Synthesis and Properties of Chromophores with a Pyrroline-Type Acceptor for Electro-Optic Polymers

Article abstract of DOI:10.1080/15421406.2014.932232

We synthesized four electro-optic (EO) chromophores, which contained an aniline donor and an acceptor of 4-cyano-5-dicyanomethylene-2-oxo-3-pyrroline (CDCOP) with alkyl groups and/or oxyethylene chains terminated by an acrylate group as the three substituents. Photopolymerization of the mixture composed of the CDCOP derivatives, a liquid acrylate monomer and a radical photoinitiator was performed, and the chromophore fixation by crosslinking was achieved although about 30% of the chromophores were degraded by the radical attacks during crosslinking. Corona poling of these CDCOP derivatives in PMMA as a matrix polymer was successful regardless of the substituents. However, poling of the CDCOP derivatives in the liquid acrylate monomer matrix during photocrosslinking was difficult, and the poling conditions should be optimized.

Full text of DOI:10.1080/15421406.2014.932232

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Synthesis and Properties of
Chromophores with a Pyrroline-Type
Acceptor for Electro-Optic Polymers
a
a
a
a
Yukichi Sato , Shunsuke Inada , Tatsunori Araki , Yoko Tatewaki &
a
Shuji Okada
a
Department of Organic Device Engineering, Graduate School of
Science and Engineering, Yamagata University, Yonezawa, Japan
Published online: 17 Nov 2014.
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To cite this article: Yukichi Sato, Shunsuke Inada, Tatsunori Araki, Yoko Tatewaki & Shuji Okada
2014) Synthesis and Properties of Chromophores with a Pyrroline-Type Acceptor for Electro-Optic
(
Polymers, Molecular Crystals and Liquid Crystals, 597:1, 73-78, DOI: 10.1080/15421406.2014.932232
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Mol. Cryst. Liq. Cryst., Vol. 597: pp. 73–78, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.932232
Synthesis and Properties of Chromophores with a
Pyrroline-Type Acceptor for Electro-Optic Polymers
YUKICHI SATO, SHUNSUKE INADA, TATSUNORI ARAKI,
∗
YOKO TATEWAKI, AND SHUJI OKADA
Department of Organic Device Engineering, Graduate School of Science and
Engineering, Yamagata University, Yonezawa, Japan
We synthesized four electro-optic (EO) chromophores, which contained an aniline donor
and an acceptor of 4-cyano-5-dicyanomethylene-2-oxo-3-pyrroline (CDCOP) with alkyl
groups and/or oxyethylene chains terminated by an acrylate group as the three sub-
stituents. Photopolymerization of the mixture composed of the CDCOP derivatives, a liq-
uid acrylate monomer and a radical photoinitiator was performed, and the chromophore
fixation by crosslinking was achieved although about 30% of the chromophores were
degraded by the radical attacks during crosslinking. Corona poling of these CDCOP
derivatives in PMMA as a matrix polymer was successful regardless of the substituents.
However, poling of the CDCOP derivatives in the liquid acrylate monomer matrix
during photocrosslinking was difficult, and the poling conditions should be optimized.
Keywords Nonlinear optics; acceptor; EO chromophore; poled polymer; crosslinking
1
. Introduction
For ultrafast and high-capacity optical data processing, high-performance electro-optic
EO) polymers are indispensable, and development of chromophores appropriate for EO
(
polymer devices is an important research topic. Chromophores with an acceptor based on 4-
cyano-5-dicyanomethylene-2-oxo-3-pyrroline (CDCOP) are known to possess the superior
nonlinear optical properties [1]. The CDCOP group has several advantages as an accep-
tor. Since CDCOP has a five-membered ring and linear substituents like cyano and oxo
groups, there is less steric hindrance between the CDCOP and the connecting aromatic rings
resulting in a plane structure responsible for effective conjugation. Compared with the simi-
lar acceptors based on 4-cyano-5-dicyanomethylene-2,2-dimethyl-2,5-dihydrofuran (TCF)
[2], the CDCOP group is a better acceptor due to the 2-oxo group. Introduction of an
additional substituent to the CDCOP group is easier than TCF groups because the CDCOP
group has a reactive nitrogen atom in the ring. When we use an amino group as a donor,
in which two substituents can be attached, and combine it with a CDCOP group to form a
donor-acceptor molecule, three substituents can be introduced into one chromophore. This
condition is suitable to fix the poled orientation of the chromophores by crosslinking. In
our previous study, an aminostilbazolium derivative with three precursor substituents for
∗
Address correspondence to Shuji Okada, Department of Organic Device Engineering, Graduate
School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Japan.
E-mail: okadas@yz.yamagata-u.ac.jp
73

74
Y. Sato et al.
Figure 1. Synthesis procedure of CDCOP derivatives and chemical structures of compounds used
in the film preparation.
crosslinking was prepared [3]. In this study, we synthesized CDCOP-based chromophores
with the substituents of simple alkyl groups and/or oxyethylene chains terminated by an
acrylate group in order to fix the chromophores in a matrix by crosslinking. Also, their
properties were investigated.
2. Experimental
CDCOP derivatives 1-4 were synthesized according to Figure 1. Compound A was obtained
referring to the literature [4]. Synthesis procedure of 4 as an example was as follows.
Substitution reaction of 2-(2-chloroethoxy)ethanol using sodium iodide in the acetone reflux
condition gave 2-(2-iodoethoxy)ethanol and its condensation reaction with acryloyl chloride
in triethylamine-dichloromethane mixture afforded B. Compound C was synthesized from
aniline and B with potassium carbonate in DMF. To a stirred solution of A (2.21 g, 9.0 mmol)
in DMF (8 mL), B (2.43 g, 9.0 mmol) in DMF (18 mL) was added dropwise and was heated
◦
◦
over night at 60 C. The mixture was cooled to −5 C, and C (3.40 g, 9.0 mmol) was added.
After adding phosphoryl chloride (2.52 mL, 27.0 mmol) dropwise, the mixture was stirred
◦
for 1 h at −5 C and then for 4 h at ambient temperature. To the reaction mixture, water (1
L) was added and the solid portion was collected by filtration. The solid was subjected to
column chromatography (silica gel) twice. The eluents used were chloroform-ethyl acetate
mixed solvents and the mixing ratios were 17:3 for the first time and 3:7 for the second
time. The final recrystallization from methanol gave 2.51 g (41%) of 4. The chemical
1
13
structures of the products were identified by H- and C-NMR spectra, IR spectra and
elemental analyses. Thermal characterizations were performed using DSC and TG/DTA.
Some chemical and physical data on 1-4 are summarized in Table 1.
We prepared two kinds of polymer films containing the CDCOP derivatives. Chemical
structures of the compounds used in the film preparation are also shown in Figure 1. In order

Chromophores with a Pyrroline-Type Acceptor
75
Table 1. Yield of the reaction in Figure 1, melting points, results of elemental anal-
ysis, absorption maximum wavelength (λmax), the molar absorption coefficient (εmax)
and solubility
Found /% (Calculated /%)
b
b
εmax
−1
/
Solubility /
mol L
a
o
b
−1
−1
No. Yield /% Mp / C
C
H
N
λmax / nm L mol cm
4
1
2
3
4
83
56
57
41
128
99
113
74
72.84 (72.70) 7.13 (7.27) 16.26 (16.30)
67.59 (67.55) 6.35 (6.45) 13.62 (13.58)
63.60 (63.88) 5.86 (5.86) 11.63 (11.64)
61.07 (61.13) 5.37 (5.42) 10.10 (10.18)
629
627
617
615
4.4 × 10
4.5 × 10
4.1 × 10
4.0 × 10
0.34
0.31
0.33
0.33
4
4
4
a
Peak temperature by DSC
In THF at room temperature
b
5
to confirm poling ability of the chromophores, PMMA (Mw = 1.2×10 , Aldrich) was used
as a polymer matrix. THF solutions of a chromophore and PMMA were spin-coated on
glass slides, and the film-coated glass slides were corona-poled between a tungsten-needle
positive electrode and a copperplate negative electrode. The distance between electrodes
was set to be 1 cm and 6 kV was applied.
For the experiments to fix the chromophores in a matrix, a liquid trifunctional monomer
of M-350 (Toagosei Co., Ltd.) and CDCOP derivatives were mixed and the mixture was
polymerized by UV irradiation at 365 nm using 2,2-dimethoxy-2-phenylacetophenone
(
DMPAP) as a photoinitiator. Sandwiched cells composed of a pair of glass slides or ITO
TM
glass plates, which have a gap of about 11 μm formed by insertion of Saran Wrap , were
used for solidification experiments or those with contact poling.
3
. Results and Discussion
Among the CDCOP derivatives synthesized, melting points were lowered by introducing
-(2-acryloyloxyethoxy)ethyl (Acr) groups instead of butyl (Bu) groups. Larger effects for
2
the temperature down of the melting points were observed in the substitution at nitrogen in
the CDCOP ring rather than in the double substitution at the amino group. This phenomenon
is interesting but the reason is not clear because the crystal structures have not been solved
and the interaction among CDCOP molecules in the crystals affecting the melting points
has not been elucidated. Temperatures corresponding to 5% weight loss under a nitrogen
◦
◦
◦
◦
atmosphere measured by TG/DTA were 294 C, 285 C, 312 C and 335 C for 1, 2, 3 and
, respectively, indicating high thermal stability of the chromophores. In general, electron
4
donating ability of Bu group is stronger than that of Acr group because of the inductive
effect by electronegative oxygen atoms in Acr group. Thus, Acr-attached amino group is
considered to be a weaker donor than Bu-attached amino group, and the resulting weaker
intramolecular charge-transfer shifted λmax to the shorter wavelengths, i.e., the λmaxs of 1
and 2 are 12-nm longer than those of 3 and 4, respectively. Similar effect was also observed
in aminostilbazolium derivatives [3]. The εmaxs of 1 and 2 are about 10% larger than those
of 3 and 4. Large differences in solubility to THF depending on substituents were not
observed.
Poling of the chromophores in PMMA was investigated. Figure 2 shows UV-visible
absorption spectra of the spin-coated PMMA films containing 10wt% of 1 or 4 before and
after poling and after post thermal treatment. In order to avoid the orientation effect by
the spin-coating process, the spectra before poling were recorded after thermal annealing

76
Y. Sato et al.
Figure 2. UV-visible absorption spectra of the spin-coated films of (a) 1 and (b) 4 dispersed in
PMMA (i) before and (ii) after poling and (iii) after post thermal treatment. The chromophore content
was 10wt%.
◦
at 120 C for 5 min. The glass transition temperatures (Tgs) of the PMMA films of 1 and
4
◦
◦
◦
were measured to be 98 C and 110 C, respectively, and poling was performed at 10 C
higher temperatures than the Tgs. The post thermal treatment was performed at each Tg
for 5 min. In both cases, the absorption decreased after poling and recovery of absorption
was observed by post thermal annealing. These results clearly indicated that decomposition
of the CDCOP derivatives during poling process was not so serious and poling of the
chromophores dispersed in PMMA was confirmed regardless of the substituents.
In order to study photopolymerization behaviors of trifunctional acrylate M-350 in-
cluding the CDCOP derivatives, photopolymerization of M-350 with DMPAP on glass
slides was first examined. Although the solidification was not sufficient under an air at-
mosphere, the complete solidification was achieved when the photopolymerization was
conducted under a nitrogen flow. Polymerization in the sandwiched cells was also effective
to eliminate the effect from oxygen in air.
Next, durability of the CDCOP derivatives under the photopolymerization conditions
was verified. UV was irradiated to M-350 containing 1 and DMPAP, which was sandwiched
between two glass slides. Changes in the UV-visible spectra are shown in Figure 3(a).
The absorption of the chromophore gradually decreased. However, in this experimental
condition, the matrix solidification was accomplished within 1 min and more than 70%
of the chromophore remained. Thus, if the conditions are much optimized, chromophore
loss may be reduced. Two effects are considered for the chromophore degradation. One is
photoirradiation. However, almost no photodegradation of the chromophore was observed
after UV irradiation for 30 min in the same experimental conditions as those in Figure 3(a)
except addition of DMPAP. Thus, the degradation is due to the other effect, i.e., radical
attacks to the chromophore. Actually, when THF solution of 1 and DMPAP in 1:1 ratio was
irradiated by UV, rapid bleaching was observed as shown in Figure 3(b) . Even in a donor-
acceptor-type azo compound, bleaching during crosslinking by radical polymerization has
been reported [5].
The effect of the crosslinkable substituents attached to the chromophores was investi-
−5
gated by simple extraction experiments. M-350 containing a chromophore (4×10 mol/g
to M-350) and DMPAP (1wt% to M-350) was inserted in the sandwiched glass-slide cells

Chromophores with a Pyrroline-Type Acceptor
77
Figure 3. UV-visible absorption spectral changes during UV irradiation at 365 nm: (a) M-350
containing 1 (0.43wt% to M-350) and DMPAP (2wt% to M-350) sandwiched between two glass
slides; (b) THF solution of 1 and DMPAP (1:1) in a 1-mm cell.
and UV was irradiated for 3 min. Then, one of the glass slides of the cells was removed
and the solidified films were immersed into acetone for 3 min. Although 1 was dissolved
in acetone from the film, the extracted portion of 4 was 13% indicating that crosslinking
between the matrix monomer and the chromophore was achieved by introducing crosslink-
able substituents to the chromophore. Chromophore 3 gave the extraction portion similar
to 4, and 2 showed the intermediate behavior between 1 and 3. Namely, more efficient
crosslinking was realized by the chromophore with more crosslinkable substituents.
In the sandwiched cells composed of two ITO glass plates, M-350 containing 1 was
introduced and a DC voltage of 100 V corresponding to about 91 kV/cm was applied
between the ITO electrodes. However, the color fade maybe due to electrochemical reactions
of 1 on the ITO electrode was observed. Thus, M-350 with DMPAP was coated on ITO glass
plates and solidified by photoirradiation to form an insulating layer on the ITO plates. These
polymer-coated ITO plates were used to fabricate sandwiched cells. Applying voltage to the
cell containing M-350 and 1 resulted in no chromophore degradation. However, detectable
absorption changes originated from chromophore orientation by the electric field was not
observed. Since the chromophore orientation in PMMA has been confirmed as mentioned
above, the poling conditions of chromophores in the monomer matrix must not be optimized.
In the present conditions, thermal molecular motions may overcome the orientational force
for the chromophores. Thus, further optimization of poling conditions by changing matrix
monomers, temperature etc. are being investigated.
Conclusion
We succeeded in synthesizing the CDCOP derivatives 1-4 with alkyl groups and/or oxyethy-
lene chains terminated by an acrylate group in the substituents. The CDCOP derivatives
in PMMA could be oriented by the corona poling regardless of the substituents. The chro-
mophore fixation by crosslinking with M-350 was achieved within 30% absorption loss of
the chromophores, which was attributed to the radical attacks to the chromophores. Chro-
mophore 4 in M-350 was effectively crosslinked and only small amount of the chromophore

78
Y. Sato et al.
was extracted from the solidified polymer. From these basic aspects on the CDCOP deriva-
tives, they will be useful EO chromophores for the device application if the optimized
poling conditions are established.
Acknowledgments
This study was supported by the program for Strategic Promotion of Innovative Research
and Development, JST. We thank to Prof. Toshikuni Kaino, Prof. Okihiro Sugihara, Dr. Ak-
ihiro Kaneko and Dr. Masataka Sato for their valuable discussions on CDCOP derivatives.
We also thank to Dr. Tatsuya Yamashita, Dr. Masaaki Tsuchimori, Dr. Osamu Watanabe
and Dr. Manabu Kagami for their useful information on photopolymers.
References
[1] Kaneko, A., Lu, Z., Wang, H., Twieg, R. J., Mao, G., Singer, K., & Kaino, T. (2005). Nonlinear
Opt. Quant. Opt., 34, 45.
[
[
2] Dalton, L. R., Sullivan, P. A., & Bale, D. H. (2010). Chem. Rev., 110, 25.
3] Ikeda, T., Tadaki, Y., Funajima, R., Tatewaki, Y., & Okada, S. (2011). Mol. Cryst. Liq. Cryst.,
539, 142.
[
[
4] Carboni, R. A., Coffman, D. D., & Howard, E. G. (1958). J. Am. Chem. Soc., 80, 2838.
5] Borbone, F., Caruso, U., Diana, R., Panunzi, B., Roviello, A., Tingoli, M., & Tuzi, A. (2009).
Org. Electron., 10, 53.