Photoresponding ionic complex containing azobenzene chromophore for use in birefringent film

Article abstract of DOI:10.1016/j.cclet.2013.11.008

A novel photoresponding ionic complex (PANDAZO) was prepared by the ionic self-assembly (ISA) of sodium polyacrylate (PANa) and azobenzene chromophores (NDAZO). The ionic complex forms an interdigitated lamellar structure with full overlap of the side chains. The optical anisotropy was investigated by using a polarization pulse laser (355 nm). Furthermore, a high photoinduced birefringence (Δn = 0.365) was measured by using a continuous 488 nm laser as the pump light.

Full text of DOI:10.1016/j.cclet.2013.11.008

Chinese Chemical Letters 25 (2014) 15–18
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Photoresponding ionic complex containing azobenzene chromophore
for use in birefringent film
a
a
a,b,
Jun Wu a, Xue-Min Lu a, , Feng Shan , Jun-Fang Guan , Qing-Hua Lu
*
*
a School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
b The State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel photoresponding ionic complex (PANDAZO) was prepared by the ionic self-assembly (ISA) of
sodium polyacrylate (PANa) and azobenzene chromophores (NDAZO). The ionic complex forms an
interdigitated lamellar structure with full overlap of the side chains. The optical anisotropy was
Received 9 June 2013
Received in revised form 11 September 2013
Accepted 16 October 2013
Available online 23 November 2013
investigated by using
a polarization pulse laser (355 nm). Furthermore, a high photoinduced
birefringence ( n = 0.365) was measured by using a continuous 488 nm laser as the pump light.
D
ß 2013 Xue-Min Lu and Qing-Hua Lu. Published by Elsevier B.V. on behalf of Chinese Chemical
Keywords:
Society. All rights reserved.
Azobenzene
Birefringence
Ionic self-assembly
Optical anisotropy
1. Introduction
application [17–19]. Therefore, design and synthesis of materials
with high photoinduced orientation and inherent high birefrin-
Materials with high birefringence have attracted great interest
for their versatile application in technologies such as organic light-
emitting diodes (OLEDs) [1], photostorage devices [2], optical
fibers [3] and telecommunication devices [4]. Different materials
are designed and synthesized to gain high birefringence including
tolane [5,6], dipheny-diacetylene [7], and azobenzene [8].
Among these materials, azobenzene and its derivatives are
actively investigated [9]. Photoinduced anisotropy (PIA) and
birefringence (PIB) were attributed to the repeated trans–cis–trans
photoisomerization cycles of azobenzene chromophores, which
endowed the materials containing azobenezene chromophores
with various applications in optical information storage [10],
polarization holographic gratings [11,12], photomechanics [13],
and so forth.
Ionic self-assembly (ISA) has emerged as an effective strategy
for the design of novel functional materials [14–16]. This approach
does not need rigorous and complicated synthetic processes
compared to all-covalent polymers. In previous reports, ionic
complexes containing azobenzene were prepared and showed
photosensitivity under light irradiation. However, the birefrin-
gence was low and far from the requirement for practical
gence by this facile method is still necessary.
Herein, we prepared a novel material (PANDAZO) with high
birefringence by ISA of sodium polyacrylate (PANa) and azoben-
zene chromophores (NDAZO). The ionic complex shows lamellar
structure. Under linear pulsed laser irradiation, the complex film
exhibits high optical anisotropy. A high photoinduced birefrin-
gence up to 0.365 is achieved by using a continuous 488 nm
linearly polarized laser as the pump light.
2. Experimental
Sodium polyacrylate (PANa) with a molar mass (Mw) of 5100
was purchased from Aldrich. Sodium nitrite, phenol, N,N-
dimethylaniline, N-methylimidazole, potassium carbonate, and
the organic solvents utilized in this work were purchased from
Sinopharm Chemical Reagent Co. and used without purification. 4-
Hydroxy-40-dimethylaminoazobenzene (1) was prepared accord-
ing to the reported method [20]. FT-IR spectra were recorded on a
Perkin-Elmer Paragon 1000 FTIR spectrometer. Normal and
polarized absorption spectra of the complex films were recorded
on a Perkin-Elmer lambda 750 UV/vis spectrometer. 1H NMR
spectra were acquired on
a Varian Mercury Plus 400 MHz
spectrometer in DMSO-d6 or CDCl3 and the chemical shifts were
referenced in ppm versus tetramethylsilane. Small-angle X-ray
diffractogram (SXRD) patterns of the casting films were recorded on
a Rigaku X-ray diffractometer D/MAX-2200/PC at a rate of 18/min
*
Corresponding authors at: School of Chemistry and Chemical Engineering,
Shanghai Jiao Tong University, Shanghai 200240, China.
E-mail addresses: xueminlu@sjtu.edu.cn (X.-M. Lu), qhlu@sjtu.edu.cn (Q.-H. Lu).
1001-8417/$ – see front matter ß 2013 Xue-Min Lu and Qing-Hua Lu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.11.008

16
J. Wu et al. / Chinese Chemical Letters 25 (2014) 15–18
over the 2u
range 1–108. Na+ and Brꢀ analysis was performed using
an FEI NanoSEM 230 scanning electron microscope equipped with
an X-Max 80 energy-dispersive spectrometer (EDS).
1600 cm-1
1366 cm-1
a
2.1. Synthesis of 4-(6-bromohexyloxy)-40-(N,N-dimethylamino)
b
c
azobenzene (2)
4-Hydroxy-40-dimethylaminoazobenzene 1 (2.41 g, 10 mmol)
was dissolved in acetone, and then 2 equivalent 1,6-dibromohex-
ane (4.83 g, 20 mmol) and 1.5 equivalent potassium carbonate
(2.1 g, 15 mmol) were added to the solution. The mixture was
refluxed for 24 h with stirring. The residue was filtered off and
washed with ethyl acetate. The organic solvent was removed from
the combined filtrate and washings under reduced pressure, and
petroleum ether (60–90 8C) was added to the concentrate. The
resulting precipitate was collected and dried. The crude product
was purified by recrystallization from ethanol. Yield: 3.1 g, 77%.
1151 cm-1
-1
1400 cm
1800
1500
1200
-1
900
Wavenumber (cm
)
Fig. 1. FT-IR spectra of (a) NDAZO, (b) PANDAZO and (c) PANa.
1H-NMR (400 MHz, CDCl3):
d 7.84 (4H, ArH), 6.97 (2H, ArH), 6.76
(2H, ArH), 4.02 (2H, OCH2), 3.43 (2H, BrCH2), 3.07 (6H, N(CH3)2),
1.91 (2H, CH2), 1.83 (2H, CH2), 1.52 (4H, CH2CH2).
chloroform/ethanol (9/1, v/v), DMSO, and N-methyl-2-pyrrolidone
(NMP). Fig. 1 shows the FT-IR spectra of NDAZO, PANDAZO, and
PANa. The stretching vibration bands of the azo moieties in the
complex were found at ca. 1600, 1366, and 1151 cmꢀ1. The
symmetric carboxylate stretch of the complex was found at
1400 cmꢀ1. This result indicated that the NDAZO unit was
successfully attached to the PANa main chain.
2.2. Synthesis of 1-(6-(4-methoxyazobenzene-40-oxy)hexyl)-3-methyl-
1H-imidazol-3-ium bromide (NDAZO)
Compound
2 (1.0 g, 2.48 mmol) was dissolved in 30 mL
tetrahydrofuran. Then 1-methylimidazole (1.63 g, 19.84 mmol)
was added to the solution. The mixture was heated to reflux for
72 h. The resulting precipitate was obtained by filtration and
washed with tetrahydrofuran and diethyl ether, respectively. The
filter cake was dried under vacuum to give NDAZO. Yield: 0.78 g,
Fig. 2 shows the 1H NMR spectra of NDAZO and PANDAZO, with
unambiguous assignments to various protons. It can be seen that
the phenyl signals of PANDAZO were slightly upfield shifted
compared to those of the small molecule NDAZO, while the
imidazolium signals were shifted downfield. In particular, a
significant downfield shift of the active proton signal ‘‘a’’ of the
imidazolium moiety was observed, which resulted from the
influence of the opposite negatively charged carboxyl on the main
chain. Furthermore, all of the proton signals of NDAZO became
broad after attachment to PANa. These results confirmed the
formation of ionic complex PANDAZO. In addition, because the
proton signals of PANa were overlapped with those of NDAZO, it
was difficult to estimate the assembly ratio of the two components
by 1H NMR. Energy-dispersive analysis of the complex indicated
the absence of Na+ and Brꢀ ions. Therefore, we could assume a 1:1
ratio of carboxylic and imidazolium moieties in the complexes.
Fig. 3 shows the small-angle X-ray diffractogram of the
PANDAZO casting film. A set of reflection peaks with a 1:2:3 ratio
of positions was seen, indicating the formation of long-range
ordered lamellar structures. The Bragg spacing d was 2.89 nm
which was in excellent agreement with the calculated length l0
(2.90 nm) of the fully extended side-chain units. This result
indicated the full overlap of the side chains in an interdigitated
packing structure, as shown in the insets of Fig. 3 [21]. In addition,
the first-order reflection peak is very weak and the second-order
reflection peak is the most intense. This might be related to
interference effects of an additional plane of symmetry in their
lamellar electron density profile [22–24].
65%. 1H NMR (400 MHz, DMSO-d6):
d 9.13 (1H, N55CH–N), 7.72 (6H,
ArH, N–CH55CH–N), 7.03 (2H, ArH), 6.80 (2H, ArH), 4.17 (2H, OCH2),
4.02 (2H, NCH2), 3.84 (3H, NCH3), 3.02 (6H, N(CH3)2), 1.76 (4H,
CH2CH2), 1.45 (2H, CH2), 1.30 (2H, CH2).
2.3. Preparation of ionic complex PANDAZO and film
Aqueous PANa solution (5 mg/mL) was added dropwise to
aqueous NDAZO solution (1 mg/mL) with a 1:1 molar charge ratio
of cations to anions. The resulting precipitate was collected by
filtration and washed thoroughly with deionized water to remove
residual salts and noncomplexed precursors, and then dried in
vacuum at 60 8C for 24 h. PANDAZO films were prepared by spin-
coating a chloroform/ethanol (9/1, v/v) solution (concentration:
20 mg/mL) onto quartz plates and glass slides (speed: 2000 rpm;
time: 20 s) after being filtered through a 0.45
mm Millipore filter.
The thickness of the resultant film was about 200–300 nm, as
measured by means of Ellipsometer.
3. Results and discussion
NDAZO unit was prepared according to Scheme 1. NDAZO unit
showed good solubility in water. Ionic complex PANDAZO was
obtained as a precipitate by adding PANa to the NDAZO solutions in
a 1:1 charge ratio. PANDAZO showed excellent solubility in
The photoinduced orientation behavior of the complex film was
investigated. An s-polarized pulsed laser (355 nm) was used as the
Scheme 1. The synthetic routes to the NDAZO and PANDAZO.

J. Wu et al. / Chinese Chemical Letters 25 (2014) 15–18
17
d
k
*
a
O
N
c
j
i
0.4
0.3
0.2
0.1
0.0
k
b
O
N
O
N
n
4
N
N
e
f
g
h
off
g
f
d
j
e
h
c
i
b
a
on
10
9
8
7
4
3
(ppm)
Fig. 2. 1H NMR spectra of PANDAZO (up) and NDAZO (down) in DMSO-d6.
0
20
40
60
80
Time (min)
Fig. 5. Photoinduced birefringence recorded in a thin film of PANDAZO with a
488 nm linearly polarized beam (165 mW/cm2) at room temperature. The points at
which the pumping light was switched on and off are marked with arrows.
= PANa main chain
= NDAZO unit
(A// ꢀ A )/(Alarge + 2Asmall), where A// and
A
are the UV/vis
?
?
absorbance parallel and perpendicular to the laser polarization
2.89 nm
direction, respectively. Alarge and Asmall is the larger and smaller
2q
absorbance of A// and A , respectively. The orientation order
?
parameter S of PANDAZO film at 395 nm was ꢀ0.59, the negative
sign of which means that the oriented direction of azobenzene was
perpendicular to the laser polarization [25].
q
3q
To investigate photoinduced birefringence,
a continuous
linearly polarized 488 nm laser with energy of 165 mW/cm2
was used as the pump light. A weak He–Ne laser at 650 nm was
2
4
6
8
used as the probe light. The birefringence (
I = I0 sin2(pDnd/
), where I is the intensity of the transmittance, I0
is the intensity of the probe beam, is the wavelength, d is the
thickness of the film. The relationship between photoinduced
birefringence and irradiation time is shown in Fig. 5. The
Dn) was calculated by
2 Theta (degree)
l
Fig. 3. SXRD profile of the complex PANDAZO at room temperature. The inset is
l
schematic representation of the layered architectures of the PANDAZO complex.
light source. The spin-coated films were fixed on an X–Y moving
platform with speeds of 0.1 mm/s and 5 mm/s in the X and Y
birefringence value (Dn) increased with the irradiation time
increasing, and reached a high saturation value (0.354) after
40 min. The low absorbance of the PANDAZO film at 488 nm
resulted in a long response time. Irradiation at shorter wavelengths
might improve the dynamics of the recording process. Finally, it is
noted that the photoinduced birefringence showed a slight
increase and reached a higher value of about 0.365 after turning
off the pump light. The ultimate high orientation may be attributed
to the rigid molecular structure and the fact that the orientation
was fixed in the aggregated phase. These results showed PANDAZO
to be an ideal photoresponding material.
directions, respectively. The incident angle
u was fixed at 158 with
respect to the normal of the PANDAZO films. Fig. 4 shows the
angular-dependent UV/vis spectra of PANDAZO at its maximum
absorption wavelength (395 nm).
A periodic change in the
absorbance with polarization angle over a period of 1808 can be
seen. The absorbance in the direction perpendicular to the laser
polarization direction (
u = 908, A ) was the highest, while that in
?
the direction parallel to laser polarization direction (
u
= 1808, A//)
was the lowest, which indicated that the preferred direction of
the azo chromophores was perpendicular to the laser polarization.
The in-plane orientation order parameter S was calculated by
4. Conclusion
We prepared
a photoresponding complex by ionic self-
90
assembly. The ionic complex PANDAZO formed interdigitated
lamellar microstructures with full overlap of the side chains.
Effective photoinduced anisotropy (S = ꢀ0.59) was observed under
irradiation by a pulsed laser. Polarized UV spectra revealed that the
preferred direction of the azobenzene chromophores was perpen-
dicular to the laser polarization direction. A high birefringence
1.5
1.0
0.5
0.0
0.5
1.0
1.5
Laser polarization
30
120
60
150
(Dn = 0.365) was induced by using a 488 nm linearly polarized
laser as the pump light.
180
0
Acknowledgments
S = -0.59
300
210
330
The authors gratefully acknowledge financial support from the
National Science Fund for Distinguished Young Scholars (No.
50925310), the National Science Foundation of China (Nos.
50902094 and 51173103), the 973 Project (Nos. 2009CB93043
and 2012CB933803), and the Excellent Academic Leaders of
Shanghai (No. 11XD1403000).
240
270
Fig. 4. Angular-dependent polarized UV/vis spectra of PANDAZO at 395 nm after
irradiation with 25 mW/cm2 by pulse polarization laser (355 nm).

18
J. Wu et al. / Chinese Chemical Letters 25 (2014) 15–18
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