Nonlinear optical side-chain polymers post-functionalized with high-β chromophores exhibiting large electro-optic property

Article abstract of DOI:10.1002/pola.24410

Electro-optic side-chain polymers have been synthesized by the post-functionalization of methacrylate isocyanate polymers with novel phenyl vinylene thiophene vinylene bridge (FTC) nonlinear optical chromophores. For this application, FTC-based chromophores were modified in their electronic donor structure, exhibiting much larger molecular hyperpolarizabilities compared with the benchmark FTC. Of these new chromophores, absorption spectra, hyper-Rayleigh scattering experiment, and thermal analysis were carried out to confirm availability as effective nonlinear optical units for electro-optic side-chain polymers. The electro-optic coefficients (r₃₃) of obtained polymers were investigated in the process of in situ poling by monitoring the temperature, current flow, poling field, and electro-optic signal. Compared with the nonsubstituted analogue, benxyloxy modified FTC chromophore significantly achieved higher nonlinear optical property, exhibiting molecular hyperpolarizability at 1.9 μm of 4600 × 10^-30 esu and an r₃₃ value of 150 pm/V at the wavelength of 1.31 μm. Synthesized electro-optic polymers showed high glass transition temperature (T_g), so that the temporal stability examination exhibited >78% of the electro-optic intensity remaining at 85 °C over 500 h. Copyright

Full text of DOI:10.1002/pola.24410

Nonlinear Optical Side-Chain Polymers Post-Functionalized with
High-b Chromophores Exhibiting Large Electro-Optic Property
XIANQING PIAO,1 XIANMIN ZHANG,1,2 YUICHI MORI,1 MASAYUKI KOISHI,1 AKINARI NAKAYA,1
SHINICHIRO INOUE,1 ISAO AOKI,3 AKIRA OTOMO,3 SHIYOSHI YOKOYAMA1,3
1Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga 815-8580, Japan
2Department of Physics, Liaoning University, 66 Chongshan Middle Road, Shenyang 110036, China
3National Institute for Information and Communications Technology, 588-2 Iwaoka, Nishi-ku, Kobe 651-2492, Japan
Received 11 August 2010; accepted 21 September 2010
DOI: 10.1002/pola.24410
Published online 11 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Electro-optic side-chain polymers have been synthe-
sized by the post-functionalization of methacrylate isocyanate
polymers with novel phenyl vinylene thiophene vinylene
bridge (FTC) nonlinear optical chromophores. For this applica-
tion, FTC-based chromophores were modified in their elec-
tronic donor structure, exhibiting much larger molecular
hyperpolarizabilities compared with the benchmark FTC. Of
these new chromophores, absorption spectra, hyper-Rayleigh
scattering experiment, and thermal analysis were carried out to
confirm availability as effective nonlinear optical units for elec-
tro-optic side-chain polymers. The electro-optic coefficients
(r33) of obtained polymers were investigated in the process of
in situ poling by monitoring the temperature, current flow,
poling field, and electro-optic signal. Compared with the non-
substituted analogue, benxyloxy modified FTC chromophore
significantly achieved higher nonlinear optical property, exhib-
iting molecular hyperpolarizability at 1.9 lm of 4600 ꢀ 10ꢁ30
esu and an r33 value of 150 pm/V at the wavelength of 1.31 lm.
Synthesized electro-optic polymers showed high glass transi-
tion temperature (Tg), so that the temporal stability examina-
tion exhibited >78% of the electro-optic intensity remaining at
85 ꢂC over 500 h.
2010 Wiley Periodicals, Inc. J Polym Sci
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Part A: Polym Chem 49: 47–54, 2011
KEYWORDS: electro-optic coefficient; electro-optic polymer;
functionalization of polymer; molecular hyperpolarizability;
NLO; poling; side-chain polymer; thermal properties
INTRODUCTION During the past several decades, there has
been growing interest in organic and polymeric electro-optic
(EO) materials due to their potential applications in telecom-
munication, digital signal processing, THz generators, and
photonic nano-micro devices.1–5 This trend of increasing
interest continues in the development of a significantly high
EO coefficient (r33); higher than 100 pm/V at telecommuni-
cation wavelengths. Such high EO performance has allowed
for Mach-Zehnder modulators with low driving voltage of
<1 V, which makes this class of devices very attractive for
high-bandwidth operation. These devices have shown optical
modulations up to 100 GHz in commercial devices and up to
1.6 THz experimentally.6–9 Such performance has demon-
strated EO activity 10 times that of the inorganic counter-
parts (LiNbO3: r33 ꢃ 32 pm/V).
characteristics to be realized, molecules with large molecular
hyperpolarizability (b), high decomposition temperatures,
and high solubility in polymer hosts are eagerly anticipated.
For increasing b values, a series of NLO chromophores via
the invention of extended polyene-bridges or conjugated
thiophene-bridge systems with strong tricyano-containing
heterocyclic electron acceptors has been reported.10–16 In
Figure 1, examples of these designs are highlighted by what
are known as phenyltetraene bridge (denoted CLD-type) and
phenyl vinylene thiophene vinylene bridge (denoted FTC-
type) chromophores. We recently achieved a significant
improvement of molecular hyperpolarizability and EO prop-
erty for FTC-based chromophores (A–C), as shown in Figure
1. These chromophores have an additional electron-donating
effect on the conjugated bridge despite being highly compa-
rable to their 4-dialkylaminoaryl analogs. The enhancement
of both microscopic b values and macroscopic r33 values is
significant for these chromophores over FTC and CLD bench-
marks. By using chromophore B or C for example, the EO
measurement in PMMA host after electric poling gained
experimental r33 over 100 pm/V, presenting about two-fold
improvement over FTC benchmarks. Though obtained EO
For promising applications of novel nonlinear optical (NLO)
chromophores, which play the key role in EO devices, a
strong need remains for improving the optical properties
and thermal stability, both at the molecular level and as
processed materials, to fulfill requirements for device fabri-
cation processes and long-term operation. For these material
Additional Supporting Information may be found in the online version of this article. Correspondence to: S. Yokoyama (E-mail: s_yokoyama@cm.
kyushu-u.ac.jp)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 47–54 (2011) 2010 Wiley Periodicals, Inc.
NONLINEAR OPTICAL SIDE-CHAIN POLYMERS, PIAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 Structures of NLO chromophores CLD, FTC, and donor-modified chromophores (A–C) with additional methyloxy, benzyl-
oxy, and TBDMS groups.
property has the higher level reported so far in the series of
FTC-based chromophores, further increment is limited by the
chromophore dipolar interaction and aggregation in the poly-
mer host at higher concentrations.
functionalized and tethered on PMMA-co-MOI backbone as
the NLO units. The nonsubstituted FTC benchmark is also,
likewise, functionalized and processed for comparison.
Toward the high EO polymer preparation and high tempera-
ture resistance, the covalent attachment of side-chain NLO
chromophores onto polymer backbones offers many advan-
tages over the host-guest materials. However, synthesis of
side-chain polymers often caused degradation of the fragile
chromophores during the polymerization process. Post-func-
tionalization of an already prepared precursor polymer by
EXPERIMENTAL
Materials and Instruments
All the reagents and solvents were purchased from commer-
cial sources and used as received. 2-Methacryloyloxyethyl
isocyanate was from Showa Denko (Japan). Column chroma-
tography was performed using silica gel (Kanto Chemical,
60N).1H and 13C NMR spectra were recorded on a JNM-
LA400 spectrometer with tetramethyl silane as a reference.
High-resolution mass spectrometry (ERMS) was performed
by JEOL MStation JMS 700. Thermo gravimetry-differential
thermal analysis (TG-DTA) was performed using a SII-TG/
NLO chromophores is
a simple, alternative synthetic
approach to classical polymerization or complicated chromo-
phore-polymer composites.17–19 First, the starting polymer
was prepared separately without exposing the NLO chromo-
phore to harsh polymerization condition. Then, NLO chromo-
phores were attached to the polymer backbone under a mild
reaction condition in high yields with a minimum release of
by-products. This strategy therefore allows optimization of a
given property, such as EO coefficients or thermal stability,
through the large variety of different materials and synthetic
conditions.
ꢂ
DTA 6200 in nitrogen atmosphere at a heating rate of 5 C/
min. Differential scanning calorimetry (DSC) was performed
using a SII-DSC 6220 in nitrogen atmosphere at a heating
ꢂ
rate of 5 C/min. The molecular weight and polydispersity of
the polymer were determined by size exclusion chromatogra-
phy using a Shodex GPC K-804L column on a JASCO LC2000
liquid chromatography system with THF as an eluent and
with a polystyrene standard calibration. UV-visible spectra
were recorded with a Shimadzu UV-vis spectrometer 1240.
FT-IR spectra were recorded with a Shimadzu FT-IR spec-
trometer 8400S.
In this article, we report details of the synthesis and optical
properties of a class of EO side-chain polymers post-func-
tionalized with high-b chromophores. Our primary efforts
have been focused on developing EO polymers with efficient
electronic structure for both increasing nonlinear optical prop-
erties and stabilizing at a higher temperature. A poly[(methyl
methacrylate)-co-(2-methacryloyloxyethyl isocyanate)] (PMMA-
co-MOI) is selected as polymeric backbone for the post-func-
tionalization system because of the simple attachment of NLO
chromophores and the expected high temperature property
through urethane-bond formation.20 Therefore, high-b chro-
mophores A and B shown in Figure 1 are mono-hydroxyalkyl
Synthesis
The mono-hydroxyalkyl donor units, chromophores, and
side-chain polymers were synthesized as shown in Schemes
1–3. The detailed synthetic procedures are as follows. 1H
and 13C NMR spectra data and thermal properties of poly-
mers were submitted in Supporting Information.
SCHEME 1 Synthesis of donors.
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SCHEME 2 Synthesis of mono-hydroxylated chromophores.
3-Methoxy-N-methylaniline (2a)
and evaporated. The ester 3a was obtained as a clear liquid
To a suspension of NaOMe (15.8 g, 292 mmol) in MeOH
(70 mL) was added m-anisidine 1a (6.0 g, 48.8 mmol). The
resulting solution was poured into a suspension of para-
formaldehyde (2.22 g, 73.9 mmol) in MeOH (25 mL). After
stirring at room temperature for 24 h, NaBH4 (1.85 g,
48.9 mmol) was added to the mixture. After reflux for 2 h,
the mixture was evaporated to 1/2 volume, treated with 1 M
KOH (200 mL), and then extracted with dichloromethane.
The organic extracts were dried over MgSO4 and evaporated
to afford 2a (5.71 g, 86.4%).
(8.24 g, 88.6%).
2-[(3-Methoxyphenyl)-N-methylamino]ethanol (4a)
A suspension of sodium borohydride (2.79 g, 73.8 mmol)
and lithium bromide (6.41 g, 73.8 mmol) in dry THF
(80 mL) was reflux under nitrogen for 1 h. A solution of the
ester 3a (8.24 g, 26.9 mmol) in THF (10 mL) was added at
room temperature and the mixture was refluxed for a fur-
ther 5 h. The solvent was evaporated and the residue was
treated slowly with 1 M KH2PO4 (200 mL) and stirred until
hydrogen evolution ceased, and then extracted with dichloro-
methane. The organic extracts were washed with saturated
aqueous NaHCO3, dried, and evaporated. The hydroxylated
donor 4a was obtained as a clear liquid (10.12 g, 72.4%).
Ethyl 2-[(3-Methoxyphenyl)-N-methylamino]acetate (3a)
A solution of 2a (7.71 g, 41.7 mmol) and ethyl bromoacetate
(8.36 g, 50 mmol) in acetonitrile (50 mL) together with
NaHCO3 (3.85 g, 45.8 mmol) was stirred and refluxed under
nitrogen for 4 h. At room temperature triethylamine (2 mL)
was added and then filtrated and evaporated. The residue
was dissolved in dichloromethane, washed with water, dried,
Pivalic Acid 2-[(3-Methoxyphenyl)-N-methylamino]ethyl
Ester (5a)
Pivaloyl chloride (7.38 g, 61.2 mmol) was added dropwise
under nitrogen to a mixture of 4a (5.54 g, 30.6 mmol) and
SCHEME 3 Synthesis of PMMA-
MOI and side-chain polymers.
NONLINEAR OPTICAL SIDE-CHAIN POLYMERS, PIAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
pyridine (7.26 g, 91.8 mmol) in toluene (100 mL). The reac-
tion mixture was stirred at room temperature for 1 h, and
then heated at 85 ꢂC for further 3 h. After cooling at room
temperature, the resulting salt was separated, and the
solvent was evaporated. The residue was dissolved in ethyl
acetate, washed with water, dried, and evaporated. The crude
product was purified by silica chromatography (ethyl ace-
tate: hexane ¼ 1:3) to give color less oil 5a (7.58 g, 93.2%).
warmed to 0 ꢂC before being quenched (H2O, 10 mL). The
reaction mixture was poured into water, extracted by
dichloromethane, washed with brine, dried, and evaporated
to give a crude oil. The oil was solved in toluene with iodide
(0.20 g). The solution was allowed to stir at room tempera-
ture for 30 min. Then, the mixture was washed with aqueous
NaHSO3 (5%) and brine, dried, and evaporated. The crude
product was purified by silica chromatography (ethyl aceta-
te:hexane ¼ 1:3) to give 10a (E form) as an orange–red solid
(4.04 g, 87.4%).
Pivalic Acid 2-[(4-Formyl-3-methoxyphenyl)-N-
methylamino]ethyl Ester (6a)
To a solution of POCl3 (2.89 g, 18.8 mmol) in DMF (15 mL)
was added 5ꢂa (5.0 g, 18.8 mmol) in DMF (5 mL) under
nitrogen at 0 C. After stirring for 16 h at room temperature,
the mixture was treated with 20% sodium acetate solution
(50 mL), and then extracted with ethyl acetate. The organic
extracts were washed with saturated aqueous NaHCO3, dried,
and evaporated. The crude product was purified by silica
chromatography (ethyl acetate:hexane ¼ 1:2) to give pale
yellow oil 6a (5.0 g, 90.6%).
5-(2-{4-[(2-Hydroxyethyl)-N-methylamino]-2-methoxy-
phenyl}vinyl)thiophene-2-carbaldehyde (11a)
To a solution of 10a (4.04 g, 9.4 mmol) in THF (200 mL) at
room temperature was added dropwise the solution of
NBu4NF4 in THF (1 M, 2.5 mL, 2.5 mmol). The mixture was
stirred at room temperature for 1 h. After removal of the sol-
vent, crude product was purified by silica chromatography
(ethyl acetate:hexane ¼ 2:1) to give 11a (E form) as an
orange–red solid (2.25 g, 75.5%).
Pivalic Acid 2-{[3-Methoxy-4-(2-thiophen-2-yl-
Chromophore (13a)
vinyl)phenyl]-N-methylamino}ethyl Ester (8a)
To a solution of 11a (1.02 g, 2.59 mmol) in THF (8 mL)
and EtOH (25 mL) was added the acceptor 12 (0.82 g,
2.59 mmol). The reaction was allowed to stir at room tem-
perature for 48 h. The product was collected by filtration,
washed with cold EtOH (20 mL, 5 times). Chromophore 13a
(E form) was purified by silica chromatography (dichloro-
methane:methanol ¼ 10:0.5) to give a dark brown solid
(1.42 g, 79.3%). HRMS (ESI) (Mþ, C33H25O3N4F3S): calcd,
614.1599; found, 614.1595 [Mþ].
To a solution of PhLi (1.64 g, 19.6 mmol, 19% in Bu2O solu-
tion) in THF (50 mL) at 0 ꢂC was added 2-thenyl triphenyl-
phosphonium chloride 7 (6.4 g, 16.3 mmol) under nitrogen.
The solution was maintained at this temperature during the
dropwise addition of a solution of 6a (4.0 g, 13.6 mmol) in
THF (15 mL). After stirring 1.5 h, the reaction mixture was
poured into water. The organic phase was extracted by ethyl
acetate, washed with brine, dried, and evaporated. The crude
product was purified by silica chromatography (ethyl aceta-
te:hexane ¼ 1:4) to give 8a (E/Z mixture) as a yellow oil
(4.03 g, 97.3%).
Chromophore (13b)
In a similar manner, as shown in Schemes 1 and 2, 13b was
synthesized from 1b. HRMS (ESI) (Mþ, C39H29O3N4F3S):
calcd, 690.1912; found, 690.1908 [Mþ].
2-(tert-Butyldimethylsiloxyethyl)[3-methoxy-4-(2-thio-
phen-2-yl-vinyl)phenyl]-N-methyl-amine (9a)
Chromophore (13c)
To a solution of NaOH (4.28 g, 107 mmol) in EtOH (25 mL)
and water (7 mL) was added 8a (4.0 g, 10.7 mmol) in EtOH
(10 mL). The solution was warmed at 40 ꢂC and stirred for
1 h, then diluted with water and extracted with ethyl acetate.
The organic extracts were washed with water, dried, and
evaporated. The resulting yellow oil was solved in DMF
(8 mL) with imidazole (1.47 g, 21.6 mL). To this mixture,
tert-butyldimethylsilyl chloride (2.4 g, 16.2 mmol) in DMF
(5 mL) was added dropwise at room temperature, and stirred
for 1.5 h. The mixture dissolved in ethyl acetate, washed with
water, dried, and evaporated. The crude product was purified
by silica chromatography (ethyl acetate:hexane ¼ 1:3) to give
9a (E/Z mixture) as a yellow oil (4.30 g, 98.6%).
In a similar manner, as shown in Scheme 2, 13c was synthe-
sized from 2-(N-methylanilino)ethanol (4c) as the bench-
mark. HRMS (ESI) (Mþ, C33H25O2N4F3S): calcd, 598.1650;
found, 598.1655 [Mþ].
Precursor Polymer (PMMA-co-MOI)
Methyl methacrylate (MMA, 2.0 g, 20.0 mmol), 2-methacry-
loyloxyethyl isocyanate (MOI, 0.78 g, 5.0 mmol), and azobisi-
sobutyronitrile (AIBN, 0.08 g, 0.49 mmol) were dissolved in
toluene (10 mL) under nitrogen. The mixture was stirred
at 65 ꢂC for 3 h to yield a viscous solution. The resulting
product was poured into hexane to induce precipitation. The
polymer was collected by filtration, washed with hexane sev-
ꢂ
eral times, and dried at 40 C in vacuo. The yield was 2.27 g
5-[2-(4-{[2-(tert-Butyldimethylsiloxyethyl)]-N-
methylamino}-2-methoxyphenyl)-vinyl]thiophene-
2-carbaldehyde (10a)
(81.6%) of a white solid.
General Procedure for Preparation of
A solution of 9a (4.30 g, 10.7 mmol) in THF (30 mL) was
Side-Chain Polymer
ꢂ
cooled to ꢁ78 C and maintained at this temperature during
PMMA-co-MOI (0.5 g, 1 equiv) and chromophore 13a (0.33
g, 40 wt % equiv) were dissolved in THF (25 mL) under
nitrogen at room temperature. A trace amount of dibutyltin
dilaurate (DBTDL) was added, and the mixture was stirred
for 24 h. To complete the reaction, MeOH (2 mL) was added
the dropwise addition of n-BuLi in hexane (1.6 M, 10 mL,
ꢂ
15.7 mmol). Following 1 h of stirring at ꢁ78 C the tempera-
ture was maintained prior to the dropwise addition of DMF
(0.95 g, 12.3 mmol). After 1 h, solution was gradually
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ARTICLE
to the mixture, and stirred for a further 12 h. The reaction
mixture was poured into methanol, and the precipitated
polymer was filtrated. To purify the NLO side-chain polymer,
precipitation was repeated several times for the removal
of unreacted chromophore. The product was dried to yield
0.78 g of a dark green solid. In a similar manner, polymers
were post-functionalized by the chromophore at different
concentrations between 30 and 50 wt % equiv. Chromo-
phores 13b and 13c (benchmark) were also used to synthe-
size the side-chain polymers.
RESULTS AND DISCUSSION
Chromophore and Polymer Syntheses
The synthesis of mono-hydroxyalkyl chromophores (13a–c)
is shown in Schemes 1 and 2. The synthesis of starting
donor 4a began from N-methyl-m-anisidine 2a, the prepara-
tion of which was approached from m-anisidine 1a by treat-
ment with NaOMe and formaldehyde in methanol, followed
by reduction with NaBH4. Compound 2a was alkylated by
ethyl bromoacetate to give ester 3a, which was smoothly
reduced to alcohol 4a with NaBH4-LiBr in THF. The reaction
of compound 4a with pivaloyl chloride was effected to pro-
tect the hydroxyl group on the donor of the chromophore.
Subsequent reaction with DMF and POCl3 afforded 4-amino-
2-methoxybenzaldehyd 6a. Thiophene bridge compound 7
was prepared in two steps to yield the Witting salts. After
the Witting condensation of 7 with aldehyde 6a, desired
donor bridge 8a was produced in high yield. The pivaloyl
protecting group on the donor end was replaced with
TBDMS group to afford compound 9a that was then formy-
lated effectively with n-BuLi and DMF to give 10a. The
TBDMS protecting group was cleaved under the acidic condi-
tion to yield mono-hydroxyalkyl donor bridge 11a. The
acceptor 12 was coupled to the conjugation bridge by
Knoevenagel condensation to afford mono-hydroxyalkyl chro-
mophore 13a. Chromophores 13b and 13c were also syn-
thesized through the similar process by using starting
compound 1b and 2-(N-methylanilino)ethanol (4c), respec-
tively. TG-DTA measurement indicated that synthesized chro-
mophores had high temperature resistance: their decomposi-
FIGURE 2 Absorption spectra of synthesized chromophores
13a, 13b, and 13c (benchmark) in chloroform.
ethyl isocyanate)] (PMMA-co-MOI), with around 20 mol % of
the isocyanate moieties. PMMA-co-MOI was prepared by
ꢂ
free-radical polymerization in toluene at 65 C using AIBN as
the initiator. After the polymerization, mono-hydroxyalkyl
chromophores were attached to the precursor polymers with
different concentrations between 20 and 50 wt % through
a simple urethane-forming reaction. By using DBTDL as a
catalyst, these post-functionalization reactions proceeded
smoothly in THF at room temperature. The reaction was
monitored using FT-IR spectroscopy. At 2272 cmꢁ1, isocya-
nate stretching vibration was observed to decrease signifi-
cantly during the reaction. To block an excess of unreacted
isocyanete after post-functionalization, MeOH was added to
complete the residual reaction. Completion of the reaction
was determined by the disappearance of the isocyanate
stretching vibration in the FT-IR spectrum.
Absorption Spectroscopy and Molecular
Hyperpolarizability of NLO Chromophores
Linear and nonlinear optical properties of synthesized chro-
mophores are summarized in Table 1. All of mono-hydroxy-
alkyl chromophores 13a–13c were well dissolved in chloro-
form and the absorption spectra were recorded to determine
the maximum absorption (kmax) of the p–p* charge-transfer
band. The benchmark 13c showed kmax at 734 nm in a
CHCl3 solution as shown in Figure 2. The addition of methyl-
oxy and benzyloxy groups on the donor unit led to a shifting
of the charge-transfer absorption to the lower energy region:
kmax to 766 and 768 nm for 13a and 13b, respectively,
implying an extra electron-donating strength to the p-conju-
gated bridges.
ꢂ
tion temperatures (Td) are around 210 C as summarized in
Table 1.
The side-chain polymers were synthesized by utilizing a co-
polymer, poly[(methyl methacrylate)-co-(2-methacryloyloxy-
TABLE 1 Physical and Optical Properties of
Mono-Hydroxy Chromophores
a
kmax
bb
Td
(ꢂC)
Chromophores
(nm)
(ꢀ10ꢁ30 esu)
13a
766
768
734
4400
4600
2800
215
210
207
The molecular hyperpolarizability (b) of chromophores was
measured in chloroform using hyper-Rayleigh scattering
(HRS) measurement at the wavelength of 1.9 lm.21,22 The
result of b values is summarized in Table 1. The technique
allows the direct evaluation of b, employing the reference
chromophore with known b value. In this study, FTC was
13b
13c (benchmark)
a
kmax are measured in chloroform.
b values are measured in chloroform using the HRS technique at the
b
laser wavelength of 1.9 lm.Td is the onset decomposition temperature
measured by TGA under nitrogen at a heating rate of 5 ꢂC/min.
NONLINEAR OPTICAL SIDE-CHAIN POLYMERS, PIAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Physical Properties of Side-Chain Polymers
respectively. Im is the measured modulated-light intensity
having the same frequency as the AC modulation field, and Ic
is the unmodulated-light intensity when phase retardation is
90ꢂ between the TE and TM mode. The intensity change was
detected using a lock-in amplifier isolating the moderated
signal, Im. Overlaying a DC poling field (Vp) with Vm, real-
time monitoring of the ratio Im/Ic allowed direct evaluation
of optimum r33 values under the controlled experimental
parameters such as current, temperature, and applied voltage.
Mw
(ꢀ104)
Chromophore
(wt %)
Tg
(ꢂC)
Td
(ꢂC)
Polymers
Mw/Mn
14a
14b
14c
4.15
3.68
2.85
2.3
2.3
2.2
35
36
34
138
135
147
295
296
282
used as the external standard.23 HRS analysis indicated that
the addition of donor group is effective in increasing the b
values. The factor of increase in the b of 13a and 13b were
1.57 and 1.64 by comparison with benchmark 13c. The
result of b enhancement by the addition of methoxy and
benzyloxy groups was consistent with the kmax shift of these
chromophores to the longer wavelengths.
A representative in situ poling process using side-chain poly-
mer 14b is shown in Figure 3(a–d). The loading concentra-
tion of the chromophore is 36 wt %. In a general experi-
ment, the sample was mounted on the poling stage and the
temperature was increased in several minutes up to around
Tg. The poling field was then applied to the sample at a
ramping rate of 8 V/min. During the poling process, the elec-
tric current was monitored to optimize poling efficiency and
to control local electrical breakdown in the films. As shown
in Figure 3(c), the current grew rapidly with the ramping of
poling field, and maximized at 480 lA when the poling field
climbed to 60 V/lm. The increment of current should be
dominantly ascribed to the chromophore alignment under
the external electric field and the rearrangement of polymer,
which is mostly accomplished around the Tg of polymer. The
monitoring of the current also enabled more efficient poling
of the sample. Generally, the breakdown of the electric field
through the polymeric film is accompanied by an abrupt
overflow of the electric current. Thus, the ramping of the
poling field ceased and maintained at 86 V/lm when the
second increasing trend of the current flow appeared in
Characterization of Side-Chain Polymers
Physicochemical properties of synthesized side-chain poly-
mers are summarized in Table 2. The loading concentration
of chromophores in the side-chain polymers (14a–14c) were
determined by measuring the absorption spectra, which
were compared with the bare chromophores (13a–13c) with
known concentration. The obtained side-chain polymers are
highly soluble in a common organic solvent such as chloro-
form, THF, cyclopentanone, or DMF. The size exclusion
chromatography analysis indicated the weight average
molecular weight (Mw) of polymers in the range of 28,500–
41,500. DSC analysis showed that the glass transition tem-
peratures (Tg) of the side-chain polymers increased as the
loading concentration of chromophores. At 34–36 wt % Tg
ꢂ
values of 14a, 14b, and 14c were 138, 135, 147 C, depend-
ing on the structure of the chromophores. TG-DTA showed
no decomposition below 280 ꢂC. These Tg and Td data were
used to determine the suitable poling temperature and
decomposition point for the process of EO measurements.
EO Properties of Side-Chain Polymers
According to the enhanced molecular hyperpolarizabilities for
chromophores 13a and 13b compared with benchmark 13c,
an improvement of macroscopic EO property for the side-
chain polymers is expected by post-functionalization reaction.
Side-chain polymers 14a–14c dissolved in cyclopentanone
(filtrated through a 0.2 lm syringe filter) were spin-coated
onto indium tin oxide (ITO) glass substrate to produce films
of about 2 lm thickness. After baking in a vacuum oven over-
night at 85 ꢂC to remove the residual solvent, a thin gold layer
with a thickness of 100 nm was sputtered on the films as the
top electrode for electric poling. The in situ r33 measurement
at the laser wavelength of 1.31 lm was performed by a modi-
fied Teng-Mann reflection method.24,25 In this measurement,
r33 signals were collected during the process of heating and
cooling in the presence of a poling field. The r33 value was
directly obtainable according to the equation:
1
2
2
2
3k Im ðn ꢁ sin hÞ
Im
Ic
r33
¼
ꢃ
(1)
sin2h
4pn2 VmIc
FIGURE 3 In situ plots of poling process on a representative
sample of side-chain polymer 14b containing 35 wt % of NLO
chromophore. (a) Temperature, (b) flowing current, (c) applied
field, and (d) r33, are plotted during the poling process.
where h, n, and Vm are the incident angle of laser, refractive
index at k ¼ 1.31 lm, and AC electric modulation field,
52
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
TABLE 3 Electro-Optic Properties of Side-Chain Polymers
Figure 3(c). The current dropped slowly after peaking, and
even the poling field maintained a constant value. This may
be due to the changing complexity of charge injection and
trapping behavior of polymer with high chromophore con-
centration. Figure 3(d) indicates that the r33 varied simulta-
neously with the applied voltage and reached its maximum
of 193 pm/V at the poling field of 86 V/lm. Such a value
leveled off under the continuous poling field at 138 ꢂC,
resulting in in situ poling efficiency of 2.3 (nm/V)2 defined
as the ratio of r33 and Vp. The sample was then cooled to
room temperature with the poling field maintained. During
the rapid cooling process, there was a notable relaxation
phenomenon of the molecular ordering as the total EO signal
decreased by 24%. At the same time, the current also
declined from 300 to 20 lA due to the remarkable increase
of resistivity of the polymeric sample. Eventually, r33 of
150 pm/V was realized at room temperature for the poled
electro-optic film after removing the poling field. Poling
efficiency of 1.7 (nm/V)2 was estimated for the polymer
14b. Measured electro-optic property is much higher than
the reported r33 values (50–70 pm/V) for host-guest NLO
polymers using standard FTC analogues in PMMA.16
Temporal
Stabilityc
(%)
b
r33
r33/Vp
(nm/V)2
Polymersa
(pm/V)
14a
14b
14c
a
97
150
75
1.2–1.4
1.6–1.8
1.0–1.2
72
78
68
Polymers contain chromophores at 35, 36, and 34 wt % for 14a, 14b,
and 14c, respectively.
b
EO coefficient measured at 1.31 lm.
c
EO property measured after 500 h at 85 ꢂC.
side-chain polymer 14b containing 36 wt % of chromophore.
The result fully reflects the varying trend of b of chromo-
phores: 13b > 13a > 13c. Though chromophore 13b has
about 5% higher b value than that of 13a, benzyloxy substit-
uent at the donor part should be intended to prevent the
strong electrostatic interaction between chromophores. This
implies that the additional donor group on the donor part of
the chromophore had a much profound effect on the elec-
tric-induced chromophore alignment, where polymer 14b
showed 50% larger poling efficiency than that for 14a and
14c. After reaching the maximum r33 value at the optimum
chromophore density, EO property tended to decrease at
higher loading range (>42 wt %). This is undoubtedly attri-
buted to the strong dipole–dipole interaction, which leads to
chromophore anti-parallel orientation that partially cancels
the acentric symmetry.
To investigate the optimum chromophore concentration for
the EO property, series of side-chain polymers was synthe-
sized with the different chromophore concentrations in the
range of 22–46 wt %. Each sample was poled around its Tg
by in situ r33 monitoring method for optimal experiments. As
shown in Figure 4, r33 values of three groups increased with
rising chromophore contents at lower concentration range.
This observation is consistent with the fact that the EO co-
efficient is directly proportional to the chromophore number
density. The resultant r33 obtained under the optimal poling
conditions are summarized in Table 3. The highest r33 values
of the films 14a, 14b, and 14c were obtained with the
chromophore concentrations of 35, 36, and 34 wt %, respec-
tively. The highest r33 value of 150 pm/V was achieved for
According to the thermal analyses by DSC and TGA, side-
chain polymers showed high Tg ranging from 135 ꢂC to
ꢂ
ꢂ
147 C and Td higher than 280 C. Though high Tg of the EO
polymer is favorable for maintaining chromophore alignment
stability, a certain reduction of EO property is inevitably
accompanied in the long-term application. Long-term align-
ment stability was recorded by heating the poled samples at
85 ꢂC and measuring the r33 values at various point of
FIGURE 4 Dependence of EO property of side-chain polymers
14a, 14b, and 14c with the different chromophore concentrations.
FIGURE 5 Temporal stability of side-chain polymers 14a, 14b, and
14c containing about 35 wt % of NLO chromophores at 85 ꢂC for 500 h.
NONLINEAR OPTICAL SIDE-CHAIN POLYMERS, PIAO ET AL.
53

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
5 Takayesu, J.; Hochberg, M.; Baehr-Jones, T.; Chan, E.; Wang,
G.; Sullican, P.; Liao, Y.; Davies, J.; Dalton, L.; Scherer, A.;
Krug, W. J Lightwave Tech 2009, 27, 440–448.
extended time as shown in Figure 5. For all of samples, elec-
tro-optic property showed an initial fast decay during the
first 20 h, whereas 68–78% of r33 value could be maintained
ꢂ
at 85 C for over 500 h. This owes to, first, the advantage of
6 Shi, Y.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.;
the side-chain polymer strategy, by which the NLO chromo-
phore is post-functionalized on the polymer backbone
through a thermally stable urethane bond. Second, the large
volume of the benzyloxy group attached on the chromophore
favors retarding the relaxation of its alignment and to ‘‘freez-
ing’’ the aligned order.
Robinson, B. H.; Steier, W. H. Science 2000, 288, 119–122.
7 Lee, M.; Katz, H. E.; Erben, C.; Gill, D. M.; Gopalan, P.; Heber,
J. D.; McGee, D. J. Science 2002, 298, 1401–1403.
8 Ermer, S.; Lovejoy, S. M.; Bedworth, P. V.; Leung, D. S.; War-
ren, H. B.; Epstein, J. A.; Girton, D. G.; Dries, L. S.; Taylor, R.
E.; Barto, R. R.; Eades, W., Jr.; Van Eck, T. E.; Moss, A. S.;
Anderson, W. W. Adv Funct Mater 2002, 12, 605–610.
CONCLUSIONS
9 Enami, Y.; Derose, C. T.; Mathine, D.; Loychik, C.; Greenlee,
C.; Norwood, R. A.; Kim, T. D.; Luo, J.; Tian, Y.; Jen, A. K.; Pey-
ghambarian, N. Nat Phot 2007, 1, 180–185.
In conclusion, we have synthesized three groups of electro-
optic side-chain polymers by using the convenient urethane-
forming post-functionalization of mono-hydroxyl chromo-
phores on PMMA-co-MOI backbone. We used FTC-based
chromophore as NLO unit, where the modification on the
donor units by attaching an additional functional group led to
a significant improvement of both microscopic hyperpolariz-
ability (b) and macroscopic electro-optic coefficient (r33). As a
result, the side-chain polymer was poled up to 150 pm/V and
exhibited about two-fold improvement over the counterpart
polymer. The enhanced donor also contributed to maintain
temporal stability of the poled side-chain polymer through
retardance of flexibility of aligned chromophores. Up to 78%
of the intensity ultimately remained after annealing at 85 ꢂC
for over 500 h. Therefore, the electro-optic side-chain polymer
not only successfully increased the r33 values by delaying
the occurrence of intermolecular interaction on the chromo-
phore concentration scale, but also considerably enhanced the
thermal stability for the real device applications.
10 He, M. Q.; Leslie, T. M.; Sinicropi, J. A. Chem Mater 2002,
14, 4662–4668.
11 Liu, S.; Haller, M. A.; Ma, H.; Dalton, L. R.; Jang, S.-H.; Jen,
A. K.-Y. Adv Mater 2003, 15, 603–607.
12 He, M.; Leslie, T. M.; Sinicropi, J. A.; Garner, S. M.; Reed, L.
D. Chem Mater 2002, 12, 4669–4675.
13 Zhang, C.; Dalton, L. R. Chem Mater 2001, 13, 3043.
14 Cheng, Y.-J.; Luo, J.; Huang, S.; Zhou, X. H.; Shi, Z.; Kim,
T.-D.; Bale, D. H.; Takahashi, S.; Yick, A.; Polishak, B. M.; Jang,
S.-H.; Dalton, L. R.; Reid, P. J.; Steier, W. H.; Jen, A. K.-Y. Chem
Mater 2008, 20, 5047–5054.
15 Davies, J. A.; Elangovan, A.; Sullivan, P. A.; Olbricht, B. C.;
Bale, D. H.; Ewy, T. R.; Isborn, C. M.; Eichinger, B. E.; Robinson,
B. H.; Reid, P. J.; Li, X.; Dalton, L. J Am Chem Soc 2008, 130,
10565–10575.
16 Kim, T.; Luo, J.; Cheng, Y.; Shi, Z.; Hau, S.; Jang, S.; Zhou,
X.; Tian, Y.; Polshak, B.; Huang, S.; Ma, H.; Dalton, L. R.; Jen,
A. K. J Phys Chem C 2008, 112, 8091–8098.
The authors acknowledge partial financial support from a
Grant-in-Aid for Young Scientists (S) and the Global Centers of
Excellence (GCOE) Program of the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan. This work was par-
tially performed under the Cooperative Research Program of
‘‘Network Joint Research Center for Materials and Devices’’.
17 Briers, D.; Koeckelberghs, G.; Picard, I.; Verbiest, T.; Persoons,
A.; Samyn, C. Macromol Rapid Commun 2003, 24, 841–846.
18 Lee, S. K.; Cho, M. J.; Jin, J.; Choi, D. H. J Polym Sci A:
Polym Chem 2007, 45, 531–542.
19 Scarpaci, A.; Cabanetos, C.; Blart, E.; Montembault, V.; Fontaine,
REFERENCES AND NOTES
L.; Rodriguez, O. F. J Polym Sci A: Polym Chem 2009, 47, 5652.
1 Hochberg, M.; Baehr-Jones, T.; Wang, G.; Shearn, M.; Har-
vard, K.; Luo, J.; Chen, B.; Shi, Z.; Lawson, R.; Sullivan, P.; Jen,
A. K. Y.; Dalton, L.; Scherer, A. Nat Mat 2006, 5, 703–709.
20 Francis, C. V.; Liu, J.; White, K. M.; Kitipichai, P. K. Ind Eng
Chem Res 1999, 38, 2–7.
21 Clays, K.; Persoons, A. Phys Rev Lett 1991, 66, 2980–2983.
22 Clays, K.; Persoons, A. Rev Sci Instrum 1992, 63, 3285–3289.
2 Seo, B.; Kim, S.; Fetterman, H.; Steier, W.; Jin, D.; Dinu, R.
J Phys Chem C 2008, 112, 7953–7958.
23 Firestone, K. A.; Lao, D. B.; Casmier, D. M.; Clot, O.; Dalton,
L. R.; Reid, P. J. Proc SPIE-Int Soc Opt Eng 2005, 5935 (Linear
and Nonlinear Optics of Organic Materials V), 49.59350P.
3 Baehr-Jones, T.; Penkov, B.; Huang, J.; Sullivan, P.; Davies,
J.; Takayesu, J.; Luo, J.; Kim, T.; Dalton, L.; Jen, A.; Hochberg,
M.; Scherer, A. Appl Phys Lett 2008, 92, 1633031-1–1633031-3
24 Teng, C. C.; Man, H. T. Appl Phys Lett 1990, 56, 1734–1736.
25 Shuto, Y.; Amano, M. J Appl Phys 1995, 77, 4632–4638.
4 Seo, B.; Kim, S.; Bortnik, B.; Fetterman, H.; Jin, D.; Dinu, R.
J Lightwave Tech 2009, 27, 3092–3106.
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