Nunchaku-like molecules containing both an azo chromophore and a biphenylene unit as a new type of high-sensitivity photo-storage material

Article abstract of DOI:10.1039/c0jm02228d

A series of nunchaku-like molecules containing both an azo chromophore and a rigid biphenylene unit (AZBP-X, X = CA, CN, and NT) was designed and synthesized in this study. The azo compounds were amorphous solids at room temperature with glass transition

Full text of DOI:10.1039/c0jm02228d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Nunchaku-like molecules containing both an azo chromophore and
a biphenylene unit as a new type of high-sensitivity photo-storage material
*
Gang Ye, Dongrui Wang, Yaning He and Xiaogong Wang
Received 11th July 2010, Accepted 19th August 2010
DOI: 10.1039/c0jm02228d
A series of nunchaku-like molecules containing both an azo chromophore and a rigid biphenylene unit
(AZBP-X, X ¼ CA, CN, and NT) was designed and synthesized in this study. The azo compounds were
amorphous solids at room temperature with glass transition temperature (Tg) values of 98, 87, and 79 ꢀC
for AZBP-CA, AZBP-CN, and AZBP-NT, respectively. Thin solid films of the azo molecular
glasses could be readily obtained by spin-coating. Photoinduced anisotropy (PIA) and surface-relief-
grating (SRG) formation were investigated by irradiating the films with a linearly polarized Ar+ laser (488
nm) beam or interfering Ar+ laser beams. Upon Ar+ laser irradiation, photoinduced orientation occurred
rapidly and the films showed significant birefringence and dichroism. When the irradiating light was
switched off, the orientation did not show relaxation, as is observed for most azo polymers, and the
birefringence even increased slightly. When irradiated with p-polarized interfering Ar+ laser beams with
modest intensity (80 mW cmꢁ2), SRGs were observed to be formed on the films in a high inscription rate.
The grating with trough depth of 380 nm could be inscribed in 1200 s, which showed the diffraction
efficiency of 27% for a He-Ne probe beam. In the initial stage of the SRG formation, a viscous flow
process was visualized for the AZBP-CA film, which underwent a transition from doublet fringes on the
grating crests to the final sinusoidal profile. Both PIA and SRG behavior showed a close correlation with
the electron-withdrawing groups of the azo chromophores. The nunchaku-like azo molecules could be
a new promising type of photo-storage materials for future applications.
believed to be related to the photoinduced trans–cis isomeriza-
Introduction
tion of azo chromophores.6,7 Although it has been actively
studied in recent years, the mechanism of SRG formation still
remains a puzzling issue requiring further elucidation.14–20
Amorphous molecular materials are organic compounds that
can exist in a stable glass state below their Tg but whose
molecular weights are much lower than those of polymers.21,22
Similar to amorphous polymers, thin solid films of amorphous
molecular materials can be prepared by spin-coating. Owing to
attractive features such as high chromophoric density and
absence of chain entanglement, amorphous molecular materials
containing azo chromophores (azo molecular glasses) are
promising as a new type of photoresponsive materials.23–25 The
well-defined structures of azo molecular glasses enable them to
be an ideal candidate for mechanism study. Several types of azo
molecular glasses have been developed through different
molecular design motifs and synthetic methods.21–30 SRGs can be
inscribed on the films with a fast forming-rate and large surface
modulation.28–30 Nevertheless, at the current stage, the types of
azo molecular glasses are still quite limited. A rigid molecular
backbone is usually required to maintain the amorphous state at
room temperature. Biphenyl or biphenylene units have been
widely used as building blocks to construct molecular glasses.21–27
Because of the rigid rod-like structure, the units can potentially
take preferential orientation induced by intermolecular interac-
tion or an external field. However, for most reported azo
molecular glasses, the biphenyl or biphenylene units and azo-
benzene are tightly anchored by rigid tris(phenyl) cores. The rigid
structure is difficult to drive by light in the photoresponsive
processes.
Polymers containing azobenzene and its derivatives (azo polymer
for short) have been intensively studied in recent years due to
their interesting properties and potential applications in the areas
such as diffractive optical elements, information storage, optical
switching, nonlinear optics, sensor and actuators.1–8 When irra-
diated with light at appropriate wavelengths, azo polymers can
show a variety of photoresponsive properties related to the
trans–cis isomerization of azo chromophores.1,2 The photoin-
duced anisotropy (PIA) and surface-relief-grating (SRG)
formation are two important properties attracting much recent
attention.5–7 Photoinduced birefringence and dichroism of azo
polymer films as typical PIA processes have been investigated by
many research groups for years.3–6,9,10 The processes are attrib-
uted to repeated trans–cis–trans photoisomerization of azo
chromophores, which forces the chromophores and related
groups to gradually line up in a direction perpendicular to the
polarization direction of the excitation.6 SRG formation is
generally referred to as the surface modulation induced by irra-
diation with interfering laser beams.5–7,11,12 SRGs on azo polymer
films can be inscribed at a temperature well below the glass
transition temperature (Tg) of the azo polymers.13 The surface
structures are stable below the Tg of the polymers and can be
erased by heating samples to a temperature above their Tg or by
irradiation with a uniform laser beam. SRG formation is also
Department of Chemical Engineering, Laboratory for Advanced Materials,
Tsinghua University, Beijing, 100084, P. R. China. E-mail: wxg-dce@mail.
tsinghua.edu.cn; Fax: +86 10 6278 1003; Tel: +86 10 6279 6171
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In this study, a new molecular design was adopted to develop
photoresponsive azo molecular glasses. The molecules were
constructed by connecting an azo chromophore and a rigid
biphenylene unit via a flexible spacer. Owing to the molecular
shape, it is designated as nunchaku-like molecules in this article.
This molecular architecture could be reminiscent of the liquid
crystal (LC) dimers, where two identical or different mesogenic
units are connected by a flexible spacer.31–33 In contrast with the
LC dimers, the aminoazobenzene unit used in this study is not
a mesogenic unit. Therefore, the nunchaku-like molecules show
phase behavior of an amorphous material rather than LC.
Transparent solid films could be obtained by spin-coating with
the azo compound solutions. The structure and properties of the
azo molecular glass materials, especially those related with their
responses to light irradiation, were investigated in detail.
reaction at 80 ꢀC for 12 h, the mixture was poured into an excess
of NaOH aqueous solution (0.1 mol Lꢁ1). The precipitate was
collected by filtration and washed adequately with deionized
water. After drying in a vacuum oven at room temperature for
24 h, the final product was obtained. Yield: 86%. 1H NMR (300
MHz, CDCl3): d 1.35 (m, 2H), 1.55 (m, 4H), 1.78 (m, 2H), 2.90
(s, 3H), 3.30 (t, 2H), 3.95 (t, 2H), 6.70 (m, 3H), 6.90 (m, 4H),
7.20 (t, 2H), 7.40 (t, 4H).
a-(N-Methyl-N-(4-(40-carboxyphenylazo)phenyl)amino)-u-(4-
hydroxylbiphenylene)-hexane (AZBP-CA) (4). 4-Aminobenzoic
acid (0.59 g, 5 mmol) was dissolved in a mixture of acetic acid (15
mL) and sulfuric acid (1 mL) at 0 ꢀC. NaNO2 (0.414 g, 6 mmol)
dissolved in 2 mL water was slowly dripped into the acidified 4-
aminobenzoic acid solution. The mixture was stirred in an ice
bath for 10 min to obtain a yellowish solution of the diazonium
salts. 4-(6-(N-methyl-N-phenylamino)hexyloxy)biphenyl-40-ol
(3) (1.5 g, 4 mmol) was dissolved in 80 mL DMF and the solution
was cooled down to 0 ꢀC. The diazonium salt solution was added
dropwise into the DMF solution and the temperature was
controlled to be below 5 ꢀC. After reaction for 5 h, the raw
product was obtained by pouring the reaction solution into 500
mL deionized water. The precipitate was collected by filtration
and washed with plenty of water until the neutral state was
achieved. After drying, the raw product was dissolved in THF
and precipitated into a suitable amount of petroleum ether. The
final product was obtained by vacuum drying at 60 ꢀC for 24 h.
Yield: 81%. 1H NMR (300 MHz, d-DMSO): d 1.39 (m, 2H), 1.47
(m, 2H), 1.63 (m, 2H), 1.76 (m, 2H), 3.08 (s, 3H), 3.50 (t, 2H),
3.99 (t, 2H), 6.88 (m, 6H), 7.43 (m, 4H), 7.83 (m, 4H), 8.10
(t, 2H), 9.41 (s, 1H), 12.90 (s, 1H). 13C NMR (600 MHz, d-
DMSO): d 25.9, 26.6, 27.0, 29.2, 38.9, 52.0, 67.9, 111.9, 115.3,
116.2, 122.2, 126.0, 127.5, 127.7, 131.0, 131.3, 132.7, 133.2, 143.0,
152.6, 155.7, 157.0, 158.0, 167.4. IR (KBr): 3200 (m; OH), 2972
(m; CH), 1725 (s; C]O), 1600, 1510 (s; benzene ring), and 1223
Experimental section
Materials synthesis
Biphenyl-4,40-diol (99%) was purchased from Tokyo Chemical
Industry Co., Ltd (TCI). N-Methylaniline, 6-chlorohexan-1-ol,
and 4-aminobenzonitrile were purchased from ACROS and
Aldrich. All other chemicals and solvents were commercially
purchased and used as received without further purification.
6-(N-Methyl-N-phenylamino)hexan-1-ol (1). N-Methylaniline
(10.752 g, 100 mmol) and 6-chlorohexan-1-ol (14.05 g, 100
mmol) were added dropwise into a mixture of K2CO3 (13.8 g, 100
mmol) and KI (18.26 g, 100 mmol) in N,N-dimethylformamide
(DMF, 100 mL) with violent stirring. After reaction at 80 ꢀC for
5 h, the mixture was poured into an excess of water. The product
was extracted from water with ethyl acetate (100 mL). The
extract was desiccated and concentrated by rotary evaporation.
The final product was obtained by silica gel chromatography
with a mixture of ethyl acetate and petroleum ether (v/v ¼ 1/2) as
the eluent. Rf ¼ 0.75 (SiO2, ethyl acetate : petroleum ether
(1 : 2)). Yield: 77%. 1H NMR (300 MHz, CDCl3): d 1.33 (m, 4H),
1.55 (m, 4H), 1.95 (br, 1H), 2.90 (s, 3H), 3.28 (t, 2H), 3.57 (t, 2H),
6.70 (m, 3H), 7.20 (t, 2H).
.
(C–O) cmꢁ1 m/z (MALDI-TOF): 523.4 (M+, calc. for
C32H33N3O4, 523.25).
a-(N-Methyl-N-(4-(40-cyanophenylazo)phenyl)amino-u-(4-
hydroxylbiphenylene)-hexane (AZBP-CN) (5). AZBP-CN
was synthesized and purified via a procedure similar to that
described above for the AZBP-CA preparation. In the reac-
tion, the diazonium salt of 4-aminobenzonitrile was reacted
with 3 to obtain AZBP-CN. Yield: 83%. 1H NMR (300 MHz,
d-DMSO): d 1.38 (m, 2H), 1.47 (m, 2H), 1.61 (m, 2H), 1.73
(m, 2H), 3.05 (s, 3H), 3.49 (t, 2H), 3.97 (t, 2H), 6.90 (m, 6H),
7.47 (m, 4H), 7.85 (m, 4H), 7.95 (t, 2H), 9.42 (s, 1H). 13C
NMR (600 MHz, d-DMSO): d 25.9, 26.6, 27.0, 29.2, 38.9,
52.1, 67.9, 111.4, 112.0, 115.3, 116.1, 119.3, 122.9, 126.4,
127.5, 127.7, 131.1, 133.1, 134.1, 143.0, 153.0, 155.4, 157.0,
158.0. IR (KBr): 3300 (m; OH), 2970 (m; CH), 2220 (s; CN),
1728 (s; C]O), 1590, 1502 (s; benzene ring), and 1230 (C–O)
cmꢁ1. m/z (MALDI-TOF): 504.4 (M+, calc. for C32H32N4O2,
504.25).
N-(6-Chlorohexyl)-N-methylaniline (2). 6-(N-Methyl-N-phe-
nylamino)hexan-1-ol (1) (10.35 g, 50 mmol) was added dropwise
into a flask containing phosphoryl trichloride (23.475 g, 150
mmol) with ice-bath cooling. Then, the mixture was slowly
ꢀ
heated to 110 C and the reaction was kept at that temperature
for 2 h. After that, the mixture was poured into an excess of
water. The product was extracted from the water using benzene
(100 mL). After desiccating and concentrating the organic phase,
the final product as yellowish liquid was obtained. Yield: 60%.
1H NMR (300 MHz, CDCl3): d 1.33 (m, 2H), 1.45 (m, 2H), 1.55
(m, 2H), 1.75 (m, 2H), 2.90 (s, 3H), 3.28 (t, 2H), 3.50 (t, 2H), 6.70
(m, 3H), 7.20 (t, 2H).
4-(6-(N-Methyl(N-phenyl)amino)hexyloxy)biphenyl-40-ol (3).
Biphenyl-4,40-diol (5.58 g, 30 mmol) and N-(6-chlorohexyl)-N-
methylaniline (2) (6.765 g, 30 mmol) were added into a mixture of
K2CO3 (6.90 g, 50 mmol) and KI (0.913 g, 5 mmol) in anhydrous
N,N-dimethylformamide (100 mL) with violent stirring. After
a-(N-Methyl-N-(4-(40-nitrophenylazo)phenyl)amino-u-(4-
hydroxylbiphenylene)-hexane (AZBP-NT) (6). AZBP-NT
was synthesized and purified via a procedure similar to that
described above for the AZBP-CA preparation. In the
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reaction, the diazonium salt of 4-nitroaniline was reacted
with 3 to obtain AZBP-NT. Yield: 89%. 1H NMR (300
MHz, d-DMSO): d 1.40 (m, 2H), 1.48 (m, 2H), 1.60 (m, 2H),
1.75 (m, 2H), 3.10 (s, 3H), 3.49 (t, 2H), 3.98 (t, 2H), 6.89 (m,
6H), 7.43 (m, 4H), 7.90 (m, 4H), 8.37 (t, 2H), 9.40 (s, 1H).
13C NMR (600 MHz, d-DMSO): d 25.9, 26.6, 27.0, 29.2,
52.0, 67.9, 112.1, 115.3, 116.1, 123.0, 125.5, 126.6, 127.5,
127.7, 131.3, 133.2, 143.2, 147.3, 153.2, 156.8, 157.0, 158.0.
IR (KBr): 3300 (m; OH), 2968 (m; CH), 1719 (s; C]O),
1605, 1518 (s; benzene ring), and 1235 (C–O) cmꢁ1. m/z
(MALDI-TOF): 524.4 (M+, calc. for C31H32N4O4, 524.24).
Results and discussion
Synthesis and characterization
The synthetic route towards the nunchaku-like molecules con-
taining both an azo chromophore and a rigid biphenylene unit is
depicted in Scheme 1. The molecules differ from each other only
in the chromophoric electron-withdrawing groups, which are
carboxyl, cyano, and nitro for AZBP-CA, AZBP-CN, and
AZBP-NT, respectively. All of the target molecules were
obtained from the same precursor (3) by an azo-coupling reac-
tion using diazonium salts of the corresponding compounds.
This reaction route could introduce azo chromophores with
different electron-withdrawing groups at the final stage of the
preparation under mild conditions. The diazonium salts readily
attack the benzene rings of the anilino groups at positions with
high electron densities. The bulkiness of the attacking groups and
resulted steric hindrance allow the electrophilic substitution to
take place exclusively at the para position. This synthetic route
can be used to prepare a series of nunchaku-like molecules
bearing different electron-withdrawing groups. The chemical
structure of the nunchaku-like molecules was confirmed by
spectroscopic analyses and MALDI-TOF measurements.
The thermal transition and phase behavior of AZBP-X (X ¼
CA, CN or NT) were investigated by differential scanning
calorimetry (DSC) and polarized optical microscopy (POM).
The DSC curves of the azo compounds are given in Fig. 1. The
glass transition temperature (Tg) values were obtained from DSC
and are summarized in Table 1. The Tg values of the AZBP-CA,
AZBP-CN and AZBP-NT are 98, 87 and 79 ꢀC, respectively.
AZBP-CA containing the 4-carboxylazobenzene moiety shows
a higher Tg than those of the other two compounds. This can be
attributed to the hydrogen bonding between the COOH groups.
POM observations indicated that before and after the glass
transition, the AZBP-X films were isotropic without preferential
molecular orientation. Thermogravimetric analysis (TGA) was
used to investigate the thermal stability of the azo compounds.
TGA curves of the azo compounds are shown in Fig. 2 and the
thermal decomposition temperature (Td) values obtained from
TGA are summarized in Table 1. The Td values of the AZBP-
CA, AZBP-CN and AZBP-NT are 198, 220, and 223 ꢀC,
respectively.
Characterization
1H NMR spectra were obtained on a JOEL JNM-ECA300 NMR
spectrometer. 13C NMR spectra were obtained on a JOEL JNM-
ECA600 NMR spectrometer. IR spectra were determined using
a Nicolet 560-IR FT-IR spectrophotometer by incorporating the
powder samples in KBr disks. Mass spectra were determined
using MALDI-TOF mass spectroscopy (BIFLEX III, Bruker
Daltonics, Inc.). The UV-vis spectra of the samples were recor-
ded using a Perkin-Elmer Lambda Bio-40 spectrophotometer.
The thermal properties of the azo molecular glasses were inves-
tigated with TA instrument (DSC 2910 and Hi-Res TGA 2950)
at a heating rate of 10 ꢀC minꢁ1 under N2 protection. The surface
profiles of the surface-relief-gratings were monitored using an
atomic force microscope (AFM) (Nanoscope IIIa, tapping
mode).
PIA and SRG study
The azo compounds were dissolved in DMF with a concentra-
tion of 0.1 g mLꢁ1. After being filtered through 0.45 mm
membrane, the solutions were spin-coated onto clean glass slides.
By adjusting the spinning rate, the thickness of the films was
controlled to be about 0.75 mm for PIA investigation and about
1.0 mm for SRG inscription. After being dried at 70 ꢀC under
vacuum for 48 h, the transparent solid films with good optical
quality were obtained, which were stored in a desiccator for
further studies. The birefringence and dichroism of the films were
induced by irradiation with a linearly polarized beam from an
Ar+ laser (488 nm, 80 mW cmꢁ2). The photoinduced birefrin-
gence was probed in a real-time manner by an unpolarized beam
from a He-Ne laser (633 nm) with low intensity, which trans-
mitted through a pair of crossed linear polarizers at ꢂ45ꢀ with
respect to the polarization direction of exciting light. The optical
setups for the experiments were similar to those reported previ-
ously.3,25,34,35 When the variation reached the saturated state, the
exciting beam was turned off to investigate the relaxation
process. The dichroism was detected by polarized UV-vis spec-
troscopy immediately after the irradiation.5 For SGR inscrip-
The UV-vis spectra of the azo compounds in DMF solutions
are shown in Fig. 3. The spectral features are typical for pseudo-
stilbene-type azo compounds.2 The absorption bands corre-
sponding to the p–p* transition appear at the longest wavelength
and the azo compounds show strong absorption in the visible
region. The positions of the absorption bands are strongly
affected by the electron-withdrawing groups on the azobenzene
moieties, which can be attributed to their inductive and con-
jugative effect on both the ground and excited states. The lmax of
AZBP-CA, AZBP-CN and AZBP-NT is 435, 469, and 493 nm
(Table 1).
tion, a linearly polarized Ar+ laser beam (488 nm, 80 mW cmꢁ2
)
was used as the light source. After the p-polarized laser beam was
expanded and collimated, half of the collimated beam was inci-
dent on the films directly and the other half of the beam was
reflected onto the films from a mirror. The diffraction efficiency
of the first order diffracted beam from the gratings was probed
with an unpolarized low power He-Ne laser beam at 633 nm in
transmission mode.11,12
Photoinduced anisotropy
Photoinduced birefringence of the azo compounds films was
investigated at 633 nm. The value of the birefringence (Dn) can be
calculated from the equation:9
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Scheme 1 Synthetic route of the nunchaku-like molecules containing both an azo chromophore and a biphenylene unit.
Fig. 1 DSC curves of the azo molecular glasses.
Fig. 2 TGA curves of the azo molecular glasses.
Table 1 Tg, Td and lmax values of AZBP-CA, AZBP-CN and AZBP-NT
the thickness of the film, and l is the wavelength of the laser beam
(633 nm). Fig. 4 shows variations of the photoinduced birefrin-
gences of the AZBP-X thin films with the irradiation time. The
birefringences increase rapidly at the initial stage and then
gradually saturate after the rapid increases. The time needed to
reach the saturated values is different for the three samples. It is
shortest for AZPB-CN, which can reach the saturated value in
less than 100 s. In contrast, AZPB-NT does not reach the satu-
rated level even in 600 s. The saturated birefringence of AZBP-
CA is the highest in the series, which can reach 0.22. The value is
much higher compared with most azo polymers and amorphous
molecular materials.4–6,25,29,36 The saturated birefringence of
Sample
Tg (ꢀC)
Td (ꢀC)
lmax (nm)a
AZBP-CA
AZBP-CN
AZBP-NT
98
87
79
198
220
223
435
469
493
a
In DMF solution.
I ¼ I0sin2(pDnd/l)
where I is the intensity of the light transmitted through the
crossed polarizer, I0 is the intensity of the incident laser beam, d is
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orientational movement of azo chromophores. Therefore, after
switching off the pumping light, the stress built-up in the system
could force some of the azo chromophores to reorientate, which
results in the birefringence decay. In contrast, for the nunchaku-
like molecules studied here, the alignment of azo chromophores
is easier to drive the flexible aliphatic spacers and even biphe-
nylene units to take the corresponding orientation. In this case,
the correlation in the molecules will not show obvious restriction
to cause the birefringence decay.
Dichroism associated with the photoinduced azo chromo-
phore orientation was also investigated. Fig. 5 shows the UV-vis
spectra of the AZBP-X films (750 nm thick) after irradiation with
the polarized Ar+ laser beam (80 mW cmꢁ2) for 20 min.
Consistent with the birefringence study, obvious dichroism is
observed for the films after the irradiation. The films possess
much stronger absorption in the direction perpendicular to the
polarization of the pumped beam. It reveals that the predomi-
nant amounts of azo chromophores have taken the orientation
perpendicular to the polarization direction of the excitation
beam.
Fig. 3 UV-vis spectra of AZBP-CA, AZBP-CN, and AZBP-NT in
DMF solutions.
By comparing the spectra given in Fig. 5, it can be seen that the
electron-withdrawing groups show a significant influence on the
dichroic behavior of the materials. AZBP-CA and AZBP-NT,
with a carboxyl and nitro substituents, show the largest and
smallest dichroism. The orientation order parameter S, which is
used to describe the degree of orientation, can be estimated from
the dichroic ratio:
S ¼ (At ꢁ A||)/(At + 2A||)
where At and A|| are the maximum absorbance in the directions
perpendicular and parallel to the polarization.5,37,38 The S values
for the AZBP-CA, AZBP-CN and AZBP-NT are calculated to
be about 0.43, 0.17 and 0.10, respectively. The order parameter
of AZBP-CA is the largest in the series and also much larger than
those of the polymeric counterparts. The observation is consis-
tent with the photoinduced birefringence results mentioned
above. Both the birefringence and dichroism are related to the
molecular orientation, but the latter is almost exclusively related
to the azo chromophore orientation. The large birefringence and
dichroism induced by the light irradiation could be explored for
optical data-storage and used to prepare optical devices such as
polarizers.
Fig. 4 Photoinduced birefringence of AZBP-X (X ¼ CA, CN and NT)
films. Linearly polarized Ar+ laser beam at 488 nm with 80 mW cmꢁ2 of
intensity was used as the pump light. After saturation, the birefringence
was further monitored for several minutes. The time when the irradiating
light is switched off is marked on the curves.
AZBP-NT is much smaller than those of its counterparts, which
is only 0.03 after irradiation for 10 min. The above results indi-
cates that the photoinduced birefringence of the molecular azo
glasses is closely related with the electron-withdrawing groups on
the azobenzene moieties. But the exact influence of the electron-
withdrawing groups on the saturated birefringence and its
developing rate could be complicated. AZBP-CA shows a much
larger saturated birefringence than that of AZBP-CN, but the
initial increase of the birefringence of the former is slower than
that of the latter.
Photoinduced surface-relief-gratings
Photoinduced SRG formation on AZBP-X films was investi-
gated by using interfering p-polarized Ar+ laser beams (80 mW
cmꢁ2) as the inscribing light and a He-Ne laser beam at 633 nm as
the probe beam. The SRG formation was monitored by
measuring the diffraction efficiencies of the probe beam in a real-
time manner. The profiles and trough depths of the gratings were
detected by AFM. Upon exposure to the interference pattern of
laser beams at the modest intensity, SRGs were observed to be
formed on films of all three azo molecular glasses. Fig. 6(a) shows
a typical AFM image of the sinusoidal surface relief structures
with regular spaces formed on the AZBP-CA film. The spatial
period depends on the wavelength and the incident angle of the
writing beams. Fig. 6(b) shows the AFM image of the surface
In contrast to azo polymers, no birefringence decay (relaxa-
tion) is observed for azo molecular glasses after switching off the
irradiating light (Fig. 4). Such decay is typical for azo polymers
and related materials.4–6,25,29,36 On the contrary, even a slight
increase in the birefringence can be observed when switching-off
the irradiating light. This unusual property can be attributed to
the structure of the nunchaku-like molecules. For azo polymers,
polymer backbones are usually not easy to drive by the
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Fig. 6 AFM images of the surface-relief-structures on AZBP-CA films,
(a) typical SRG; (b) pattern composed of two-set fringes recorded
orthogonally to each other at the same location.
For a comparative study, the films of three azo molecular
glasses were exposed to the laser light when the experimental
conditions, such as laser intensity, incident angle and film
thickness were controlled to be identical. Fig. 7 shows the first-
order diffraction efficiency (DE) as a function of irradiation time
for AZBP-X (X ¼ CA, CN and NT). Result shows that the SRG-
forming rate and saturated level are closely related to the
electron-withdrawing groups of the azo chromophores. AZBP-
CA shows the fastest inscription rate during the SRG formation
among the series. Its diffraction efficiency can reach 27% after
irradiation for 1200 s. The saturated depths of AZBP-CA,
AZBP-CN, AZBP-NT measured by AFM are 380, 353, and
315 nm after irradiation for 1200 s. A similar dependence of the
SRG formation behavior on the electron-withdrawing groups
has been observed for epoxy-based azo polymers, hyperbranched
Fig. 5 UV-vis spectra of 750 nm thick films of (a) AZBP-CA, (b) AZBP-
CN, (c) AZBP-NT, after irradiation by the polarized Ar+ laser (80 mW
cmꢁ2) for 20 min. Curve 1: perpendicular to the polarization direction of
the laser beam; curve 2: parallel to the polarization direction of the laser
beam.
azo
polymers,
and
dendritic
azobenzene-containing
compounds.30,34,39 The SRG-forming rate and modulated
amplitude depend on the experimental parameters such as light
polarization manner, film thickness and grating space period.
Under the same conditions, the SRG formation rate of AZBP-
CA is much faster than that of BP-AZ-CA, an epoxy-based
polymer bearing the same type of azo chromophores.34 In this
study, the SRGs were inscribed with p-/p-polarized interfering
beams. The saturated surface modulation and diffraction
relief structures composed of two-set fringes orthogonal to each
other at the same location. Like those reported for azo polymer
films, the SRGs are stable below the Tg of the amorphous
materials and can be removed by optical erasure or by heating
samples to a temperature above their Tg.
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Fig. 8 Diffraction efficiency varying with time in the initial stage of the
SRG formation for the AZBP-CA film. The points marked correspond to
the time to obtain the AFM images given in Fig. 9.
Fig. 7 Diffraction efficiency as a function of the irradiation time for
AZBP-X.
efficiency of AZBP-CA are obviously larger than those of typical
azo polymers obtained under the same experimental condi-
tions.11–14 It indicates that the response to the light irradiation is
significantly influenced by the molecular architecture. The
nunchaku-like molecular structure is more favorable for the
light-driven movement.
An interesting feature of the diffraction efficiency (DE) vari-
ation can be observed for the AZBP-CA film (Fig. 7). When the
writing beams are switched on, DE starts to increase rapidly at
the first stage. After about 50 s, the increase slows down and
a platform can be seen for the DE–time curve, which appears that
the saturated level has been obtained. However, after irradiation
for another 100 s, the diffraction efficiency starts to increase
again with a relatively slower rate. It finally reaches a much
higher and truly saturated DE after irradiation for 1200 s. The
DE variation with irradiation time can be understood by
considering the contribution from the polarization (birefrin-
gence) grating and SRG.40,41 When the diffraction efficiency from
both gratings is comparable, the phase shift between the gratings
has a considerable effect on the dynamics of diffraction effi-
ciency.41 The rapid DE increase in the initial stage can be
attributed to the formation of the polarization grating caused by
the molecular orientation. In order to verify this point, the DE
variation in the first 300 s is plotted in Fig. 8 and the AFM
images corresponding to the marked time periods (10, 35, 45, 60,
80 s) are given in Fig. 9. In the stage of the DE rapid increase, the
SRG formed is shallow as it is a relatively slow process.
Comparing the results given in Fig. 4 and Fig. 9, it can be seen
that both polarization grating and SRG will increase in the
period from 50 to 150 s. The DE platform for this time period can
only be attributed to the phase shift of polarization grating and
SRG in the space. In this case, the diffraction efficiency can not
reflect the real inscription depth. As AZBP-CA shows the most
significant birefringence induced by the polarized light, the
superimposed effect from both gratings can be clearly observed.
When the surface modulation is large (such as 200 nm), the
diffraction effect from the bulk refractive index grating can be
ignored.40 The viscous flow of the material involved in the surface
structure formation could disrupt the birefringence grating. The
Fig. 9 Development of the surface morphology of the AZBP-CA film at
the initial stage.
DE increase with a relative slow rate after irradiation for 150 s
can be mainly attributed to the diffraction contribution from
SRG.
Fig. 9 reveals more details of the SRG formation process of
AZBP-CA, which is the most efficient one in the series. After
exposing the film to the interfering beams for 10 s, the grating
starts to form on the surface. Because of the small modulation
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5 J. A. Delaire and K. Nakatani, Chem. Rev., 2000, 100, 1817.
amplitude, the profile is hardly perceivable by AFM (Fig. 9(a)).
After irradiation for 35 s, the surface relief structure can be
clearly seen (Fig. 9(b)). However, in contrast to the final SRG,
there is a shallower trough on every relief crest. In order to
identify the difference, it is named as shallow trough to distin-
guish it from the true trough. The separation between the true
troughs corresponds to the grating period and is determined by
the incident angle and the wavelength of the laser beam. It can be
estimated from AFM that the depth of the shallow trough is
about 5 nm and the depth of the true grating trough is about
12 nm. After irradiation for 45 s, the depth of the true trough
obviously increases but the depth of the swallow trough is almost
the same (Fig. 9(c)). When the irradiation time further increases,
the modulation amplitude of the grating keeps increasing and the
shallow trough on the relief crest gradually disappears (Fig. 9(d)
and 9(e)). The result presents a visualized picture of the SRG
formation on the film through viscous flow. During the process,
the material in the bright areas of the interfering fringes becomes
soft and starts to migrate to the dark area at the initial stage. As
the irradiation time increases, the mass-migration process grad-
ually develops and spreads to the dark regions of the fringes. This
observation can be used to better understand the SRGs forma-
tion mechanisms.
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€
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A series of nunchaku-like azo compounds (AZBP-X, X ¼ CA,
CN, and NT) was synthesized in this work. The materials showed
amorphous solid behavior at room temperature without special
treatment such as quenching. The type of the electron-
withdrawing groups of the azo chromophores played an impor-
tant role to influence the photoresponsive abilities of the
materials. Some of the materials (such as AZBP-CA) showed
a very sensitive response to the light irradiation as exhibited by
the photoinduced birefringence, dichroism, and SRG formation.
In contrast to azo polymers, when the irradiating light was
switched off, no orientation relaxation occurred and the bire-
fringence even increased slightly. SRG on the AZBP-CA film
showed a high diffraction efficiency of 27% for the probed beam.
The nunchaku-like molecules as an amorphous material are
promising for the high PIA efficiency, rapid SRG formation rate
and large surface modulation. The molecular design using the
flexible aliphatic spacer between the functional and rigid units
can be further explored for materials development and better
understanding of the structure–property relationship.
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Acknowledgements
The financial support from the NSFC under Projects 50533040
and 20774055 is gratefully acknowledged.
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