New multifunctional sulfonamide-containing azobenzene chromophores: Synthesis and photochromic properties

Article abstract of DOI:10.1016/j.dyepig.2011.07.008

The work presents synthesis and characterization of novel sulfadimethoxine and sulfabenzamide azo derivatives as well as the kinetic study of their trans-cis-trans isomerization. Spectral properties and kinetic constants were determined for the chromophor

Full text of DOI:10.1016/j.dyepig.2011.07.008

Dyes and Pigments 92 (2012) 1018e1024
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New multifunctional sulfonamide-containing azobenzene chromophores:
Synthesis and photochromic properties
*
ꢀ
Sonia Zielinska, Maria Larkowska, Stanis1aw Kucharski
Wrocław University of Technology, Department of Polymer Engineering and Technology, 50-370 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The work presents synthesis and characterization of novel sulfadimethoxine and sulfabenzamide azo
derivatives as well as the kinetic study of their transecisetrans isomerization. Spectral properties and
kinetic constants were determined for the chromophores dispersed in poly(methyl methacrylate-co-
butyl methacrylate) deposited on glass substrates in form of transparent, thin films. The maximum
absorption bands were observed within wavelength in range of 446e457 nm for N-disubstituted ami-
noazobenzene derivatives and 361e362 nm for phenolic chromophores. For the phenolic azo derivatives
of sulfadimethoxine and sulfabenzamide the photoisomerization induced with UV light followed the first
order kinetics, while for other compounds the process combined of two parallel reactions with different
rate constants was observed. The rate constants of thermal relaxation in the dark were usually few times
smaller than those determined for photoinduced transecis isomerization. The change of real part of
refractive index upon illumination determined by ellipsometry was within the range of 0.0043e0.0138.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 10 March 2011
Received in revised form
13 July 2011
Accepted 16 July 2011
Available online 23 July 2011
Keywords:
Azobenzene chromophores
Photoisomerization kinetics
Refractive index modulation
Transecis isomerization
Sulfonamide derivatives
Photochromism
1. Introduction
sulfamethoxazole, isoxazole, imidazole and many other heterocy-
clic compounds have been published [16e26]. These chromophores
Azobenzene-containing chromophores have attracted attention
in recent years due to their unique properties arising from revers-
ible conformational changes accompanying light-induced trans-
ecisetrans isomerization. Illumination of azobenzene derivatives
with UV or visible light yields a photostationary composition that is
temperature and wavelength dependent [1]. They are one of the
most popular class of the photochromic compounds due to
simplicity and cost-effectiveness of their synthesis and purification
accompanied by versatile, unique properties. These features make
azobenzene derivatives promising materials for many applications,
including holographic data storage [2,3], liquid crystal displays
[4,5], surface relief gratings (SRGs) formation [6e8] and nonlinear
optical processes [9e11]. Numerous studies were dedicated to
various systems containing azobenzene chromophores. Among the
azobenzenes the simple derivatives of p-nitroaniline, like Disperse
Red dyes, have been most widely investigated [12e16]. However,
other more sophisticated structures containing heterocyclic rings
were also designed and synthesized. The papers concerning prop-
erties of the azobenzene derivatives of sulfisomidine,
were successfully incorporated into various systems, like Lang-
muireBlodgett films [17,18], vinyl and addition polymers
[16,19e22] or hybrid organic-inorganic materials based on silicon
[23,24]. Different kinds of properties of materials mentioned above
have been investigated, including photochromism, refractive index
modulation [19,22], second harmonic generation [17,18] or surface
relief grating formation [25,26]. The results of the studies turned
out to be promising and encouraged us to continue the investiga-
tions in that field.
The aim of this study was to determine susceptibility of the
photochromic dyes vs. light in polymer matrix used as a continuous
phase. Two kinds of the dyes were selected for the synthesis. The
first ones were azo derivatives containing hydroxyphenyl group at
the side of electron donor part of the molecule and absorbing in UV
range. The second ones were similar derivatives containing ami-
nophenyl group at analogous place and these dyes absorbed light in
visible range of the spectrum.
The reversible transecisetrans isomerization of the chromo-
phores was investigated using ellipsometric technique and UVeVis
spectroscopy for “guestehost” systems with poly(methyl methac-
rylate-co-butyl methacrylate) as a polymer matrix. Kinetic study
was performed to provide full characteristics of photochromic
properties for all compounds. Due to the presence of reactive
* Corresponding author. Tel./fax: þ1 48 71 3202862.
E-mail address: stanislaw.kucharski@pwr.wroc.pl (S. Kucharski).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.07.008

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S. Zielinska et al. / Dyes and Pigments 92 (2012) 1018e1024
1019
hydroxyl group in molecules of all chromophores, they can be easily
transformed into vinyl monomers or siloxane precursors and then
into macromolecular or hybrid photochromic materials.
1H NMR (DMSOed6): 3.11 ppm, s, 3H (CH3); 3.57 ppm, dd, 4H
(OCH2CH2N); 4.80 ppm, s, 1H (OH); 6.84 ppm, d, 2H (CH in phe-
nylene ring in ortho position to amino group); 7.48 ppm, t, 2H (CH
in phenyl ring in meta position to carbonyl group); 7.62 ppm, t, 1H
(CH in phenyl ring in para position to carbonyl group); 7.79 ppm, d,
2H (CH in phenylene ring in meta position to amino group);
7.85 ppm, d, 2H (CH in phenyl ring in ortho position to carbonyl
group); 7.91 ppm, d, 2H (CH in phenylene ring in ortho position to
sulfonyl group); 8.11 ppm, d, 2H (CH in phenylene ring in meta
position to sulfonyl group); 12.63 ppm, br.s., 1H (NH). Elemental
analysis calcd for C22H22N4O4S: C 60.26%, H 5.06%, N 12.78%, S
2. Materials and methods
2.1. Materials
Sulfabenzamide (TCI Europe), sulfadimethoxine (TCI Europe),
phenol (POCH S.A. Gliwice Poland), 2-(methylphenylamino)ethanol
(SigmaeAldrich), 2-(N-ethylanilino)ethanol (SigmaeAldrich), N-
phenyldiethanolamine (SigmaeAldrich), poly(methyl methacry-
late-co-butyl methacrylate) (Mw ¼ w75 000) (SigmaeAldrich),
sodium nitrite (POCH S.A. Gliwice Poland), sodium acetate (POCH
S.A., Gliwice, Poland) were used as received.
7.31%. Found:
C 60.20%, H 4.82%, N 12.36%, S 7.50%. M.p.:
159.8e162.3 ꢀC.
2.2.5. N-[4-[(E)-[4-[(E)-[4-[ethyl(2-hydroxyethyl)amino]phenyl]
azo]phenyl]azo]phenyl] sulfonylbenzamide (SBAe)
2-(N-ethylanilino)ethanol (4.0 mmol, 0.66 g) was dissolved in
2 ml of methanol and cooled to 0 ꢀC. Then it was added to solution
of diazonium salt (SBACl) and stirred for 2 h. The reaction mixture
was neutralized with sodium acetate and filtered under vacuum.
The crude product was washed several times with water and
recrystallized from ethanol. SBAe was obtained in a yield of 92%.
1H NMR (DMSOed6): 1.13 ppm, t, 3H (CH3); 3.57 ppm, m, 6H
(CH2); 4.81 ppm, br.s., 1H (OH); 6.84 ppm, d, 2H (CH in phenylene
ring in ortho position to amino group); 7.48 ppm, t, 2H (CH in
phenyl ring in meta position to carbonyl group); 7.62 ppm, t, 1H (CH
in phenyl ring in para position to carbonyl group); 7.79 ppm, d, 2H
(CH in phenylene ring in meta position to amino group); 7.85 ppm,
d, 2H (CH in phenyl ring in ortho position to carbonyl group);
7.91 ppm, d, 2H (CH in phenylene ring in ortho position to sulfonyl
group); 8.11 ppm, d, 2H (CH in phenylene ring in meta position to
sulfonyl group); 12.62 ppm, br.s., 1H (NH). Elemental analysis calcd
for C23H24N4O4S: C 61.05%, H 5.35%, N 12.38%, S 7.09%. Found: C
61.00%, H 5.31%, N 11.98%, S 7.27%. M.p.: 157.6e160.3 ꢀC.
2.2. Synthesis and characterization
2.2.1. 4-[(E)-[4-(benzoylsulfamoyl)phenyl]azo]benzenediazonium
chloride (SBACl)
The mixture of sulfabenzamide (4.0 mmol, 1.10 g), concentrated
hydrochloric acid (2 ml) and water (9 ml) was heated to 50 ꢀC and
then cooled to 0 ꢀC in an ice-brine bath. The solution of sodium
nitrite (4.5 mmol, 0.31 g) in water (1 ml) was added dropwise to the
reaction mixture keeping the temperature between 0 and 5 ꢀC. The
solution of diazonium salt was stirred for 30 min maintaining a low
temperature and was further used in coupling reactions.
2.2.2. 4-[(E)-[4-[(2,6-dimethoxypyrimidin-4-yl)sulfamoyl]phenyl]
azo]benzenediazonium chloride (SDMCl)
The diazonium salt in the form of SDMCl was synthesized as
described above using sulfadimethoxine (4.0 mmol, 1.24 g) instead
of sulfabenzamide.
2.2.3. N-[4-[(E)-[4-[(E)-(4-hydroxyphenyl)azo]phenyl]azo]phenyl]
sulfonylbenzamide (SBAph)
2.2.6. N-[4-[(E)-[4-[(E)-[4-[bis(2-hydroxyethyl)amino]phenyl]azo]
phenyl]azo]phenyl] sulfonylbenzamide (SBAf)
Phenol (4.5 mmol, 0.42 g) was dissolved in 5 ml of 6 wt % water
solution of sodium hydroxide and was cooled to the temperature
below 5 ꢀC. Then to the solution of phenol was slowly added
solution of diazonium salt (SBACl). The reaction mixture was stirred
for 2 h and then was filtered under vacuum. The crude product was
washed several times with water and recrystallized from ethanol.
SBAph was obtained in a yield of 88%.
The solution of N-phenyldiethanolamine (4.0 mmol, 0.72 g) in
methanol (3 ml) was cooled to temperature below 5 ꢀC. Then it was
slowly added to the solution of diazonium salt (SBACl) keeping
temperature between 0 and 5 ꢀC. The reaction mixture was stirred
for 2 h. Product of coupling was precipitated by neutralization of
mixture to pH 7 using sodium acetate. The crude product was
filtered under vacuum and washed several times with water. SBAf
was recrystallized from ethanol. The yield of product was 91%.
1H NMR (DMSOed6): 3.43 ppm, t, 8H (CH2); 4.86 ppm, s, 2H
(OH); 6.84 ppm, d, 2H (CH in phenylene ring in ortho position to
amino group); 7.48 ppm, t, 2H (CH in phenyl ring in meta position to
carbonyl group); 7.62 ppm, t, 1H (CH in phenyl ring in para position
to carbonyl group); 7.79 ppm, d, 2H (CH in phenylene ring in meta
position to amino group); 7.85 ppm, d, 2H (CH in phenyl ring in
ortho position to carbonyl group); 7.91 ppm, d, 2H (CH in phenylene
ring in ortho position to sulfonyl group); 8.11 ppm, d, 2H (CH in
phenylene ring in meta position to sulfonyl group); 12.62 ppm, br.s.,
1H (NH). Elemental analysis calcd for C23H24N4O5S: C 58.96%, H
5.16%, N 11.96%, S 6.84%. Found: C 58.83%, H 5.04%, N 11.35%, S 7.07%.
M.p. 117.0e119.6 ꢀC.
1H NMR (DMSOed6): 6.84 ppm, d, 2H, (CH in phenylene ring in
ortho position to hydroxyl group); 7.48 ppm, t, 2H (CH in phenyl
ring in meta position to carbonyl group); 7.62 ppm, t, 1H (CH in
phenyl ring in para position to carbonyl group); 7.79 ppm, d, 2H (CH
in phenylene ring in meta position to hydroxyl group); 7.85 ppm, d,
2H (CH in phenyl ring in ortho position to carbonyl group);
7.91 ppm, d, 2H (CH in phenylene ring in ortho position to sulfonyl
group); 8.11 ppm, d, 2H (CH in phenylene ring in meta position to
sulfonyl group); 10.55, s, 1H (OH); 12.62 ppm, br.s., 1H (NH).
Elemental analysis calcd for C19H15N3O4S: C 59.83%, H 3.96%, N
11.02%, S 8.41%. Found: C 59.50%, H 3.50%, N 10.38%, S 8.09%. M.p.:
263.5e265.2 ꢀC.
2.2.4. N-[4-[(E)-[4-[(E)-[4-[2-hydroxyethyl(methyl)amino]phenyl]
azo]phenyl]azo] phenyl]sulfonylbenzamide (SBAm)
2.2.7. N-(2,6-dimethoxypyrimidin-4-yl)-4-[(E)-[4-[(E)-(4-
hydroxyphenyl)azo]phenyl] azo]benzenesulfonamide (SDMph)
SDMph was synthesized as described in the synthesis of SBAph
using SDMCl instead of SBACl. The product was obtained in a yield
of 83%.
The cold solution of 2-(methylphenylamino)ethanol (4.1 mmol,
0.62 g) in water (2 ml) was poured into solution of diazonium salt
(SBACl) and stirred for 2 h. Then reaction mixture was neutralized
with sodium acetate and filtered under vacuum. The crude product
was washed several times with water and recrystallized from
ethanol. SBAm was obtained in a yield of 90%.
1H NMR (DMSOed6): 3.72 ppm, s, 3H (OCH3 in 2,6-
dimethoxypirimidin-4-yl ring in C6 position); 3.78 ppm, s, 3H

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S. Zielinska et al. / Dyes and Pigments 92 (2012) 1018e1024
1020
(OCH3 in 2,6-dimethoxypirimidin-4-yl ring in C2 position);
5.97 ppm, s, 1H (CH in 2,6-dimethoxypirimidin-4-yl ring in C5
position); 6.84 ppm, d, 2H (CH in phenylene ring in ortho position to
hydroxyl group); 7.77 ppm, d, 2H (CH in phenylene ring in meta
position to hydroxyl group); 7.87 ppm, d, 2H (CH in phenylene ring
in ortho position to sulfonyl group); 8.04 ppm, d, 2H (CH in phe-
nylene ring in meta position to sulfonyl group); 9.98 ppm, s, 1H
(OH); 11,77 ppm, br.s., 1H (NH). Elemental analysis calcd for
C18H17N5O5S: C 52.04%, H 4.12%, N 16.86%, S 7.72%. Found: C 52.18%,
H 3.67%, N 16.52%, S 7.84%. M.p.: 153.0e155.8 ꢀC.
4-yl ring in C6 position); 3.77 ppm, s, 3H (OCH3 in 2,6-
dimethoxypirimidin-4-yl ring in C2 position); 4.85 ppm, s, 1H (OH);
5.96 ppm, s, 1H (CH in 2,6-dimethoxypirimidin-4-yl ring in C5 posi-
tion); 6.83 ppm, d, 2H (CH in phenylene ring in ortho position to amino
group); 7.76 ppm, d, 2H (CH in phenylene ring in meta position to
amino group); 7.86 ppm, d, 2H (CH in phenylene ring in ortho position
to sulfonyl group); 8.04 ppm, d, 2H (CH in phenylene ring in meta
position to sulfonyl group); 11.78 ppm, br.s., 1H (NH). Elemental
analysis calcd for C22H26N6O5S: C 54.31%, H 5.39%, N 17.27%, S 6.59%.
Found: C 54.50%, H 5.22%, N 16.77%, S 6.25%. M.p.: 135.8e137.9 ꢀC.
2.2.8. N-(2,6-dimethoxypyrimidin-4-yl)-4-[(E)-[4-[(E)-[4-[2-
hydroxyethyl(methyl)amino] phenyl]azo]phenyl]azo]
benzenesulfonamide (SDMm)
2.2.10. 4-[(E)-[4-[(E)-[4-[bis(2-hydroxyethyl)amino]phenyl]azo]
phenyl]azo]-N-(2,6-dimethoxy-pyrimidin-4-yl)benzenesulfonamide
(SDMf)
SDMm was obtained according to the procedure described in
the synthesis of SBAm using SDMCl instead of SBACl. The yield of
product was 85%.
SDMf was obtained according to the procedure described in the
synthesis of SBAf using SDMCl instead of SBACl. The yield of
product was 83%.
1H NMR (DMSOed6): 3.08 ppm, s, 3H (CH3N); 3.56 ppm, dd, 4H
(NCH2CH2O); 3.74 ppm, s, 3H (OCH3 in 2,6-dimethoxypirimidin-4-
yl ring in C6 position); 3.78 ppm, s, 3H (OCH3 in 2,6-
dimethoxypirimidin-4-yl ring in C2 position); 4.80 ppm, s, 1H
(OH); 5.95 ppm, s, 1H (CH in 2,6-dimethoxypirimidin-4-yl ring in
C5 position); 6.85 ppm, d, 2H (CH in phenylene ring in ortho
position to amino group); 7.78 ppm, d, 2H (CH in phenylene ring in
meta position to amino group); 7.88 ppm, d, 2H (CH in phenylene
ring in ortho position to sulfonyl group); 8.04 ppm, d, 2H (CH in
phenylene ring in meta position to sulfonyl group); 11.78 ppm, br.s.,
1H (NH). Elemental analysis calcd for C21H24N6O5S: C 53.38%, H
5.12%, N 17.79%, S 6.79%. Found: C 53.12%, H 4.94%, N 16.67%, S
6.82%. M.p.: 177.8e180.7 ꢀC.
1H NMR (DMSOed6): 3.57 ppm, m, 8H (NCH2CH2O); 3.72 ppm, s,
3H (OCH3 in 2,6-dimethoxypirimidin-4-yl ring in C6 position);
3.78 ppm, s, 3H (OCH3 in 2,6-dimethoxypirimidin-4-yl ring in C2
position); 4.86 ppm, s, 2H (OH); 5.97 ppm, s, 1H (CH in 2,6-
dimethoxypirimidin-4-yl ring in C5 position); 6.84 ppm, d, 2H
(CH in phenylene ring in ortho position to amino group); 7.77 ppm,
d, 2H (CH in phenylene ring in meta position to amino group);
7.87 ppm, d, 2H (CH in phenylene ring in ortho position to sulfonyl
group); 8.04 ppm, d, 2H (CH in phenylene ring in meta position to
sulfonyl group 2); 11.77 ppm, b.s., 1H (NH). Elemental analysis:
calcd for C22H26N6O6S: C 52.58%, H 5.21%, N 16.72%, S 6.38%. Found:
C 52.00%, H 5.41%, N 16.25%, S 6.53%. M.p.: 163.6e165.4 ꢀC.
2.3. Preparation of the films
2.2.9. N-(2,6-dimethoxypyrimidin-4-yl)-4-[(E)-[4-[(E)-[4-[ethyl(2-
hydroxyethyl)amino] phenyl]azo]phenyl]azo]benzenesulfonamide
(SDMe)
The guest-host thin polymer films containing azo dyes were
prepared by spin coating technique. The synthesized chromo-
phores were dissolved in 1.2 ml of tetrahydrofuran in an amount of
12 mg. Then 28 mg of poly(methyl methacrylate-co-butyl meth-
acrylate) was added to each photochromic solution. The polymer
mixtures containing azo dyes were ultrasonicated for 15 min and
SDMe was obtained according to the procedure described in the
synthesis of SBAe using SDMCl instead of SBACl. The yield of
product was 89%.
1H NMR (DMSOed6): 1.10 ppm, t, 3H (CH3); 3.49e3.58 ppm, m, 6H
(CH2eNeCH2CH2); 3.71 ppm, s, 3H(OCH3 in 2,6-dimethoxypirimidin-
filtered through syringe filter with a pore size of 0.45
mm. The
+ Cl
N2
NH2
a
O
R1
HCl, CH3COOH
NaNO2
N
R2
SO2
SO2
OCH3
N
OCH3
HN
HN
R1
R2
R1
SDMph
SDMm
SBAph
SBAm
OH
CH2
N
+ Cl
R2
HO
CH2
CH3
N2
b
N
CH2
N
SDMe
SDMf
CH3
SBAe
SBAf
N
HO
+
CH2
CH2
SO2
HN
CH2
N
CH2
HO
CH2
R1
CH2
OH
SO2
HN
R1
Fig. 1. Scheme of azo dyes synthesis a) diazotization reaction; b) coupling reaction.

ꢀ
S. Zielinska et al. / Dyes and Pigments 92 (2012) 1018e1024
1021
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
materials were deposited onto glass plates in form of thin trans-
parent films. The resulting films were kept in dark before
measurements. The films were illuminated with UV tube lamp (8 W
power) or with white light filtered to remove radiation below
400 nm. The white light power was 2 mW per square centimeter.
2.4. Measurements
1H NMR and 1H e 1H COSY NMR spectra were recorded with
300 MHz BRUKER Advance DRX-300 apparatus using DMSOed6 as
solvents and TMS as an internal standard.
UVeVis spectra were recorded with Hitachi
U
1900
spectrophotometer.
Melting points were determined with Electrothermal 9100
apparatus.
Ellipsometer EL X-02C of DRE Ellipsomerterbau GmbH
(Germany) operated at an incident angle of 70ꢀ and using a linearly
polarized laser beam of 632 nm and ca. 3 mW power was used to
measure the changes of refractive index of thin photochromic films
before and after illumination with light. The measurements were
carried out in air at room temperature.
SDMf
10 min. of illumination
20 min. of relaxation
300
350
400
450
500
550
600
wavelength [nm]
Fig. 2. Spectral changes of SDMf in p(MMA-co-BMA) (-,-) after 10 min of illumina-
tion with white light (
) and after 20 min of thermal relaxation in the dark (
)
recorded at 25 ꢀC.
3. Results and discussion
HOMOeLUMO gap for phenol derivatives was ca. ꢁ7.6 eV while for
the other compounds it was in the range of ꢁ6.98 to ꢁ7.09 eV.
These results indicate that 2-(N-ethylanilino)ethanol is the
strongest electron donor, what is in good agreement with
measured data, as the maximum absorption band of chromophores
SBAe and SDMe, containing that donor group, is shifted towards
longer wavelengths as compared with other amino chromophores.
Phenol was found to be much weaker electron donor in comparison
to N-disubstituted aminophenyl groups resulting in distinct UV-
shift of SBAph and SDMph maximum absorption band. The pres-
ence of alkyl groups bound to nitrogen atom in aminophenyl group
increases the donor strength. And additionally, the electron donor
ability of 2-(methylphenylamino)ethanol is more pronounced than
that of N-phenyldiethanolamine.
3.1. Photoisomerization characteristics
All chromophores were synthesized through the welleknown
reaction path given in Fig. 1. The synthesis consisted of two types of
reaction occurring at temperatures below 5 ꢀC. In the first one the
diazotization of a primary amine to diazonium salt took place
(Fig. 1a). The second reaction was a coupling of the diazonium salt
with a tertiary amine or phenol (Fig. 1b). The dyes so obtained in
good yields of 83e92% were colored crystalline solids. The purities
and structures of synthesized chromophores were confirmed by 1H
NMR. The 1H NMR peaks assignments were verified with use of 1H
e
1H COSY NMR spectroscopy.
Kinetic aspects of transecisetrans isomerization of sulfonamide
The azobenzene-containing chromophores in solution usually
obey the first order kinetics described by the following equation
[1]:
chromophores were studied using UVeVis spectroscopy. The azo-
benzene derivatives of sulfabenzamide and sulfadimethoxine were
dispersed in poly(MMA-co-BMA) matrix and were examined as
thin films deposited onto glass substrates. Chromophore to poly-
mer host weight ratio and preparation conditions were the same
for all samples.
AN ꢁ At
ln
¼ ꢁk1t
(1)
AN ꢁ A0
All chromophores underwent reversible transecis isomerization
during illumination with light. Examples of spectral changes after
10 min of irradiation and 20 min of thermal relaxation in the dark
are presented in Figs. 2 and 3. The maximum absorption band of
phenol derivatives (SBAph and SDMph) was observed at
361e362 nm, while for other derivatives it was placed within
visible range of wavelengths at ca. 450 nm. The exact values of lmax
are presented in Table 1. To explain the influence of chemical
structure of the chromophore molecule on the position of
maximum absorption band, the approximate values HOMO and
LUMO values were calculated. The HOMOeLUMO gap was chosen
as an indicator of electron donoreacceptor interaction due to
simplicity of its reliable estimation with quantum chemical
methods.
The geometries of SBAph, SBAf, SBAm and SBAe were optimized
using MOPAC2009 program with PM6 semi-empirical method [27].
The optimized structures were further used to determine the
energy of highest occupied molecular orbital (HOMO), which
according to Koopmans’ theorem [2], is a good estimation of
molecule ionization potential. The estimated values of ionization
potentials were found to be equal to 9.38, 8.72, 8.64 and 8.58 eV,
ascribed to SBAph, SBAf, SBAm and SBAe, respectively. The
1,2
1,0
0,8
0,6
0,4
0,2
SBAph
10 min. of illumination
20 min. of relaxation
0,0
300
325
350
375
400
425
450
wavelength [nm]
Fig. 3. Spectral changes of SBAph in p(MMA-co-BMA) (-,-) after 10 min of illumi-
nation with UV light (
) and after 20 min of thermal relaxation in the dark (
)
recorded at 25 ꢀC.

ꢀ
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S. Zielinska et al. / Dyes and Pigments 92 (2012) 1018e1024
Table 1
4,0
Kinetic data for transecis photoisomerization determined by UVeVis measurements
for chromophores in p(MMA-co-BMA) matrix.
SBAm
SBAf
SBAe
SBAph
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
Sample
lmax [nm]
a
k1 ꢂ 103 [sꢁ1
]
1 ꢁ
a
k2 ꢂ 103 [sꢁ1
]
SDMm
SDMf
SDMe
SBAm
SBAf
SBAe
SBAph
SDMph
450
448
454
448
446
457
361
362
0.66
0.65
0.59
0.65
0.58
0.85
1
148.5
81.9
69.6
116.7
74.4
167.3
11.7
0.34
0.35
0.41
0.35
0.42
0.15
e
9.80
7.96
7.16
4.84
9.93
8.30
e
1
20.8
e
e
where AN represents absorbance at the photostationary state, A0
represents absorbance at initial state (before irradiation) and At
represents absorbance of the sample at time t.
The same chromophores incorporated into polymer matrix
usually exhibit deviation from simple first-order kinetics; what
suggests that in these conditions the photoreaction is a combina-
tion of two processes [2]. The transecis photoisomerization results
in a change of molecular shape of the azo chromophore around
diazo group. In a dense polymeric milieu the molecular movement
can be hindered and therefore this influence should be taken into
account as a slow part of the material response. The fraction of each
process can be estimated from the following equation:
0
20
40
60
80
100 120 140 160 180 200 220
time [s]
Fig. 5. Kinetics of sulfabenzamide derivatives photoisomerization induced by illumi-
nation with white (SBAm, SBAe, SBAf) or UV (SBAph) light at 25 ꢀC.
concentration, the change of shape is not hindered by the solvents
and therefore the first order kinetics is observed. In our case, the
concentration of the chromophore in polymer matrix is, by no
accident, high to ensure good photochromic response. The molec-
ular movement of the chromophore molecules in such dense
environment must be hindered by rheological properties of the
material, particularly if photoreactions take place at the tempera-
ture below Tg. It is also probable that polar dye molecules interact
not only with copolymer chains of the matrix but also between
themselves. Due to the presence of hydroxyl group(s) in dye
molecules the interactions in materials by formation of hydrogen
bonds between material components should also be considered.
However the system is still limited by free volume available in rigid
polymer matrix, therefore the rate of photoreaction and photo-
conversion is not large.
The kinetic behavior for pairs of chromophores containing the
same donor and different acceptor groups is comparable. Consid-
ering the compounds with amino donor groups, the fraction of fast
photoisomerization stage in total conversion of the system is
significantly larger than that of the slow process. The rate constant
of fast process (k1) has the smallest values for SDMe and SBAf
samples, while the highest values are observed for SDMm and
SBAe. The plots presented in Figs. 4 and 5 show that the deviation
from the first order kinetics is similar for all the systems except
SBAm, where the deviation is even more pronounced due to the
smallest value of slow process rate constant (k2).
AN ꢁ At
¼
a
$expðꢁk1$tÞ þ ð1 ꢁ
a
Þ$expðꢁk2$tÞ
(2)
AN ꢁ A0
where k1 is the rate constant of the fast reaction step, k2 is the
rate constant of the slow reaction step and describes the fraction of
a
fast photoisomerization stage in total conversion of the system [2].
Equation (2) was used to determine the kinetic constants for
transecis isomerization induced by irradiation with white or UV
light. All parameters determined for photoisomerization process
are presented in Table 1.
The kinetic behavior of phenolic chromophores (SBAph and
SDMph) was different from that of other compounds. Transecis
isomerization of these molecules dispersed in poly(MMA-co-BMA)
matrix followed first-order kinetics and could be described by
equation (1), while for other samples the deviation from first-order
kinetics was observed and the transecis isomerization was
described with equation (2). The transecis isomerization of azo
chromophores involves a change of molecular shape of the chro-
mophores. In solutions of organic solvents, usually of low
3,5
SDMm
SDMf
3,0
As it was mentioned, the dyes SDMph and SBAph containing
phenol rest in the molecule isomerize according to first order
kinetics, as shown in Figs. 4 and 5. The explanation of this
phenomenon may be as follows. The reaction rate of the first stage
is comparable with that of the second stage, ascribed to rheological
SDMe
SDMph
2,5
2,0
1,5
1,0
0,5
0,0
Table 2
Kinetic data for cisetrans thermal relaxation in the dark determined by UVeVis
measurements for chromophores in p(MMA-co-BMA) matrix.
Sample
AN/A0
a0
k01 ꢂ 103½sꢁ1
ꢃ
1 ꢁ a0
k02 ꢂ 103½sꢁ1
ꢃ
SDMm
SDMf
SDMe
SBAm
SBAf
SBAe
SBAph
SDMph
0.754
0.776
0.792
0.786
0.814
0.864
0.529
0.516
0.41
0.36
0.64
0.49
0.40
0.74
0.12
0.05
4.64
5.34
5.20
7.65
8.05
62.3
4.16
3.46
0.59
0.64
0.36
0.51
0.60
0.26
0.88
0.95
0.185
0.241
0.287
0.175
0.151
0.360
0.101
0.058
0
20
40
60
80
100
120
140
160
180
time [s]
Fig. 4. Kinetics of sulfadimethoxine derivatives photoisomerization induced by illu-
mination with white (SDMm, SDMe, SDMf) or UV (SDMph) light at 25 ꢀC.

ꢀ
S. Zielinska et al. / Dyes and Pigments 92 (2012) 1018e1024
1023
Table 3
3,0
2,5
2,0
1,5
1,0
0,5
0,0
Changes of real part of refractive index of photochromic films.
Sample SBAph SBAm SBAe SBAf SDMph SDMm SDMe SDMDf
nr 0.0115 0.0122 0.0043 0.0125 0.0108 0.0086 0.0113 0.0138
SBAm
SBAf
SBAe
SBAph
D
higher for sulfabenzamide derivatives, while the rate constants of
slow reaction k02 are generally higher for sulfadimethoxine deriv-
atives. Both kinds of rate constants have the smallest values for
phenolic chromophores; SBAph and SDMph. Plots presenting the
kinetics of thermal relaxation are shown in Figs. 6 and 7.
3.2. Refractive index modulation
Ellipsometry is an effective and nondestructive method, which
allows estimating variations of refractive index and thickness of
thin films occurring during photochromic cycles. Ellipsometry is
based on measurement of a change in polarization of light reflected
from a material surface. The linearly polarized light reflects from
the sample and changes its polarization into elliptical one. The
parameters measured by ellipsometry are called ellipsometric
0
1000
2000
3000
4000
5000
6000
time [s]
Fig. 6. Kinetics of sulfabenzamide derivatives thermal relaxation in the dark at 25 ꢀC.
material movement, so that the whole process might be treated as
it were first order reaction. Evidence that we have to do also with
slower reaction stage in the case of these dyes could be found in
next section, describing relaxation kinetics.
The kinetics of thermal relaxation occurring in the dark is
described by the following equation [2]:
parameters
J and D. J indicates amplitude and D is phase shift of
polarized light. The change in light polarization is defined as:
rp
rs
r
¼ tanð
J D
Þexpði Þ ¼
(4)
where rp and rs are the complex Fresnel reflection coefficients for
polarized light parallel and perpendicular to the plane of incidence,
À
Á
À
Á
A0 ꢁ At
¼
a
$exp ꢁk01$t þ ð1 ꢁ
a
Þ$ ꢁk02$t
(3)
A0 ꢁ AN
respectively.
J and D are used to describe the optical properties
which determine how light interacts with a material. The complex
where k01 is the rate constant of the fast reaction stage, k02 is the rate
~
refractive index ðnÞ consists of the real part (nr) and imaginative
constant of the slow reaction stage,
a describes the fraction of fast
part of refractive index (ik) where k is extinction coefficient.
process in total conversion of the system, A0 stands for initial
absorbance value (before irradiation), At is absorbance at time t and
AN describes absorbance at photostationary state.
~
n ¼ nr þ ik
(5)
The values derived from estimation of experimental data for
relaxation process with equation (3) as well as AN/A0 ratios are
presented in Table 2. The rate constants for thermal relaxation
process are significantly smaller than those determined for light-
induced photoisomerization process. The fraction of fast process
in total conversion of the system is dominating only for chromo-
phores containing ethyl(2-hydroxyethyl)amino group (SBAe and
SDMe), for other systems the fraction of the slow process is pre-
vailing. Values of rate constant k01 referring to fast process are
The measurements of ellipsometric parameters were carried out
using thin photochromic polymer films during periodic illumina-
tion with light. The changes of
J and D were observed for all
chromophores what allowed determining the changes of refractive
indices of the film before and after illumination. The changes of the
refractive indices (
0.0138 for SDMf (Table 3). The course of modification of parameter
, which include four photochromic cycles, is shown on Fig. 8. Each
Dnr) were in the range from 0.0086 for SDMm to
J
cycle consists of transecis isomerization and thermal relaxation
process. Transecis isomerization was observed under illumination
with diode light of wavelength 395 nm (growth curve). Thermal
relaxation occurred in the absence of illumination (decay curve).
3,0
SDMm
SDMf
SDMe
SDMph
2,5
23,66
off
off
23,64
23,62
23,60
23,58
23,56
23,54
23,52
23,50
23,48
23,46
23,44
off
off
2,0
on
on
on
on
1,5
1,0
0,5
0,0
III
IV
II
I
0
200
400
600
800
1000
1200
1400
1600
0
1000
2000
3000
4000
5000
6000
t [s]
time [s]
Fig. 8. Ellipsometry of SDMm. The changes of
photochromic cycles.
J parameter of film during four
Fig. 7. Kinetics of sulfadimethoxine derivatives thermal relaxation in the dark at 25 ꢀC.

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S. Zielinska et al. / Dyes and Pigments 92 (2012) 1018e1024
1024
molecular materials, 4-[bis(9,9-dimethylfluoren-2-yl)amino]-40-cyanoazo-
4. Concluding remarks
benzene and 4-[bis(9,9-dimethylfluoren-2-yl)-amino]-40-nitroazobenzene.
Dyes Pigments 2010;84:102e7.
The sulfonamide-containing azobenzene chromophores were
synthesized in good yields with an easy procedure. All compounds
investigated in a “guestehost” system in poly(methyl methacry-
late-co-butyl methacrylate) polymer matrix underwent reversible
transecis photoisomerization. The kinetic study based on UVeVis
measurements of the compounds in question indicated that the
kind of electron-donor group affected the rate of isomerization and
thermal relaxation processes as well as the position of maximum
absorption band. The reaction run of hydroxyphenyl containing
dyes (absorbing in UV) followed first order kinetics as the rate of
photoreaction (first stage) was probably of the same range as that
ascribed to material rheological behavior (slow stage).
The rate constants for transecis isomerization were found to be
higher for sulfadimethoxine derivatives than for sulfabenzamide-
containing chromophores except for the SBAe sample, for which
the isomerization rate constant had the highest value. For the
thermal relaxation in the dark the results were reverse e the process
was faster for the sulfabenzamide derivatives. The refractive index
modulation observed during illumination was moderate but the
change of dielectric properties of materials was distinct qualifying
the dyes as potentially usable in holographic applications.
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Due to the presence of reactive hydroxyl group in chemical
structure of the chromophores, they can be further used for the
synthesis of polymer or hybrid organiceinorganic materials.
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Acknowledgements
[19] Ortyl E, Kucharski S. Kinetics of refractive index changes in polymeric
photochromic films. Macromol Symp 2004;212:321e6.
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new chiral photochromic copolymers. Opt Mater 2009;32:198e206.
[21] Koszykowska M, Tokarek M, Kucharski S. Synthesis and photochromic prop-
erties of poly [N-vinyl-2-(phenylazo)-imidazole] derivatives in the near UV
range. Mater Sci Poland 2009;27:699e708.
This work was supported by the grant of Chemistry Department
of Wroc1aw University of Technology, Wroc1aw, Poland
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