modes. These data are useful for the characterization of Methyl Red azo dye as a liquid crystal photo-aligning material.
DOI:
10.18083/LCAppl.2022.4.63
For citation:
Kozenkov V. M., Belyaev V. V., Chausov D. N. Photo-induced anisotropy of methyl red azo dye in polymer film. Liq. Cryst. and their Appl., 2022, 22 (4), 63-72.
Corresponding author: dn.chausov@mgou.ru © Kozenkov V. M., Belyaev V. V., Chausov D. N., 2022
Научная статья
УДК 538.9;535.012;536-1;773.93;773.74
ФОТОИНДУЦИРОВАННАЯ АНИЗОТРОПИЯ АЗОКРАСИТЕЛЯ МЕТИЛОВОГО КРАСНОГО В ПОЛИМЕРНОЙ ПЛЕНКЕ
Владимир Маркович Козенков1, Виктор Васильевич Беляев1,2, Денис Николаевич Чаусов
'Московский государственный областной университет (МГОУ), Мытищи, Россия 2Российский университет дружбы народов (РУДН), Москва, Россия 3Институт общей физики имени А. М. Прохорова РАН, Москва, Россия 4Московский финансово-промышленныйуниверсит ет «Синергия», Москва, Россия
3,4*
И Н Ф О Р М А Ц И Я
А Н Н О Т А Ц И Я
История статьи:
Поступила 22.08.2022 Одобрена 20.09.2022 Принята 25.09.2022
Ключевые слова:
метиловый красный
краситель,
слои азокрасителя,
ЖК-фотоориентирующий
материал,
фотоиндуцированная оптическая анизотропия, вращательная диффузия
Методом измерения дихроичных спектров и фотоиндуцированного двулучепреломления проведено исследование фотоиндуцированной анизотропии (ФА) метилового красного (МК) красителя, введенного в твердые пленки полимера полиметиметакрилат (ПММА). Обратимую ФА образца МК измеряли во время процесса фотоактивного облучения. Фотоиндуцированный порядок увеличивается с увеличением мощности экспозиции. Показано замедление темновой релаксации ФА при более высоких воздействиях энергии облучения. Выполнена оценка вращательной диффузии МК при различных режимах воздействия. Эти данные важны для характеристики азокрасителя метилового красного в качестве материала для фотоориентации жидких кристаллов и современных жидкокристаллических системах отображения информации.
DOI:
10.18083/LCAppl.2022.4.63
Для цитирования:
Козенков В. М., Беляев В. В., Чаусов Д. Н. Фотоиндуцированная анизотропия азокрасителя метилового красного в полимерной пленке // Жидк. крист. и их практич. использ. 2022. Т. 22, № 4. С. 63-72.
*Адрес для переписки: dn.chausov@mgou.ru © Козенков В. М., Беляев В. В., Чаусов Д. Н., 2022
Introduction
Photo-induced optical anisotropy (PIA) in organic dyes' films became an important effect for many applications in optical engineering. One of azo dyes (Methyl Red or MR) is a model object in investigations of PIA kinetics as a medium for optical information recording and processing as well as a material for liquid crystals (LC) photoalignment. The PIA in Methyl Red azo dye embedded into a polymer matrix (poly(vinyl alcohol), poly(methyl methacrylate) (PMMA) or poly (vinyl acetate)) [1-3] was used in experiments for optical data processing [4-6] and anisotropic planar waveguides [7, 8]. This azo dye was used for the forced Rayleigh scattering (FRS) study of probe diffusion in polymer solutions [9]. The FRS method is helpful to determine the electric field dependence of thermal and mass diffusion [10-12] and electrohydrodynamics of MR dye molecules in liquid crystals [13]. A MR-doped LC was used as a highresolution recording holographic medium [14-16]. The reference [17] describes the method of holographic grating formation in MR-doped surface-stabilized liquid crystal film. Such gratings can be formed in typical nematic LC as well as in chiral mesophases, such as blue phases [18].
The MR molecules can be also used for ferroelectric LC photoalignment [19-21]. The photo-induced optical anisotropy of photoalignment layer strongly effects the performance of LC devices [22]. The MR dye can be also used in composite LC materials with different nanoparticles [23, 24]. The material dielectric anisotropy is strongly affected by doping ratio. The anisotropy value decreases with increasing dye concentration.
The paper presents the kinetics of reversible photo-induced birefringence of MR azo dye layers, embedded into PMMA polymer.
Experimental
Sample preparation
The azo-dye 2-[4-(dimethylamino)phenylazo]benzoic acid (Methyl Red, MR) (melting point 179-182 °C) was used in our experiments. The formula of MR azo dye is shown in Fig. 1. The MR free acid form is taken from Dainippon Ink & Chemicals Inc. In our
experiment we used the substance with carboxylic group in the orto-position.
Fig. 1. Methyl Red azo dye
In view of conformations, Methyl red can exist in one of three isomeric forms - ortho, meta, and para [25]. The orientation of LC on the films made of different MR isomers is described in [25]. There was an evidence of weak photoinduced adsorption of meta- and para-methyl red to the during photoexcitation. The role of the MR photoisomerization in the actual molecules' reorientation process is considered in [26]. The extraordinarily large nonlinear response is obtained that is accompanied by modification of the charge density on the irradiated surface.
A photosensitive film based on 5 wt. % of MR azo dye in PMMA was prepared by dissolving in dichlorethane (DCE) and then spin-coating on ITO-covered glass substrate at 3500 rpm during 40 s. The layer thickness was about 0.5 |im. The thick films of 15±0.5 |im were made by dip-coating from a solution of MR and PMMA dissolved in DCE. All films were cured for 40 min at T = 90 °C. As the illumination source, Ar+ laser (Xexp = 457 nm) was used.
The birefringence photo-induced by Ar+ laser was measured in situ within the region, where MR/PMMA films were transparent. He-Ne laser (A = 632.8 nm) was used in situ for recording purpose. As the birefringence of thin film photo-aligning layer is small, we constructed a special registration setup [27, 28]. It is based on photo-elastic modulator, allowing to measure small values of photoinduced birefringence with the accuracy of ±0.001 rad [27]. The photo-induced phase retardation was:
r = 2nXdAn, (1)
where A is the wavelength of probe light, d is layer thickness, 5 = dAn is the photo-induced birefringence of the azo dye layer. The experiment was performed at a temperature between 20 °C and 22 °C.
The samples demonstrated a good performance stability. Problems of photodegradation and biodegradation of MR are considered in [29-31].
Results and Discussion
PIA determined from dichroic spectra
A typical example of the optically induced dichroism of the absorption spectra of the thin MR/PMMA film before and after exposure to a linear polarized Ar+ laser (kexp = 457 nm) light with various power is shown in Fig. 2. According to [32], the order parameter can be estimated as
S =
(Al - DP
Dt + 2 D,
(2)
The order parameter was -0.007 and -0.032 for the illumination energy H = 0.24 J/cm2 and H = 3.84 J/cm2, respectively.
There is no significant shape difference between the five spectra (Fig. 2). Simultaneous decrease of the absorptions D± and D\\ under the influence of activating radiation testifies to the photochemical mechanism of photoinduced anisotropy. Indeed, it is well known that aromatic azo compounds exhibit «trans—-cis» photoisomerization [33].
Fig. 2. Dichroic spectra of o-MR dye, embedded into PMMA polymer for various exposure energy. The optical density (D)
without illumination is the curve (1). The curves (2) and (5) are the optical density D± and D for the illumination energy H = 0.24 J/cm2 and H = 3.84 J/cm2, respectively. The symbols ± and II indicate the polarization directions of the probe light perpendicular and parallel to the light polarization of induced illumination
However, the structure of MR azo dye is very similar to azobenzene except for carboxyl group in ortho position in one of the benzene rings (Fig. 1). This group is responsible for the shift of strong n-n* transitions into the visible region and formation of intramolecular #-bond with the nearest nitrogen atom of the azo group -N=N-. The intramolecular #-bond provides the photochemical stability of MR molecule and the quantum yields of «trans—-cis» photochemical reaction goes to zero.
We revealed a small bleaching of the MR/PMMA film after an hour exposure by the non-filtered radiation of the Hg lamp. This effect occurs practically without variation of the absorption spectrum in the photostationary state. Such deformation is not
observed in the case of reversible «trans—cis» photoisomerization. Moreover, we observed a very low-intensity band in the long-wave region of the spectrum (X ~ 560 nm), which is not typical for the photo-induced «cis» isomer of azo dyes [34]. We believe that the effect can take place due to the photo-induced ordering of MR molecules under the action of non-polarized light in such a way that their transition dipole moments become parallel to the direction of light propagation. Similar effect of the photo-induced molecular ordering was found in some photochemically stable azo dyes in polymer matrix [35], amorphous film of azo dyes formed by vacuum evaporation [36] or spin-coating [37] and Langmuir-Blodgett films [38].
PIA determined from photo-induced birefringence
Figure 3, a shows the dependence of photo-induced birefringence versus exposure energy, which increases with the exposure time and saturates at some stationary level, getting larger for the higher power of exposure light. This behavior complies with the model of photo-induced anisotropy in rigid molecular structures with rotational Brownian diffusion in the field of polarized light [39].
Dark relaxation of photo induced birefringence
When the activating illumination is switched off, the photo-induced birefringence gradually decreases, and approaches the nonzero steady state level in 10-20
minutes. The value of this level increases for the higher exposure power (Fig. 3, b). After that, a certain small optical anisotropy remains for a long time and can be erased by a non-polarized or circularly polarized light of second exposure or by thermal annealing.
The dark relaxation process in o-MR/PMMA layer can be explained by two simultaneous processes: dark cis isomerization of azo dye molecules [32] and their rotational Brownian diffusion [38]. The kinetics of dark relaxation in Fig. 3, b can be described using minimum two exponential functions (Table).
5 = S0 + Sx exp(-tXi) + S2 exp(-tXi). (3)
1.2
0.8
0.4
0.0
- ' :>' — —----' S.-' ________ _____ —~ ___ ^ 1 ^ _ , , __ nj ■ —>«♦" .....
i */ - - 0.2 mW/cm2
« ♦ - 0.5 mW/cm2
: • -1.0 mW/cm2
i ■ - 2.0 mW/cm2
50
100 150 Energy (mJ/cm2)
200
1.0
0.8
S 0.6
0.4
- 0.2 mW/cm2
- 0.5 mW/cm2
- 1.0 mW/cm2
- 2.0 mW/cm2
250
0.2
600
b
a
Fig. 3. The dependence of photo-induced birefringence of MR/PMMA layer on the power density Pexp. (a). The values of the power density are shown in Fig. 3, a. Relaxation the photo-induced phase retardation of MR/PMMA sample after switching off the illumination source with different powers, shown in the Fig. 3, b. The exposure energy was 600 mJ/cm2
Table. Normalized photo-induced birefringence values ô
Pexp, mW/cm2 So Si Ti S2 T2
2.0 0.238 0.390 17.2 0.332 178.0
1.0 0.287 0.338 17.9 0.364 161.6
0.5 0.320 0.299 21.0 0.374 196.2
0.2 0.375 0.263 30.3 0.345 147
The faster component of the relaxation time t decreases with the increase of power of illumination Pexp. From these results of kinetic characteristics it can be concluded that the photo-induced optical anisotropy in o-MR/PMMA layers takes place not only due to the photochromic processes of «trans—-cis» isomerization
in o-MR azo dye molecules. The small illumination power promotes the formation of o-MR molecular domains of large size, thus hindering the process of dark relaxation. When the power is sufficiently high, the diffusion involves small molecular domains and becomes faster.
This behavior is in agreement with the model of photo-induced anisotropy of rigid molecular structures with rotatory Brownian diffusion in the field of polarized light [38, 39]. Photoinduced birefringence arises is increasing with the increase of power density Pexp and under of the coefficient of rotation diffusion of the azo molecule D = const, where
D = kT/6m\a, (4)
and k is Boltzmann coefficient, T is absolute temperature, n is macroscopic viscosity of a film, a is the characteristic size of molecule or molecular cluster. The relation (4) states that the coefficient of rotation diffusion is independent on the light illumination power density and energy [38].
From (2), the nmax value under Imeas = 632.8 nm for the photosensitive film based on 5 wt. % of o-MR / PMMA is 0.02. Really, the birefringence n(<meas) is related to the dichroism D = D\\ - Dj., and it can be evaluated using the Kramers-Kronig transformation [39]:
D(u')
n(wmeas)
f CP
= Jv
'2 —
■dtà'
(5)
where P is the principal value of integral, <meas is the measurement frequency of light, c is the speed of light, and represents the whole spectrum.
From the analysis of the values the order parameter S and the birefringence nmax under Xmeas- = 632.8 nm, it is clear that the photoinduced ordering of transparency for activating illumination monomeric fragments of polymeric matrix. Absorption light molecules of oMR is an important contributor to the overall birefringence.
The data in Fig. 3, b allow also evaluating the MR rotational diffusion D in PMMA as well its dependence on the illumination intensity Pexp.
The orientational relaxation time t is determined as the time of reduction of photo-induced birefringence, and the rotational diffusion D can be established as t-1. Evaluation data are presented in Fig. 4. The dependence D(Pexp) in Fig. 4 can be described by the expression: D = 1/t = Do+D^pE-E = Do+DspE2, where Do is the rotational diffusion at zero exposure, De is a coefficient describing the rotational diffusion acceleration under action of the light wave electric field of strength E, p is the MR molecule's polarizability. The product pE2 is dimensionless because [p] = C-m2-Tl, [E] = V-m"1.
10J
1 —1—T, s
\ 1 2 —— D, s"1 2 S
0.0
0.4
0.8
1.2 mW ■ cm":
1.6
2.0
0.006
0.005
0.004 '
0.003
0.002
0.001
Fig. 4. The orientational relaxation time t and rotational diffusion D dependences vs illumination intensity Pexp measured in the MR/PMMA layer
Cyclic forming of PIA
Fig. 5 shows cyclic photoinduction and dark relaxation (birefringence) in thin MR/PMMA film for two values of the power density of activating light illumination. We observe the following: (i) increase of photosensitivity in cyclic process, (ii) slowing down of
the rate of dark relaxation and (iii) appearance of a continuously growing background until saturation is eventually reached (Fig. 5). These photokinetic curves correspond to typical dependences obtained for other substances including the MR [25].
Fig. 5. Optical induced and dark relaxed phase retardation of MR/PMMA film in a cyclic process. A thin layer of the MR/PMMA was illuminated by the activating light having two values of power (0.2 and 2.0 mW/cm2). The exposure and
relaxation time in each cycle was 125 and 625 s accordingly
These experimental data contradict with Todorov et al. [40], where the «recording-self-erasure» cycles in MR/polymer matrix were perfectly reproducible. We have also shown more complicated behavior of dark relaxation in MR/PMMA film (Fig. 6). After partial erasure of the photo-induced birefringence during the secondary illumination with the light of orthogonal
polarization, the birefringence level can be decreased or even increased depending on the birefringence initial value. The effect can be explained by the photo-physical process of photo-orientation and the consequent dark self-ordering of MR molecules in polymer matrix [28, 41].
Fig. 6. Optical writing-partially optical erasure and subsequent dark relaxation processes of the photo-induced birefringence in thin MR/PMMA film for different times of optical erasure
Similar effect was observed in [15]. The diffraction efficiency enhancing up to 11.5 % was obtained in Methyl Red doped liquid crystal grating by re-illumination of the sample. From the change of the phase retardation presented in Fig. 6, we can evaluate the MR rotational diffusion in every cycle. It equals to
0.002.s-1. This value is a few orders higher than for liquid crystals (108 s-1 [42]).
Conclusion
The photo-induced anisotropy (PIA) of Methyl Red embedded into PMMA polymer (MR/PMMA) solid films was investigated by studying the dichroic spectra and photo-induced birefringence. The MR order parameter calculated from the dichroic spectra is small, but it becomes higher for the larger illumination power. The photo-induced birefringence increases with the power density of activated illumination. When the source is switched off, the birefringence value relaxes to some saturated non-zero value. The characteristic relaxation time decreases with the illumination energy. The MR rotational diffusion is 0.001-0.002 s-1. It increases proportionally to the writing light intensity.
The photo-induced optical anisotropy in MR/PMMA solid film can be explained mainly by photo-physical process of active photo-ordering and photoorientation of MR molecules and its surroundings. These data are important for the characterization of Methyl Red azo dye as the material for LC photo-aligning or LC holography elements.
Acknowledgments: The study was performed under partial support of Russian Science Foundation, project No. 20-19-00201.
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Вклад авторов: 1Козенков В. М. - проведение исследований, разработка концепции научной работы, методологии и реализации исследования, редактирование текста статьи. 2Беляев В. В. - подготовка иллюстраций, консультация по вопросам планирования, методологии и реализации исследования, редактирование текста статьи. 3Чаусов Д. Н. - консультация по вопросам планирования работ, подготовка иллюстраций, написание текста статьи.
2Belyaev V.V. - preparation of illustrations, consultation on planning, methodology and implementation of research, editing of the text of the article.
3Chausov D.N. - consultation on work planning, preparation of illustrations, writing the text of the article.
Авторы заявляют об отсутствии конфликта интересов.
The authors declare no conflicts of interests.
1orcid - отсутствует
2https://orcid.org/0000-0003-0553-9358
3https://orcid.org/0000-0002-1287-6427
Contribution of the authors: 1Kozenkov V.M. - conducting research, developing the concept of scientific work, methodology and implementation of research, editing the text of the article.
Поступила 22.08.2022, одобрена 20.09.2022, принята 25.09.2022 Received 22.08.2022, approved 20.09.2022, accepted 25.09.2022.
