Synthesis, structural characterization and computational study of NLO-responsive chromophores and second-order coefficients of thermally crosslinked polymers

Article abstract of DOI:10.1039/c9nj03389k

This paper emphasizes the preparation of nonlinear optical (NLO) responsive chromophores and their corresponding polymers. Initially, the carboxyl acid group-based precursors of the chromophores containing strong acceptors such as nitro-substituted thiazo

Full text of DOI:10.1039/c9nj03389k

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DOI: 10.1039/C9NJ03389K
Synthesis, structural characterization and computational study of
NLO responsive chromophores and second-order coefficients of
thermally crosslinked polymers
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Radha V. Doddamani, Padmeshwary S. Rachipudi, Nandini A. Pattanashetti,
Mahadevappa Y. Kariduraganavar*
This paper emphasizes on the preparation of nonlinear optical (NLO) responsive
chromophores and their corresponding polymers. Initially, the carboxyl acid groups based
precursors of the chromophores containing strong acceptors like nitro-substituted thiazoles
and thiadiazole were prepared and subjected to condensation reaction with a monomer to
yield a series of polymers. Prior to the synthesis of polymers, thermally crosslinkable
monomer, 2,6-di-(octy-1-nyl)-p-phenylenediamine was synthesized by following the
Sonagashira coupling reaction. All these synthesized precursors, monomer and their
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corresponding polymers were characterized using FTIR, H NMR, CHN analyzer and UV-
Vis spectrophotometer. Simultaneously, the molecular structures of the precursors were
designed and optimized using Gaussian 16 (Revision A.03) program. The dipole moment,
energy band gap between HOMO and LUMO, polarizability and first order
hyperpolarizability of the precursors were calculated theoretically using DFT approach and
the values were in the range of 9.4321-15.3354 D, 1.3709 - 1.4146 eV, 53.40 - 64.80×10-24
esu and 3703.35 - 5254.90×10-32 esu, respectively. The inherent viscosities (η inh) of the
polymers were measured by Ubbelohde viscometer and were in the range of 0.2453 - 0.2860
dl/g. Thermal behavior of the polymers was investigated using DSC and TGA. The glass
transition temperature of the polymers was in the range of 190 - 218 ºC. Thicknesses and
refractive indices of the thin films were determined by ellipsometer and the values were in
the range of 0.100 - 0.127 µm and 1.211 – 1.426, respectively. The molecular orientation in
thin polymer films was induced by corona poling, and this was ascertained using both UV-
Vis spectrophotometer and atomic force microscope. The second harmonic generation (SHG)
of the poled polymers was determined using Nd-YAG laser. The SHG coefficients (d33) were
ranged between 98.30 and 106.04 pm/V at 532 nm. Among the polymers synthesized,
polymer containing dinitro-substituted thiazole demonstrated an excellent SHG coefficient as
high as 106.04 pm/V. Further, none of the polymers showed SHG decay below 100 ºC, and
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retained 95% of their SHG value even up to 600 h. Based on the enhanced NLO efficiVeienwcAyrticle Online
DOI: 10.1039/C9NJ03389K
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and long temporal stability, particularly the polymer containing dinitro-substituted thiazole
could be a promising candidate for photonic devices.
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Department of Chemistry, Karnatak University, Dharwad 580003, India. E-mail: mahadevappayk@gmail.com
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1. Introduction
Nonlinear optical materials have created a tremendous interest among the scientific
community in view of their wide range of applications in telecommunications, optical
switching, sensor protections, information storages, etc.1-5 Thus, there is a huge demand to
design and develop novel nonlinear optical materials with large second order coefficients.6-8
So far, a wide range of NLO responsive materials have been investigated, among which
organic materials are much focused due to their excellent optical and electronic properties,
which can be further tailored through structural modification and achieve better
characteristics to improve the nonlinear optical activity for industrial processing.9-12
The synthesis of organic materials, which contain electron-donor (D) and electron-
acceptor (A) units linked through a π-conjugated bridge (D-π-A molecules as push–pull
systems) have been a subject of considerable interest.13-16 In such materials, the first order
hyperpolarizability depends on the conjugated-electronic structure, viz., its length and shape,
and on the specific character of the donor and acceptor groups. In synthesizing such
molecules, a strategy that is being proved is the introduction of heteroaromatic molecule so as
to reduce the aromaticity and ionization potential along the conjugation path. This led to an
increased hyperpolarizability compared to six membered rings. Thus, chromophores
particularly containing penta aromatic heterocycles or their derivatives, such as thiophene,17-
19 thiazole,20,21 benzothiazole22 and benzoxazole,23 are among the most studied heterocycles.
Further, it is also necessary to give importance to other ancillary properties that could play a
vital role in enhancing the processability of materials and performance, which includes film
making, poling efficiency, durability, thermal and photochemical stability.24
In spite of achieving excellent properties, rapid development of organic NLO devices
is hindered by several shortcomings, like relatively low temporal and thermal stability of
SHG coefficients. Unlike conventional inorganic materials, organic molecules suffer due to a
loss of NLO activity with time. This is primarily because of the relaxation of chromophore
alignments achieved through electrical field poling due to their intermolecular electrostatic
interactions. To circumvent this, several approaches have been investigated. They are:
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development of hybrid organic-inorganic systems or high glass transition temperatureVi(ewTAgr)ticle Online
DOI: 10.1039/C9NJ03389K
polymers such as polyimides or polyurethanes25. In such kind of systems, the necessity of
orienting chromophores at high temperatures could result to degradation of chromophores.
The basic idea in designing such polymers is that, after crosslinking the motion of free-
volume available in the polymer matrix could be frozen. This is achieved by introducing a
cross-linkable functionalities within the polymer matrix to arrest the relaxation of
chromophore molecules after performing thermal curing during poling process.26-28 Through
this approach, it is reasonably possible to obtain materials with a high degree of chromophore
alignment. At the same time, high temporal stability of the polar order could be achieved
owing to an establishment of crosslinking networks in the polymer matrix.29
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Understanding this Liang and Dalton30 synthesized the heterocyclic polymers with
high degree of cross-1inking and achieved SHG coefficients upto 85 pm/V. Based on
bismaleimide chemistry Jeng et. al,31 developed a series of thermally stable polymers and
obtained SHG coefficients as high as 57.0 pm/V. Similarly, Gao et. al,32 prepared a novel
Diels–Alder crosslinking polymer with a large electro-optic coefficient of about 96.3 pm/V.
This polymer also showed excellent long term stability upto 250 h.
Realizing the importance of the aforementioned approaches employed in arresting the
relaxation of molecular orientation and the type of NLO responsive molecules chosen, we
have designed the nitro substituted thiazoles and thiadiazole chromophores, and optimized
their molecular structures using Gaussian 16 (Revision A.03) program. The resulting
molecules were subjected to DFT calculation for the determination of dipole moment, energy
band gap between HOMO and LUMO structures, polarizability and hyperpolarizability. The
obtained results prompted us to synthesize chromophores’ precursors via azo-coupling
reaction of 4-(4-nitrophenyl)-1,3-thiazol-2-amine, 5-nitro-4-(4-nitrophenyl)-1,3-thiazol-2-
amine and 5-(4-nitrophenyl)-1,3,4-thiadiazol-2-amine with 2,2'-(phenylimino)diacetic acid
and followed by reduction reaction with thionyl chloride to yield two acetyl chloride end
groups. In order to have thermally crosslinkable groups in the polymer backbone, 2,6-di-1-
octynyl-p-phenylenediamine as a monomer was synthesized33 and subjected to
polymerization with the synthesized chromophores’ precursors. The physicochemical
properties of the resulting precursors, monomer and their corresponding polymers were
investigated using different techniques. The induced molecular alignment was confirmed
using UV-vis spectrophotometer and atomic force microscope. The thermal stability of the
NLO responsive polymers was assessed by differential scanning calorimeter and
thermogravimetric analysis. The second-order coefficients determined by Nd-YAG laser
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technique were discussed in terms of withdrawing ability of the acceptorsD,OmI: 10o.l10e3c9u/Cla9NrJ03389K
orientation, role of crosslinkable segments in arresting molecular relaxation, etc. The results
were also correlated with the theoretical values calculated using the DFT approach.
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2. Results and discussion
2.1. Synthesis and characterization of chromophores
The NLO chromophores containing substituted thiazoles and thiadiazole were synthesized via
azo-coupling reaction of 4-(4-nitrophenyl)-1,3-thiazol-2-amine, 5-nitro-4-(4-nitrophenyl)-
1,3-thiazol-2-amine and 5-(4-nitrophenyl)-1,3,4-thiadiazol-2-amine, respectively with 2,2'-
(phenylimino) diacetic acid and followed by reduction reaction with thionyl chloride to
obtain two acetyl chloride end groups allowing their use in further polymerization reactions.
All the synthesized chromophores were highly reactive owing to the presence of
diacetyl chloride groups and were used directly for polymerization with the monomer. Hence,
the characterization of chromophores was not performed. However, diacetic acid derivatives
(compounds 2, 4 and 6), which are the precursors of chromophores, were subjected to
different characterization techniques for elucidation of their structures. The FTIR spectra,
illustrated in Figure 1, clearly state that the frequencies of hydroxyl group stretching and azo-
group stretching respectively appeared around 3400 cm-1 and in the range of 1428-1504 cm-1.
The strong bands exhibited in the range of 1500-1604 and 1332-1348 cm-1 was respectively
assigned to asymmetric and symmetric stretching of nitro groups, which signify the presence
of an azo-group in the structure. The data of 1H NMR spectra depicts that the assigned peaks
were consistent with the proposed structures of the compounds 2, 4 and 6. For instance, the
1H NMR spectrum of compound 6 presented in Figure 2 showed two doublets at 8.02 ppm
and 8.29 ppm for total eight aromatic protons, a singlet at 7.69 ppm for two hydroxyl protons
and a singlet at 3.56 ppm corresponding to two alkyl protons. The DMF solutions of the
compounds 2, 4 and 6 showed strong UV absorption maxima at 362, 425 and 503 nm,
respectively. The analytical data also confirmed the expected chemical structures. The NLO
chromophores were the push - pull type compounds (see Scheme 1) with N,N-acetyl chloride
group as the electron donor and nitro group as the electron acceptor linked through an azo -
conjugated bridge. These compounds were soluble in many of the organic solvents such as
dichloromethane, THF, cyclohexanone, DMF, N,N-dimethylacetamide (DMAc), DMSO and
N-methyl-2-pyrrolidinone (NMP).
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Fig. 1 The FTIR spectra of compounds 2, 4 and 6.
Fig. 2 The 1H NMR spectrum of compound 6.
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Scheme 1 Synthesis of chromophores a, b and c.
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2.2. Synthesis and characterization of monomer and crosslinkable polymers
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The synthesized NLO chromophores a, b and c were condensed with the synthesized
monomer 9 to obtain the polymers with crosslinking groups.
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To prepare the monomer, we followed the acetylene Sonagashira coupling reaction
catalyzed by palladium catalyst (see Scheme 2) in an inert atmosphere.33 The resulting
monomer was purified by column chromatography using chloroform/methanol (25/1, v/v) as
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eluent. The resulting monomer was pure as evidenced from the FTIR and H NMR data. The
polymerization was performed in dichloroethane (see Scheme 3). The intrinsic viscosity data
of polymers determined in DMSO was in the range of 0.2453 - 0.2860 dl/g. The FTIR spectra
of all the resulting polymers showed a strong -C=O band at lower frequencies between 1644
and 1685 cm-l. These frequencies were much lower than those generally obtained for -C=O
band (around 1715 - 1730 cm-l). This is mainly due to the formation of amide bonds. For
instance, polymer (Pa) exhibited the absorption band of -C=O around 1600 cm-1 (Fig. 3).
However, its corresponding precursor (compound 2) exhibited its -C=O band at 1717 cm-l
(see Fig. 1). The -C≡C groups showed the absorption band at 2250 cm-l, while nitro groups
showed the bands in the range of 1505 to 1600 and 1350 to 1300 cm-l. The data of 1H NMR
spectra of all the polymers showed peak broadening and other spectral features were
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consistent with the structures of polymers. Thus, both the FTIR and H NMR spectral data
clearly, supported the proposed structures of polymers presented in Scheme 3. The
spectroscopic features of the polymers reported here also indicated that the chromophore was
not decomposed during the transfer process of functional groups from the acetic acid moiety
to acetyl chloride during polymerization. Further, the ethynyl groups in the polymer
backbone underwent a thermal intramolecular reaction with the amide groups to form a
heterocyclic structure and/or an intermolecular reaction with the amide groups on the other
polymer chains to form the crosslinks. The resulting polymers were soluble in many common
organic solvents, such as THF, acetone, dioxane, DMF, cyclohexanone, DMAc, DMSO and
NMP. Thus, one could easily process these polymers into optical quality thin films for device
applications. However, after thermal curing and poling, the crosslinks were established in the
polymers. This is very well supported from FTIR data. For instance, the FTIR spectra of
polymer Pa before poling and after poling are presented in Fig. 3. It is observed that the
absorption band of -C≡C appearing at 2250 cm-1 decreased dramatically when polymer Pa
was thermally treated under vacuum for 40 min at 230 ºC.
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Scheme 2 Synthesis of monomer.
Scheme 3 Synthesis of polymers Pa, Pb and Pc.
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Fig. 3 The FTIR spectra of polymer Pa before poling and after poling.
2.3. Molecular modeling
To understand the electronic transition and charge delocalization within the push-pull
chromophores, it is important to study the molecular orbitals of the resulting compounds 2, 4
and 6 in gas-phase.34 The electron-state-density distribution of HOMO and LUMO of the
optimized geometries of the compounds 2, 4 and 6 were computed and presented in Table 1.
As can be seen from the structures, both HOMOs and LUMOs were delocalized over the
entire molecule, and the electron density was equally distributed in two parts of the molecule.
It is well known that HOMO energy characterizes the electron giving ability, while LUMO
energy characterizes the electron withdrawing ability.35 The energy gap between HOMO and
LUMO exhibits the molecular chemical stability and it plays a critical role in determining the
intra-molecular electrical transport properties. Obviously, it indicates the electron
delocalization within the compounds. The conjugated organic molecular systems are
generally described by a small energy gap between the HOMO and LUMO. The calculated
band gaps are included in Table 1, and the values of energy gap for compounds 2, 4 and 6
were in the range of 1.3709 – 1.4146 eV. The small energy gap obtained between HOMO and
LUMO indicated the occurrence of charge transfer within molecules, and thus proves that the
molecules can be easily polarized.36,37 The DFT computational methods always provide the
reproducible results to understand the optical properties of materials.38,39As commonly used
in literature,40 the calculated physicochemical properties are expressed as dipole moment (µ),
average polarizability (α) and first order hyperpolarizability (β) are as follows:
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2
2
2
µ푥 + µ푦 + µ푧
µ =
(1)
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(αxx + αyy + αzz)
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α =
(2)
(β xxx + βyyx + βzzx)2 + (β yyy + β yxx + βzzy)2 + (βzzz + βzxx + βzyy
=
)
2
βtot
(3)
Table 2 summarizes, the values obtained from the above equations and are in the range
9.4321-15.3354 D, 53.40 - 64.80×10-24 esu and 3703.35 - 5254.90×10-32 esu, respectively.
The order was found to be: Compound 4 > Compound 6 > Compound 2. From this order, it
can be analyzed that the compound 4 exhibited the highest order owing to the presence of two
nitro groups and a thiazole ring as compared to one nitro and a thiadiazole ring in case of
compound 6 and one nitro and a thiazole ring in case of compound 2. This must be due to
additional electron withdrawing ability of nitro groups. Similarly, the electron-rich nitrogen
atom reduced the aromaticity and ionization potential of the heteroaromatic thiazole ring.41
Table 1 Optimized molecular orbital structures, molecular orbital surfaces of the HOMO and
LUMO, their energies and band gap.
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Optimized structures
of compounds
Molecular orbital
surfaces of the HOMO surfaces of the LUMO
Molecular orbital
HOMO LUMO Band
(eV)
(eV)
gap
(eV)
DR1
-2.8125 -1.0730 1.7395
-3.0344 -1.6198 1.4146
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-3.1117 -1.7014 1.4103
Table 2 B3LYP/6-311++(G)(d,p) results of physicochemical properties of chromophores’
precursors
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2.4. Thermal properties of crosslinkable polymers
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Compound
Dipole Moment, µ
(Debye)
Polarizability,
α (1024 esu)
49.90
Hyperpolarizability,
β (1032 esu)
1679.31
3703.35
5254.90
4251.55
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DR1
5.5004
9.4321
2
4
6
53.40
15.3354
12.7467
64.80
62.95
The thermal properties of polymers were evaluated using both DSC and TGA, and the
resulting DSC thermograms are shown in Fig. 4. The glass transition temperatures of the
polymers ranged from 190 - 218 ºC and the thermograms showed broad exothermic peaks.
This behaviour could be attributed to thermal intra- and intermolecular reactions. The ethynyl
groups in the polymer backbone underwent thermal intramolecular reactions with the amide
groups and formed the heterocyclic structures. Similarly, the ethynyl groups underwent
intermolecular reaction with the amide groups on other polymer chains and formed the
crosslinks. This led to the formation of rigid polymer networks and thereby the induced
dipole orientation was arrested so as to obtain the highest second-order coefficients.
Fig. 4 The DSC thermograms of polymers.
The TGA curves of the polymers are shown in Fig. 5. As evident from the data
presented in Table 3, it is observed that the polymers developed here have exhibited good
thermal stability. At 10% gravimetric loss, all the polymers showed initial decomposition
temperature (Td) between 265 and 330 ºC. This clearly revealed that both intra- and
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intermolecular reactions greatly contributed towards the enhancement of thermal stability of
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the polymers.
In addition to the thermal stability, which was attributed to the rigid polymeric
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network structures, the role of chromophores with heterocyclic rings should also be
considered. Among the polymers developed, polymer containing chromophore b exhibited
the highest Tg value compared to the polymers having chromophore a and c. This implies that
higher the substitution of nitro groups, better is the thermal stability. Further, thiadiazole
containing polymer (Pc) exhibited relatively higher thermal stability compared to the
polymer containing thiazole moieties. This might be due to one extra nitrogen atom present in
the thiadiazole ring.
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Fig. 5 The TGA thermograms of polymers.
Table 3 The thermal properties of polymers
Polymers
aTg
bDegradation temperature (ºC)
bResidue (%)
(C)
at 850 oC
10% loss
20% loss
305
40% loss
Pa
Pb
Pc
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218
205
265
330
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395
635
565
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aGlass
transition
temperature
410
350
measured with DSC under N2 at a scan rate of 10 ºC/min.
bDegradation temperature measured with TGA under N2 at a scan rate of 10 ºC/min.
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2.5. Nonlinear optical properties of polymers
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The NLO properties of the polymers were studied using SHG method. To induce non-
centrosymmetric order, the spin coated polymer films were corona poled at a temperature of
5 - 10 ºC below the glass transition temperature. After electric poling, the dipole moments of
the NLO chromophores were aligned along the direction of the applied electric field. This
was confirmed by measuring the absorbance of the polymer films before and after poling. For
instance, the UV-vis spectra of the polymer Pb of both poled and unpoled films are shown in
Fig. 6. An abrupt decrease in the absorbance was noticed after poling. A similar behavior was
observed for polymers Pa and Pc. It can be ascribed to two reasons. Firstly, an alignment of
molecules along the direction of the field poling, i.e., perpendicular to the plane of the film
reduces the absorption cross-section for light at normal incidence. Secondly, this might be
due to partial degradation of chromophores during the poling process. In the present study,
the decomposition temperatures of all the synthesized polymers are well above the poling
temperature (near Tg) and hence the second reason can be ruled out. Further, the electrical
poling caused a slight blue shift in the absorption peak, and this is due to the birefringence
effect.42,43
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Fig. 6 The UV-vis spectra of polymer Pb before and after poling.
From the change in absorbance, the order parameter of the poled polymer films
could be estimated, which signifies the efficiency of the poling. The order parameter was
calculated by using equation 4:
Ф = 1-A1/A0
(4)
where A0 and A1 are the absorbance of the polymer films before and after poling,
respectively. The order parameter of the poled polymer film (Pb) was found to be 0.76,
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which suggests that molecules were effectively aligned along the direction of the field poling.
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8
The calculated order parameters of the remaining two polymer films are also included in
Table 4. From the data, it is noticed that the polymer Pb demonstrated the highest order
parameter among the polymers studied. This trend is in accordance with the values of dipole
moments, polarizability and hyperpolarizability values calculated from the DFT approach
(see Table 2).
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To further justify the alignments of NLO chromophores along the direction of field
poling, we have investigated the domain structure of NLO polymer films using atomic force
microscope. Figure 7 illustrates the AFM images of the spin-coated polymer (Pa) film before
and after poling. Before poling, the surface of the film was clean and flat. However, when it
was subjected to poling, the clean and flat film was dramatically changed to numerous hills
and valleys on the surface. This clearly justifies that after poling, the NLO chromophores
were effectively aligned along the direction of the field poling. Similarly, AFM scans were
carried out for the remaining samples before and after poling, but the images are not
presented to minimize the number of figures. All these data suggest that the polymers
prepared in the present work were efficiently poled and as a result, the chromophores were
effectively aligned along the direction of the applied field.
Fig. 7 The AFM images of the polymer Pb before and after poling.
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To know the temperature at which maximum second-order NLO response in the
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polymers is showed, where the molecules align to a maximum extent along the direction of
the applied field also known as optimum poling temperature (Topt) was selected by in situ
poling and temperature ramping technique.44 Topt’s of the resulting polymers were measured
and the values are included in Table 4. The Fig. 8 illustrates the SHG signals of Pb as a
function of temperature. It is observed that the SHG signal was increased steadily until Topt
was reached, and then the SHG signals decreased rapidly. Based on these Topt’s values, the
non-centrosymmetric alignment of the NLO chromophores in the polymer films was induced
with the corona poling method. After poling, films undergo crosslinking and the films are
insoluble in chloroform and DMF.
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Fig. 8 Topt of the polymer Pb
To assess the second order NLO activity of the resulting polymers, the SHG
coefficients (d33) for the poled films were estimated based on the following equation:45

2
2


lc,k
d33,s
d36,k
s
k
Is


F
(5)

Ik ls
where d36,k is the d36 of KDP crystal, which is 0.40 pm/V; Is and Ik are the SHG intensities of
the polymer sample and KDP crystal, respectively; lc,k is the coherence length of the KDP
crystal (11.4 µm); ls is the thickness of the polymer sample; and F is the correction factor,
which is approximately equal to 1.2 when lc,k >> ls. The calculated d33 values of these
polymers and thicknesses of the films were included in Table 4.
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Table 4 Physical and NLO properties of polymers
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8
b
Polymers η inha
λmax
Topt
Фc
lsd
(µm)
nd
d33
(dl/g)
(nm)
(oC)
(pm/V)
9
Pa
Pb
Pc
0.2738
0.2860
0.2453
280
377
480
195
220
209
0.6361
0.7631
0.6888
0.100 1.426 98.30
0.109 1.249 106.04
0.127 1.211 99.01
b
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aInherent viscosity of polymers: concentration of 0.5 g/dl in DMSO at 25 ºC. Wavelength at
the maximum absorbance of Polymer films after corona poling. Order parameter. Film
thickness and refractive index measured using Ellipsometer.
c
d
Among the poled films, polymer containing chromophore b (Pb) demonstrated the
highest d33 values compared to polymers having chromophores a and c (Pa and Pc). From
this, it is realized that the additional nitro group present in the nitro-substituted thiazole
chromophore greatly enhanced the d33 value up to 106.04 pm/V. Between thiadiazole and
thiazole moieties, thiadiazole exhibited slightly higher d33 value of 99.01 pm/V compared to
d33 value of thiazole moiety (98.30 pm/V). This trend is similar to the values of dipole
moment, polarizability and hyperpolarizability calculated from the DFT approach.
We have also monitored the temporal stability of the SHG signals for all the polymers
at room temperature as well as at 110 ºC. As expected, the cured and poled polymers indeed
exhibited a large and stable optical nonlinearity. The data generated as a function of time are
presented in Fig. 8. At room temperature, all the polymers were stable and obviously there
was no decay in the d33 values. However, at 110 ºC in air after the initial decay of less than
2% of the original signal, more than 98% of the SHG signal remained stable even up to the
measured time of around 600 h for all the polymers regardless of the chromophores attached.
Fig. 9 Temporal stability of SHG signals of the polymers.
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The dynamic thermal stability of all the polymers was investigated, where in the real
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time decay of the SHG signals in air was monitored as a function of temperature. As shown
in Fig. 9, the SHG signal of polymer Pb was suddenly decreased when temperature reached
near 222 ºC. This suggests that the polymer Pb has high thermal stability up to 220 ºC.
However, it was found that the same magnitude of the SHG signal could be recovered from
the same sample after repoled by corona discharging, signifying that the film was not
damaged even after experiencing the high temperature. Similar behaviour was observed for
the remaining two polymers (Pa and Pc), this indeed signifies that crosslinking plays an
important role in stabilizing the dipole orientation. The long-term temporal stability of the
polymers showed that the synthesized polymers are promising candidate for device
fabrication.
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Fig. 10 SHG signals of polymer Pb as a function of temperature.
From the results, we noticed that regardless of the chromophores attached all the
polymers exhibited the excellent property. This is mainly because, the backbone unit
established more rigidity in the polymer network due to formation of crosslinked network
after thermal curing and poling.
3. Experimental section
3.1. Materials and methods
Thiosemicarbazide, p-aminoacetanilide, anhydrous triethylamine, bis(triphenylphosphine)
palladium(II) chloride, copper(I) iodide (CuI), triphenylphosphine, 1-octyne, p-acetophenone,
1,2-dichloroethane and anhydrous magnesium sulfate were procured from Sigma-Aldrich, St.
Louis, Missouri, USA. Benzene, thiourea, iodine, ammonia, sulphuric acid, N,N-
dimethylformamide (DMF), anhydrous tetrahydrofuran (THF), dimethyl sulfoxide (DMSO),
nitric acid, benzoic acid, phosphorus oxychloride, sodium hydroxide, ethyl acetate, sodium
carbonate, hydrogen chloride, chloroform, methanol, diethyl ether and ethanol were
purchased from S. D. Fine Chemicals Ltd., Mumbai, India. Before use diethyl ether was dried
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by adding sodium wire overnight. All the chemicals were of the reagent grade DaOnId: 10u.1s0e39d/Ca9NsJ03389K
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received unless otherwise stated.
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3.2. Instrumentation
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Fourier transform infrared (FTIR) spectra were recorded with a Nicolet 6700 Thermo Fischer
Scientific Spectrometer using KBr pellet. The proton nuclear magnetic resonance (1H NMR)
spectra were recorded with a Bruker Advance III 400 MHz instrument. The deuterated
dimethyl sulfoxide (DMSO-d6) and tetramethylsilane were respectively used as solvent and
as reference for recording NMR spectra. Microwave reaction was carried out in a Microwave
Reactor, CEM, Discover. Ultraviolet-visible (UV–vis) spectra were recorded with a JASCO
V670 Spectrophotometer. The thermal stability of the polymers was investigated using
Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC), with a
Perkin–Elemer Diamond Q600 TGA/DTA and TA Instruments, Waters LLC, DSC Q20,
respectively. Thermograms were recorded at a heating rate of 10 ºC/min in a nitrogen
atmosphere.
Elemental analyses of the synthesized chromophores’ precursors and monomer were
performed with a Perkin–Elemer PE 2400 CHN Elemental Analyzer. The inherent viscosity
of the polymers was determined at 25 ºC using an Ubbelohde Viscometer. The polymer
solutions of 5 wt% prepared in DMSO were filtered through a 0.20 µm Teflon membrane
filter and the resulting solutions were used to prepare the thin films onto an indium–tin oxide
(ITO) glass substrate using a spin coating technique at a rate of 1800–2000 rpm. The films
thus prepared were dried for 5 h in a vacuum oven at 80 ºC to remove the residual solvents.
The morphology of thin films before and after poling was investigated using a Nanosurf®
Atomic Force Microscope (AFM). The melting points of the samples were recorded with
open capillary tubes on a Mel-Temp Apparatus. The thicknesses (ls) and refractive indices (n)
of the polymer films prepared by spin coating technique on a silicon wafer substrate were
measured with J. A. Woollam Ellipsometer M-2000-U at a wavelength of 632.98 nm with
fixed angles of 65º, 70º and 75º. The data were modeled with β-spline film single layer and
were fitted for all polymers with the same model with silicon as substrate layer. The values of
n and ls were solved with a minimum mean square error.
The dried polymer film was heated up to a temperature 5-10 ºC near the glass
transition temperature (Tg) and corona poled with an intense dc electric field. After poling for
1 h, the temperature of the film was brought to room temperature in presence of electric field.
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The poling conditions were as follows: high voltage, 6 KV at the needle point; gap distance
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between needle and film 0.8 cm; poling current < 0.43 mA.
The second harmonic measurements were performed utilizing the setup described in
our previous papers.46,47 A Mode-Locked Nd:YAG laser (Continuum Minilite-I, 6 ns pulse
duration, 28 mJ maximum energy at 1064 nm, 10 Hz repetition rate) was used as a
fundamental light source. The second harmonic signals generated by the p-polarized
fundamental wavelength (1064 nm) were detected by fast photodiode (FDS010, rise time 0.9
ns, Thorlabs) and an oscilloscope (Tektronix TDS 724D, Digital Phosphor Oscilloscope) with
a frequency of 500 MHz. While measuring the SHG, the polymer sample was held at 45º
angle to the incident laser beam in order to get a maximum SHG output. As a standard,
potassium dideuterium phosphate (KDP) crystal was used as a reference sample.
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3.3. Computational details
All the computational calculations were carried out using Gaussian 16 (Revision A.03)
program.48 Optimization of the molecular structures of the precursors of chromophores in
gas-phase and calculations of band gap between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) energies, dipole moment,
polarizability and hyperpolarizability were carried out using Density Functional Theory
(DFT) approach with the Becke-3-Lee-Yang-Parr (B3LYP) functional levels for the 6-
311++(G)(d,p) basis set.
3.4. Synthesis of chromophores
3.4.1. Synthesis of 4-(4-nitrophenyl)-1,3-thiazol-2-amine 1
A mixture of p-nitroacetophenone (1.42 g, 8.6 mmol), thiourea (1.3 g, 17.2 mmol) and iodine
(2.2 g, 8.6 mmol) was placed in an open vessel containing Teflon coated magnetic bar. This
vessel was placed in a cavity of the microwave reactor and subjected to microwave (MW)
irradiation (50 W) at 140 ºC for 10 min. The crude mixture was cooled to 40 ºC and it was
then triturated, filtered and washed with ether. The crude product was dispersed in warm
water and pH of the solution was maintained between 11 and 12 using ammonium hydroxide.
The resulting orange coloured precipitate was filtered and crystallized from ethanol:water
(1:4) mixture to obtain 4-(4-nitrophenyl)-1,3-thiazol-2-amine 1.
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3.4.2. Synthesis of 2,2'-[(4-{(E)-[4-(4-nitrophenyl)-1,3-thiazol-2-yl]diazenyl}phenyl)imino]
5
6
diacetic acid 2
A suspension of 4-(4-nitrophenyl)-1,3-thiazol-2-amine 1 (1.20 g, 5.4 mmol) was prepared in
cold sulphuric acid and kept in an ice bath. To this, the cold solution of NaNO2 (0.42 g, 6
mmol) prepared in water was added drop-wise while stirring. After the addition, the reaction
mixture was stirred for 30 minutes at low temperature. To this diazonium salt, 2,2'-
(phenylimino)diacetic acid (1.14 g, 5.4 mmol) dissolved in ethanol was added and formed a
red coloured suspension. The suspension was subsequently neutralized with potassium
carbonate solution to obtain the precipitate of azo compound. The compound was collected
by filtration, washed and recrystallized from DMF to yield red crystalline solids, and was
designated as compound 2 (1.68 g, yield: 70 %). M.p.: 132-135 ºC. Maximum absorption
wavelength (λmax; DMF): 362 nm. FT-IR (KBr, cm-1): 3425 (O–H), 2858 (aliph. C–H), 1717
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1
(C=O), 1596, 1515 (arom. C=C), 1626 (C=N), 1504 (N=N), 1604, 1348 (N=O). H NMR
(DMSO-d6, δ): 4.31 (s, 4H, -CH2), 6.77 - 6.82 (s, 2H, -OH), 7.86 - 7.88 (d, 4H, Ar-H), 8.28 -
8.35 (d, 4H, Ar-H), 8.49 (s, 1H, -CH) ppm. Anal. Calcd. for C19H15N5O6S: C (51.70%), H
(3.43%), N (15.87%), O (21.75%), S (7.26%); Found: C (51.68%), H (2.98%), N (16.07%).
3.4.3. Synthesis of 2,2'-({4-[(E)-(4-phenyl-1,3-thiazol-2-yl)diazenyl]phenyl}imino)diacetyl
chloride a
The mixture of compound 2 (1.289 g, 3.60 mmol), thionyl chloride (20 ml) and benzene (5
ml) was refluxed on a water bath until a clear solution was obtained. After 5 h, thionyl
chloride and benzene were removed by distillation and the residual thionyl chloride was
pumped out. The resulting compound a was directly used for polymerization.
3.4.4. Synthesis of 5-nitro-4-(4-nitrophenyl)-1,3-thiazol-2-amine 3
Under stirring condition, 4-(4-nitrophenyl) thiazole-2-amine 1 (4.4 g, 20.0 mmol) was slowly
dissolved in 10 ml of conc. H2SO4 while maintaining the temperature between 8 and 12 ºC.
The resulting solution was cooled below 5 ºC and to this, nitric acid (1 ml, 20.0 mmol) was
added slowly while stirring at such a rate that the temperature remains constant. The reaction
mixture was then brought to room temperature and kept overnight. The precipitate thus
collected was filtered and washed with water till the pH of the filtrate became neutral. The
resulting yellow colored product was designated as 3.
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3.4.5. Synthesis of 2,2'-[(4-{(E)-[5-nitro-4-(4-nitrophenyl)-1,3-thiazol-2-yl]diazenyl}
5
6
phenyl)imino]diacetic acid 4
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9
The compound 4 was obtained by diazocoupling reaction following the procedure adopted for
2,2'-[(4-{(E)-[4-(4-nitrophenyl)-1,3-thiazol-2-yl]diazenyl}phenyl)imino]diacetic acid 2, but
utilizing the 5-nitro-4-(4-nitrophenyl)-1,3-thiazol-2-amine 3 in place of compound 1. The
yield was found to be 2.16 g (yield: 82 %). M.p.:122 - 125 ºC (d). Maximum absorption
wavelength (λmax; DMF): 425 nm. FT-IR (KBr, cm-1): 3667-3444 (O–H), 2860 (aliph. C–H),
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1725 (C=O), 1599, 1562 (arom. C=C), 1698 (C=N), 1428 (N=N), 1525, 1348 (N=O). H
NMR (DMSO-d6, δ): 3.48 (s, 8H, -CH2), 4.01 (s, 2H, -OH), 7.93 - 7.96 (d, 4H, Ar-H), 8.27 -
8.30 (d, 4H, Ar-H) ppm. Anal. Calcd. for C19H14N6O8S: C (46.92%), H (2.90%), N (17.28%),
O (26.31%), S (6.59%); Found: C (46.52%), H (3.10%), N (17.98%).
3.4.6. Synthesis of 2,2'-[(4-{(E)-[5-nitro-4-(4-nitrophenyl)-1,3-thiazol-2-yl]diazenyl}phenyl)
imino]diacetyl chloride b
The chromophore b was synthesized similar to the procedure adopted for chromophore a, but
utilizing the compound 4 with the same scales. The compound b was directly used for
polymerization.
3.4.7. Synthesis of 5-(4-nitrophenyl)-1,3,4-thiadiazol-2-amine 5
To a mixture of p-nitrobenzoic acid (0.99 g, 6 mmol) and thiosemicarbazide (0.54 g, 6
mmol), POCl3 (2.76 g, 1.8 mmol) was added drop-wise for 30 min while maintaining the
temperature between 0 and 5 ºC. After addition, the temperature of the reaction was raised
and refluxed for 4 h. The reaction mixture was then brought to room temperature and 50 ml
of water was added. The pH of the reaction solution was adjusted in the range of 8 to 9 with a
solution of 15% NaOH. The precipitated crude product was filtered, washed with water, dried
and finally recrystallized from ethanol to afford compound 5.
3.4.8.
Synthesis
of
2,2'-[(4-{(E)-[5-(4-nitrophenyl)-1,3,4-thiadiazol-2-
yl]diazenyl}phenyl)imino]diacetic acid 6
The compound 6 was prepared by following the procedure adopted for compounds 2 and 4,
but utilizing the compound 5 with the same scales. The yield was found to be 2.05 g (yield:
85 %). M.p.: 140-143 ºC. Maximum absorption wavelength (λmax; DMF): 503 nm. FT-IR
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(KBr pellet, cm-1): 3434 (O–H), 2854 (aliph. C–H), 1726 (C=O), 1445 (N=N),D1O5I:0100.1,03193/C392NJ03389K
1
(N=O). H NMR (DMSO-d6, δ): 7.69 (s, 2H, -OH), 8.02 (d, 4H, Ar-H), 8.29 (d, 4H, Ar-H),
3.56 (s, 2H, -CH2) ppm. Anal. Calcd. for C18H14N6O6S: C (48.87%), H (3.19%), N (19.00%),
O (21.70%), S (7.25%); Found: C (48.65%), H (3.22%), N (18.73%).
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3.4.9.
Synthesis
of
2,2'-[(4-{(E)-[5-(4-nitrophenyl)-1,3,4-thiadiazol-2-
yl]diazenyl}phenyl)imino]diacetyl chloride c
The chromophore c was prepared by following the procedure employed for chromophores a
and b, but utilizing the compound 6 with the same scales. The compound c was directly used
for the polymerization.
The reaction pathways for the synthesis of chromophores a, b and c are illustrated in
Scheme 1.
3.5. Synthesis of monomer
3.5.1. Synthesis of 3,5-dibromo-p-aminoacetanilide 7
3,5-Dibromo-p-aminoacetanilide 7 was prepared by brominating p-aminoacetanilide (25 g,
0.17 mol) with a mixture of 5 ml bromine and 35 ml of acetic acid.29 The obtained crude
product with good yield was subjected to sublimation process to get a pure white crystalline
solid.
3.5.2. Synthesis of 2,6-dibromo-p-phenylenediamine 8
3,5-Dibromo-p-aminoacetanilide 7 (18 g, 58.44 mmol) was suspended in 150 ml of conc.
hydrochloric acid and refluxed for 1 h, until no starting material could be detected by TLC.
After completion of the reaction, mixture was diluted with a sufficient quantity of water
(almost 4 - 5 folds). The residue was filtered-off and washed thoroughly three times with 100
ml of 2 N HCl. The resulting dark violet coloured filtrate was neutralized with aqueous
Na2CO3 solution and the resulting precipitate was filtered-off. After drying, this crude
product was subjected to crystallization with ethyl acetate to yield 2,6-dibromo-p-
phenylenediamine as brown crystalline solid.
3.5.3. Synthesis of 2,6-di-(octy-1-nyl)-p-phenylenediamine 9
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For the preparation of monomer 9, the following procedure was adopted witDhOI:th10e.10u39s/uC9aNlJ03389K
precautions for rigorous exclusion of air and moisture under nitrogen atmosphere.
A compound 2,6-dibromo-p-phenylenediamine 8 (3.99 g, 15 mmol) was suspended in
60 ml of dry triethylamine. To this suspension, bis(triphenylphosphine)palladium(II) chloride
(0.28 g, 0.4 mmol), CuI (3.05 g, 1.6 mmol), triphenylphosphine (1.05 g, 4 mmol) and 1-
octyne (3.97 g, 36 mmol) were added and refluxed for 6 h. The completion of the reaction
was monitored by TLC (CHCl3/MeOH = 9/1). The disubstituted p-phenylenediamines 9 was
recognized by their blue fluorescence upon irradiation with UV light of 365 nm wavelength
in a UV chamber. The initially formed mono-substituted product spot had just disappeared in
the TLC, the reaction was suddenly quenched by adding 50 ml of diethyl ether. In a
separating funnel an extra 50 ml of ether was added and ethereal solution was washed twice
with water. The ether phase was dried with MgSO4 and subjected to rota-evaporator at room
temperature to obtain the product as yellow oil. FTIR (KBr, cm-1): 2250 (C≡C), 2854 (aliph.
C–H), 1522 (-NH2), 1596, 1516 (arom. C=C).1H NMR (DMSO-d6, δ): 8.6 (s, 2H, Ar-H), 4.2
(s, 2H, NH2), 4.6 (s, 2H, NH2), 2.27 (t, 4H, CH2), 1.31-1.41 (m, 16H, CH2), 0.83 - 0.87 (t,
6H, CH3) ppm. Anal. Calcd. for C22H32N2: C (81.43%), H (9.94%), N (8.63%); Found: C
(80.83%), H (9.98%), N (8.50%).
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The reaction pathway for the synthesis of monomer is illustrated in Scheme 2.
3.6. Synthesis of polymers
3.6.1. Synthesis of polymer Pa
Chromophore a (0.95 g, 2.0 mmol) was dissolved in 1,2-dichloroethane (8 ml). To this,
monomer 9 (0.65 g, 2.0 mmol) and triethylamine (1 ml) were added while maintaining the
inert atmosphere. The resulting mixture was heated at 60 ºC for 3 h and then poured into
methanol. The precipitate was collected and washed with methanol. To further purify the
polymer, the collected solid was then redissolved in chloroform, filtered and reprecipitated by
adding methanol. FTIR (KBr, cm-1): 1684 (C=O), 1355 (C-N), 1600 (C=C), 1622 (C=O),
1585, 1502 (arom. C=C), 1628 (C=N), 1518 (N=N), 1596, 1346 (N=O). lH NMR (DMSO-d6)
0.8 (s, 4H, -CH2), 1.1 (m, 20H, -CH2), 1.9 (t, 6H, -CH3), 3.9 (s, 3H, -NH), 7.0 - 8.4 (m, 10 H,
Ar-H) ppm.
A similar procedure was followed for the synthesis of polymers Pb and Pc and hence,
only the spectral data of polymers Pb and Pc are given under respective headings.
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3.6.2. Synthesis of polymer Pb
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FTIR (KBr, cm-1): 1676 (C=O), 1369 (C-N), 1610 (C=C), 1676 (C=O), 1596, 1516 (arom.
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l
C=C), 1625 (C=N), 1440 (N=N), 1505, 1300 (N=O). H NMR (DMSO-d6, δ): 1.1 (s, 4H, -
9
CH2), 1.9 (m, 20H, -CH2), 2.6 (t, 6H, -CH3), 3.6 (s, 3H, N-H), 6.6 - 8.1 (m, 10H, Ar-H) ppm.
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3.6.3. Synthesis of polymer Pc
FTIR (KBr, cm-1): 1644 (C=O), 1379 (C-N), 1613 (C=C), 1644 (C=O), 1594, 1521 (arom.
l
C=C), 1648 (C=N), 1456 (N=N), 1505, 1335 (N=O). H NMR (DMSO-d6, δ): 0.9 (s, 4H, -
CH2), 1.1 (m, 20H, -CH2), 1.27 (s, 6H, -CH3), 4.6 (s, 3H, -NH), 6.4 - 8.5 (m, 10H, Ar-H)
ppm.
The reaction pathways for the synthesis of polymers Pa, Pb and Pc are illustrated in
Scheme 3.
4. Conclusions
In summary, we have successfully synthesized NLO responsive chromophores having nitro
substituted thiazoles and thiadiazole. Similarly, thermally crosslinkable monomer, 2,6-di-
(octy-1-nyl)-p-phenylenediamine, was synthesized by following the Sonagashira coupling
reaction. The monomer was subjected to condensation with the synthesized chromophores’
precursors to yield a series of polymers. Meanwhile, the molecular structures of precursors
were designed and optimized using Gaussian 16 program. The dipole moment, energy band
gap between HOMO and LUMO structures, polarizability and the first order
hyperpolarizability of the precursors were calculated using the DFT approach and were found
to be in the range of 9.4321 - 15.3354 D, 0.10075 - 0.12375 eV, 53.40 - 64.80×10-24 esu and
3703.35 - 5254.90×10-32 esu, respectively. All the synthesized polymers exhibited the
excellent Tg values ranging from 190 - 218 ºC. Apparently, this is due to an establishment of
rigid structure in the polymer network owing to the presence of thermal crosslinking groups
in the polymer backbone. The order parameter of polymers was found to be in the range of
0.64 - 0.76, suggesting that the molecules were effectively aligned along the direction of field
poling. Thus, SHG coefficients of all the polymers were exhibited in the range of 98.30 -
106.04 pm/V at 532 nm. Among the polymers developed, polymer containing chromophore b
demonstrated the highest d33 value of 106.04 pm/V. This was mainly attributed to dinitro
substituted thiazole ring. Further, it is observed that no decay in the SHG signals was noticed
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below 110 ºC irrespective of the nature of chromophores condensed. This indeed signifies
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that the cross-linking groups played an important role in stabilizing the dipole orientation. All
these evidences are in good agreement with the data calculated from the DFT approach. At
the end, we conclude that among the polymers, polymer Pb could be a potential candidate for
photonic devices.
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Acknowledgements
The authors wish to thank the UGC, New Delhi, for providing the financial support under
CPEPA Program [Contract No. 8-2/2008(NS/PE)].This work is also supported by the DST-
PURSE-Phase-II Program [F. No. SR/PURSE Phase 2/12(G)].
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508x252mm (72 x 72 DPI)