Substituent engineering of electrooptic chromophores to suppress their aggregation

Article abstract of DOI:10.1080/15421406.2019.1648039

Chromophores composed of 4-cyano-5-dicyanomethylene-2-oxo-3-pyrroline (CDCOP) and a donor have been synthsized for electrooptic applications. Three substituents of [5-(4-aminophenyl)-2-thienyl]-substituted CDCOP 1 and (4-aminophenyl)-substituted CDCOP 2 w

Full text of DOI:10.1080/15421406.2019.1648039

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Substituent engineering of electrooptic
chromophores to suppress their aggregation
Kazuki Kashiwabara, Shunsuke Inada, Takamasa Noguchi, Yukichi Sato,
Kohei Kikuchi, Masato Imai, Ryohei Yamakado & Shuji Okada
To cite this article: Kazuki Kashiwabara, Shunsuke Inada, Takamasa Noguchi, Yukichi Sato,
Kohei Kikuchi, Masato Imai, Ryohei Yamakado & Shuji Okada (2019) Substituent engineering of
electrooptic chromophores to suppress their aggregation, Molecular Crystals and Liquid Crystals,
686:1, 70-77, DOI: 10.1080/15421406.2019.1648039
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Published online: 10 Oct 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2019, VOL. 686, NO. 1, 70–77
https://doi.org/10.1080/15421406.2019.1648039
Substituent engineering of electrooptic chromophores to
suppress their aggregation
Kazuki Kashiwabaraa, Shunsuke Inadaa, Takamasa Noguchia, Yukichi Satoa, Kohei
Kikuchia, Masato Imaia, Ryohei Yamakadob, and Shuji Okadaa,b
aGraduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510,
b
Japan; Graduate School of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa
992-8510, Japan
KEYWORDS
ABSTRACT
EO chromophore; polymer
dispersion; poled
polymer; crosslink
Chromophores composed of 4-cyano-5-dicyanomethylene-2-oxo-3-
pyrroline (CDCOP) and a donor have been synthsized for electrooptic
applications. Three substituents of [5-(4-aminophenyl)-2-thienyl]-sub-
stituted CDCOP 1 and (4-aminophenyl)-substituted CDCOP 2 were
modified to suppress the aggregation for effective poling. To 1,
bulky and branched 2-ethylhexyl groups were introduced instead of
linear alkyl groups. Less aggregation was confirmed in the polymer
dispersion films, and the corona poling behaviors were examined. To
2, we introduced norbornyl epoxide groups as bulky and thermally
crosslinkable moieties. The aggregation was suppressed as well, and
the thermal crosslinking of the epoxide with acid anhydride in the
presence of a base was investigated.
1. Introduction
The first-order electrooptic (EO) effect, which is classified in a second-order nonlinear
optical effect, causes refractive index changes of matrials by applying electric field. By
using this effect, the devices for optical switching triggered by the electrical modulation
have been fabricated. For ultrafast and high-capacity information optical processing in
the next generation, EO materials should have low dielectric constants and inorganic
materials do not satisfy this point. Meanwhile, organic materials with low dielectric con-
stants can follow high-frequency modulation. In addition, polymers with EO chromo-
phores [1,2] have adventages on processabllity into optical waveguides. For the EO
chromophores, thr large first hyperpolarizability (b) as well as the large dipole moment
(l) to align the chromophores in a polar manner by the poling process are required,
and various chlomophores have been developed. Since 4-cyano-5-dicyanomethylene-2-
oxo-3-pyrroline (CDCOP) is a strong acceptor, its chromophores combined with donors
are also good candidates for EO applications [3–10]. On the other hand, their efficient
poling is still an unresolved issue. One of the problems is aggregation of the chromo-
phores in the matrix polymers. In the present study, we report substituent engineering
CONTACT Shuji Okada
okadas@yz.yamagata-u.ac.jp
Graduate School of Organic Materials Science, Yamagata
University, 4-3-16 Jonan, Yonezawa 992-8510, Japan.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
ß 2019 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
71
Figure 1. Synthesis scheme of 1 and structures of 2.
of the CDCOP chromophores for less aggregation. Subsituents of the EO chromophores
were converted from simple linear alkyl chains to branched substituents (1a in Figure
1) or bulky crosslinkable substituents (2c in Figure 1) to suppress aggregation without
loosing compatibility with the matrix materials. Their aggregation behaviors were
mainly elucidated from the optical properties.
2. Experimental
Synthesis scheme of 1a and 1b is shown in Figure 1. Synthesis procedure of 1a from 4-
iodoaniline 3 is described below.
N,N-Bis(2-ethylhexyl)-4-iodoaniline 4a. To a mixture of 3 (3.29 g, 15 mmol) and
potassium carbonate (6.22 g, 45 mmol) in DMF (30 mL), 2-ethyl-1-iodohexane (10.8 g,
45 mmol) was added, and the mixture was stirred at 80 ˚C for 24 h. Then, water was
added to the mixture and it was extracted with ethyl acetate-hexane mixture. The com-
bined organic layer was dried over anhydrous sodium sulfate and filtered. Solvent in the
filtrate was removed in vacuo, and the residue was purified by column chromatography
(silica gel/hexane) to give 1.27 g (19%) of 4a as a yellow oil. 1H-NMR (400 MHz,
CDCl3, d) 7.40 (2H, d, J ¼ 8.1 Hz), 6.42 (2H, d, J ¼ 8.1 Hz), 3.00–3.24 (4H, m),
1.68–1.80 (2H, m), 1.17–1.40 (16H, m), 0.80–0.95 (12H, m);13C-NMR (100 MHz,
CDCl3, d) 147.68, 147.66, 137.41, 114.94, 75.45, 56.24, 36.51, 36.50, 30.60, 30.58, 28.66,
28.64, 23.86, 23.16, 14.07, 10.67, 10.65. Underlined numbers are sets of 13C peaks at the
same position in the diastereomers. 2.38 g (48%) of N-(2-ethylhexyl)-4-iodoaniline was
1
also obtained as a yellow oil. H-NMR (400 MHz, CDCl3, d) 7.39 (2H, d, J ¼ 8.7 Hz),
6.37 (2H, d, J ¼ 8.7 Hz), 3.51 (1H, broad s) 2.96 (2H, d, J ¼ 6.4 Hz), 1.53 (1H, m),
1.23–1.43 (8H, m), 0.90 (6H, t, J ¼ 7.6 Hz); 13C-NMR (100 MHz, CDCl3, d) 148.16,
137.67, 114.78, 77.12, 46.82, 38.89, 31.21, 28.91, 24.39, 23.05, 14.07, 10.87.
N,N-Bis(2-ethylhexyl)-4-(2-thienyl)aniline 5a. To 0.37 g (0.32 mmol) of tetrakis(triphe-
nylphosphine)palladium(0) under a nitrogen atmosphere, 1.40 g (3.2 mmol) of 4a in
toluene (15 mL), 0.53 g (4.1 mmol) of 2-thiopheneboronic acid in ethanol (10 mL) and
1.31 g (9.5 mmol) of potassium carbonate in water (3 mL) were added, and the mixture
was stirred at 90 ˚C for 5 h. After solvent evaporation of the mixture in vacuo, brine

72
K. KASHIWABARA ET AL.
(200 mL) was added to the residue and the mixture was extracted with chloroform. The
combined organic layer was dried over anhydrous magnesium sulfate and filtered.
Solvent in the filtrate was removed in vacuo, and the residue was purified by column
chromatography (silica gel, hexane-chloroform (8:2)) to give 0.82 g (65%) of 5a as a yel-
1
low oil. H-NMR (400 MHz, CDCl3, d) 7.44 (2H, d, J ¼ 7.4 Hz), 7.08–7.14 (2H, m), 7.01
(1H, m), 6.64 (2H, d, J ¼ 7.4 Hz), 3.15–3.32 (4H, m), 1.80 (2H, m), 1.19–1.44 (16H, m),
0.82–0.96 (12H, m); 13C-NMR (100 MHz, CDCl3, d) 147.69, 147.67, 145.49, 127.74,
126.80, 122.28, 121.43, 120.38, 112.57, 56.35, 36.75, 36.73, 30.65, 30.63, 28.69, 28.67,
23.89, 23.20, 14.10, 10.69, 10.67. Underlined numbers are sets of 13C peaks at the same
position in the diastereomers.
4-Cyano-5-dicyanomethylene-1-(2-ethylhexyl)-3-(5-(4-(bis(2-ethylhexyl)amino)phenyl)-
2-thienyl)-2-oxo-3-pyrroline 1a. To 4-cyano-5-dicyanomethylene-3-hydroxy-2-oxo-3-
pyrroline disodium salt [11] (506 mg, 2.2 mmol) in DMF (5 mL), 581 mg (2.42 mmol) of
2-ethyl-1-iodohexane was added and the mixture was stirred at 60 ˚C overnight. Then,
it was cooled by an ice bath and 799 mg (2.0 mmol) of 5a was added. Next, 0.5 mL
(5.4 mmol) of phosphoryl chloride was added dropwise, and the mixture was further
stirred at 0 ˚C for 1 h and at ambient temperature for 4 h. Water was added to the reac-
tion mixture and the resulting precipitates were filtered. Filtered crude material was
purified by column chromatography (silica gel/chloroform) to give 1a as a dark purple
1
solid. H-NMR (400 MHz, CDCl3, d) 8.56 (1H, d, J ¼ 4.7 Hz), 7.66 (2H, d, J ¼ 9.1 Hz),
7.44 (1H, d, J ¼ 4.7 Hz), 6.72 (2H, d, J ¼ 9.1 Hz), 3.93 (2H, d, J ¼ 7.7 Hz), 3.24–3.44 (4H,
m), 1.79–1.92 (3H, m), 1.17–1.44 (24H, m), 0.84–0.97 (18H, m); 13C-NMR (100 MHz,
CDCl3, d) 166.43, 155.30, 151.13, 140.08, 136.33, 128.74, 124.52, 119.34, 113.18, 113.07,
112.93, 111.56, 92.83, 60.50, 56.36, 45.81, 38.42, 37.27, 30.62, 30.59, 29.16, 28.62, 28.60,
27.87, 23.90, 23.06, 22.94, 22.57, 22.55, 13.97, 13.91, 10.62, 10.60, 9.95. Underlined num-
bers are sets of 13C peaks at the same position in the diastereomers.
Compound 1b with linear alkyl groups was synthsized via the procedure simiar to 1a.
1
1b: Mp. 113 ˚C; H-NMR (400 MHz, CDCl3, d) 8.58 (1H, d, J ¼ 4.6 Hz), 7.65 (2H, d,
J ¼ 9.2 Hz), 7.43 (1H, d, J ¼ 4.6 Hz), 6.67 (2H, d, J ¼ 9.2 Hz), 4.03 (2H, t, J ¼ 7.8 Hz),
3.38 (4H, t, J ¼ 7.8 Hz), 1.56–1.72 (6H, m), 1.28–1.46 (14H, m), 0.98 (3H, t, J ¼ 7.3 Hz),
0.92 (6H, t, J ¼ 6.4 Hz); 13C-NMR (100 MHz, CDCl3, d) 166.08, 154.85, 150.85, 140.12,
136.04, 128.85, 124.44, 119.01, 113.22, 112.96, 112.09, 111.59, 92.40, 59.78, 51.30, 41.71,
31.57, 31.47, 27.32, 26.70, 22.61, 19.48, 14.01, 13.59.
Compound 2c with epoxy groups in Figure 1 was synthesized as follows. At first, via
the similar procedure to prepare the derivatives of 2d with butyl groups [3,7–9], 2e
with 2-(acryloyloxy)ethyl groups as R and R’ was synthesized. After the Diels-Alder
reaction between 2e and cyclopentadiene, 2f whose substituents were terminated by a
norbornene moiety was obtained. Finaly, C ¼ C bonds of the norbornene moieties of 2f
1
were epoxydized by m-chloroperbenzoic acid. 2c: H-NMR (400 MHz, CDCl3, d) 8.46
(2H, d, J ¼ 9.2 Hz), 6.93–7.02 (2H, m), 4.28–4.48 (8H, m), 3.79–3.92 (4H, m), 3.06–3.18
(6H, m), 2.64–2.84 (5H, m), 2.47–2.55 (3H, m), 2.34 (1H, m), 1.75–1.88 (3H, m),
1.55–1.63 (3H, m), 1.36–1.44 (2H, m), 1.25 (1H, m), 0.78–0.94 (3H, m).13C-NMR
(100 MHz, CDCl3, d) 174.27, 174.09, 173.33, 173.22, 166.68, 155.01, 153.01, 152.98,
142.18, 133.29, 133.27, 116.21 (two carbons), 112.73, 110.87, 97.58, 97.54, 61.60, 61.04,
60.84, 51.24, 51.18, 50.52, 50.49, 50.44, 49.28, 48.65, 48.56, 44.33, 41.97, 41.94, 41.22,

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
73
41.10, 40.51, 39.79, 39.66, 36.63, 36.61, 36.35, 29.73, 29.62, 27.91, 27.88, 26.98, 26.94,
23.92, 23.88. Underlined numbers are sets of 13C peaks at the same position in the dia-
stereomers or in substituents R and R’.
Spin-coated films of the chromophores dispersed in polymers were prepared as fol-
lows. In the spectral measurements depending on concentration of 1a and 1b, 5 mg of
the mixture of the chromophore and PCZ-300 (poly(oxycarbonyloxy-1,4-phenylenecy-
clohexane-1,1-diyl-1,4-phenylene), Mitsubishi Gas Chemical) in a given weight concen-
tration was dissolved in 0.5 mL of chloroform. In the poling experiments, 1a (1 mg) and
PCZ-300 (5 mg) were dissolved in 1 mL of chloroform. For 2c and 2d, 1 mg of the
chromophore and 10 mg of PMMA (poly(methyl methacrylate), Mw ¼ 1.2 ꢀ 105,
Aldrich) were dissolved in 1 mL of chloroform. Glass slides, on which the solution was
dropped, were spun at 500 rpm for 10 s and then at 1000 rpm for 30 s. For the corona
poling, the spin-coated film on the glass slide was put on a grounded copper plate,
which was placed on a temperature-controllable heater. A positive voltage to the copper
plate was applied from a tungsten needle setting 1 cm above the film.
3. Results and discussion
The l and b values were calculated by using PM5 (MOPAC 2002 Ver. 2.5.0) for the
structure optimized by using D-VWN DFT (DGauss Ver. 4.1). The l and b of 1 with
methyl groups as R and R’ were obtained to be 12.6 D and 341 ꢀ 10ꢁ30 esu, respect-
ively, while those of 2 with methyl groups as R and R’ were 10.6 D and 127 ꢀ 10ꢁ30
esu, respectively. Namely, lb of 1 is aproxymately 3.2 times as large as that of 2. Thus,
the effect on alkyl chain types was examined on 1 because the molecular length of 1 is
longer than that of 2 and aggregation formation of 1 seemed to be easier than that of 2.
When 10 wt% of 1b with linear alkyl groups was dispersed in the polymer, absorption
maximum wavelength (kmax) was observed at 729 nm (Figure 2 (b)). However, in the
40 wt% sample, the blue-shifted sharp band appeared at 625 nm due to formation of the
H-type aggregates. On the other hand, 1a did not show such a prominent peak due to
H-type aggregation although blue shift of the kmax was observed, i.e., the kmax at
739 nm of the 10 wt% sample gradually moved to 680 nm in the 40 wt% sample (Figure
2 (a)). Bulkiness of the branched alkyl groups seemed to suppress the aggregation prop-
erty of the chromophore. In addition, since we used racemic 2-ethyl-1-iodohexane as a
reagent, 1a is a mixture of three pairs of enantiomers to reduce crystallinity. In fact, the
melting point range of 1a is broad approximately between 100 and 117 ˚C and this is
considered to be another factor for less aggregation. When the films in Figure 2 (a)
were heated at 150 ˚C for 30 min, disaggregation was promoted and chromophore
orientation induced during the spin-coating process was randomized to some extent,
resulting in absorption maximum shifts to longer wavelengths, i.e., the kmax of the
10 wt% and 40 wt% samples were observed at 757 nm and 715 nm, respectively (Figure 2
(c)). The spin-coated films of the 40 wt% samples of 1a, 1b and 1a after heating at 150
˚C for 30 min were observed by AFM (Figure 3). When the samples of 1a and 1b were
compared, the average surface roughnesses (Ras) were 0.63 nm and 0.93 nm, respectively,
and the maximum hight differences (Rzs) were 5.76 nm and 8.88 nm, respectively.
Smaller values for 1a seemed to be related to better misibility of 1a to the polymer. For

74
K. KASHIWABARA ET AL.
Figure 2. UV-visible-NIR absorption spectra of 1: (a) Absorption changes depending on content of 1a
in PCZ-300; (b) absorption changes depending on content of 1b in PCZ-300; (c) absorption of 1a in
PCZ-300 after heating at 150 ˚C for 30 min; (d) absorption changes of 1a in PCZ-300 before and after
corona poling. The preheated film was poled applying 4 kV at 140 ˚C for 5 min, and then postheated
at 140 ˚C for 10 min to relax the orientation.
Figure 3. AFM topographic images of spin-coated films of 1: (a) 1a (40 wt%) in PCZ-300, (b) 1b
(40 wt%) in PCZ-300 and (c) 1a (40 wt%) in PCZ-300 after heating at 150 ˚C for 30 min. The vertical
and horizontal lines in the lower left corners in each image indicate 1 lm length.
the heated sample of 1a, fine heterogeneous structures became ambiguous. On the other
hand, large undulation, which was represented by the Ra of 1.74 nm and Rz of 25.8 nm,
was observed although the mechanism for such thermal material transport is not clear.
Corona poling was performed for the films of 1a in PCZ-300, whose grass transition
temperature was observed at approxymately 140 ˚C. For example, Figure 2 (d) shows
absorption spectral changes of 1a in PCZ-300 before and after corona poling applying
4 kV at 140 ˚C for 5 min. In this condition, the oriented chromophores were randa-
mized after postheating at 140 ˚C for 10 min to recover the absorption to the preheating
stage, indicating almost no damage of the chromophore. However, when the applied

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
75
Figure 4. Absorption spectra of 2: UV-visible absorption spectra of (a) 2c and (b) 2d in chloroform
solution (dashed lines) and the absorption changes depending on content of the chromophores in
PMMA; (c) IR absorption spectra of 2c, MHHPA and the mixture of 2c, MHHPA and DMBA (100:150:1)
before and after curing.
voltage increased resulting in the poling current of more than 5 lA and/or the poling
time increased, the chromophore degradation became prominent.
In addition to the aggregation problem, orientational relaxation of poled structures is
another issue and crosslink of the chromophores is one of the solutions. In our previous
studies, it was found that CDCOP chromophores were degraded especially by radical
polymerization reactions stimulated by a photoinitiator [7], and thermal crosslinking
process seemed to be better [8]. Epoxy group is a typical thermal crosslinkable moiety
and it has been also applied to fix polar orientation of the nonlinear optical chromo-
phores [12]. Thus, we synthesized 2c with epoxide groups incorporated in the bulky nor-
bornyl moiety to avoid aggregation of the chromophores. Due to the synthesis
procedure of 2c, the norbornyl substituents were confirmed to contain endo and exo
stereoisomers, which were identified by the 13C-NMR spectra [13]. Consequently, 2c is a
mixture of twenty pairs of enantiomers. This also affects lowering the crystallinity and
the mixture has the broad melting point range approximately between 100 and 118 ˚C.
Figure 4 (a) shows UV-visible absorption spectra of 2c in chloroform and the absorption
changes depending on content of 2c in PMMA. The kmax of the PMMA films was
observed at 608 nm, which was not affected by the chromophore concentration. The
kmax of the chloroform solution was almost the same at 612 nm. On the other hand, the
kmax of 2d with linear alkyl substituents showed a hypsochromic shift from 658 nm in
chloroform to 616 nm in the 60 wt% PMMA film (Figure 4 (b)). This spetral difference is
again considered to be originated from the aggregation property, i.e., 2d is apt to form
the aggregates compared with 2c.

76
K. KASHIWABARA ET AL.
The crosslink reaction of 2c was first examined by using boron trifluoride diethyl
etherate as a Lewis acid. However, cleavage of ester groups preferentially occurred, and
production of the corresponding alcohol derivative of 2c and derivatives of 3-oxatricy-
clo[3.2.1.02,4]octane-6-carboxylic acid including 2-hydroxy-4-oxatricyclo[4.2.1.03,7]-
nonan-5-one were confirmed by the NMR spectra. Thus, the crosslinking was
performed in a basic condition using 4-methylhexahydrophthalic anhydride (MHHPA)
as a hardener and N,N-dimethylbenzylamine (DMBA) as an accelerator. Chromophore
2c, MHHPA and DMBA were mixed in 100:150:1 molar ratio and the mixture was
cured first at 120 ˚C for 2 h and then at 150 ˚C for 48 h. The IR spectra of the mixture
before and after curing are shown in Figure 4 (c) together with those of 2c and
MHHPA for comparison. In the spectrum of the mixture before curing, the peaks a at
1859 cmꢁ1 and b at 1788 cmꢁ1 can be assigned to symmetric and asymmetric C ¼ O
stretching of MHHPA, respectively. The peak c at 901 cmꢁ1 is for C-O stretching of the
acid anhydride framework of MHHPA. The peak d at 850 cmꢁ1 is assignable to asym-
metric ring deformation of the epoxide groups of 2c. After curing, these peaks were
apparently weakened indicating that the crosslink reaction between the epoxide groups
of 2c and MHHPA progressed. It was also confirmed by the fact that no chloroform-
soluble portion was detected from the cured mixture. Since other characteristic IR peaks
remained even after curing, the above crosslinking process seemed to be appropriated
for orientation fixing of this chromophore.
4. Conclusion
In this study, the substituents of the CDCOP chromophores were modified to suppress
the aggregation. For the thiophene-inserted CDCOP chromophores, introduction of 2-
ethylhexyl groups was effective to avoid aggregation compared with the case of linear-
alkyl substitution especially in the films with high chromophore content. In the experi-
ments on orientation of the chromophore by corona poling and its relaxation by post-
heating, clear decrease of absorbance and its almost quantitative recovery was observed,
and there was no inconvenience due to bulkyness of the branched alkyl groups and ser-
ious damages of the chromophore. For the CDCOP chromophores directly bonded to
the aniline moiety, norbornyl epoxide groups were introduced to satisfy both bulkiness
to suppress aggregation and crosslink reactivity. The epoxide groups were confirmed to
crosslink thermally with MHHPA in the presence of DMBA without serious damages of
the chromophore. Effective poling process during crosslinking is in progress, and the
thiophene-inserted CDCOP chromophore with norbornyl epoxide groups will be exam-
ined in the future study.
Acknowledgements
The authors thank Mr. Kota Kuwamoto for his experimental support. This study was partly sup-
ported by the program of JST.
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