Substituent modification of electro-optic chromophores with 4-cyano-5-dicyanomethylene-2,5-dihydro-1H-pyrrol-2-one as an acceptor

Article abstract of DOI:10.1080/15421406.2016.1200935

Electro-optic chromophores with 4-cyano-5-dicyanomethylene-2,5-dihydro-1H-pyrrol-2-one (CDCOP) and aniline moieties as an acceptor and a donor, respectively, were synthesized with modification of three substituents, among which two were attached to the aniline moiety (R) and one was introduced to the CDCOP ring (R′). Butyl and hexyl groups were introduced as R and R′, and higher melting points were observed when R and R′ were the same. For the chromophore fixation, thermal crosslinking based on cyanate trimerization was performed. The chromophore with cyanate moiety in R′ was combined with bisphenol A dicyanate, and the thermal chromophore fixation was confirmed with almost no chromophore degradation.

Full text of DOI:10.1080/15421406.2016.1200935

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Substituent modification of electro-
optic chromophores with 4-cyano-5-
dicyanomethylene-2,5-dihydro-1H-pyrrol-2-one as
an acceptor
Shunsuke Inada, Tatsunori Araki, Yukichi Sato, Yoko Tatewaki & Shuji Okada
To cite this article: Shunsuke Inada, Tatsunori Araki, Yukichi Sato, Yoko Tatewaki & Shuji
Okada (2016) Substituent modification of electro-optic chromophores with 4-cyano-5-
dicyanomethylene-2,5-dihydro-1H-pyrrol-2-one as an acceptor, Molecular Crystals and Liquid
Crystals, 636:1, 24-29, DOI: 10.1080/15421406.2016.1200935
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1200935
Published online: 01 Nov 2016.
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Date: 27 November 2016, At: 21:50

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Substituent modification of electro-optic chromophores with
-cyano--dicyanomethylene-,-dihydro-H-pyrrol--one
as an acceptor
Shunsuke Inadaa, Tatsunori Arakia, Yukichi Satoa, Yoko Tatewakib, and Shuji Okadaa
aGraduate School of Science and Engineering, Yamagata University, Yonezawa, Japan; bDivision of Applied
Chemistry, Tokyo University of Agriculture and Technology, Koganei, Japan
ABSTRACT
KEYWORDS
EO chromophore; polymer
dispersion; cyanate;
polycyanurate; poled
polymer
Electro-optic chromophores with 4-cyano-5-dicyanomethylene-2,5-
dihydro-1H-pyrrol-2-one (CDCOP) and aniline moieties as an acceptor
and a donor, respectively, were synthesized with modification of three
substituents, among which two were attached to the aniline moiety (R)
and one was introduced to the CDCOP ring (Rꢀ). Butyl and hexyl groups
were introduced as R and Rꢀ, and higher melting points were observed
when R and Rꢀ were the same. For the chromophore fixation, thermal
crosslinking based on cyanate trimerization was performed. The
chromophore with cyanate moiety in Rꢀ was combined with bisphenol
A dicyanate, and the thermal chromophore fixation was confirmed with
almost no chromophore degradation.
1. Introduction
Electro-optic (EO) devices are expected to be used for ultrafast optical information pro-
cessing [1]. One of the promising materials for the EO devices is polymers with chro-
mophores composed of a π-conjugation system terminated with electron accepting and
donating groups. 4-Cyano-5-dicyanomethylene-2,5-dihydro-1H-pyrrol-2-one (or 4-cyano-
5-dicyanomethylene-2-oxo-3-pyrroline (CDCOP)) is a strong acceptor, and CDCOP deriva-
tives combined with a donor have been known as a series of dyes more than fifty years ago
[2, 3]. Recently, they are received as useful EO chromophores [4–6]. In order to obtain high-
performance EO polymers, the chromophores should be molecularly dispersed in the matrix
polymer for efficient electrical poling although chromophores often form aggregates in the
polymer [7]. Also, the polar structure generated by electrical poling should be fixed to avoid
temporal relaxation.
In this study, CDCOP derivatives with an aniline moiety were synthesized with modifica-
tion in the alkyl substituents and these properties were investigated. A cyanate group, which
is thermally trimerized to form cyanurate (Figure 1) [8, 9], was also introduced as one of the
substituents to fix the chromophores in the polymer matrix of polycyanurate (PCN).
CONTACT Shuji Okada
okadas@yz.yamagata-u.ac.jp
Science and Engineering, Yamagata University, -- Jonan, Yonezawa -, Japan.
Department of Organic Device Engineering, Graduate School of
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[215]/25
Figure . Chemical structures and synthesis schemes of CDCOP derivatives and polycyanurate (PCN). Hx and
Ar indicate hexyl and aryl groups, respectively.
2. Experimental
Chromophores 1 and 2 with a CDCOP moiety were synthesized according to the reaction
scheme in Figure 1. N,N-Dialkylaniline derivatives 4 were prepared by heating the mixture
containing aniline 3, alkyl iodide and potassium carbonate in DMF. Ester 6 was obtained
by esterification of 4-hydroxybenzoic acid 5 in ethylene chlorohydrin with sulfuric acid as a
catalyst. Halogen exchange of 6 from chlorine to iodine was performed by sodium iodide in
acetone to give 7. A one-pot reaction including N-alkylation of 8 and coupling between the
resulting N-alkyl compound and 4 gave CDCOP derivatives 1a and 2a. For 1b–1d, coupling
reaction between aniline and CDCOP acceptor 8 and N-alkylation were performed stepwise.
The chemical structures of the products were mainly identified by 1H- and 13C-NMR spectra.
Synthesis procedures of 2a and 2b are described below.
To 0.23 g (1 mmol) of 8 [10] in DMF (2 mL), 0.32 g (1.1 mmol) of 7 was added and the
mixture was stirred at 60 °C for 2 days. The mixture was cooled by a salt-ice bath and 0.23 g
(1.1 mmol) of N,N-dibutylaniline was added. Next, 0.28 mL (3 mmol) of phosphoryl chloride
was added dropwise, and the mixture was stirred for 1 h after addition completion. The bath
was removed and the mixture was further stirred for 4 h at ambient temperature. Then, water
(600 mL) was poured into the mixture and stirring was continued overnight. The mixture
was filtered and the filtered solid was purified using a column chromatography (silica gel,
chloroform-ethyl acetate (9:1)) to give 0.25 g (46%) of 2a as a violet solid: Mp 156 °C; 1H-
NMR (400 MHz, CDCl3, δ) 0.99 (6H, t, J = 7.3 Hz), 1.40 (4H, tq, J = 7.3, 7.3 Hz), 1.65 (4H, tt,
J = 8.0, 7.3 Hz), 3.45 (4H, t, J = 8.0 Hz), 4.55 (4H, s), 6.10 (1H, brs), 6.73 (2H, d, J = 9.4 Hz),
6.85 (2H, d, J = 8.7 Hz), 7.88 (2H, d, J = 8.7 Hz), 8.47 (2H, d, J = 9.4 Hz); 13C-NMR (100 MHz,
CDCl3, δ) 13.81, 20.19, 29.63, 40.64, 51.38, 59.37, 61.77, 93.69, 110.71, 113.00, 113.43, 113.92,
115.43, 115.95, 121.54, 132.04, 133.89, 141.55, 153.38, 156.00, 160.41, 165.95, 167.38; IR (KBr)
3400, 2958, 2931, 2871, 2219, 1725, 1606, 1550, 1268, 1230, 1205, 1164, 1110, 1093 cm−1.
To 0.35 g (0.65 mmol) of 2a in anhydrous acetone (4.0 mL) cooled by a salt-ice bath, 0.16 g
(1.5 mmol) of cyanogen bromide in anhydrous acetone (1 mL) was added at once. Triethy-
lamine (1 mL) was added dropwise, and the mixture was stirred for 2 h at ambient temper-
ature. The precipitates were collected by filtration and purified by column chromatography
(silica gel, chloroform-ethyl acetate (9:1)) to give 0.13 g (36%) of 2b: Mp 158 °C; 1H-NMR

26/[216]
S. INADA ET AL.
(400 MHz, CDCl3, δ) 1.00 (6H, t, J = 7.2 Hz), 1.41 (4H, tq, J = 7.5, 7.2 Hz), 1.66 (4H, tt,
J = 7.7, 7.5 Hz), 3.46 (4H, t, J = 7.7 Hz), 4.57 (2H, t, J = 4.4 Hz), 4.63 (2H, t, J = 4.4 Hz),
6.74 (2H, d, J = 9.5 Hz), 7.36 (2H, d, J = 9.1 Hz), 8.11 (2H, d, J = 9.1 Hz), 8.49 (2H, d, J =
9.5 Hz); 13C-NMR (100 MHz, CDCl3, δ) 13.78, 20.16, 29.60, 40.45, 51.39, 59.43, 62.66, 93.36,
107.75, 111.35, 113.07, 113.47, 113.82, 115.44, 115.89, 128.20, 132.40, 133.80, 141.14, 153.41,
155.69, 155.80, 164.42, 167.44; IR (KBr) 3442, 2960, 2933, 2283, 2258, 2237, 2219, 1731, 1606,
1552, 1504, 1396, 1363, 1205, 1095 cm−1; Found: C, 63.60%; H, 5.86%; N, 11.63%. Calcd for
C32H30N6O4: C, 63.88%; H, 5.86%; N, 11.64%.
Poly(methyl methacrylate) (PMMA, Mw = 1.2 × 105, Aldrich) and poly(oxycarbonyloxy-
1,4-phenylenecyclohexane-1,1-diyl-1,4-phenylene) (PCZ-300, Mitsubishi Gas Chemical),
which is an amorphous polycarbonate, were used as polymer dispersion media of chro-
mophores 1a–1d. Poly(acrylic acid) (PAA, Mw = 1.0 × 105, Aldrich) was used to form a
passivation layer of the thin films.
Thin films of PCN were obtained as follows. At first, a soluble prepolymer (PP) of PCN
was prepared. The mixture of dicyanate 9 (100 mg) and zinc acetylacetonate (ZAA, 0.25 mg)
was heated in ethanol at 80 °C for 5 min and the following solvent evaporation gave PP. Then,
1a or 2b was mixed to PP in the ratio of 0.1 mmol of the chromophore to 1 g of PP, and ZAA
(0.25 wt% to PP) was added. The mixture was dissolved in THF and spin-coated on glass
slides. The films were heated at 120 °C for 30 min.
For corona poling, a grounded copper plate was placed on a temperature-controllable
heater, and a spin-coated film on a glass slide was put on the plate. A tungsten needle was
set 1 cm above the film and a positive voltage was applied.
3. Results and discussion
The melting points of 1a, 1b, 1c and 1d were found to be 129 °C, 93 °C, 91 °C and 118 °C,
respectively. It is interesting that the chromophores with the same alkyl group as R and Rꢀ
have the melting points higher than 118 °C while those with different alkyl groups have lower
melting point of around 90 °C. In general, high melting point suggests large intermolecu-
lar interaction of the component molecules. In the chromophore dispersion system in poly-
mers, anti-parallel molecular aggregation often becomes a crucial issue to reduce the poling
efficiency. Thus, introduction of different alkyl groups to chromophores may diminish inter-
molecular interaction resulting in one of the good solutions to avoid aggregate formation.
In order to investigate the intermolecular interaction of these chromophores, single crystal
growth and the crystallographic analyses are in progress.
Figure 2 shows UV-visible spectra of 1a. Figure 2 (a) is comparison of the spectra in chlo-
roform, PMMA and PCZ-300. The concentration of 1a was 5 wt% for both polymers. Absorp-
tion of these spectra is normalized by absorbance at each maximum. Absorption maximum
wavelengths (λmaxs) of 1a in chloroform, PCZ-300 and PMMA were 658 nm, 652 nm and
637 nm, respectively. Although the λmaxs are different among them, this fact seems to be
caused by difference in the surrounding media of the chromophores. Meanwhile, widths of the
absorption bands are not so different between chloroform solution and polymer mixtures. The
λmaxs of the CDCOP derivatives 1b–1d with different alkyl groups in PMMA with 5 wt% con-
centration were almost the same as those of 1a. These results indicate that the chromophores
are nearly dispersed in the molecular state in these polymers.
Figure 2 (b) shows UV-visible absorption spectra of 1a in PMMA (10 wt%). For the
as-prepared spin-coated film, the λmax is slightly shifted to shorter wavelength by 4 nm
compared with that of 5 wt% dispersion, but the chromophore aggregation seemed to be

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[217]/27
Figure . UV-visible spectra of 1a. The spectra in Figure (a) were measured in (i) chloroform solution, (ii)
PCZ- and (iii) PMMA and were normalized by absorbance at λmax. Chromophore concentration of 1a in
PCZ- and PMMA was  wt%. Figure (b) is the spectra of 1a in PMMA ( wt%) for corona poling: (iv) just
after spin-coating, (v) after corona poling (see text in detail) and (vi) after post-heating.
not so prominent from the absorption shift. After corona poling applying 6 kV/cm at 110
°C for 10 min, the absorbance of the absorption band became small (Figure 2 (iv) → (v)).
Since absorption increase by post-heating at 110 °C for 10 min was observed (Figure 2
(v) → (vi)), the chromophores in the polymer should be oriented in a polar manner after
poling. However, the absorbance did not completely recovered after post-heating indicating
that a part of chromophores were degraded during poling process. This damage could be
diminished by covering the film by a passivation layer described later.
For 2b with cyanate group in Rꢀ, the thermal behavior was first investigated to know the
crosslinking temperature. The solution of 2b, PP and ZAA for spin-coating was put on an
aluminum pan for DSC measurement. As shown in Figure 3 (a), a broad exothermic peak
originated from the crosslinking reaction of cyanate trimerization was observed starting from
approximately 60 °C. In the measuring condition, the peak was recorded at around 140 °C
Figure . DSC thermogram (a) and FT-IR spectra (b) of the mixture of 2b, PP and ZAA. In Figure (b), spectrum
(i) is for before heating and spectrum (ii) is for after heating at  °C for  min.

28/[218]
S. INADA ET AL.
Figure . UV-visible spectra of the spin-coated films composed of 2b, PP and ZAA. Figure (a) is for chro-
mophore extraction by acetone: (i) just after spin-coating, (ii) after heating at  °C for  min and (iii) after
the heat treatment followed by soaking in acetone for  min. Figure (b) is for corona poling: (iv) just after
spin-coating, (v) after corona poling (see text in detail) and (vi) after post-heating.
and the exothermic reaction ceased at around 200 °C. Thus, the crosslinking reaction of the
mixture was conducted at 120 °C, which was included in the exothermic peak range. In the FT-
IR spectra, the CࣕN stretching vibration peaks at 2237 and 2272 cm−1 decreased by heating
(Figure 3 (b)). The peak decreasing ratio was almost saturated after heating time of 20 min.
Figure 4 shows UV-visible absorption spectra of spin-coated films containing 2b, PP and
ZAA. The λmax of the film before heating was observed at 665 nm (Figure 4 (i)). After heating
at 120 °C for 20 min, the λmax was shifted to 656 nm (Figure 4 (ii)). It should be noted that the
absorbance of the chromophore was almost preserved even after the crosslinking reaction in
the present case. It is contrastive to acrylate-modified CDCOP derivatives, whose absorption
bands degraded during the crosslinking via radical polymerization [6]. When the crosslinked
film treated at 120 °C for 30 min was soaked in acetone for 3 min, 31% of the chromophore
remained (Figure 4 (iii)). Meanwhile, in the same condition using 1a instead of 2b, almost all
chromophores were extracted. This large difference is caused by chromophore crosslinking
when 2b was used.
Corona poling was performed for the spin-coated films containing 2b. When an electric
field of 6 kV/cm was applied at 30 °C, absorption degradation due to the sample damage was
observed. Thus, PAA dissolved in water (3.3 wt%) was spin-coated on the film as a passivation
layer. After drying under reduced pressure, the film did not show degradation by corona pol-
ing at 30 °C. Then, corona poling of the film applying the same voltage was conducted with
increasing temperature from 60 °C to 180 oC for 15 min followed by decreasing temperature
to 40 oC for 5 min. After poling, the absorption band of the chromophore reduced (Figure 4
(iv) → (v)) indicating chromophore orientation perpendicular to the substrate. Since second-
harmonic generation (SHG) of the film was observed, the polar structure was also confirmed.
However, post-heating at 100 °C for 5 min resulted in absorption recovery (Figure 4 (vi)).
From this result, we can conclude that the chromophore was not seriously damaged during
the poling process but the polar orientation was instable at high temperatures. The thermal
stability of orientation may be improved to some extent by optimizing curing process. How-
ever, 2b is connected only by one arm to a PCN network and the essential solution seems to
be more connection points of the chromophore, i.e., introduction of cyanate groups in R.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[219]/29
4. Conclusion
Two series of the CDCOP derivatives were synthesized. For the derivatives with alkyl groups,
large melting point difference was found between those of R = Rꢀ and those of R=Rꢀ, suggest-
ing different molecular interactions in the crystalline state. On the other hand, these deriva-
tives mixed with 5 wt% concentration in PMMA or PCZ-300 showed the absorption spectra
similar to those in solution indicating their molecularly dispersed states in the polymers. For
the derivative with a cyanate group in Rꢀ, fixation of the chromophore was achieved by heat-
ing with the dicyanate-based prepolymer. The heating process for crosslinking via cyanate
trimerization was found to be useful because of almost no degradation of the chromophore.
Since one cyanate group per chromophore was insufficient to maintain the chromophore ori-
entation, introduction of the cyanate group in both R and Rꢀ should be worth investigating.
Acknowledgments
This study was supported by the program for Strategic Promotion of Innovative Research and Devel-
opment, JST. We thank to Dr. Freddy S. Tan and Prof. Okihiro Sugihara for their experimental support
on the SHG measurement.
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