Synthesis and characterization of Y-type polymers for second-order nonlinear optical applications

Article abstract of DOI:10.1002/pola.26439

A series of dicyanomethylene-substituted polymers having Y-type molecular architecture were synthesized by Knoevenagel condensation reaction. The polymers were found to be soluble in organic solvents like tetrahydrofuran and chloroform. From gel permeatio

Full text of DOI:10.1002/pola.26439

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Synthesis and Characterization of Y-Type Polymers for Second-Order
Nonlinear Optical Applications
Balakrishna Kolli,1 Sarada P. Mishra,1 Akshaya K. Palai,1 Tapan Kanai,1
M. P. Joshi,2 S. Raj Mohan,2 T. S. Dhami,2 L. M. Kukreja,2 A. B. Samui1
1Polymer Science and Technology Centre, Naval Materials Research Laboratory, Shil-Badlapur Road,
Ambernath, Thane 421506, Maharashtra, India
2Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore 452013,
Madhya Pradesh, India
Correspondence to: A. B. Samui (E-mail: absamui@gmail.com)
Received 30 May 2012; accepted 17 October 2012; published online 8 November 2012
DOI: 10.1002/pola.26439
ABSTRACT: A series of dicyanomethylene-substituted polymers
having Y-type molecular architecture were synthesized by
Knoevenagel condensation reaction. The polymers were found
to be soluble in organic solvents like tetrahydrofuran and chlo-
roform. From gel permeation chromatography, the molecular
weights of the polymers were found to be in the range of
15,300–33,800 g/mol. Thermal analysis showed that the poly-
mers were stable up to 350 ꢀC with glass transition tempera-
ture (Tg) in the range of 129–212 ꢀC. These polymers were
found to form good optical quality films. The order parameter
was calculated to be in the range of 0.01–0.48. Atomic force
microscopy indicated prominent morphology changes due to
alignment of dipoles after poling. By using Nd:YAG laser of
1064 nm, angular dependence and temperature dependence of
second-harmonic generation intensity were investigated. The
geometry optimization, shape of polymers, and restricted
torsion angle between acceptor and donor substituents (push–
C
V
pull system) were calculated.
2012 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2013, 51, 836–843
KEYWORDS: atomic force microscopy (AFM); corona poling; dif-
ferential scanning calorimetry (DSC); dipole alignment; glass
transition; NLO; order parameter; polymers; relaxation; second-
harmonic generation; UV–vis spectroscopy
INTRODUCTION Polymer materials with large nonlinear opti-
cal (NLO) responses are of great interest because of their
vast applications in optoelectronic technology, such as tele-
communications, optical data storage, and optical informa-
tion processing.1 Because of their good processability, ultra-
fast response, chemical flexibility, and large nonlinearity,
polymeric materials are more attractive when compared
with the traditional inorganic materials like lithium niobate
and potassium dihydrogen phosphate.2 NLO polymers can be
synthesized by either doping chromophore into a polymer
matrix3 or by attaching a chromophore covalently to a
polymer.4 In the former process, there is a possibility of
decomposition of chromophore at high temperature and
immiscibility issue with the host system.5 Therefore, attach-
ing a chromophore covalently to polymer chain is the
preferred way for synthesizing NLO polymer by researchers.
and synthesized was having high glass transition tempera-
ture (Tg; e.g., polyimide,7 polyurethane,8 and polyester9) and
also by crosslinking the polymer under poling.10 In general,
the activity of NLO chromophore depends on the donor–
acceptor strength and the conjugation length. This aspect
has been well studied in various reports.11 Various substitu-
ents on benzene ring have been tried along with oxygen
atom as donor. NLO polymers containing other aromatic/
nonaromatic system has also been explained in the litera-
ture.12 An heteroaromatic system like phenothiazine is con-
sidered to be a highly efficient donor group as it contains
both nitrogen and sulfur having lone pair of electrons.
The control of function through the primary structure of a
molecule is the basis of designing unique architecture such
as dendritic architecture.13
Taking all these parameters into consideration, Lee and co-
workers14 synthesized various T- and Y-shaped polymers to
achieve processable linear polymers similar to that of side-
chain systems. Issam has reported that azomethine-based
Y-type polyurethane showed reasonable enhancement in
thermal stability (Tg) up to 155 ꢀC.15,16 They observed that
Different types of donors, acceptors, and p-bridges are used
to achieve better and long-term properties.6 The challenging
issue is the designing of materials with stable alignment of
dipoles after poling. In the quest of stable alignment of
dipoles, the most common polymer architecture designed
C
V
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the main advantage of this Y shape is that the stability and
processability were similar to the main-chain and side-chain
polymers, respectively. Two-dimensional Y-type polymers
based on 4-(dicyanomethylene)-2-methyl-6-(p-(dimethylami-
no)styryl)-4H-pyran (DCM dye) have also been reported.17,18
Theoretical investigations of DCM derivatives have well
encouraged the second-order NLO study.19 In addition, a
large number of donor–acceptor assemblies via self-organiza-
tion of supramolecular helical dendrimer design and their
unique synthesis were reported by Percec et al.20 Carbazole,
naphthalene, and pyrene derivatives have been introduced at
the apex as donor groups, and 4,5,7-trinitrofluorene-2-
carboxylic acid as donor group glycols were used as spacer.
In this way, the charge carrier mobility was found to
increase by five orders of magnitude. In another report by
the same group, induction of p-stacking of donors at the
center of a supramolecular helical pyramidal column was
realized by attaching flexible spacer to electron-donor
molecules of semifluorinated first-generation self-assembling
dendrons.21 The self-assembling of functional dendrons into
supramolecular helical pyramidal columns, containing
p-stacks of the electron-acceptor groups having their long
axis perpendicular to the axis of the column, was also
reported by Percec et al.22 It was anticipated that the simple
and versatile strategy for producing conductive p-stacks of
aromatic groups will lead to new classes of supramolecular
materials of interest for electronic and optoelectronic
applications.
the polymers, their other properties such as thermal stability,
Tg, surface morphology, and their second-harmonic genera-
tion (SHG) activity were investigated.
EXPERIMENTAL
Materials
4-Hydroxybenzaldehyde (98%; Fluka, Switzerland), vanillin
(99%; S.D. Fine Chemicals, India), phenothiazine (98%; Lancas-
ter, England), and 2,6-dimethyl-4H-pyran-4-one, syringaldehyde,
and malononitrile were purchased from Sigma-Aldrich, Ger-
many, and used as received. Piperidine and pyridine were dried
by standard procedure. Compounds 2-(2,6-dimethyl-4H-pyran-
4-ylidene)malononitrile (1)18(e) and 4,40-[hexane-1,6-diylbis
(oxy)]bis(3-methoxybenzaldehyde) (3)27 were synthesized as
per the literature procedure.
Characterization Techniques
Fourier transform infrared (FTIR) spectra were recorded on
Perkin-Elmer 1600 series FTIR spectrophotometer using KBr
pellets. UV–visible spectra were taken on a Varian (Cary
500) scan UV–Vis–NIR spectrophotometer. 1H NMR spectra
were recorded either on 400 MHz or 500 MHz Bruker spec-
trometer using CDCl3 as solvent, and 13C NMR spectra were
recorded on 300 MHz Jeol spectrometer. Chemical shifts
were determined by using tetramethylsilane as an internal
standard. Molecular weight distributions were determined
by using gel permeation chromatography on Waters 2690
separations module using a Waters 2410 refractive index
detector with tetrahydrofuran (THF) as an eluent (flow rate
¼ 1 mL/min) and polystyrene as calibration standards.
Thermogravimetric analyzer (TGA; TA Instruments His Res
TGA 2950) was used for thermal analysis, with a heating
As heteroaromatic moieties have less aromatic stabilization
energy that enhances the nonlinearity, phenothiazine-based
materials were studied by Choi and coworkers.23 Polar-
ordered thin films were reported for molecules containing
phenothiazine.24 Phenothiazine is an electron-donating group
that can facilitate the charge transport of the carrier.25
Molecular orbital calculation was performed to find out the
quantum mechanical parameters. Its strong donor properties
were calculated from absorption spectroscopy and electro-
chemical analysis. The strong donor property of phenothia-
zine was also confirmed from absorption spectroscopy and
theoretical calculation.26 In the current work, we synthesized
different Y-type polymers based on the donor–acceptor–
donor–type repeating unit having different aromatic moieties
acting as donors and dicyanomethylene acting as an acceptor
through conjugated bridge. The donor group is directly
linked to the polymer backbone at both the ends, and the
acceptor dicyanomethylene group is attached to the pyran
ring. The donor is varied from different substituted benzene
to phenothiazine.
ꢀ
rate of 20 C/min in N2 atmosphere. The thermal transitions
were determined by using differential scanning calorimeter
(TA Instruments) with 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 an electrothermal 9100 melting point
apparatus.
Thin Film Preparation
For making thin film of the polymers, indium tin oxide (ITO)
glass substrates were precleaned thoroughly with dimethyl-
formamide (DMF), distilled water, methanol, and acetone in
ultrasonic bath. About 3 wt % chloroform solution of poly-
mer was filtered through 0.25-l PTFE filter, and then the so-
lution was spin coated on an ITO glass slide at a rotation
speed of 1200 rpm for 60 s at room temperature. Finally,
the cast films were dried in vacuum for 12 h at room tem-
perature to remove the trace amount of the solvent. The film
thicknesses were found to be in the range of 0.54–1.13 lm.
In the earlier study of Y-type NLO polymer, the chromophore
contains nitrogen atom in the donor moiety. Furthermore, in
the conventional method, the chromophore was built first,
which was then polymerized by attaching with other reac-
tants. However, in this study, the nitrogen in the donor group
was replaced by oxygen, and during polymerization, the
Y-type structure was developed. The strategy coupled with
long alkane chain in the polymer is expected to give better
solubility characteristics. After confirming the structures of
Corona Poling and SHG Measurement
The polymer-coated ITO glass was mounted on a heating
stage. The distance of the film was maintained to be 1 cm
from the tip of the tungsten needle. An electric field of 4.8
kV was applied through the needle at a temperature slightly
higher than the Tg of polymer. The poled polymer film
was characterized by using UV–vis spectroscopy. The order
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parameter (U) was calculated by using the following equa-
Yield: 47%; mp: 152 ꢀC; FTIR (KBr): m (cmꢁ1) 3060, 2926,
2850, 2583, 1596, 1333, 1247, 752. 1H NMR (CDCl3, 400
MHz): d (ppm) 7.14 (m, 8H), 6.91 (m, 8H); 3.82 (t, J ¼ 12.0 Hz,
4H), 1.79 (m, 4H), 1.46 (m, 4H). 13C NMR (CDCl3, 75 MHz):
d (ppm) 145, 127.4, 127.1, 124, 122, 115, 47, 26.6, 26.4.
tion:28
U ¼ 1 ꢁ A1=A0;
where A0 and A1 are the absorbance of the polymer film
before and after poling, respectively.
Synthesis of 10,100-(Hexane-1,6-diyl)bis(10H-phenothia-
zine-3-carbaldehyde) (6)
The second-order NLO effect of poled films was found out
by SHG experiment. For measuring the SHG intensity, 1064-
nm-pulsed Nd:YAG laser of pulse width 5 ns and 10 Hz repe-
tition rate, with laser energy of 5 mJ, was used. In this pro-
cess, poled films were mounted on rotational stage for
checking angular dependence of the SHG intensity. The fun-
damental beam from the generated SHG signal was blocked
by IR filter. The SHG signal was detected by a photomulti-
plier tube, and the signal was averaged by using an
oscilloscope.
Compound 5 (0.7 g, 1.45 mmol) was taken in a round-bottomed
flask containing 15 mL of dichloroethane and 1 mL of DMF. The
solution was cooled in an ice bath, and then 0.7 mL of POCl3
was added slowly. The ice bath was removed; the reaction mix-
ture was refluxed for 9 h and poured into an aqueous sodium
acetate solution. The compound was extracted with chloroform,
washed with water, dried, and evaporated. The crude product
was purified by column chromatography.
Yield: 52%; mp: 140 ꢀC; FTIR (KBr): m (cmꢁ1) 3055, 2931,
2855, 2721, 1687, 1592, 1366, 1166, 748. 1H NMR (CDCl3,
400 MHz): d (ppm) 9.79 (s, 2H), 7.63–7.56 [m (br), 4H],
7.17–6.83 (m, 10H), 3.88 (t, J ¼ 4.0 Hz, 4H), 1.8 (m, 4H),
1.47 (m, 4H). 13C NMR (CDCl3, 75 MHz): 189.9, 150, 143,
131, 129, 128, 127.6, 127.5, 125, 124, 123, 116, 114, 47,
26.5, 26.2.
Synthesis of Monomers and Polymers
Synthesis of 4,40-[Hexane-1,6-diylbis(oxy)]
dibenzaldehyde (2)
4-Hydroxybenzaldehyde (2.0 g, 0.016 mmol) and potassium
carbonate (6.0 g, 0.049 mmol) were taken in a 100-mL
round-bottomed flask containing 25 mL of DMF and stirred
for 15 min at room temperature. 1,6-Dibromohexane (2.0 g,
8.19 mmol) was then charged into it and stirred for 24 h by
maintaining the temperature at 90 ꢀC. The reaction mixture
was precipitated in excess water, and the product was
extracted thrice with chloroform. The crude compound was
recrystallized from ethanol and chloroform (9:1 v/v)
mixture.
Representative Example of Polymerization
2-(2,6-Dimethyl-4H-pyran-4-ylidene)malononitrile (1; 0.1 g,
0.58 mmol) and Compound 2 (0.18 gm, 0.58 mmol) were
charged in a 25-mL round-bottomed flask connected to a
Schlenk tube. To the solution, 3 mL of dry pyridine and 0.1
mL of piperidine (1.16 mmol) were charged and refluxed for
60 h. The polymer was precipitated in excess methanol, and
the solid was separated by filtration. It was purified by soxh-
lation with methanol to remove oligomers. The polymer was
dried under vacuum for 24 h at room temperature.
Yield: 85%; mp: 110 ꢀC; FTIR (KBr): m (cmꢁ1) 2947, 2846,
1
2753, 1686, 1598, 1254, 1158. H NMR (CDCl3, 400 MHz): d
(ppm) 9.8 (s, 2H), 7.84 (d, J ¼ 8.0 Hz, 4H), 7.0 (d, J ¼ 8.0
Hz, 4H), 4.08 (t, J ¼ 8.0 Hz, 4H), 1.88 (m, 4H), 1.59 (m, 4H).
13C NMR (CDCl3, 75 MHz): d (ppm) 190, 165, 131.9, 131.5,
114, 68, 28, 25.
Polymer P1. Yield: 85%; FTIR (KBr, cmꢁ1): 3033, 2936, 2860,
1
2199, 1604, 1580, 1427, 1249. H NMR (CDCl3, 500 MHz): d
(ppm) 7.54–7.40 (m, 4H), 7.0–6.86 (m, 4H), 6.84–6.78 (m, 2H),
6.76–6.69 (m, 2H), 4.08–3.80 (m, 4H), 1.9–1.4 (m, 8H).
Synthesis of 4,40-[Hexane-1,6-diylbis(oxy)]
bis(3,5-dimethoxybenzaldehyde) (4)
A similar procedure was followed to synthesize Compound 2.
Polymer P2. Yield: 89%; FTIR (KBr, cmꢁ1): 2997, 2936,
2855, 2199, 1644, 1583, 1422, 1266. 1H NMR (CDCl3, 500
MHz): d (ppm) 7.45–7.35 (m, 2H), 7.03–6.89 (m, 4H), 6.84–
6.58 (m, 6H), 4.13–3.68 (m, 10H), 2.10–1.4 (m, 8H).
Yield: 85%; mp: 80 ꢀC; FTIR (KBr): m (cmꢁ1) 3011, 2972,
2854, 2746, 1682, 1581, 1236, 1038. 1H NMR (CDCl3, 400
MHz): d (ppm) 9.86 (s, 2H), 7.26 (s, 4H), 4.10 (t, J ¼ 8.0 Hz,
4H), 3.91 (s, 12H), 1.81 (m, 4H), 1.55 (m, 4H). 13C NMR
(CDCl3, 75 MHz): d (ppm) 191, 153, 143, 131, 106, 73, 56,
30, 25.
Polymer P3. Yield: 70%; FTIR (KBr, cmꢁ1): 2999, 2932,
2855, 2199, 1694, 1583, 1417, 1227. 1H NMR (CDCl3, 500
MHz): d (ppm) 7.13–7.08 (m, 4H), 6.66–6.61 (m, 4H), 6.45–
6.38 (m, 2H), 4.09–3.69 (m, 16H), 1.90–1.38 (m, 8H).
Polymer P4. Yield: 94%; FTIR (KBr, cmꢁ1): 3060, 2936,
2844, 2204, 1638, 1533, 1466, 1255. 1H NMR (CDCl3, 500
MHz): d (ppm) 7.7–7.0 (m, 14H), 6.9–6.5 (m, 6H), 3.90–3.49
(m, 4H), 1.9–1.1 (m, 8H).
Synthesis of 1,6-Di(10H-phenothiazin-10-yl)hexane (5)
Phenothiazine (2.0 g, 0.01 mol) and sodium hydroxide (2.4 g,
0.06 mol) were taken in a round-bottomed flask containing
25 mL of DMSO, and then the mixture was stirred for 20 min.
1,6-Dibromohexane (1.22 g, 5.02 mmol) was then added
RESULTS AND DISCUSSION
ꢀ
slowly, and the temperature was maintained at 70 C for 6 h.
The reaction mixture was cooled to room temperature and
added into water. The compound was extracted with chloro-
form, washed with water, dried in MgSO4, and evaporated.
The crude material was purified by column chromatography.
Synthesis and Characterization
Synthetic routes for monomers and polymers are shown in
Schemes 1 and 2, respectively. First, 2-(2,6-dimethyl-4H-py-
ran-4-ylidene)malononitrile (1) was obtained by reacting
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SCHEME 1 Synthesis of monomers.
2,6-dimethyl-4H-pyran-4-one with malononitrile. p-Hydroxy-
benzaldehyde, vanillin, syringaldehyde, and phenothiazine
were treated with 1,6-dibromohexane in the presence of
base to afford Compounds 2–5. Compound 5 was further
subjected to standard Vilsmeier–Haack reaction to get alde-
hyde 6 in good yield.29 All the monomers showed character-
istic peaks in 1H NMR and particularly the peak around
9.7–9.8 ppm, confirming the presence of aldehyde proton.
FTIR spectra of monomers showed characteristic peaks near
1680 cmꢁ1 for carbonyl stretching. Figure 1 shows a strong
peak near 2200 cmꢁ1, which is characteristic of cyano group.
All the monomers (2–4 and 6) were then reacted with
equivalent moles of Compound 1 in pyridine and a catalytic
amount of piperidine to afford the polymers P1–P4 with
SCHEME 2 Synthesis of Y-type polymers.
FIGURE 1 IR spectra of polymers.
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TABLE 1 Polymer Characterization Data
kmax (nm)a
Mw
PDIb
Tg (ꢀC)c
Ud
b
Polymer
P1
P2
P3
P4
a
417
340
337
459
33,800
19,600
18,200
15,300
2.4
1.7
1.7
1.5
172
129
133
212
0.30
0.20
0.07
0.48
Wavelength at which maximum absorption was observed.
Calculated from gel permeation chromatography.
b
c
Glass transition temperature of polymers found from differential scan-
ning calorimetry.
d
Order parameter measured from UV–vis spectra.
1
good yield. In H NMR, appearance of a peak at 6.9–6.6 ppm
confirmed the presence of stilbene double bond. A small
peak for aldehyde proton was also observed, indicating that
the polymers formed were not of very high molecular
weight.
FIGURE 3 UV–vis absorption spectra of polymer P1.
The polymers were soluble in organic solvents like THF and
chloroform with molecular weights in the range of 15,300–
33,800 g/mol and polydispersity index 1.5–2.4 (Table 1).
The TGA thermogram showed weight loss of 10% around a
temperature range of 366–374 ꢀC (Fig. 2). The higher ther-
mal stability may be due to the two-dimensional structures
of the chromophores in the polymer, which has been well
explained by Hsiue and coworkers.30 The Tg of the polymers
(P1–P2) were in the range of 129–212 ꢀC. A drop in the Tg
of polymers P2 and P3 from that of P1 may be due to the
combined effect of lower molecular weight and the presence
of methoxy group substitutions on the benzene ring. The
polymer P4 showed enormous enhancement in Tg despite its
lowest molecular weight, which may be due to the con-
densed nitrogen-containing aromatic ring31 and two-dimen-
sional structure of the polymer.
structure has influenced the absorption pattern of the poly-
mer as the acceptor remained same. The wavelength of max-
imum absorbance (kmax) depends on the conjugation length
and the donor strength.
Corona Poling and Optical Properties
Optically transparent thin films of the polymers (P1–P4)
were cast from chloroform solution over ITO glass slides for
measuring their SHG intensity. Initially, the thin films of poly-
mers are in centrosymmetric fashion due to the dipole–
dipole interaction. After applying the electric fiꢀeld (4.8 kV),
the temperature was gradually increased to 10 C above the
Tg. The polymer film was then maintained for 30 min under
the applied voltage. Finally, the film was cooled to room tem-
perature in the presence of electric field to freeze the aligned
dipoles in noncentrosymmetric fashion, which was confirmed
by the electronic spectra. The representative electronic spec-
tra (Fig. 3) showed decrease in absorption after poling due
to birefringence.32 From change in absorption pattern, the
order parameter (U) was calculated and found to be 0.07–
0.48, which is better than the azo polymer in our earlier
report33 (Table 1). Figure 4 shows surface morphology of
the polymer P3 obtained by atomic force microscopy. Figure
4(b) shows the development of well-defined hills and valleys
after poling. This change in original micrograph indicates
alignment of dipoles, which is well known in the literature.34
Figure 3 shows the electronic spectra of the thin film of
polymer P1. The absorption data of the polymers in thin
film are shown in Table 1. It can be observed that the donor
The second-order NLO properties were studied by SHG
method. By using Nd:YAG laser of IR wavelength (1064 nm),
the angular dependence was studied. Figure 5 shows the
angular dependence of SHG intensity of polymer P1. It can
be observed that the SHG intensity increases with the angle
of incidence and reaches a maximum at around 50ꢀ. Then it
decreases with further increase in the angle of incidence.
This is due to maximum interaction of laser with dipole at a
particular angle.
By increasing the temperature of the poled film, the dynamic
thermal stability of poled dipoles was also studied. For this,
the temperature of the poled film was increased at the rate
FIGURE 2 TGA thermogram of polymers P1–P4.
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FIGURE 4 Atomic force microscopy of P3 before (a) and after (b) poling.
of 4 ꢀC/min (from room temperature). Figure 6 shows the
dynamic thermal stability of polymers P1, P2, and P4. It can
be seen that SHG intensity of P2 is stable up to 100 ꢀC,
Forcite calculates single-point energy and performs geometry
optimization, that is, energy minimization of polymers and
periodic systems.35 The study focuses on atomistic simula-
tions of polymer; the main interest being the performance of
classical models. The polymer-consistent force field devel-
oped for synthetic polymers forms the basis for the simula-
tions. The simulated structures of the polymers are shown in
Figure 7.
ꢀ
whereas P1 is stable up to 110 C. This could be due to the
higher Tg value of P1 when compared with P2 (also P3,
which relaxed very fast and attained less SHG intensity).
This results in higher segmental mobility of P2 making the
dipoles relax easily. However, when the system changes to
heteroaromatic moiety containing polymer P4, the SHG in-
The shapes of polymers P1–P3 are found to be zigzag in na-
ture where repeating unit is Y-type. The acceptor (pull unit)
groups and donor (push unit) groups are found orthogonal
(opposite regiomers) to each other. The orientation of these
groups has large contribution to the net dipole moment and
consequently on SHG intensity of polymer. In case of P4,
although Y shape is maintained, the shape of the polymer is
similar to a ribbon.
ꢀ
tensity remains stable up to almost 165 C.
Theoretical Calculations
To examine the effect of monomethoxy- and dimethoxy-substi-
tuted benzene and phenothiazine in the geometry of designed
polymers on the torsion angle, energies, and so forth, compu-
tational study was executed. Although correlation between ex-
perimental and theoretical data differs slightly for various
methods, they are useful for trend predictions in the molecular
design of new compounds for NLO applications.
The calculated energy of the polymers strongly depends on
the conformational statistics of the polymer chains and the
force field. The total energy can be given as follows:36
X
X
V ¼
Vdiag: terms
þ
V
cross terms
valence energy
valence energy
X
þ
V
nonbonded energy
The total energies for all the four types of dimers are sum-
marized in Table 2. The energy values are found to be at
maximum in case of P3. It is important to note that polymer
P3 having methoxy substituents on benzene ring has large
steric effect, which might be responsible for high energy
value. The polymer P1 has the lowest energy value, indicat-
ing its maximum stability. However, P4 has the energy
slightly higher than P1, indicating high stability as well as
high electron-donating ability. This imparts higher order pa-
rameter and stability of SHG signal over longer period. The
restricted torsion angle determined for all polymers are
summarized in Table 3. The torsion angle of donor unit and
FIGURE 5 SHG intensity of poled film P1 as a function of inci-
dent angle.
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FIGURE 6 SHG intensity of poled polymers P1 and P2 (a) and P4 (b) as a function of temperature.
FIGURE 7 Simulation results in material studio (polymer-consistent force field, atom based): (a) P1; (b) P2; (c) P3; and (d) P4.
acceptor unit do not deviate much from coplanarity, indicat-
ing sufficient high second-order nonlinearity.
mers were above 129 ꢀC and ranging up to 212 ꢀC. These
polymers easily formed good optical quality films from their
chloroform solution. The solid films remained stable at high
electric field (4.8 kV) during poling. Electronic spectra
showed decrease in absorption due to poling, and particu-
larly for polymer P4, the order parameter was quite high at
0.48. In addition, polymer P4 also exhibited higher SHG
intensity and high thermal stability.
CONCLUSIONS
Four Y-type polymers were successfully synthesized by Knoe-
venagel condensation. The polymers were thermally stable
up to 350 ꢀC. The glass transition temperatures of all poly-
TABLE 3 Restricted Torsion Angle of Polymers
TABLE 2 Calculated Energies of Dimers from Modeling Study
Torsion Angle
P1
P2
3.09
P3
5.18
P4
2.2
Polymer
Total Energy (kcal/mol)
C2-C1-C13-C15
C4-C5-C12-C14
C3-C7-C8-N9
1.64
7.59
1.86
5.83
P1
P2
P3
P4
81.568
166.806
229.702
149.509
4.21
132.92
25.66
2.42
26.06
33.20
11.53
19.14
43.97
C3-C7-C10-N11
842
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