Responsive behavior of 4-(N-maleimido)azobenzene in polymers with aromatic main chain and side chain linked units

Article abstract of DOI:10.1590/S0103-50532010000300014

Polymer matrices with aromatic main chain and side chain linked units (polystyrene, polysulfone) were employed to construct thin films containing maleimidoazobenzene molecules in order to develop new materials with responsive properties. The photochromic behavior of the active compound was studied both in the solid and solution state for comparison. The thermal cis-trans isomerization of azobenzene chromophores in polymer films follows two different kinetic pathways. A double-exponential relaxation process (fast and slow) took place in polystyrene matrix under 70 °C while the thermal isomerization at 70 °C and higher follows a first-order kinetics. The cis isomer content corresponding to the photostationary state was found to be lower in polymer films than in solution.

Full text of DOI:10.1590/S0103-50532010000300014

J. Braz. Chem. Soc., Vol. 21, No. 3, 489-495, 2010.
Printed in Brazil - ©2010 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Responsive Behavior of 4-(N-maleimido)azobenzene in Polymers with Aromatic
Main Chain and Side Chain Linked Units
Anton Airinei,* Elena Rusu and Virgil Barboiu
Petru Poni Institute of Macromolecular Chemistry, Romanian Academy, Aleea Grigore Ghica Voda,
41A, 700487 Iasi, Romania
Matrizes poliméricas com cadeia principal aromática e cadeias laterais formadas por unidades
de poliestireno e polissulfona foram empregadas para construir filmes finos contendo moléculas
de maleimidoazobenzeno, a fim de desenvolver novos materiais com propriedades responsivas. O
comportamento fotocrômico do composto ativo foi estudado no estado sólido e em solução, para
comparação. A isomerisação térmica cis-trans de cromóforos de azobenzeno em filmes finos de
polímeros segue duas etapas cinéticas diferentes. Um processo de relaxação de segunda ordem
(rápido e lento) ocorre em matriz de poliestireno abaixo de 70 oC, enquanto a isomerização térmica
a 70 oC ou em temperatura mais alta segue cinética de primeira ordem.A quantidade de isômero cis
correspondente ao estado fotoestacionário foi mais baixa em filmes do polímero do que em solução.
Polymer matrices with aromatic main chain and side chain linked units (polystyrene,
polysulfone) were employed to construct thin films containing maleimidoazobenzene molecules
in order to develop new materials with responsive properties. The photochromic behavior of the
active compound was studied both in the solid and solution state for comparison. The thermal cis-
trans isomerization of azobenzene chromophores in polymer films follows two different kinetic
pathways.A double-exponential relaxation process (fast and slow) took place in polystyrene matrix
under 70 oC while the thermal isomerization at 70 oC and higher follows a first-order kinetics. The
cis isomer content corresponding to the photostationary state was found to be lower in polymer
films than in solution.
Keywords: azobenzene, photoisomerization, UV-Vis spectroscopy, thin polymer film
Introduction
appropriate UV light, the stable trans form of azobenzene
and its derivatives can be converted to the metastable cis
form.As response to irradiation with visible light or thermal
treatment in dark cis isomer will subsequently relax back
to the trans state. Moreover, associated photoisomerization
processes can take place and thus some of its properties
can be controlled. During the trans-cis-trans isomerization
processes significant changes of properties like the radiation
absorption, molecular dimensions and dipole moment
occur.1,11,12 The incorporation of photochromic moieties in
solid polymers has been explored in order to obtain new
photoresponsive materials and optical devices. Several
approaches were used to insert azobenzene chromophores
into polymer matrices: randomly incorporation (guest-host
structures), binding to main or side chains, or formation of
thin-layer structures like Langmuir-Blodgett films.2-5,13-15
Azobenzene-containing polymers have been investigated
with regard to different phenomena associated with the
photochromic nature of the azobenzene moiety. Their
One of the most interesting and studied systems that
can respond to changes in environmental conditions are
those that contain the azobenzene chromophores. Their
potential use in optics or optoelectronics, such as for
information storage, optical switching devices, surface
relief gratings, nonlinear optics, etc has been motive the
attention of the researchers and end users.1-10 Large area of
applications of azobenzene compounds is possible due to
both its photoinduced properties and wide variety of the
systems that it contained. Azobenzene chromophore has
unique optical and photochemical properties. It undergo a
reversible well know process with wide importance in pure
and applied chemistry and in nature, namely trans-to-cis
isomerization. This fundamental process can be controlled
by both light and thermal relaxation. So, irradiated with
*e-mail: airineia@icmpp.ro

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Responsive Behavior of 4-(N-maleimido)azobenzene in Polymers
J. Braz. Chem. Soc.
conformational modifications induced by photo- or
thermal isomerization of the azobenzene chromophore
give the possibility to control some physical and chemical
properties of the materials including viscosity, solubility,
pH, refractive index, conductivity, dielectric permittivity,
mechanical properties, etc.1-3,12,16
will be discussed. Two commercial polymers were chosen
as matrix component of the systems under study. One was
polystyrene that has a long hydrocarbon chain, with a phenyl
groupattachedtoeveryothercarbonatom.Theotherpolymer
matrixwaspolysulfonethatcontainsaromaticmoietiesalong
polymer chains. The two polymers are well known for their
ability to form photostable transparent films. Despite the fact
that polysulfone is a high performance polymer and widely
used in the preparation of the membranes and composites its
application as matrix for azobenzene chromophores lacks in
the literature. The azobenzene compound, 4-(N-maleimido)
azobenzene belongs to the azobenzene spectroscopic first
class, as described Rau.12 The photoresponsive behavior
of the 4-(N-maleimido)azobenzene in polymer matrix was
compared with that observed in dilute solution. The kinetic
parameters were evaluated from the temporal evolution of
p-p* absorption band intensity and the results were presented
herein.
Doping azobenzene compounds into polymer matrices
is a convenient inclusion technique. Besides opportunity
to use a presynthetized polymer the development of doped
azobenzene systems become interesting for studying
the behavior of chromophore and owing to the actual
tendency to obtain composites with very small content of
active component. A variety of polymers and azobenzene
compounds is possible to be used. The choosing of a
polymer is related to its film forming ability because for
many practical applications is of interest to use solid films.
On other hand it is necessary to take into account solubility
of azobenzene compounds in order to obtain homogeneous
and transparent films. Although it is known that the nature
and morphology of a polymer influence the photophysical
and photochemical behavior of a chromophore incorporated
in a polymer matrix, the way in which a photochromic
process is influenced by polymer properties is not completely
explained. The isomerization reactions in solid state may be
differentfromthoseinsolutionduetotheinfluenceofthefree
volume local distribution in the matrix and the restrictions of
chain mobility. For example, a non-first-order progress of the
reaction is frequently observed in solid state.The thermalcis-
trans isomerization of azobenzene groups is characterized
by first-order kinetics for polymers in solution or above their
glass transition temperature. On the other hand, deviations
from the first-order kinetics were found for the thermal
relaxation of azobenzene moities in polymer matrices in
glassy state. In this case the thermal back process has been
described by two or three exponentials3,12,17,18 or by the
Kohlrausch-Williams-Watt stretched exponential model.19
In the fundamental and applied chemistry maleimide
derivatives are placed on an important place. Due to
their dienophilic nature (Diels-Alder reaction, “click
chemistry”), reactivity in Michael-addition reactions
(especially for located S atoms in the protein and peptide
structures), ability to undergo photocycloaddition,
polymerization and crosslinking reactions this type of
compounds is widely employed as substrates in chemical
and biological applications. The electron-withdrawing
nature of the two carbonyl groups present in their structure
generate a very electron-deficient double bond, that than
explicate the reactivity of maleimides.20
Experimental
Equipment and reagents
Electronic absorption spectra were obtained on a
SPECORD M42 Carl Zeiss Jena spectrophotometer.
Samples as polymer films and as chloroform solutions using
10 mm quartz cells fitted with poly(tetrafluoroethylene)
stoppers were investigated. The initial absorbance in the
case of the solution samples was taken between 0.8 and 1.0.
The photochromic experiments were carried out at room
temperature using a 350 medium pressure mercury arc lamp
and suitable glass filters were used to obtain the irradiation
light.An intensity of 40-45 W m-2 for 365 nm radiation was
as a rule used. The polymer films were prepared on quartz
supports by casting from dichloroethane solution by mixing
equal portions of a 5 wt% polymer solution and a 0.2 wt%
solution of low-mass azobenzene. The films were dried
o
under reduced pressure at 50 C for 24 h. The thickness
of the films was about 120 mm. The thermal relaxation of
the samples under study was followed by monitoring the
temporal modification of the absorbance corresponding
p-p* transition at constant temperature. The reverse cis-
trans isomerization was also observed by irradiation with
436 nm visible light.
Synthesis of 4-(N-maleimido)azobenzene (MAz)
Maleic anhydride (Fluka) was purified by distillation.
4-Aminoazobenzene (Merck) was used as received.
Chloroform (Merck) was spectrophotometric grade.
All other chemicals were used as supplied. Polystyrene
In the present paper the photochromic behavior and the
kinetics of trans-cis-trans isomerization of low-molecular
weight azobenzene chromophores in polymer matrixes

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Airinei et al.
491
(Aldrich, Mw = 42 500) and polysulfone UDEL 1700
(Union Carbide, Mw = 38 000) were used as host polymers.
The guest component was prepared as follows:
behavior of the azobenzene moieties. Figure 1 shows
the absorption spectra of 4-(N-maleimido)azobenzene
(MAz) recorded at several values of the irradiation time.
The appearance of a strong absorption band centered
at around 327 nm and another absorption band of low
intensity at around 440 nm is evidenced in chloroform
solution. These new absorption bands can be assigned to
the p-p* and n-p* electronic transitions of the azobenzene
chromophore.1,11,12 Under 365 nm irradiation the intensity
of the p-p* absorption band gradually decreased with the
irradiation time, while that of the n-p* absorption band
increased. Simultaneously, the absorption maximum
of p-p* band is slightly shifted to shorter wavelengths
because the molecular conjugation decreases when the
cis content increases. The two typical isosbestic points
observed at around 285 and 390 nm, respectively,
confirms that only two absorbing species were present
in the reaction medium, namely trans and cis isomers
of maleimidoazobenzene. The trans-cis photoreaction
reached a stationary state after ca. 15 min of irradiation,
when the cis fraction is about 0.70.
To a well stirred solution of maleic anhydride (15 mmol,
1.47 g) in 10 mL of dry acetone, 4-aminoazobenzene
(15 mmol, 2.96 g) dissolved in dry acetone (15 mL) was
added under nitrogen over a period of 20 min at ice bath
temperature.After complete addition, the reaction mixture
was stirred for 3 h at room temperature. The insoluble
maleamic acid was separated by filtration, washed with
acetone and then added to a solution of sodium acetate in
acetic anhydride (30 mL, 0.025 mol) in order to cyclize to
the corresponding maleimide. The reaction was conducted
under nitrogen at 80 oC for 4 h. The mixture was poured
into a saturated aqueous solution of NaHCO3 and then
the precipitate was washed three times with water and
dried at 50 oC under vacuum. Pure MAz was obtained as a
bright orange crystalline solid after recrystalization from
chloroform. Yield 85%; mp 169 oC.
Nitrogen analysis calculated for C16H11N3O2: N,
15.15%. Found: N, 15.10%.
1H NMR (DMSO-d6) d (ppm): 7.25 (s, 2H, CH=CH),
7.6-7.7 (m, 5H, Ar), 7.95 (d, 2H, Ar, adjacent to azo), 8.05
(d, 2H, Ar, adjacent to imide).
The photoisomerization kinetics of azobenzene
chromophores in MAz was established by monitoring the
absorbance of p-p* absorption band. The experimental data
were analysed according to the equation:21
Results and Discussion
ln(Ao – A∞)/(At – A∞) = k1t
(1)
The guest compound of N-substituted maleimide
type was prepared by reacting 4-amino azobenzene
with maleic anhydride. On the first stage of the reaction,
maleic anhydride dissolved in acetone was condensed at
room temperature with 4-aminoazobenzene (in equimolar
amounts) to give azobenzenemaleanilic acid and then the
anhydride cycle was reformed in the presence of acetic
anhydride at 80 oC. The synthetic procedure of azoactive
compound is illustrated in Scheme 1.
where t is the irradiation time, Ao, At and A∞ are the trans
form absorbances corresponding to the time 0, t and
photostationary state, respectively. The rate constant k1 has
the expression k1 = ktc + kct = I(ftcet + fctec)ln10, where ktc
and kct are the rate constants for the trans-cis and cis-trans
photoisomerization reactions, respectively, ftc and fct are the
quantum yields of the same processes, et andec are the molar
absorptivities of the trans and cis species at the irradiation
wavelength, and I is the incident radiation intensity. The
plots of ln (Ao - A∞)/(At - A∞) versus irradiation time were
linear for all samples (solution and film) as shown in
Figure 2. A value of 6.37 × 10-3 s-1 was obtained for the
rate constant (k1) for photoisomerization of azobenzene
moieties in MAz in chloroform solution.
Photoisomerization of 4-(N-maleimido)azobenzene
molecules in solution
Ultraviolet visible absorption spectroscopy was
utilized in order to investigate the photoisomerization
Scheme 1. Synthesis of maleimidoazobenzene derivative.

492
Responsive Behavior of 4-(N-maleimido)azobenzene in Polymers
J. Braz. Chem. Soc.
ln(A∞ – Ao)/(A∞ – At) = k2t
(2)
where k2 is the rate constant of the recovery reaction. The
o
value of k2 for MAz was found 3.33 × 10-5 s-1 at 50 C.
From theArrhenius plot of the rate constants, the following
values of activation energy and the preexponential factor
were evaluated: Ea = 18.06 kcal mol-1 and Z = 6.3 × 107 s-1.
The value found for the activation energy of cis-trans
isomerization of MAz is rather similar to those of other
azobenzene compounds.11,23-26
Figure 1. Change in electronic absorption spectra during trans–cis
photoisomerization of 4-(N-maleimido)azobenzene. Arrows indicate the
directional changes of the spectra. Irradiation times: 0, 30, 60, 90, 120,
180, 300, 420, 900 s.
Figure 3. Variation of UV-Vis spectra of MAz during the thermal
o
relaxation process at 60 C. Inlet: Cis-trans isomerization kinetics in
solution.Arrows indicate the directional changes of the spectra. Relaxation
times: 0, 35, 70, 125, 225, 345, 585 min.
Photoisomerization in polymer matrix
As known, the isomerization reactions of azobenzene
chromophores in rigid media depend very much on the
local environment and free volume distribution around the
chromophores.27-29
Figure 2. Photoisomerization kinetics for MAz in solution () and in
polystyrene matrix ().
Figure 4 illustrates the changes in the electronic
absorption spectra of the 4-(N-maleimidoazobenzene)
dispersed in polystyrene during irradiation with UV
radiation of 360 nm. One can see that the polymer films
exhibit similar absorption maxima as liquid samples,
namely at around 333 and 440 nm, assigned to the p-p*
and n-p* transitions, respectively. The p-p* absorption
band decreases in intensity with the irradiation time due
to the trans-cis photoisomerization process of azobenzene
moieties. A photostationary state is reached after 20 min
when the conversion is about 60%. As in the case of liquid
samples, there are isosbestic points on the both sides of the
p-p* absorption band, and the absorption maximum shifted
to shorter wavelengths. It is to be noticed that the cis content
of the stationary state was a little lower in polymer film
than in solution.A photoisomerization inhibition should be
expected in solid samples because the free volume around
the chromophore is smaller than in liquid samples due to
a closer packing of the polymer chains in film.
Thermal cis-trans isomerization in solution
In the dark, the cis isomer slowly reverted to the original
trans form that is thermodynamically more stable. The
absorption spectrum gradually recovered to the before
irradiation pattern. The thermal back isomerization was
monitored at several temperatures and the results are shown
o
in Figure 3 for MAz, at 60 C. The absorption band at
327 nm increased in intensity to the starting value before
irradiation. The retention of isosbestic points shows that a
real cis-trans isomerization occurs thermally. The cycle of
trans-cis photoisomerization and cis-trans isomerization
can be repeated several times without significant
modification in the intensity of p-p* absorption band.
It is known that the thermal cis-trans isomerization
of azo species follows first-order kinetics.22,23 The kinetic
equation in this case should be:

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cis-trans isomerization takes place, namely one where
a fraction of cis azo molecules quickly convert to trans
species and another one where the remainder isomerizes
more slowly. This fact seems to be in agreement with
experimental data obtained for other azoaromatic polymer
systems.29,31 The fast process can be ascribed to the trapping
of cis chromophores in a strained environment, that
determines their rapid return to trans species as compared
to the relaxed cis isomers. The slow isomerization can be
attributed to the cis species which are not environmentally
stressed to isomerize. The slow isomerization process
would depend on the coupling interaction between the
azobenzene moieties and polymer segments and on the
mobility of the polymer chain segments.32 The two rate
constants for MAz containing polystyrene films were
1.34 × 10-5 s-1 and 6.43 × 10-4 s-1 at 50 oC.
Figure 4. Spectral changes during trans-cis photoisomerization for a
polymer film containing maleimidoazobenzene. Arrows indicate the
directional changes of the spectra. Irradiation times: 0, 30, 60, 120, 180,
540 s.
The change in absorbance of the p-p* band during
UV irradiation is consistent with a first-order process
(Figure 2). The rate constant (k1) for the azochromophore
photoisomerization was found to be 8.34 × 10-3 s-1 for
polystyrene film and 7.54 × 10-3 s-1 for polysulfone film.
One observes that the photoisomerization rates in solution
and in polymer films are of the same order of magnitude.
These results suggest that there is no significant influence
of the polymer main chain structure and packing on the
trans-cis photoisomerization process in solution as well
as in polymer film. Similar observations have been made
for other azobenzene-containing polymers.25,30 However a
deviation from the first-order law was observed in the later
stages of the photoprocess in polymer films and it has been
explained by a retardation of cis isomer formation due to
some restrictions in the chromophore mobility.
On the other hand the kinetics of the thermal cis-trans
o
reaction in polystyrene films at 70 C and higher could
be satisfactorily described with equation 2 (Figure 5).
The activation energy for the first process of the cis-trans
recovery evaluated from Arrhenius plot, is lower for film
samples (Ea = 15.55 kcal mol-1) than that found in solution.
Also the corresponding estimated preexponential factor
(Z = 3.3 × 106 s-1) is also smaller. Such behavior can be
explained by assuming that the structures of the cis isomers
are different in solution and in polymer matrix. Also, we
found out that the kinetics of thermal recovery in MAz
containing polysulfone film at 65 oC is also described by
a first-order kinetics (Figure 5), with a rate constant of
4.34 × 10-4 s-1.
These values of rate constants are similar in magnitude
to those of other systems containing azobenzene units
covalently bonded or incorporated in polystyrene. The
rate constants corresponding to thermal isomerization
of 2-ethoxyazobenzene or 2,2′-azonaphthalene were
1.41 × 10-4 (55 oC) and 1.33 × 10-4 s-1 (45 oC), respectively,33
while for 4-dimethylaminoazobenzene chromophore in
polystyrene two rate constants were found out 2.50 × 10-4
and 5.33 × 10-5 s-1 (30 oC), respectively.3
The trans-cis photoisomerization in polymer films was
also performed by irradiation with 334 nm UV light. The
changes in the electronic absorption spectra show similar
trends as described above, the photoisomerization rate
constant being 6.38 × 10-3 s-1.
Thermal cis-trans izomerization in polymer matrix
The spectral changes show a recovery to the initial
spectral pattern. The thermal reversion to the original trans
form in polystyrene film was observed after about 230 min
at 70 oC. The thermally induced cis-trans isomerization of
the azobenzene molecules incorporated in polymer films was
monitored at different temperatures. Representative data are
shown in Figure 5. One observes that the process occurring
o
below 70 C deviates from the first-order kinetics. In this
case a biexponential equation 3 containing two first-order
termsdescribesthekineticbehaviorofthethermalrecovery.31
(A∞ – Ao)/(A∞ – At) = aexp(–k2t) + (1 – a)exp(–k3t) (3)
The back reaction process in MAz containing polymer
films was also carried out by irradiation with blue light
at 436 nm (Figure 6). The increase in intensity of the
p-p* absorption was very quickly during irradiation
and the absorption spectrum was completely restored
to that of nonirradiated sample after only 360 s. The
corresponding cis-trans isomerization rate constant was
where k2 and k3 are the rate constants of the fast and slowly
isomerizing processes, respectively, and a the fraction of
fast isomering species.
It is supposed that the biexponential equation 3 suggests
two different local environments in which the thermal

494
Responsive Behavior of 4-(N-maleimido)azobenzene in Polymers
J. Braz. Chem. Soc.
isomerization of azobenzene chromophores in polymer films
has been satisfactorily described by first-order kinetics at
70 oC and higher, while two first-order rate processes occur
at temperatures below 70 oC. By irradiation with visible
light at 436 nm, the cis-trans process occurs very quickly
(in 360 s) in comparison with thermal relaxation (few days
o
at 30 C). These results may be useful in design of new
polymeric containing-azobenzene materials with enhanced
photoresponsive properties and they' could be promising
materials in some optical applications.
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Received: July 2, 2009
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Web Release Date: December 3, 2009