Crosslinkable nonlinear optical dendrimers synthesized by Diels-Alder reaction

Article abstract of DOI:10.1166/jnn.2012.5357

Delivered by Ingenta to: Elsevier Crosslinkable NLO dendrimers based on azobenzene-type chromophores were synthesized by a Diels-Alder reaction. Their thermal and optical properties were investigated before and after poling at a high temperature. We found that the dendrimers retained good thermal stability up to 260 °C after curing at 130 °C for 10 min based on thermal analysis. Through in-situ poling and curing processes, the highest NLO activity of the dendrimers was d₃₃ = 1.7×10^-6 esu at 1064 nm, which was determined by a Maker fringe experiment. Copyright

Full text of DOI:10.1166/jnn.2012.5357

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 730–736, 2012
Crosslinkable Nonlinear Optical Dendrimers
Synthesized by Diels-Alder Reaction
Su-Hwan Bae1, Hye-Min Kim1, Hae Sik Min2, Doseok Kim2, and Tae-Dong Kim1ꢀ∗
1Department of Advanced Materials, Hannam University, Daejeon 305-811, Korea
2Department of Physics, Sogang University, Seoul 121-742, Korea
Crosslinkable NLO dendrimers based on azobenzene-type chromophores were synthesized by
a Diels-Alder reaction. Their thermal and optical properties were investigated before and after poling
ꢀ
at a high temperature. We found that the dendrimers retained good thermal stability up to 260 C
after curing at 130 ꢀC for 10 min based on thermal analysis. Through in-situ poling and curing
processes, the highest NLO activity of the dendrimers was d33 = 1ꢀ7×10−6 esu at 1064 nm, which
was determined by a Maker fringe experiment.
Keywords: Second-Order Nonlinear Optics, Dendrimer, Crosslinking, Diels-Alder Reaction,
Poling.
1. INTRODUCTION
monodisperse structures can lead to interesting charac-
teristics and unusual physical properties.16–20 It has been
reported that NLO dendrimers can significantly improve
optical nonlinearity by reducing electrostatic interaction
between NLO chromophores.21ꢁ22 Multiple NLO chro-
mophore building blocks can be placed into the dendrimers
to construct a precise molecular architecture with a pre-
determined chemical composition. The most important
advantage of NLO dendrimers is their capability of max-
imizing chromophore concentration without phase separa-
tion and aggregation. Here we describe the synthesis and
characterization of crosslinkable NLO dendrimers facil-
itated with a Diels-Alder reaction (Fig. 1). In order to
conduct Diels-Alder reactions for the NLO dendrimer,
maleimide- and anthryl-containing NLO chromophores
were prepared. Through in-situ poling and curing pro-
cesses, the highest NLO activity showed 1.7×10−6 esu of
second harmonic generation coefficient (d33ꢂ at 1064 nm
by a Maker fringe experiment.
Organic second-order nonlinear optical (NLO) materi-
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als have been extensively studied for their potential
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applications for information technologies in telecom-
Copyright: American Scientific Publishers
munications, computing, network sensing, and THz
generation/detection.1–6 It is well established that dipo-
lar NLO chromophores consisting of electron donor and
acceptor groups contribute to large second-order optical
nonlinearity arising from their intramolecular charge trans-
fer and need to be assembled into a noncentrosymmet-
ric lattice in order to obtain macroscopic nonlinearity.
A major challenge for designing efficient dipolar NLO
chromophores is the optimization of the ꢀ-conjugation
length as well as the strength of electron donors and
acceptors.7ꢁ8 It is also highly desirable to covalently
incorporate NLO chromophores into polymer networks
and harden the matrix through a crosslinking reaction to
improve both their thermal and mechanical properties.9–11
High performance NLO polymers were recently demon-
strated by Diels-Alder reaction for postfunctionalization
and lattice hardening to improve NLO properties and ther-
mal stability.12–15 It involves a ring forming coupling reac-
tion between a dienophile and a conjugated diene, and can
be generally described by a symmetry allowed concerted
mechanism that does not include reactive radical or ionic
intermediate species.
2. EXPERIMENTAL DETAILS
2.1. Chemicals
All reagents were purchased from commercial sources
and used without further purification unless otherwise
noted. 3-Maleimidopropionic acid (MPA),23 4-(anthracen-
9-ylmethoxy)-4-oxobutanoic acid (AMBA),24 (2-amino-
5-nitrophenyl)methanol (ANM),25 and 4-(dimethylamino)
pyridinium-4-toluenesulfonate (DPTS)26 were synthesized
according to the literature methods. The reaction solvents,
such as dichloromethane and tetrahydrofuran, were freshly
On the other hand, dendrimers are a relatively new class
of macromolecules and are different from conventional
linear, crosslinked, or branched polymers. Their globular,
∗Author to whom correspondence should be addressed.
730
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 1
1533-4880/2012/12/730/007
doi:10.1166/jnn.2012.5357

Bae et al.
Crosslinkable Nonlinear Optical Dendrimers Synthesized by Diels-Alder Reaction
O
O
O
O
N
N
O
N
N
O
O
O
+
O
Diels-Alder
Reaction
O
N
N
O
O
Maleic-containing
chromophore
Anthryl-containing
chromophore
Crosslinked NLO network
Fig. 1. Crosslinked NLO network by Diels-Alder reaction.
distilled prior to use. All reactions were carried out under
an inert nitrogen atmosphere.
eluent to afford DRAN (Yield: 85%). 1H-NMR (300 MHz,
CDCl3ꢁ ppm): ꢃ 8.41 (d, 2 H), 8.32 (d, 2 H), 8.23 (d, 2 H),
7.89 (d, 4 H), 7.65 (d, 2 H), 7.03 (d, 2 H), 6.14 (s, 1 H),
5.78 (s, 2 H), 5.34 (s, 2 H), 2.64 (t, 2 H).
2.1.1. Synthesis of (E)-2,2ꢁ-(4-((2-(hydroxymethyl)-
4-nitrophenyl)diazenyl)phenylazanediyl)
Diethan-Ol (DR-OH)
2.1.4. Synthesis of DR-2G
Ice-cooled solution containing ANM (0.5 g, 3.0 mmol)
and 3% HCl solution (30 mL) was added dropwise to
sodium nitrite (0.28 g, 4.0 mmol) in water (4 mL).
N-phenyldiethanolamine (0.54 g, 3.0 mmol) in 7% HCl
solution was then added. The resulting mixture was stirred
The solution of DRMI (0.12 g, 0.15 mmol) in THF
(80 mL) was refluxed and stirred for 1 hour to dissolve
the DRMI completely. DRAN (0.03 g, 0.02 mmol) in
20 mL of THF was slowly added to the reaction mix-
ture and kept reflux for overnight. The crude product
was purified by chromatography using ethyl acetate and
methanol (9:1) as an eluent to afford DR-2G (Yield: 35%).
ꢀ
for 2 hours at 5 C and then neutralized with 10% NaOH
solution. A deep red precipitate was collected and washed
1
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three times with ethanol/water (1:3) to give DR-OH (Yield:
H-NMR (300 MHz, CDCl3, ppm): ꢃ 8.41 (d, 2 H), 8.32
IP: 178.57.66.140 On: Wed, 27 Apr 2016 03:49:47
85%). 1H-NMR (300 MHz, CDCl3, ppm): ꢃ 8.41 (d, 2 H),
(d, 2 H), 8.23 (d, 2 H), 7.65 (d, 2 H), 7.31 (d, 2 H),
7.19 (t, 3 H), 6.94 (s, 2 H), 5.94 (s, 2 H), 5.34 (s, 1 H),
4.31 (t, 2 H), 3.87 (t, 2 H), 3.63 (t, 2 H), 3.14 (d, 2 H),
2.64 (t, 2 H), 2.51 (t, 2 H). MALDI-TOF-MS (matrix,
2-(4-hydroxyphenylazo)benzoic acid (HABA)): calcd for
Copyright: American Scientific Publishers
8.32 (d, 2 H), 8.23 (d, 2 H), 7.65 (d, 2 H), 7.03 (d, 2 H),
5.39 (t, 3 H), 4.79 (t, 3 H), 3.76 (t, 3 H), 3.54 (t, 3 H).
2.1.2. Synthesis of Maleimide-Containing
Chromophore DRMI
C
188H167N25O56 3672.47; found 3670.64 [M+].
DCC (0.2 g, 0.10 mmol) with a catalytic amount of DPTS
was added to the solution of DR-OH (0.10 g, 0.30 mmol)
and MPA (0.14 g, 0.80 mmol) in THF (30 mL). The reac-
tion mixture was allowed to stir at room temperature for 6
hours under the nitrogen atmosphere. After filtration of the
resultant urea, the solution was concentrated under reduced
pressure. The crude product was purified by column chro-
matography using ethyl acetate and hexane (2:1) as an
2.2. Characterization
1H NMR spectra (300 MHz) were taken on a Varian
300 spectrometer and mass spectra were recorded on
a JMS-AX505WA mass spectrometer. UV/vis spectra were
obtained on a Perkin-Elmer spectrophotometer. Differen-
tial scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) were performed on a TA instruments Q50
1
ꢀ
eluent to afford DRMI (Yield: 73%). H-NMR (300 MHz,
at a ramping rate of 10 C/min under a nitrogen atmo-
CDCl3, ppm): ꢃ 8.41 (d, 2 H), 8.32 (d, 2 H), 8.23 (d, 2 H),
7.65 (s, 2 H), 7.03 (d, 2 H), 6.94 (s, 2 H), 5.34 (s, 2 H),
4.31 (t, 2 H), 3.87 (t, 2 H), 3.63 (t, 2 H), 2.61 (m, 4 H).
sphere. Atomic force microscopy (AFM) was performed
using a Digital Instruments Nanoscope IV operated in
tapping mode (∼350 kHz frequency, Si tip).
2.1.3. Synthesis of Anthryl-Containing
Chromophore DRAN
2.3. Film Preparation and SHG Measurements
Pinhole-free films were prepared by spin-coating onto
indium tin oxide (ITO) glass substrates from cyclopen-
tanone solution (12 wt% of solid contents) after filtration
through a 0.2 ꢄm of poly(tetrafluoroethylene) (PTFE) fil-
ter. The substrate was dried under high vacuum at 40 C
for overnight to remove residual solvents. The thickness of
This was synthesized with a similar procedure as described
for compound DRMI using DR-OH (0.10 g, 0.30 mmol)
and AMBA (0.80 g, 3.00 mmol) instead of DR-OH and
MPA. The crude product was purified by column chro-
matography using ethyl acetate and hexane (2:1) as an
ꢀ
J. Nanosci. Nanotechnol. 12, 730–736, 2012
731

Crosslinkable Nonlinear Optical Dendrimers Synthesized by Diels-Alder Reaction
Bae et al.
the films were found to be around 0ꢅ8∼1ꢅ2 ꢄm. For second
AMBA, to afford DRMI or DRAN, respectively. The
resulted N-acrylurea and anhydride byproduct can be
removed by repetitive precipitation after a catalyzed-
esterification reaction. The crosslinkable NLO system is
able to be generated from blending with a 1:1 mixture of
DRMI and DRAN. After spin-coating the mixture solu-
tion onto glass substrates, the highly reactive anthracene
groups in DRAN react with maleimide groups in DRMI
at elevated temperatures. Through this solid phase Diels-
Alder reaction, it forms a chromophore-embedded network
system. A multi-arm NLO dendrimer, DR-2G, was syn-
thesized by a solution reaction between DRAN and an
excess of DRMI also facilitated with the Diels-Alder reac-
tion (Scheme 2). The structure and purity of all intermedi-
harmonic generation (SHG) measurements, the films were
ꢀ
poled at 130 C for 30 minutes under an applied voltage
of 6 kV with a corona electrode using a tungsten needle.
A Q-switched Nd:YAG laser (ꢆ = 1064 nm), with a pulse
width of 10 ns and a repetition rate of 10 Hz, was used
as the fundamental light source and Y -cut quartz was used
as a reference. SHG coefficients, d33, were derived from
analysis of a Maker fringe method.27
3. RESULTS AND DISCUSSION
The synthetic scheme of maleimide-containing DRMI
and anthryl-containing DRAN chromophores is shown
in Scheme 1. The hydroxy-functionalized chromophore,
DR-OH, was reacted with an acid compound, MPA or
1
ates and final compounds have been verified by H NMR,
UV/vis and mass spectroscopy, and thermal analysis.
HO
OH
N
N
NH2
HO
OH
N
OH
N
NO2
OH
ANM
DROH
NO
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2
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Copyright: American Scientific Publishers
O
O
O
O
O
N
N
O
O
O
O
N
O
OH
O
O
O
O
O
DRAN
NO2
DROH
O
O
O
N
O
O
N
N
O
O
O
O
COOH
N
O
N
O
O
N
O
N
O
DRMI
NO2
Scheme 1. Synthesis of anthryl-containing DRAN and maleimide-containing DRMI chromophores.
J. Nanosci. Nanotechnol. 12, 730–736, 2012
732

Bae et al.
Crosslinkable Nonlinear Optical Dendrimers Synthesized by Diels-Alder Reaction
NO2
O
N
N
N
O
O
O
O
O
O
N
O
N
O
O
O
O
O
O
N
O
O
O
O
O
O
O
N
N
N
O
O
DRAN
(1eq)
N
O
O
O
O
O
Diels-Alder
reaction
N
+
O
O
N
O
N
N
O
O
O
NO2
THF
O
DRMI
(excess)
NO2
N
O
O
O
O
N
O
N
O
O
O
O
N
N
O
N
DR-2G
Mol Wt: 3,672.47
O
O
NO2
N
O
Scheme 2. Synthesis of a DR-2G dendrimer.
DR-2G has excellent solubility in common organic sol-
obtained from the as-prepared film. The typical absorp-
tion bands for the anthracene moiety are observed at 354,
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vents, such as acetone, ethyl acetate, tetrahydrofuran,
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372, and 392 nm. As seen from Figure 2, upon increas-
toluene, and chlorinated solvents. Therefore, DR-2G can
be directly spin-coated to form a monolithic molecular
glass and exhibits high film quality without any aggrega-
tion or phase separation. It can further form a crosslinking
network with two equivalents of DRAN in the thin film.
Figure 2 shows the UV-vis absorption spectra of a thin
film for DRMI/DRAN (1:1 mixture). The main absorp-
tion peak at 476 nm is due to the intramolecular charge-
transfer band of the chromophores. The solid line was
ꢀ
ing the time at 130 C, these bands are reduced by the
Diels-Alder cycloaddition reaction between the anthryl-
and maleimide-functionalized chromophores. These bands
have completely disappeared within 5 minutes during
annealing, indicating the high reactivity of the Diels-Alder
reaction in the solid state. It was found that the UV-vis
absorption spectra of DR-2G blended with DRAN showed
ꢀ
similar behavior during annealing at 130 C. Interestingly,
DR-2G has a higher absorption coefficient than DRMI and
DRAN, as listed in Table I. This is probably due to reduc-
ing chromophore aggregation for the DR-2G dendrimer in
the solution and solid states.
Thermal analysis of DRMI/DRAN (1:1 mixture) and
DR-2G/DRAN (1:2 mixture) by DSC exhibited an
exothermic transition above 130 ꢀC, at which tempera-
ture Diels-Alder cycloaddition and a crosslinking network
were induced. Upon the second heating of these samples,
0.10
As-prepared
30 sec
60 sec
120 sec
240 sec
300 sec
0.08
0.06
0.04
0.02
0.00
Table I. Thermal and optical properties of DRMI, DRAN, and DR-2G.
Sample
Tga (ꢀC)
Tdb (ꢀC)
ꢆcmax_sol (nm)
ꢇd (L/mol·cm)
DRMI
DRAN
DR-2G
–
78
56
218
204
230
460
470
465
3ꢅ1×104
2ꢅ9×104
3ꢅ8×104
–0.02
400
500
600
700
800
Wavelength (nm)
Notes: aꢁbDetermined by DSC and TGA curves at a heating rate of 10 ꢀC · min
under nitrogen atmosphere; cAbsorption maxima in dichlromethane; dAbsorption
coefficient.
Fig. 2. UV-vis absorption spectra of a thin film for DRMI/DRAN (1:1
mixture) upon increasing a time at 130 ꢀC.
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Crosslinkable Nonlinear Optical Dendrimers Synthesized by Diels-Alder Reaction
Bae et al.
(a)
100
80
60
40
DRAN
DRMI
20
DRMI/DRAN (1:1)
0
100
200
300
400
500
Temperature (ºC)
Fig. 3. TGA thermograms of DRMI, DRAN, and DRMI/DRAN (1:1
mixture) at a heating rate of 10 ꢀC/min under nitrogen.
(b)
we did not observe any significant thermal transition up
to 250 ꢀC before decomposition of the chromophores.
They were very tough and were completely insoluble
in any organic solvent. The initial decomposition tem-
perature (Tdꢂ with 5% of weight loss was measured by
TGA for DRMI, DRAN, DRMI/DRAN (1:1 mixture), and
DR-2G/DRAN (1:2 mixture). As shown in Figureꢀ3, the
Tds of DRMI and DRAN were found to be at 218 C and
ꢀ
204 C, respectively. The cured crosslinked networks of
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DRMI/DRAN (1:1 mixture) and DR-2G/DRAN (1:2 mix-
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ture) significantly enhanced their thermal stabilities over
Copyright: American Scientific Publishers
ꢀ
50 C compared with DRMI and DRAN.
The NLO properties of DRMI/DRAN (1:1 mixture) and
DR-2G/DRAN (1:2 mixture) were studied by the SHG
method. To induce noncentrosymmetric polar order, the
thin films wereꢀtreated with corona poling (external field of
6 kV ) at 130 C for 30 minutes. Starting from a soft and
very soluble composite film, the poled/cured network film
became very tough and gained good solvent resistance due
to Diels-Alder crosslinking. Both poling fields and times
were varied to optimize the polar order and crosslink-
ing. Although the UV-vis absorption spectra showed effec-
tive crosslinking network induced within 5 min, we found
improved poling efficiency as increased time until 30 min.
Furthermore we did not observe any NLO activity of
crosslinked films applied with the voltage let than 5 kV of
poling fields.
Due to such a high voltage applied at a high tempera-
ture, the poled films may possess unevenly distributed sur-
face morphology. Therefore, we examined the film surface
by AFM before and after poling. As shown in Figure 4(a),
the AFM image of the DRMI/DRAN (1:1 mixture) before
the poling shows a highly uniform film with a surface
roughness of 1∼2 nm. We observed aggregates evenly dis-
tributed with a size of 10∼30 nm and aligned to the pol-
ing direction (Fig. 4(b)). However, there is little change
for the surface roughness of the poled film. This result
differs from previous studies that show numerous hills
Fig. 4. AFM images of the DRMI/DRAN (1:1 mixture) film (a) before
and (b) after poling.
and valleys in the surface structure of corona-poled poly-
mers and suggest that crosslinked network films may be
more stable under poling. UV-vis absorption spectra before
and after poling for the DRMI/DRAN (1:1 mixture) are
shown in Figure 5. The solid line was obtained from the
ꢀ
film treated with heating to 130 C without any external
field applied. The dashed line was obtained from the same
ꢀ
film with the corona poling applied with 6 kV at 130 C
for 30 minutes. The decrease of the peak absorbance is
caused by the alignment of the chromophore dipoles along
the poling direction, which is the incident light direction
as well. Knoesen and coworkers have described the axial
orientation of the chromophores by the order parameter,
ꢈ = 1 − ꢉA/A0ꢂ, where A0 and A are respectively the
absorbance maxima for the unpoled and poled samples
at normal incidence.28 From the calculation, the estimated
order parameter was 0.38 for DRMI/DRAN (1:1 mixture).
It is noted that DR-2G/DRAN (1:2 mixture) exhibited the
order parameter of 0.22. The reduced value suggests a high
734
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Bae et al.
Crosslinkable Nonlinear Optical Dendrimers Synthesized by Diels-Alder Reaction
Table II. Thermal, optical, and NLO properties of DRMI/DRAN (1:1
mixture) and DR-2G/DRAN (1:2 mixture).
0.25
Before Poling
After Poling
Sample
Tda (ꢀC) ꢆbmax_film (nm) ꢊc
d3d1 (esu)
d3d3 (esu)
0.20
0.15
0.10
0.05
0.00
DRMI/DRAN
(1:1 mixture)
DR-2G/DRAN
(1:2 mixture)
260
263
476
481
0.38 3.6×10−7 1.7×10−6
0.22 3.2×10−7 1.1×10−6
Notes. aDetermined by TGA curves at a heating rate of 10 ꢀC·min under nitrogen
atmosphere; bAbsorption maxima in a thin solid film; cOrder parameter; dSHG
coefficients derived from the analysis of a Maker fringe technique.
d33/d31 for DR-2G/DRAN (1:2 mixture) is in good agree-
ment. Evaluation of high-temperature and temporal align-
ment stability of the crosslinked NLO films is on progress
by using various poling and annealing conditions.
400
600
Wavelength (nm)
Fig. 5. UV-vis absorption spectra of the DRMI/DRAN (1:1 mixture)
film before and after poling.
4. CONCLUSION
concentration of the maleimide exterior moieties in the
DR-2G accelerated Diels-Alder reaction with DRAN at
lowered temperatures resulted in a poor poling order.
SHG measurements were performed at a fundamental
wavelength of 1064 nm with a Q-switched Nd-YAG laser
and OPO. The SHG intensity of the films was measured
using a standard Maker fringe technique. Figure 6 shows
the relationship between the SHG intensity and the inci-
dent angle for DRMI/DRAN (1:1 mixture). The SHG coef-
We have successfully synthesized two crosslinkable
NLO dendrimers, DRMI/DRAN (1:1 mixture) and
DR-2G/DRAN (1:2 mixture). Through the solid phase
Diels-Alder reaction, they formed chromophore-embedded
network systems. Their thermal and optical properties
were investigated before and after poling at a high tem-
perature. The cured crosslinked networks significantly
enhanced their thermal stability more than 50 ꢀC compared
ficients can be calculated fromDtehleivaenrgeudlabrydIenpgeenndteanctoe:oUf niversity of New South Wales
with DRMI and DRAN. For the DRMI/DRAN (1:1 mix-
IP: 178.57.66.140 On: Wed, 27 Apr 2016 03:49:47
ture), the highest NLO activity was d33 = 1ꢅ7 × 10−6 esu
the SHG intensity compared to a Y -cut quartz as a ref-
erence signal. To obtain the d31 and d33 values, both
Copyright: American Scientific Publishers
with an order parameter of 0.38, which was obtained from
s-polarized and p-polarized IR laser beams were directed
at the samples. The NLO properties of DRMI/DRAN (1:1
mixture) and DR-2G/DRAN (1:2 mixture) are summarized
in Table II. The values of d31 and d33 for DRMI/DRAN
(1:1 mixture) were 3.6×10−7 and 1.7×10−6 esu, respec-
tively. However we found a reduced order parameter
and lowered SHG coefficient (d33 = 1ꢅ1 × 10−6 esu) for
DR-2G/DRAN (1:2 mixture). Since the ratio of d33/d31
is predicted to be about 3 in the isotropic model, 3.4 of
in-situ poling and curing processes.
Acknowledgments: This work was supported by
the Korea National Research Foundation (NRF-2009-
0065894). Tae-Dong Kim acknowledges University
Research Program in Hannam University (2011).
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Received: 4 November 2010. Accepted: 1 February 2011.
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