Thin layers of low molecular azobenzene materials with effective light-induced mass transport

Article abstract of DOI:10.1021/la9040562

The very effective significant corrugation of a film surface (SRG) formation in the novel azobenzene-containing materials with three azobenzene units was investigated. The materials were prepared by amide formation through reaction of 1,3,5-benzenetricarbonyl trichloride (BTC) with amino-substituted azobenzene derivative in anhydrous dimethoxyethane at room temperature in the presence of triethylamine in analogy to the reaction of BTC and aniline. The thickness of the films was in the range of 50-2000 nm as measured by profilometer. The photo-induced orientation was investigated by exposing the films to the linearly polarized light of 488 nm with spatially homogeneous intensity of 300 mW cm^-1. The formation of three-armed 4-aminoazobenzene (AAB) product was proved by ESI-mass spectroscopy. While the inscription rate for the latter polymer material becomes zero as the film thickness approaches 100 nm, the inscription rate of 0.0027min^-1 was observed for the material based on BTC + AAB.

Full text of DOI:10.1021/la9040562

pubs.acs.org/Langmuir
© 2010 American Chemical Society
Thin Layers of Low Molecular Azobenzene Materials with Effective
Light-Induced Mass Transport
Leonid M. Goldenberg,*,† Lazar Kulikovsky,† Olga Kulikovska,‡ Jaroslaw Tomczyk,‡ and
Joachim Stumpe‡
†Institute of Thin Film Technology and Microsensorics, Kantstrasse 55, 14513 Teltow, Germany and
‡Fraunhofer Institute for Applied Polymer Research, Science Campus Golm, Geiselbergstrasse 69,
14476 Potsdam, Germany
Received October 26, 2009. Revised Manuscript Received November 30, 2009
Azobenzene-containing film forming materials have been well-
known for the photoinduction of optical anisotropy and genera-
tion of surface relief gratings (SRG).1 The phenomena are
promising for application in optics and photonics, including
diffractive optical elements, optical data storage and communi-
cations.2 The formation of SRG is advantageous for many
applications due to its efficiency, all-optical character and
reversibility.2
difficulties with purification and low solubility of the final
material.4,5 These materials produced via noncovalent interac-
tions instead of covalent bonds were summarized recently.5
From the other hand only very recently the possibility of a
simple method of synthesis ofsidechain epoxy-based azobenzene-
containing oligomers directly in the film has been demonstrated
for the first time.6a One more example is fabrication of colorless
SRG, which was demonstrated in a new easy urea-bond linked
material.6b The success of different material approaches moti-
vated further investigation aiming materials with good film
forming properties in a wide range of thicknesses for the efficient
formation of SRG. In this sense the glass forming low molecular
weight materials7 present a promising solution. High azobenzene
loading is intrinsic to them. Second, their low molecular weight is
suggested to reduce chain entanglement and thus favorably
influence the SRG formation. Notice that the latter suggestion
has been rather adopted from the reology of polymers then
confirmed by the experimental observations on SRG formation.8
A few results on comparison between low molecular weight and
polymer materials are rather contradictory.9a,9d Obviously the
comparison has been complicated by different experimental
conditions.
Although a number of dendritic azobenzene materials—often
associated with complicated synthesis—has been published, in
most of cases SRG formation has not been reported. Thus, the
approach of a low molecular weight material is still promising for
SRG formation. Other than already mentioned complicated
synthesis the main problem to overcome is a strong tendency to
crystallization or aggregation restricting film forming properties.
We report here the very effective SRG formation in the novel
azobenzene-containing materials with three azobenzene units
bound to the core.
The key to the photoinduced mass transport (the basic process
of SRG formation) is the efficient multiple cycling between E and
Z isomers of azobenzene moieties upon irradiation. Beyond this
local process, the micrometer scale motion takes place that tags
the passive polymer matrix and causes significant corrugation of a
film surface (SRG).1 Though the effect has been demonstrated in
a plenty of azobenzene-containing materials, there is still no clear
understanding of the grating formation mechanism. Most theo-
ries are based on a driving force directly applied to the azobenzene
unit while differing in the nature of this force. Alternatively so-
called phototriggered mechanism was proposed to explain the
SRG formation in some LC azobenzene-containing materials.3
Interestingly, in the materials with phototriggered mechanism the
surface corrugation as deep as double film thickness could be
achieved making these materials superior to others in terms of
sensitivity. However, their practical applications may berestricted
by small film thickness and thus by correspondingly low values of
diffraction efficiency (DE) and surface modulations reached. In
this sense the supramolecular materials achieving a record modu-
lation depth of 1650 nm4a are superior to other azobenzene
containing materials. Moreover, the supramolecular mate-
rials overcome the disadvantages such as complicated synthesis,
*Corresponding author. E-mail: lengold@gmx.de.
(1) (a) Ichimura, K. Chem. Rev. 2000, 100, 1847. (b) Natansohn, A.; Rochon, P.
Chem. Rev. 2002, 102, 4139.
(2) (a) Viswanathan, N. K.; Kim, D. Y.; Bian, S.; Williams, J.; Liu, W.; Li, L.;
Samuelson, L.; Kumar, J.; Tripathy, S. K. J. Mater. Chem. 1999, 9, 1941. (b)
Yoshino, K.; Takeda, H.; Kasano, M.; Satoh, S.; Matsui, T.; Ozaki, R.; Fujii, A.; Ozaki,
M.; Kose, A. Macromol. Symp. 2004, 212, 179. (c) Harada, K.; Itoh, M.; Yatagai, T.;
Kamemaru, S. Optic. Rev. 2005, 12, 130. (d) Ye, C.; Wong, K. Y.; He, Y.; Wang, X. Opt.
Exp. 2007, 15, 936.
(3) (a) Ubukata, T.; Seki, T.; Ichimura, K. Adv. Mater. 2002, 12, 1675. (b) Zettsu,
N.; Fukuda, T.; Matsuda, H.; Seki, T. Appl. Phys. Lett. 2003, 83, 4960. (c) Zettsu, N.;
Seki, T. Macromolecules 2004, 37, 8692. (d) Zettsu, N.; Ogasawara, T.; Arakawa, R.;
Nagano, S.; Ubukata, T.; Seki, T. ibid 2007, 40, 4607. (e) Zettsu, N.; Ogasawara, T.;
Mizoshita, N.; Nagano, S.; Seki, T. Adv. Mater. 2008, 20, 516.
(5) Stumpe, J.; Kulikovska, O.; Goldenberg, L. M.; Zakrevskyy, Y. In Azobenzene-
Containing Polymers and Liquid Crystals; Zhao, Y., Ikeda, T., Eds.; Wiley: New York,
2009, Chapter 2, P47.
(6) (a) Goldenberg, L. M.; Kulikovsky, L.; Kulikovska, O.; Stumpe, J. J. Mater.
Chem. 2009, 19, 6103. (b) Goldenberg, L. M.; Kulikovsky, L.; Kulikovska, O.; Stumpe,
J. J. Mater. Chem. 2009, 19, 8068; (c) Goldenberg, L. M.; Kulikovsky, L. Unpublished
results.
(7) (a) Shirota, Y. J. Mater. Chem. 2005, 15, 75. (b) Tanino, T.; Yoshikawa, S.;
Ujike, T.; Nagahama, D.; Moriwaki, K.; Takahashi, T.; Kotani, Y.; Nakano, H.; Shirota,
Y. J. Mater. Chem. 2007, 17, 4953.
€
(8) Gharagozloo-Hubmann, K.; Kulikovska, O.; Borger, V.; Menzel, H.;
(4) (a) Kulikovska, O.; Goldenberg, L. M.; Stumpe, J. Chem. Mater. 2007, 19,
3343. (b) Kulikovska, O.; Goldenberg, L. M.; Kulikovsky, L.; Stumpe, J. Chem. Mater.
2008, 20, 3528. (c) Stumpe, J.; Goldenberg, L.; Kulikovska, O. European Patent EP
1632520-A1; (d) Kulikovska, O.; Kulikovsky, L.; Goldenberg, L. M.; Stumpe, J. Proc.
SPIE 2008, 6999, 69990I. (e) Kulikovsky, L.; Kulikovska, O.; Goldenberg, L. M.;
Stumpe, J. ACS Appl. Mater. Interface 2009, 1, 1739. (f) Goldenberg, L. M.;
Kulikovska, O.; Stumpe, J. Langmuir 2005, 21, 4794.
Stumpe, J. Macromol. Chem. Phys. 2009, 210, 1809.
(9) (a) Seo, E.-M.; Kim, M. J.; Shin, Y.-D.; Lee, J.-S.; Kim, D.-Y. Mol. Cryst.
Liq. Cryst. 2001, 370, 143. (b) Kim, M. J.; Seo, E.-M.; Vak, D.; Kim, D.-Y. Chem.
Mater. 2003, 15, 4021. (c) Chun, C.; Kim, M. J.; Vak, D.; Kim, D.-Y. J. Mater. Chem.
2003, 13, 2904. (d) Chun, C.; Seo, E.-M.; Kim, M. J.; Shin, Y.-D.; Kim, D.-Y. Opt.
Mater. 2007, 29, 970. (e) Lee, E. U.; Jung, K. M.; Cho, M. J.; Kim, K. H.; Choi, D. H.
Macromol. Res. 2008, 16, 434.
2214 DOI: 10.1021/la9040562
Published on Web 01/19/2010
Langmuir 2010, 26(4), 2214–2217

Goldenberg et al.
Letter
Figure 1. Reaction scheme.
The materials were prepared by amide formation through
reaction of 1,3,5-benzenetricarbonyl trichloride (BTC) with amino-
substituted azobenzene derivative in anhydrous dimethoxyethane
at room temperature in the presence of triethylamine (Figure 1) in
analogy to the reaction of BTC and aniline.10 Recently, a similar
compound has been patented for the use in dielectric films.11 With
our recipe the crude solution after filtration could be used strait
away for the optical film preparation without further purification.
In spite of the azobenzene loading close to 75% by weight no
crystallization was visually observed in the films for at least 5
months. The thickness of the films was in the range of 50-2000 nm
as measured by profilometer. The reaction could be also performed
with other amino- and hydroxy-substituted azobenzene. Not all
tested derivatives render film-forming properties, indicating that
not in all cases the product was amorphous.
For 4-aminoazobenzene (AAB), the formation of three-armed
product was proved by ESI-mass spectroscopy (m/z = 748.2836).
The presence of compounds with two and one azobenzene units
(m/z = 569.1967 and 392.9200) and some initial 4-aminoazoben-
zene (m/z = 196.9667) was also detected. By 1H NMR spectros-
copy it was estimated that ca. 10% of unreacted AAB (H2N, ca. 4
ppm) and respectively ca. 10% of -COCl groups in BTC are left
building compounds with two and/or one azobenzene arms. For
comparison the product was also separated by column chroma-
tography (silica gel, eluent CH2Cl2, then ethyl acetate) and
recrystallized from THF:EtOH (70% yield), mp 317 °C. It was
characterized by 1H NMR [(500 MHz, DMSO-d6, ppm): 10.97 (s,
3H), 8.83 (s, 3H), 8.11 (d, 6H), 7.99 (d, 6H), 7.89 (d, 6H), 7.53-7.60
(m, 9H) (Figure 2)], 13C NMR [(75 MHz, DMSO-d6, ppm): 164.7,
151.9, 148.0, 141.9, 135.2, 131.0, 129.3, 128.9, 123.5, 122.3, 120.5
(see Supporting Information)], FTIR [(KBr, cm-1): 1677 and 1533
(amide)], ESI-mass spectroscopy [calculated for C45H34N9O3,
748.2785; found, 748.2804; [M þ Hþ], 749.2781], and elemental
analysis (Anal. Calcd for C45H34N9O3: C, 72.28; H, 4.48; N, 16.86.
Found: C, 70.98; H, 4.58; N, 16.88). Optical films were prepared
with the pure compound and compared in their holographic
efficiency with the crude formulation (see below, Figure 6).
The photoinduced orientation has been investigated by expos-
ing the films to the linearly polarized light of 488 nm with spatially
homogeneous intensity of 300 mW cm-1 as described before.4,6
The induced birefringence (Figure 3) was switched between two
states by changing the polarization plane of the irradiating light.
After irradiating light was switched off a slight relaxation of
Figure 2. 1H NMR spectrum of main product of reaction between
BTC and AAB.
Figure 3. Induction of birefringence Δn in a 1.97 μm thick film of
the material BTC þ AAB under linearly polarized homogeneous
irradiation at 488 nm, switching and relaxation of birefringence:
the light was turned on at points A (polarization state A) and B
(polarization state B) and turned off at the point C.
birefringence occurred with remaining value of Δn = 0.042. This
result appears to correspond to the general understanding of the
orientation process in the case when azobenzene is attached with-
out flexible spacer and three chromophores might compensate
each other in their movement. It is also confirmed by other
examples: three-armed material with flexible spacer9d exhibited a
stable photoorientation while three and four-armed materials with
rigidly connected azobenzenes showed only Δn ∼ 0.01.9b,9c
(10) (a) Miller, T. M.; Neenan, T. X. Chem. Mater. 1990, 2, 346. (b) Bayliff, P. M.;
Feast, W. J.; Parker, D. Polym. Bull. 1992, 29, 265.
(11) Arayama, K.; Hiraoka, H.; Iwato, K. US Patent application, 2008 0161532.
(12) Fukuda, T.; Matsuda, H.; Shiraga, T.; Kimura, T.; Kato, M.; Viswanathan,
N. K.; Kumar, J.; Tripathy, S. K. Macromolecules 2000, 33, 4220.
Langmuir 2010, 26(4), 2214–2217
DOI: 10.1021/la9040562 2215

Letter
Goldenberg et al.
Figure 4. (a) DE of zeroth, first, and second diffraction orders with irradiation time for a grating in a 240 nm thick film of the material based
on 4-aminoazobenzene. (b) AFM image of the inscribed SRG.
Figure 5. (a) Total DE with irradiation time for gratings in the films of the material based on BTC þ AAB (thickness and final SRG
modulation Δh are indicated). (b) dependence of inscription rate (slope of total DE curve) on film thickness for the material based on BTC þ
AAB (open circles) and material described earlier6b,6c (squares).
Holographic patterning—by irradiation with interference pat-
tern at 488 nm—was performed in orthogonal ((45°) polariza-
tion configuration as described in our previous papers.4,6 This
polarization configuration has been proved to be the most
effective for the SRG inscription. Grating formation was moni-
tored in situ by diffraction of a weak probe beam as it is shown in
Figure 4a for a 240 nm thick film. The inscribed relief grating was
then verified by AFM measurement; the surface topology is
shown in Figure 4b. A relief height of 350 nm was measured that
correlates with the observed DE. The achieved relief height is
noteworthy as it significantly exceeds the initial film thickness. To
our best knowledge it is the first demonstration of such a deep
corrugation of the film surface in the relatively thin films achieved
by light-induced mass transport in azobenzene-containing mate-
rials. This fact may be of high importance for the understanding
of the light-induced mass transport as it is opposite to the current
opinion in the literature that the mass transport can not be
performed efficiently in thin films. A certain limit of thickness
at the lower side is usually observed; afterward, the mass tran-
sport essentially slows down.12 Furthermore, we investigated
the dynamics of grating formation depending on film thickness
in the material based on AAB. Some representative kinetics are
shown in Figure 5a along with the film thicknesses and corre-
sponding relief heights. The data are summarized in Figure 5b,
where the inscription rate (slope of total DE curve) is shown with
the film thickness. A clear tendency is seen—typically for the
azobenzene-containing materials the gratings are formed essen-
tially faster in thicker films until saturation is reached. However,
it is noteworthy that in thinner films the surface corruga-
tions achieved are up to 1.5 times higher than the initial film
thicknesses. Even in films with thickness below 100 nm, a
modulation depth of 90 nm was achieved though at higher
intensity and longer irradiation time. This means that only few
monolayers of the material are left in the valleys of SRG after-
ward. A comparison of the inscription rates for the material under
study and another material6b—with the typical for azobenzene-
containing polymer dependency on the film thickness—is exhib-
ited in Figure 5b. While the inscription rate for the latter polymer
material becomes zero as the film thickness approaches 100 nm,
the inscription rate of 0.0027 min-1 was observed for the material
based on BTC þ AAB. We believe that high loading of azoben-
zene is responsible for the SRG enhancement in thin films. The
understanding of this fact requires further investigations invol-
ving a theoretical consideration in terms of laminar flow.
Furthermore, we investigated the material formulations with
other azobenzene derivatives. We estimated their ability to form
films (glass forming properties) and the efficiency of SRG
inscription. The latter was concluded from the kinetics of the
total DE (Figure 6). The films with 4-amino-40-(dimethylamino)-
azobenzene were even faster than the parent formulation (based
on AAB). The formulation with 4,40-diamino-derivative yielded
only crystalline films. The formulation with Fast Garnet GPC
(Figure 6) and pure compound based on AAB were significantly
slower than the parent formulation. We also succeeded in using
hydroxyl azobenzene derivative Disperse Red 13 with rather fast
inscription kinetics (Figure 6); similar by structure compound
was obtained recently by azo-coupling13 and appeared to be
quite effective for holography. A surprising difference between
(13) He, Y.; Gu, X.; Guo, M.; Wang, X. Opt. Mater. 2008, 31, 18.
2216 DOI: 10.1021/la9040562
Langmuir 2010, 26(4), 2214–2217

Goldenberg et al.
Letter
et al.7b For example, for the pure compound based on BTC þ
AAB a high temperature melting (310 °C) was observed during
first heating, while the second heating revealed the glass transition
at ca. 140 °C followed by polymorphism. The latter is characteri-
zed by two crystallization peaks and then by corresponding two
melting peaks indicating two phases (see Supporting In-
formation). Microscopic observation of the heated samples
confirmed a melting point at ca. 317 °C. The fast cooling led to
the amorphous and the slow cooling to the crystalline phase.
Correspondingly to the DSC behavior the second heating under
microscope observation of amorphous phase led to the crystal-
lization at ca. 208 °C. The detailed correlation of the phase
behavior with the light-induced processes may be an interesting
topic for further investigation.
In summary, SRG inscription in the films based on easily
prepared three arms azobenzene-containing low molecular weight
glass forming materials is distinguished by high efficiency. Un-
usually, the induced surface corrugation can essentially exceed
the initial film thickness and the inscription is still effective even
in the films with thickness below 100 nm, which has not yet
been reported for light-induced mass transport. This material
property opens a completely new possibility to investigate the
light-induced mass transport in the confinement conditions of a
very thin film that was never reached before. Second, it allows
application of new methods of investigations restricted to the thin
films, for example surface plasmon assisted nanolithography.14 A
further unique feature is a possibility to realize material transport
in the true nanoscale, i.e., in films with thickness in nanometer
range.
Figure 6. Total diffraction efficiency curves for ca. 300 nm thick
films of formulation with AAB, crude and pure, 4-amino-40-
(dimethylamino)azobenzene, Fast Garnet GPC, and Disperse
Red 13 as indicated.
crude and pure BTC þ AAB products could be explained by the
material phase difference. Generally the question of the influence of
the material purity on the SRG formation has been rather under-
evaluated in the literature. However, in the case of low molecular
weight materials with high azobenzene loading it may be of high
importance. Also the thermal stability of the induced structures will
be influenced by the morphological state of the film.
Thermal stability of SRG in the materials under study was
quite different. SRG written in the films of the material based on
BTC þ AAB were stable until 110-120 °C and then disappeared.
SRG written in the material based on BTC þ 4-amino-
40-(dimethylamino)azobenzene were very stable remaining half
of their DE upon heating at high temperature (260 °C). Such high
thermal stability of SRG is unusual and was found before only for
ionic4 and urea-bond linked materials.6b Correspondingly, the
DSC measurements revealed a complicated phase behavior of the
materials having different phase transitions during different
heating cycles. Similar behavior has been reported by Shirota
Supporting Information Available: Figures showing the
1
DSC curves and H and 13C NMR spectra for pure com-
pound based on BTC and AAB. This material is available
free of charge via the Internet at http://pubs.acs.org.
€
(14) Konig, T.; Goldenberg, L. M.; Kulikovska, O.; Kulikovsky, L.; Stumpe, J.;
Santer, S. Soft Matter, manuscript in preparation.
Langmuir 2010, 26(4), 2214–2217
DOI: 10.1021/la9040562 2217