NLO properties of poled azocarbazole-epoxy composites

Article abstract of DOI:10.1080/15421401003719811

Azocarbazole dyes are organic molecules that have already shown potential in nonlinear optical applications in host-guest systems. These molecules should be covalently bound to a host polymer network. This can be easily achieved if the host polymer is an

Full text of DOI:10.1080/15421401003719811

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NLO Properties of Poled Azocarbazole-
Epoxy Composites
Jacek Niziol a , Jan Pielichowski b & Ewa Gondek c
a Faculty of Physics and Applied Computer Science, AGH – University
of Science and Technology , Cracow, Poland
b Department of Polymer Science and Technology , Cracow
University of Technology , Cracow, Poland
c Institute of Physics, Cracow University of Technology , Cracow,
Poland
Published online: 28 May 2010.
To cite this article: Jacek Niziol , Jan Pielichowski & Ewa Gondek (2010) NLO Properties of Poled
Azocarbazole-Epoxy Composites, Molecular Crystals and Liquid Crystals, 522:1, 249/[549]-254/[554],
DOI: 10.1080/15421401003719811
To link to this article: http://dx.doi.org/10.1080/15421401003719811
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Mol. Cryst. Liq. Cryst., Vol. 522: pp. 249=[549]–254=[554], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003719811
NLO Properties of Poled Azocarbazole-
Epoxy Composites
JACEK NIZIOL,1 JAN PIELICHOWSKI,2
AND EWA GONDEK3
1Faculty of Physics and Applied Computer Science, AGH – University
of Science and Technology, Cracow, Poland
2Department of Polymer Science and Technology, Cracow University
of Technology, Cracow, Poland
3Institute of Physics, Cracow University of Technology, Cracow, Poland
Azocarbazole dyes are organic molecules that have already shown potential in nonlinear
optical applications in host-guest systems. These molecules should be covalently
bound to a host polymer network. This can be easily achieved if the host polymer is
an epoxy resin. The work reports synthesis and properties of four new azocarbazoles
dyes bearing a hydroxy function, which enables binding to epoxy resin.
Keywords Azocarbazoles; epoxy resin composites; nonlinear optics
Introduction
For the least three past decades the organic systems were extensively studied for
their non-linear optics (NLO) properties. The most popular systems consist of a host
polymer doped with physically dispersed guest dye molecules [1]. The host polymer
should be optically transparent and without NLO traits. In this way, all non-linear
properties arise from dispersed NLO dye molecules. NLO performance of such mate-
rials is usually attenuated by either aggregation or crystallization of dye molecules. A
typical remedy to this inconvenience is substitution of an aliphatic or other bulky side
group to the dye, which prevents long range organisation. The other approach
depends on chemical binding of the dye molecule to the host polymer backbone. This
procedure is almost effortless if the host polymer initially exists in from a liquid epoxy
resin. NLO dye molecules (possessing appropriate ligands), can be chemically incor-
porated in the polymer network during epoxy resin setting process. Chemical binding
of the dye molecule together with epoxy system leads to superior properties over the
same dye but dispersed only physically [2]. There exists a variety of ligands enabling
binding to epoxy resins, among the other the hydroxyl group. In this work are
reported synthesis and properties of four azocarbazole dyes bearing such function.
Properties of these dyes were theoretically calculated prior to the synthesis and after-
wards confronted with experimental findings.
Address correspondence to Jacek Niziol, Faculty of Physics and Applied Computer Science,
AGH – University of Science and Technology, Cracow, Poland. E-mail: niziol@agh.edu.pl
249=[549]

250=[550]
J. Niziol et al.
Dye Synthesis
All chemicals were purchased from Aldrich or Fluka and used as supplied. The
azocarbazole dyes were obtained in two-stage reaction. Since there was no common
solvent for necessary reactants, the reaction has to be carried out in a biphasic
system of immiscible liquids. In the first step hot, concentrated hydrochloric acid
was added into suspension of nitroaniline derivative in water and solution containing
sodium nitrite in water was slowly added. After 30 min solution was filtered to
remove residues of aniline derivative. In the second step the solution, now containing
diazonium salt, was poured to solution of N-hydroxyalkyl derivatives of carbazole in
a mixture of n-buthanol and iso-buthanol. Both solutions were vigorously stirred to
provide large contact interface between two phases containing reactants. During the
coupling reaction (Fig. 1), maintained for 24 hours in 40ꢀC. Their progress was
monitored through TLC. The synthesised dye precipitated and it was filtered off.
According to the method described above four compounds the following compounds
were synthesised.
3-(20-Chloro-40-nitrophenylazo)-9-(b-hydroxyethyl)-carbazole (dye 1). yield ¼ 80%,
m.p. ¼ 251ꢀC
IR (KBr) n [cmꢁ1], 3397 (nO-H), 3097, 2955 (nAr-H), 1626, 1599 (nC¼C), 1460,
1437 (nC-H), 1529, 1240 (nN¼N), 1041, 748 (nC-Cl), 723 (cCH
)
2
1H-NMR (300 MHz), d, [ppm], 1,29 (d,2H), 2,47 (d,2H), 3,90 (d,2H), 4.44
(t,2H), 7,52 (dt,1H), 7,67 (dt,1H), 7,76 (dd,1H), 7,89 (d,1H), 7,92 (m,3H), 8,56
(d,1H), 8,80 (d,1H)
3-(20,40-Dinitrophenylazo)-9-(b-hydroxyethyl)-carbazole
m.p. ¼ 247ꢀC
(dye
2). yield ¼ 61%,
IR (KBr) n [cmꢁ1], 3364 (nO-H), 3094, 2942 (nAr-H), 1599, 1224 (nN¼N), 1532,
1339 (nNO ), 1130, 1049 (nC-O), 752 (cCH
)
2
2
1H-NMR (300 MHz), d, [ppm], 1,24 (d,2H), 2,47 (d,1H), 3,89 (d,2H), 4,43
(t,2H), 7,52 (dt,1H), 7,66 (dt,1H), 7,78 (dd,1H), 7,89 (d,1H), 7,95 (m,3H), 8,55
(d,1H), 8,79 (d,1H)
3-(40-Cyanophenylazo)-2-hydroxy-9-carbazole (dye 3). yield ¼ 46%, m.p. ¼ 235ꢀC
IR (KBr) n [cmꢁ1], 3364 (nO-H), 3098, 2960 (nAr-H), 2229 (nCN), 1600, 1240
(nN¼N), 1463, 1418 (nC-H), 747 (cCH
)
2
1H-NMR (300 MHz), d, [ppm], 1,28 (d,1H), 6,96 (dt,1H), 7,25 (dt,1H), 7,49
(dt,1H), 7,78 (dd,1H), 7,87 (d,1H), 8,01 (m,3H), 8,64 (d,1H)
Figure 1. Scheme of the synthesis of azocarbazole dyes. dye (1): R1 ¼ Cl, R3 ¼ NO2,
R4 ¼ C2H2OH, dye (2) R1 ¼ NO2, R3 ¼ NO2, R4 ¼ C2H2OH, dye (3) R3 ¼ CN, R5 ¼ OH, dye
(4) R2 ¼ CN, R3 ¼ CN, R5 ¼ OH.

NLO Properties of Poled Azocarbazole-Epoxy Composites 251=[551]
3-(30,40-Dicyanophenylazo)-2-hydroxy-9-carbazole (dye 4). yield ¼ 47%, m.p. ¼
245ꢀC
IR (KBr) n [cmꢁ1], 3304 (nO-H), 3100, 2957 (nAr-H), 2228 (nCN), 1595, 1243
(nN¼N), 1629 (nC¼C), 1466, 1394 (nC-H), 756 (cCH
)
2
1H-NMR (300 MHz), d, [ppm], 1,28 (d,1H), 6,95 (dt,1H), 7,25 (dt,1H), 7,48
(dt,1H), 7,96 (d,1H), 8,10 (m,3H), 8,63 (d,1H)
Theoretical Calculations
Theoretical calculations were supported by Hyperchem package, version 7.5. The
integral geometry optimization of the molecules and its complexes was performed
in frame of molecular mechanics (MMþ) force field, where the lowest energy confor-
mations are obtained. The MMþ-force field method was chosen because is very fast,
may be used for large organic molecules and gives simultaneously information about
total energies and molecular geometries. Geometry optimization was accomplished
through AM1 (Austin Model 1) within restricted Hartree-Fock (RHF) level. As a
criterion of self-consistent convergence during the numerical procedure it was chosen
that the value of energy gradient should not exceeding 0,001 kcal=mol.
Intensities of optical absorption I(x) were determined according to formula (1):
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢃꢀ
2
~
n
hW iꢀh r W i
ꢀ
X X
r
ki
ji
IðxÞ ꢂ x
ð1Þ
ꢁ
ꢂ
2
2
i¼x;y;z ꢀh2 x ꢁ xjk þðC=2Þ
k¼1
ꢃ
~
where Wj and Wk are wave function of j-th and k-th energy levels, iꢀh rr stands for
transition dipole momentum of molecule, ꢀhxjk ¼ (Ej ꢁ Ek) is energy difference
between the ground and the excited states and x represents frequency of the incident
electromagnetic wave.
Molecular nonlinear hyperpolarizabilities b and c were calculated using the
model with so-called multilevel excitation resumed in Eqs. (2) and (3), where: Kb
and Kc are constants, g – number of transition from ground state, Eg – transition
energy, lðigÞ – transition dipole component, MkðgÞ – excited state dipole component,
Mkð0Þ – ground (reference) dipole component, H – damping
ꢀ
ꢀ
ꢀꢀ
ꢀ
ꢀ
ꢃ
ꢄ
ðgÞꢀꢀ
E2 l
lðgÞ MðkgÞ ꢁ Mðk0Þ
N
ꢀ
ꢀꢀ
ꢀ
X
g
i
j
ꢃ
ꢄꢃ
ꢄ
bkij ¼ Kb
ð2Þ
ð3Þ
Eg2 ꢁ 4E2 þ H2 Eg2 ꢁ E2 þ H2
g¼1
ꢀ
ꢀ
ꢀ
ꢀꢀ ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀ
ꢀ
ꢀ
ꢀꢀ ꢀꢀ
lðgÞ: lðgÞ
M
ðgÞ ꢁ Mkð0Þ
M
ðgÞ ꢁ Mlð0Þ
:
N
ꢀꢀ ꢀꢀ
X
i
j
k
l
ꢃ
ꢄ
cklij ¼ Kc
Eg2 ꢁ 4E2 þ H2
g¼1
Experimental
Absorption spectra in UV-vis wavelength range were measured in solvents of
different polarity – from the least polar cyclohexane to the most polar acetonitrile.

252=[552]
J. Niziol et al.
Solid state thin films, containing the studied dyes, were prepared as follows. At
first, the dyes were dissolved in a low molecular mass epoxy resin Epidian 6 (from
Organika Sarzyna – Poland) in proportion providing 5% per resin repeating unit.
Next a stoichiometric amount of the setting agent (triethylenetetramine) was added,
and the mixture was smeared over ITO covered glass substrate. After complete
setting, samples were oriented by corona-poling, at 75ꢀC, 7 keV applied for 5 minutes
and for additional ð22Þminuteðs2Þuntil the temperature didn’t drop to the ambient.
Components v333 and v3333 of second and third order nonlinear susceptibilities
tensors were measured using reflection electro-optic method, adapted from [3]. This
approach was sufficient supposing Ct1 symmetry of the samples [4]. The sample was
covered with vacuum evaporated aluminium layer, which played simultaneously the
role of electrode and reflecting mirror. Next, the sample was subjected to superposed
constant EDC and modulated EAC electric fields. This field modulated intensity of the
reflected laser beam according to Eq. (4), where const express a geometrical factor
depended on refractive indices, incidence angle, sample thickness and initial intensity
of the laser beam. The reflected laser beam intensity was measured by a photodiode
and modulated portion of the signal retrieved through lock-in technique.
DIðxÞ ¼ const ꢄ EACðvð2Þ ꢁ 2EDCv3ð33Þ33Þ
ð4Þ
333
Results and Discussion
Data obtained in results of numerical calculations performed prior to the synthesis,
i.e., absorption spectra, dipole moments, and parameters describing molecules
stability are collected in Table 1. Calculations were performed for isolated molecules
and factors affecting peaks broadening were neglected, so one can compare only
peaks position. Keeping this fact in mind, it may be concluded that simulated
absorption spectra corresponded well to experimental findings as it illustrates
example shown in (Fig. 2). Calculated dipole moments were substantially smaller
in case of dyes 3 and 4, i.e., where hydroxy group were directly bound to the
carbazole moiety. This property corresponded to observed much lower solubility
of dyes 1 and 2 in less polar solvents.
Absorption edge of the studied materials appeared (in solution and in epoxy
matrix) at c.a. 560 nm, what provided correct electro-optic measurements without
attenuation due to optical absorption. From the other side, such absorption makes
these dyes applicable for diffraction grating inscription, using YAG laser [5]. The
Table 1. Numerical simulation results – binding energy, heat of formation, and
dipole moment
Parameter=dye
dye 1
dye 2
dye 3
dye 4
Binding energy (kcal=mol)
Heat of formation (kcal=mol)
ꢁ4545.47
ꢁ4665.29
ꢁ4338.56
ꢁ4782.35
417.73
330.14
253.54
249.95
Total dipole (D)
4.20
4.89
2.57
0.59

NLO Properties of Poled Azocarbazole-Epoxy Composites 253=[553]
Figure 2. Comparison of theoretical and experimental spectra of dye 1.
observed solvatochromic effect was not very important, attaining not more then c.a.
60 nm of shift due to the increased solvent polarity (between cyclohexane and acet-
onitrile). Absorption spectra of all studied dyes are shown together in (Fig. 3).
In Table 2 are presented calculated molecular hyperpolarizabilities b and c
ð2Þ
ð2Þ
compared to experimentally found v and v . Although there does not exist a
direct link between nonlinear optical properties of individual molecules and contain-
333
3333
ing them bulk materials, some interesting conclusions can be drawn. Parameters
v
ð2Þ
measured for macroscopic samples are more or less in same proportion with
3333
calculated c for appropriate dyes. This is not the case for v and b. Ratio between
ð2Þ
333
Figure 3. Absorption spectra of the studied dyes in solution in acetone. Spectra were
normalized to the unity for the fundamental transition.

254=[554]
J. Niziol et al.
Table 2. Calculated molecular hyperpolarizabilities b and c and experimentally
found macroscopic parameters v and v
ð2Þ
ð2Þ
333
3333
ð2Þ
ð2Þ
vð323Þ3=b
[ꢅ10ꢁ15
(e)
vð323Þ33=c
[ꢅ1014]
(f)
v
b
[a.u.]
(a)
c
[m5 ꢄ Vꢁ2
(b)
v
[m=V]
(c)
3333
333
[m2 ꢄ Vꢁ2
]
]
]
Molecule
(d)
dye 1
dye 2
dye 3
dye 4
340
856
1961
854
7,16 ꢄ 10ꢁ36
4,25 ꢄ 10ꢁ36
2,07 ꢄ 10ꢁ35
1,25 ꢄ 10ꢁ35
2,9 ꢄ 10ꢁ11
5,1 ꢄ 10ꢁ11
1,2 ꢄ 10ꢁ11
8,1 ꢄ 10ꢁ12
2,8 ꢄ 10ꢁ21
1,4 ꢄ 10ꢁ21
1,1 ꢄ 10ꢁ20
9,1 ꢄ 10ꢁ21
85
60
6.1
9.5
3.9
3.3
5.3
7.3
them holds separately for dyes containing hydroxyethyl (1 and 2) and hydroxy (3
and 4) side groups. In the last group values of v are lower in spite of higher values
ð2Þ
333
of b. A possible explanation can be their lower dipole moment – a factor directly
linked to corona poling efficiency. This hypothesis is hard to be accepted because
oðf2Þbig difference between dipole moments of dye 3 and dye 4, while ratio of their
v333 and b holds. More likely dye molecules bound to epoxy matrix through hydro-
xyethyl group (i.e., more distanced form polymer network) can easier change their
orientation during the poling process, what resulted in more important orientation
on macroscopic scale.
Conclusions
There was presented efficient method of synthesis of new azocarbazole dyes of non-
linear optical properties. Obtained experimental data, although applied corona pol-
ing was not optimized, situate these dyes between typically reported so far in the
literature. It was found that dyes bearing hydroxyethyl group are more efficient
NLO dopants in systems based on solid state epoxy resin.
References
[1] Nalwa, H. S. & Miyata, S. (1997). Nonlinear Optics of Organic Molecules and Polymeric
Materials, CRC-Press: ISBN 0849389232.
[2] Niziol, J., Essaidi, Z., Bednarz, M., & Sahraoui, B. (2009). Optical Materials, 31(3),
501–504.
[3] Rohl, P., Andress, B., & Nordman, J. (1991). Appl. Phys. Lett., 59(22), 2793.
[4] Prasad, P. N. & Williams, D. J. (1991). Introduction to nonlinear optical effects in molecules
and polymers, Wiley: ISBN 0-471-51562-0.
[5] Czaplicki, R., Krupka, O., Essaidi, Z., El-Ghayoury, A., Kajzar, F., Grote, J. G., &
Sahraoui, B. (2007). Optics Express, 15(23), 15268.