Self-structured surface patterns on molecular Azo glass films induced by laser light irradiation

Article abstract of DOI:10.1021/la9041056

In this work, formation of photoinduced self-structured surface pattern and its correlation with chromophoric structures were studied by using a series of star-shaped azo compounds, which exist as stable molecular glass at room temperature. For the synthesis, a star-shaped precursor (Tr-AN) was prepared by a ring-open reaction between 1, 3, 5-triglycidyl isocyanurate and N-methylaniline. The star-shaped azo compounds were then synthesized through azo-coupling reactions between the precursor and diazonium salts of 4-nitroaniline, 2-methyl-4-nitroaniline, and 4-aminobenzonitrile, respectively. Through these steps, three azo compounds were obtained to bear different donor-acceptor type azo chromophores at the peripheral positions. The formation of the photoinduced self-structured patterns was investigated by irradiating solid thin films of the molecular azo glass with a uniform laser beam (532 nm, 200 mW/cm²) at normal incidence. For comparison, formation of surface-relief-gratings (SRGs) was also investigated by exposing the thin films to an interference pattern of the laser beams (532 nm, 80 mW/cm²). The results show that SRGs can be inscribed on the films of all three star-shaped azo compounds, but self-structured surface patterns is only observed on film of the azo compound containing 4-cyanoazobenzene moieties (Tr-AZ-CN) under the same irradiation condition. The electron-withdrawing groups, which control the absorption band position, show a significant influence on both the self-structured pattern formation and SRG inscription rate. Under proper experimental conditions, both self-structured surface pattern and SRG can simultaneously be observed on the Tr-AZ-CN films. The observations can lead to a deeper understanding of the photoinduced effects, especially their correlation with molecular structures.

Full text of DOI:10.1021/la9041056

pubs.acs.org/Langmuir
© 2009 American Chemical Society
Self-Structured Surface Patterns on Molecular Azo Glass Films Induced by
Laser Light Irradiation
Jianjun Yin, Gang Ye, and Xiaogong Wang*
Department of Chemical Engineering, Laboratory for Advanced Materials, Tsinghua University, Beijing, P. R.
China, 100084
Received October 28, 2009. Revised Manuscript Received November 24, 2009
In this work, formation of photoinduced self-structured surface pattern and its correlation with chromophoric structures
were studied by using a series of star-shaped azo compounds, which exist as stable molecular glass at room temperature. For
the synthesis, a star-shaped precursor(Tr-AN) was prepared by a ring-open reactionbetween 1, 3, 5-triglycidyl isocyanurate
and N-methylaniline. The star-shaped azo compounds were then synthesized through azo-coupling reactions between the
precursor and diazonium salts of 4-nitroaniline, 2-methyl-4-nitroaniline, and 4-aminobenzonitrile, respectively. Through
these steps, three azo compounds were obtained to bear different donor-acceptor type azo chromophores at the peripheral
positions. The formation of the photoinduced self-structured patterns was investigated by irradiating solid thin films of the
molecular azo glass with a uniform laser beam (532 nm, 200 mW/cm2) at normal incidence. For comparison, formation of
surface-relief-gratings (SRGs) was also investigated by exposing the thin films to an interference pattern of the laser beams
(532 nm, 80 mW/cm2). The results show that SRGs can be inscribedonthefilms ofall threestar-shaped azocompounds, but
self-structured surface patterns is only observed on film of the azo compound containing 4-cyanoazobenzene moieties
(Tr-AZ-CN) under the same irradiation condition. The electron-withdrawing groups, which control the absorption band
position, show a significant influence on both the self-structured pattern formation and SRG inscription rate. Under proper
experimental conditions, both self-structured surfacepattern and SRG can simultaneously be observed on the Tr-AZ-CN
films. The observations can lead to a deeper understanding of the photoinduced effects, especially their correlation with
molecular structures.
Introduction
polymers. The SRG formation has been studied for side-chain
azo polymers, main-chain azo polymer, and hyperbranched azo
polymers.2,3,13 Many research results show that the SRG forma-
tioniscausedbythe mass-migration onazo polymer film surfaces,
where the photoinduced trans-cis isomerization of azo chromo-
phores plays a critically important role.2,3 The SRG formation
has been rationalized by considering different driven forces such
as the isomerization-driven free volume expansion in bulk,14 the
force based on the dipolar interaction with the optically induced
electric field gradient,15 a translational wormlike diffusion caused
by the photoisomerization of the azobenzene chromophores,16 a
mean-field force caused by the molecule alignment,17 and photo-
mechanical effect occurring in thin films.18 Although the intensive
investigations have been carried out in recent years, the mecha-
nism of SRG formation has not been fully understood until now.
On the basis of this effect, new photofabrication methods have
been developed for preparing various surface structures.19-21
In recent years, polymers containing azobenzene and its
derivatives (azo polymers for short) have been intensively investi-
gated for their unique photoresponsive properties.1-4 A variety of
photoinduced responses have been observed for azo polymers,
such as phase transition,5 chromophore orientation,6 surface-
relief-grating (SRG) formation,7,8 and photomechanical defor-
mation of thin films.9-12 SRG formation is generally referred to
the appearance of sinusoidal surface patterns on azo polymer
films upon irradiation with interfering laser beams.2,3,7,8 The
surface patterns with modulated depth in submicrometer scale
can be formed on azo polymer films at a temperature well below
the glass transition temperatures (Tgs) of the polymers. SRGs are
stable below the Tgs of the polymers and can be removed by
heating samples to a temperature above their Tgs. The surface
patterns are also erasable by an optical method such as irradiation
with a circularly polarized laser beam below the Tgs of the
*Corresponding author: wxg-dce@mail.tsinghua.edu.cn.
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Langmuir 2010, 26(9), 6755–6761
Published on Web 12/11/2009
DOI: 10.1021/la9041056 6755

Article
Recent researches have revealed another interesting photo-
Yin et al.
SRG inscription were studied by using the amorphous molecular
materials. Both photoinducedsurface deformation effects showed
close correlation with the chromophore structures. Under the
same test conditions, only molecular azo glass containing
4-cyanoazobenzene moieties (Tr-AZ-CN) exhibited the ability
to form hexagonal surface structures when irradiated with a
uniform laser beam at normal incidence. Under proper experi-
mental conditions, both self-structured surface pattern and SRGs
were observed to be formed on the same Tr-AZ-CN film. The
experimental detail, results and discussion will be presented in the
following sections.
induced effect on azo polymer film surfaces, which is known as
spontaneous surface pattern formation through light irradia-
tion.22 It is characterized by the spontaneous formation of
submicrometer hexagonal patterns on the azo polymer films after
irradiation with a uniform single laser beam at normal incidence.
The patterns comprise regularly spaced pillar-like structures with
saturated height about 100 nm. The influence of experimental
parameters, such as polarization of the laser beam, irradiation
time and intensity, has been investigated.23 In comparison with
the extensive exploration of the SRG formation, research effort
devoted to the photoinduced self-structured pattern formation is
rather limited. Although it has been understood that the azo
chromophore also plays a key role in the process, the influence of
different molecular structures on this effect is still unclear at the
moment. To our knowledge, formation of the self-structured
surface pattern has been observed only for a poly(methyl
methacrylate)-based copolymer (DR1MA/MMA 35/65) among
few others.22-24
Molecular glass, also named as “amorphous molecular materi-
als”, is a new type of glassy materials related to some well-
designed low-molecular-weight organic compounds.25,26 The
amorphous molecular materials show glass-transition behavior
similar to amorphous polymeric materials. On the other hand,
molecular glass possesses well-defined structures and often shows
better reproducible properties compared with polymers. By using
the common spin-coating method, thin solid films with a smooth
surface can be feasibly prepared from the amorphous molecular
materials. Formation of SRGs on molecular azo glass films has
recently been reported by several groups.27-30 Results show that
SRGs can be inscribed on molecular azo glass films with a faster
rate compared to those observed for azo polymers. Very recently,
it has been reported that complex periodic superstructures can be
induced under high interferential illumination of small-molecule-
based photochromic materials.31 The superstructures show an
amplitude much larger than the initial film thickness. A logical
inference could be that the self-structured surface patterns
should be more easily observed on films of molecular azo glass.
Because of the well-defined structures, molecular azo glass
could be a desirable type of materials for studying the possible
influence ofthe molecular structuresonthe self-structured pattern
formation.
Experimental Section
Tr-AN Synthesis. 1,3,5-Triglycidyl isocyanurate (2.97 g,
0.01 mol) and N-methylaniline (4.28 g, 0.04 mol) were mixed
and slowly heated in the reactor. After the solid was melted, the
mixture was continuously stirred and kept at 110 °C for 8 h. The
product was dissolved in THF (50 mL) and then precipitated
with petroleum ether. The solid was collected by filtration and
dried in a vacuum oven at 70 °C for 24 h. The crude product
was further purified by column chromatography (CH2Cl2, the
1
first component). Yield: 85%. H NMR (DMSO-d6), δ (ppm):
2.92 (s, 9H), 3.20 (m, 3H), 3.45 (m, 3H), 3.68 (m, 3H), 3.90
(m, 3H), 4.07 (m, 3H), 4.91 (d, 3H), 6.57 (t, 3H), 6.66 (d, 6H), 7.12
(t, 6H).
Tr-AZ-CN Synthesis. 4-Aminobenzontrile (0.59 g, 5 mmol)
was mixed with of sulfuric acid (0.5 mL) and glacial acetic acid
(12 mL). Diazonium salt was prepared by slowly adding an aqueous
solution of sodium nitrite (0.4 g, 5.78 mmol in 1 mL of water) into
the 4-aminobenzontrile solution. The mixture was stirred at 5 °C for
5 min and then added dropwise into a solution of Tr-AN (0.618 g,
1 mmol) in DMF (60 mL). The solution was stirred at 0 °C for 8 h
and then poured into plenty of water. The precipitate collected by
filtration was dissolved in THF and precipitated with petroleum
ether. The final product was vacuum-dried at 70 °C for 24 h. Yield:
87%. DSC: Tg 123 °C. MS: MW 1006.43 (calcd 1006.11). IR (KBr,
cm-1): 3440 (O-H), 2910 (-CH2), 2224 (CtN), 1693 (CdO),
1597, 1516 (benzene ring). 1H NMR (DMSO-d6), δ (ppm): 3.13 (s,
9H), 3.40 (m, 3H), 3.70 (m, 3H), 3.80 (m, 3H), 3.95 (m, 3H), 4.16 (m,
3H), 5.21 (d, 3H), 6.86 (d, 6H), 7.80 (d, 6H), 7.83 (d, 6H), 7.95 (d,
6H). UV-vis (DMF): 463 nm.
Tr-AZ-NT Synthesis. This compound was synthesized by
the azo-coupling reaction between Tr-AN and diazonium salt of
4-nitroaniline via similar method mentioned above. Yield: 85%.
DSC: Tg 119 °C. MS: MW 1066.40 (calcd. 1066.07). IR (KBr,
cm-1): 3430 (O-H), 2920 (-CH2), 1700 (CdO), 1600, 1510
In this study, three star-shaped azo compounds were synthe-
sized to contain push-pull type azo chromophores with different
structures. The self-structured surface pattern formation and
1
(benzene ring), 1345 (-NO2). H NMR (DMSO-d6), δ (ppm):
3.14 (s, 9H), 3.42 (m, 3H), 3.70 (m, 3H), 3.81 (m, 3H), 3.96 (m,
3H), 4.17 (m, 3H), 5.22 (d, 3H), 6.88 (d, 6H), 7.82 (d, 6H), 7.88 (d,
6H), 8.32 (d, 6H). UV-vis (DMF): 491 nm.
(22) Hubert, C.; Debuisschert, C. F.; Maurin, I.; Nunzi, J. M.; Raimond, P. Adv.
Mater. 2002, 14, 729–732.
Tr-AZ-mNT Synthesis. This compound was synthesized
by the azo-coupling reaction between Tr-AN and diazonium salt
of 2-methyl-4-nitroaniline via similar method mentioned above.
Yield: 87%. DSC: Tg 109 °C. MS: MW 1108.42 (calcd 1108.15).
IR (KBr, cm-1): 3440 (O-H), 2918 (-CH2), 1693 (CdO), 1601,
1518 (benzene ring), 1338 (-NO2). 1H NMR (DMSO-d6),
δ (ppm): 2.64 (s, 9H), 3.14 (s, 9H), 3.41 (m, 3H), 3.70 (m, 3H),
3.80 (m, 3H), 3.96 (m, 3H), 4.17 (m, 3H), 5.20 (d, 3H), 6.86 (d,
6H), 7,59 (d, 3H), 7.79 (d, 6H), 8.06 (d, 3H), 8.32 (s, 3H). UV-vis
(DMF): 489 nm.
(23) Hubert, C.; Fiorini-Debuisschert, C.; Rocha, L.; Raimond, P.; Nunzi, J. M.
J. Opt. Soc. Am. B 2007, 24, 1839–1846.
(24) (a) Kandjani, S. A.; Barille, R.; Dabos-Seignon, S.; Nunzi, J. M.; Otryl, E.;
Kucharski, S. Mol. Cryst. Liq. Cryst. 2006, 466, 99–109. (b) Kandjani, S. A.; Barille,
R.; Dabos-Seignon, S.; Nunzi, J. M.; Otryl, E.; Kucharski, S. Opt. Lett. 2005, 30, 1986–
1988.
(25) (a) Shirota, Y. J. Mater. Chem. 2000, 10, 1–25. (b) Ishow, E.; Bellaïche, C.;
Bouteiller, L.; Nakatani, K.; Delaire, J. A. J. Am. Chem. Soc. 2003, 125, 15744–15745.
(c) Shirota, Y. J. Mater. Chem. 2005, 15, 75–93.
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(27) (a) Nakano, H.; Takahashi, T.; Kadota, T.; Shirota, Y. Adv. Mater. 2002,
14, 1157–1160. (b) Nakano, H.; Tanino, T.; Takahashi, T.; Ando, H.; Shirota, Y.
J. Mater. Chem. 2008, 18, 242–246. (c) Ando, H.; Tanino, T.; Nakano, H.; Shirota, Y.
Mater. Chem. Phys. 2009, 113, 376–381.
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4021–4027. (b) Chun, C.; Kim, M. J.; Vak, D.; Kim, D. Y. J. Mater. Chem. 2003, 13,
2904–2909.
Film Preparation. Solid films of the molecular azo glass with
smooth surfaces were prepared by spin-coating. The homoge-
neous solutions of the azo compounds were obtained by dissol-
ving a suitable amount of the solids in anhydrous N,N-
dimethylformamide (DMF). The solutions were filtered through
0.45 μm membranes and spin-coated onto glass slides. The film
thickness was controlled to be in a range from 200 to 400 nm by
adjusting the solution concentrations (in range from 7% to 10%
(29) Ishow, E.; Lebon, B.; He, Y. N.; Wang, X. G.; Bouteiller, L.; Galmiche, L.;
Nakatani, K. Chem. Mater. 2006, 18, 1261–1267.
(30) He, Y. N.; Gu, X. Y.; Guo, M. C.; Wang, X. G. Opt. Mater. 2008, 31, 18–27.
ꢀ
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Adv. Funct. Mater. 2009, 19, 796–804.
6756 DOI: 10.1021/la9041056
Langmuir 2010, 26(9), 6755–6761

Yin et al.
Article
Table 1. Tgs and Tds of the Star-Shaped Azo Compounds
azo compounds
Tg (°C)
Td (°C)
Tr-AZ-CN
Tr-AZ-NT
Tr-AZ-mNT
123
119
109
261
271
221
Figure 1. The chemical structure of Tr-AZ-CN, Tr-AZ-NT,
and Tr-AZ-mNT.
Figure 2. UV-vis spectra of the star-shaped azo compounds in
DMF solutions.
(weight %)) and the spinning speed. The films were dried at
60-70 °C under vacuum for 48 h.
azo-coupling reactions of the precursor. The precursor
(Tr-AN) was synthesized from 1,3,5-triglycidyl isocyanurate
(TGIC) and N-methylaniline (AN). The ring open reaction was
carried out at 110 °C, whichwas above the meltingpoint ofTGIC.
This reaction temperature could avoid possible reaction between
the OH groups formed during the reaction and the unreacted
epoxide rings, which often occurs at a higher temperature. The
addition of excessive amount of N-methylaniline (molar ratio to
epoxide group of TGIC) ensured all the three epoxide groups of
TGIC to be completely reacted. The remaining N-methylaniline
was removed by petroleum extraction after the reaction.
Laser Irradiation. A linearly or circularly polarized beam
from a diode-pumped frequency doubled solid state laser
(532 nm) was used as the light source. The laser beam with
intensity of 200 mW/cm2 was obtained after being spatially
filtered, expanded, and collimated. For inducing the spontaneous
surface pattern, thelaser beamwas incidentperpendicularly to the
film surfaces. SRG inscription was carried out by the experi-
mental setup and conditions similar to those reported before.7,8
A
p-polarized beam from the diode-pumped frequency doubled solid
state laser at 532 nm with intensity about 80 mW/cm2 was used as
the writing beam. The writing beam was split by a mirror such that
a half of the beam reflected onto the film surface was coincident
with the other half beam to form an interference pattern. The
formation of SRGs was probed with an unpolarized low power
He-Ne laser beam at 633 nm by measuring the diffraction
efficiency of the first-order diffracted beam in a real time mode.
Characterization. IR spectra were determined using a Nicolet
560-IR FT-IR spectrophotometer by incorporating samples in
KBr disks. 1H NMR spectra were recorded using a JEOL JNM-
ECA300 NMR spectrometer. Thermal phase transitions of the
azo compounds were tested using TA Instruments DSC 2920 with
a heating rate of 10 °C /min in nitrogen atmosphere. UV-vis
absorption spectra were recorded on a Perkin-Elmer Lamba Bio-
40 spectrophotometer. The surface images of the photoinduced
self-structured surface structures and surface-relief-gratings were
monitored using an atomic force microscope (AFM) (Nanoscope
IIIa, tapping mode) in the tapping mode.
The target azo compounds were synthesized by azo-coupling
reactions between Tr-AN and the diazonium salts of 4-amino-
benzonitrile, 4-nitroaniline, and 2-methyl-4-nitroaniline, respec-
tively. The analytical results indicated that the conversion of azo-
coupling reactions was near 100%. In the 1H NMR spectrum of
Tr-AN, the anilino moieties show the resonance signals at 7.12,
6.66, and 6.57 ppm attributed to the protons at meta, ortho, and
para positions of the amino groups. After azo-coupling reactions,
the following changes in the 1H NMR spectra were observed for
anilino moieties in Tr-AN. The resonance at 6.57 ppm, corres-
ponding to the protons at para positions of the amino groups,
completely disappeared. The resonance signals of protons at the
ortho and meta positions shifted to lower magnetic field. These
spectral changes can be attributed to the incorporation of the
electron-withdrawing groups and increase of the conjugation
length after forming the azo linkages. The additional resonance
signals corresponding to the newly introduced benzenoid protons
Results and Discussion
1
In this study, three star-shaped azo compounds (Tr-AZ-CN,
Tr-AZ-NT, and Tr-AZ-mNT) were synthesized to study
photoinduced formation of the self-structured surface patterns.
The chemical structure of Tr-AZ-CN, Tr-AZ-NT, and
Tr-AZ-mNT is given in Figure 1. The star-shaped architecture
is adopted to avoid the crystallization occurring for most small
organic molecules. The strong dipole-dipole interaction between
the azo chromophores increases the glass transition temperature
(Tg) of the materials, which can maintain a stable amorphous state
well above room temperature. Because of the structural character-
istics, the star-shaped azo compounds show typical behavior of
molecular glass. As the only difference among the azo compounds
exists in the chromophoric structure, its influence on the photo-
induced pattern formation can be studied in a feasible way.
Synthesis and Characterization. The syntheses included
the preparation of a precursor with the core structure and
appeared at a lower magnetic field. From H NMR and other
analytical results given in the Experimental Section, it is con-
cluded that Tr-AZ-CN, Tr-AZ-NT, and Tr-AZ-mNT with
proposed structures have been successfully synthesized.
The glass transition temperatures (Tgs) and decomposition
temperatures (Td s) of the star-shaped azo compounds were
obtained from the thermal analyses and are given in Table 1.
Tgs of Tr-AZ-CN, Tr-AZ-NT, and Tr-AZ-mNT are 123,
119, and 109 °C, and Td s are 261, 271, and 221 °C. Figure 2 gives
UV-vis absorption spectra of the azo compounds in DMF
solutions. The spectra show typical characteristics of the pseu-
dostilbene type azo chromophores,32 where the absorption bands
corresponding to the π-π* transition appear in visible region.
(32) Rau, H. In Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press:
Boca Raton, FL, 1990; Vol. II, Chap. 4.
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DOI: 10.1021/la9041056 6757

Article
Yin et al.
Because of the strong π-π* absorption, the weak n-π* absorp-
tion bands cannot be seen, which is located at longer wavelength
side of the λmaxs. The λmaxs of the star-shaped azo compounds are
strongly affected by the p-substituents on the azobenzene units
due to their inductive and conjugative effect on both the ground
and excited states. The λmaxs of the azo compounds in DMF
solutions and as solid films are compared in Table 2. λmaxs of the
star-shaped azo compounds as spin-coated films show blue-shifts
compared with those of the corresponding DMF solutions. All
three azo compounds could be easily dissolved in polar organic
solvents such as tetrahydrofuran (THF), N,N-dimethylforma-
mide (DMF), and N,N-dimethylacetamide (DMAc). Solid thin
films of the azo compounds with smooth surfaces could be
obtained through spin-coating and were used for the light
irradiation study.
Photoinduced Self-Structured Pattern Formation. To
study self-structured surface pattern formation, the solid films
of molecular azo glass were irradiated with a uniform laser beam
(532 nm, 200 mW/cm2) at the normal incidence. Figure 3 shows
typical AFM images and their two-dimensional Fourier trans-
form (2D-FT) images of the photoinduced surface patterns on the
Tr-AZ-CN film after irradiation for 15 min. The surface
patterns comprise regularly spaced peaks with distance in range
from 420 to 460 nm. Some localized hexagonal arrangement of
Table 2. λmaxs of the star-shaped azo compounds in DMF solutions
and as spin-coated films
azo
compounds
λmax (nm) (in DMF
solutions)
λmax (nm) (as spin-coated
films)
Tr-AZ-CN
Tr-AZ-NT
Tr-AZ-mNT
463
491
489
438
459
463
Figure 3. Typical AFM images (10 μm  10 μm) and their two-dimensional Fourier transform (2D-FT) images of the photoinduced self-
structured patterns formed on Tr-AZ-CN films with different incident laser beam polarizations: (a) circularly polarized, 2D-view; (b)
circularly polarized, 3D-view; (c) circularly polarized, 2D-FT images; (d) linearly polarized, 2D-view; (e) linearly polarized, 3D-view; (f )
linearly polarized, 2D-FT images. The thickness of the films was 250 nm. The intensity of the laser beam was 200 mW/cm2 and the irradiation
time was 15 min.
6758 DOI: 10.1021/la9041056
Langmuir 2010, 26(9), 6755–6761

Yin et al.
Article
the peaks can be recognized from the images. The saturated
amplitude of the surface modulation depends on the polarization
conditions of the laser light. It is about 120 nm in the case of the
light with circular polarization (Figure 3, parts a and b) and about
80 nm for linear polarization (Figure 3, parts d and e). The
periodic orders of the surface structures on a large scale can be
seen from the 2D-FT images (Figure 3, parts c and f ). Other
significant differences can also be observed for irradiation with
circularly and linearly polarized light. In the case of irradiation
with the linearly polarized laser beam, the arrangement of the
hexagonal patterns shows some correlation with the polarization
direction. But no similar correlation is observed for the surface
patterns formed upon the irradiation with the circularly polarized
light, where the localized hexagonal patterns have a random
orientation. After irradiation with the linearly polarized light,
some of the peaks seem to be elongated in two directions which
are about (60°to the polarization direction (Figure 3, parts d
and e). In contrast, no such deformation is observed for the
irradiation with circularly polarized light (Figure 3, parts a and b).
The above observations on the self-structured pattern formation
are consistent with those reported for DR1MA/MMA 35/65.22,23
For three types of molecular azo glass studied in this work,
photoinduced self-structured surface patterns were only ob-
served to be formed on Tr-AZ-CN films upon the irradiation
with the laser beam (532 nm, 200 mW/cm2) for 20 min. Under
the same light irradiation condition, no similar self-structured
patterns were formed on the films of Tr-AZ-NT and
Tr-AZ-mNT. AFM observation only revealed random peaks
with an average height about several nanometers, which
corresponded to the original surface roughness of the films.
Tr-AZ-CN is distinguished from the other two azo com-
pounds only by the electron-withdrawing groups of the azo
chromophores. The electron-withdrawing groups show a sig-
nificant influence on the absorption band position, where the
Figure 4. Typical AFM images (10 μm  10 μm) of the surface-
relief-gratings formed on Tr-AZ-mNT films: (a) 2D-view; (b)
3D-view. The thickness of the films was 300 nm. The intensity of
the laser beam was 80 mW/cm2 and the irradiation time was 1000 s.
λ
maxs of Tr-AZ-CN and Tr-AZ-NT are 463 and 491 nm in
DMF solutions (Table 2). Comparing with the absorption
band positions (Figure 2), it can be seen that the excitation
wavelength is near the maximum absorption for Tr-AZ-NT
and Tr-AZ-mNT, but approaches the absorption band tail
for Tr-AZ-CN. The modulation efficiency of DR1MA/
MMA 35/65 has been studied by using lasers with different
wavelength.22,23 It shows that the modulation efficiency of the
spontaneous patterns become more important when the ex-
citation wavelength is approach the edge of the absorption
band of the azo dye.23 In above two cases, although the relative
positions of the excitation wavelengths and absorption bands
were adjusted by different ways, the results obtained are
consistent with each other. Compared with the azo polymer,
Tr-AZ-CN shows an obviously faster rate to form the
photoinduced self-structured surface patterns.
Patterns Induced by Interfering Laser Beams. Solid films
of the molecular azo glass were also used to study the surface-
relief-grating (SRG) formation. SGRs were inscribed by exposing
the films to an interference pattern of two p-polarized laser beams
at 532 nm. Upon the light irradiation, SRGs were observed to be
formed on films of all three azo compounds studied in this work.
Figure 4 shows a typical AFM three-dimensional image of the
SRG formed on the Tr-AZ-mNT film, which appears as
regularly spaced sinusoidal surface structures. The spatial period
of the grating is about 1.5 μm, which can be changed by adjusting
the intersection angle between the two interfering beams. SRGs
formed on the films of the other two azo compounds show a
similar morphological feature. Similar to SRGs formed on azo
polymer films, the SRGs are stable at a temperature below the Tgs
Figure 5. First-order diffraction efficiency as a function of the
irradiation time for Tr-AZ-CN, Tr-AZ-NT, and
Tr-AZ-mNT.
of the azo compounds and can be removed by heating samples
above their Tgs. The diffraction efficiency of the SRGs can be
used to characterize the pattern formation.7,8,30 Figure 5 shows
the relationship between the first-order diffraction efficiency of
SRGs and the irradiation time for the star-shaped azo com-
pounds. Tr-AZ-CN, Tr-AZ-NT, and Tr-AZ-mNT show
significantly different inscription rates of the SRG formation.
Tr-AZ-CN, which contains 4-cyanoazobenzene moieties, ex-
hibits the fastest inscription rate for the SRG formation among
the three compounds. It can reach the saturated diffraction
efficiency in less than 700 s. The other two azo compounds, which
have the same electron-withdrawing groups (-NO2), also show
different SRG-forming rates. Tr-AZ-mNT, which has a methyl
substituent in ortho-positions of the azo group, shows the faster
rate to form SRG.
Under proper light irradiation condition, both SRGs and self-
structured surface patterns can be formed on the same piece of
Tr-AZ-CN films. Figure 6 shows typical AFM images of the
surface of Tr-AZ-CN film after the irradiation with the inter-
fering laser beams (80 mW/cm2) for 1000 s. The SRG formed has
Langmuir 2010, 26(9), 6755–6761
DOI: 10.1021/la9041056 6759

Article
Yin et al.
Figure 6. Typical AFM images (10 μm  10 μm) of the surface patterns formed on Tr-AZ-CN films by irradiation with interfering laser
beams. The thickness of the films was 300 nm. The intensity of the laser beam was 80 mW/cm2 and the irradiation time was 1000 s.
a spatial period of 1.5 μm and modulation depth of 150 nm. The
self-structured surface patterns can be clearly seen in the troughs
of the grating. The modulation amplitude of the self-structured
peaks is about 45 nm, which is much smaller than the modulation
amplitude of the grating. As mentioned above, the self-structured
pattern formation is a relatively slow process compared with the
SRG formation. The appearance of both photoinduced patterns
on the same surface can be rationalized by considering two
dynamic processes with different time scales. In the first process,
SRG is formed through mass-migration in the grating vector
direction. In the second process, the self-structured pattern is
gradually formed in the trough areas of the saturated SRG. It was
observedthatthe appearanceofthe self-structuredpattern needed
some threshold intensity. No self-structured pattern was formed
when the laser beam had the lower intensity such as 50 mW/cm2.
Similar to the case using uniform laser beam, the self-structured
pattern induced by the interfering laser beams was only observed
on Tr-AZ-CN films.
substituent effect has been observed for azo polymer with the
high chromophore density.14
Comparing the SRG formation, less has been known about
photoinduced self-structured pattern formation concerning
mechanism and its correlation with molecular structure.
Experimental conditions, including irradiation wavelength,
irradiation time, irradiation power, incidence angle, and sam-
ple backside reflection, have been fully investigated by using an
azo copolymer (DR1MA/MMA 35/65).22,23 The current ob-
servations based on molecular azo glass are consistent with
those results. The photoinduced self-structured patterns show
features obviously different with the parallel ripple structures
on polymer surfaces induced by irradiation with a single beam
of excimer laser or Nd:YAG pulsed laser, which has been
known as laser-induced periodic surface structure (LIPSS) for
decades.37-40 LIPSS has been explained by the models based
on electromagnetic field theory such as the surface-scattered
wave model.41 However, the self-structured patterns observed
by the current study and those already reported on azo
copolymers did not show direct correlation with LIPSS. Two
important points related with the current investigation worth
further discussions. First, the observation of self-structured
patterns on the molecular glass (Tr-AZ-CN) film indicates
that the pattern formation is not directly correlated with
polymeric backbone. The polymeric chain could play an im-
portant role to influence the pattern formation rate, but it is not
a necessary factor to cause the self-structured patterns. Second,
similar to the SRG formation, the electron-withdrawing group
of the azo chromophore shows a significant influence on the
self-structured pattern formation. This result indicates that the
surface pattern formation can also have a close correlation with
the excitation manner and photoisomerization pathway of
the azo chromophores. Obviously, to fully understand this
interesting photoinduced phenomenon, especially its explana-
tion at molecular level, more thorough investigations will be
needed.
Discussion
Formation of SRGs on azo polymer and molecular glass films
has been intensively investigated in the past decades. Previous
reports have indicated that the electron-withdrawing group has a
significant influence on the SRG-forming rate.33-35 The substi-
tuent effect could be attributed to the influence on absorption
band position and its correlation with the excitation wavelength.
It has been indicated that the excitation at n-π* absorption band,
which located at longer wavelength side of the λmaxs near the tail
of the strong π-π* absorption band, can more efficiently induce
the trans-cis isomerization of azo dyes with push-pull struc-
tures.36 The accelerated trans-cis-trans isomerization cycle
could be the main cause to increase the SRG-forming rate. The
current study also shows that introducing methyl groups on the
azo chromophore can accelerate the SRG formation, which can
be seen by comparing the SRG-forming rates of Tr-AZ-mNT
and Tr-AZ-NT. The effect could beattributed to the decreaseof
the dipole interaction and increase of free volume because of the
methyl substitution. This point can be seen from the lower Tg of
Tr-AZ-mNT compared with that of Tr-AZ-NT. Similar
Conclusion
A new class of star-shaped azo compounds was prepared
and used to study the photoinduced surface pattern formation.
(33) He, Y. N.; Wang, X. G.; Zhou, Q. X. Polymer 2002, 43, 7325–7333.
(34) He, Y. N.; Yin, J. J.; Che, P. C.; Wang, X. G. Eur. Polym. J. 2006, 42, 292–
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(35) Wang, D. R.; He, Y. N.; Deng, W.; Wang, X. G. Dyes Pigments 2009, 82,
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(36) Kim, M. J.; Lee, J. D.; Chun, C.; Kim, D. Y.; Higuchi, S.; Nakayama, T.
Macromol. Chem. Phys. 2007, 208, 1753–1763.
(37) (a) Bolle, M.; Lazare, S.; Le Blanc, M.; Wilmes, A. Appl. Phys. Lett. 1992,
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6760 DOI: 10.1021/la9041056
Langmuir 2010, 26(9), 6755–6761

Yin et al.
Article
For the molecular azo glass, the type of the azo chromophores
plays an important role to influence both the SRG inscription
rate and photoinduced self-structured pattern formation.
Compared with the other two compounds, the azo compound
containing 4-cyanoazobenzene moieties (Tr-AZ-CN) shows
a much faster rate to form SRG. Under the same laser
illumination condition (200 mW/cm2, 20 min), the photo-
induced self-structured surface pattern is only observed on
Tr-AZ-CN films. By irradiation with interfering laser beams
(80 mW/cm2), both photoinduced self-structured pattern and
SRG can simultaneously be observed on the Tr-AZ-CN film
surfaces.
Langmuir 2010, 26(9), 6755–6761
DOI: 10.1021/la9041056 6761