Synthesis and properties of liquid-crystalline cyclic siloxanes containing azo dye groups

Article abstract of DOI:10.1080/15421400600700331

The effects of incorporating an azo dye moiety as a nonlinear optically active chromophore with benzoyloxyphenyl-substituted mesogens to a cyclic siloxane backbone are reported. The liquid-crystalline (LC) cyclic oligosiloxanes were characterized by

Full text of DOI:10.1080/15421400600700331

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Synthesis and Properties
of Liquid-Crystalline Cyclic
Siloxanes Containing Azo Dye
Groups
Joanna T. Sołtysiak a & Ewa Białecka-Florjańczyk b
a Industrial Chemistry Research Institute , Warsaw,
Poland
b Warsaw Agriculture University, Institute of
Chemistry , Warsaw, Poland
Published online: 20 Dec 2006.
To cite this article: Joanna T. Sołtysiak & Ewa Białecka-Florjańczyk (2006)
Synthesis and Properties of Liquid-Crystalline Cyclic Siloxanes Containing
Azo Dye Groups, Molecular Crystals and Liquid Crystals, 460:1, 23-33, DOI:
10.1080/15421400600700331
To link to this article: http://dx.doi.org/10.1080/15421400600700331
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Mol. Cryst. Liq. Cryst., Vol. 460, pp. 23–33, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600700331
Synthesis and Properties of Liquid-Crystalline Cyclic
Siloxanes Containing Azo Dye Groups
Joanna T. Sołtysiak
Industrial Chemistry Research Institute, Warsaw, Poland
´
Ewa Białecka-Florjanczyk
Warsaw Agriculture University, Institute of Chemistry,
Warsaw, Poland
The effects of incorporating an azo dye moiety as a nonlinear optically active
chromophore with benzoyloxyphenyl-substituted mesogens to a cyclic siloxane
backbone are reported. The liquid-crystalline (LC) cyclic oligosiloxanes were char-
acterized by 1H NMR, FT-IR, and UV-Vis spectra. The mesogenic properties of the
oligomers were studied by differential thermal analysis, polarizing microscopy,
and X-ray diffraction. The results show that all the cyclic oligosiloxanes exhibit
the smectic phase except for the cyclic oligosiloxane with azo dye–only substituents,
for which the LC state was not evident. The LC cyclic oligosiloxanes, when
irradiated at 365 nm in chloroform solution, undergo a trans=cis isomerization
of the azo dye moiety.
Keywords: azo dye; cyclic siloxanes; liquid crystals
1. INTRODUCTION
During the past decade, scientific research in the polysiloxane area
has focused on the synthesis of new oligosiloxanes with special proper-
ties (i.e., liquid-crystalline behavior). Special attention was attributed
to the oligosiloxanes containing dichroic dye moieties because of their
potential application, particularly in reversible, optical, high-density
data recording and data storage [1,2]. Cyclic siloxane oligomers
are materials of significant technological interest for their potential
applications in the fields of nonlinear optics. The siloxane backbone
provides a high degree of flexibility to the oligomer and results in a
liquid-crystalline (LC) state at a relatively low temperature.
Address correspondence to Joanna T. Sołtysiak, Industrial Chemistry Research Insti-
tute, Rydygiera 8, Warsaw 01-793, Poland. E-mail: joanna.soltysiak@ichp.pl
23

24
J. T. Sołtysiak
SCHEME 1 General formula of LC cyclic oligosiloxanes COSi1–COSi6.
Side-chain LC cyclic oligosiloxanes containing photochromic side
groups were first described by Ortler et al. [3]. In the past few
years, a number of new LC siloxanes with photochromic azobenzene-,
spiropyrane-, and other azo-moieties in side chains were synthesized
and used for reversible optical storage [4–6].
In this article, a series of novel photochromic side-chain LC cyclic
tetramethyltetrasiloxanes containing an azo group (as shown in
Scheme 1) were synthesized and characterized. The influences of a
lateral attachment of the photochromic azo-moieties on the packing
behavior of rod-like mesogens around the cyclic siloxane ring and
changes in the thermal properties with varying compositions of the
mesogens are discussed. Photochemical behavior of cyclic siloxanes
in solution was also investigated.
2. EXPERIMENTAL
Materials and reagents were of commercial-grade quality and used
without further purification unless otherwise noted. Toluene used in
the hydrosilylation reaction was first refluxed over sodium and then
distilled. All other solvents were purified by standard methods. The
hydrosilylation catalyst [platinum tetramethyldivinyl disiloxane com-
plex in xylene (11% wt)] was obtained from GE Silicon.
2.1. Synthesis
The alkene mesogenic side chain precursors: 40-methoxyphenyl 4-
(undec-10-enyloxy)benzoate, 40-decyloxyphenyl 4-(undec-10-enyloxy)-
benzoate mesogenic groups, and 4-(undec-10-enyloxy)benzoic acid
were prepared by a conventional method described in earlier papers
[7,8]. Esterification of the 4-(undec-10-enyloxy)benzoic acid with

Synthesis and Properties of Liquid-Crystalline
25
SCHEME 2 Synthetic route of LC cyclic oligosiloxanes.
4[4-(phenylazo)-1-naphthylazo]phenol] (Disperse Orange 13 from
Aldrich) was performed with N,N0-dicyclohexylcarbodiimide using a
previously reported method [9]. The structures of all the mesogens
1
were confirmed by means of FT-IR and H NMR. Attachment to the
cyclic tetramethyltetrasiloxanes, shown in Scheme 2, was performed
using standard hydrosilylation chemistry with a platinum catalyst.
Full experimental details of the hydrosilylation reaction, used to pre-
pare the cyclic siloxane oligomers, are given in Ref. [9]. Hydrosilyla-
tion reactions were carried out in toluene at 50–60ꢀC under argon.
The FT-IR spectra of cyclic oligosiloxanes revealed the complete disap-
pearance of the Si-H stretching band at 2163 cmꢁ1. 1H NMR also
confirmed the structure of the cyclic oligosiloxanes. The absence of
remaining Si-H functions and of mesogens was confirmed by the
1
absence of the corresponding signals in the H NMR spectra (Si-H:
4.7 ppm, CH2¼CH-: 4.8–5.0 ppm and 5.7–6.0 ppm). Upon completion,
the reaction solution was filtered into methanol to precipitate the pro-
duct. This was done repeatedly until thin-layer chromatography (TLC)
showed that no residual alkene remained in the product.
2.2. Characterization Techniques
1H NMR was collected on a Varian Inova (500 MHz) spectrometer,
using deuterated chloroform as a solvent and tetramethylsilane as
the internal standard. FT-IR spectra of the synthesized mesogens
and oligomers in the solid state by the KBr method or, if liquid,
by casting films on NaCl discs, were obtained with Perkin-Elmer
Spectrum 1000 spectrometer. Elemental analysis performed using a
Euro Vector 3018 instruments for analysis of carbon, hydrogen, and
nitrogen contents.
Differential scanning calorimetry (DSC) thermograms were
obtained using a Perkin Elmer DSC-7, with a TAC 7=PC interface

26
J. T. Sołtysiak
and a controlled cooling accessory. The transition temperatures were
taken from DSC traces of samples annealed by cooling from the
isotropic melt corresponding to the maxima and the onset points of
the enthalpy peaks for oligomers and low-molecular-weight samples,
respectively, at a heating rate of 10ꢀC=min.
Texture analysis was carried out by polarized optical microscopy
(POM) on a Biolar polarization microscope equipped with a Mettler
FP82HT heating stage and a Mettler FP-90 processor. Samples for
POM analysis were prepared by dissolving about 1-5 mg of oligomer
solution in volumes of 0.3 ml of CHCl3 followed by casting two to three
droplets of solution on a microscope slide cover. The films were not
homogeneous in thickness; oligomer films with thicknesses between
1 and 5 mm exhibited the most recognizable textures.
Solutions of the azo monomer and oligomers in chloroform were
irradiated with a UV lamp (30 W) at 365 nm (20ꢀC), and corresponding
UV measurements were performed with a Philips PU 8740 UV-Vis
spectrometer.
X-ray diffraction measurements have been performed using a
Philips X’Pert diffractometer fitted with a Guinier symmetrical focus-
ing transmission photographic camera. In the case of the diffract-
ometer, copper radiation filtered by a nickel absorption filter was
applied, where as the Guinier camera worked with cobalt radiation.
Diffractometer smectic-phase measurements were carried out using
the reflecting method from flat samples aligned by glass substrate.
The chosen temperature was maintained within ꢂ0.1ꢀC.
3. RESULTS AND DISCUSSION
3.1. Addition of Azo Dye Moiety to Cyclic Siloxane
The spectroscopic analysis confirmed the predicted molecular struc-
tures of the mesogenic units R1–R3 and LC cyclic oligosiloxanes
COSi1–COSi6. Theoretical comonomers ratios and the final oligomer
compositions determined by 1H NMR for cyclic oligosiloxanes
COSi1–COSi4 are shown in Table 1. The aromatic protons from phe-
nyl benzoate and naphthalene moieties at 7.46–8.09 ppm and the
methoxy (O-CH3) resonance at 3.8 ppm from mesogen R2 were used
to calculate compositions using integral areas. The results show a good
agreement between the initial monomer ratio and oligomer composi-
tions, indicating reactivity of the azo-monomer is similar to that of
the mesogen R2. In case of oligomers COSi5–COSi6, the structure of
the oligomers was confirmed by elemental analysis, because 1H
NMR analysis in this case is not sensitive enough. Because only the

Synthesis and Properties of Liquid-Crystalline
27
TABLE 1 Compositions of Hydrosilylation Products as Determined by
1
Elemental Analysis and H NMR of the Cyclic Oligosiloxanes
Elemental analysis (%)
AZO-1=R2 ratioa
AZO-1=R3 ratiob
Oligomer
C
H
N
Theory Determined Theory Determined
COSi1
COSi2
COSi3
COSi4
COSi5
COSi6
1.0
1.0
0.75
0.50
0.25
0.78
0.54
0.23
79.97
79.33
9.26
8.31
2.50
4.70
0.25
0.50
0.23
0.55
aComposition of hydrosilation products as determined by 1H NMR of the cyclic
oligosiloxanes.
bComposition of hydrosilation products as determined by elemental analysis.
azo-monomer contains a nitrogen element, the cyclic oligosiloxanes
could be determined by the elemental analysis of the nitrogen. The
data of the elemental analysis for the cyclic oligosiloxanes COSiO5–
COSiO6 and the calculated molar fraction of the azo unit in cyclic
oligosiloxanes are listed in Table 1.
3.2. Mesomorphic Properties
The mesomorphic properties of all cyclic oligomers COSi1–COSi6 and
their precursor Azo-1 were investigated by POM and DSC. For the azo
monomer, a defined melting point was observed by POM and no LC
behavior could be detected, where as nonpolar mesogens R2 and R3
exhibited LC behavior. Thus, the smectic A1 phase in the ranges of
51–132ꢀC and 70–89ꢀC was observed for R2 and for R3 precursors,
respectively. Details of preparation and the results of POM measure-
ments have been published previously [7,8]. Freshly cast and
unheated films of the cyclic oligosiloxanes with azo dye moiety without
any treatment are dark between crossed polarizers but exhibit
birefringence domains when heated above the melting transition tem-
perature. Oligomers from COSi2 to COSi4 exhibit a nonclassical tex-
ture, probably due to the reduced mobility of the side chain as the
result of the steric hindrance. COSi6 and COSi7 oligomers exhibit very
small fan textures after annealing for 24 h.
The values of phase-transition temperatures as well as their enthal-
pies are summarized in Table 2.
All the thermograms show two endothermic peaks, one of Tm at
lower temperature and the other ascribed to the mesophase–isotropic

28
J. T. Sołtysiak
TABLE 2 Transition Temperatures (ꢀC) of Cyclic Oligosiloxanes
Transition
Oligomer
temperatures (ꢀC)
DHS-I (J=g)
COSi1
COSi2
COSi3
COSi4
COSi5
COSi6
Cr 113 I
Cr 42 SmA 73 I
Cr 38 SmA 80 I
Cr 37 SmA 86 I
Cr 39 SmA 103 I
Cr 41 SmA 101 I
2.4
2.9
1.5
2.7
2.4
Note. Cr, crystalline phase; SmA, smectic phase A; I, isotropic phase.
phase transition (Ti) (Fig. 1). With increasing azo-mesogen content in
siloxane ring, the LC phase range is decreased, because the crystalli-
zation temperature is slightly increased and at the same time the
clearing temperature is decreased. This indicates that the order neces-
sary for the formation of a mesophase is disturbed by the azo-mesogen.
Additionally, the mesogenic properties of these cyclic oligosiloxanes
depend on the length of the end terminal group in 4-alkoxyphenyl ben-
zoate mesogen. When comparing oligomers COSi4 and COSi5 with the
same azo group content but different terminal groups in nonpolar
mesogens, both mesophase transition and clearing temperatures of
COSi5 with a decyloxy terminal group are higher than those of COSi4
with a methoxy terminal group. This suggests that the presence of
mesogenic moieties of different lengths (d ¼ 3.85 nm for azo-dye and
d ¼ 2.85 nm for methoxy mesogen (calculated by means of Hyper-
Chem) makes the mesophase formation difficult.
FIGURE 1 DSC curves of the cyclic oligosiloxanes COSi2, COSi3, and COSi4.

Synthesis and Properties of Liquid-Crystalline
29
The presence of a mesogenic moiety of comparable length d
(ꢃ4.0 nm in the case of decyloxy substituent) facilitates the molecular
ordering favoring the LC properties.
The identification of the type of mesophase was also characterized
by temperature-controlled X-ray diffraction of the powder samples.
In general, the X-ray diagrams in low-angle range exhibit one or two
sharp reflections that can be indexed as the 001 reflections of the
layered smectic structure of parameter d (thickness of the smectic
layers), and a diffuse band at wide angles, associated with the lateral
packing, indicating the formation of a smectic phase. Furthermore, the
first reflection intensity becomes weaker with the increase of the azo-
group content in the siloxane ring as compared with the wide-angle
reflection intensity, which is unchanged.
3.3. Photochemical Properties
The photochemical properties of all compounds were studied at 20ꢀC for
their solutions using the UV=Vis Philips PU8750 spectrometer in the 200
to 600-nm spectral range. The solution was prepared using CHCl3
(0.2 g=l) and kept in the dark overnight for further experiments. The
measurements were performed using a 0.2 cm path length quartz
cuvette, 365 nm with UV lamp (nonpolarized source). The lamp power
was 30 mW, and intensity at sample level was 2.4 mW=cm2. Oligomers
COSi2ꢁCOSi4, as well as their Azo-1 low-molecular-weight precursor
FIGURE 2 Characteristic UV-Vis spectra for COSi2 in chloroform.

30

Synthesis and Properties of Liquid-Crystalline
31
show, in chloroform solution, a strong UV absorption band centred
at about 320 nm (band B at Fig. 2) and a broad band around 425 nm
(band C). These two bands are attributed to the p–pꢄ and n–pꢄ electronic
transitions of the trans-azobenzene chromophore, respectively [7].
The band A at 276 nm is related to the absorption of aromatic meso-
genic cores. Thus, the isomerization process of the azo moieties can be
traced easily by a change in relative intensity of the B and C bands.
The band B is related to monotonic decrease of the molar extinction coef-
ficient (e) of the maximum from 15,700 to 10,200 with increasing content
of azo dye units in the siloxane rings in series COSi2–COSi4 (Table 3).
A slight bathochromic shift is also observed with decreasing content
of azo dye moiety in siloxane rings. This shift could be at least partially
responsible for the particular behavior of the n–pꢄ electronic transition
band. The observed bathochromic and hypochromic shifts have been
reported for optical polymers, and they have been related to interac-
tions between neighboring chromophores [10,11]. All oligomers in sol-
ution undergo a trans-cis photoisomerization of the azo dye moiety at
365 nm in the p–pꢄ absorption region. The intensity of the p!pꢄ
absorption band decreases, whereas that of the n!pꢄ increases with
irradiation time. These phenomena are similar to those observed in
other side chain polymers containing an azo group [12,13].
The dependence of time–relative absorbance (A=A0 where A0 and A
are the absorbance of the sample at time 0 and t, respectively) indi-
cates that the photoinduced isomerization rates in chloroform of the
oligomers COSi3 and COSi5 are lower than those of the azo monomer
(Azo-1) (Fig. 3) [14,15]. This result indicates that the cyclic oligomer
FIGURE 3 Decrease of the relative absorbance (A=A0) in dependence of time
upon irradiating with 365-nm light for AZO-1 (&), COSi3 (.), and COSi5 (~) in
chloroform at 20ꢀC.

32
J. T. Sołtysiak
structure can affect the photoisomerization rate. It could be connected
with the fact that the length-to-width ratio for the cyclo(oligosilox-
anes) with azo groups (azo-mesogen contain naphthalene and three
phenyl rings) is higher than the classical azo-polymers that contain
only azobenzene groups. Additionally, the influence of the neighboring
nonpolar groups could affect this relation between time and relative
absorbance as well. Further study is needed for a complete under-
standing of the molecular packing mode. These results will be pub-
lished elsewhere.
4. CONCLUSIONS
In this article, a series of novel cyclic tetramethyltetrasiloxanes with
azo dye moiety was prepared and investigated. The cyclic siloxanes
containing up to 75% azo-dye are LC, whereas at higher content the
cyclic oligosiloxane is crystalline. Furthermore, an increase of azo
dye moiety of the cyclic oligosiloxanes leads to narrowing the tempera-
ture range of the LC phase. The results also show that the solutions of
the azo monomer and cyclic siloxanes in chloroform can undergo
photoisomerization upon irradiation with 365-nm UV.
ACKNOWLEDGMENTS
The research was supported by State Committee of Scientific Research
of the Polish government. The authors also thank Prof. J. Przedmojski
from Warsaw University of Technology for his assistance in the X-ray
diffraction.
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