Click chemistry-based synthesis of azo polymers for second-order nonlinear optics

Article abstract of DOI:10.1002/pola.25928

Four linear polymers containing pendant azo moiety were synthesized through click chemistry for second-order nonlinear optical study. The polymers were found soluble in most of the polar organic solvents such as tetrahydrofuran (THF), chloroform, and dimethyl formamide (DMF). The polymers showed thermal stability up to 300 °C and glass transition temperatures (T_g) in the range of 120-140 °C. The molecular weights (M_w) of these polymers (measured by gel permeation chromatography) were in the range 37,900-55,000 g/mol. The polymers were found to form optically transparent films by solution casting from THF solution. Order parameters were calculated from UV-vis absorption spectra. The morphology changes in the films after poling were characterized by atomic force microscopy. The angular dependence, temperature dependence, and time dependence of second harmonic generation (SHG) intensity were obtained by using 1064 nm Nd:YAG laser. The SHG intensity remained unchanged up to 95 °C. At room temperature, it remained stable up to 8 days after initial drop of about 14%.

Full text of DOI:10.1002/pola.25928

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Click Chemistry-Based Synthesis of Azo Polymers for Second-Order
Nonlinear Optics
Balakrishna Kolli,1 Sarada P. Mishra,1 M. P. Joshi,2 S. Raj Mohan,2 T. S. Dhami,2 A. B. Samui1
1Polymer Science and Technology Centre, Naval Materials Research Laboratory, Shil-Badlapur road, Ambernath 421506, India
2Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
Correspondence to: A. B. Samui (E-mail: absamui@gmail.com)
Received 17 November 2011; accepted 19 December 2011; published online 5 February 2012
DOI: 10.1002/pola.25928
ABSTRACT: Four linear polymers containing pendant azo moiety
were synthesized through click chemistry for second-order non-
linear optical study. The polymers were found soluble in most of
the polar organic solvents such as tetrahydrofuran (THF), chloro-
form, and dimethyl formamide (DMF). The polymers showed
thermal stability up to 300 ꢀC and glass transition temperatures
(Tg) in the range of 120–140 ꢀC. The molecular weights (Mw) of
these polymers (measured by gel permeation chromatography)
were in the range 37,900–55,000 g/mol. The polymers were found
to form optically transparent films by solution casting from THF
solution. Order parameters were calculated from UV–vis absorp-
tion spectra. The morphology changes in the films after poling
were characterized by atomic force microscopy. The angular de-
pendence, temperature dependence, and time dependence of
second harmonic generation (SHG) intensity were obtained by
using 1064 nm Nd:YAG laser. The SHG intensity remained
unchanged up to 95 ꢀC. At room temperature, it remained stable
C
V
up to 8 days after initial drop of about 14%. 2012 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 50: 1572–1578, 2012
KEYWORDS: atomic force microscopy; differential scanning
calorimetry; nonlinear polymer; thin films; UV-Vis spectroscopy
INTRODUCTION During the past two decades, attention has
been paid by researchers for development of novel materials
for second-order nonlinear optics (NLO) because of their
applications in several areas including telecommunications,
optical modulation, optical data storage, frequency doubling,
and optical switching.1 A good NLO polymer should have the
properties, such as better optically transparency, high optical
thresholds to laser power, and large NLO coefficient.2 In this
regard, several inorganic materials such as lithium niboate
(LiNbO3), potassium dihydrogen phosphate, and so on are
used for this purpose. However, apart from stability their
properties are inferior as compared with organic counter-
parts. At the same time, although the organic materials are
superior in chemical resistivity, good processability, lighter in
weight and mechanical endurance, their stability remains a
concern. To address the stability problem, various
approaches such as crosslinking,3 incorporation of polyi-
mide,4 and polyurethane5 linkage were also adopted. Various
molecular designs for side chain NLO moiety were
attempted, such as T-type,6 Y-type,7 H-type,8 and so on. The
Tg of polymer and interaction between the chromophores
are of prime concern in enhancing the relevant properties.9
The optimization of properties is the ultimate goal in devel-
oping a suitable NLO polymer. In our approach, attempts
have been made to incorporate ring/fused aromatic ring in
the main chain for enhancing rigidity.
With the above idea in mind, we tried to follow a suitable
scheme so that rigid moieties are incorporated during pro-
gress of polymerization. There are several ways of synthesiz-
ing such types of polymers with various functional groups for
tuning the optoelectronic properties. Percec et al. reported a
series of synthetic strategies from linear to ring structure to
hyperbranched polymers.10 Further the same author elabo-
rated a synthetic method for the preparation of linear mono-
disperse LC oligoethers and polyethers via a repetitive 2n geo-
metric growth approach.11 The synthesis strategy along with
the phase behavior was also dealt with.12 The synthesis of
macrocyclic and hyperbranched polymers was also reported
during the same period.13 One of the reaction procedures
called ‘‘Click’’ chemistry introduced by sharpless14 has been
popularized among the synthetic chemist due to its remark-
able features such as quantitative yields, mild reaction condi-
tion, tolerance toward a broad range of functional groups etc.
Another important aspect is the use of water as a cosolvent in
this method. By using this methodology, various polymers are
reported in various fields such as organic chemistry,15 drug
discovery,16 dye-sensitized solar cells,17 side chain liquid crys-
talline polymers,18 and various types of linear as well as
dendronized NLO polymers.19
In the present work, we synthesized four linear polymers
containing azo chromophore as a dipole by copper-catalyzed
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click chemistry. Polymerization was carried out between dia-
zido azo chromophore (which is similar to 4-N,N0-bis(2-
hydroxyethyl) amino-40-nitro azo benzene called DR19) and
diyne moieties. Two of the synthesized polymers possess
good film forming ability and their optically transparent
films were fabricated for further analysis without blending
with any other polymer like polycarbonate or PMMA. The
polymers were easily soluble in common organic solvents
and well characterized. In addition, their thermal, optical,
and NLO properties were investigated.
Yield: 5.4 g (80.5 %); mp: 155 ꢀC; FTIR (KBr): m (cmꢁ1
)
1600 (AC¼¼CA), 1510, 1350 (ANO2), 1180 (CAN); 1H NMR
(CDCl3, 500 MHz): d (ppm) 8.42 (d, J ¼5.0 Hz, 2H), 8.02 (d, J
¼ 15 Hz, 4H), 6.87 (d, J ¼ 5 Hz, 2H), 3.96 (t, J ¼ 5 Hz, 4H),
3.80 (t, J ¼5 Hz, 4H); 13C NMR (CDCl3, 125 MHz): d (ppm)
156.3, 149.7, 147.7, 144.6, 126.1, 124.6, 122.8, 111.6, 53.4,
40.1.
Synthesis of 4-Nitro-40-[bis (2-azidoethyl)amino]azo
benzene (3)
The compound 2 (1 g, 2.72 mmol) and sodium azide (0.53 g,
8.1 mmol) were charged in a round-bottomed flask contain-
ing 15 mL of dimethyl sulfoxide and heated to 90 ꢀC for 24
h. The reaction mixture was poured in to excess dilute hy-
drochloric acid (2 N, 50 mL) and extracted with dichlorome-
thane. The organic phase was washed with aqueous NaHCO3
solution till neutral, dried over MgSO4, and evaporated in
vacuo. The crude compound was purified by column chroma-
tography using 10% ethylacetate in petroleum ether as an
eluent to get dark red colored product.
EXPERIMENTAL
Materials
N,N-Bis(2-hydroxyethyl)aniline, sodium azide, sodium ascor-
bate, copper(II) sulfate, and thionyl chloride were purchased
from Sigma-Aldrich and used without further purification. All
solvents were used as received without further purifications
unless otherwise stated. The compounds N,N-bis(2-chloroethy-
l)aniline (1),16 1-(2-ethylhexyloxy)-2,5-diethynyl-4-methoxy-
benzene (4), 1,4-bis(2-ethylhexyloxy)-2,5-diethynylbenzene (5),
9-(2-ethylhexyl)-3,6-diethynyl-9H-carbazole (6), and 9,9-bis(2-
ethylhexyl)-2,7-diethynyl-9H-fluorene (7) were synthesized as
per reported procedure.14,20
Yield: 73%; mp: 90 ꢀC; FTIR (KBr): m (cmꢁ1) 2130 (AN3),
1600 (AC¼¼CA), 1510, 1340 (ANO2), 1140 (CAN); 1H NMR
(CDCl3, 500 MHz): d (ppm) d 8.36 (d, J ¼ 5 Hz, 2H), 7.97 (d,
J ¼6.5 Hz, 2H), 7.96 (d, J ¼ 6.0 Hz, 2H), 6.84 (d, J ¼ 10 Hz
2H), 3.74 (t, J ¼ 5 Hz, 4H), 3.62 (t, J ¼ 4.5 Hz, 4H); 13C NMR
(CDCl3, 125 MHz): d (ppm) 156.4, 149.9, 147.7, 144.4, 126.1,
124.4, 122.7, 111.9, 50.7, 48.7.
Instrumentation
FTIR spectra were recorded on Perkin-Elmer 1600 series
Fourier Transform infrared spectrophotometer using KBr
pellets. UV–visible spectra were taken on a Cary 500 Scan
UV–vis–NIR spectrophotometer. 1H and 13C NMR spectra
were recorded on a 500 MHz Bruker-FT NMR spectrometer
using CDCl3 as solvent. Molecular weights of the polymers
were determined by using gel permeation chromatography
(GPC; on waters 2690) with tetrahydrofuran (THF) as an
eluent and polystyrene as standards. Thermogravimetric ana-
lyzer (TGA; TA instruments His Res TGA 2950) was used for
determining thermal stability with a heating rate of 20 ꢀC/
min in N2 atmosphere. Differential scanning calorimeter
(DSC; TA instruments) was used to determine the glass tran-
sition temperature (Tg) with a heating rate of 10 ꢀC/min in
N2 atmosphere. Thickness of the film was measured by using
Alpha step surface profiler. Melting point of the monomers
was recorded by melting point apparatus (Electro thermal
9100).
Synthesis of Polymers
Synthesis of P1
Compound 3 (0.15 g, 0.35 mmol) and diyne 4 (0.1 g, 0.35
mmol) were taken in a round-bottomed flask containing a
mixture of H2O and THF (3 mL, THF: H2O: 1:2). The solution
was degassed for 10 min followed by the addition of sodium
ascorbate (0.14 g, 0.73 mmol) and then copper(II) sulfate
(0.1 g, 0.38 mmol). The reaction mixture was allowed to stir
at room temperature for 50 min under N2. The polymer was
precipitated in methanol and filtered. It was dissolved in
THF and passed through alumina column to remove catalyst.
The THF solution was concentrated and precipitated with
methanol. The precipitate obtained was filtered and dried
under vacuum to obtain 0.08 g of polymer P1 as a dark red
solid.
Yield: 32%. FTIR (KBr): m (cmꢁ1) 2960, 2920, 2860, 2100,
1600, 1510, 1340, 1140; 1H NMR (CDCl3, 500 MHz): d
(ppm) 8.3 (br, 2H), 8.0–7.9 (br, 6H), 7.2 (br, 1H), 6.8
(br, 2H), 4.5 (br, 3H), 4.0–3.8 (br, 4H), 3.4 (br, 2H), 1.6–0.8
(br, 17H).
Synthesis of Monomers
Synthesis of 4-Nitro-40-[bis(2-chloroethyl)amino]azo ben-
zene (2)
The diazonium salt was prepared by adding aqueous sodium
nitrite (1.51 g, 22.0 mmol) solution to p-nitroaniline (2.53 g,
18.34 mmol) in hydrochloric acid (9 N, 40 mL) at 5 C. The
Polymer P2
The polymer P2 was synthesized by similar procedure from
chromophore 3 (0.137 g, 0.35 mmol) and compound 5 (0.14
g, 0.35 mmol).
ꢀ
diazonium salt was then added slowly to a ethanolic solution
of N,N-bis(2-chloroethyl)aniline (2.53 g, 18.34 mmol) by
maintaining the temperature below 10 C. The reaction mix-
Yield: 84 %; FTIR (KBr): m (cmꢁ1) 2940, 2920, 2850, 2360,
2340, 2100, 1600, 1510, 1340, 1100; 1H NMR (CDCl3, 500
MHz): d (ppm) 8.3 (br, 2H), 8.0–7.9 (br, 6H), 7.2 (br, 1H), 6.8
(br, 2H), 4.5 (br, 3H), 4.0–3.8 (br, 4H), 3.4 (br, 2H), 1.6–0.8
(br, 17H).
ꢀ
ture was allowed to stir for 3 h after which the pH of the so-
lution was adjusted to neutral by the addition of aqueous
NaOH solution. The precipitate formed was filtered off and
recrystallized in ethanol to obtain a dark red solid.
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SCHEME 1 Synthesis of Diazide monomer.
Polymer P3
eration (SHG). In this process, poled films were mounted on
rotational stage. For measuring the SHG intensity, 1064-nm
pulsed Nd:YAG laser of pulse width 5 ns and 10 Hz repeti-
tion rate with laser energy 5 mJ was passed through poled
films. The fundamental beam was blocked after passing
through the film by an IR filter. The SHG signal was detected
by a photomultiplier tube (PMT) and the signal averages by
using an oscilloscope.
The polymer P3 was synthesized by similar procedure from
chromophore 3 (0.128 g, 0.35 mmol) and compound 6 (0.11
g, 0.35 mmol).
Yield: 6% (only soluble fraction); FTIR (KBr): m (cmꢁ1) 2940,
2920, 2850, 2360, 2340, 2100, 1600, 1500, 1340, 1100.
Polymer P4
The polymer P4 was synthesized by similar procedure from
chromophore 3 (0.136 g, 0.35 mmol) and compound 7
(0.159 g, 0.35 mmol).
RESULTS AND DISCUSSIONS
Synthesis and Characterization of Polymers
In this study, one of the monomer was widely varied by
changing the aromatic unit as well as the type of substitution
for finding the effect on NLO property. The synthesis of
monomers and the polymerizations were performed as
shown in Schemes 1 and 2, respectively. The monomers
were synthesized by adopting the standard diazotization pro-
cedure. Initially, N,N-bis(2-hydroxyethyl)aniline was chlori-
nated in presence of POCl3 to obtained compound 1 in quan-
titative yield.22 Then compound 1 was treated with p-nitro
aniline under diazotization condition to obtained compound
2 in good yield. Compound 2 was then reacted with sodium
Yield: 59%; FTIR (KBr): m (cmꢁ1) 2960, 2920, 2850, 2360,
2340, 2100, 1600, 1500, 1340, 1100; 1H NMR (CDCl3, 500
MHz): d (ppm) 8.3 (br, 2H), 8.0–7.9 (br, 6H), 7.8–7.7 (br,
2H), 7.3 (s, 1H), 6.9 (br, 2H), 4.6 (br, 4H), 4.0 (br, 4H), 2.1
(br, 4H), 1.6–0.5 (br, 30H).
Thin Films Preparation, Poling, and Second Harmonic
Generation Measurement
For preparing the polymeric films, the indium-tin oxide
(ITO)-coated glass slides [(2.5 cm ꢂ 3.5 cm) (r ¼ 10–15 X/
sq)] were precleaned stepwise with N,N-dimethyl formamide,
distilled water, methanol, and acetone, respectively, in ultra-
sonic bath. The solutions of the polymers (50 mg/mL in
THF) were filtered through 0.25l PTFE filter for removing
undissolved particles. The polymer solutions were then spin
coated on ITO-coated glass substrates at a speed of 1000
rpm for 2 min. The spin coated films on ITO-coated glass
plates were then corona poled. Poling conditions were main-
tained at high voltage (4.8 kV) while the tip was at a dis-
tance of 1 cm at a temperature very close to the Tg of poly-
mer. After half an hour the film was cooled to RT while
voltage was on. From poling experiment the order parameter
(F) was calculated from their electronic absorption spectra
by using the following equation21
U ¼ 1 ꢁ A1=A0;
where A1 and A0 are the absorbance of poled and unpoled
films, respectively.
The second-order optical nonlinearity of the linear click poly-
mers were then determined from their second harmonic gen-
SCHEME 2 Click polymerization of diazide and diynes.
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TABLE 1 Polymeric Characterization Data
PDIb Tg (ꢀC) ls (lm) fe
a
b
c
d
Polymer kmax (nm) Mw
P2
P4
a
445
445
55,000 2.1
37,900 2.3
120
140
0.54
0.46
0.10
0.18
Wavelength at which maximum absorption observed for polymer in
thin film on ITO.
b
Determined by GPC in THF (polystyrene standard).
c
Glass transition temperature of polymers detected by DSC.
Film thickness measured by profilometer.
d
e
Order parameter measured by UV–vis spectra.
lymerization duration. During polymerization, the intensity
of the peak gradually decreased and finally disappeared indi-
cating the progress of polymerization reaction. The structure
of the polymer was confirmed from 1H NMR spectra, which
showed broad peaks having different chemical shift than that
of the monomers (Fig. 2). The molecular weight of polymer
was determined by GPC using THF as solvent and was found
to be in the range of 37,900–55,000 g/mol. Polymerization
beyond 50 min lead to polymer, which could not be analyzed
as it was not soluble in solvents. This may be due to high
molecular weight. For most of the polymers synthesized
thermal stability (by TGA; not shown) was found to be quite
reasonable as only 3 wt % loss is observed above 290 ꢀC.
The higher thermal stability of the polymers could be due to
the presence of large number of nitrogen atom as well as
the triazole ring. DSC study of the polymers showed glass
transition temperatures (Tg) in the range of 120–140 ꢀC
(Table 1).
FIGURE 1 FTIR spectra of dichloroazo compound(2), diazide
azo monomer (3) and polymer (P4).
azide to get the diazide monomer 3, which was used for syn-
thesizing polymers. All the triple-bonded compounds
(diynes) containing different aromatic moiety were synthe-
sized following the reported procedure and shown in
Scheme 2. The diazide and diynes monomers were then
polymerized by using copper-catalyzed 1,3-cycloaddition to
get the polymers in quantitative yield. Most of the polymer-
ization reactions were found to be completed within 50 min.
It was observed that the polymerization reaction continued
for longer time results in the formation of insoluble product,
which may be due to the formation of high molecular weight
polymers.
Polymerizing through click chemistry is not only helpful in
synthesizing novel polymers within shorter reaction time but
also introduce a triazole ring between the chromophores and
the aromatic ring. The formation of ring structure in step-
wise synthesis was also reported by Percec et al.10–13 The
ring structure in the main chain obviously enhances the ri-
gidity which is very much desired in such type of materials
to have better thermal stability.
For studying the SHG activity, the polymers were cast on
ITO-coated glass. It is observed that polymers P2 and P4
form good quality film, whereas polymers P1 and P3 form
poor quality film. The formation of bad quality film for poly-
mers P1 and P3 could be due to the presence of less
branches making the polymer more rigid having restricted
solubility. These kinds of observations were also reported by
Chen et al. for the click polymers containing carbazole in the
main chain.23
From the FTIR spectroscopy (Fig. 1) for compound 3, a
sharp peak near 2130 cmꢁ1 indicates the presence of azide
group. This peak almost disappeared after the 50 min of po-
FIGURE 2 1H NMR spectra of polymer P4.
FIGURE 3 UV–vis absorption spectrum of polymer P4.
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FIGURE 4 Atomic force micrography of polymer P4. (A) Before poling and (B) After poling.
NLO Properties of Polymers
By using Nd:YAG laser of fundamental wavelength 1064 nm,
angular dependence of SHG intensity was measured. Figure
5 shows the angular dependence of SHG intensity for poly-
mer P4. The SHG intensity gradually increases with the inci-
dent angle, which is followed by a decrease. It means that at
a particular angle (36ꢀ) for P4 the dipoles interact to maxi-
mum extent with the fundamental frequency. After confirm-
ing the maximum SHG intensity at a particular angle (y),
same angle was maintained for further measurements.
As polymers P1 and P3 did not form good quality films, only
polymers P2 and P4 were taken for NLO characterizations.
Films of both the polymers P2 and P4 were cast using 5 wt
% (w/v) THF solutions over ITO substrate. For achieving
noncentrosymmetric environment, polymer films were co-
rona poled by applying 4.8 kV. After applying the field, the
temperature was raised to Tg, that is, 135 ꢀC (which is very
close to Tg) for polymer P4 and maintained for 30 min. The
noncentrosymmetric arrangements of the dipoles are con-
firmed by checking their absorption spectra, which shows
decrease in the intensity of absorption due to poling. Figure 3
shows the change in absorption for polymer P4 due to
poling. From the change in absorption, the order parameters
(F) calculated are found to be 0.1 and 0.18 for P2 and P4,
respectively. Figure 4 shows atomic force microscopy image
of a spin-coated film, before and after poling for polymer P4.
It is found that the smoothness of the surface reduces after
poling which is due to the alignment of dipoles along the
field direction.
Dynamic thermal stability of SHG intensity for polymers P2
and P4 was studied by increasing the temperature at a rate
ꢀ
of 4 C/min. Figure 6 shows the dynamic thermal stability of
SHG intensity for polymers P2 and P4. From the figure it is
clear that polymer P4 shows better dynamic thermal stabil-
ity than P2. For polymer P2, the drop of SHG intensꢀity
ꢀ
started at 70 C where as for polymer P4 it started at 95 C.
This could be due to the presence of a large condensed aro-
matic ring (fluorine) in case of P4 when compared with P2
where benzene is the aromatic ring present. However, after
100 ꢀC, a sharp decrease in SHG intensity was observed for
FIGURE 5 SHG Intensity of poled polymer P4 as a function of
FIGURE 6 SHG Intensity of poled polymer P2 and P4 as a func-
incident angle.
tion of temperature.
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FIGURE 7 Temporal stability of SHG signals of polymer P4 at
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CONCLUSIONS
The NLO polymers containing azo chromophore as side
chain and aromatic ring in the main chain were synthe-
sized through copper-catalyzed click chemistry. The poly-
mers formed good optically transparent films from THF
solution. The Tg of the polymers was found to be above
100 ꢀC, which ensured reasonable stability of poled poly-
mer at RT. UV–vis spectra showed decrease in absorbance
after poling, which confirms the alignment of dipoles in
the polymer film. Dynamic thermal stability of P4 recorded
continuous SHG intensity versus temperature and a fall
started after 95 ꢀC. Temporal stability of these polymers
were studied at RT up to 8 days. It showed about 14%
initial drop in SHG intensity that remained stable over the
time studied.
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