The role of conductivity and molecular mobility on the photoanisotropic response of a new azo-polymer containing sulfonic groups

Article abstract of DOI:10.1016/j.jphotochem.2019.112268

We report the preparation of a new light-responsive side-chain terpolymer containing azobenzenes, as chromophoric components, sulfonic groups, as polar components, and methacrylate groups, as film forming components, and we provide a detailed characterisa

Full text of DOI:10.1016/j.jphotochem.2019.112268

Journal Pre-proof
The role of conductivity and molecular mobility on the photoanisotropic
response of a new azo-polymer containing sulfonic groups
Sakinah Mohd Alauddin, Nurul Fadhilah Kamalul Aripin, T.S.
Velayutham, Irakli Chaganava, Alfonso Martinez-Felipe
PII:
S1010-6030(19)31387-5
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112268
JPC 112268
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
14 August 2019
24 October 2019
27 November 2019
Please cite this article as: Alauddin SM, Aripin NFK, Velayutham TS, Chaganava I,
Martinez-Felipe A, The role of conductivity and molecular mobility on the photoanisotropic
response of a new azo-polymer containing sulfonic groups, Journal of Photochemistry and
amp; Photobiology, A: Chemistry (2019),
doi: https://doi.org/10.1016/j.jphotochem.2019.112268
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Alauddin et al.
Main manuscript - Revised
The role of conductivity and molecular mobility on the photoanisotropic
response of a new azo-polymer containing sulfonic groups.
Sakinah Mohd Alauddin1,2, Nurul Fadhilah Kamalul Aripin1,2, T.S. Velayutham2,3, Irakli
Chaganava4, 5, Alfonso Martinez-Felipe6, *
1 Faculty of Chemical Engineering, University of Technology MARA, 40450 Shah Alam, Selangor
Darul Ehsan, Malaysia.
2Fundamental and Frontier Sciences in Nanostructure Self-Assembly Center, Department of
Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia.
3 Low Dimensional Material Research Center, Department of Physics, Faculty of Science, University
of Malaya, 50603 Kuala Lumpur, Malaysia.
4 Laboratory of Holographic Recording and Processing of Information, Institute of Cybernetics, GTU. 5
Sandro Euli str., 0186 Tbilisi. Georgia.
5 Georgian State Teaching University of Physical Education and Sport.49, Chavchavadze ave. 0179,
Tbilisi. Georgia.
6 Chemical and Materials Engineering Research Group, School of Engineering, University of
Aberdeen, King's College, Old Aberdeen AB24 3UE, UK.
* Corresponding author: a.martinez-felipe@abdn.ac.uk
Graphical abstract
1

Alauddin et al.
Main manuscript - Revised
Highlights





Facile preparation of a new light-responsive side-chain terpolymer with sulfonic
groups.
Photoisomerisation of methoxyazobenzenes is controlled by steric effects and
interactions.
The terpolymer shows high effective photoanistropy rate by the presence of polar
groups.
Reorientations of the sulfonic groups take place at low temperatures, in the glass
phase.
The terpolymer has p-type conductivity, which can promote photoisomerisation.
Abstract
We report the preparation of a new light-responsive side-chain terpolymer containing
azobenzenes, as chromophoric components, sulfonic groups, as polar components, and
methacrylate groups, as film forming components, and we provide a detailed
characterisation of its thermal parameters, photoanisotropic character, dielectric response
and optoelectronic properties. The poly[(4-methoxyazobenzene -4’-oxy) methacrylate]-co-
poly[2-acrylamido-2-methyl-1-propanesulfonic acid]-co-poly[methyl methacrylate],
MeOAzB/AMPS/MMA, was prepared by radical copolymerisation, possesses high thermal
stability and is amorphous. Its high glass transition (Tg = 151oC) is explained by steric
effects (induced by the bulky azobenzenes) and hydrogen bonding (promoted by the polar
sulfonic groups) near the polymeric backbone, which may also reduce the photoanisotropic
efficiency. The transition between the energy levels of the azobenzenes (MeOAzB) in the
terpolymer is controlled by p-type conductivity, and can be associated to motions in the side-
chains containing the sulfonic groups (AMPS), which are locally activated below the
MeOAzB/AMPS/MMA glass transition.
Keywords: azo-polymers; dielectric spectroscopy; UV-Vis spectrophotometry;
photoanisotropy; trans-to-cis photoisomerisation; conductivity.
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Main manuscript - Revised
1. Introduction
Azobenzenes are undoubtedly some of the most utilised and investigated components as
light-responsive materials, based on their trans-to-cis photoisomerisation driven by
irradiation with UV light, normally around ~ 365 nm, see Fig. 1 1, 2. This process is
reversible, and the cis-to-trans relaxation to the ground state can occur by thermal activation,
and can also be promoted by irradiation at longer wavelengths. Due to the molecular
anisotropy of the trans isomers, azobenzenes can assist the formation of liquid crystalline
phases, and are used to disrupt the order in microstructures by switching to the non-linear
cis isomer 3. The application of light allows for spatial control with high efficiency, and a wide
variety of so-called azo-polymers have been extensively prepared and used for data storage
4-11, in plasmonic metasurfaces for photopatterning of molecular orientations12, as
electrolytes with enhanced ionic transport 13-18, in formulations for controlled drug delivery 19-
23 and as energy harvesters 24, 25
.
N
N
N
N
Figure 1. Azobenzene isomers: trans (left) and cis (right). Trans-to-cis isomerisation,
triggered by UV-Vis light radiation (hv), and thermally activated cis-to-trans relaxation ().
Photoanisotropy is an attractive strategy to exert directional control on the morphological
changes of azo-compounds that respond to actinic linearly polarised light. In addition,
operation in a narrow band of the electromagnetic spectrum allows to encode a photo signal
with very high density26. Photoanisotropic materials are particularly relevant in the
production of polarisation-holographic elements 27, which can carry out analyses and
transformations of light in real time and to operate in a wide spectral range. As a result, a
single polarisation-holographic element can replace an entire set of conventional elements in
polarisation optics.
Organic media based on polymers and functional azo-dyes are increasingly popular as
holographic elements, and recently, we have developed side-chain copolymers containing
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Alauddin et al.
Main manuscript - Revised
azobenzenes that show promising, but unusual, photochromic response in terms of
photoanisotropy (퐴푒푓푓 푀푎푥) and induction velocity (퐴푒푓푓·, s-1) 28. The polarity of these
̇
materials relies on the presence of terminal cyano groups attached to the azobenzene units,
which increase their photosensitivity and accelerate the response to actinic light through
intramolecular push-pull effects. Unfortunately, uncontrolled self-aggregation resulted in
inhomogeneous samples with low values of optical isotropy. In order to yield homogeneous
films with thickness around 20 m, it was then necessary to mix the azo-materials with a
non-chromophore polymer, resulting in poorer photoanisotropic parameters.
In the present work, we investigate new photoanisotropic polymers that contain azobenzene
and polar groups in different side-chains of a poly(methacrylate)-based terpolymer: the
poly[(4-methoxyazobenzene -4’-oxy) methacrylate]-co-poly[2-acrylamido-2-methyl-1-
propanesulfonic acid]-co-poly[methyl methacrylate], MeOAzB/AMPS/MMA, 1 in Fig. 2. The
methoxyazobenzene units, MeOAzB, promote optoelectronic response, whilst the 2-
acrylamido-2-methyl-1-propanesulfonic acids, AMPS, contain polar sulfonic groups.
Methyl(methacrylate) groups, MMA, are introduced into the main chain in order to enhance
the film forming properties. Our target is to enhance the optoelectronic performance of new
polymeric azo-derivatives by the inclusion of the AMPS groups, based on the high
conductivity observed in analogous materials 29, 30. With this strategy, we aim to facilitate
mutual integration of polar components that can accelerate the light response of
azobenzenes by forming specific interactions, while maintaining design flexibility. Here we
have evaluated such effects by a full structural, dielectric and conductivity analysis of
MeOAzB/AMPS/MMA.
4

Alauddin et al.
Main manuscript - Revised
H N
O CH
3
2
Phenol
KOH
NaCl
NaNO
2
HO
N
N
O CH
2
O
3
CH
N
3
H C
2
Cl
THF
N
DMAP
CH
H C
3
3
CH
3
O
O
N
O CH
3
3
5
H C
2
CH
3
O
CH
O
3
CH
H C
SO H
3
2
H C
4
3
2
O
CH
NH
2
CH
3
CH
3
H
H
H C
CH
3
3
CH
CH
O
CH
2
2
2
X
Y
Z
C
O
C
O
C
O
O
HN
H C
CH
3
1
CH
3
3
CH
2
SO H
3
N
N
OCH
3
Figure 2. Synthetic route of poly[(4-methoxyazobenzene -4’-oxy) methacrylate]-co-poly[2-
acrylamido-2-methyl-1-propanesulfonic acid]-co-poly[methyl methacrylate],
MeOAzB/AMPS/MMA, 1. X=44, Y=18 and Z=38, represent the molar % of each monomeric
unit in the polymer chain
5

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Main manuscript - Revised
2. Experimental section
Synthesis of poly[(4-methoxyazobenzene -4’-oxy) methacrylate]-co-poly[2-acrylamido-2-
methyl-1-propanesulfonic acid]-co-poly[methyl methacrylate], MeOAzB/AMPS/MMA, 1.
The MeOAzB/AMPS/MMA terpolymer was prepared following the route shown in Fig. 2.
The synthesis of 4-hydroxy-4’-methoxyazobenzene, 2, is described in detail elsewhere 31-33
N,N-dimethylaminopyridine (DMAP) (40 mg, 0.3 mmol), triethylamine (1.06 g, 10.5 mmol)
.
and 2 (1.14 g, 5 mmol), were dissolved in 75 mL of THF. The mixture was cooled using ice,
and methacrylic chloride (1.15 g, 11 mmol) was added dropwise. The resulting reddish
solution was stirred at 40°C for 24 h, and after cooling to room temperature, the reaction
mixture was poured into water and the precipitate collected and recrystallised from ethanol,
obtaining 4-(methoxyazobenzene -4’-oxy) methacrylate, 3. Yield: 52 %; 1H-NMR (DMSO-
d6, ): 6.9 - 7.8 (m, aromatic, 8 H; J = 3.0, 8.9, 8.8 and 9.0 Hz), 5.7, 6.3 (s, CH2=C-, 2H), 3.8
(s, ArOCH3, 3H), 2.0 (s, CH3C(COO), 3H), see Fig. ESI1.
Methyl(methacrylate), MMA, 4, was purchased from Acros, and purified by washing with
NaOH and water, followed by drying with anhydrous MgSO4. 2-Acrylamido-2-methyl-1-
propanesulfonic acid, AMPS, 5 (99% purity), was purchased from Sigma Aldrich and used
without further purification. All other reagents were used as received from Sigma Aldrich.
The terpolymer MeOAzB/AMPS/MMA, 1, was prepared by free radical polymerisation, by
dissolving 3, (0.6 g, 2 mmol), 4 (0.4 g, 2 mmol) and 5 (0.2 g, 2 mmol), in dimethylformamide
(12.1 g), and 1,10- azobis(cyclohexane carbonitrile) (0.0132 g) was added as initiator. The
reaction mixture was flushed with nitrogen for 45 min, then heated at 80oC in the absence of
oxygen, to initiate polymerisation. After 24 h, the reaction was terminated by precipitation into
diethyl ether. The polymer was purified by subsequent precipitations from dichloromethane
into diethyl ether. Yield (MeOAzB/AMPS/MMA): 65.0% in terms of MeOAzB equivalents; 1H-
NMR (CDCl3, ): 8.5 (s, NH, 1H) 6.9, 7.8 (m, aromatic, 8H), 3.8 (s, ArOCH3, 3H), 3.6 (s, CH3-
OCO, 3H), 2.8 (s, CH2SO3, 2H), 2 - 0.7 (m, main chain, 7H).
Characterisation techniques
The chemical structures of the terpolymer and the intermediates were verified by nuclear
magnetic resonance, 1H-NMR, using a 400 MHz Bruker AVANCE III NMR spectrometer, and
by infrared spectroscopy, IR, using a Perkin Elmer Spectrum One FT-IR spectrometer
1
equipped with an Attenuated total reflectance (ATR). For the H-NMR experiments, either
deuterated chloroform (CDCl3) or dimethyl sulfoxide (DMSO-d6) was used as the solvent. IR
measurements were taken on powder samples, at room temperature. The IR spectra were
6

Alauddin et al.
Main manuscript - Revised
obtained in the 4000 – 400 cm-1 range, with a 4 cm-1 accuracy, as the average of 64 scans.
The average molecular weights, Mw and Mn, polydispersity, Mw/ Mn, and degree of
polymerisation, DP, of MeOAzB/AMPS/MMA, were measured using a Shimadzu 20A setup
with a refractive index detector (Shimadzu RID-10A), equipped with a PL polypore column.
Samples were eluted in tetrahydrofuran with a flow rate of 1 ml·min-1 at 25oC. Polystyrene
standards were used for calibration.
The phase behaviour of MeOAzB/AMPS/MMA was determined by differential scanning
calorimetry, DSC, using a Mettler Toledo DSC1 module. Around 4 mg of the sample were
heated from 25oC to 200oC, held at 200oC for 3 minutes, cooled to 25oC, held for 3 minutes,
and then reheated again to 200oC. All scans were conducted at a rate of ±10oC·min-1 under
nitrogen atmosphere. Phase identification was confirmed by an Olympus BX51 polarised
optical microscope, POM. The thermal stability of the terpolymer was assessed by
thermogravimetric analysis, TGA, using a Mettler Toledo TGA/DSC1 module from 25oC to
800oC, under 10 ml·min-1 flowrate of nitrogen.
The ultraviolet-visible (UV-Vis) absorbance spectra of MeOAzB/AMPS/MMA was recorded at
room temperature, on a film cast on a quartz substrate, and in a 4.9·10-5 M THF solution, using
a Perkin Elmer Lambda 750UV-VIS-NIR spectrometer in the 200 to 800 nm wavelength range.
The terpolymer samples were measured before (ground state) and immediately after (excited
state) being exposed for 10 minutes with UV light (= 365 nm, intensity = 50 µW/cm2).
Samples were then kept in the dark, and were measured at various intervals until the spectra
recovered its original shape (relaxation).
The photometric setup described in Fig. 3 was used to study the photoanisotropy kinetics of
MeOAzB/AMPS/MMA under polarised actinic light. A diode-pumped solid-state laser, DPSS,
emitted linearly polarised light in the blue spectral range (445 nm wavelength), and the probing
beam was generated by a laser at a 635 nm wavelength, with an angle of 45 degrees relative
to the polarisation beam. The light transmitted through the terpolymer was then analysed by
using an appropriate filter, with the photodetector oriented orthogonally to the polarisation
direction of the probing beam, in order to record optical anisotropy. The dynamics of light
response upon exposure was recorded in terms of the effective anisotropy (퐴푒푓푓) and effective
∂A푒푓푓
̇
⁄
anisotropy rate (퐴푒푓푓
=
). It should be noted that the material shows minimal
∂t
absorption at the probe beam wavelength (635 nm), hence, any induced linear birefringence
is prominent in our experiments and dichroism effects can be neglected 34.
7

Alauddin et al.
Main manuscript - Revised
Figure 3.
Experimental setup to determine the photoanisotropy kinetics of
MeOAzB/AMPS/MMA.
The dielectric measurements were carried out on a metal-insulator-metal (MIM) structure
device. Firstly, MeOAzB/AMPS/MMA was dissolved in dichloromethane (DCM), to yield a 5
% weight solution of the terpolymer. Glass substrates were pre-cleaned by sonicating for 10
min in soap water, acetone, isopropanol and distilled water, and dried with nitrogen gas. The
clean glasses were then pre-coated with aluminium via thermal evaporation, and the
MeOAzB/AMPS/MMA DCM solution was spin coated on the substrate to yield a homogeneous
transparent thin film, which was dried in an oven at 80°C for 24 hours, in order to remove
residual solvent. The thickness of the resulting film was 0.9 ± 0.1 m, measured by a KLA
Tencor P-6 mechanical profilometer. The aluminium top electrode was then deposited on the
film to produce the MIM structure device, which was subsequently used for dielectric analysis.
Measurements were carried out in the frequency range of 0.01 Hz to 1 MHz, by using a
combination of two apparatus: a home-made dielectric spectrometer (0.01–104 Hz), and an
Agilent 4294A impedance analyser (102–106 Hz). The spectra were obtained in isothermal
steps on heating from T=-100°C to 165°C, and the isothermal frequency sweeps were
performed after the sample was stabilised at the corresponding temperature for 10 min.
Cyclic voltammetry (CV) was used to calculate the molecular energy levels of
MeOAzB/AMPS/MMA, using a Versa STAT 3 potentiostat in 0.1 M of potassium chloride (KCl)
solution, at room temperature. 1 mM potassium ferricyanide was used as supporting
electrolyte in KCl solution, ITO as the working electrode, a platinum gauze electrode as the
counter electrode, and Ag/AgCl as the reference electrode. The CV measurement was carried
out at 10 mV·s-1 scan rate.
8

Alauddin et al.
Main manuscript - Revised
3. Results and discussion
Structural and thermal characterisation
MeOAzB/AMPS/MMA was synthesised according to Fig. 2, and the molar percentages of
monomeric units, i.e., (4-methoxyazobenzene-4’-oxy)methacrylate, MeOAzB, X=44%, 2-
acrylamido-2-methyl-1-propanesulfonic acid, AMPS, Y=18%, and methyl(methacrylate),
1
MMA, Z=38%, were experimentally determined by H-NMR spectroscopy and assessed by
IR. More specifically, the composition was calculated using the integrals of the 7-8 ppm signal
assigned to the azobenzene in MeOAzB (8H), the 2.7 ppm signal corresponding to the
methylene group of AMPS (2H), and the 3.6 ppm signal associated to the methyl group of
MMA (3H), see Fig. ESI2(a). These results are consistent with the appearance of the
corresponding IR characteristic bands in Fig. ESI2(b). The molecular weight of the terpolymer
was determined by GPC, Fig. ESI2(c), showing unimodal distribution. Considering the lower
reactivity ratio of the AMPS monomer35, 36, we cannot rule out that MeOAzB/AMPS/MMA may
contain tapered chains initiated by polymerisation of methacrylate units, and we will treat our
material as a statistical terpolymer13. The calculated molecular weights are Mn = 66751 g·mol-
1
and Mw = 101209 g mol-1, with a polydispersity value of Mw/Mn = 1.52, and degree of
polymerisation of DP = 326.
The thermogravimetric and derivative thermogravimetric curves of MeOAzB/AMPS/MMA (TG
and DTG, respectively) are shown in Fig. 4(a). Thermal decomposition of the terpolymer
proceeds through two main weight loss processes: degradation of side chains, in the
250oC/350oC range (temperature at 5% weight loss is T5%~280oC), followed by breakage of
the copolymer main chain, in the 350oC/550oC range. This thermal degradation profile is
similar to that reported on analogous 10-MeOAzB/AMPS/MMA terpolymers, 6, but the
absence of long alkyl spacers (Cn, n=10) between the azobenzene group and the polymer
backbone in MeOAzB/AMPS/MMA seems to destabilise its side chains13, 37, 38
.
9

Alauddin et al.
Main manuscript - Revised
CH
2
O
(CH )
O
H C
C
O
N
N
2 10
3
OCH
3
X
CH
2
CH
3
NH
O
CH SO H
2 3
H
C
O
Y
CH
3
3
CH
2
H C
C
O
CH
3
Z
6, liquid crystalline 10-MeOAzB/AMPS/MMA terpolymers reported in 13.
Fig. 4(b), on the other hand, depicts the thermogram obtained by differential scanning
calorimetry, DSC, corresponding to the second heating scan of MeOAzB/AMPS/MMA. The
curve displays a pseudo-second order transition, associated to the glass transition of the
polymer, at Tg ~ 151oC, and there are no visible first order thermal transitions. Polarised
optical microscopy, POM, did not reveal other thermal events and we then can rule out the
formation of liquid crystalline phases. The terpolymer is therefore amorphous and forms a
glass at T < Tg.
Under some circumstances, the molecular anisotropy of the azobenene groups can promote
liquid crystal behaviour, and in the past we reported the formation of smectic phases by the
13, 39
10-MeOAzB/AMPS/MMA analogues, 6
.
The absence of alkyl spacers in
MeOAzB/AMPS/MMA, however, seems to inhibit the capability of the MeOAzB groups to
decouple from the backbone motions and ultimately rearrange into mesomorphic structures40,
41. Due to the proximity of the azobenzene bulky groups to the main chain, the segmental
motions of the polymer backbone may also be impeded, and MeOAzB/AMPS/MMA exhibits a
higher glass transition compared to the 10-MeOAzB/AMPS/MMA terpolymers (Tg ~ 70oC) 13.
This limitation in backbone mobility must be further embraced by the existence of hydrogen
bonding between the AMPS groups 42, which potentially affects the MeOAzB motions too, as
we will discuss with detail in the next sections.
10

Alauddin et al.
Main manuscript - Revised
(b)
(a)
wt/%
dwt/dT %·C-1
100
0
Tg=150.6oC
80
60
40
20
0
-0.1
-0.2
-0.3
-0.4
0
100 200 300 400 500 600 700 800
T/oC
Figure 4. Thermal analysis of MeOAzB/AMPS/MMA: (a) thermogravimetric (TG, solid line)
and derivative thermogravimetric (DTG, dotted line) curves; (b) differential scanning
calorimetric (DSC) thermogram obtained during the second heating scan, indicating the
glass transition, Tg; hf is specific heat flow.
Light responsive behaviour
We now study the light response of the MeOAzB/AMPS/MMA terpolymer, and Fig. 5 shows
its UV-Vis spectra measured for: (a) a THF solution (tetrahydrofuran), and (b) a film cast on
quartz. Prior to UV exposure, see crosses in Fig. 5, an intense band appears centred at ~349
nm, corresponding to the * transition of the azobenzene trans conformer, together with a
much weaker absorption band at ~450 nm, associated with the symmetric forbidden n*
transition43. These bands confirm the light responsive behaviour of MeOAzB/AMPS/MMA,
induced by the presence of the MeOAzB units, which is the majority monomeric component
in the terpolymer (X=44%, molar %).
11

Alauddin et al.
Main manuscript - Revised
(a)
(b)
Figure 5. UV-Vis spectra of MeOAzB/AMPS/MMA obtained for: (a) a 4.9·10-5 M THF solution;
(b) a film cast on quartz. Crosses correspond to the non UV-exposed sample, and solid lines
correspond to measurements taken at different relaxation times after illumination with 365 nm
light for 10 minutes, keeping the sample in the dark. Dotted arrows indicate the effect of
thermal relaxation.
Irradiation with light at 365 nm promotes fast trans-to-cis photoisomerisation, resulting in a
dramatic increase of the * transition band, and a simultaneous (and less acute) increase
of n*. It is worth noting that, upon irradiation, the quartz film shows larger residual
absorbance of the ~349 nm band (0.35%, Fig. 5(b)), compared to that observed in solution
(almost zero, Fig. 5(a)), suggesting that a fraction of azobenzenes might not undergo trans-
to-cis isomerisation in the bulk. The two samples were then kept in the dark, and the UV-Vis
spectra were measured at different time intervals, in order to monitor the cis-to-trans thermal
relaxation. Whilst the spectra measured in solution took 48 hours to relax back into the original
state, the film needed 72 hours, evidencing that the molecular processes are slower in the
bulk. We note that MeOAzB/AMPS/MMA requires longer times than the liquid crystalline 10-
MeOAzB/AMPS/MMA analogues13, probably due to the stronger coupling between the side-
chain azobenzenes and the skeletal main chain.
It is clear that the photoisomerisation kinetics of MeOAzB/AMPS/MMA is highly sensitive to
steric effects involving the azobenzenes43, which may restrict the molecular mobility in the
terpolymer. In principle, we could argue that such restrictions could also hinder cis-to-trans
isomerisation, but the proximity of the MeOAzB units to the backbone may also induce steric
strains in the cis form that could ultimately endow in faster thermal relaxation rates 44-46. These
two competing effects must somehow offset to yield the average light response in Fig. 5, and
12

Alauddin et al.
Main manuscript - Revised
the back relaxation process of the terpolymer follows first-order kinetics in both solution and
the bulk, see Fig. ESI3, albeit much slower in the latter.
The effective photoanisotropy of MeOAzB/AMPS/MMA, 퐴푒푓푓, has been calculated in the form
of light-induced birefringence, excluding dichroism effects 34, 47
,
1
2
2 휋
휆
Eq. 1
퐴푒푓푓
=
[1 − cos(
푑Δ푛)]
where 휆 is the wavelength of the inducing light, d is the thickness of the medium, and Δ푛 is
the optical birefringence48-50. According to Fig. 6(a), MeOAzB/AMPS/MMA shows a limited
degree of light induced birefringence. Its response has been compared to those of a solid
solution with the azo-chromophore non-covalently bound to the polymer matrix, MY-26, 7, and
a polymer with nitro-azomonomers built-in, BiN-GP, 8, 28
7, MY-26
8, BiN-GP
The 퐴푒푓푓 values of MeOAzB/AMPS/MMA fall between those exhibited by MY-26 and BiN-GP.
Interestingly, and compared to these model compounds, our terpolymer shows very high
̇
photoanisotropy induction rate, 퐴푒푓푓, calculated as the derivative of 퐴푒푓푓 at the initial moments
of isomerisation, see Fig. 6(b),
휕퐴푒푓푓
̇
Eq. 2
⁄
퐴푒푓푓
=
휕푡
These results demonstrate the sensitivity of MeOAzB/AMPS/MMA to actinic radiation, and
therefore its potential to be used in linearly polarised systems, but also confirm the limited
isomerisation efficiency of the azobenzenes in this terpolymer. It is worth mentioning that the
13

Alauddin et al.
Main manuscript - Revised
UV-Vis absorbances of MY-26 and BiN-GP at  = 445 nm are similar to that of
MeOAzB/AMPS/MMA, see Fig. 5(b) and Fig. ESI4, suggesting that the corresponding
photoanisotropies are comparable.
(a)
(b)
휕퐴푒푓푓
̇
⁄
퐴푒푓푓
=
퐴푒푓푓
휕푡
20
15
10
5
100
MeOAzB/AMPS/MMA
80
60
40
20
0
MeOAzB/AMPS/MMA
MY-26
MY-26
BIN-GP
BIN-GP
0
0
100
200
300
400
500
0
2
4
6
8
10
t / s
t / s
Figure 6.
Kinetic curves corresponding to the photoanisotropy induction of
MeOAzB/AMPS/MMA (solid line) and two reference materials (MY-26 and BIN-GP, dotted
lines): (a) effective anisotropy, 퐴푒푓푓; (b) effective anisotropy rate immediately after light
̇
application (t = 10 s), 퐴푒푓푓
.
We believe that the photonic response of MeOAzB/AMPS/MMA must be substantially
enhanced by the strong electrostatic interactions between the sulfonic acids and the
51
azobenzenes
.
More specifically, Fig. 6 suggests that the sulfonic groups in
MeOAzB/AMPS/MMA promote more effective light response than the dense network of ester
linkages (CO.O) and the nitro terminations (NO2) in BIN-GP. The high 퐴푒푓푓 values can be
then explained by the ionisable character of the SO3H groups, compared to the partial
polarisation achieved by CO.O and NO2 52. Whilst MeOAzB/AMPS/MMA shows smaller 퐴푒푓푓
values than MY-26, the terpolymer presents greater design flexibility, since the relative
position of the sulfonic and the azobenzene units can be tailored, at least to some extent.
Ultimately, the preparation of azo-polymers offers the exciting possibility to tune the light
response via polymerisation parameters, such as, monomer composition and distribution,
molecular weight or polymerisation degree 53, 54. The high photoanisotropy induction rate
̇
exhibited by MeOAzB/AMPS/MMA, 퐴푒푓푓, will be further discussed during the next sub-section.
Molecular mobility and conductivity. Dielectric response
We now apply dielectric spectroscopy with the aim to correlate the light-response of
MeOAzB/AMPS/MMA to the local environment of the azobenzene groups in the terpolymer
14

Alauddin et al.
Main manuscript - Revised
structure. The temperature-frequency dependence of the dielectric loss factor, ’’, is
summarised in Fig. 7(a), displaying several dielectric processes illustrated in Fig. 8. More
specifically, five relaxations, associated to dipole reorganisations, are observed and are
labelled as , , x and in increasing temperature order, see Fig. 7(a) and 7(b). At low
temperatures, -100oC/50oC , the  and  relaxations are visible and are assigned to motions
of the terminal methoxy and azobenzene groups, respectively, in the MeOAzB chains 55, 56
In the high temperatures range, T ≥150oC, the  relaxation is observed, corresponding to
the onset of main-chain segmental motions, and associated to the glass transition of the
terpolymer (Tg). At higher temperatures, T>Tg, more free volume is available, and the
relaxation is activated, associated to the rotation of the MeOAzB side chains along the
.
main chain axis 56. This process has been ascribed previously to the onset of direct current,
DC, conductivity through smectic phases in azobenzene-based polymers15, and we will
return to this observation later.
These relaxations (, , and ) are typical of comb-shaped poly(methacrylate)s 55, 57-60, and
we note that the so-called 1 relaxation was not unveiled in our measurements, normally
ascribed to flip-flop motions of the polarisable carboxyl groups (CO.O) that activate the 
relaxation in the glass state61-63. Instead, we have observed an additional relaxation at
intermediate temperatures between the  and  processes, see Fig. 7(b) for tan()=’’/’.
This process, hereinafter labelled as x, is not observed in other polymers containing only
MeOAzB and/or MMA units15, and must be then associated to the presence of AMPS
groups. Unfortunately, it was not possible to monitor x through the whole experimental
temperature-frequency range, probably due to the low concentration of sulfonic groups in the
terpolymer (Y=18%).
(a)
x
(b)
(c)
tan()
log(t/s)
’’

x

x





log(f/Hz)
T/oC
T/oC
1/(T/K)
Figure 7. Summary of the dielectric response of MeOAzB/AMPS/MMA: (a) temperature and
frequency dependence of the loss factor, ’’; (b) temperature dependence of tan()=’’/’ at
15

Alauddin et al.
Main manuscript - Revised
selected frequencies; (c) Arrhenius plots showing the relaxation times, t, corresponding to the
maxima in the ’’ curves for different relaxations.

H C
3
H
H C
3
CH
O
CH
O
CH
O
2
2
2
C
O
C
C
O

HN


CH
H C
CH
3
3
3
CH
2
x?
N
N


SO H
3
OCH
3
Figure 8. Schematic representation of the dielectric relaxations (, , x , and ) and
molecular motions of MeOAzB/AMPS/MMA (was not observed experimentally in this
work15).
We have studied the thermal activation of the ,  and dielectric processes via their
relaxation times, corresponding to the maxima of the ’’ curves obtained after fitting the
experimental results to a sum of empirical Havriliak-Negami (HN) functions64,
휀
휎푑푐
휀∗ ꢀ휔 = 휀∞ + ∑
푘
+
(
)
Eq. 3
(1 + (ꢀ휔ꢁ)푚)푛
ꢀ휔
푝
(
)
where ω is the angular frequency, 휀∞ is the instantaneous permittivity, ∆ɛ is the dielectric
relaxation strength, τ is the relaxation time, m and n are parameters describing the width and
asymmetry of the distribution of relaxation times, k is the number of relaxations, σdc is the direct
current conductivity and p is a parameter describing the contribution of conductivity to the
dielectric permittivity. ꢀ = −1 is the imaginary unit. Both  and processes show Arrhenius
√
behaviour, see Fig. 7(c), and their activation energies, 퐸푎, can be obtained according to,
16

Alauddin et al.
Main manuscript - Revised
Eq. 4
퐸푎
푅
1
푇
ꢁ(푇)푚푎푥 = ꢁ표exp(
∙
)
where ꢁ표 = ꢁ푚푎푥 at 푇 → ∞ , and R is the absolute gas constant. The results for
MeOAzB/AMPS/MMA are summarised in Table 1, and both relaxations show slightly higher
퐸푎 values than other analogous azo-polymers that contain flexible spacers, the 10-
MeOAzB/AMPS/MMA terpolymers discussed above. This may reflect the motional restrictions
of the MeOAzB units in our terpolymer compared to comb-shaped poly(methacrylate)s 15. The
linearity of the  and processes in Fig. 7(c) is consistent with their predominant local
character, even though the high Ea values denote some sort of cooperativity by the influence
of the intermolecular environment on the dielectric response. Our results are also consistent
with the activation energy values obtained for epoxy-based oligomers containing
nitroazobenzene chromophore fragments, labelled as FIII, CFAO and CFMAO in Table 1, and
whose structures are shown in Fig. ESI565, 66
.
Contrarily, the t-temperature dependence of the process clearly deviates from linearity, and
can be described by the Vogel-Fulcher-Tamman equation67, 68
,
퐵
ꢁ(푇)푚푎푥 = ꢁ표exp(
)
Eq. 5
푇 − 푇0
where t0 represents the pre-exponential term, 푇0 is the Vogel temperature, which is associated
to the cessation of motions of polymeric segments, and B is an apparent activation energy.
The high B value confirms the complex phenomena associated to this relaxation, which involve
segmental motions in the MeOAzB/AMPS/MMA main chain. These motions may be impeded
by the presence of the bulky MeOAzB units and the formation of hydrogen bonds by the AMPS
groups, as mentioned above.
17

Alauddin et al.
Main manuscript - Revised
Table 1. Kinetic parameters of the different dielectric relaxation processes in
MeOAzB/AMPS/MMA, according to Eq. 4 and 5. We include values obtained for reference
comb-shaped poly(methacrylate)s (see structures in Fig. ESI5).
- process
- process
- process
Ea
Ea
B
To
Sample acronym*
-log(to/s)
-log (to/s)
-log(to/s)
(kJ·mol-1)
(kJ·mol-1)
(K)
(K)
1705 300
868 305
2698 274
1496 336
1279 318
11.2
16.7
-
MeOAzB/AMPS/MMA
0.22MeOAzB/MMA15
FIII65
32.9
11.4
16.7
14.2
16.5
15.9
59.1
17.9
17.3
11.7
10.9
9.7
34.7
-
54.6
46.0
61.9
51.9
CFAO66
26.8
25.1
12.4
12.3
CFMAO66
* As per the corresponding references.
The  relaxation, on the other hand, could not be analysed with detail in our experimental
temperature range, but can be associated to the onset of DC conductivity processes 15, dc,
which we have studied through the real component of the complex conductivity, 휎∗ = 휎′ +
ꢀ휎′′,
휎′ = 휔휀푂휀′′
Eq. 6
where 휀푂 = 8.85 ∙ 10−12 퐹 ∙ 푚−1 is the permittivity of free space.
The log(’) vs log(f/Hz) plots of MeOAzB/AMPS/MMA are depicted in Fig. 9, between T=-
10oC and T=165oC. The low conductivity values could be somehow expected, considering
the small concentration of polarisable sulfonic groups in the terpolymer (Y=18%). At high
temperatures, a DC process, dc,1, is distinguishable at around T~140oC from the plateaus in
Fig. 9. This process is ascribed to the  relaxation related to the onset of segmental
motions in the polymer backbone occurring near the glass transition.
Interestingly, at lower temperatures, T=35oC, the log(’) curves start to develop inflexions in
the low frequency range, suggesting the presence of an additional direct current conductivity
process, dc,2. The temperature-frequency range of this DC process is similar to the so-
called x relaxation, and we believe that the dc,2 conductivity could be then ascribed to
dipole reorganisations of AMPS groups that are locally activated in the polymer glass, at T <
Tg, hence ascribed to the sulfonic acids in the terpolymer. It is possible that such local
conductivity effects facilitate motions of the azobenzenes in the terpolymer (responsible of
̇
the  relaxation) and ultimately enhance the photoanisotropy induction rate, 퐴푒푓푓. Both
18

Alauddin et al.
Main manuscript - Revised
conductivity processes, dc,1 and dc,2, are further promoted in the vicinity of the glass
transition, Tg=150oC, by the increase of free volume available around the main chain.
log (f/Hz)
-2
0
2
4
6
-4
-5
-6

dc,2(T=165oC) ~10-6.5 S cm-1
-7

dc,1(T=165oC)~10-8.2 S cm-1
-8
-9
-10
-11
log (’/S·cm-1)
Figure 9. Double logarithmic plots of the real component of the complex conductivity, σ′, of
MeOAzB/AMPS/MMA, as a function of the frequency, measured in isothermal steps (°C) on
heating from the glass (T=-10oC) to the rubbery state (T=165oC); estimation of σdc,1 and σdc,2
at T=165°C. Dotted arrow indicates direction on heating.
The relationships between MeOAzB/AMPS/MMA molecular mobility and conductivity are
now further investigated by its electrochemical response and molecular energy levels, via
cyclic voltammetry, CV, see Fig. 10(a). The highest occupied molecular orbital (HOMO),
lowest unoccupied energy levels (LUMO) and the electrochemical band gap (퐸푔푒푙) were
determined by taking the known EHOMO of ferrocene (Fc) (4.8 eV below the vacuum level) as
reference value,
퐸퐻푂푀푂 = −푒[퐸푂푋/푅퐸퐷 + 4.8 − 퐸퐹푂퐶
]
Eq. 7
Eq. 8
퐸퐿푈푀푂 = 퐸퐻푂푀푂 + 퐸푔푒푙
19

Alauddin et al.
Main manuscript - Revised
where 퐸푂푋/푅퐸퐷 is the first onset oxidation and reduction potential of MeOAzB/AMPS/MMA and
퐸퐹푂퐶 is the external standard potential of ferrocene/ferrocenium (Fc/Fc+) ion couple. The EFOC
was measured to be 0.16 eV in KCl solution. EHOMO = -5.37 eV and ELUMO = -3.42 eV, are the
HOMO and the LUMO energy levels, respectively, where the onset oxidation is EOX = 0.57 V
and the onset reduction is ERED = -1.38 V, obtained from the potential curves of the polymer.
Reduction starts from the electron transporting segments, which are most probably the N
atoms in the azobenzene chromophore and AMPS, while we believe that oxidation must start
from the hole transporting segments located at the sulfonic terminations. The electrochemical
band gap calculated from the HOMO/LUMO is 퐸푔푒푙~1.95 eV, and the energy band diagram of
MeOAzB/AMPS/MMA is proposed in Fig. 10(b).
An optical band gap of 퐸푔표푝푡 = 3.03 eV can also be calculated from the onset of the UV-Vis
absorption spectra,
c
퐸푔표푝푡=h
Eq. 9
Edge
where h is the Planck constant, c is the speed of light and Edge = 409 nm is the terpolymer
optical absorbance band edge obtained from the spectra. The larger value of 퐸푔표푝푡 (respect to
퐸푔푒푙) can probably be explained by reduction and oxidation progressing in the full conjugated
system, rather than in isolated electron and hole transporting 69, 70, and hence, the bandgap
measured directly from the CV might be underestimated.
(a)
(b)
-3.42 eV
LUMO
Al
-4.3 eV
ITO
-4.8 eV
-5.37 eV
HOMO
Fig. 10. (a) Cyclic voltammogram of a MeOAzB/AMPS/MMA film coated on ITO substrate,
on 1 mM K4Fe(CN)6 in 0.1 M potassium chloride solution; sweep rate 10 mV·s-1; (b) Energy
band diagram (hole transporting segments, −; electron transporting segments, ---).
20

Alauddin et al.
Main manuscript - Revised
4. Conclusions
The MeOAzB/AMPS/MMA terpolymer exhibits promising light-responsive behaviour in the
glass phase (T<Tg), driven by trans-to-cis photoisomerisation of the methoxyazobenzene
units, triggered by actinic UV light exposure at 356 nm. Our results suggest that the high
̇
photoanisotropic rate observed for MeOAzB/AMPS/MMA, 퐴푒푓푓, may be promoted by
interactions between the MeOAzB and AMPS units, located in different side-chains of the
terpolymer structure, and associated to p-type conductivity involving the polarisable sulfonic
groups of AMPS.
Even though it is challenging to draw conclusions on the molecular dynamic processes in
non-equilibrium glasses43, 71, our results open new fronts to optimise the photoresponse of
azo-polymers for optoelectronic applications. More specifically, the role of the polymer
backbone rigidity and chemical composition, the inclusion of liquid crystalline phases via
block-copolymerisation, the promotion of hydrogen bonding72, and the molecular origin and
optimisation of the newly reported sub-glass x process, will be the focus of new
investigations taking MeOAzB/AMPS/MMA as a reference.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the University Malaya under the UMRG grant RP038B-17AFR.
NFKA and AMF would like to thank the Royal Academy of Engineering, U.K., and Academy
of Science, Malaysia, for the grant NRCP1516/4/61, from the Newton Research
Collaboration Programme, and the University of Aberdeen, for the award of the grant
SF10192, corresponding to the Global Challenge Research Fund Internal Pump Priming
Fund Round 2. AMF would like to acknowledge the Carnegie Trust for the Universities of
Scotland for the award of one Scottish Research Incentive Grant (RIG008586).
21

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Main manuscript - Revised
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