Functional poly(phenylacetylene)s carrying azobenzene pendants: Polymer synthesis, photoisomerization behaviors, and liquid-crystalline property

Article abstract of DOI:10.1016/j.polymer.2011.09.026

Functional poly(phenylacetylene)s (PPAs) bearing different azobenzene pendants were synthesized in desirable yields and molecular weight by using organorhodium complexes [Rh(diene)Cl]₂ as catalysts. The structure of the derived azobenzene-functionalized PPAs was characterized by NMR, IR, and UV spectroscopic techniques. Their photoinduced isomerization behavior was monitored with UV-visible spectroscopy. The thermal stability was evaluated by TGA technique. Polarized optical microscope (POM) observations indicated that the PPAs constructed by linking azobenzene moieties via a longer flexible alkyl spacer to PPA backbone showed typical liquid-crystalline property and the mesophase was assigned to SmA phase. Their phase transition behaviors were further investigated by differential scanning calorimetric (DSC) measurements. The molecular packing modes were analyzed by using X-ray diffraction (XRD) measurement and theoretical simulation. These results revealed some details about the interactions between the polymer backbone, flexible alkyl spacer, and azobenzene functional moiety, which are constructive to design and synthesize novel functional conjugated polymers.

Full text of DOI:10.1016/j.polymer.2011.09.026

Polymer 52 (2011) 5290e5301
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Functional poly(phenylacetylene)s carrying azobenzene pendants: Polymer
synthesis, photoisomerization behaviors, and liquid-crystalline property
Xiao A.-Zhanga, Hui Zhaoa, Yuan Gaoa, Jiaqi Tonga, Liang Shana, Yufei Chena, Shuang Zhanga, Anjun Qina,
,
a,b,
**
Jing Zhi Suna , Ben Zhong Tang
*
a Department of Polymer Science and Engineering, Institute of Biological and Medical Macromolecules, MOE of Key Laboratory of Macromolecular Synthesis and Functionalization,
Zhejiang University, Hangzhou 310027, China
b Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Functional poly(phenylacetylene)s (PPAs) bearing different azobenzene pendants were synthesized in
desirable yields and molecular weight by using organorhodium complexes [Rh(diene)Cl]2 as catalysts.
The structure of the derived azobenzene-functionalized PPAs was characterized by NMR, IR, and UV
spectroscopic techniques. Their photoinduced isomerization behavior was monitored with UVevisible
spectroscopy. The thermal stability was evaluated by TGA technique. Polarized optical microscope (POM)
observations indicated that the PPAs constructed by linking azobenzene moieties via a longer flexible
alkyl spacer to PPA backbone showed typical liquid-crystalline property and the mesophase was assigned
to SmA phase. Their phase transition behaviors were further investigated by differential scanning
calorimetric (DSC) measurements. The molecular packing modes were analyzed by using X-ray diffrac-
tion (XRD) measurement and theoretical simulation. These results revealed some details about the
interactions between the polymer backbone, flexible alkyl spacer, and azobenzene functional moiety,
which are constructive to design and synthesize novel functional conjugated polymers.
Received 18 June 2011
Received in revised form
6 September 2011
Accepted 14 September 2011
Available online 19 September 2011
Keywords:
Functional poly(phenylacetylene)s
Azobenzene
Liquid crystal
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
because the rigidity of the conjugated polymer backbone was once
considered as a diverse factor to the orderly alignment of mesogens
Polyacetylene is the archetypal conjugated polymer [1e3], and
its substituted derivatives have demonstrated a spectrum of
advanced functions, such as liquid-crystalline and light-emitting
properties, photoconductivity, optical nonlinearity, chain helicity,
self-assembling ability, and biocompatibility [4e12]. The liquid-
crystalline polyacetylenes are one of research topics in our group.
We have designed and synthesized a series of liquid-crystalline
polyacetylenes, which contain biphenyl, phenylcyclohexyl, phenyl
benzoate, cholesteryl, ergosteryl or stigmasteryl as mesogenic
cores, and have gained valuable information on the structur-
eeproperty relationships involved in these polymers [4e6,13e18].
For example, before the systematic investigation on polyacetylene-
based polymeric liquid-crystalline, side-chain liquid-crystalline
polymers with stiff backbones have received little attention,
and could result in processing difficulties associated with their high
thermal transition temperature. In our previous work, however, it
was found that the rigid polyacetylene backbones played an active
and constructive role in the alignment and packing processes of the
mesogenic pendants and the amelioration of thermal stability.
Azobenzene is a well-known mesogenic functionality and
azobenzene-containing polymers have been intensively explored
over the past two decades due to their versatile properties,
including liquid crystallinity [19e45], optical nonlinearity [46e49],
surface relief grating (SRG) formation [36e45,50e62], energy
transfer [63e68], and self-assembly [69e74]. Upon exposure to UV
and visible light, the azobenzene undergoes photoinduced isom-
erization between the trans and cis isomers [75e83], which
bestows the azobenzene-containing polymers with interesting
photoresponsive properties. Of them, homopolymers and copoly-
mers of azobenzene-containing acrylates have gained the most
extensive studies due to their facile preparation through free or
controlled radical polymerization.
* Corresponding author. Tel./fax: þ86 571 87953734.
** Corresponding author. Department of Chemistry, The Hong Kong University of
Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China. Tel.: þ852
23587375; fax: þ852 23581594.
The introduction of azobenzene moieties into the side chain of
polyacetylene, however, is an untouched research area. In this
work, we report our studies on the poly(phenylacetylene)s (PPAs)
E-mail addresses: sunjz@zju.edu.cn (J.Z. Sun), tangbenz@ust.hk (B.Z. Tang).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.09.026

X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
5291
carrying azobenzene pendants. By linking the azobenzene and
2.3. Monomer synthesis
trifluoromethyl-capped azobenzene moieties to the PPA skeleton
with proper flexible alkyl spacers, four azobenzene-containing
PPAs have been designed and synthesized (Chart 1, P1 and P2).
Their chemical structures have been fully characterized. The
thermal stability, the photoinduced isomerization behaviors of the
azobenzene functionalities in PPAs side chains, and the mesomor-
phic properties of the monomers and derived polymers are
demonstrated.
Monomers 1(m) and 2(m) (m ¼ 6, 12) were prepared according
to the synthetic routes shown in Scheme 1. The detailed experi-
mental procedures for the syntheses of compounds 9, 10, 8(m)e
3(m) and monomers 1(m), 2(m) are given below.
2.3.1. Synthesis of (E)-4-(phenyldiazenyl)phenol (9)
In a 250 mL round-bottom flask dissolve 11 (0.93 g, 10 mmol) in
concentrated hydrochloric acid (37%, 2.5 mL, 30 mmol) and
deionized water (20 mL), apply heat if necessary. The temperature
was maintained at 0e5 ꢀC in an ice bath, an aqueous solution of
sodium nitrite (0.72 g, 10.5 mmol) was added dropwise through
a dropping funnel to the cooled solution. After stirring at 0e5 ꢀC for
15 min, an ethanol solution of phenol (0.94 g, 10 mmol) was added
slowly, and then the mixture was stirred for 1 h in ice bath. Add
sodium bicarbonate to adjust pH to 7, then the mixture was further
stirred for 0.5 h. The resultant precipitate was filtered and the crude
product was purified by column chromatography using a mixture of
petroleum ether and ethyl acetate (10:1 by volume) as eluent. A
yellow solid of 9 was obtained in 89.7% yield. 1H NMR (400 MHz,
2. Experimental
2.1. Materials
Tetrahydrofuran (THF; Labscan) was distilled under normal
pressure from sodium benzophenone ketyl under nitrogen imme-
diately prior to use. Triethylamine (TEA) was distilled and dried
over potassium hydroxide. Other solvents were purified by stan-
dard procedures. Aniline (11), 4-(trifluoromethyl)aniline (12),
phenol, 1,6-dibromohexane, 1,12-dibromododecane, 4-iodophenol,
piperidine, copper (I) iodide, triphenylphosphine, dichlorobis(-
triphenylphosphine)palladium(II), and trimethyl-ethynylsilane
were purchased from Aldrich and used as received without
further purifications. Sodium nitrite, hydrochloric acid, sodium
bicarbonate, potassium carbonate, potassium hydroxide, and
potassium iodide were purchased from SCRC and used as received.
Organorhodium complexes [Rh(nbd)Cl]2 (nbd ¼ 2,5-norbornad-
iene) and [Rh(cod)Cl]2 (cod ¼ 1,8-cyclooctadiene) were prepared
in our laboratories by literature methods [84].
CDCl3, d, TMS, ppm): 7.86e7.91 (4H, m, ArH ortho to eN]Ne), 7.53
(3H, d, ArH meta and para to eN]Ne) and 6.98 (2H, d, ArH ortho to
phenolic OH).
2.3.2. (E)-4-((4-(trifluoromethyl)phenyl)diazenyl)phenol (10)
This intermediate was prepared from 12 by a procedure similar
to that used for preparation of 9. A yellow solid was obtained in
86.5% yield. 1H NMR (400 MHz, CDCl3,
d, TMS, ppm): 7.90e7.97 (4H,
m, ArH ortho to eN]Ne), 7.77 (2H, d, ArH ortho to CF3) and 6.99
2.2. Instrumentation
(2H, d, ArH ortho to phenolic OH).
The infrared (IR) spectra were measured on a PerkineElmer 16
PC FT-IR spectrophotometer. The 1H and 13C NMR spectra were
taken in CDCl3 on a Bruker ARX 400 NMR spectrometer. The
2.3.3. (E)-1-(4-(6-bromohexyloxy)phenyl)-2-phenyldiazene (7(6))
In
a 250 mL round-bottom flask equipped with a reflux
condenser were placed 1,6-dibromohexane (3.05 g, 12.5 mmol),
potassium carbonate (0.76 g, 5.5 mmol) and acetone (20 mL), and
then 10 mL acetone solution of 9 (0.99 g, 5 mmol) was added
dropwise through a dropping funnel to the refluxing mixture. The
resultant mixture was refluxed for 24 h. The solid was removed by
filtration and the filtrate was concentrated under reduced pressure.
The crude product was purified by column chromatography using
a mixture of petroleum ether and ethyl acetate (50:1 by volume) as
eluent. A light yellow solid of 7(6) was obtained in 75.5% yield. 1H
chemical shifts were reported in ppm on the
methylsilane (TMS;
¼ 0 ppm) was used as internal reference for
the NMR analysis. The UVevis absorption spectra were recorded
on Milton Roy Spectronic 3000 Array spectrophotometer.
d scale, and tetra-
d
a
Elemental analysis was conducted on a ThermoFinnigan Flash
EA1112 apparatus. Mass spectra were measured on a GCT Premier
CAB 048 mass spectrometer. Molecular weights (Mw and Mn) and
polydispersity indices (PDI or Mw/Mn) of the polymers were tested
in THF with a Waters Associate gel permeation chromatography
(GPC) system. The molecular weight was calibrated by a set of
monodisperse polystyrene standards covering molecular weight
range of 103e107.
Under a nitrogen atmosphere, the TGA analysis was performed
on a PerkineElmer Pyris thermogravimetric analyzer TGA 6 at
a heating rate of 20 ꢀC/min. The texture of liquid crystal was
observed with an Olympus BX 60 polarized optical microscope
(POM) equipped with a Linkam TMS 92 hot stage. The images were
taken with a DP70 camera (Olympus). Differential scanning calo-
rimetric (DSC) measurements were undertaken using a Setaram
DSC 92 at a heating or cooling rate of 10 ꢀC/min ranging from 50 to
250 ꢀC. X-ray diffraction (XRD) measurements were achieved at
room temperature on a Philips PW 1830 powder diffractometer
NMR (400 MHz, CDCl3, d, TMS, ppm): 7.94 (4H, m, ArH ortho to
eN]Ne), 7.49 (2H, t, ArH meta to eN]Ne), 7.43 (1H, t, ArH para to
eN]Ne), 7.01 (2H, d, ArH ortho to phenolic group), 4.08 (2H, t,
OCH2), 3.43 (2H, t, CH2Br), 1.85e1.95 (4H, d, [CH2]2) and 1.27e1.64
(4H, t, [CH2]2).
2.3.4. (E)-1-(4-(6-bromohexyloxy)phenyl)-2-(4-(trifluoromethyl)
phenyl)diazene (8(6))
This intermediate was prepared from 10 by a procedure similar
to that used for preparation of 7(6). A yellow solid of 8(6) was ob-
tained in 76.8% yield. (400 MHz, CDCl3, d, TMS, ppm): 7.97 (4H, m,
ArH ortho to eN]Ne), 7.77 (2H, d, ArH ortho to CF3), 7.00 (2H, d,
ArH ortho to OCH2), 4.09 (2H, t, OCH2), 3.41 (2H, t, CH2Br),1.85e1.92
(4H, d, [CH2]2) and 1.51e1.56 (4H, t, [CH2]2).
using the monochromatized X-ray beam from the nickel-filtered
ꢀ
Cu:K
a
radiation with a wavelength of 1.5406 A (scanning rate
2.3.5. (E)-1-(4-(12-bromododecyloxy)phenyl)-2-phenyldiazene
(7(12))
0.05 deg/s, scan range 2ꢀe30ꢀ). The polymer samples for the XRD
measurements were prepared by freezing the molecular arrange-
ments in the liquid-crystalline state with liquid nitrogen. The
polymer films were prepared by spin-coating their 1,2-
dichloroethane solutions (3.6 mg/mL) onto quartz substrates at
a rotation rate of 1500 r/min.
This intermediate was prepared from 9 by a procedure similar to
that used for preparation of 7(6). 1,12-dibromododecane was used
instead of 1,6-dibromohexane. A light yellow solid of 7(12) was
obtained in 67.5% yield. 1H NMR (400 MHz, CDCl3,
d
, TMS, ppm):
7.92 (4H, m, ArH ortho to eN]Ne), 7.53 (3H, d, ArH meta and para

5292
X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
to eN]Ne), 7.00 (2H, d, ArH ortho to OCH2), 4.06 (2H, t, OCH2),
3.43 (2H, t, CH2Br), 1.79e1.88 (4H, m, OCH2CH2 and CH2CH2Br) and
1.24e1.47 (16H, m, [CH2]8).
CDCl3, d, TMS, ppm): 7.94 (4H, m, ArH ortho to eN]Ne), 7.49 (4H,
m, ArH meta to eN]Ne and ArH ortho to I), 7.44 (1H, t, ArH para to
eN]Ne), 7.00 (2H, d, ArH ortho to OCH2), 6.67 (2H, d, ArH meta to
I), 4.05 (2H, t, CH2O), 3.93 (2H, t, CH2O), 1.81 (4H, m, [CH2]2),
1.27e1.57 (4H, t, [CH2]2).
2.3.6. (E)-1-(4-(12-bromododecyloxy)phenyl)-2-(4-
(trifluoromethyl)phenyl)diazene (8(12))
This intermediate was prepared from 10 by a procedure similar
to that used for preparation of 7(6). 1,12-dibromododecane was
used instead of 1,6-dibromohexane. A yellow solid of 8(12) was
2.3.8. (E)-1-(4-(6-(4-iodophenoxy)hexyloxy)phenyl)-2-(4-
(trifluoromethyl)phenyl)-diazene (6(6))
This intermediate was prepared from 8(6) by a procedure
similar to that used for preparation of 5(6). A light yellow solid of
obtained in 63.4% yield. 1H NMR (300 MHz, CDCl3,
d, TMS, ppm):
7.97 (4H, m, ArH ortho to eN]Ne), 7.77 (2H, d, ArH ortho to CF3),
7.00 (2H, d, ArH ortho to OCH2), 4.07 (2H, t, OCH2), 3.43 (2H, t,
CH2Br), 1.80e1.88 (4H, m, OCH2CH2 and CH2CH2Br) and 1.30e1.45
(16H, m, [CH2]8).
6(6) was obtained in 80.5% yield. 1H NMR (400 MHz, CDCl3,
d, TMS,
ppm): 7.97 (4H, m, ArH ortho to eN]Ne), 7.77 (2H, d, ArH ortho to
F3C), 7.56 (2H, d, ArH ortho to I), 7.00 (2H, d, ArH ortho to OCH2),
6.69 (2H, d, ArH meta to I), 4.09 (2H, t, OCH2), 3.96 (2H, t, CH2O),
1.54e1.89 (8H, m, [CH2]2).
2.3.7. (E)-1-(4-(6-(4-iodophenoxy)hexyloxy)phenyl)-2-
phenyldiazene (5(6))
2.3.9. (E)-1-(4-(12-(4-iodophenoxy)dodecyloxy)phenyl)-2-
phenyldiazene (5(12))
In
a 250 mL round-bottom flask equipped with a reflux
condenser, 4-idophenol (1.1 g, 5 mmol), KOH (0.31 g, 5.5 mmol) and
KI (0.25 g, 1.5 mmol) were dissolved in 100 mL of acetone/DMSO
(9:1 by volume) mixture with stirring. To the reaction mixture was
added of 7(6) (1.80 g, 5 mmol) and the resultant mixture was
refluxed for 24 h. The solid was removed by filtration and the
filtrate was concentrated under reduced pressure. The mixture was
washed by excess, dilute hydrochloric acid and extracted with
chloroform for 3 times. The chloroform solution was then washed
by saturated NaHCO3 and NaCl solutions. The mixture was dried
over 4.5 g of magnesium sulfate. The crude product was condensed
and purified by column chromatography using a mixture of
petroleum ether and ethyl acetate (5:1 by volume) as eluent. A light
yellow solid of 5(6) was obtained in 90.8% yield. 1H NMR (400 MHz,
This intermediate was prepared from 7(12) by a procedure
similar to that used for preparation of 5(6). A light yellow solid of
5(12) was obtained in 84.5% yield. 1H NMR (400 MHz, CDCl3,
d, TMS,
ppm): 7.94 (4H, m, ArH ortho to eN]Ne), 7.43e7.55 (5H, m, ArH
meta and para to eN]Ne, ArH ortho to I), 7.00 (2H, d, ArH ortho to
OCH2), 6.67 (2H, d, ArH meta to I), 4.06 (2H, t, OCH2), 3.92 (2H, t,
CH2O), 1.71e1.86 (4H, m, [CH2]2), 1.30e1.50 (16H, t, [CH2]2).
2.3.10. (E)-1-(4-(12-(4-iodophenoxy)dodecyloxy)phenyl)-2-(4-
(trifluoromethyl)phenyl)-diazene (6(12))
This intermediate was prepared from 8(12) by a procedure
similar to that used for preparation of 5(6). A light yellow solid of
6(12) was obtained in 88.6% yield. 1H NMR (400 MHz, CDCl3,
d, TMS,
R
R
R
R
N
1. H+,NaNO2,0-5oC
2. phenol, NaHCO3
Br(CH2)mBr,reflux
4-iodophenol, KI, KOH
acetone/DMSO, reflux
N
N
N
K2CO3,acetone
N
O
N
NH2
O
11 (R=H)
OH
(CH2)m
12
(R=CF3)
(CH2)m
O
9
(R=H)
(R=CF3)
Br
10
7
(m) (R=H)
8(m) (R=CF3) (m=6, 12)
I
5
(m) (R=H)
6(m) (R=CF3) (m=6, 12)
PdCl2(PPh3)2,CuI,PPh3,TEA
R
N
Si
O(CH2)mO
N
R
SiMe3
rt,24h
3
(m) (R=H)
4(m) (R=CF3) (m=6, 12)
N
KOH, rt
THF/MeOH
HC
C
O(CH2)mO
N
1(m) (R=H)
2(m) (R=CF3) (m=6, 12)
Scheme 1.

X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
5293
ppm): 7.97 (4H, m, ArH ortho to eN]Ne), 7.77 (2H, d, ArH ortho to
F3C), 7.55 (2H, d, ArH ortho to I), 6.99 (2H, d, ArH ortho to OCH2),
6.69 (2H, d, ArH meta to I), 4.07 (2H, t, OCH2), 3.97 (2H, t, CH2O),
1.50e1.90 (20H, m, [CH2]2).
poured into 1000 mL of 1 M HCl solution. The mixture was
extracted by DCM 3 times and the organic layers were collected.
The solvents were removed under reduced pressure, and the crude
product was purified by column chromatography using a mixture of
petroleum ether and ethyl acetate (5:1 by volume) as eluent. A light
yellow solid of 1(6) was obtained in 81.4% yield.
2.3.11. (E)-1-phenyl-2-(4-(6-(4-((trimethylsilyl)ethynyl)phenoxy)
hexyloxy)phenyl)-diazene (3(6))
Characterization data: 1H NMR (400 MHz, CDCl3,
d, TMS, ppm):
In a 250 mL two-necked round-bottom flask were added
PdCl2(PPh3)2 (14 mg, 0.02 mmol), CuI (3.8 mg, 0.02 mmol), PPh3
(10.5 mg, 0.04 mmol) and 15 mL THF solution of 5(6) (1.0 g,
2 mmol) under nitrogen. 25 mL TEA and 15 mL piperidine were
injected to better dissolve the catalysts. After the catalysts were
completely dissolved, trimethylsilylacetylene (0.24 g, 2.4 mmol)
was injected into the flask and the mixture was stirred at room
temperature for 12 h. The solid was removed by filtration and
washed with diethyl ether. The filtrate was then concentrated by
a rotary evaporator. The crude product was purified by column
chromatography using a mixture of petroleum ether and ethyl
acetate (20:1 by volume) as eluent. A light yellow solid of 3(6) was
7.92 (4H, m, ArH ortho to eN]Ne), 7.40e7.51 (5H, m, ArH meta
and para to eN]Ne, ArH ortho to ^), 7.00 (2H, d, ArH ortho to
OCH2), 6.81e6.83 (2H, d, ArH meta to ^), 4.05 (2H, t, OCH2), 3.97
(2H, t, CH2O), 3.00 (1H, s, ^CH), 1.81e1.84 (4H, m, [CH2]2),
1.25e1.60 (4H, t, [CH2]2). 13C NMR (400 MHz, CDCl3,
d, TMS, ppm):
161.6, 159.4 (aromatic carbons attached to eN]Ne), 152.7, 148.9
(aromatic carbons attached to eOe), 133.6 (aromatic carbons ortho
to ^), 130.3 (aromatic carbon para to eN]Ne), 129.0 (aromatic
carbons meta to eN]Ne), 124.7 and 122.5 (aromatic carbons ortho
to eN]Ne), 114.7, 114.4 and 113.9 (aromatic carbons ortho and
para to eOe), 83.7 (^C attached to benzene), 75.7 (^CeH), 68.1
and 67.8 (carbons attached to eOe), 29.1 (OCH2CH2), 25.8 (CH2). IR
obtained in 92.2% yield. 1H NMR (400 MHz, CDCl3,
d
, TMS, ppm):
(KBr), n
(cmꢁ1): 3285 (HC^C), 2949 and 2875 (HeCH), 1400 (N]
7.93 (4H, m, ArH ortho to eN]Ne), 7.38e7.51 (5H, m, ArH meta
and para to eN]Ne, ArH ortho to ^), 7.00 (2H, d, ArH ortho to
OCH2), 6.83 (2H, d, ArH meta to ^), 4.08 (2H, t, OCH2), 3.97 (2H, t,
CH2O), 1.84 (4H, m, [CH2]2), 1.25e1.58 (4H, t, [CH2]2), 0.24 (9H, s, Si
[CH3]3).
N). Calcd for C26H26N2O2 (398.5): C, 78.36; H, 6.58; N, 7.03. Found:
C, 78.14; H, 6.63; N, 7.11.
2.3.16. (E)-1-(4-(6-(4-ethynylphenoxy)hexyloxy)phenyl)-2-(4-
(trifluoromethyl)phenyl)-diazene (2(6))
This monomer was prepared from 4(6) by a procedure similar to
that used for preparation of 1(6). A yellow solid of 2(6) was ob-
tained in 88.5% yield.
2.3.12. (E)-1-(4-(trifluoromethyl)phenyl)-2-(4-(6-(4-
((trimethylsilyl)ethynyl)phenoxy)-hexyloxy)phenyl)diazene (4(6))
This intermediate was prepared from 6(6) by a procedure
similar to that used for preparation of 3(6). A yellow solid of 4(6)
Characterization data: 1H NMR (400 MHz, CDCl3,
d, TMS, ppm):
7.95 (4H, m, ArH ortho to eN]Ne), 7.72e7.75 (2H, d, ArH ortho to
CF3), 7.41 (2H, m, ArH ortho to ^), 7.00 (2H, d, ArH ortho to OCH2),
6.80 (2H, d, ArH meta to ^), 4.07 (2H, t, OCH2), 3.98 (2H, t, CH2O),
was obtained in 91.4% yield. 1H NMR (400 MHz, CDCl3,
d, TMS,
ppm): 7.97 (4H, m, ArH ortho to eN]Ne), 7.74e7.77 (2H, d, ArH
ortho to CF3), 7.38e7.41 (2H, m, ArH ortho to ^), 7.00 (2H, d, ArH
ortho to OCH2), 6.80 (2H, d, ArH meta to ^), 4.11 (2H, t, OCH2), 4.00
(2H, t, CH2O), 1.85 (4H, m, [CH2]2), 1.25e1.58 (4H, t, [CH2]2), 0.24
(9H, s, Si[CH3]3).
3.00 (1H, s, ^CH), 1.85 (4H, m, [CH2]2), 1.25e1.57 (4H, t, [CH2]2). 13
C
NMR (400 MHz, CDCl3, d, TMS, ppm): 161.6,159.4 (aromatic carbons
attached to eN]Ne), 152.7, 148.9 (aromatic carbons attached to
eOe), 133.6 (aromatic carbons ortho to ^), 131.5 (aromatic carbon
para to eN]Ne), 126.3 (aromatic carbons meta to eN]Ne), 124.7
and 122.5 (aromatic carbons ortho to eN]Ne), 114.7, 114.4 and
113.9 (aromatic carbons ortho and para to eOe), 83.7 (^C attached
to benzene), 75.7 (^CeH), 68.1 and 67.8 (carbons attached to
2.3.13. (E)-1-phenyl-2-(4-(12-(4-((trimethylsilyl)ethynyl)phenoxy)
dodecyloxy)phenyl)-diazene (3(12))
This intermediate was prepared from 5(12) by a procedure
similar to that used for preparation of 3(6). A yellow solid of 3(12)
eOe), 29.1 (OCH2CH2), 25.8 (CH2). IR (KBr), n
(cmꢁ1): 3285 (HC^C),
2949 and 2875 (HeCH), 1400 (N]N). Calcd for C27H25F3N2O2
was obtained in 83.5% yield. 1H NMR (400 MHz, CDCl3,
d
, TMS,
ppm): 7.94 (4H, m, ArH ortho to eN]Ne), 7.37e7.53 (5H, m, ArH
meta and para to eN]Ne, ArH ortho to ^), 7.00 (2H, d, ArH ortho
to OCH2), 6.82 (2H, d, ArH meta to ^), 4.06 (2H, t, OCH2), 3.96 (2H, t,
CH2O),1.87 (4H, m, [CH2]2),1.30e1.45 (16H, d, [CH2]8), 0.24 (9H, s, Si
[CH3]3).
(466.49): C, 69.52; H, 5.40; N, 6.01. Found: C, 69.05; H, 5.34; N, 6.08.
2.3.17. (E)-1-(4-(12-(4-ethynylphenoxy)dodecyloxy)phenyl)-2-
phenyldiazene (1(12))
This monomer was prepared from 3(12) by a procedure similar
to that used for preparation of 1(6). A light yellow solid of 1(12) was
obtained in 98.0% yield.
2.3.14. (E)-1-(4-(trifluoromethyl)phenyl)-2-(4-(12-(4-
((trimethylsilyl)ethynyl)phenoxy)-dodecyloxy)phenyl)diazene
(4(12))
Characterization data: 1H NMR (400 MHz, CDCl3,
d, TMS, ppm):
7.92 (4H, m, ArH ortho to eN]Ne), 7.39e7.51 (5H, m, ArH meta and
para to eN]Ne, ArH ortho to ^), 7.00 (2H, d, ArH ortho to OCH2),
6.82 (2H, d, ArH meta to ^), 4.04 (2H, t, OCH2), 3.97 (2H, t, CH2O),
3.00 (1H, s, ^CH), 1.75e1.84 (4H, m, [CH2]2), 1.25e1.47 (16H, t,
This intermediate was prepared from 6(12) by a procedure
similar to that used for preparation of 4(6). A yellow solid of 4(12)
was obtained in 94.6% yield. 1H NMR (400 MHz, CDCl3,
d, TMS,
ppm): 7.97 (4H, m, ArH ortho to eN]Ne), 7.75e7.77 (2H, d, ArH
ortho to CF3), 7.37e7.40 (2H, m, ArH ortho to ^), 6.99 (2H, d, ArH
ortho to OCH2), 6.80 (2H, d, ArH meta to ^), 4.10 (2H, t, OCH2), 4.00
(2H, t, CH2O), 1.86 (4H, m, [CH2]2), 1.26e1.50 (16H, t, [CH2]2), 0.24
(9H, s, Si[CH3]3).
[CH2]8). 13C NMR (400 MHz, CDCl3,
d, TMS, ppm): 161.6, 159.4
(aromatic carbons attached to eN]Ne), 152.7, 148.9 (aromatic
carbons attached to eOe), 133.6 (aromatic carbons ortho to ^),
130.3 (aromatic carbon para to eN]Ne), 129.0 (aromatic carbons
meta to eN]Ne), 124.7 and 122.5 (aromatic carbons ortho to eN]
Ne), 114.7, 114.4 and 113.9 (aromatic carbons ortho and para to
eOe), 83.7 (^C attached to benzene), 75.7 (^CeH), 68.1 and 67.8
2.3.15. (E)-1-(4-(6-(4-ethynylphenoxy)hexyloxy)phenyl)-2-
phenyldiazene (1(6))
In a 250 mL round-bottom flask were added 3(6) (0.94 g,
2 mmol), KOH (0.68 g, 12 mmol), 50 mL THF, and 50 mL methanol.
The mixture was stirred at room temperature for 12 h and then
(carbons attached to eOe), 29.1 (OCH2CH2), 25.8 (CH2). IR (KBr),
n
(cmꢁ1): 3285 (HC^C), 2949 and 2875 (HeCH), 1400 (N]N). Calcd
for C32H38F3N2O2 (482.66): C, 79.63; H, 7.94; N, 5.80. Found: C,
79.42; H, 7.96; N, 5.92.

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X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
2.3.18. (E)-1-(4-(12-(4-ethynylphenoxy)dodecyloxy)phenyl)-2-(4-
(trifluoromethyl)-phenyl)diazene (2(12))
transferred to the monomer solution using a hypodermic syringe.
The mixture was stirred at room temperature under nitrogen for
24 h and was then diluted with 10 mL of THF and added dropwise
to a 500 mL methanol through a cotton filter under stirring. The
precipitate was allowed to stand for 24 h and then filtered with
a Gooch crucible. The polymer was washed with methanol for three
times and then dried in a vacuum oven at 25 ꢀC to a constant
weight.
This monomer was prepared from 4(12) by a procedure similar
to that used for preparation of 1(6). A light yellow solid of 2(12) was
obtained in 94.2% yield.
Characterization data: 1H NMR (400 MHz, CDCl3,
d, TMS, ppm):
7.95 (4H, m, ArH ortho to eN]Ne), 7.72e7.75 (2H, d, ArH ortho to
CF3), 7.41 (2H, m, ArH ortho to ^), 7.00 (2H, d, ArH ortho to OCH2),
6.80 (2H, d, ArH meta to ^), 4.07 (2H, t, OCH2), 3.98 (2H, t, CH2O),
3.00 (1H, s, ^CH), 1.85 (4H, m, [CH2]2), 1.25e1.57 (4H, t, [CH2]2). 13
C
2.4.1. Characterization data for P1(6)
NMR (400 MHz, CDCl3,
d
, TMS, ppm): 161.6,159.4 (aromatic carbons
Brown powdery solid; yield 84.7%. Mw: 44,700; Mw/Mn: 1.93
attached to eN]Ne), 152.7, 148.9 (aromatic carbons attached to
eOe), 133.6 (aromatic carbons ortho to ^), 131.5 (aromatic carbon
para to eN]Ne), 126.3 (aromatic carbons meta to eN]Ne), 124.7
and 122.5 (aromatic carbons ortho to eN]Ne), 114.7, 114.4 and
113.9 (aromatic carbons ortho and para to eOe), 83.7 (^C attached
to benzene), 75.7 (^CeH), 68.1 and 67.8 (carbons attached to
(GPC, polystyrene calibration; Table 1, no. 2). 1H NMR (400 MHz,
CDCl3, d, TMS, ppm): 7.80e7.90 (ArH ortho to eN]Ne), 7.39e7.50
(ArH meta and para to eN]Ne), 6.36, 6.60, 7.00 (ArH ortho to
OCH2, ArH ortho and meta to ]), 5.80 (HeC]C), 3.70e4.06 (OCH2),
1.26e1.84 (CH2]2). 13C NMR (400 MHz, CDCl3,
d, TMS, ppm): 161.6,
159.4 (aromatic carbons attached to eN]Ne), 152.7, 148.9
(aromatic carbons attached to eOe), 130.3 (aromatic carbon para to
eN]Ne), 129.0 (aromatic carbons meta to eN]Ne), 124.7 and
122.5 (aromatic carbons ortho to eN]Ne), 114.7, 114.4 and 113.9
(aromatic carbons ortho and para to eOe), 68.1 and 67.8 (carbons
eOe), 29.1 (OCH2CH2), 25.8 (CH2). IR (KBr), n
(cmꢁ1): 3285 (HC^C),
2949 and 2875 (HeCH), 1400 (N]N). HRMS (CI): m/z 551.2859
[(M þ 1)þ, calcd 551.2807, see Figure S1].
attached to eOe), 29.1 (OCH2CH2), 25.8 (CH2). IR (KBr),
2949 and 2875 (HeCH), 1400 (N]N).
n
(cmꢁ1):
2.4. Polymerization preparation
All the phenylacetylene derivatives polymerizations and
manipulations were carried out under dry nitrogen using Schlenk
techniques in a vacuum-line system except for the purification of
the polymers, which was done in air. The synthetic route is shown
as Scheme 2 and typical experimental procedures for polymeriza-
tion of 1(6) are given below as an example.
In a 20 mL baked Schlenk tube with a side arm was added 99 mg
(0.25 mmol) of monomer 1(6). The tube was evacuated under
vacuum and flushed with dry nitrogen three times through the side
arm. Then 1.5 mL of THF was injected into the tube to dissolve the
monomer. The catalyst solution was prepared in another tube by
dissolving [Rh(nbd)Cl]2 or [Rh(cod)Cl]2 in 1 mL of THF with one
drop of TEA added. After aging for 15 min, the catalyst solution was
2.4.2. Characterization data for P1(12)
Brown powdery solid; yield 85.5%. Mw: 76,600; Mw/Mn: 1.79
(GPC, polystyrene calibration; Table 1, no. 7). 1H NMR (400 MHz,
CDCl3, d, TMS, ppm): 7.84e7.92 (ArH ortho to eN]Ne), 7.41e7.52
(ArH meta and para to eN]Ne), 6.36, 6.60, 7.00 (ArH ortho to
OCH2, ArH ortho and meta to ]), 5.80 (HeC]C), 3.87e4.03 (OCH2),
Table 1
Polymerization of monomers 1(6), 2(6), 1(12), and 2(12).a
Catalystb
[M]0
(M)
[Cat.]
(mM)
Yield
(%)
Mw
Mw/
Mn
c
No
c
Monomer 1(6)
1
2
3
[Rh(cod)
Cl]2
[Rh(nbd)
Cl]2
[Rh(nbd)
Cl]2
0.10
0.20
0.10
2.00
8.50
2.50
55.4
84.7
65.4
33,700
44,700
60,600
1.59
1.92
1.60
H
C C
[Rh(diene)Cl]2
THF/Et3N, N2, 24 h
room temperature
Monomer 2(6)
4
5
6
[Rh(nbd)
Cl]2
[Rh(nbd)
Cl]2
[Rh(cod)
Cl]2
0.10
0.10
0.10
6.00
2.00
2.00
97.4
84.2
61.1
42,300
66,500
45,500
1.58
1.90
1.69
O
O
(CH2)m
(CH2)m
O
O
Monomer 1(12)
7
[Rh(nbd)
Cl]2
[Rh(nbd)
Cl]2
[Rh(nbd)
Cl]2
[Rh(cod)
Cl]2
0.05
0.10
0.10
0.10
2.50
6.00
2.00
2.00
85.5
89.0
79.3
66.1
76,600
41,300
92,200
57,700
1.79
1.47
1.90
1.90
8
9
N
N
N
R
N
R
10
Monomer 2(12)
11
[Rh(nbd)
Cl]2
[Rh(cod)
Cl]2
0.10
0.10
2.00
2.00
85.1
60.7
86,600
66,300
1.95
1.70
12
P1(m) (R = H)
1(m) (R = H)
a
Carried out at room temperature in an atmosphere of dry nitrogen for 24 h in
2
2
P (m) (R = CF3)
(m) (R = CF3)
THF/Et3N.
b
c
(m = 6, 12)
(m = 6, 12)
Abbreviations: nbd ¼ 2.5-norborndiene and cod ¼ 1,5-cyclooctadiene.
Determined by gel permeation chromatograph (GPC) in THF on the basis of
Scheme 2.
a polystyrene calibration.

X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
5295
1.25e1.81 (CH2]2). 13C NMR (400 MHz, CDCl3,
d
, TMS, ppm): 161.6,
monomers were readily polymerized by [Rh(nbd)Cl]2 and [Rh(cod)
Cl]2 in a THF/TEA mixture. These polymerizations proceeded well in
THF and produced polymers with high Mw value and accepted yield
of w55.4e97.4% (see Table 1).
159.4 (aromatic carbons attached to eN]Ne), 152.7, 148.9
(aromatic carbons attached to eOe), 130.3 (aromatic carbon para to
eN]Ne), 129.0 (aromatic carbons meta to eN]Ne), 124.7 and
122.5 (aromatic carbons ortho to eN]Ne), 114.7, 114.4 and 113.9
(aromatic carbons ortho and para to eOe), 68.1 and 67.8 (carbons
3.2. Structural characterization
attached to eOe), 29.1 (OCH2CH2), 25.8 (CH2). IR (KBr),
2949 and 2875 (HeCH), 1400 (N]N).
n
(cmꢁ1):
The obtained polymers were characterized with multiple spec-
troscopic methods. All the polymers gave satisfactory analysis data
corresponding to their expected molecular structures (see
Experimental section for details). An example of the IR spectrum of
P1(6) is shown in Fig. 1; the spectrum of its monomer 1(6) is also
given in the same figure for comparison. The monomer exhibits
absorption band at 3285 cmꢁ1, which is ascribed to ^CH stretching
vibrations. The band completely disappears in the spectrum of
P1(6), indicating that the acetylene triple bond of 1(6) has been
fully consumed by the rhodium-catalyzed polymerization reaction.
NMR spectroscopy is an effective method of characterizing the
polymer structures. 1H NMR spectra of polymer P1(6) and its
monomer 1(6) are shown in Fig. 2. As can be seen from the 1H NMR
2.4.3. Characterization data for P2(6)
Dark brown powdery solid; yield 84.2%. Mw: 66,500; Mw/Mn:
1.90 (GPC, polystyrene calibration; Table 1, no. 5). 1H NMR
(400 MHz, CDCl3, d, TMS, ppm): 7.78e7.96 (ArH ortho to eN]Ne),
7.60e7.65 (ArH ortho to CF3), 6.36, 6.60, 7.00 (ArH ortho to OCH2,
ArH ortho and meta to ]), 5.80 (HeC]C), 3.70e4.06 (OCH2),
1.26e1.84 (CH2]2). 13C NMR (400 MHz, CDCl3,
d, TMS, ppm): 161.6,
159.4 (aromatic carbons attached to eN]Ne), 152.7, 148.9
(aromatic carbons attached to eOe), 130.3 (aromatic carbon para to
eN]Ne), 129.0 (aromatic carbons meta to eN]Ne), 124.7 and
122.5 (aromatic carbons ortho to eN]Ne), 114.7, 114.4 and 113.9
(aromatic carbons ortho and para to eOe), 68.1 and 67.8 (carbons
spectrum of P1(6), there is no resonance peak at
associated with the acetylene proton (Fig. 2B). A new peak associ-
ated with the resonance of olefinic protons appears at w 5.80. The
d w 3.00, which is
attached to eOe), 29.1 (OCH2CH2), 25.8 (CH2). IR (KBr),
2949 and 2875 (HeCH), 1400 (N]N).
n
(cmꢁ1):
d
polymerization has thus transformed the acetylenic triple bond of
1(6) to olefinic double bond of P1(6). This accordingly causes
upfield-shift in the resonance peaks of the phenyl protons, now
2.4.4. Characterization data for P2(12)
Dark brown powdery solid; yield 85.1%. Mw: 86,600; Mw/Mn:
1.95 (GPC, polystyrene calibration; Table 1, no. 11). 1H NMR
occurring at
d 6.60 and 6.36 (Fig. 2B, peaks b and c).
The 13C NMR spectra of polymer P1(6) and its monomer 1(6) are
shown in Fig. 3. The acetylene carbon atoms of 1(6) resonate at
(400 MHz, CDCl3, d, TMS, ppm): 7.88e7.95 (ArH ortho to eN]Ne),
7.74e7.76 (ArH ortho to CF3), 6.36, 6.60, 7.00 (ArH ortho to OCH2,
ArH ortho and meta to ]), 5.80 (HeC]C), 3.95e4.06 (OCH2),
d
83.7 and 75.7, which completely disappear in the spectrum of
1.26e1.84 (CH2]2). 13C NMR (400 MHz, CDCl3,
d, TMS, ppm): 161.6,
P1(6). The resonance peaks of the polyene backbone carbon atoms
of P1(6) are not distinguishable, probably because the polyene
backbone are buried in the long side groups. Like the IR spectra, the
NMR spectra duly confirm that the triple bond of the monomer has
been consumed by the polymerization reaction, and the expected
azobenzene-containing PPAs have been successfully derived.
159.4 (aromatic carbons attached to eN]Ne), 152.7, 148.9
(aromatic carbons attached to eOe), 130.3 (aromatic carbon para to
eN]Ne), 129.0 (aromatic carbons meta to eN]Ne), 124.7 and
122.5 (aromatic carbons ortho to eN]Ne), 114.7, 114.4 and 113.9
(aromatic carbons ortho and para to eOe), 68.1 and 67.8 (carbons
attached to eOe), 29.1 (OCH2CH2), 25.8 (CH2). IR (KBr),
2949 and 2875 (HeCH), 1400 (N]N).
n
(cmꢁ1):
3.3. Photoisomerization behaviors
3. Results and discussion
UVevis spectroscope was used to monitor the photo-
isomerization behaviors of the obtained polymers. The absorption
spectra of the pristine polymers in THF solution show a strong band
3.1. Polymer synthesis
By linking azobenzene to the phenyl ring through flexible
spacers of six carbon atoms (1(6) and 2(6)) and twelve carbon
atoms (1(12) and 2(12)), we obtained four azobenzene-containing
phenylacetylene derivatives. These monomers were prepared
through multi-step reactions shown in Scheme 1. All the reactions
proceeded smoothly, and the desired monomers were isolated in
satisfactory yields (w81e98%). All of the intermediates and
monomers were characterized by standard spectroscopic methods,
from which satisfactory analysis data were obtained (see
Experimental section for details).
CH
A
water
To transform the monomers to polymers, we tried to use orga-
norhodium complexes [Rh(diene)Cl]2, which are widely used as
effective catalysts to polymerize phenylacetylene derivatives. We
first attempted to use [Rh(cod)Cl]2 to polymerize monomer 1(6) in
a THF/TEA mixture. The result was, however, not very satisfactory:
polymer P1(6) was obtained in a yield of w55.4%, which was lower
in comparison with the analogous polymerizations of
phenylacetylene-based monomers, although the molecular weight
was moderate (Mw w 33,700, see Table 1, no. 1). Then we turned to
catalyst [Rh(nbd)Cl]2. Delightfully, the polymerization reaction
gave a better result, affording a polymer in a higher yield of w84.7%
with a higher Mw value of w44,700 (see Table 1, no. 2). All the other
B
CH
2
4000 3000 2000
Wavenumber (cm
1600 1200
800
400
−1
)
Fig. 1. FT-IR spectra of (A) monomer 1(6) and (B) its polymer P1(6) (sample from
Table 1, no. 2).

5296
X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
i
j
1.0
a
b
c
d
e
f
g
h
m
1(6)
N
HC
C
OCH2CH2(CH2)2CH2CH2O
N
P 1(6)
P 1(12)
P 2(6)
P 2(12)
k
l
0.8
0.6
0.4
0.2
0.0
b,l,m
A
f
a
h d
i c
j,k
e,g
0.2
0.1
0.0
*
*
*
i
j
b
c
d
e
f
g
h
m
N
N
400
500
600
C
C
OCH2CH2(CH2)2CH2CH2O
k
l
a H
n
B
250 300 350 400 450 500 550 600
Wavelength (nm)
l,m
*
j,k
e,g,f
h d
i
Fig. 4. Absorption spectra of polymers P1(6) (sample from Table 1, no. 2), P1(12)
(Table 1, no. 7), P2(6) (Table 1, no. 5), P2(12) (Table 1, no. 11) and monomer 1(6) in THF
at room temperature. The concentration of solution is 5 ꢂ10ꢁ5 M.
b c
a
*
9
8
7
6
5
4
3
Chemical shift (ppm)
2
1
0
at around 440 nm is due to nep* transition of the azobenzene
pendants and the stronger absorption of P1(6) in the long wave-
length region is evidently corresponding to the absorption of its
polyene backbone.
Fig. 2. 1H NMR spectra of (A) 1(6) and (B) its polymer P1(6) (sample from Table 1, no.
2) in chloroform-d. The solvent peaks are marked with asterisks.
Under proper ultraviolet irradiation, azobenzene compounds
undergo isomerization from trans- to cis-forms, which is referred as
photoisomerization. To examine the photoisomerization behavior
of the polymers, we monitored the changes in absorption features
of the monomers and their polymers in proper solutions by
recording at different time intervals until cis-rich photostationary
state was reached. Fig. 5 displays the recorded photoisomerization
behavior of P1(6). After irradiation with 365 nm UV light (8 W), the
absorption band at around 350 nm decreased continuously, while
the absorption intensity at around 440 nm increased gradually.
Furthermore, two isosbestic points at 313 and 408 nm were
observed. The change of the absorption bands induced by UV
irradiation is indicative of the photoisomerization of polymers
carrying azobenzene pendants from the trans to the cis form.
UVevis absorption changes of the monomers and other
azobenzene-containing polymers were also recorded under
at round 350 nm and weak broad tail at wavelengths longer than
400 nm (Fig. 4), which can be readily assigned by comparing with
the absorption spectra of their monomers. The spectrum of 1(6) is
also given in Fig. 4 as a standard. The strong band of 1(6) at round
350 nm is ascribed to the
pe
p* transition of the trans-form of
azobenzene moieties. The polymers all exhibit an identical
absorption band in the same wavelength region, indicating the
pristine polymers take trans-configuration in THF solution. From
the magnified spectra shown in the inset of Fig. 4, it can be seen
that the absorption intensity of P1(6) at wavelengths longer than
400 nm is stronger than its monomer 1(6). The weaker band of 1(6)
q r
m n
g,k
p
s
d e
g
h
i
j
k
a b
N
l
o
N
f
c
HC
C
OCH2CH2(CH2)2CH2CH2O
1.0
*
q
e,m
c
d
A
i
r
n
0.8
h,j
0s
a
o,f p l
s
b
0.6
q r
p
s
m n
d e
c
g h
i
j k
N
N
f
l
O
o
bC
H
OCH2CH2(CH2)2CH2CH2
0.4
Ca
420s
n
q
*
B
0.2
n
r
e,m
o
f
p l
h,j
g,k
i
s
0.0
250 300 350 400 450 500 550 600
Wavelength (nm)
180 160 140 120 100 80
Chemical shift (ppm)
60
40
20
Fig. 5. UVevis absorption changes of P1(6) (sample from Table 1, no. 2) under different
time interval during the irradiation time with 365 nm UV light (8 W) in THF at room
temperature. The concentration of solution is 5 ꢂ10ꢁ5 M.
Fig. 3. 13C NMR spectra of (A) 1(6) and (B) its polymer P1(6) (sample from Table 1, no.
2) in chloroform-d. The solvent peaks are marked with asterisks.

X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
5297
different time interval during the irradiation time with 365 nm UV
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
UV irradiation time (min)
0
light in THF, and the results are shown in Figures S2eS8.
The kinetics of transecis photoisomerization of polymers P1(6),
P1(12), P2(6), P2(12) and monomers 1(6) and 1(12) in THF solution
were evaluated by analyzing the absorbance of the pep
* transition
5
of the trans-form of azobenzene. The first-order rate constant was
determined by the formula (1).
10
15
20
heat at 90oC
for 5 min
ꢁket ¼ ln½ðAN ꢁ AtÞ=ðAN ꢁ A0Þꢃ
(1)
where ke and t are the rate constant of photoisomerization and the
time of UV-light irradiation, and A0, At, and AN represent the
absorbance at around 350 nm at the time of beginning, time t, and
time infinite, respectively. Extracting experimental data from Fig. 5,
the photoisomerization kinetics of these polymers and monomers
are plotted in Fig. 6. All the polymers and the monomers display
linear relationship between the natural logarithm of specific
absorbance difference, or (AN ꢁ At)/(AN ꢁ A0) and UV-light illumi-
nation time. The linear dependence suggests that the photo-
isomerization of the polymers as well as the monomers follows the
first-order kinetics, and the rigid polyene backbone has not
changed the basic kinetics of the azobenzene moieties attaching to
it via a flexible alkyl spacer.
25
30
250
300
350
400
450
500
550
Wavelength (nm)
Fig. 7. UVevis absorption changes of P2(12) (sample from Table 1, no. 11) film during
the irradiation with UV light (365 nm, 8 W) at room temperature (rt) and further
heating at 90 ꢀC for 5 min. The irradiation time is marked as numeral.
moiety must have taken trans-form when the polymer film was
quenched from its isotropic state. Indeed, only the trans-form
azobenzene is regarded as a kind of mesogen to form liquid crystal.
The UV spectra of polymer films of P1(6), P1(12) and P2(6) under
UV irradiation and heating were also tested, and the effect of irra-
diation time on absorbance intensity changes at 354 nm is shown
in Fig. 8. Despite their structural differences, four polyacetylenes
adopted similar isomerization behaviors due to the fact that the
azobenzene moieties were equally suppressed so as to undergo
identically slow isomerization.
But the polymer backbone has inevitably influenced the rate of
photoisomerization. Deduced from Fig. 6, the photoisomerization
rate constant, ke, of polymers P1(6), P1(12), P2(6), and P2(12) is
0.013, 0.014, 0.015, and 0.017 sꢁ1, respectively. The average rate
constant of the polymers is about 0.015 sꢁ1, which is lower than the
literature value derived from experiments conducted under similar
conditions. When compared with monomers 1(6) and 1(12)
(0.022 sꢁ1), the average ke of the polymers decreases by nearly 32%.
This is reasonable that the trans-to-cis transition resistance existed
in small molecules is lower than that in macromolecules.
We also studied the photoisomerization behavior of four poly-
mers as film state. Fig. 7 shows the UV spectra of P2(12) film under
UV irradiation and further heating for a certain time. It is clear that
the isomerization of polymer film from trans- to cis-rich photo-
stationary state is 4.3 times slower than the corresponding polymer
solution (see Figure S8). We can also noted that, by heating the film
at 90 ꢀC for 5 min, the cis-rich P2(12) was largely transformed to
trans-form. Therefore, it is understandable that the azobenzene
3.4. Thermal stability
Azobenzene is a widely used mesogen in the construction of
thermotropic liquid-crystalline polymers, and heating is necessary
for the generation of the mesophases. Therefore, the thermal
stability of the polymers is one of the key parameters and we
evaluated the thermal stability of P1(6), P2(6), P1(12), and P2(12) by
thermal gravimetric analysis (TGA) technique. The recorded ther-
mograms for the polymers are shown in Fig. 9; the Td values for
P1(6), P2(6), P1(12), and P2(12) are picked up from the TGA curves
0
-1
-2
0.35
0.30
UV irradiation
0.25
-3
P 1(6)
P 1(12 )
P 2(6)
heat
0.20
-4
0.15
P 2(12 )
1(6)
1(12 )
P 1(6)
P 2(6)
P 1(12 )
-5
0.10
-6
P 2(12 )
0.05
0
50
100 150 200 250 300
Time (s)
0
5
10
15
20
25
30
UV irradiation time (min)
Fig. 6. First-order transecis isomerization kinetic of polymers P1(6) (sample from
Table 1, no. 2), P1(12) (Table 1, no. 7), P2(6) (Table 1, no. 5), P2(12) (Table 1, no. 11) and
monomers 1(6) and 1(12) in THF solution under different time interval at room
temperature. The concentration of solution is 5 ꢂ10ꢁ5 M.
Fig. 8. Absorbance intensity changes at 354 nm of polymer films of P1(6) (sample from
Table 1, no. 2), P1(12) (Table 1, no. 7), P2(6) (Table 1, no. 5), P2(12) (Table 1, no. 11)
under UV irradiation (365 nm, 8 W) at rt and further heating at 90 ꢀC for 5 min.

5298
X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
100
80
60
40
20
0
A
B
g
i
g
SmA
74.8
72.0
87.5
P1(12)
P2(12)
T (o C)
5
295
293
310
277
P1(12)
SmA
i
i
P1(6)
P2(6)
P1(12 )
P2(12 )
P2(12)
80.8
i
SmA
92.2
SmA
g
0
100
200
300
400
500
600
102.9
o
90.5
Temperature ( C)
Fig. 9. TGA thermograms of P1(6) (sample from Table 1, no. 2), P1(12) (Table 1, no. 7),
P2(6) (Table 1, no. 5), and P2(12) (Table 1, no. 11) measured under nitrogen at a heating
rate of 20 ꢀC/min.
60
80
100
120 60
Temperature, C
80
100
120
o
Fig. 11. DSC thermograms of P1(12) (Table 1, no. 7) and P2(12) (Table 1, no. 11)
recorded under nitrogen during the (A) first cooling and (B) second heating scans at
a scan rate of 10 ꢀC/min.
as 295, 293, 310, and 277 ꢀC, respectively. These data indicate that
these azobenzene-containing polymers have higher thermal
stability than their parent polymer, or poly(phenylacetylene) (PPA),
which shows a decomposition temperature (Td, defined as the
temperature at which the polymer loses 5% of its original weight) of
225 ꢀC [85]. The improved thermal stability can be ascribed to the
“jacket effect” of the bulky azobenzene pendants attached to the
PPA backbone [4e6,86e89]. In comparison with the azobenzene-
containing polyacrylates, the increase in decomposition tempera-
ture is more pronounced. For example, Zhang and colleagues
microscope (POM) was used to examine the mesomorphic behav-
iors of these azobenzene-containing polymers. Fig. 10 shows the
POM microphotographs of the textures of P1(12) and P2(12)
recorded in the heatingecooling cycles. When P2(12) was cooled
from its isotropic state, very small bâtonnets emerged from the
homotropic dark background, forming an anisotropic mesomorphic
texture. This indicates that P2(12) is a liquid-crystalline polymer.
The texture contains tiny bands, whose directors are perpendicular
to the long axes of the bâtonnets. Such in-domain bands are
frequently observed during the formation of the focal-conic
textures in liquid-crystalline polyacetylenes [13e18], suggesting
that the mesophase of P2(12) is in a nature smectic A phase. In an
attempt to grow bâtonnets to bigger focal-conic texture, after
cooling P2(12) to 91.7 ꢀC, we annealed the sample at the same
temperature and observed fan-shaped patterns with a diameter of
recently reported
a series of polymethacrylate-based photo-
responsive side-chain liquid-crystalline polymers with azobenzene
mesogen; the decomposition temperatures of these polymers are
all lower than 280 ꢀC [32]. The elevated decomposition tempera-
ture of the obtained azobenzene-containing polyphenylacetylenes
is desirable because high thermal stability is required by many
high-tech applications.
35 mm at the annealing time of 30 min (Fig. 10B). When P1(12) was
3.5. Liquid crystallinity
cooled from its melting state, many anisotropic entities emerged
from the background (Fig. 10C). However, the development of these
tiny entities into a typical SmA texture was difficult. For both P1(6)
After checking the thermal stabilities of the polymers, we pro-
ceeded to study their liquid crystallinity. Polarized optical
Fig. 10. Mesomorphic textures observed on cooling (A) P2(12) to 91.7 ꢀC, (B) P2(12) to 91.7 ꢀC and followed by annealing for 30 min, (C) P1(12) to 74.5 ꢀC from their isotropic states
at a cooling rate of 3 ꢀC/min.

X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
5299
Table 2
as revealed by the POM images (Figures S9 and S10). These results
indicate that the length of the flexible spacers plays a key role in the
formation of observable mesophases.
Thermal properties of polymers P1(6), P2(6), P1(12) and P2(12).a
Polymer
T, ꢀC
Differential scanning calorimetry (DSC) measurements were
conducted to evaluate the thermal transition temperatures. Fig. 11
shows the DSC thermograms of P1(12) and P2(12) recorded under
nitrogen during the first cooling and second heating scans and the
DSC curves for the corresponding monomers are displayed in
Figures S11 and S12. In the first cooling circle of P1(12), a sharp
exothermic peak associated with i-SmA is observed at 80.8 ꢀC. The
mesophase is stable in a temperature range over 8.8 ꢀC before the
polymer solidifies at 72.0 ꢀC. The associated g-SmA and SmA-i
transitions are observed at 74.8 ꢀC and 87.5 ꢀC, respectively. The
transition profiles of P2(12) are similar to those of P1(12). In its first
cooling circle, P2(12) enters the SmA phase from its isotropic state
at 102.9 ꢀC and the corresponding SmA-i transition is detected at
92.2 ꢀC. The thermal properties of four polymers are summarized in
Table 2. It is clear that both P1(6) and P2(6) had only one thermal
transition, consistent with the POM results. However, P1(12) and
P2(12) owned two transitions, indicative of the existence of liquid-
crystalline state.
Cooling
Heating
P1(6)
P2(6)
P1(12)
P2(12)
i 102.9 k(g)
i 113.3 k(g)
i 80.8 SmA 72.0 k(g)
i 102.9 SmA 90.5 k(g)
k(g) 108.1 i
k(g) 121.3 i
k(g) 74.8 SmA 87.5 i
k(g) 92.2 SmA 105.4 i
a
Data taken from the DSC thermograms recorded under nitrogen in the first
cooling and second heating scans; abbreviations: k ¼ crystalline state, g ¼ glassy
state, SmA ¼ smectic A phase, i ¼ isotropic liquid.
and P2(6), the POM observation gave a disappointing result that
they were not liquid crystalline. This is probably due to the shorter
flexible spacer and the stiff polymer backbone, which hinder the
alignment of the azobenzene mesogen. In contrast, the two
monomers 1(12) and 2(12) both showed liquid-crystalline behavior,
XRD analyses were performed in an attempt to verify the mes-
ophase assignment and to learn more about the molecular packing.
The Bragg reflections at 2
spacing of molecular orientational order, and the reflections at 2
q
of 3e5ꢀ can be ascribed to the layer
q
around 20ꢀ associate with the intermesogenic organization within
the layers [18,20,90]. The appearance of a broad or sharp peak
serves as a qualitative indication of the degree of order. As can be
seen from Fig. 12, the diffraction pattern of P1(12) shows a diffuse
peak centered at 2
of 4.56 A (d2). This distance is associated with the lateral packing
q
¼ 19.46ꢀ, corresponding to an average distance
ꢀ
arrangement of the azobenzene mesogen. In the small-angle
region, a Bragg diffraction at 2
q
¼ 5.29ꢀ is observed and the calcu-
ꢀ
lated layer distance is 16.69 A (d1), which is tentatively assigned to
the half of the fully extended molecular length of two layers azo-
benzene mesogens (see Fig. 13).
As to P2(12), the Bragg diffraction at wide angle region appears
ꢀ
ꢀ
at 2q
¼ 20.28 , corresponding to an average distance of 4.38 A (d2).
It indicates that the lateral packing arrangement of the CF3-azo-
benzene mesogens is closer than that of azobenzene mesogens
(4.56 A). The observed shorter intermesogen distance can be
Fig. 12. XRD patterns of liquid-crystalline samples of P2(12) (Table 1, no. 11) and P1(12)
(Table 1, no. 7). The samples were heated to 200 ꢀC, cooled to 90 ꢀC for P2(12) and
70 ꢀC for P1(12) at a rate of 5 ꢀC/min, and then kept for 30 min before being rapidly
quenched by liquid nitrogen.
ꢀ
explained by the fact that the electron-withdrawing effect of the
Fig. 13. Illustrations of the molecular configuration after energy minimization: (A) and (B) are top and side view of P1(12), and (C) and (D) are top and side view of P2(12),
respectively. Ten repeat units are used in calculation. The carbon, hydrogen, oxygen, nitrogen and fluoride atoms are shown in gray, white, red, blue and green, while the carbon
atoms in backbone are shown in pink color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5300
X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
trifluoromethane group at the end of azobenzene mesogen
enhances the molecular dipole and induces the adjacent azo-
benzene mesogens to come close. In small-angle region, the
N
R
C
O(CH2)mO
N
diffraction peak centers at 2
q
¼ 4.06ꢀ, corresponding to an inter-
H C
ꢀ
layer distance of 21.75 A (d1). This distance is longer than that
observed for P1(12), which can be reasonable interpreted by the
fact that an CF3-azobenzene mesogen has a larger size than an
azobenzene mesogen.
n
P1(m) (R=H)
2
To confirm the analysis of the XRD data and get further insight
into the mesogen arrangement in solid states, theoretical calcula-
tion has been performed on both to P1(12) and P2(12) with Mate-
rials Studio 5.0 program. The calculation results are displayed in
Fig. 13. For both P1(12) and P2(12), the side view of the macro-
molecular configuration simulated by using 10 repeat units
suggests that the side chains stretch out at the two sides of the
polyene backbone and they lay in the same plane as the polymer
main-chain (Fig. 13A and B). The top views disclose more structural
information about molecular packing. As shown by Fig. 13C and D,
the side and main-chains of the polymers take a fishbone-like
arrangement. For P1(12), the tilt angles between the side chains
and main-chain can be largely divided into two groups; the smaller
group is averaged to be 41.88ꢀ, and the larger one is 54.08ꢀ
(Fig. 11C). While for P2(12), the average tilted angle is 59.45ꢀ
(Fig. 13D). In addition, the calculated results also show that the CF3-
azobenzenes have higher degree of regularity in mesogen packing
than the azobenzenes. These data imply that, in comparison with
P1(12), P2(12) has more extended side chains’ configuration, and
this is in good agreement with the XRD data. Based on the calcu-
lation results and experimental data, the smectic structure
observed for P1(12) and P2(12) can be illustrated as Fig. 14. A single
polymer chain takes a fishbone configuration with a mesogen at the
end of each side chain. The mesogens on the adjacent polymer side
chains take an interdigital arrangement; such an arrangement
results from the dipoleedipole interaction between the adjacent
mesogens, which leads to the minimum of total free energy of the
whole system. The macromolecular packing shown in Fig. 14 is not
P (m) (R=CF3) (m=6, 12)
Chart 1.
only in good consistent with the XRD data and simulation results,
but also constructive to the formation of smectic liquid crystal
mesophase.
4. Conclusions
In summary,
a
series of azobenzene-containing poly(-
phenylacetylene) derivatives (P1(6), P1(12), P2(6), P2(12)) have
been successfully synthesized in high yields by using [Rh(nbd)Cl]2
and/or [Rh(cod)Cl]2 as catalysts; their structures have been char-
acterized with spectroscopic characterization techniques including
FT-IR, 1H NMR, 13C NMR, and UVevisible absorption spectroscopy.
Typical photoisomerization behavior was observed for all these
azobenzene-containing PPAs by monitoring the feature variations
of the UVevisible absorption spectra of the polymer solutions. The
rigid PPA main-chain had not prohibited the photoisomerization
due to the existence of flexible alkyl spacers between PPA main-
chain and the azobenzene moieties, but only lowered the transi-
tion rate. The longer alkyl spacer corresponded to a higher isom-
erization rate.
The incorporation of azobenzene moieties into polymer side
chains also bestowed P1(12) and P2(12) with liquid-crystalline
property. The flexibility of the alkyl spacer played a key role in the
formation of the mesophase. POM observations indicated that only
P1(12) and P2(12) exhibited liquid crystal property, but P1(6) and
P2(6) did not, although these two polymer also possessed azo-
benzene mesogens. The mesomorphic texture in the POM images
suggested that the mesophase formed by P1(12) and P2(12) could
be assigned to SmA phase. The phase transition behaviors were
measured by DSC technique. The g-SmA and SmA-i transitions
observed for P1(12) occurred at 74.8 ꢀC and 87.5 ꢀC, respectively.
While for P2(12), it entered the SmA phase from isotropic state at
102.9 ꢀC and the corresponding SmA-i transition was detected at
92.2 ꢀC in the cooling process. The XRD patterns of P1(12) and
P2(12) both displayed sharp diffraction peaks, confirming the
assignment of SmA phase. The XRD peaks at w5ꢀ and w20ꢀ were
correlated with the reflections from ordered layer-by-layer stacking
of macromolecules and the ordered packing of adjacent mesogens,
respectively. Combining with the theoretical calculation results, an
illustration of the molecular arrangement was tentatively
proposed. The present investigation not only obtained a few novel
azobenzene-functionalized polyphenylacetylene derivatives, but
also demonstrated the interdependent properties between three
key structural parameters of rigid polymer main-chain, flexible
alkyl spacer, and photoisomerization-capable mesogen, thus
provides helpful information to the designation and preparation of
conjugated polymers with desirable opto-electronic properties.
Acknowledgments
The work reported in this paper was partially supported by the
National Science Foundation of China (Project Nos. 50873086,
Fig. 14. Schematic presentation of the Smectic A structure of P2(12).

X. A.-Zhang et al. / Polymer 52 (2011) 5290e5301
[40] Ubukata T, Hara M, Ichimura K, Seki T. Adv Mater 2004;16:220e3.
5301
20974098, and 21074113), the Ministry of Science & Technology of
China (Project No. 2009CB623605), the Research Grants Council of
Hong Kong (603509 and HKUST2/CRF/10), and the University
Grants Committee of Hong Kong (AoE/P-03/08). B.Z.T. thanks the
support from the Cao Guangbiao Foundation of Zhejiang University.
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