Novel, side-on, PVK-based nonlinear optical polymers: Synthesis and NLO properties

Article abstract of DOI:10.1016/j.dyepig.2009.07.006

A novel, poly(vinylcarbazole)-based nonlinear optical polymer (side-on type), in which the chromophore was attached to the polymer backbone at the centre of the D-π-A bridge, was synthesized conveniently via the post-Knoevenagel condensation reaction. The structure of chromophore was further modified by the introduction of an isolation spacer (carbazole group) to improve poling efficiency. The ensuing polymer displayed good solubility and high thermal stability and its poled film exhibited a relatively large d₃₃ value of ≤56.0 pm/V.

Full text of DOI:10.1016/j.dyepig.2009.07.006

Dyes and Pigments 84 (2010) 134–139
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journal homepage: www.elsevier.com/locate/dyepig
Novel, side-on, PVK-based nonlinear optical polymers: Synthesis and NLO
properties
,
Zhong’an Li a, Li Wang a, Bi Xiong a, Cheng Ye b, Jingui Qin a, Zhen Li a
*
a Department of Chemistry, Wuhan University, Wuhan 430072, China
b Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel, poly(vinylcarbazole)-based nonlinear optical polymer (side-on type), in which the chromophore
Received 2 June 2009
Received in revised form
8 July 2009
Accepted 10 July 2009
Available online 21 July 2009
was attached to the polymer backbone at the centre of the D-p-A bridge, was synthesized conveniently
via the post-Knoevenagel condensation reaction. The structure of chromophore was further modified by
the introduction of an isolation spacer (carbazole group) to improve poling efficiency. The ensuing
polymer displayed good solubility and high thermal stability and its poled film exhibited a relatively
large d33 value of ꢀ56.0 pm/V.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Poly(vinylcarbazole)
Nonlinear optics
Side-on
Isolation groups
Synthesis
Post functionalization
1. Introduction
enhanced dramatically [16,17]. Dalton and Jen et al. prepared
a series of dendrimers and polymers containing dendronized
Considerable efforts have focused on organic nonlinear optical
(NLO) polymers over the past few decades, owing to such
compounds displaying advantages over inorganic materials,
including large NLO effects, mechanical endurance, low driving
voltage and ease of processing [1–5]. In order to realize their huge
potential for use in photonic devices, such as telecommunications,
optical data storage and THz generation, NLO materials should
demonstrate large macroscopic optical nonlinearity, high physical
and chemical stabilities as well as good optical transparency [1–11].
However, the achievement of all of these requirements simulta-
neously presents a major obstacle, one of note being that high
chromophores, which displayed very large macroscopic optical
nonlinearity [18–22]. The author’s research group is interested in
the design and synthesis of novel NLO materials and the relation-
ship between structure and properties [23–32]; recent papers
[26–32] have revealed that the macroscopic nonlinearity of NLO
polymers could be boosted several times by binding suitable
‘isolation groups’ to NLO chromophore moieties [26–32].
A novel, postfunctionalization method was used to prepare
a PVK-based NLO polymer, which exhibited good long term stability
of the NLO effect (the NLO activity remains unchanged at 120 ꢁC for
over 1000 h after a minor initial drop), but it possessed relatively
low d33 value (20 pm/V) [33]. A series of PVK-based NLO polymers
with sulfonyl-groups as acceptor in the chromophore moieties
were prepared, in which different isolation spacers were intro-
duced, and the polymers exhibited good comprehensive properties
[34], confirming that PVK was a good parent polymer to be further
functionalized for the preparation of promising PVK-based NLO
materials.
molecular nonlinearity (b) of a chromophore cannot be efficiently
translated into high macroscopic NLO activity of a polymer.
Generally, during the poling-induced noncentrosymmetric align-
ment of chromophores, which is required to secure macroscopic
NLO effects, the poling efficiency of the materials is significantly
reduced owing to the strong intermolecular, dipole–dipole inter-
actions of the highly polar chromophore [12–15]. By controlling the
shape of the chromophore moieties according to the site isolation
principle, such interactions can be minimized and poling efficiency
In contrast, in the majority of side-chain NLO polymers reported,
the typical linkage position on the D-p-A system was at the end of
the donor or acceptor side in the chromophore groups, which could
be termed as end-on side-chain polymers [23,24,33,34]. Although
very scarce, there is another type of NLO polymers (side-on type),
namely, one in which the site for the introduction of the
* Corresponding author. Tel.: þ86 27 62254108; fax: þ86 27 68756757.
E-mail address: lizhen@whu.edu.cn (Z. Li).
0143-7208/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.07.006

Zhong’an Li et al. / Dyes and Pigments 84 (2010) 134–139
135
chromophore within the polymer backbone is at the centre of the
bridge [35–38]. Jen et al. prepared a series of this type of side-chain
dendronized NLO polymers by covalently attaching the chromo-
p
a Shimadzu UV-2550 spectrometer. Gel permeation chromatog-
raphy (GPC) was used to determine the molecular weights of
polymers. GPC analysis was performed on a Waters HPLC system
equipped with a 2690D separation module and a 2410 refractive
index detector. Polystyrene standards were used as calibration
standards for GPC. DMF was used as eluent, and the flow rate was
1.0 mL/min. EI-MS spectra were recorded with a Finnigan PRACE
mass spectrometer. Elemental analyses were performed by a CAR-
LOERBA-1106 micro-elemental analyzer. Thermal analysis was
performed on NETZSCH STA449C thermal analyzer at a heating rate
of 10 ꢁC/min in nitrogen at a flow rate of 50 cm3/min for ther-
mogravimetric analysis (TGA). The thermal transitions of the
polymers were investigated using a METTLER differential scanning
calorimeter DSC822e under nitrogen at a scanning rate of 10 ꢁC/
min. The thermometer for measurement of the melting point was
uncorrected. The thickness of the films was measured with an
Ambios Technology XP-2 profilometer.
phore moieties to the centre of the
p-bridge in the polymeric
backbone; the ensuing compounds displayed high dielectric
strength, excellent optical quality, good processability, and reliable
poling behaviors [39]. The current research group also has carried
out similar work, and obtained several NLO polymers with good
performance [28,29]. Thus, considering the low poling efficiency of
our previous PVK-based polymers, we sought to apply this side-on
mode to PVK-based NLO polymers. This paper concerns the
synthesis of a novel, side-on PVK-based NLO polymer, in which the
linkage position on the chromophore group was in the middle of its
D-p-A bridge, and, as such, differs from previously reported PVK-
based polymers [33,34]. Carbazole moieties were used as isolation
groups, which have been confirmed as the suitable isolation groups
for the nitro-based chromophores in our previous work [26],
according to the concept of ‘‘suitable isolation groups’’. The resul-
tant polymer demonstrated excellent solubility, good thermal
ability, and relatively high d33 value (56.0 pm/V). Herein, we would
like to report the synthesis, characterization and NLO properties of
this PVK polymer Schemes 1 and 2.
2.3. Synthesis of compound 2
3-Bromoaniline (1) (5.16 g, 0.03 mol) and bromoethane (9.81 g,
0.09 mol) were dissolved in t-BuOH (50 mL) in the presence of
potassium carbonate (13.80 g, 0.10 mol) as an acid acceptor with
potassium iodide (0.60 g, 3.60 mmol) as catalyst. The resultant
mixture was stirred at 100 ꢁC for 2 days. Then the residue was
filtered, and most of the solvent in the filtrate was removed under
reduced pressure. The crude product was purified with column
chromatography on silica gel using ethyl acetate/petroleum ether
(1/5) as eluent to afford brown oil 2 (3.00 g, 43.9%). 1H NMR (CDCl3)
2. Experimental section
2.1. Materials
Tetrahydrofuran (THF) was dried over and distilled from K–Na
alloy under an atmosphere of dry nitrogen. N,N-Dimethylforma-
mide (DMF) was dried over and distilled from CaH2 under an
atmosphere of dry nitrogen. Dichloromethane (CH2Cl2) was dried
over anhydrous CaCl2 and fresh distilled before use. Poly(N-vinyl-
carbazole) (PVK) was purchased from Aldrich, and its weight-
averaged molecular weight was estimated to be 1.10 ꢂ 106. PVK-
CHO (P0) was synthesized according to our previous work [33].
Compounds 3 and 5 were synthesized according to our previous
work [26,40]. All other reagents were used as-received.
d
(ppm): 1.15 (t, J ¼ 7.5 Hz, 6H, –CH3), 3.33 (q, J ¼ 6.6 Hz, 4H, –N–
CH2–), 6.55 (d, J ¼ 8.7 Hz, 1H, ArH), 6.77 (m, 2H, ArH), 7.04
(t, J ¼ 8.1 Hz, 1H, ArH).
2.4. Synthesis of chromophore 4
4-Nitroaniline (3) (105 mg, 0.53 mmol) was dissolved in a water
solution of 35% hydrochloric acid. The mixture was cooled to 0–5 ꢁC
in an ice bath, and then a solution of sodium nitrite (41 mg,
0.59 mmol) in water was added to the above solution dropwise.
After stirred below 5 ꢁC for 15 min, a solution of 1 (114 mg,
0.50 mmol) in ethanol was added slowly. The mixture was left in
the ice bath for another 1 h, some sodium bicarbonate was added to
adjust the pH value to about 7.0. The reaction mixture was stirred
for another 0.5 h, the red precipitate was filtered, washed with
water. The crude product was purified by recrystallization from
2.2. Instrumentation
1H and 13C NMR spectra were measured on a Varian Mercury300
spectrometer using tetramethylsilane (TMS;
d
¼ 0 ppm) as internal
standard. The Fourier transform infrared (FTIR) spectra were
recorded on a PerkinElmer-2 spectrometer in the region of 3000–
400 cmꢃ1 on NaCl pellets. UV–vis spectra were obtained using
NH2
NH2
N
Br
O(CH2)2OH
K2CO3/KI
t-BuOH
N
NO2
1. NaNO2
+
BrCH2CH3
+
3
N
N
4
2. HCl, 0-5 oC
O
Br
Br
1
2
NO2
CH2CH2OH
N
N
N
HO
B
OH
NCCH2COOH
,N
Pd(PPh3)4 a2CO3
THF/H2O, 80 oC
4
+
N
N
+
N
DCC, DMAP,CH2Cl2
N
N
N
5
O
C4H9
HOH2CH2CO
7
NCH2CCOH2CH2C
O
6
NO2
NO2
Scheme 1.

136
Zhong’an Li et al. / Dyes and Pigments 84 (2010) 134–139
0.5
0.5
N
n
N
0.5
7
0.5
N
n
piperidine
N
O2N
O
CN
O
N
P0
O
CHO
N
P1
N
N
Scheme 2.
ethanol/water to afford deeply red powder (120 mg, 55.0%).
148.01,151.45,154.63, 163.30. MS (EI), m/z [Mþ]: 646.5, calcd: 646.3.
Mp ¼ 138–140 ꢁC. 1H NMR (CDCl3)
d
(ppm): 1.23 (t, J ¼ 7.5 Hz, 6H,
C37H38N6O5 (EA) (%, found/calcd): C, 68.20/68.71; H, 6.17/5.92; N,
–CH3), 3.44 (q, J ¼ 7.2 Hz, 4H, –N–CH2–), 4.01 (br, s, 2H, –O–CH2–),
4.36 (t, J ¼ 6.9 Hz, 2H, –O–CH2–), 6.62 (dd, J ¼ 1.5, 8.7 Hz, 1H, ArH),
6.97 (s, 1H, ArH), 7.77 (m, 2H, ArH), 7.94 (m, 2H, ArH).
12.62/12.99. UV–vis (DMF, 2.5 ꢂ10ꢃ5 mol/L): lmax: 515 nm; 3max
:
2.68 ꢂ 104 molꢃ1 L cmꢃ1
.
2.7. Synthesis of polymer P1
2.5. Synthesis of compound 6
PVK-CHO (P0) (27 mg) was dissolved in DMF (2 mL), then
a solution of chromophore 7 (105 mg, 0.16 mmol) dissolved in THF
(2 mL) and a catalytic amount of piperidine were added under an
atmosphere of dry nitrogen. The reaction mixture was stirred at
45 ꢁC for 48 h, then dropped into methanol. The obtained precipi-
tate was filtered and washed with methanol for several times. The
resultant product was collected and dried under vaccum at 40 ꢁC
(60 mg, 55.1%). Mw ¼ 7.35 ꢂ105, Mw/Mn ¼ 1.31 (GPC, polystyrene
A mixture of compound 4 (110 mg, 0.26 mmol), 3-N-(n-butane)
carbazole boronic acid (5) (88 mg, 0.30 mmol), sodium carbonate
(276 mg, 2.60 mmol), THF (9 mL)/water (3 mL), and Pd(PPh3)4
(10 mg) was carefully degassed and charged with nitrogen. Then
the reaction mixture was stirred at 80 ꢁC for 30 h. After cooled to
room temperature, the mixture was extracted by CHCl3. The crude
product was purified by chromatography on silica gel using ethyl
acetate/petroleum ether (1/1) as eluent to afford deeply red solid
calibration). IR (thin film),
746, 722 (carbazole), 1519, 1335 (–NO2). 1H NMR (DMSO-d6)
(ppm): 0.7–1.0 (–CH3), 1.0–1.3 (–CH3), 1.4–1.9 (–CH2–), 3.6–3.8
y
(cmꢃ1): 2220 (CN), 1719 (C]O), 1671,
(138 mg, 93.9%). Mp ¼ 167–168 ꢁC. 1H NMR (CDCl3)
d (ppm): 0.98 (t,
d
J ¼ 7.2 Hz, 3H, –CH3) 1.29 (t, J ¼ 6.6 Hz, 6H, –CH3), 1.45 (m, 2H,
–CH2–), 1.92 (m, 2H, –CH2–), 3.53 (q, J ¼ 6.6 Hz, 4H, –N–CH2–), 4.00
(br, s, 2H, –O–CH2–), 4.37 (m, 4H, –N–CH2– and –O–CH2–), 6.74 (dd,
J ¼ 2.7, 9.0 Hz, 1H, ArH), 6.85 (s, 1H, ArH), 7.21 (m, 1H, ArH), 7.47 (m,
4H, ArH), 7.62 (d, J ¼ 8.7 Hz, 1H, ArH), 7.69 (dd, J ¼ 1.5, 9.0 Hz, 1H,
ArH), 7.93 (m, 2H, ArH), 8.06 (d, J ¼ 7.8 Hz, 1H, ArH), 8.23 (s, 1H,
ArH).
(–NCH2–), 4.1–4.4 (–NCH2– and –OCH2–), 4.7–4.9 (–CH2–), 6.7–6.9
(ArH), 7.0–7.2 (ArH), 7.3–7.6 (ArH), 7.7–7.9 (ArH), 8.0–8.3 (ArH).
UV–vis (DMF, 0.01 mg/mL): lmax (nm): 516 nm.
3. Results and discussion
3.1. Synthesis and characterizations
2.6. Synthesis of compound 7
The detailed synthetic procedure of chromophore 7 is presented
in Scheme 1, and the overall route was simple and with moderate
yield. 3-Bromo-bisethyl-aminobenzene (2) was prepared by the
reaction between 3-bromoaniline (1) and bromoethane, similar to
our previous work [26]. Under the normal azo coupling reaction
conditions, chromophore 4 with two functional groups (aryl
bromine and hydroxyl group) was conveniently yielded, which
could react with 3-N-(n-butane) carbazole boronic acid (5) to afford
chromophore 6 with high yield, through Suzuki coupling reaction.
Then, the cyanoacetylated chromophore 7 was prepared success-
fully by the reaction of chromophores 6 with cyanoacetic acid
under mild conditions. Finally, by Knoevenagel condensation
reaction, the PVK-based NLO polymer (P1) was conveniently
obtained between the partially formylated PVK (P0) and the cor-
responding chromophore (7) with the satisfied yield (55.1%).
Chromophore 6 (128 mg, 0.22 mmol), cyanoacetic acid (23 mg,
0.28 mmol), dicyclohexylcarbodiimide (DCC) (71 mg, 0.34 mmol),
4-(N,N-dimethyl)aminopyridine (DMAP) (5 mg, 0.04 mmol) were
dissolved in proper dry CH2Cl2 and stirred at room temperature for
24 h. The precipitate was filtered and the crude product was puri-
fied by column chromatography using ethyl acetate/dichloro-
methane (1/20) as eluent to afford deeply red solid (120 mg, 84.5%).
Mp ¼ 126–128 ꢁC. IR (thin film),
y
(cmꢃ1): 1754 (C]O), 1518, 1331
(–NO2). 1H NMR (CDCl3)
d
(ppm): 0.99 (t, J ¼ 7.2 Hz, 3H, –CH3) 1.30
(t, J ¼ 6.6 Hz, 6H, –CH3), 1.45 (m, 2H, –CH2–), 1.90 (m, 2H, –CH2–),
3.54 (m, 6H, –CH2CN and –N–CH2–), 4.35 (t, J ¼ 7.2 Hz, 2H, –N–CH2–
), 4.48 (t, J ¼ 4.5 Hz, 2H, –O–CH2–), 4.72 (t, J ¼ 4.5 Hz, 2H, –O–CH2–),
6.80 (m, 2H, ArH), 7.22 (m, 1H, ArH), 7.47 (m, 4H, ArH), 7.64 (dd,
J ¼ 1.8, 9.0 Hz, 1H, ArH), 7.69 (dd, J ¼ 2.4, 9.0 Hz, 1H, ArH), 7.85 (d,
J ¼ 2.4 Hz, 1H, ArH), 7.86 (d, J ¼ 9.6 Hz, 1H, ArH), 8.06 (d, J ¼ 7.8 Hz,
1H, ArH), 8.24 (s, 1H, ArH). 13C NMR (CDCl3)
d
(ppm): 13.08, 14.18,
3.2. Structure characterization
20.86, 24.96, 31.45, 43.25, 45.16, 64.90, 68.30, 107.89, 109.13, 110.85,
111.37, 112.41, 113.03, 118.01, 118.10, 118.42, 119.25, 120.42, 122.60,
122.98, 123.22, 126.00, 129.08, 130.53, 140.26, 141.11, 141.65, 147.54,
The chromophores and polymer were characterized by spec-
troscopic methods, and all gave satisfactory spectral data (see

Zhong’an Li et al. / Dyes and Pigments 84 (2010) 134–139
137
Experimental section for detailed analysis data). As shown in Fig. 1,
1.2
1.0
0.8
0.6
0.4
0.2
0.0
the strong absorption band of the formyl groups at 1690 cmꢃ1 in P0
disappeared after the reaction between P0 and chromophore 7, and
the new absorption bands in P1 at about 1719 and 2220 cmꢃ1, were
ascribed to the carbonyl stretching vibration of a conjugated
carboxylic ester and the nitrile stretching vibration respectively,
while the typical absorption peak of nitro groups appeared at about
1519 and 1330 cmꢃ1. All these changes indicated that the chro-
mophore moieties were successfully bonded to the polymer back-
bone. In addition, in the 1H NMR spectra of P1, the absorption peak
of the aldehyde groups disappeared, proving the almost complete
conversion of the formyl groups of P0 to chromophore-function-
alized PVK-based polymer, due to high reactivity of the formyl
groups towards the active methylene species in the cyanoacetate
via the Knoevenagel condensation reaction.
P0
P1
7
P1 was well soluble in common polar organic solvents, such as
DMF, DMSO, and NMP, since the introduced isolation groups
suppressed the polymer inter-chain entanglements. The UV–vis
absorption spectra of P0, P1 and the corresponding chromophore
(7) in DMF solutions are shown in Fig. 2. The polymers exhibited
weak absorption bands of the carbazole group at about 324 nm,
and the maximum absorption wavelength of the azo moieties was
at about 515 nm (listed in Experimental section). Moreover, the
maximum absorption wavelength at 407 nm should be attributed
to the charge transfer of [2-cyano-3-[9-alkyl substituted] carba-
zolyl]-acrylate chromophore, indicating the success of polymer
reaction.
270
320
370
420
470
520
570
620
670
Wavelength (nm)
Fig. 2. UV–vis spectra of N,N-dimethylformamide (DMF) solutions of polymers P0–1
(0.02 mg/mL) and chromophores 7 (2.5 ꢂ10ꢃ5 mol/L).
Calculation of the SHG coefficients (d33) for the poled films is based
The molecular weights of polymer were determined by gel
permeation chromatography (GPC) with DMF as an eluent, with
polystyrene standards as calibration standards, and the Mw was
7.35 ꢂ105 and Mw/Mn was 1.31. As shown in Fig. 3, the thermal
stability, determined by TGA, displayed that the polymer was
thermal stable up to 235 ꢁC, and the glass transition temperature of
P1 was tested to be 116 ꢁC using a Setaram differential scanning
calorimeter (DSC). Compared to our other PVK-based polymers, P1
exhibited relatively low glass transition temperature, the possible
reason should be that this shape of chromophores created much
more free volume, which decreased the rigidity of PVK backbone in
some degree.
on the equation given below:
sffiffiffiffi
d33;s
d11;q
Islc;q
Iq ls
¼
F
(1)
where d11,q is d11 of the quartz crystals, which is equal to 0.45 pm/V.
Is and Iq are the SHG intensities of the sample and the quartz,
respectively, lc,q is the coherent length of the quartz, ls is the
thickness of the polymer film, and F is the correction factors of the
apparatus and equals to 1.2 when lc >> ls. From the experimental
data, the d33 value of P1 was calculated to be 56.0 pm/V at funda-
mental wavelength of 1064 nm. And by using the approximation
two-level model, the nonresonant d33 value was estimated to be
3.0 pm/V. In comparison with our reported PVK-based polymers
[33,34], here, P1 exhibited relatively high NLO effect, related to the
isolation effects and its unique approach of the chromophore
attaching into polymer-chain. Firstly, carbazole groups were
3.3. NLO properties
To evaluate the NLO activity of the polymer, its poled thin films
were prepared. The convenient technique to study the second-
order NLO activity is to investigate the second harmonic generation
(SHG) processes characterized by d33
,
an SHG coefficient.
100
P0
75
CHO
50
CN
P1
C=O
-NO
2
25
4000
3000
2000
1600
1200
800
40
140
240
340
440
Wavenumber cm-1
Temperature (°C)
Fig. 1. IR spectra of polymers P0 and P1.
Fig. 3. DSC and TGA curves of P1 at a heating rate of 10 ꢁC/min under N2 atmosphere.

138
Zhong’an Li et al. / Dyes and Pigments 84 (2010) 134–139
Acknowledgements
2.4
2.0
1.6
1.2
0.8
0.4
0.0
We are grateful to the National Science Foundation of China (No.
20674059), the Program for NCET, the National Fundamental Key
Research Program and Hubei Province for financial support.
P1
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¼ 1 ꢃ A1=A0
(2)
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F
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