New second-order nonlinear optical polyphosphazenes: Convenient postfunctionalization synthetic approach and application of the concept of suitable isolation group

Article abstract of DOI:10.1016/j.dyepig.2009.09.008

The concept of suitable isolation group was applied to the synthesis of nonlinear optical polyphosphazenes that contained different isolation groups attached to the side chains of polyphosphazenes using a post-functional strategy. The loading density of NLO chromophore moieties in the polymers was relatively high (0.60 per unit) and all polymers displayed good processability and relatively good NLO effects, as exemplified by an SHG coefficient (d₃₃) of 67.3 pm V^-1 recorded for the polyphosphazene that contained the largest isolation group.

Full text of DOI:10.1016/j.dyepig.2009.09.008

Dyes and Pigments 84 (2010) 229–236
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New second-order nonlinear optical polyphosphazenes: Convenient
postfunctionalization synthetic approach and application
of the concept of suitable isolation group
,
Qi Zeng a, Guofu Qiu b, Cheng Ye c, Jingui Qin a, Zhen Li a
*
a Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China
b College of Pharmacy, Wuhan University, Wuhan 430072, China
c Organic Solids Laboratories, Institute of Chemistry, The 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:
The concept of suitable isolation group was applied to the synthesis of nonlinear optical poly-
phosphazenes that contained different isolation groups attached to the side chains of polyphosphazenes
using a post-functional strategy. The loading density of NLO chromophore moieties in the polymers was
relatively high (0.60 per unit) and all polymers displayed good processability and relatively good NLO
effects, as exemplified by an SHG coefficient (d33) of 67.3 pm Vꢀ1 recorded for the polyphosphazene that
contained the largest isolation group.
Received 24 June 2009
Received in revised form
11 September 2009
Accepted 14 September 2009
Available online 1 October 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Polyphosphazene
Nonlinear optics
Postfunctionalization
Azo coupling
Suitable isolation groups
Structural modification
1. Introduction
chromophore moieties and glass transition temperature (Tg), have
been reported [26–32]. As a promising candidate for NLO poly-
Polyphosphazenes possess a unique inorganic backbone based
on a repeating unit of alternating phosphorus and nitrogen atoms,
bearing two substituent units on each phosphorous. Since Allcock
et al. [1] synthesised the first soluble and stable polyphosphazene
in 1965, literally, hundreds of divers examples of such compounds
have been reported and their manifold applications as functional
polymeric materials, such as fire-resistant polymers, biomedical
materials, responsive membranes, battery electrolytes, fuel cell
membranes, superhydrophobic surfaces, as well as nonlinear
optical (NLO) polymers, reported [2–12].
Second-order nonlinear optical materials have attracted much
attention owing to their potential application in photonics
because of their characteristic large NLO coefficients, ultrafast
response time and ease of integration [13–24]. In 1991, a second-
order nonlinear optical (NLO) chromophore, Dispersed red-1
(DR-1), was attached to the backbone of polyphosphazene by
Allcock et al. [25]. Improvement in the properties of NLO poly-
phosphazene, including NLO coefficient, loading density of NLO
meric materials, polyphosphazenes offer many advantages, such
as high thermo-oxidative and photochemical stability and trans-
parency of the backbone over the range, 200 nm to near-IR;
additionally, the two reactive sites on each repeat unit enable
good synthesis flexibility. However, the improvement in NLO
properties achieved thus far has not been significant enough to
satisfy all required NLO applications because of two reasons
namely, the difficulty of preparing soluble NLO polyphosphazenes
of high loading density of NLO chromophore moieties and
synthesis difficulties caused by strong dipole–dipole interactions
between the highly polar chromophoric moieties in the polymeric
system. Recent research has demonstrated that according to the
site isolation principle, the NLO effect of polymeric materials can
be improved dramatically by the introduction of isolation spacers
to the azo chromophores, which weaken the intermolecular
dipole–dipole interactions between the chromophoric moieties
[33–36]. The current authors have prepared different kinds of
NLO polymers, in which the size of the isolation groups ranged
from small to larger [37–49]. It was found that the macroscopic
nonlinearity of polymers could be boosted greatly by introducing
‘‘suitable isolation group’’. Interestingly, our recent research has
* 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.2009.09.008

230
Q. Zeng et al. / Dyes and Pigments 84 (2010) 229–236
revealed that the both the solubility and processibility of the
polymers can be improved after isolation groups have been
introduced [47,48].
with water; contact with water liberates flammable gases; explo-
sive when mixed with oxidizing substances; may form explosive
peroxides) (0.64 g, 5.5 mmol) in 80 mL of THF, and the mixture was
stirred at 60 ꢁC for 2 days. Then NaOCH2CH3 (0.79g, 11.6 mmol) was
added, and the resultant mixture stirred at 60 ꢁC for another 2 days.
The mixture was poured into water (400 mL), and the white solid
was filtered, washed with water, and air-dried. The solid was dis-
solved in THF, and the insoluble residue was filtered out. The filtrate
was evaporated to remove the bulk of THF. Then the solid was
isolated and further purified by several precipitations from THF into
methanol. The solid was vacuum-dried at 40 ꢁC to yield the product
(1. 20 g, 74.0%). Mw ¼ 1 460 000, Mw/Mn ¼ 2.73 (GPC, polystyrene
It was therefore considered that the introduction of isolation
groups within the side chain of polyphosphazenes may solve the
two problems mentioned above, namely, enhance macroscopic
nonlinearity and improve solubility. This paper concerns the
synthesis of a series of NLO polyphosphazenes (P2–4), that bear
NLO chromophoric side chains with different isolation spacers,
using a postfunctionalization strategy. The loading density of the
chromophore moieties in the polymers was as high as 0.60 per unit,
a level of substitution that could not easily be realised using the
conventional direct approach for the preparation of functional
polyphosphazenes. All polymers were soluble in polar organic
solvents and demonstrated good processability. Second harmonic
generation (SHG) experiments confirmed that after the introduc-
tion of isolation groups, the SHG coefficients (d33) of P2–P4
increased from 33.2 to 67.3 pm Vꢀ1, in accordance with an increase
in the size of the bound isolation group.
calibration). 1H NMR (CDCl3):
d (ppm): 0.3–1.1 (–CH3), 2.5–3.5
(–OCH2– and –NCH2–), 3.5–4.2 (–OCH2–), 6.0–6.6 (ArH), 6.6–7.2
(ArH).
2.4. Synthesis of P1
P0 (0.70 g) was dissolved in N-methylpyrrolindone (NMP,
3.4 mL), and then 2-(2-hydroxyethoxy)-4-nitrobenzenediazonium
fluoroborate (caution: author needs to add cautionary advice here)
(0.71 mg, 2.4 mmol) was added under cooling with an ice bath. The
color of the solution changed to red immediately. After stirring for
8 h at 0 ꢁC, excess anhydrous potassium carbonate was added, and
the mixture was stirred for additional 1 h and then filtered. The
residue was washed with DMF, the filtrates were collected, and
DMF was removed under reduced pressure. Some methanol was
added dropwise to precipitate the polymer. The solid was further
purified by several precipitations from DMF into methanol. The
solid was dried in a vacuum at 40 ꢁC to yield a red product P1
2. Experimental
2.1. Instrumentation
1H and 13C NMR spectra were measured on
Mercury300 spectrometer using tetramethylsilane (TMS;
¼ 0 ppm) as internal standard. The Fourier transform infrared
a Varian
d
(FTIR) spectra were recorded on a PerkinElmer-2 spectrometer in
the region of 3000–400 cmꢀ1 on NaCl pellets. UV–visible spectra
were obtained using a Schimadzu UV-2550 spectrometer. GPC
analysis was performed on an Agilent 1100 series HPLC system
and a G1362A refractive index detector. Polystyrene standards
were used as calibration standards for GPC. THF was used as an
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 CARLOERBA-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 thermogravimetric 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 thick-
ness of the films was measured with an Ambios Technology XP-2
profilometer.
(0.92 g, 89%). 1H NMR (DMSO-d6):
(–CH2O), 3.8–4.2 (–CH2O, –NCH2–), 6.2–6.8 (ArH), 6.8–7.0 (ArH),
7.0–7.3 (ArH), 7.3–7.6 (ArH), 7.6–7.9 (ArH). IR (thin film),
(cmꢀ1):
1339 (–NO2). UV–vis (DMF, 0.02 mg/mL): lmax (nm): 480 (nm). IR
(thin film),
(cmꢀ1): 1339 (–NO2).
d (ppm): 0.4–1.1 (–CH3), 3.4–3.8
y
y
2.5. General procedure for the synthesis of P2–P4
P1 (220 mg, 1.00 equiv) was dissolved in DMF (2 mL), then
a solution (2 mL) of compound 2 or 3 or 4 (10.0 equiv), 4-(N,N-
dimethyl)aminopyridine (DMAP) (caution: toxic; incompatible
with acids and oxidants) (10.0 equiv) and dicyclohexylcarbodiimide
(DCC) (caution: moisture sensitive; combustible; incompatible
with strong oxidizing agents; avoid exposure to air or moisture)
(10.0 equiv) was added. The resultant mixture was stirred at room
temperature for 72 h under an atmosphere of dry nitrogen and then
filtered to remove the insoluble solid. The filtrate was added
dropwise to methanol to precipitate the polymer, which was
further purified by several precipitations from THF into methanol,
and dried in a vacuum to a constant weight.
2.2. Materials
Tetrahydrofuran (THF) was dried over and distilled from K–Na
alloy under an atmosphere of dry nitrogen. N-Methylpyrrolidone
(NMP; caution: decomposes upon exposure to light; combustible;
incompatible with strong oxidizing agents, strong acids, reducing
agents, bases) was dried over and distilled from CaH2 under an
atmosphere ofdry nitrogen. Compounds9 were obtainedasreported
in our previous work [49]. All other reagents were used as received.
P2: red powder (196 mg, 78.2%). Mw ¼ 8 570 000, Mw/Mn ¼ 2.08
(GPC, polystyrene calibration). 1H NMR (DMSO-d6):
d (ppm):
0.4–1.1 (–CH3), 3.4–4.2 (–NCH2–), 4.2–4.7 (–OCH2CH2–), 6.2–6.8
(ArH), 6.8–7.0 (ArH), 7.0–7.3 (ArH), 7.3–7.6 (ArH), 7.6–7.9 (ArH). IR
(thin film), y
(cmꢀ1): 1340 (–NO2). UV–vis (DMF, 0.02 mg/mL): lmax
(nm): 480 (nm).
2.3. Synthesis of P0
P3: red powder (201 mg, 75.8%). Mw ¼ 5 260 000, Mw/Mn ¼ 1.11
(GPC, polystyrene calibration). 1H NMR (DMSO-d6):
d (ppm): 0.4–
N-Ethyl-N-hydroxyethylaniline (1.91 g, 11.6 mmol) reacted with
sodium hydride (caution: corrosive; reacts violently with water,
liberating hydrogen; incompatible with water, acids, alcohols,
strong oxidizing agents) (0.46 g, 60% in paraffine, 11.6 mmol) in THF
(20 mL) at 60 ꢁC overnight, then the resultant solution of the
sodium salt of compound 1 was added to a solution of poly(di-
chlorophosphazene) (caution: toxic; flammable; reacts violently
1.1 (–CH3), 3.4–4.2 (–NCH2–), 4.2–4.8 (–OCH2CH2–), 6.0–6.6 (ArH),
6.8–7.4 (ArH), 7.4–8.2 (ArH), 8.4–8.7 (ArH). IR (thin film),
y
(cmꢀ1):
1338 (–NO2). UV–vis (DMF, 0.02 mg/mL): lmax (nm): 480 (nm).
P4: red powder (245 mg, 87.2%). Mw ¼ 6 300 000, Mw/Mn ¼ 3.42
(GPC, polystyrene calibration). 1H NMR (DMSO-d6):
d (ppm): 0.4–1.1
(–CH3), 3.4–4.2 (–NCH2–), 4.2–4.8 (–OCH2CH2–), 4.8–5.2 (–CH2N<),
6.0–6.6 (ArH), 6.8–7.3 (ArH), 7.3–7.7 (ArH), 7.7–8.0 (ArH). IR (thinfilm),

Q. Zeng et al. / Dyes and Pigments 84 (2010) 229–236
231
y
(cmꢀ1): 1338 (–NO2). UV–vis (DMF, 0.02 mg/mL): lmax (nm): 480
(d, J ¼ 8.7 Hz, 1H, ArH), 7.91 (m, 5H, ArH), 8.22 (d, J ¼ 6.6 Hz, 1H,
(nm).
ArH), 8.94 (d, J ¼ 7.5 Hz, 1H, ArH). 13C NMR (CDCl3)
d (ppm): 12.8,
45.0, 63.3, 68.9, 107.8, 110.9, 111.2, 117.7, 124.8, 126.1, 126.4, 126.8,
128.1, 128.7, 130.8, 131.6,133.7, 134.0, 144.3, 148.0,151.3, 155.0, 167.6.
(EI), m/z [Mþ]: 512.4, calcd: 512.2. C29H28N4O5 (EA) (%, found/
calcd): C, 68.13/67.96; H, 5.71/5.51; N, 10.99/10.93.
2.6. Synthesis of chromophore 5
9 (327 mg, 1.65 mmol) were dissolved in an aq solution of 35%
hydrochloric acid. The mixture was cooled to 0–5 ꢁC in an ice bath,
and then a solution of sodium nitrite (114 mg) in water was added
dropwise. After stirred below 5 ꢁC for 15 min, a solution of N,
N-diethylaniline (224 mg, 1.5 mmol) in ethanol was added slowly.
The mixture was stirred in the ice bath for another 1 h, some
sodium bicarbonate was added to adjust the pH value to 7.0. The
reaction mixture was further stirred for 0.5 h, the precipitate was
filtered, washed with water. The crude product was purified by
column chromatography on silica gel using ethyl acetate/chloro-
form (1/4) as an eluent to afford red powder (495 mg, 92.1%). 1H
Chromophore 8. 5 (64 mg, 0.18 mmol), 4 (81 mg, 0.36 mmol).
Purified by column chromatography on silica gel using dichloro-
methane as an eluent to afford red powder (93 g, 91.9%). IR (thin
film),
y d (ppm): 1.24
(cmꢀ1): 1339 (–NO2). 1H NMR (CDCl3)
(t, J ¼ 5.1 Hz, 6H, –CH3), 3.47 (d, J ¼ 6.6 Hz, 4H, –NCH2–), 4.38
(t, J ¼ 4.5 Hz, 2H, –OCH2–), 4.65 (t, J ¼ 5.1 Hz, 2H, –CH2CO–), 5.04
(s, 2H, –CH2N<), 6.68 (d, J ¼ 6.6 Hz, 2H, ArH), 7.30 (m, 7H, ArH), 7.75
(d, J ¼ 2.4 Hz, 1H, ArH), 7.87 (m, 4H, ArH), 8.04 (d, J ¼ 8.1 Hz, 1H,
ArH). 13C NMR (CDCl3)
d (ppm): 12.8, 29.9, 44.9, 45.1, 63.6, 68.5,
108.5, 110.6, 111.3, 117.7, 120.0, 120.6, 123.4, 126.1, 126.8, 140.7, 144.3,
147.8, 147.9, 151.5, 154.7, 168.6. (EI), m/z [Mþ]: 565.4, calcd: 565.2.
C32H31N5O5 (EA) (%, found/calcd): C, 68.31/67.95; H, 5.98/5.52; N,
12.41/12.38.
NMR (CDCl3)
d
(ppm): 1.24 (t, J ¼ 6.6 Hz, 3H, –CH3), 3.47
(d, J ¼ 6.6 Hz, 4H, –NCH2–), 3.97 (t, J ¼ 5.4 Hz, 2H, –CH2OH–), 4.34
(t, J ¼ 4.5 Hz, 2H, –OCH2–), 6.71 (d, J ¼ 9.3 Hz, 2H, ArH), 7.69
(d, J ¼ 9.6 Hz, 1H, ArH), 7.83 (d, J ¼ 8.7 Hz, 2H, ArH), 7.91 (d,
J ¼ 3.9 Hz, 2H, ArH). MS (EI), m/z [Mþ]: 358.3 calcd: 358.2.
2.8. Preparation of polymer thin films
The polymers were dissolved in THF (concentration w3 wt%)
and the solutions were filtered through syringe filters. Polymer
films were spin-coated onto indium-tin-oxide (ITO)-coated glass
substrates, which were cleaned by DMF, acetone, distilled water
and THF sequentially in ultrasonic bath before use. Residual solvent
was removed by heating the films in a vacuum oven at 40 ꢁC.
2.7. General procedure for synthesis of chromophores 6–8
5 (1.00 equiv), the acid (2, 3 or 4) (2.00 equiv), dicyclohex-
ylcarbodiimide (DCC) (2.00 equiv), 4-(N,N-dimethyl)aminopyridine
(DMAP) (0.20 equiv) were dissolved in dry dichloromethane, then
stirred at room temperature for 20 h. the precipitate was filtered,
and the crude product was purified by column chromatography.
Chromophore 6. 5 (64 mg, 0.18 mmol), 2 (44 mg, 0.36 mmol).
Purified by column chromatography on silica gel using dichloro-
methane as an eluent to afford red powder (82 mg, 99.0%). IR (thin
2.9. NLO measurement of poled films
The second-order optical nonlinearity of the polymers was deter-
mined by in-situ second harmonic generation (SHG) experiment using
a closed temperature-controlled ovenwith opticalwindows and three
needle electrodes. The films were kept at 45ꢁ to the incident beam and
poled inside the oven, and the SHG intensity was monitored simul-
taneously. Poling conditions were as follows: temperature: different
for each polymer (Table 1); voltage: 7.0 kV at the needle point; gap
distance: 0.8 cm. The SHG measurements were carried out with
a Nd:YAG laser operating at a 10 Hz repetition rate and an 8 ns pulse
width at 1064 nm. A Y-cut quartz crystal served as the reference.
film),
y d (ppm): 1.24
(cmꢀ1): 1339 (–NO2). 1H NMR (CDCl3)
(t, J ¼ 7.2 Hz, 6H, –CH3), 3.46 (d, J ¼ 6.6 Hz, 4H, –NCH2–), 4.61
(t, J ¼ 4.5 Hz, 2H, –OCH2–), 4.79 (t, J ¼ 5.1 Hz, 2H, –CH2CO–), 6.66
(d, J ¼ 7.1 Hz, 2H, ArH), 7.39 (m, 3H, ArH), 7.54 (m, 1H, ArH), 7.71
(d, J ¼ 8.7 Hz, 1H, ArH), 7.84 (d, J ¼ 8.7 Hz, 2H, ArH), 7.89 (d,
J ¼ 7.2 Hz,1H, ArH), 7.98 (s,1H, ArH), 8.06 (d, J ¼ 7.5 Hz,1H, ArH). 13C
NMR (CDCl3) d (ppm): 12.9, 45.1, 63.4, 69.1, 111.3, 117.8, 126.8, 128.6,
130.0, 133.3, 144.3, 148.0, 148.1, 151.4, 155.0, 166.7. (EI), m/z [Mþ]:
462.4, calcd: 462.2. C25H26N4O5 (EA) (%, found/calcd): C, 65.06/
64.92; H, 5.25/5.67; N, 12.01/12.11.
Chromophore 7. 5 (64 mg, 0.18 mmol), 3 (62 g, 0.36 mmol).
Purified by column chromatography on silica gel using dichloro-
methane as an eluent to afford red powder (90 mg, 98.1%). IR (thin
3. Results and discussion
3.1. Synthesis
film),
y d (ppm): 1.21
(cmꢀ1): 1340 (–NO2). 1H NMR (CDCl3)
(t, J ¼ 5.4 Hz, 6H, –CH3), 3.42 (d, J ¼ 7.2 Hz, 4H, –NCH2–), 4.66
(t, J ¼ 4.5 Hz, 2H, –OCH2–), 4.90 (t, J ¼ 4.5 Hz, 2H, –CH2CO–), 6.57
(d, J ¼ 9.6 Hz, 2H, ArH), 7.44 (m, 2H, ArH), 7.52 (m, 2H, ArH), 7.73
The synthetic route to polyphosphazenes was shown in Scheme 1.
2-(2-Hydroxyethoxy)-4-nitrobenzenediazonium fluoroborate was
prepared by the similar method as reported previously [50].
Table 1
Polymerization results and characterization data.
no.
Yield (%)
Mw
Mw/Mn
lmaxb (nm)
lmaxc (nm)
Tf (ꢁC)
ls (mm)
d33h (pm/V)
d33 (N) (pm/V)
a
a
d
Tg (ꢁC)
e
Td (ꢁC)
g
i
P2
P3
P4
78.2
75.8
87.2
8 570 000
5 260 000
6 300 000
2.08
1.11
3.42
471 (495)
472 (496)
472 (496)
480 (506)
480 (508)
480 (508)
63
70
91
250
242
261
56.7
68.2
99.5
0.46
0.38
0.32
33.2
40.6
67.3
4.92
6.03
9.97
a
Determined by GPC in THF on the basis of a polystyrene calibration.
The maximum absorption wavelength of polymer solutions in THF, while the maximum absorption wavelength of the corresponding small chromophore molecules in
b
diluted THF solutions are given in the parentheses.
c
The maximum absorption wavelength of polymer solutions in DMF, while the maximum absorption wavelength of the corresponding small chromophore molecules in
diluted DMF solutions are given in the parentheses.
d
Glass transition temperature (Tg) of polymers detected by the DSC analyses under nitrogen at a heating rate of 10 ꢁC/min.
e
the 5% weight loss temperature of polymers detected by the TGA analyses under argon at a heating rate of 20 ꢁC/min.
f
the best poling temperature.
film thickness.
Second harmonic generation (SHG) coefficient.
The nonresonant d33 values calculated by using the approximate two-level model.
g
h
i

232
Q. Zeng et al. / Dyes and Pigments 84 (2010) 229–236
ONa
N
OH
+
N
1
NaH
Cl
P
CH3CH2ONa
1
P
N
P N
N
P0
n
b
a
n
a=0.79
b=0.21
O
OCH2CH3
Cl
N
P
N
P
N
P
N
P
N
P
N
P N
c
b
n
c
d
b n
d
O
O
OCH2CH3
O
O
OCH2CH3
N
N
N
N
2-4)
RCOOH(
DCC, DMAP
N2BF4
O(CH2)2OH
P2: R=Bz
P3: R=Np
2: R=
(Bz)
(Np)
(Cz)
P4
: R=Cz
N
N
N
N
O
c=0.30
d=0.49
b=0.21
O
O
3
: R=
NO2
O
R
OH
N
NO2
NO2
P1
4: R=
Scheme 1.
Poly(dichlorophosphazene), a highly reactive macromolecular inter-
mediate, wasprepared by the thermal ring-opening polymerization of
phosphonitrile chloride trimer [1]. P0 was obtained by nucleophilic
substitution reaction from Poly(dichlorophosphazene). As the aniline
group could not react with all the chlorine atoms because of the steric
shielding effect [51], an excess of NaOCH2CH3 was added into the
reaction mixture to replace all the remaining chlorine atoms
completely at the end of the substitution reaction.
Through post-azo coupling reaction, the azobenzene struc-
ture in the side chain of polyphosphazene P1 was constructed
efficiently. Different from our previous work, a reactive group-
hydroxyl was introduced into the side chain of polyphosphazene
for the further functionalization of the chromophore moieties,
which could be utilized in the following esterification between
P1 and different carboxylic acids, including benzoic acid (Bz),
1-naphthoic acid (Np) and N-carbazolylacetic acid (Cz). These
acids were selected as the isolation moieties with different bulk
sizes to modify the azobenzene side chains of P1. The two-step
post-functional polymer reactions, post-azo coupling and
esterification reaction, underwent smoothly under very mild
conditions while the purification procedure was simple, which
was another proof that the post-functional strategy could be an
efficient method to modify the polymer structure.
It should be pointed out that the loading density of the chro-
mophore moieties in these polyphosphazenes was really high. As
mentioned in the introduction part, in 1991, DR-1 was first linked to
the phosphorus–nitrogen backbone for the synthesis of new
nonlinear optical (NLO) polymeric materials, and the loading of DR-
1 was 33% [25]. As the loading of DR-1 was relatively low, the
synthetic procedure was not easy by a direct reaction, Allcock et al.
improved their synthetic methods to raise the loading up to 46% via
reactions that involved functionalized poly(organophosphazene)s.
However, soluble polyphosphazenes with higher percentages of
DR-1 moieties could not be obtained by this method. They ascribed
these results to the intrinsically high polarity of the donor–acceptor
structure of DR-1, the low solubility of the sodium salt of the DR-1
alcohol, and the close packing or stacking of rigid aromatic side
groups [25,29]. In 2003, we developed a convenient approach to
a polyphosphazene with a high loading density of DR-1 moieties,
demonstrating the power and flexibility of the postfunctionaliza-
tion strategy [31]. Here, through the combination of two post-
functional reactions, post-azo coupling and post esterification
reactions, soluble polyphosphazenes P2–4 were conveniently
obtained, which bear high loading density of chromophore moieties
in the side chains. This further confirmed that the post-functional
strategy could be an alternative approach to functional polymers, in
N
N
6: R=Bz
7: R=Np
8: R=Cz
NH2
N
O(CH2)2OH
2-4)
RCOOH(
+
N
DCC, DMAP
N
N
N
O
O
O
NO2
9
OH
O
R
NO2
NO2
5
Scheme 2.

Q. Zeng et al. / Dyes and Pigments 84 (2010) 229–236
233
addition to the normal synthetic route of the polymerization of the
corresponding monomers.
Also, we prepared three model compounds having the similar
structure with those attached to the polyphosphazene backbone
(Scheme 2). Thus, a comparative study on some properties could be
performed between the model compounds and polymers.
3.2. Structural characterization
In the IR spectra of P0–4 (Fig. 1), the 1250–1200 cmꢀ1 bands
were attributed to an intense P]N stretching vibration and the
750 cmꢀ1 band to an in-phase P-N-P stretch. In comparison with
P0, P1–4 showed a new strong absorption band at 1338 cmꢀ1
,
which should be ascribed to the absorption of nitro groups, con-
firming the successful introduction of nitro groups into the side
chain of polyphosphazenes through the post-azo coupling reaction.
As for all the polymers, the chemical shifts in corresponding 1H
NMR spectra verified the proposed structure demonstrated in
Scheme 1. The molar concentration of chromophore moieties in P1
could be calculated by analyzing the 1H NMR peak integration. And
its UV–vis spectra gave the similar result. The new absorption peak
in the range of 7.3–7.9 ppm should be ascribed to the nitrobenzene
moieties. The absorption peak at 4.8–5.0 ppm might be assigned to
the hydroxyl group due to its disappearance during heavy water
exchange. After the esterification reaction, the absorption peak of
the ethylene groups between two oxygen atoms had a downfield
shift. For example, in P3 (Fig. 2), the absorption peak at 3.67 ppm
shifted to 4.47 ppm and the peak of the hydroxyl group dis-
appeared, indicating that the hydroxyl group reacted with the
added acid completely. Compared with P0, the new absorption
peaks in P2–4 at 4.0–5.0 ppm and downfield should be ascribed to
the nitrobenzene moieties, which were consistent with 1H NMR
spectra of chromophore 6–8 (Figs. 3 and 4).
Fig. 2. 1H NMR spectra of P1 (A) and P3 (B) in DMSO-d6. The solvent peaks are marked
with asterisks (*).
difference should be caused only by the different size of the isola-
tion groups. In comparison with their corresponding model
compounds, the maximum absorption wavelengths of P2–4 were
blue shifted, which might be due to the electronic interaction
between the polymer backbone and the NLO chromophores in side
chains.
The molecular weights of polymers were determined by gel
permeation chromatography (GPC) with THF as an eluent and
polystyrene standards as calibration standards. All the results were
summarized in Table 1, and most of the polymers possessed similar
molecular weights, which would perhaps facilitate the comparison
of their properties on the same level. Though the molecular weights
were high, P2–4 still showed good solubility in common polar
organic solvents such as THF, DMF, and DMSO. The excellent film-
forming property of them was beneficial to be spin-coated into thin
solid films with proper thickness for the following NLO properties
measurement. Their TGA thermograms were shown in Fig. 6, with
the 5% weight loss temperatures of polymers listed in Table 1. P4
exhibited better thermal stability than other polymers, indicating
that the introduction of the carbazolyl group to the chromophore
would benefit its stability against heating. The glass transition
temperatures (Tg) of the polymers were investigated using
a differential scanning calorimeter (Table 1). The glass transition
temperatures of P2–4 were low due to the flexible backbone of
polyphosphazenes.
Polymers P2–4 were soluble in common polar organic solvents
such as THF, DMF, and DMSO, much better than P1, which could
hardly dissolve in THF. This should be ascribed to the introduction
of the isolation groups, as previous cases and confirming the
important role the bonded isolation group again. Fig. 5 showed UV–
vis spectra of P2–4 in DMF, and the maximum absorption wave-
lengths for the p–p* transition of the azo moieties at 480 nm were
nearly unchanged for all three polymers, which means that the
introduction of isolation groups did not affect the push-pulls
structure of the azobenzene chromophore moieties. That is to say,
the electronic properties of the chromophore moieties in poly-
phosphazenes were just the same before and after the attachment
of the isolation groups. So for the NLO properties of P2–4, the
3.3. NLO properties
The NLO activity of the polymers were measured similar as we
reported previously [37–46,49], which was studied by investigating
P0
P1
P3
-NO
2
4000
3000
2000
Wavenumber (cm-1)
1600
1200
800
Fig. 3. 1H NMR spectra of P0 (A) in CDCl3 and P2–4 (B, C, D) in DMSO-d6. The solvent
Fig. 1. IR spectra of polymers P0, P1 and P3.
peaks are marked with asterisks (*).

234
Q. Zeng et al. / Dyes and Pigments 84 (2010) 229–236
100
90
80
70
60
P2
P3
P4
Fig. 4. 1H NMR spectra of 6–8 (A, B, C) in CDCl3. The solvent peaks are marked with
50
100
150
200
250
300
350
asterisks (*).
Temperature (oC)
Fig. 6. TGA thermograms of P2–P4 measured in argon at a heating rate of 10 ꢁC/min.
the second harmonic generation (SHG) processes characterized by
d33, an SHG coefficient. And from the experimental data, the d33
values of P2–P4 were calculated at the 1064 nm fundamental
wavelength (Table 1). The NLO properties of all the polymers were
tested at the same time to avoid systematic errors.
D
E
ꢀ
ꢁ
1
2
2
u
d33
¼
N
b
f 2 f u
cos3
q
(1)
where N is the number density of the chromophore,
b
is its first
To show the NLO results more visually, we compared the d33
values of the polymers using that of P2 as reference in Fig. 7. As
expected, the d33 values for all of three polymers were much higher
than previous ones, which should be benefit from the introduction
of isolation groups to interrupt the intermolecular dipole–dipole
interactions in some degree. From P2 to P4, the size of isolation
group enlarged gradually, while the d33 values increased from 33.2
pm/V for P2 to 67.3 pm/V for P4. As discussed above, the similar
maximum absorption wavelengths in UV–vis spectra indicated the
similar electronic properties of the chromophore moieties in pol-
yphosphazenes. Thus the different NLO properties of the polymers
should be caused by difference among the isolation groups, espe-
cially related to their size. However, the introduction of different
isolation groups would surely lead to the different molar weights of
the obtained chromophores. According to the one-dimensional
rigid orientation gas model: [52]
hyperpolarizability, f is the local field factor, 2
u is the double
frequency of the laser, q>
u
is its fundamental frequency, and <cos3
is the average orientation factor of the poled film. Here, the active
NLO chromophore moieties are the same, which exhibit the same
first hyperpolarizability (b), thus under identical experimental
conditions, d33 should be proportional to the number density of the
chromophore moieties in the polymers. Therefore, we considered
the different concentrations of the active chromophore moieties in
the polymers, using the tested d33 values dividing the concentra-
tion of the active chromophore moieties in the polymers, and
comparing the results again with that of P2 as references. As
a result, the same trend between curve A and curve B could be
found, which coincided well with the site isolation principle. Also,
we calculated the d33 (N) values of P2–4 by using the approximate
A
B
C
1.2
P2
P3
2.0
1.5
1.0
P4
0.9
0.6
0.3
0.0
Cz
Bz
Np
Fig. 7. The analysis of the obtained NLO data of polymers P2–P4. A: comparison of the
d33 values of the polymers; B: comparison of the calculated d33 values, which were
obtained by using the tested d33 values dividing the concentration of the active
chromophore moieties of the polymers; C: comparison of the calculated d33 (N) values
according to the approximate two-level model, using P2 for P2–P4 as a reference.
400
450
500
550
600
650
Wavelength (nm)
Fig. 5. UV–Vis spectra of DMF solutions of P2–P4 at the concentration of 0.02 mg/mL.

Q. Zeng et al. / Dyes and Pigments 84 (2010) 229–236
235
[13] Lee M, Katz HE, Erben C, Gill DM, Gopalan P, Heber JD, et al. Broadband
modulation of light using an electro-optic polymer. Science 2002;298:
1401–3.
[14] Shi Y, Zhang C, Zhang H, Bechtel JH, Dalton LR, Robinson BH, et al. Low half-
wave voltage polymer electro-optic modulators achieved by controlling
chromophore shape. Science 2000;288:119–22.
[15] Burland DM, Miller RD, Walsh CA. Second-order nonlinearity in poled-
polymer systems. Chemical Reviews 1994;94:31–75.
[16] Ma H, Jen AKY. Functional dendrimers for nonlinear optics. Advanced Mate-
rials 2001;13:1201–5.
[17] Moerner WE, Jepsen AG, Thompson CL. Photorefractive polymer. Annual
Review of Materials Science 1997;27:585–623.
[18] Barclay GG, Ober CK. Liquid crystalline and rigid-rod networks. Progress in
Polymer Science 1993;18:899–945.
[19] Marks TJ, Ratner MA. Design, synthesis, and properties of molecule-based
assemblies with large second-order optical nonlinearities. Angewandte
Chemie International Edition 1995;34:155–73.
[20] Marder SR, Kippelen B, Jen AKY, Peyghambarian N. Design and synthesis of
chromophores and polymers for electro-optic and photorefractive applica-
tions. Nature 1997;388:845–51.
two-level model. It was found that the d33 (N) values similarly
increased corresponding to the increasing of the size of the isola-
tion group.
Coinciding with the ‘‘site isolation principle’’, in our case, the
NLO effect increased with the size of the isolation group. And the
carbazolyl moieties should be the most suitable isolation groups for
these NLO polyphosphazenes with azobenzene chromophore side
chains reported here. Compared with P2, the d33 value of P4 with
carbazolyl moieties was two times higher. The introduction of
isolation groups weakened the strong intermolecular dipole–dipole
interactions among the polar chromophore moieties, and benefited
to the poling-induced noncentrosymmetric alignment under elec-
tric field. Similar as our previous cases, the introduction of carba-
zolyl moieties as isolation group minimized the electronic
interactions greatly, leading to the best NLO property of P4.
[21] Zyss J. Octupolar organic systems in quadratic nonlinear optics: molecules and
materials. Nonlinear Optics 1991;1:3–18.
[22] Bai Y, Song N, Gao JP, Sun X, Wang X, Yu G, et al. A new approach to highly
electrooptically active materials using cross-linkable, hyperbranched chro-
4. Conclusion
Different isolation groups with different size were introduced to
the side chain of NLO polyphosphazenes through the post-functional
strategy, by applying the concept of ‘‘suitable isolation group’’. The
reaction conditions were mild and the purification procedure was
very simple. All the polymers demonstrated good stability, film-
forming ability and solubility. Also, we studied the effect of the
different size of isolation spacer on the resultant NLO properties. The
NLO effects for P2–4 increased with the size of the isolation groups,
with the d33 value of P4 up to 67.3 pm/V. Meanwhile, it was proved
that the introduction of an additional reactive group on the side
chain, was an efficient strategy to modify the structure of polymers
and consequently to optimize their properties.
mophore-containing oligomers as
American Chemical Society 2005;127:2060–1.
a macromolecular dopant. Journal of
[23] Andreu R, Blesa MJ, Carrasquer L, Garin J, Orduna J, Villacampa B, et al. Two-
Photon absorption at telecommunications wavelengths in a dipolar chromo-
phore with a pyrrole auxiliary donor and thiazole auxiliary acceptor. Journal of
American Chemical Society 2005;127:7282–3.
[24] Wang Q, Wang LM, Yu LP. Development of fully functionalized photorefractive
polymers. Macromolecular Rapid Communications 2000;21:723–45.
[25] Allcock HR, Dembek AA, Kim C, Devine RLS, Shi Y, Steier WH, et al. Second-
order nonlinear optical poly(organophosphazenes): synthesis and nonlinear
optical characterization. Macromolecules 1991;24:1000–10.
[26] Carriedo GA, Catuxo LF, Alonso FJG, Elipe PG, Gonzalez PA. Preparation of
a new type of phosphazene high polymers containing 2,2‘-dioxybiphenyl
groups. Macromolecules 1996;29:5320–5.
[27] Rojo G, Martin G, Lopez FA, Carriedo GA, Alonso FJG, Martinez JIF. Second-
harmonic response and relaxation behavior of high glass-transition temper-
ature polyphosphazene films. Chemistry of Materials 2000;12:3603–10.
[28] Li Z, Qin J, Yang Z, Ye C. Synthesis and nonlinear optical properties of poly-
phosphazenes with binaphtholyl and indole side groups. Journal of Applied
Polymer Science 2007;104:365–71.
Acknowledgements
We acknowledge the financial support from the National
Natural Science Foundation of China (Project Nos. 20674059).
[29] Allcock HR, Ravikiran R, Olshavsky MA. Synthesis and characterization of
hindered polyphosphazenes via functionalized intermediates: exploratory
models for electro-optical materials. Macromolecules 1998;31:5206–14.
[30] Allcock HR, Kim C. Liquid crystalline phosphazenes. High polymeric and cyclic
trimeric systems with aromatic azo side groups. Macromolecules
1989;22:2596–602.
References
[1] Allcock HR, Kugel RL. Synthesis of high polymeric alkoxy-and arylox-
yphosphonitriles. Journal of American Chemical Society 1965;87:4216–7.
[2] Allcock HR. Recent developments in polyphosphazene materials science.
Current Opinion in Solid State & Materials Science 2006;10:231–40.
[3] Allcock HR, Steely LB, Kim SH, Kim JH, Kang BK. Plasma surface functionali-
zation of poly[bis(2,2,2-trifluoroethoxy)phosphazene] films and nanofibers.
Langmuir 2007;23:8103–7.
[4] Laurencin CT, El-Amin SF, Ibim SE, Willoughby DA, Attawia M, Ambrosio AMA,
et al. A highly porous 3-dimensional polyphosphazene polymer matrix for
skeletal tissue regeneration. Journal of Biomedical Materials Research
1996;303:133–8.
[5] Nair L, Bhattacharyya S, Bender JD, Greish YE, Brown PW, Allcock H, et al.
Fabrication and otimization of methylphenoxy substituted polyphosphazene
nanofibers for biomedical applications. Biomacromolecules 2004;5:2212–20.
[6] Allcock HR, Ambrosio AMA. Synthesis and characterization of pH-sensitive
poly(organophosphazene) hydrogels. Biomaterials 1996;17:2295–302.
[7] Barrett EW, Phelps MVB, Silva RJ, Gaumond RP, Allcock HR. Patterning
poly(organophosphazenes) for selective cell adhesion applications. Bio-
macromolecules 2005;6:1689–98.
[8] Allcock HR, Phelps MVB, Barrett EW, Pishko MV, Koh W. Ultraviolet photo-
lithigraphic development for polyphosphazene hydrogel microstructures for
potential use in microarray biosensors. Chemistry of Materials 2006;18:
609–13.
[31] Allcock HR, Kim C. Liquid crystalline phosphazenes bearing biphenyl meso-
genic groups. Macromolecules 1990;23:3881–7.
[32] Li Z, Qin J, Tang H, Liu Y. Postfunctionalization strategy for developing poly-
phosphazene with a high loading of highly polar molecules in the side arms.
Journal of Applied Polymer Science 2003;89:2989–93.
[33] Fre´chet JMJ, Hawker CJ, Gitsov I, Leon JW. Dendrimers and hyperbranched
polymers: two families of three-dimensional macromolecules with similar but
clearly distinct properties. Journal of Macromolecular Science: Pure and
Applied Chemistry 1996;33:1399–425.
[34] Hecht S, Fre´chet JMJ. Dendritic encapsulation of function: applying nature’s
site isolation principle from biomimetics to materials science. Angewandte
Chemie International Edition 2001;40:74–91.
[35] Ma H, Chen BQ, Sassa T, Dalton LR, Jen AKY. Highly efficient and thermally
stable nonlinear optical dendrimer for electrooptics. Journal of American
Chemical Society 2001;123:986–7.
[36] Luo J, Liu S, Haller M, Liu L, Ma H, Alex JKY. Design, synthesis, and properties of
highly efficient side-chain dendronized nonlinear optical polymers for elec-
tro-Optics. Advanced Materials 2002;14:1763–8.
[37] Li Z, Li Z, Di C, Zhu Z, Li Q, Zeng Q, et al. Structural control of the side-chain
chromophores to achieve highly efficient nonlinear optical polyurethanes.
Macromolecules 2006;39:6951–61.
[38] Li Z, Zeng Q, Li Z, Dong S, Zhu Z, Li Q, et al. An attempt to modify nonlinear
optical effects of polyurethanes by adjusting the structure of the chromophore
moieties at the molecular level using "Click" chemistry. Macromolecules
2006;39:8544–6.
[9] Tang H, Pintauro PN. Polyphosphazene membranes. IV. Polymer morphology and
proton conductivity in sulfonated poly[bis(3-methylphenoxy)phosphazene]
films. Journal of Applied Polymer Science 2001;79:49–59.
[10] Lvov SN, Zhou XY, Benning L, Wei XJ, Fedkin MV, Allcock HR, et al. Proceedings
of symposium on energy engineering in the 21 Century 2000; p. 1483–8.
[11] Allcock HR, Crane CA, Morrissey CT, Nelson JM, Reeves SD, Honeyman CH, et al.
"Living" cationic polymerization of phosphoranimines as an ambient
temperature route to polyphosphazenes with controlled molecular weights.
Macromolecules 1996;29:7740–7.
[39] Zeng Q, Li Z, Li Z, Ye C, Qin J, Tang BZ. Convenient attachment of highly polar
azo chromophore moieties to disubstituted polyacetylene through polymer
reactions by using "click" chemistry. Macromolecules 2007;40:5634–7.
[40] Li Z, Li P, Dong S, Zhu Z, Li Q, Zeng Q, et al. Controlling nonlinear optical effects
of polyurethanes by adjusting isolation spacers through facile postfunctional
polymer reactions. Polymer 2007;47:3650–7.
[41] Li Z, Dong S, Yu G, Li Z, Liu Y, Ye C, et al. Novel second-order nonlinear
optical main-chain polyurethanes: adjustable subtle structure, improved
thermal stability and enhanced nonlinear optical property. Polymer
2007;47:5520–9.
[12] Allcock HR, de Denus CR, Prange R, Nelson JM. Synthesis of trifluoromethyl
and methyl phosphazene polymers: differences between polymerization and
initiator/terminator properties. Macromolecules 1999;32:7999–8004.

236
Q. Zeng et al. / Dyes and Pigments 84 (2010) 229–236
[42] Li Z, Zeng Q, Yu G, Li Z, Ye C, Liu Y, et al. New azo chromophore-containing
conjugated polymers: facile synthesis by using "Click" chemistry and
enhanced nonlinear optical properties through the introduction of suitable
isolation groups. Macromolecular Rapid Communications 2008;29:136–41.
[43] Li Z, Yu G, Li Z, Liu Y, Ye C, Qin J. New second-order nonlinear optical polymers
containing the same isolation groups: optimized syntheses and nonlinear
optical properties. Polymer 2008;49:901–14.
[44] Li Q, Li Z, Ye C, Qin J. Nonlinear optical chromophores with pyrrole moieties as
the conjugated bridge: enhanced NLO effects and interesting optical behavior.
Journal of Physical Chemistry B 2008;112:4928–33.
[47] Li Z, Yu G, Dong S, Wu W, Liu Y, Ye C, et al. The role of introduced isolation
groups in PVK-based nonlinear optical polymers: enlarged nonlinearity,
improved processibility, and enhanced thermal stability. Polymer
2009;50:2806–16.
[48] Li Z, Dong S, Li P, Li Z, Liu Y, Ye C, et al. New PVK-based nonlinear optical
polymers: enhanced nonlinearity and improved transparency. Journal of
Polymer Science Part A: Polymer Chemistry 2008;46:2983–93.
[49] Zhu Z, Li Q, Zeng Q, Li Z, Li Z, Qin J, et al. New azobenzene-containing poly-
urethanes: post-functional strategy and second-order nonlinear optical
properties. Dyes and Pigments 2008;78:199–206.
[45] Li Q, Yu G, Huang J, Liu H, Li Z, Ye C, et al. Polyurethanes containing indole-
based non-linear optical chromophores: from linear chromophore to H-Type.
Macromolecular Rapid Communications 2008;29:798–803.
[46] Li Q, Li Z, Zeng F, Gong W, Li Z, Zhu Z, et al. From controllable attached
isolation moieties to possibly highly efficient nonlinear optical main-chain
polyurethanes containing indole-based chromophores. Journal of Physical
Chemistry B 2007;111:508–14.
[50] Starkey EB, Smith LI, Ungnade HE. p-Dinitrobenzened. Organic Syntheses
1943;2:225–7.
[51] Allcock HR. Phosphazene high polymers. In: Allen SG, Bevington JC, editors.
Comprehensive polymer science. Oxford: Pergamon Press; 1989. p. 525–32.
[52] Moylan CR, Miller RD, Twieg RJ, Lee VY, McComb IH, Ermer S, et al. Defeating
tradeoffs for nonlinear optical chromophores. Proceedings of SPIE
1995;2527:150–62.