Comparative Studies of the Formation of Surface Relief Grating. Amorphous Molecular Material vs Vinyl Polymer

Article abstract of DOI:10.1246/cl.2003.710

Comparative studies have been performed on the formation of surface relief grating (SRG) using an amorphous molecular material, 4-[di(biphenyl-4-yl)amino] azobenzene (DBAB), and a vinyl polymer containing DBAB as a pendant chromophore. It was found that S

Full text of DOI:10.1246/cl.2003.710

710
Chemistry Letters Vol.32, No.8 (2003)
Comparative Studies of the Formation of Surface Relief Grating.
Amorphous Molecular Material vs Vinyl Polymer
Hiroyuki Ando, Toru Takahashi, Hideyuki Nakano, and Yasuhiko Shirotaꢀ
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565-0871
(Received May 12, 2003; CL-030407)
Comparative studies have been performed on the formation
to make direct comparison of SRG-forming properties between
azobenzene-based photochromic amorphous molecular materi-
als and polymers containing the same azobenzene chromophore
as the pendant group under the same experimental conditions. It
is also required that these two materials for comparison should
exhibit nearly the same photochromic behavior. In the present
study, we have investigated SRG formation using an azoben-
zene-based photochromic amorphous molecular material, 4-
[di(biphenyl-4-yl)amino]azobenzene (DBAB), and a new vinyl
polymer containing DBAB as the pendant chromophore,
poly[1-(40-{N-(biphenyl-4-yl)-N-[4-(phenylazo)phenyl]amino}-
biphenyl-4-yl)ethylene] (PVDBAB).14
of surface relief grating (SRG) using an amorphous molecular
material, 4-[di(biphenyl-4-yl)amino]azobenzene (DBAB), and
a vinyl polymer containing DBAB as a pendant chromophore.
It was found that SRG was formed more rapidly for the photo-
chromic amorphous molecular material than for the correspond-
ing vinyl polymer.
Formation of surface relief grating (SRG) by irradiation of
amorphous films of azobenzene-functionalized polymers with
two coherent laser beams has been a topic of current inter-
est.1–7 SRG, which is formed by the mass transport induced
by the photoisomerization of the azobenzene chromophore,
may find potential applications for erasable and rewritable holo-
graphic memory, polarization discriminator, and waveguide
coupler. It has been suggested that the existence of a polymer
chain connecting the azo-chromophore plays an important role
for the formation of SRG, since molecularly doped polymer sys-
tems, where low molecular-weight azo-dyes are dispersed in a
polymer binder, exhibit poor SRG-forming properties relative
to azobenzene-functionalized polymers.3,5
CH CH2
n
N
N
N
N
N
N
DBAB
PVDBAB
DBAB was prepared by the method reported in our previ-
ous paper.9 It readily forms an amorphous glass with a glass-
transition temperature (Tg) of 68 ꢁC. A new vinyl monomer,
40-{N-(biphenyl-4-yl)-N-[4-(phenylazo)phenyl]amino}-4-vinyl-
biphenyl (VDBAB), was synthesized by Suzuki coupling reac-
tion of styrene-4-boronic acid and 4-[N-(4-bromophenyl)-N-(bi-
phenyl-4-yl)amino]azobenzene (BrPBAB) with Pd catalyst in
THF and identified by various spectroscopy, mass spectrometry,
and elemental analysis.15 BrPBAB was synthesized by the reac-
tion of 4-(phenylamino)azobenzene and 4-iodobiphenyl in the
presence of Cu, 18-crown-6, and K2CO3 in mesitylene, fol-
lowed by bromination with NBS in CHCl3/CH3COOH. The
corresponding vinyl polymer, PVDBAB, was synthesized by
radical polymerization of VDBAB in toluene solution using
AIBN as an initiator. The weight-average molecular weight
and the molecular weight distribution of the obtained polymer
were 42000 and 2.6, respectively, as determined by GPC using
polystyrene as a standard. The Tg of PVDBAB was ca. 140 ꢁC,
as determined by differential scanning calorimetry.
PVDBAB shows absorption spectra similar to those of
DBAB9 in solution (ꢀmaxðlog eÞ ¼ 435 nm (4.4) for DBAB
and 437 nm (4.4) for PVDBAB in toluene) and as amorphous
film. DBAB and PVDBAB exhibited almost the same photo-
chromic behavior as amorphous films prepared by spin coating
from benzene solution. The cis-fraction at the photostationary
state was ca. 0.5 for both amorphous films of DBAB and
PVDBAB. The cis-trans thermal isomerization reactions of
DBAB and PVDBAB as amorphous films were also almost
the same with each other. The reactions were successfully ana-
We have performed a series of studies on the creation of
amorphous molecular materials.8 As a part of these studies,
we have proposed a new concept, ‘‘photochromic amorphous
molecular materials,’’9 and created novel families of such amor-
phous molecular materials.9–12 Photochromic amorphous mo-
lecular materials can form uniform amorphous films by spin
coating and vacuum deposition. Studies of SRG formation using
azobenzene-based photochromic amorphous molecular materi-
als are of interest and significance from the viewpoints that such
amorphous molecular materials, which are free from polymer
chains and their entanglement, may constitute a new class of
SRG-forming materials, permitting ready studies of the correla-
tion between molecular structure and SRG formation.
In the light of these considerations, we have investigated
SRG formation using an azobenzene-based photochromic amor-
phous molecular material, 4-[bis(9,9-dimethylfluoren-2-yl)ami-
no]azobenzene (BFlAB), and found that it formed SRG with a
relatively high diffraction efficiency of ca. 23% for a probe laser
beam of 633 nm and a modulation depth of ca. 280 nm, when its
amorphous film was irradiated with two coherent Arþ laser
beams (488 nm).13 This result indicates that the existence of a
polymer chain is not necessarily required for the formation of
SRG and that azobenzene-based photochromic amorphous mo-
lecular materials constitute promising candidates for SRG for-
mation.
It is then of great interest and significance to elucidate the
difference in SRG-forming properties between amorphous mo-
lecular materials and polymers. For this purpose, it is necessary
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.8 (2003)
711
lyzed by the first-order kinetics consisting of two components
ð½cisꢂt=½cisꢂ0 ¼ f1 expðꢃk1tÞ þ f2 expðꢃk2tÞÞ with the kinetic
parameters of f1 = 0.03, f2 = 0.97, k1 = 0.14 minꢃ1 and k2
= 3:6 ꢄ 10ꢃ3 minꢃ1 for DBAB and f1 = 0.06, f2 = 0.94, k1
= 0.14 minꢃ1, and k2 = 4:3 ꢄ 10ꢃ3 minꢃ1 for PVDBAB at
30 ꢁC.
Almost the same photochromic behavior of DBAB and
PVDBAB allowed us to compare their SRG-forming properties.
When the amorphous films of DBAB and PVDBAB (thickness:
ca. 15 mm) were irradiated with coherent, linearly polarized two
Arþ laser beams (488 nm, 10 mW), SRGs were formed. By the
use of two beams with the polarization directions of þ45ꢁ and
ꢃ45ꢁ with regard to p-polarizaition, a diffraction efficiency of
ca. 8% for a probe laser beam (633 nm) and a modulation depths
of ca. 200 and 310 nm were obtained for DBAB and PVDBAB,
respectively. Figure 1 shows the AFM image of the irradiated
sample of PVDBAB. Almost the same diffraction efficiency ob-
tained for DBAB and PVDBAB, irrespective of the smaller
modulation depth for DBAB, may result from a larger refractive
index of the amorphous molecular material DBAB relative to
the vinyl polymer PVDBAB.
Figure 2. Irradiation time dependence of diffraction effi-
ciency for (a) DBAB and (b) PVDBAB amorphous films.
DBAB and PVDBAB were comparable with each other but that
SRG was formed more rapidly for the amorphous molecular
material than for the corresponding vinyl polymer.
References and Notes
1
2
3
4
5
P. Rochon, E. Batalla, and A. Natansohn, Appl. Phys. Lett., 66,
136 (1995).
D. Y. Kim, S. K. Tripathy, L. Li, and J. Kumar, Appl. Phys.
Lett., 66, 1166 (1995).
P. Lefin, C. Fiorini, and J.-M. Nunzi, Opt. Mater., 9, 323
(1998).
L. Andruzzi, A. Altomare, F. Ciardelli, R. Solaro, S. Hvilsted,
and P. S. Ramanujam, Macromolecules, 32, 448 (1999).
N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu,
L. Li, L. Samuelson, J. Kumar, and S. K. Tripathy, J. Mater.
Chem., 9, 1941 (1999).
6
7
8
9
T. Ubukata, T. Seki, and K. Ichimura, Adv. Mater., 12, 1675
(2000).
K. G. Yager and C. J. Barret, Curr. Opin. Solid State Mater.
Sci., 5, 487 (2001).
Y. Shirota, J. Mater. Chem., 10, 1 (2000), and references cited
therein.
Y. Shirota, K. Moriwaki, S. Yoshikawa, T. Ujike, and H.
Nakano, J. Mater. Chem., 8, 2579 (1998).
Figure 1. AFM image of the surface of the amorphous film of
PVDBAB after irradiation with two coherent writing laser
beams (488 nm, 10 mW) with polarization directions of
þ45ꢁ and ꢃ45ꢁ with regard to p-polarizaition.
10 H. Utsumi, D. Nagahama, H. Nakano, and Y. Shirota, J. Mater.
Chem., 10, 2436 (2000).
11 H. Utsumi, D. Nagahama, H. Nakano, and Y. Shirota, J. Mater.
Chem., 12, 2612 (2002).
12 Y. Shirota, H. Utsumi, T. Ujike, S. Yoshikawa, K. Moriwaki,
D. Nagahama, and H. Nakano, Opt. Mater., 21, 249 (2003).
13 H. Nakano, T. Takahashi, T. Kadota, and Y. Shirota, Adv. Ma-
ter., 14, 1157 (2002).
14 H. Ando, T. Takahashi, H. Nakano, and Y. Shirota, Preprints of
the 83rd Annual Meeting of The Chemical Society of Japan,
Tokyo, March, 2003, Vol. 1, p 43.
15 VDBAB: yield 60%. mp: 149 ꢁC. MS: m=z 527 (Mþ). 1H NMR
(750 MHz, THF-d8): d (ppm) = 7.86 (d, 2H, J ¼ 7:7 Hz), 7.85
(d, 2H, J ¼ 8:9 Hz), 7.65 (d, 2H, J ¼ 8:7 Hz), 7.63 (d, 2H,
J ¼ 8:4 Hz), 7.63 (d, 2H, J ¼ 8:1 Hz), 7.62 (d, 2H,
J ¼ 8:3 Hz), 7.49 (d, 2H, J ¼ 8:3 Hz), 7.47 (dd, 2H,
J ¼ 7:6; 7:7 Hz), 7.41 (t, 1H, J ¼ 7:6 Hz), 7.40 (dd, 2H,
J ¼ 7:3; 8:1 Hz), 7.29 (t, 1H, J ¼ 7:3 Hz), 7.27 (d, 2H,
J ¼ 8:7 Hz), 7.27 (d, 2H, J ¼ 8:4 Hz), 7.21 (d, 2H,
J ¼ 8:9 Hz), 6.75 (dd, 1H, J ¼ 11:0; 17:6 Hz), 5.70 (d, 1H,
J ¼ 11:0 Hz), 5.21 (d, 1H, J ¼ 17:6 Hz). 13C NMR
(188 MHz, THF-d8): d (ppm) = 153.9, 151.4, 148.4, 147.2,
147.1, 141.3, 140.6, 137.9, 137.5, 137.5, 137.3, 131.1, 129.8,
129.5, 128.8, 128.7, 127.9, 127.5, 127.4, 127.4, 126.5, 126.5,
125.1, 123.3, 122.8, 113.7. Calcd for C38H29N3: C, 86.50; H,
5.54; N, 7.96%. Found: C, 86.45; H, 5.62; N, 7.84%.
It is noteworthy that SRG is formed more rapidly for
DBAB than for PVDBAB. As Figure 2 shows, whereas the dif-
fraction efficiency reached the maximum in 4 min upon irradia-
tion for DBAB, irradiation for more than 15 min was required
for PVDBAB under the same conditions. It is thought that more
facile mass transport for the amorphous molecular material,
which is free from the restriction by the polymer chain and its
entanglement, relative to the vinyl polymer rather than the dif-
ference in their Tgs is responsible for the faster formation of
SRG for DBAB. In fact, we have found that the irradiation time
required for the diffraction efficiency reaching the maximum for
other amorphous molecular materials with different Tgs, BFlAB
(Tg: 97 ꢁC)13 and 4-{bis[9,9-di(4-tolyl)fluoren-2-yl]amino}azo-
benzene (Tg: 144 ꢁC), was nearly the same with that for DBAB.
The increase in the diffraction efficiency after the turn-off of the
writing beams is thought to be due to the increase of the refrac-
tive index caused by the thermal isomerization from the photo-
generated cis-form to the trans-form.
In summary, comparative studies of the formation of SRG
using an azobenzene-based photochromic amorphous molecular
material, DBAB, and a vinyl polymer containing DBAB as a
pendant chromophore, PVDBAB, have been performed. It
was found that the diffraction efficiencies of SRGs formed for
Published on the web (Advance View) July 14, 2003; DOI 10.1246/cl.2003.710