Photorefractive response of polymeric composites with pendant triphenylamine moiety

Article abstract of DOI:10.1021/ma051113b

The photorefractive property of newly synthesized polymeric composite, poly(4-diphenylamino)styrene (PDAS), as a triphenylamine pendant moiety is analyzed. The photorefractive performance of PADS composites including a sensitizer, a nonlinear optical dye and a plasticizer was also investigated. Photorefractive organic composites have the high potential applications such as holographic optical devices for data storage and real-time image processing. Large symmetric two-beam coupling, which is caused by out-of-phase refractive index modulation with respect to illumination pattern, is a unique feature of photorefractive materials.

Full text of DOI:10.1021/ma051113b

Macromolecules 2005, 38, 7521-7523
7521
Notes
Photorefractive Response of Polymeric
Composites with Pendant Triphenylamine
Moiety
Naoto Tsutsumi,* Takehiro Murao, and
Wataru Sakai
Department of Polymer Science & Engineering, Kyoto
Institute of Technology, Matsugasaki, Sakyo, Kyoto
606-8585, Japan
Received May 30, 2005
Revised Manuscript Received July 1, 2005
Introduction
Information and data processing technology have
been widely developed. Among them, photorefractive
organic composites have high potential applications
such as holographic optical devices for data storage and
real-time image processing. Since the first report on1
photorefractive polymer composite had been published,
many studies of photorefractive organic materials have
been investigated, which has been well reviewed in the
2
,3
articles. We have also developed photorefractive re-
4
sponse of poly(vinylcarbazole) (PVCz) composite and
5
,6
related materials. It was obtained the response time
-
1
t ) 0.004 s and optical gain ) 31.7 cm at 80 V/µm for
the composites using tolyldiphenylamine links as the
7
hole transporting units. Large asymmetric two-beam
coupling (asymmetric energy transfer), which is caused
by out-of-phase refractive index modulation with respect
to the illumination pattern, is a unique feature of
photorefractive materials. Refractive index modulation
due to photochromism, photoisomerization, thermo-
refraction, and photobleaching is in-phase to illumina-
tion pattern, which cannot give the asymmetric energy
transfer.
In this paper, we present a newly synthesized pho-
torefractive vinyl polymer with pendant moiety of
triphenylamine, poly(4-diphenylamino)styrene (PDAS).
The photorefractive performance of PDAS composites
including a sensitizer, a nonlinear optical dye, and a
plasticizer was investigated in comparison with that of
conventional composite based on PVCz.
Figure 1. Structural formulas of chemicals used in this study.
a column chromatography with dichloromethane as an devel-
oping solvent at room temperature. The melting point of DAS
was 91.3 °C. DAS monomer (0.5 g/mL) was polymerized in
tetrahydrofuran (THF) with a 2,2′-azobis(isobutyronitrile) as
an initiator in a vacuum at 60 °C for 12 h to prepare PDAS.
1
Obtained PDAS was reprecipitated in acetone. H nuclear
3
magnetic resonance (NMR) spectra in CDCl are as follows:
δ (ppm) ) 7.3-6.95 (m, 14H, Ar), 6.65 (q, 1H, CH), 5.65 (d,
H, CH ), 5.15 (d, 2H, CH ) for DAS. δ (ppm) ) 6.9 (br, 14H,
Ar), 2.0 (br, H, -CH〈), 1.55 (br, 2H, -CH -) for PDAS. 2,4,7-
2
2
2
2
Trinitro-9-fluorenone (TNF) was used as a sensitizer and
4-azacycloheptylbenzylidene-malononitrile (7-DCST) as a
nonlinear optical dye. Diphenyl phthalate (DPP) was used as
g
a platicizer to decrease glass transition temperature (T ) of
composites. 7-DCST was synthesized using a method in ref 9.
TNF and DPP were used as received. Figure 1 shows the
structural formulas of these materials.
Sample Preparation. Composites with a given weight
ratio of PDAS, TNF, 7-DCST, and DPP were dissolved in THF.
Composite films were cast from THF solution and dried on a
heated plate at 70 °C for 24 h and subsequently in a vacuum
at 70 °C for 24 h. Dried composites were melt-pressed between
two indium-tin oxide (ITO) glass plates with a proper spacer.
Film thickness was ca. 50 or 100 µm.
Four-Wave Mixing Diffraction Measurement. Diffrac-
tion grating of the sample film was measured using a degener-
ate four-wave mixing (DFWM) technique. Holographic grating
was written by p-polarized two beams of He-Ne laser (λ )
632.8 nm, 10 mW) intersected in the sample. Two writing
Experimental Part
Materials. PDAS was prepared via the synthetic method
already been reported in the literature.8 The following is a
summary of synthetic route of PDAS. Triphenylphosphonium
methylide was synthesized with methyltriphenylphosphonium
bromide (12.5 g, 0.035 mol) and 1.6 M n-butyllithium (21.9
mL, 0.035 mol) in dehydrated tetrahydrofuran (THF) solution
for 4 h under mixing. Then, adding 4-(diphenylamino)benzal-
dehyde (9.56 g, 0.035 mol) to this solution under mixing for 2
h, 4-diphenylaminostyrene (DAS) was synthesized via the
Wittig reaction. The obtained crude DAS was purified using
*
To whom all correspondence should be addressed: Fax (+81)-
7
5-724-7805; e-mail tsutsumi@kit.ac.jp.
1
0.1021/ma051113b CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/28/2005

7522 Notes
Macromolecules, Vol. 38, No. 17, 2005
Table 1. Absorption Coefficient, Photocurrent, and
Refractive Index at 632.8 nm for Each Composite
absorption
photocurrent
refractive
index
coefficient (cm-1)
(µA/(W cm ))
2
PDAS 39
21.6
14.7
22.8
22.9
1.45
0.55
1.26
0.11
1.6638
1.6664
1.6529
1.6842
PDAS 39.5
PDAS 49
PVCz 49
beams intersect in the sample at incidence angles of R
A
)
4
0.54° and R ) 59.46° in air. The s-polarized reading (probe)
B
beam from the same source propagates in the direction
opposite to one of the writing beams (beam B) is diffracted by
the photorefractive grating in the sample film, and the
diffracted signal propagates in the direction opposite to that
of beam A and is reflected off by a beam splitter. The diffracted
signal is then detected by a photodiode detector.
Two-Beam Coupling Measurement. Photorefractive cou-
pling gain coefficients of the sample were measured using the
two-beam coupling (2BC) technique with p-polarized He-Ne
laser (λ ) 632.8 nm, 10 mW). The same geometric configura-
tion of p-polarized two beams crossing as DFWM was used
except that no probe beam was used. The intensity of two
crossing beams were measured using photodiode detectors to
evaluate gain coefficients (optical gain).
Figure 2. Time profile of photocurrent for PDAS and PVCz
composites after a step illumination of manochromated xenon
light. Photoillumination starts at time ) 0.
Characterization. The NMR spectrum was measured with
TMS as an internal standard at 20 °C using a Varian model
Gemini-200. The number-average molecular weight (M
n
) and
weight-average one (M ) of PDAS were measured using a gel
w
gave a faster photoconductive response. The faster
response and larger photocurrent of PDAS composite
suggest the faster drift mobility of hole in PDAS
composite due to lower ionization potential of tripheny-
lamine moiety in PDAS. Larger TNF content also leads
to the larger photocurrent. These photoconductive prop-
erties of PDAS contribute to the good photorefractive
properties.
Figure 3 shows the dependence of diffraction ef-
ficiency on applied electric field for each composite.
Increase of applied electric field leads to the increase of
diffraction efficiency. It is shown that the higher dif-
fraction efficiency is measured for composite with higher
TNF content. This result shows the larger content of
charge transfer complex due to increase of TNF content
is directly related to the larger space-charge field for
higher diffraction efficiency.
The observed grating buildup dynamics is given by a
exponential function of the kind η ) η0[1 - exp(-t/τη)].
Table 2 summarizes the diffraction efficiency (η0),
grating buildup time (τη), and optical gain (Γ0). As shown
in Table 2, compared with the PVCz composite, the
PDAS composite has a faster response of beam diffrac-
tion. PDAS 39 has a faster response time of 0.4 s,
whereas PVCz 49 has a slower response time of 1.2 s at
the same applied electric field. These faster responses
are also ascribed to faster drift mobility of hole in PDAS
composite due to lower ionization potential of the
triphenylamine moiety in PDAS.
permeation chromatograph, Shodex GPC SYSTEM-21, with
a serially connected columns of G5000Hxl and G3000Hxl. The
UV-vis absorption spectrum was recorded on a Shimadzu UV-
2101PC spectrophotometer. Differential scanning calorimetry
was carried out on a TA Instruments DSC 2920 differential
scanning calorimeter with a heating rate of 10 °C/min to
determine the glass transition temperature of sample. The
m-line method, in which an evanescent field is responsible for
the guided-wave mode at discrete mode angle, was employed
to determine refractive indices of sample films. The laser
source is a polarized He-Ne laser (632.8 nm). A prism of
TaFD21 (HOYA Glass) with high refractive index (1.92588 at
6
32.8 nm) was coupled to film with an air gap. Photoconduc-
tivity was measured using an electrometer at applied voltage
of 400 V under an illumination of monochromated xenon light.
Electrooptic measurement was performed with a He-Ne laser
to determine Pockels coefficient at λ ) 632.8 nm.
Results and Discussion
Four different PDASs with different Mn of 5000, 9000,
4 000, and 20 000 with ratio of Mw/Mn of 2.3, 2.2, 1.6,
1
and 1.6, respectively, were synthesized. Glass transition
temperatures of PDAS are 142.6, 141.3, 140.4, and 134.7
°C in turn as decreasing molecular weight. Composites
of PDAS with lowest molecular weight gave the best
quality film for photorefractive performance. Then
PDAS with Mn of 5000 was used in the present work.
Four types of sample films were prepared. PDAS/7-
DCST/DPP/TNF (39/15/45/1), PDAS/7-DCST/DPP/TNF
(
39.5/15/45/0.5), PDAS/7-DCST/DPP/TNF (49/10/40/1),
Refractive index modulation is calculated from the
diffraction efficiency for transmission grating geometry
using an equation of
and PVCz/7-DCST/DPP/TNF (49/10/40/1) were prepared
and denoted as PDAS 39, PDAS 39.5, PDAS 49, and
PVCz 49, respectively. PVCz 49 was prepared in com-
parison with the present newly synthesized PDAS
composites.
2
η ) sin [K∆n cos θ ]
(1)
s
s
G
Table 1 summarizes the absorption coefficient, pho-
tocurrent, and refractive index (n) at 632.8 nm for each
composite. It is noted that PDAS composite gave more
than 10 times as much photocurrent as PVCz one does.
Photoconductive responses after a step illumination of
monochromated xenon light for PDAS and PVCz com-
posites are shown in Figure 2. The PDAS composite
where K ) πd/λ(cos θA cos θB)1/2, ∆ns is refractive index
modulation, θG is the angle between direction of grating
vector and applied electric field, d is film thickness, λ
is wavelength of laser beam () 632.8 nm), and θA and
θB are internal refraction angles of beam A and beam
B in the film, respectively.

Macromolecules, Vol. 38, No. 17, 2005
Notes 7523
Table 3. Electrooptic Coefficient, Refractive Index
Modulation, Space-Charge Field, and Photorefractive
Phase Shift in the Matrix
E0 (V/µm)
r33 (pm/V)
∆ns
Ψ (deg)
-
-
-
-
4
4
4
4
PDAS 39
PDAS 39.5
PDAS 49
PVCz 49
61.2
61.2
63.2
65.6
2.2
2.0
1.5
1.8
3.9 × 10
2.6 × 10
4.0 × 10
2.9 × 10
90
74
45
50
model, if large trap densities are assumed, smaller
values of the phase shift will be obtained.11 Cyclic
volutammetry showed that 7-DCST has oxidation po-
tential of 1.49 V (vs Ag/AgCl electrode) lower than 1.66
V for ethylcarbazole as a model compound of PVCz.
Triphenylamine as a model compound of PDAS has
oxidation potential of 1.38 eV. The lower oxidation
potential means lower ionization potential. The lower
ionization potential of 7-DCST suggested that 7-DCST
works as a hole trap in PVCz composite but not in PDAS
composite. This is consistent with the smaller photo-
current and slower response of photoconductivity in
PVCz composite and larger photocurrent and faster
response of photoconductivity in PDAS composite.
7
-DCST does not work as any trap sites in PDAS
composite.
Conclusion
Photorefractive properties of newly synthesized PDAS
as a triphenylamine pendant moiety have demon-
strated. Better photoconductive properties such as large
photocurrent and fast photoconductive response of
PDAS composites are consistent with their better pho-
torefractive response. This approach provides one of the
promised candidates of the future holographic data
optical storage memory.
Figure 3. Diffraction efficiency as a function of applied
electric field: (a) PDAS 39 and PDAS 39.5; (b) PDAS 49 and
PVCz 49.
Table 2. Photorefractive Parameters Obtained DFWM
and 2BC Measurements
E0 (V/µm)
η0 (%)
τη (s)
Γ0 (cm-1)
Acknowledgment. The authors thank Dr. Nishio,
Core Technology Center, Nitto Denko Corporation,
Japan, for the special gift of 7-DCST. N.T. also thanks
Mr. Nakajima for measuring cyclic volutammetry of
PDAS 39
PDAS 39.5
PDAS 49
PVCz 49
61.2
61.2
63.2
65.6
0.89
0.39
0.94
0.83
0.4
0.9
0.7
1.2
20.5
12.4
14.1
11.0
7
-DCST, triphenylamine as model compound of PDAS,
The phase shift, out-of-phase refractive index modu-
lation with respect to the illumination pattern, is a
unique feature of photorefractive effect in the devices.
The phase shift of Ψ is evaluated from the refractive
index modulation (∆n) and optical gain (Γ0) with an eq
and ethylcarbazole as PVCz.
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MA051113B