Stable holographic gratings with small-molecular trisazobenzene derivatives

Article abstract of DOI:10.1021/ja9091038

We present a series of small-molecular trisazobenzene chromophores, including, for instance, 1,3,5-tris{[4-[4-[(4-cyanophenyl)azo]phenoxy]butyryl] amino}benzene that feature a remarkably stable light-induced orientation in initially amorphous thin-film ar

Full text of DOI:10.1021/ja9091038

Published on Web 12/17/2009
Stable Holographic Gratings with Small-Molecular
Trisazobenzene Derivatives
Klaus Kreger,† Pascal Wolfer,‡ Hubert Audorff,§,| Lothar Kador,§,|
Natalie Stingelin-Stutzmann,‡,⊥ Paul Smith,‡ and Hans-Werner Schmidt*,†,|
Macromolecular Chemistry I, Institute of Physics, and Bayreuther Institut fu¨r
Makromoleku¨lforschung (BIMF), UniVersity of Bayreuth, D-95440 Bayreuth, Germany,
Department of Materials, ETH Zu¨rich, CH-8093 Zu¨rich, Switzerland, and Department of
Materials, Imperial College London, London SW7 2AZ, United Kingdom
Received May 29, 2009; E-mail: hans-werner.schmidt@uni-bayreuth.de
Abstract: We present a series of small-molecular trisazobenzene chromophores, including, for instance,
1,3,5-tris{[4-[4-[(4-cyanophenyl)azo]phenoxy]butyryl]amino}benzene that feature a remarkably stable light-
induced orientation in initially amorphous thin-film architectures. It is demonstrated that for optimal
performance it is critical to design chemical structures that allow formation of both an amorphous and a
liquid-crystalline phase. In the present approach, the liquid-crystalline feature was introduced by inserting
decoupling spacers between a trisfunctionalized benzene core and the three azobenzene moieties, as
well as adding polar end groups to the latter. To compensate for the deleterious reduction of the glass
transition temperature associated with the spacers in the compounds, polar units were incorporated between
the benzene core and the side groups. Intriguingly, the molecular glasses that feature a latent
liquid-crystalline phase display a remarkable “postdevelopment”, i.e., an increase of the amplitude of
refractive index modulation in holographic experiments after writing of optical gratings is arrested, exceeding
20% for the previously mentioned derivative. Thus, these nonpolymeric, azobenzene-containing compounds
presented in this work appear to be attractive candidates for fabrication of stable holographic volume gratings.
Introduction
of this technique, optical gratings have been successfully
inscribed and highly useful optical characteristics and long-
Owing to the rich photochemistry of azobenzene chro-
mophores, a large variety of both small-molecular and
polymeric compounds have been synthesized for use as smart
light-responsive materials in various potential applications.1
One feature of azobenzene-containing species is their well-
known trans-to-cis isomerization.2,3 In addition, exposure to
linearly polarized light of thin films of these compounds leads
to in-plane orientation of chromophores by repeated trans
-cis-trans isomerization cycles.4-6 In holographic experi-
ments, photoinduced alignment in such thin-film architectures
obtained with two superimposed lasers was extensively
explored for use in high-density data storage.7-10 On the basis
term stability of light-induced orientation have been dem-
onstrated for selected macromolecular speciessespecially
those that feature liquid-crystalline (LC) phases.11,12 By
contrast, related small-molecular compounds have been less
explored. The latter have the advantage, among other things,
of relatively simple synthesis, being of higher purity, resulting
in well-defined structures and corresponding physicochemical
properties and providing the option of homogeneously
incorporating them into polymeric matrixes. Hence, in this
study we set out to focus on those azobenzene chromophores
of low molecular weight and in particular those that are prone
to form molecular glasses.
Molecular glasses represent an emerging class of materials
which are particularly investigated for potential application in
optoelectronic devices, photoconductor drums, and photo-
lithography.13-17 These compounds combine high glass transi-
tion temperatures (Tg) with pronouncedscritically importants
stable amorphous phases.18-20 According to Naito et al., these
features are met by small molecules exhibiting low entropic
contributions, resulting in a high Tg, and low enthalpic contribu-
† Macromolecular Chemistry I, University of Bayreuth.
‡ ETH Zu¨rich.
§ Institute of Physics, University of Bayreuth.
| BIMF, University of Bayreuth.
⊥ Imperial College London.
(1) Zhao, Y.; Ikeda, T. Smart Light-ResponsiVe Materials: Azobenzene-
Containing Polymers and Liquid Crystals; Wiley: Hoboken, NJ, 2009.
(2) Rau, H. Photoisomerization of Azobenzenes. In Photochemistry and
Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1990;
Vol. 2, p 119.
(3) Zimmerman, G.; Chow, L.-Y.; Paik, U.-J. J. Am. Chem. Soc. 1958,
80, 3528.
(9) Nikolova, L.; Todorov, T.; Ivanov, M.; Andruzzi, F.; Hvilsted, S.;
Ramanujam, P. S. Appl. Opt. 1996, 35, 3835.
(4) Dumont, M. Mol. Cryst. Liq. Cryst. 1996, 282, 437.
(5) Hagen, R.; Bieringer, T. AdV. Mater. 2001, 13, 1805.
(6) Yager, K. G.; Barrett, C. J. J. Photochem. Photobiol., A 2006, 182,
250.
(10) Sabi, Y.; Yamamoto, M.; Watanabe, H.; Bieringer, T.; Haarer, D.;
Hagen, R.; Kostromine, S. G.; Berneth, H. Jpn. J. Appl. Phys. 2001,
40, 1613.
(7) Eich, M.; Wendorff, J. H.; Reck, B.; Ringsdorf, H. Macromol. Chem.
Rapid Commun. 1987, 8, 59.
(11) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139.
(12) Hvilsted, S.; Andruzzi, F.; Kulinna, C.; Siesler, H. W.; Ramanujam,
P. S. Macromolecules 1995, 28, 2172.
(8) Berg, R. H.; Hvilsted, S.; Ramanujam, P. S. Nature 1996, 383, 505.
9
10.1021/ja9091038  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 509–516 509

A R T I C L E S
Kreger et al.
Table 1. Chemical Structures and Thermal Properties of the
Trisazobenzene Trisamides and Trisazobenzene Trisesters
Explored
tions, minimizing the tendency for crystallization of a vitrified
supercooled liquid.21,22 Furthermore, it was demonstrated that
hydrogen bonding between the molecular species may enhance
the stability of an amorphous phase in thin films of small
molecules.23
In recent years, photochromic molecular glasses have been
investigated by several research groups with particular interest
in the formation of surface relief gratings (SRGs).24-27 In those
studies, the light-induced mass transport over micrometer
distances driven by the photoisomerization of the above-
mentioned azobenzene moieties was demonstrated and charac-
terized for the purposes of optical information storage and
surface structuring. Recently, we investigated this feature for a
series of azobenzene-containing molecular glasses, and struc-
ture-property relationships between the height of the SRGs and
the optical susceptibility of the azobenzene moieties were
unveiled.28 Whereas those compounds were designed to ef-
ficiently form surface relief gratings with modulation heights
up to 600 nm, practical use of azobenzene-containing molecular
glasses for fabrication of Volume gratings has been limited, since
photoinduced molecular alignment in thin films typically was
lost over a period of days or less. Owing to the promising optical
performance, designing photochromic small molecules that can
be applied for long-term stable volume gratings remains a
challenge of fundamental interest.
On the basis of the design of molecular glasses featuring latent
mesogenic character, we present azobenzene derivatives that
are suitable for volume holographic data storage with remarkably
stable gratings.
In the approach discussed, compounds that comprise three
photoactive side groups that are linked, but decoupled with
spacers of different lengths, to a central core in a C3-symmetric
pattern are synthesized.29 Moreover, two trisazobenzene deriva-
tives were prepared as reference compounds in which the
chromophores are linked directly to the core. To compensate
the reduction of the glass transition temperatures due to the
decoupling spacers, polar moieties, i.e., amides or esters, were
incorporated between the side groups and the core. Especially,
the firstsexhibiting the potential to form hydrogen bondsswill
lead to higher glass transition temperatures as well as melting
points. In addition to spacers, introduction of terminal dipolar
substituents, e.g., methoxy or cyano groups, was employed as
a Tg
) glass transition temperature (determined with differential
scanning calorimetry (DSC) first-heating thermograms employing
samples that were previously quenched from the melt in liquid
nitrogen). b Key: G, glass; Cr, crystalline; LC, liquid-crystalline; Iso,
isotropic melt (transitions correspond to peak temperatures of DSC
second-heating/cooling thermograms).
another effective tool to enhance the mesogenic character of
the molecules.
On the basis of a systematic study of a range of the above-
described trisazobenzene derivatives, we demonstrate that it
is critical for optimal holographic performance to design
structures that feature a liquid-crystalline phase at elevated
temperatures.
(13) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(14) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck,
J.; Spreitzer, H.; Gratzel, M. Nature 1998, 395, 583.
(15) Dai, J.; Chang, S. W.; Hamad, A.; Yang, D.; Felix, N.; Ober, C. K.
Chem. Mater. 2006, 18, 3404.
Results and Discussion
Synthesis. An overview of the chemical structures of the
compounds synthesized in this work is given in Table 1.
Typically, three-arm star-shaped molecules were built up by
either an amidation or an esterification reaction, both represent-
ing largely uncomplicated reaction types and, hence, conve-
niently allowing introduction of different polar groups or
hydrogen-bonding moieties.
In detail, the molecules based on trisamide derivatives were
synthesized by a 3-fold amidation of acid chlorides containing
side groups together with 1,3,5-triaminobenzene. Molecules
based on trisester derivatives were obtained in a similar manner,
however utilizing 1,3,5-trihydroxybenzene (cf. Scheme 1). When
the molecules were based on 1,3,5-trimesic acid, i.e., when the
direction of the amide or ester linkage was reversed, the
photoactive side groups comprised amino groups or hydroxyl
functions, respectively.
(16) De Silva, A.; Felix, N. M.; Ober, C. K. AdV. Mater. 2008, 20, 3355.
(17) Van der Auweraer, M.; De Schryver, F. C.; Borsenberger, P. M.;
Bassler, H. AdV. Mater. 1994, 6, 199.
(18) Shirota, Y. J. Mater. Chem. 2000, 10, 1.
(19) Strohriegl, P.; Grazulevicius, J. V. AdV. Mater. 2002, 14, 1439.
(20) Shirota, Y. J. Mater. Chem. 2005, 15, 75.
(21) Naito, K.; Miura, A. J. Phys. Chem. 1993, 97, 6240.
(22) Naito, K. Chem. Mater. 1994, 6, 2343.
(23) Lebel, O.; Maris, T.; Perron, M.-E.; Demers, E.; Wuest, J. D. J. Am.
Chem. Soc. 2006, 128, 10372.
(24) Nakano, H.; Takahashi, T.; Kadota, T.; Shirota, Y. AdV. Mater. 2002,
14, 1157.
(25) Chun, C.; Kim, M.-J.; Vak, D.; Kim, D. Y. J. Mater. Chem. 2003,
13, 2904.
(26) Ishow, E.; Lebon, B.; He, Y.; Wang, X.; Bouteiller, L.; Galmiche, L.;
Nakatani, K. Chem. Mater. 2006, 18, 1261.
(27) Kim, M.-J.; Seo, E.-M.; Vak, D.; Kim, D.-Y. Chem. Mater. 2003, 15,
4021.
(28) Audorff, H.; Walker, R.; Kador, L.; Schmidt, H.-W. J. Phys. Chem.
B 2009, 113, 3379.
The design of the functional side groups was chosen in such
a way that they could either be directly linked to the core or
(29) Junge, D. M.; McGrath, D. V. J. Am. Chem. Soc. 1999, 121, 4912.
9
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Holographic Gratings with Trisazobenzene Derivatives
A R T I C L E S
Scheme 1. Overview of the Synthesis Routes to Trisazobenzene Trisamide and Trisazobenzene Trisester Derivatives 1-10
exhibit aliphatic spacers with three and five methylene units,
effectuating a decoupling of the chromophores from the central
core. In addition, different azobenzene chromophores were
synthesized, i.e., azobenzene, (cyanoazo)benzene, and (meth-
oxyazo)benzene derivatives. This set allowed the creation of
several star-shaped compounds in a construction-kit principle
where the required properties, e.g., the glass transition temper-
ature, film-forming behavior, and photoactive characteristics,
conveniently could be evaluated. In a typical procedure, the
photoactive chromophores were formed by a common azo-
coupling reaction. The aliphatic spacers with terminal ethyl
esters were introduced by etherification and subsequent saponi-
fication to the corresponding acids. Azobenzene compounds with
carbonic acids were, finally, converted to reactive acid chloride
species. In addition, side groups with amino or hydroxyl function
in the spacer moieties were utilized if the core was based on
trimesic acid derivatives. An amino function was obtained by
introducing a t-Boc-protected aminopropyl spacer with subse-
quent cleavage of the protecting group, whereas in the case of
hydroxyl-functionalized spacer moieties bromopropanol was
used as the reagent for the etherification (for details see the
Supporting Information). Mono- and bisubstituted products, as
well as residual photochromic side groups, were analyzed by
thin-layer chromatography (TLC) and oligomeric size-exclusion
chromatography (SEC). Subsequently, they were removed by
common techniques, e.g., flash chromatography and recrystal-
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Kreger et al.
Figure 1. Differential scanning calorimetry (second-cooling thermograms, recorded under a N2 atmosphere at a rate of 10 °C/min) and corresponding
variable-temperature polarizing optical microscopy of the trisazobenzene derivatives indicate a liquid-crystalline phase for compounds 4-10 (polarizer/
analyzer position illustrated with white arrows). Representative images were taken, respectively, above, between, and below the relevant transition temperatures.
lization. Especially, oligomeric SEC was found to be a powerful
tool to monitor the reaction progress. From the monosubstituted
to the trissubstituted species isolated peaks can be observed due
to the increase of their hydrodynamic volume. All compounds
LC phase prior to melting. In contrast to species 4-10,
compounds 1 and 3 displayed vitrification and the trisester
2 was found to crystallize upon cooling. The above findings
indicate that our design approach toward molecular glasses
with liquid-crystalline character was successful with respect
to thermal properties.
1
were fully characterized by H nuclear magnetic resonance
(NMR), 13C NMR, infrared (IR) spectroscopy, and elemental
analysis (EA).
Photochemical Behavior. Determination of the photochemi-
cal behavior of the different compounds in tetrahydrofuran
(THF) solutions, allowing us to study the trans-to-cis as well
as the cis-to-trans isomerization, was employed as a first tool
toward the optical characterization of thin films. Reassuringly,
we found a strong photoresponse in solutions of all species
in a series of ultraviolet/visible (UV/vis) light absorption
spectra. This observation has its origin in the well-known
trans-to-cis isomerization of the azobenzene moieties when
exposed to unpolarized, monochromatic UV light. The pho-
tochemical reaction continued until a photostationary cis-
rich state was reached. Subsequently, the corresponding back-
relaxation to the thermodynamically more stable trans state
was recorded when the samples were stored in the dark
(Figure 2). Whereas during exposure to UV light the π,π*
absorbance decreased drastically, the n,π* absorbance increased,
while the reverse process occurred when the solutions were kept
in the dark. Characteristically for reversible photochemical reac-
tions, distinct isosbestic points were present in the sequences of
UV/vis spectra.
Holography. For analyzing the suitability of the trisazoben-
zene derivatives for photoinduced volume gratings, their
response in the solid state was investigated in a set of
holographic experiments. To this end, thin, amorphous films
were produced by spin-coating THF solutions of the azobenzene
compounds. On the basis of this technique, films of good quality
and homogeneous optical properties were fabricated. To increase
the film thickness to a level required in holographic experiments,
the viscosity of the solutions was enhanced by adding ∼5 wt
% (with respect to the solute) ultra-high-molecular-weight
polystyrene (UHMW-PS), resulting in active layers of ∼400
nm average thickness. In contrast to the UV light used for
Thermal Properties. The thermal properties of the trisazoben-
zene derivatives were explored with differential scanning
calorimetry (DSC) and supported by variable-temperature
polarizing optical microscopy. The results of these studies are
presented in Table 1 and Figure 1. In general, higher glass
transition (Tg), melting (Tm), and crystallization (Tc) temperatures
were found for the amide homologues when compared to their
ester counterparts. This difference illustrates the influence of a
potential hydrogen bond formation in the former species. With
increasing spacer length, however, this feature is reduced and,
owing to strongly increased conformational arrangement
possibilities, the influence of the spacers becomes more
dominant, leading to a reduction in glass transition temper-
ature with increasing length, as observed for compound series
1, 4, and 9 and 2, 7, and 10. All trisazobenzene derivatives
discussed featured a Tg above ambient, and hence, quenching
to room temperature resulted in the formation of thermody-
namic nonequilibrium amorphous glasses that were found to
be stable at ambient conditions. All compounds that were
designed with decoupling spacers between the central core
and the side groups, i.e., 4-10, had liquid-crystalline
mesophases featured when they were cooled from the
isotropic phase at a relatively moderate rate of 10 °C/min
(cf. Table 1 and Figure 1). The values of the isotropic to
liquid-crystalline transition enthalpies were found to be
relatively small and ranged from -1.2 to -4.0 kJ/mol. In
combination with typical defects in the Schlieren textures
as observed in the polarizing microscope, this suggests the
presence of nematic phases for trisazobenzenes 4-10. When
the compounds were heated from the solidified state at the
same rate, on the other hand, only compound 8 featured an
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Holographic Gratings with Trisazobenzene Derivatives
A R T I C L E S
Table 2. Refractive Index Modulation and Writing Time of Species
1-10
1
2
3
4
5
6
7
8
9
10
a
n1 × 10-3
tb (s)
14
6
8
2
3
5
8
29 23
43 41 27
23
∼0.5
5
20 ∼350 10 12 ∼200
a Maximum amplitude of refractive index modulation at inscription of
holographic gratings. b Writing time to maximum of refractive index
modulation (argon ion laser, 488 nm, 1 W/cm2).
Figure 3. Refractive index modulation of ∼400 nm thin films of small-
molecular trisazobenzene compounds, prepared from THF solutions by spin-
coating onto glass substrates, as a function of time: left, amplitude of
refractive index modulation n1, normalized to 1 when the writing laser was
stopped, of 1, 3, 5, 8, and 9; right, absolute values for 3 and 5 demonstrating
the postdevelopment of the latter species on a logarithmic time scale (writing
laser off indicated by arrows).
Figure 2. cis-to-trans isomerization of compounds 3 (top) and 5 (bottom)
in THF solutions monitored with UV/vis spectroscopy: left, spectra taken
after irradiation with UV light (365 nm) for different exposure times; right,
thermal back-relaxation in the dark indicated in hours (solid lines), as well
as spectra before and after 120 s of irradiation with UV light (dashed lines).
The reversible character of the isomerization is emphasized by isosbestic
points at 304 and 407 nm for derivative 3 and at 317 and 425 nm for
compound 5.
intriguingly, highly stable photo-orientation was observed for
4-10. These compounds, which feature spacers and additional
terminal groups, also displayed a remarkable “postdevelop-
ment”si.e., an increase of n1 after writing of the holographic
gratings was arrestedsexceeding, for instance, 20% for deriva-
tive 5. As a consequence of overexposure and the resulting
decrease of contrast between irradiated and unexposed areas,
the refractive index modulation features a maximum during
inscription. Thus, this amplification of n1 is a very useful tool
for fabricating holographic intensity gratings with better contrast
than achievable with optical writing methods only. To our
knowledge, this highly beneficial phenomenon so far only has
been reported for macromolecular systems.31,32
The present library of trisazobenzene compounds permitted
us to conduct a cursory examination of the influence of certain
key elements of the molecular architectures of these materials
on their performance. Comparison of the characteristics of 1
and 2 with those of species 3-10, for instance, demonstrates
that the introduction of a spacer promotes, in most cases, the
long-term stability of the photoinduced molecular alignment (cf.
Table 2 and Figure 3).
irradiating the solutions in the UV/vis examinations discussed
above, here an argon ion laser at 488 nm was employed.
Exposure to this source led to repeated trans-cis-trans
isomerization cycles of the chromophores in the film. Hence,
in-plane orientation of the azobenzene moieties perpendicular
to the polarization of the interfering laser beams was induced
in irradiated areas.
All compounds showed photo-orientationshere expressed as
the amplitude of the refractive index modulation (n1) between
oriented and nonoriented domains of the optical intensity
gratings inscribed in holographic experiments. That optical
characteristic was analyzed by the diffraction efficiency of the
grating employing a laser diode of 685 nm wavelength, which
is outside the absorption band of the azobenzene moieties, thus
avoiding affecting the inscribed pattern. Details describing the
experimental setup for the generation of holographic gratings
have been published elsewhere.30
An exception is compound 3, which does not possess a polar
end group and shows a rapid decay of the molecular orientation.
This is in contrast to the behavior of all other spacer-decoupled
Typical for molecular glasses, n1 was found to decrease for
1-3 after the writing beams were shut off. However, most
(31) Stumpe, J.; La¨sker, L.; Fischer, T.; Rutloh, M.; Kostromin, S.;
Ruhmann, R. Thin Solid Films 1996, 284-285, 252.
(32) Zilker, S. J.; Huber, M. R.; Bieringer, T.; Haarer, D. Appl. Phys. B:
Lasers Opt. 1999, 68, 893.
(30) Ha¨ckel, M.; Kador, L.; Kropp, D.; Schmidt, H.-W. AdV. Mater. 2007,
19, 227.
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finally distilled again. NMP was stirred for 3 days over CaH2
and finally distilled off. Triethylamine was treated in the same
manner. Phloroglucine, 1,3,5-benzenetricarbonyl trichloride, and
(phenylazo)phenol, were commercially available.
azobenzene derivatives, indicating that these substituents have
a positive effect on the stability of photoaligned patterns.
Increasing spacer length, inversion of the amide or ester
sequence in the side groups, and substitution of an amide by
an ester or vice versa, did, however, not affect the presence of
the stability but did have an effect on the absolute values of
the refractive index modulation n1, writing time t to a maximum
of refractive index modulation, and degree of postdevelopment.
Accordingly, the formation of hydrogen bonds in the amide
homologues cannot be the origin of the observed stability found
for selected species.
Whereas n1 typically was found to be higher for compounds
featuring a liquid-crystalline phase at elevated temperatures,
a clear correlation among n1, t, and the detailed molecular
structure appears to be absent (cf. Table 2). Hence, the above
observations suggest that two components, i.e., spacers and
end groups, are crucial for stable photoinduced alignment
of the trisazobenzene derivatives explored. Interestingly, we
find that species featuring these specific architectural elements
form molecular glasses with a latent liquid-crystalline phase
(cf. Table 1 and Figure 3).
Characterization and Sample Preparation. Characterization
1
Methods. H NMR and 13C NMR spectra were recorded with a
Bruker AC 300 spectrometer (300 and 75 MHz, respectively).
Fourier transform infrared (FTIR) spectra were recorded with a
BIO-RAD Digilab FTS-40.
Oligomeric size exclusion chromatography (Oligo-SEC) mea-
surements were performed utilizing a Waters 515-HPLC pump with
stabilized THF as the eluent at a flow rate of 0.5 mL/min. A 20 µL
volume of a solution with a concentration of approximately 1 mg/
mL was injected into a column setup, which consists of a guard
column (Varian, 5 × 0.8 cm, mesopore gel, particle size 3 µm)
and two separation columns (Varian, 30 × 0.8 cm, mesopore gel,
particle size 3 µm). The compounds were monitored with a Waters
486 tunable UV detector at 254 nm and a Waters 410 differential
RI detector.
EA was performed with a HEKAtech elemental analyzer, EA
3000 (Euro Vector CHNS), charged with tungsten oxide and copper.
Detection was performed utilizing a GC setup equipped with a
thermal conductivity detector.
DSC was conducted under a N2 atmosphere at a scan rate of 10
°C/min with a Mettler Toledo DSC 822e instrument. For the
determination of the glass transition temperatures Tg of the
trisazobenzene derivatives explored, amounts of about 3 mg ((0.2
mg) each compound were sealed in Al standard 40 µL crucibles,
heated to 260 °C (except 1, 310 °C), and subsequently quenched
from the melt in liquid nitrogen. For all other phase transition
temperatures, similar amounts were analyzed employing identical
sample holders.
Glass transition temperatures Tg of all compounds were
determined from DSC first heating thermograms of the melt-
quenched samples. All other phase transitions were taken to be
the peak temperatures in the DSC second-heating and -cooling
thermograms.
Thin-Film Preparation and Optical Analysis. Thin films of
all compounds were spin-coated at ambient with a Laurell WS-
400B-6NPP/LITE spin-coater at 800 rpm from ∼2 wt % THF
solutions (total content) onto glass slides. To increase the film
thickness to a level required in holographic experiments, ∼5 wt
% (with respect to the solute) UHMW-PS was added to the
solutions.
UV/vis absorption spectra of about 0.01 mg/mL THF solutions
of all compounds were recorded after different exposure times to
UV light (365 nm) with a Perkin-Elmer Lambda 900 spectropho-
tometer. Subsequently, the thermal back-relaxation in the dark was
analyzed in a second series of measurements.
Polarizing optical microscopy was carried out with a Leica
DMRX polarizing microscope equipped with a Mettler Toledo
FP82HT hot stage. The thermal behavior of the compounds was
examined applying a continuous N2 flow and a scan rate of 10
°C/min.
Conclusions
In summary, azobenzene-containing low-molecular-weight
compounds have been presented; those that feature liquid-
crystalline behavior appear to be attractive nonpolymeric
candidates for fabrication of stable holographic volume
gratings. Different structural elements in the design of these
star-shaped molecules allowed us to tailor the phase behavior
of the compounds explored. Liquid-crystalline character was
induced by integrating decoupling spacers between the
benzene core and the three azobenzene moieties, as well
as additional polar end groups. The glassy nature of the
trisazobenzene derivatives was controlled with polar linkages,
i.e., amide or ester bonds, between the core and the spacers.
In solutions, reversible trans-to-cis isomerization was ob-
served for all compounds. In a set of holographic experiments
with amorphous films, however, stable intensity gratings only
were obtained with species exhibiting decoupling spacers and
additional end groups, i.e., liquid-crystalline molecular glasses
4-10. For the same compounds an unusual postdevelopment
of the refractive index modulation was demonstrated. By
contrast, typical for molecular glasses, no stable volume
gratings were obtained with reference compounds 1-3,
featuring either decoupling spacers or polar end groups only.
These results indicate that it is critical for fabricating stable
holographic gratings to design structures that feature liquid-
crystalline behavior at elevated temperatures but still allow
formation of stable amorphous films at ambient. Nevertheless,
several characteristics observed in our investigations of the
trisazobenzene derivatives, as well as the alignment mech-
anism on a molecular level, remain unclear and require further
detailed studies.
Holographic experiments were performed using the blue-green
line (488 nm) of an argon ion laser. A beam splitter generated two
coherent parts of the laser beam, which were brought to interference
in the sample plane. The diameter of the s-polarized laser beams
was about 1 mm; the intensity of each beam was adjusted to 0.5
W/cm2. The angle of incidence relative to the surface normal was
(14°, resulting in a grating period of about 1 µm. A laser diode at
685 nm, a wavelength range outside the absorption band of the
azobenzene moiety, was used for in situ monitoring of the
diffraction efficiency without affecting the writing process. To
determine the diffraction efficiency of the grating, the powers of
the transmitted and diffracted beams were measured with two
photodiodes. For improving the signal-to-noise ratio, the laser diode
was modulated and lock-in detection was used.
Experimental Section
Materials. The synthesis of 1,3,5-triaminobenzene is described
elsewhere.33 All other chemicals were commercially available from
Aldrich and Acros and used without further purification. UHMW-
PS (Mw ≈ 30 000 kg/mol) was purchased from Polysciences, Inc.,
Warrington, PA. THF was refluxed for 3 days over CaH2,
distilled, refluxed for another three days over potassium, and
(33) Schmidt, H.-W.; Smith, P.; Blomenhofer, M. PCT Int. Appl. WO 02/
46300A2, 2002.
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514 J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010

Holographic Gratings with Trisazobenzene Derivatives
A R T I C L E S
Synthesis and Characterization of Trisazobenzene Deriva-
tives. General Preparation Method 1. In a flame-dried flask 1.4
mmol of 1,3,5-triaminobenzene, 0.1 g of LiCl, and 1 mL of
triethylamine were dissolved in 20 mL of dry NMP. Then the
solution was cooled to 0 °C, and 4 equiv (5.5 mmol) of azobenzene-
based acid chloride was added. The reaction mixture was allowed
to warm to room temperature and subsequently stirred for 12 h at
60 °C. The solution was cooled to room temperature and poured
into ice-water. The solid was filtered off and washed several times
with water.
The synthesis of 1,3,5-tris{[6-[4-[(4-methoxyphenyl)azo]phenoxy]-
hexanoyl]amino}benzene (9) was carried out according to general
preparation method 2, but performed at room temperature. Recrystal-
1
lization from THF/EtOH yielded a yellow powder (87%). H NMR
(DMSO-d6): δ ) 1.45 (m, 6H), 1.62 (m, 6 H); 1.74 (m, 6 H), 2.30
(t, 6 H), 3.82 (s, 9 H), 4.04 (t, 6 H), 7.06 (m, 12 H), 7.64 (s, 3H),
7.80 (m, 12 H), 9.86 (s, 3H) ppm. 13C NMR (DMSO-d6): δ ) 25.0,
25.1, 28.3, 36.3, 55.5, 67.8, 114.5, 114.9, 124.0, 124.1 (2C), 139.5,
146.0, 146.1, 160.9, 161.4, 171.2 ppm. IR (KBr): 3282, 3050, 2942,
1667, 1599, 1501, 1441, 1318, 1248, 1148, 1105, 1028, 841 cm-1
.
El. vol. (Oligo-GPC): 15.54 mL. Elemental analysis (N, C, H):
calcd, 11.50, 69.02, 6.34; found, 11.00, 68.59, 6.49.
Trisamides based on trimesic acid were prepared in a similar
manner.
1,3,5-Benzenetrisamide Based on 1,3,5-Trimesic Acid. The
synthesis of benzene-1,3,5-tricarboxylic acid tris{4-[4-[(4-methoxy-
phenyl)azo]phenoxy]butyl}amide (6) was carried out according to
general preparation method 1. After flash chromatography in THF/
General Preparation Method 2. In a flame-dried flask 1.4 mmol
of phloroglucine and 1 mL of triethylamine were dissolved in 30
mL of dry THF. Then the solution was cooled to 0 °C, and 4 equiv
(5.5 mmol) of 4-azobenzene-based acid chloride was added. The
reaction mixture was allowed to warm to room temperature and
refluxed for 12 h at 60 °C. The formed suspension was cooled to
room temperature, and the triethylammonim salt was filtered off
and washed several times with THF. Subsequently, the solvent was
evaporated off.
1
hexane (3/1) a yellow solid was obtained (yield 13%). H NMR
(DMSO-d6): δ ) 2.03 (quintet, 6 H), 3.48 (m, 6 H), 3.83 (s, 9 H),
4.15 (t, 6 H), 7.09 (m, 12 H), 7.81 (m, 12H), 8.43 (s, 3 H), 8.84
(m, 3H) ppm. 13C NMR (DMSO-d6): δ ) 28.7, 36.5, 55.6, 66.7,
114.6, 115.0, 124.2 (2C), 128.6, 135.1, 146.1, 146.2, 160.9, 161.5,
165.7 ppm. IR (KBr): 3428, 3071, 2932, 2837, 1651, 1599, 1501,
1468, 1246, 1147, 1030, 841 cm-1. El. vol. (Oligo-GPC): 15.81
mL. Elemental analysis (N, C, H): calcd, 12.45, 67.64, 5.68; found,
11.48, 67.26, 5.99.
1,3,5-Benzene Trisester Based on 1,3,5-Trihydroxybenzene. The
synthesis of 1,3,5-tris{[4-[(4-methoxyphenyl)azo]benzoyl]oxy}benzene
(2) was carried out according to general preparation method 2. The
crude product was recrystallized from THF/EtOH, yielding an
orange solid (50%). 1H NMR (CDCl3): δ ) 3.90 (s, 9 H), 7.03 (d,
6 H), 7.24 (s, 3H), 7.97 (dd, 12 H), 8.32 (d, 6 H) ppm. 13C NMR
(CDCl3): δ ) 56.0, 114.7, 123.0, 125.7, 130.2, 131.7, 147.4, 151.9,
156.3, 163.2, 164.4, 166.5 ppm. IR (KBr): 3027, 2844, 1741, 1600,
1501, 1453, 1252, 1130, 1064, 835 cm-1. El. vol. (Oligo-GPC):
16.14 mL. Elemental analysis (N, C, H): calcd, 9.99, 68.57, 4.32;
found, 9.83, 68.79, 4.40.
The synthesis of 1,3,5-tris{[4-[4-[(4-methoxyphenyl)azo]phenoxy]-
butyryl]oxy}benzene (7) was carried out analogously to general
preparation method 2, but performed at room temperature. After
flash chromatography in CH2Cl2/THF (40/1) a yellow solid (yield
40%) was obtained. 1H NMR (CDCl3): δ ) 2.26 (quintet, 6H), 2.79
(t, 6 H), 3.89 (s, 9 H), 4.14 (t, 6 H), 6.86 (s, 3 H), 6.99 (m, 12H),
7.89 (m, 12 H) ppm. 13C NMR (CDCl3): δ ) 24.4, 30.9, 55.5, 66.7,
112.8, 114.1, 114.7, 124.3 (2C), 147.0, 147.1, 151.0, 160.7, 161.6,
170.8 ppm. IR (KBr): 3071, 2942, 2837, 1761, 1601, 1500, 1454,
1369, 1317, 1248, 1147, 1030, 943, 843 cm-1. El. vol. (Oligo-GPC):
15.78 mL. Elemental analysis (N, C, H): calcd, 8.28, 67.45, 5.36;
found, 8.22, 67.37, 5.41.
Trisesters based on trimesic acid were prepared in a similar
manner.
1,3,5-Benzenetrisamide Based on 1,3,5-Triaminobenzene. The
synthesis of 1,3,5-tris{[4-[(4-methoxyphenyl)azo]benzoyl]amino}-
benzene (1) was carried out according to general method 1. The
crude product was recrystallized from THF/EtOH, yielding an
orange solid (39%). 1H NMR (DMSO-d6): δ ) 3.88 (s, 9 H), 7.16
(d, 6 H), 7.96 (dd, 12 H), 8.13 (s, 3H), 8.20 (d, 6 H), 10.5 (s, 3H)
ppm. 13C NMR (DMSO-d6): δ ) 56.6, 109.9, 115.6, 122.9, 125.8,
130.0, 137.2, 140.1, 147.1, 154.5, 163.2, 165.7 ppm. IR (KBr):
3308, 2838, 1654, 1600, 1540, 1503, 1418, 1258, 1139, 1052, 855,
836 cm-1. El. vol. (Oligo-GPC): 16.05 mL. Elemental analysis (N,
C, H): calcd, 15.04, 68.81, 4.69; found, 14.44, 67.82, 4.76.
The synthesis of 1,3,5-tris{[4-[4-(phenylazo)phenoxy]butyryl]ami-
no}benzene (3) was carried out according to general preparation
method 1. After flash chromatography in ethyl acetate/cyclohexane
1
(2/3) a yellow solid (yield 24%) was obtained. H NMR (DMSO-
d6): δ ) 2.04 (quintet, 6 H), ∼2.5 (6H), 4.10 (t, 6 H), 7.10 (d, 6 H),
7.5 (m, 9H), 7.66 (s, 3H), 7.83 (m, 12 H), 9.97 (s, 3H) ppm. 13C
NMR (DMSO-d6): δ ) 24.6, 32.7, 67.4, 115.0, 122.2 (2C), 124.6,
129.3, 130.8, 139.5, 146.1, 152.0, 161.4, 170.6 ppm. IR (KBr):
3312, 3219, 3040, 2942, 2868, 1651, 1603, 1555, 1500, 1452, 1325,
1252, 1142, 1053, 839 cm-1. El. vol. (Oligo-GPC): 15.90 mL.
Elemental analysis (N, C, H): calcd, 13.67, 70.34, 5.58; found,
13.19, 70.56, 5.69.
The synthesis of 1,3,5-tris{[4-[4-[(4-methoxyphenyl)azo]phenoxy]-
butyryl]amino}benzene (4) was carried out according to general
preparation method 1. After recrystallization from THF/MeOH a
The synthesis of 1,3,5-tris{[6-[4-[(4-methoxyphenyl)azo]phenoxy]-
hexanoyl]oxy}benzene (10) was carried out according to general
preparation method 2. Recrystallization from THF/EtOH yielded a
1
yellow solid (yield 32%) was obtained. H NMR (DMSO-d6): δ )
1
2.05 (quintet, 6H), ∼2.5 (6 H), 3.83 (s, 9H), 4.11 (t, 6 H), 7.09 (d,
12 H), 7.67 (s, 3H), 7.82 (m, 12 H), 9.97 (s, 3H) ppm. 13C NMR
(DMSO-d6): δ ) 25.0, 33.0, 55.9, 67.8, 114.8, 115.3, 124.4 (2C),
139.8, 146.4, 146.5, 158.5, 161.1, 161.8, 171.0 ppm. IR (KBr):
3289, 3071, 1667, 1599, 1501, 1455, 1318, 1250, 1148, 1028, 941,
841 cm-1. El. vol. (Oligo-GPC): 15.73 mL. Elemental analysis (N,
C, H): calcd, 12.45, 67.64, 5.68; found, 12.09, 67.13, 5.76.
yellow powder (86%). H NMR (CDCl3): δ ) 1.62 (m, 6 H), 1.84
(m, 12 H), 2.58 (t, 6 H), 3.87 (s, 9 H), 4.04 (t, 6 H), 6.84 (s, 3H),
6.98 (dd, 12 H), 7.87 (dd, 12 H) ppm. 13C NMR (CDCl3): δ ) 24.9,
26.0, 29.2, 34.6, 55.9, 68.2, 113.1, 114.5, 115.0, 124.7, 124.7, 147.3,
147.4, 151.5, 161.4, 161.9, 171.6 ppm. IR (KBr): 3050, 2942, 2870,
1765, 1599, 1501, 1462, 1422, 1318, 1250, 1127, 1024, 918, 843
cm-1. El. vol. (Oligo-GPC): 15.55 mL. Elemental analysis (N, C,
H): calcd, 7.65, 68.84, 6.05; found: 7.61, 69.33, 6.23.
The synthesis of 1,3,5-tris{[4-[4-[(4-cyanophenyl)azo]phenoxy]-
butyryl]amino}benzene (5) was carried out according to general
preparation method 1. After flash chromatography in THF/hexane
1,3,5-Benzene Trisester Based on 1,3,5-Trimesic Acid. The
synthesis of benzene-1,3,5-tricarboxylic acid tris{4-[4-[(4-methoxy-
phenyl)azo]phenoxy]butyl} trisester (8) was carried out according
to general method 2. After flash chromatography in toluene/THF a
1
(3/1) a yellow solid (yield 23%) was obtained. H NMR (DMF-d7):
δ ) 2.19 (quintet, 6H), 2.65 (t, 6 H), 4.26 (t, 6 H), 7.23 (d, 6 H),
7.87 (s, 3H), 7.9-8.1 (m, 24 H), 10.06 (s, 3H) ppm. 13C NMR
(DMF-d7): δ ) 25.2, 33.1, 68.3, 113.4, 115.6, 118.9, 123.4 (2C),
125.8, 134.2, 140.5, 147.0, 155.0, 163.2, 171.6 ppm. IR (KBr):
3341, 3090, 2844, 2226, 1698, 1599, 1501, 1418, 1254, 1138, 1043,
847 cm-1. El. vol. (Oligo-GPC): 15.48 mL. Elemental analysis (N,
C, H): calcd, 16.86, 68.66, 4.85; found, 16.23, 68.53, 4.96.
1
yellow solid was obtained (yield 63%). H NMR (CDCl3): δ )
2.32 (quintet, 6 H), 3.88 (s, 9 H), 4.20 (t, 6 H), 4.62 (t, 6 H), 7.00
(m, 12 H), 7.88 (m, 12 H), 8.87 (s, 3H) ppm. 13C NMR (CDCl3):
δ ) 28.6, 29.7, 55.1, 62.9, 64.6, 114.1, 114.6, 124.3 (2C), 131.3,
134.6, 147.0, 147.1, 160.6, 161.6, 164.8 ppm. IR (KBr): 3073, 2965,
2880, 1730, 1599, 1500, 1421, 1319, 1236, 1147, 1028, 985, 839
9
J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010 515

A R T I C L E S
Kreger et al.
cm-1. El. vol. (Oligo-GPC): 15.91 mL. Elemental analysis (N, C,
H): calcd, 8.28, 67.45, 5.36; found, 8.01, 67.08, 5.47.
D. Hanft, and C. Lo¨ffler for their invaluable assistance in material
synthesis.
Acknowledgment. P.W. acknowledges the Materials Research
Center (MRC) at ETH Zu¨rich for initiating the project with seed
funding. N.S.-S. thanks the Royal Society for a Seed Fund Research
Grant. Financial support for this work at the University of Bayreuth
by the German Science Foundation (Grant SFB 481, B2) is
gratefully acknowledged. We are also indebted to S. Ganzleben,
Supporting Information Available: Synthesis of side groups
and additional figures. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA9091038
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516 J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010