Push-pull chromophore with phenylene and thiophene as conjugation bridge for electro-optic applications

Article abstract of DOI:10.1080/15421400601021505

We designed and synthesized a push-pull type chromophore (PT-TCF) containing phenylene and thiophene ring as a conjugation bridge. Bulky alkyl chain was substituted into the phenylene bridge to minimize the intermolecular interaction among the neighbor mo

Full text of DOI:10.1080/15421400601021505

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Push-Pull Chromophore with
Phenylene and Thiophene as
Conjugation Bridge for Electro-
Optic Applications
Dong Wook Kim a , Jin Joo Choi a , Jong-Sun Lim a
Changjin Lee a
&
a Advanced Materials Division , Korea Research
Institute of Chemical Technology , Daejon, Korea
Published online: 12 Mar 2007.
To cite this article: Dong Wook Kim , Jin Joo Choi , Jong-Sun Lim & Changjin Lee
(2007) Push-Pull Chromophore with Phenylene and Thiophene as Conjugation Bridge
for Electro-Optic Applications, Molecular Crystals and Liquid Crystals, 463:1, 43/
[325]-53/[335], DOI: 10.1080/15421400601021505
To link to this article: http://dx.doi.org/10.1080/15421400601021505
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Mol. Cryst. Liq. Cryst., Vol. 463, pp. 43=[325]–53=[335], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400601021505
Push-Pull Chromophore with Phenylene and Thiophene
as Conjugation Bridge for Electro-Optic Applications
Dong Wook Kim
Jin Joo Choi
Jong-Sun Lim
Changjin Lee
Advanced Materials Division, Korea Research Institute of Chemical
Technology, Daejon, Korea
We designed and synthesized a push-pull type chromophore (PT-TCF) containing
phenylene and thiophene ring as a conjugation bridge. Bulky alkyl chain was sub-
stituted into the phenylene bridge to minimize the intermolecular interaction
among the neighbor molecules and to enhance solubility of the chromophore in sol-
vents and a polymer host. 1H NMR and 1H-1H COSY spectroscopy confirmed that
the chromophore was in all trans-ethenyl conformation. The chromophore exhib-
ited fairly high melting point of 188ꢀC. The chromophore was thermally stable
up to 220ꢀC in the nitrogen atmosphere. The relatively high melting point and
decomposition on-set may be attributable to the rigid bridge component. The chro-
mophore was blended with an optically transparent polymer host to make a thin
film and then its macroscopic optical nonlinearities were evaluated. In a prelimi-
nary result, the polymer films containing PT-TCF as a concentration of 50 wt%
exhibited electro-optic coefficient as large as 22 pm=V taken at 1.55 lm.
Keywords: bulky alkyl chain; electro-optic coefficient; nonlinear optical chromophore;
thermal stability; trans-ethenyl conformation
INTRODUCTION
Since the first nonlinear optical (NLO) polymer was reported in 1982, a
lot of researchers have concentrated their efforts on designing and pre-
paring highly efficient NLO organic and polymeric materials. The NLO
polymers inherit such superior properties as fast response time, small
This work was financially supported by the MOST for NT-IT Fusion Strategy of Adv.
Tech.
Address correspondence to Dong Wook Kim, Advanced Materials Division,
Korea Research Institute of Chemical Technology, Daejon, 305-600, Korea. E-mail:
dongwook@krict.re.kr
43=[325]

44=[326]
D. W. Kim et al.
dispersion of the refractive index from dc to optical frequencies, and pro-
cessability, when compared to the lithium niobate currently used in the
commercial electro-optic modulators [1–7]. A recent work successfully
demonstrated the electro-optic modulator based on a NLO polymer with
drive voltage below 1 V as well as bandwidth over 150 GHz [8].
Experimental data and molecular simulation results in the past
decades have suggested that NLO chromophores should have a
push-pull structure, in which an electron donor in one end is connec-
ted with an electron acceptor in the other end through a conjugation
bridge [1–2]. While aminobenzene is commonly adopted as a donor,
a variety of candidates have still competed for an acceptor. One of
the known efficient acceptors is 2-cyanomethylene-3-cyano-4,5,5-
trimethyl-2,5-dihydrofuran (TCF) and its derivatives. A recent paper
presented a highly efficient NLO chromophore, CLD which has amino-
benzene and TCF connected with a polyene bridge [9]. It has some
drawback in thermal and photo stability mainly because its polyene
structure is susceptible to a radical attack, although the chromophore
exhibits EO coefficient as large as 60 pm=V at 1.3 mm [10–11].
While there is certain trade-off between optical nonlinearity and
stability of the chromophores, high NLO activity without stability is
useless in the practical view. Since aromatic groups such as phenylene
and thiophene rings are reported to possess much better stability than
the polyene, we have chosen the aromatic rings as a bridge for our
chromophore PT-TCF as shown in Scheme 1. 2-Ethylhexyl group
was attached to the bridge so as to keep the chromophore molecule
from contacting closely its neighboring molecules and hence minimize
its intermolecular electrostatic interaction [9]. The alkyl chains may
also play a major role in suppressing formation of cis-isomer as well
as offering high solubility in solvents and polymer medium, which is
critical in fabricating quality films. The alkyl chains were designed
to connect to the phenylene ring through ether linkage because direct
link between the alkyl group and the ring results in benzylic carbon,
which is vulnerable to a radical attack. The resulting chromophore
was blended with an optically transparent polymer host and was fab-
ricated into thin films. Then its macroscopic electro-optic activity was
evaluated by a simple reflection method proposed by Teng [12].
EXPERIMENTAL
Materials
Methylhydroquinone, 2-ethylhexylbromide, triethylphosphite, 4-
(diethylamino)benzaldehyde, 2-methylthiophene, sodium hydride

Push-Pull Chromophore for Electro-Optic Applications
45=[327]
SCHEME 1 Synthesis of the NLO chromophore PT-TCF. Reaction conditions:
(i) NBS, BPO, CCl4, 20 h, reflux; (ii) triethyl phosphite, 16 h, 160ꢀC; (iii) acet-
onitrile, K2CO3, reflux, 3 days; (iv) (a) NBS, AIBN, CCl4, 2 h, reflux and (b)
NBS, THF, 1 h, reflux; (v) triethyl phosphite, 2 h, 160ꢀC; (vi) NaH, THF, 0ꢀC
for 15 min and reflux for 11 h; (vii) butyl lithium, DMF, THF, 3 h, rt.; (viii)
LDA, THF, 5 h, rt.; (ix) butyl lithium, DMF, THF, 16 h, rt.; (x) piperidine,
THF, overnight, rt.
(95%), n-butyllithium (2.5 M solution in hexane), lithium diisopropyl-
amide (LDA, 2.0 M solution in heptane=tetrahydrofuran=ethylbenzene)
and piperidine were purchased from Aldrich and used as received.

46=[328]
D. W. Kim et al.
N-bromosuccinimide (NBS), benzoyl peroxide (BPO) and 2,20-azobis(2-
methylpropionitrile) (AIBN) were obtained from Aldrich and used after
purification by recrystallization. Poly[bisphenol A carbonate-co-4,40-
(3,3,5-trimethylcyclohexylidene)diphenol] (APC) was purchased from
Aldrich and purified by reprecipitating the polymers in methanol several
times. All solvents used in this work were freshly dried by distillation. 2-
Cyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) was
prepared according to the literature [13].
Synthesis of NLO Chromophore
Synthesis of Diethyl(5-bromothiophen-2-yl)
Methylphosphonate (1)
Compound 1 was purified by flash column chromatography with
hexane=ethyl acetate (1:5 by volume) to afford compound 1 as a yellow
oil (yield: 24%): 1H NMR (CDCl3, ppm) d 6.91–6.88 (d, J ¼ 3.6 Hz, 1H),
6.74–6.72 (t, J ¼ 3.6 Hz, 1H), 4.14–4.04 (m, 4H), 3.31–3.24 (d,
J ¼ 20.7 Hz, 2H), 1.30 (t, 6H).
Synthesis of 1,4-bis-(2-ethyl-hexyloxy)-2-methyl-benzene (2)
Compound 2 was purified by a column chromatography (ethylacetate=
1
hexane ¼ 1=10). Colorless liquid was obtained in 66% yield: H-NMR
(CDCl3, ppm) d 6.72 (3H), 3.77 (4H, d), 2.21 (3H, s), 1.68–0.9 (30H, m).
Synthesis of 1-bromo-4-bromomethyl-2,5-bis-(2-ethyl-
hexyloxy)-benzene (3)
Compound 3 was purified by a column chromatography (methylene
1
chloride=hexane ¼ 1=5) to produce a colorless liquid in 51% yield: H-
NMR (CDCl3, ppm) d 7.06 (1H, s), 6.88 (1H, s), 4.49 (2H, s), 3.84 (4H,
d), 1.68–0.9 (30H, m).
Synthesis of Diethyl-[2,5-bis(2-ethyl-hexyloxy)-4-
bromophenyl] methylphosphonate (4)
Compound 4 was purified by column chromatography (ethylacetate=
hexane ¼ 1=3) to give a colorless liquid in 95% yield: 1H-NMR (CDCl3,
ppm) d 7.04 (1H, s), 6.98 (1H, s), 4.02 (4H, t) 3.84 (4H, d), 3.23 (2H, d)
1.72–0.93 (36H, m).
Synthesis of 4-[2,5-bis(2-ethyl-hexyloxy)-4-bromostyryl]-N,N-
diethylbenzenamine (5)
Compound
5
was purified by
a
column chromatography
(ethylacetate=hexane ¼ 1=10) to produce a yellow liquid in 84% yield:
1H-NMR (CDCl3, ppm) d 7.40 (2H, d), 7.15 (1H, d), 7.09 (1H, s), 7.05

Push-Pull Chromophore for Electro-Optic Applications
47=[329]
(1H, s), 7.01 (1H, d), 6.67 (2H, d), 3.92 (2H, d), 3.84 (2H, d), 3.39 (4H, q),
1.77–0.95 (36H, m).
Synthesis of 2,5-bis(2-ethyl-hexyloxy)-4-[4-
(diethylamino)styryl] benzaldehyde (6)
Compound
6
was purified by column chromatography
(ethylacetate=hexane ¼ 1=10) to produce an orange liquid in 87%
1
yield: H-NMR (CDCl3, ppm) d 10.43 (1H, s), 7.43 (2H, d), 7.31 (1H,
s), 7.25 (1H, s), 7.21 (1H, s), 7.15 (1H, s), 6.68 (2H, d), 4.01 (2H, d),
3.92 (2H, d), 3.40 (4H, q), 1.79–0.97 (36H, m).
Synthesis of 4-{2,5-bis(2-ethylhexyloxy)-4-[(E)-2-(5-
bromothiophen-2-yl)vinyl]styryl}-N,N-diethylbenzenamine (7)
Compound 7 was purified by flash column chromatography with
hexane=ethyl acetate (20:1 by volume) to give compound 7 as yellow
oil (yield: 83%): 1H NMR (CDCl3, ppm) d 7.41–7.38 (d, J ¼ 8.7 Hz,
2H), 7.29–7.24 (d, J ¼ 16.4 Hz, 1H), 7.21–7.16 (d, J ¼ 15.4 Hz, 1H),
7.16 (s, 1H), 7.09 (s, 1H), 7.09–7.03 (d, J ¼ 15.7 Hz, 1H), 7.03–6.98
(d, J ¼ 16.7 Hz, 1H), 6.93–6.91 (d, J ¼ 3.9 Hz, 1H), 6.75–6.74
(d, J ¼ 3.8 Hz, 1H), 6.67–6.64 (d, J ¼ 8.7 Hz, 1H), 3.95–3.89 (m, 4H),
3.40–3.33 (m, 4H), 1.84–1.74 (m, 2H), 1.59–1.19 (m, 16H), 1.17–1.02
(t, 6H), 1.00–0.86 (m, 12H).
Synthesis of 5-{2,5-bis(2-ethylhexyloxy)-4-[4-
(diethylamino)styryl]styryl}thiophene-2-carbaldehyde (8)
Compound 8 was purified by flash column chromatography with
hexane=ethyl acetate (10:1 by volume) to afford compound 8 as an
1
orange oil (yield: 38%): H NMR (CDCl3, ppm) d 9.83 (s, 1H), 7.65–
7.64 (d, J ¼ 3.9 Hz, 1H), 7.55–7.49 (d, J ¼ 16.2 Hz, 1H), 7.42–7.39
(d, J ¼ 8.7 Hz, 2H), 7.29–7.24 (d, J ¼ 16.2 Hz, 1H), 7.12–7.10
(d, J ¼ 5.1 Hz, 1H), 7.10 (s, 1H), 7.11–7.07 (d, J ¼ 14.2 Hz, 1H), 7.02
(s, 1H), 6.68–6.65 (d, J ¼ 8.7 Hz, 2H), 3.98–3.91 (m, 4H), 3.42–3.35
(m, 4H), 1.83–1.74 (m, 2H), 1.60–1.35 (m, 16H), 1.21–1.16 (t, 6H),
1.02–0.87 (m, 12H).
Synthesis of 4-((1E)-2-(5-(2,5-bis(2-ethylhexyloxy)-4-
(4-(diethylamino)styryl)styryl)thiophen-2-yl)vinyl)-2-
(dicyanomethylene)-2,5-dihydro-5,5-dimethylfuran-3-
carbonitrile (PT-TCF)
PT-TCF was purified by flash column chromatography with hexane=
ethyl acetate (1:1 by volume) and recrystallization from dichlorometha-
ne=hexane mixture to afford PT-TCF as a dark purple crystal
1
(yield: 39%): H NMR (CDCl3, ppm) d 7.81–7.76 (d, J ¼ 15.7 Hz, 1H),

48=[330]
D. W. Kim et al.
7.50–7.44 (d, J ¼ 16.2 Hz, 1H), 7.43–7.40 (d, J ¼ 8.2 Hz, 2H), 7.40–7.39
(d, J ¼ 3.4 Hz, 1H), 7.35–7.30 (d, J ¼ 16.2 Hz, 1H), 7.29–7.24 (d,
J ¼ 16.4 Hz, 1H), 7.14–7.08 (d, J ¼ 16.9 Hz, 1H), 7.11 (s, 1H), 7.10–
7.08 (d, J ¼ 3.9 Hz, 1H), 6.69–6.66 (d, J ¼ 9.0 Hz, 2H), 6.62–6.57 (d,
J ¼ 15.7 Hz, 1H), 4.00–3.91 (m, 4H), 3.43–3.36 (m, 4H), 1.87–76 (m,
8H), 1.65–1.38 (m, 16H), 1.22–1.17 (t, 6H), 1.03–0.91 (m, 12H).
Instruments
1H NMR spectra were recorded on Bruker DPX-300 spectrometer at
300 MHz. 1H-1H COSY and 13C NMR spectra were obtained with
Bruker AMX-500 spectrometer at 500 MHz and 125 MHz, respectively.
Differential Scanning Calorimetry (DSC) and thermal gravimetric
analysis (TGA) were performed on TA instruments DSC-2910 and
TGA-2950 in a nitrogen atmosphere, respectively. UV=Vis absorption
spectra of the chromophore solution and the thin film samples were
taken by Shimadzu UV-2550 Spectrophotometer. Refractive Index of
the polymer films was measured with a prism coupler (Sairon Co.,
SPA-4000) at 1550 nm. Thickness of the films was measured by using
a surface profiler (Tencor Ins., Alpha-Step 500).
Measurement of Electro-Optic Coefficient
Host-guest films were prepared by spincoating the solution of PT-TCF
and APC dissolved in cyclopentanone on ITO glasses. The chromo-
phore was well blended up to 50 wt.% with the host polymer. The spin-
coated films were thoroughly dried in the vacuum oven at 100ꢀC for
2 h. The dark blue films were about 1 mm thick. 100 nm-thick gold elec-
trodes were deposited on the top of the films in a thermal evaporator.
Then we carried out a contact poling process in order to align the
chromophore molecules in a noncentrosymmetric way, which were
randomly dispersed in the polymer host. The samples were heated
on a hot plate with a temperature controller to around the glass tran-
sition temperature of the polymer films, remained in the temperature
for about 60 min, and cooled down to room temperature under the
electric field. The amplitude of the modulated light was measured
by the reflection method proposed by Teng and Man and then the
electro-optic coefficient (r33) was calculated from the amplitude [12].
RESULTS AND DISCUSSION
Synthetic procedure of the NLO chromophore PT-TCF began with a
build-up of conjugation bridge of dialkoxy-substituted phenylene and

Push-Pull Chromophore for Electro-Optic Applications
49=[331]
thiophene rings as shown in Scheme 1. Wittig-Horner reaction is an
efficient coupling way between aldehyde and phosphonate. Knoevena-
gel condensation is a simple and high yield coupling method between
aldehyde and activated methylene. So the phenylene and thiophene
bridge was designed to have not only phosphonate functionality for
coupling with the donor (aminobenzaldehyde), but also bromide
terminal which was inert in the Wittig-Horner coupling but was able
to easily transform to aldehyde for Knoevenagel condensation with the
acceptor (TCF). 2-Ethylhexyl group was attached to the bridge to keep
the chromophore molecule from interacting closely with its neighbor-
ing ones and hence minimize its intermolecular electrostatic interac-
tion. The alkyl chains may also play a major role in suppressing
formation of cis-isomer as well as offering high solubility in solvents
and polymer medium, which is critical in fabricating quality films.
The alkyl chains were designed to connect to the phenylene ring
through ether linkage because direct coupling between the alkyl group
and the ring results in benzylic carbon, which is vulnerable to a radical
attack.
2-Methylthiophene was brominated by treatment of N-bromosucci-
nimide and then transformed to phosphonium salt (compound 1).
A Williamson etherification of methylhydroquinone with 2-ethylhexyl-
bromide gave 1,4-bis-(2-ethyl-hexyloxy)-2-methyl-benzene (compo-
und 2). Subsequent bromination using N-bromosuccinimide and
2,20-azobis(2-methylpropionitrile) in carbon tetrachloride produced
1-bromo-4-bromomethyl-2,5-bis-(2-ethyl-hexyloxy)-benzene (compound 3).
Phosphonate (compound 4) was obtained from compound 3 by a
Michalis-Arbuzov reaction. The phenylene bridge was coupled with
4-diethylamino-benzaldehyde via Wittig-Horner reaction. The ter-
minal bromide of the compound 5 was transformed to aldehyde by
treatment with n-butil-lithium and DMF according to the Bouveault
method. The compound 6 was coupled with the thiophenephosphonate
(compound 1) to give compound 7. After the terminal bromide was
again changed to aldehyde, the compound 8 was reacted with the
acceptor group of 2-(3-cano-4,5,5-trimethyl-5H-furan-2-ylidene)-
malononitrile (TCF) to result in the NLO chromophore PT-TCF. The
chromophore was thoroughly purified by column chromatography
and recrystallization to yield a dark purple crystal.
The chemical structure and purity were confirmed by such instru-
1
1
1
ments as H NMR, H-1H COSY and 13C NMR. H NMR spectrum in
Figure 1 shows that all the relevant proton peaks are appeared with-
out any impurity peaks and the integration is in good accord with the
calculated one. H-1H COSY is more helpful to identify the compli-
1
cated molecular structure from the information about what protons

50=[332]
D. W. Kim et al.
FIGURE 1 1H NMR spectrum of PT-TCF.
correlate to certain protons in the molecule. We could easily recog-
nize the protons attached with the carbon-carbon double bond con-
necting the phenylene bridge and the aminobenzene donor in
Figure 2, in which the proton at 7.81–7.76 ppm correlates to one at
6.62–6.57 ppm. As large as 15.7 Hz coupling constant indicates that
the ethenyl moiety has a trans-conformation. The ethenyl protons
in the double bond between the phenylene and the thiophene are
observed at 7.29–7.24 ppm and 7.14–7.08 ppm with 16.4 Hz of coup-
ling constant. The peaks of ethenyl protons in the double bond bridg-
ing the thiophene and the acceptor are appeared at 7.50–7.44 ppm
and 7.35–7.30 ppm with 16.2 Hz. Such NMR data clearly suggested
that we obtained the chromophore in all trans conformation. This
is because the bulky alkyl chain may suppress the formation of a
cis isomer.
Thermal behavior of the chromophore was traced by DSC and TGA.
In spite of the bulky alkyl chains, PT-TCF exhibited fairly high melt-
ing point of 188ꢀC. TGA thermogram in Figure 3 shows that there is no
thermal decomposition up to 220ꢀC in the nitrogen atmosphere. The
high melting point and decomposition on-set may be attributable to
the rigid bridge, phenylene and thiophene.
Overall procedures to evaluate the macroscopic optical nonlinear-
ity of the chromophore are composed of spincoating, drying, poling
and measurement of the amplitude of the modulated light. APC
was used for a host polymer which was optically transparent
and possessed a high Tg (181ꢀC, determined by DSC). Such charac-
teristics of the chromophore as solubility, thermal stability and
purity have a significant influence on quality of the polymer film,

Push-Pull Chromophore for Electro-Optic Applications
51=[333]
FIGURE 2 1H-1H COSY NMR spectrum of PT-TCF at 500 MHz.
which subsequently determine the macroscopic optical nonlinearity.
PT-TCF has a quite good solubility in solvents and is well
blended with the polymer host. We could prepare optical quality
polymer films containing up to 50 wt% of the chromophore. The
spin-coated films were thoroughly dried in a vacuum oven at
100ꢀC for 2 h.

52=[334]
D. W. Kim et al.
FIGURE 3 TGA thermogram of PT-TCF.
A 100 nm-thick gold was deposited on top of the film through a
shadow mask in a thermal evaporator. The gold was utilized as an
electrode. Then the film was contact poled under as high as
100 V=mm electric field at 95ꢀC for 1 h. The reflection technique pro-
posed by Teng is assumed a simple and efficient way to determine
the electro-optic coefficient of the thin films containing NLO chromo-
phores among others [12]. The amplitude of the modulated light
was measured and then the electro-optic coefficient (r33) was derived
from a certain equation. In a preliminary result, the polymer films
containing PT-TCF as a concentration of 50 wt% exhibited 22 pm=V
taken at 1.55 mm.
In conclusion, we designed and synthesized a new NLO chromo-
phore with thermal stability and relatively high EO coefficient. 1H
1
NMR and H-1H COSY clearly suggested that the chromophore was
in all trans conformation. The chromophore exhibited fairly high melt-
ing point of 188ꢀC and was thermally stable up to 220ꢀC in the nitrogen
atmosphere. The relatively high melting point and decomposition
on-set may be attributable to the rigid phenylene and thiophene
bridge. The polymer films containing PT-TCF as a concentration of
50 wt% exhibited electro-optic coefficient as large as 22 pm=V taken
at 1.55 mm.

Push-Pull Chromophore for Electro-Optic Applications
53=[335]
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