Azo monomers exhibiting low layer shrinkage at the SmA-SmC transition and trans-cis light-induced isomerization

Article abstract of DOI:10.1002/cphc.201200585

Methacrylic monomers containing a (phenylene)azobenzene unit substituted with a lateral cyano group and alkyl chains of different length are synthesized and characterized by NMR techniques. Their liquid-crystalline properties are studied by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. All monomers exhibit a mesomorphic behavior that extends over wide temperature ranges with nematic and orthogonal or tilted smectic-type mesophases, depending on the length of the terminal chain. The smectic structures are determined to be single-layered with a low layer shrinkage (<5 %) at the SmA-SmC transition. This atypical behavior is attributed to the combination of a high smectic order promoted by both π-π and bond dipole-bond dipole interactions between cyano-substituted central cores, and a low correlation between neighboring layers arising from dispersive forces between the end groups (methacrylic group and alkyl chain) of the monomer. On the other hand, the trans-cis isomerization of monomers is induced in solution by irradiating with a UV lamp. High cis-isomer contents (≥96 %) are obtained at the photostationary state, which is reached in a relatively short time (40 s). Copyright

Full text of DOI:10.1002/cphc.201200585

DOI: 10.1002/cphc.201200585
Azo Monomers Exhibiting Low Layer Shrinkage at the
SmA–SmC Transition and trans–cis Light-Induced
Isomerization
Tonatiuh Garcꢀa, Leticia Larios-Lꢁpez, Rosa Julia Rodrꢀguez-Gonzꢂlez, Josꢃ Romꢂn Torres-
Lubiꢂn, and Dꢂmaso Navarro-Rodrꢀguez*[a]
Methacrylic monomers containing a (phenylene)azobenzene
unit substituted with a lateral cyano group and alkyl chains of
different length are synthesized and characterized by NMR
techniques. Their liquid-crystalline properties are studied by
differential scanning calorimetry, polarizing optical microscopy,
and X-ray diffraction. All monomers exhibit a mesomorphic be-
havior that extends over wide temperature ranges with nemat-
ic and orthogonal or tilted smectic-type mesophases, depend-
ing on the length of the terminal chain. The smectic structures
are determined to be single-layered with a low layer shrinkage
(<5%) at the SmA–SmC transition. This atypical behavior is at-
tributed to the combination of a high smectic order promoted
by both p–p and bond dipole–bond dipole interactions be-
tween cyano-substituted central cores, and a low correlation
between neighboring layers arising from dispersive forces be-
tween the end groups (methacrylic group and alkyl chain) of
the monomer. On the other hand, the trans–cis isomerization
of monomers is induced in solution by irradiating with a UV
lamp. High cis-isomer contents (ꢀ96%) are obtained at the
photostationary state, which is reached in a relatively short
time (40 s).
1. Introduction
Azobenzene liquid crystals combine two physical properties of
practical interest: the spontaneous self-organization of mole-
cules in mesomorphic states and the reversible photoinduced
trans–cis isomerization of the azo group.[1–3] Such coexistence
can produce a significant synergistic effect in properties, for in-
stance in the light-induced birefringence of films irradiated
with linearly polarized light.[4,5] It turns out that the liquid-crys-
talline (LC) order improves the photoinduced alignment of azo-
benzenes through a coordinated motion of molecular do-
mains, and also stabilizes it once the light is turned off,[6] al-
though the trans–cis conformational switching could affect or
disrupt the LC order.[7,8] In this concern, much investigation has
been conducted on side-chain LC polymers with the azo chro-
mophores as pendant groups.[9–11] There are several ways in
which the azo chromophore can be inserted into a polymer
chain, and the simplest one is probably by polymerizing mono-
mers bearing this group.[12,13] Some of these “azo monomers”
are liquid crystals and their photoisomerization in a mesomor-
phic state has been used to enhance the molecular align-
ment.[13,14] This strategy avoids the use of prealigned substrates
and/or the application of rubbing or some other surface treat-
ment that promotes molecular orientation in a preferential di-
rection,[15,16] and also allows the bulk alignment through the
action of the photoactive azobenzene moieties.[17]
gular phenomenon has become an important problem in the
theory of liquid crystals and a key issue for applications in
which a minor layer contraction in externally induced molecu-
lar reorientations is desired.[18] Theoretical and experimental
evidence on de Vries-like materials is extensively documented
elsewhere.[19,20]
Most de Vries liquid crystals have been characterized by
X-ray diffraction (XRD) to validate the unusual, small variation
of the smectic period (d) at the SmA–SmC transition.[21] Such
a small variation of d was first explained by Adrian de Vries
himself through a “diffuse cone model” in which molecules in
a smectic A phase are tilted in a random distribution in azimu-
thal directions, whereas in a smectic C phase the molecules re-
orient into a specific azimuthal direction; as the average tilt
angle is similar in both phases, no layer contraction occurs in
the transition from one phase to the other.[22] Some other ex-
perimental techniques such as nuclear magnetic resonance
(NMR) spectroscopy have recently been used to characterize
de Vries-like liquid crystals. By this technique an average tilt
angle of molecular moieties in the SmA phase was con-
firmed.[23] Also, a strong magnetic effect on the orientational
[a] T. Garcꢀa, L. Larios-Lꢁpez, R. J. Rodrꢀguez-Gonzꢂlez, Dr. J. R. Torres-Lubiꢂn,
Dr. D. Navarro-Rodrꢀguez
There is a kind of liquid crystal known as “de Vries material”
that shows low or no layer shrinkage at the smectic A to smec-
tic C (SmA–SmC) transition. This behavior arises from the fact
that the tilt angle of molecules in a smectic A phase is already
large, and no further inclination occurs in the transition to
a smectic C phase but only a molecular reorientation. This sin-
Departamento de Materiales Avanzados
Centro de Investigaciꢁn en Quꢀmica Aplicada
Blvd. Enrique Reyna H. 140
Saltillo Coahuila 25253 (Mꢃxico)
E-mail: damaso@ciqa.mx
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201200585.
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D. Navarro-Rodrꢀguez et al.
order of the aromatic core within the stability region of the
SmA phase was observed. From these observations a variation
of the diffuse cone model was proposed, which consisted of
smectic layers with locally ordered distribution of molecules in
azimuthal directions, namely the cluster diffuse model.[24] In
the present study, XRD results, coupled with microscopic opti-
cal observations, and supported by unperceived differential
scanning calorimetry (DSC) enthalpies at the SmA–SmC transi-
tion, let us determine a de Vries-like behavior in the studied
monomers.
highly segregating elements such as those mentioned above
helps to satisfy such conditions, although there are some ex-
ceptions to this unwritten rule. In the already mentioned mon-
omers only weak segregating elements are present and they
are probably associated with the low layer-shrinkage behavior.
We have synthesized a series of methacrylic monomers bearing
a cyano-substituted (phenylene)azobenzene core that also ex-
hibits a low layer shrinkage at the SmA–SmC transition. These
monomers without highly segregating groups show some
structural similarities to those reported by Gonzꢂlez-Henrꢀquez
et al., and herein we propose that their de Vries-like behavior
arises from comparable chemical interactions. The photoin-
duced properties of these azo monomers are also presented
and discussed.
The simple reorientation of molecules at the SmA–SmC tran-
sition in de Vries-like liquid crystals is advantageous for practi-
cal applications. For instance, high electroclinic effects (re-
sponse, tilt angle, and birefringence) can be achieved at low
voltages as compared with typical electroclinic materials.[25,26]
Furthermore, the low layer contraction avoids the formation of
a chevron structure and zigzag defects that usually occur in
SmA–SmC transitions of conventional ferroelectric (SmC*)
liquid crystals and that are detrimental for the optical quality
of electro-optical devices. The reorientation of molecules is
also interesting in de Vries LC elastomers, the low birefringence
value of which arises from the high tilt angle of molecules (ran-
domly oriented in azimuthal directions) in the smectic A
phase.[27] Under strain, the correlation in tilt alignment could
be extended, thus allowing the elastomer to deform at low
energy cost. Compared to a classical elastomer, a de Vries elas-
tomer would display large birefringence under minor stress.
For azo polymers the light-induced isomerization changes the
shape of mesogens from rodlike to kinklike, thereby destabiliz-
ing the LC order.[28] The concept was transferred to LC elasto-
mers to produce contraction, expansion, and/or bending by ir-
radiation at constant temperature. The reisomerization of the
azo groups makes the actuation reversible. All these character-
istics in a single molecule open up the scope to explore both
new properties and potential applications.
2. Results and Discussion
2.1. Synthesis
Monomers were prepared as reported previously.[32] In the
present work, only the last reaction is shown (Scheme 1),
which consists of the esterification of methacrylic acid with
a
bromoalkyl derivative. All bromoalkyl derivatives (here
named mesogens) have shown interesting LC properties and
for this reason they were included for discussion. Monomers
and mesogens have a (phenylene)azobenzene unit substituted
with a lateral CN group. In these molecules the terminal chain
has 6, 10, or 12 carbon atoms, whereas the “spacer” has six
methylene groups. These monomers were named Mon6,
Mon10, and Mon12 and their corresponding mesogens Mes6,
Mes10, and Mes12.
2.2. NMR Characterization
Monomers and mesogens here synthesized were fully charac-
1
terized by H and 13C NMR spectroscopy. This full characteriza-
Molecules with different composition and geometry, such as
chiral smectic materials and smectic liquid crystals bearing
highly segregating elements (e.g. siloxane and fluorinated
tails), have been reported to show a low layer-shrinkage be-
havior.[29,30] To the best of our knowledge, there is only one
report due to Gonzꢂlez-Henrꢀquez et al. on monomers showing
no interlayer contraction at the SmA–SmC transition.[31] It con-
cerns methacrylic monomers bearing an azobenzene unit, sub-
stituted with one lateral OH group and two alkyl chains at the
4- and 4’-positions. It was stated that for a smectic A phase of
de Vries-like materials there may exist a high lamellar order,
with both a low orientational correlation of molecules within
layers and a weak coupling between layers.[22] The presence of
tion was necessary taking into account that some authors
differ in the assignment of hydrogen atoms in similar azoben-
zene cores.[33,34] 1H, 13C, COSY (HÀH COrrelation SpectroscopY),
HMQC (CÀH Heteronuclear Multiple Quantum Correlation), and
HMBC (CÀH Heteronuclear Multiple Bond Correlation) NMR
techniques allowed us to assign unequivocally the chemical
shifts to the whole molecule. For the sake of simplicity, here
1
only the H and 13C NMR spectra of one mesogen (Mes6) were
selected for discussion (Figure 1). The chemical shifts (d) and
coupling constants (nJH–H) are expressed in terms of ppm and
hertz, respectively. Signals of the aromatic hydrogen atoms H2,
H16, H19; H15, H20, and methylene groups 8, 22, and 27 alpha
Scheme 1. Last step in the synthesis of mesogens and monomers bearing a (phenylene)azobenzene unit substituted with a CN lateral group. HQ = hydroqui-
none.
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Azo Monomers Exhibiting Low Layer Shrinkage
trum three triplets are clearly distinguished: those centered at
d=4.10 (t, 3JH8–H9 =6.4 Hz) and 4.04 ppm (t, 3JH22–H23 =6.3 Hz)
correspond to the methylene groups alpha to the oxygen
atom (H8 and H22, respectively), and that centered at d=
3
3.43 ppm corresponds to the protons H27 (t, JH27–H26 =6.4 Hz)
of the methylene group alpha to the bromine atom. All ali-
phatic hydrogen atoms located in the low-frequency region of
the spectrum (between d=0.9 and 2.0 ppm) were assigned
through the combination of the NMR techniques described in
the Experimental Section. For more details refer to the Sup-
porting Information.
The 13C NMR spectrum (broad band decoupling) shows 27
out of a total of 31 signals expected for Mes6, which means
that four carbon-atom pairs are chemically equivalent (Fig-
ure 1B). Such carbon-atom pairs (C15, C16, C19, and C20) are
those in close vicinity to the C14ÀC4 and C18ÀN bonds. The
two signals located at the highest frequencies of the spectrum
correspond to C21 (d=161.8 ppm) and C1 (d=160.47 ppm),
and are affected by the inductive effect of the corresponding
oxygen atoms. The signal at d=116.3 ppm, which corresponds
to C7 (cyano group) and is assigned by HMBC (see Supporting
Information), showed a three-bond distance correlation with
H5. The quaternary carbon atoms C21, C1, C17, C18, C14, C4,
and C6 were also unequivocally assigned by means of HMBC.
From d=33.7 to 14.0 ppm the spectrum shows ten signals cor-
responding to all the carbon atoms of the aliphatic chains,
except those alpha to the oxygen atoms. C11 and C12 were
not possible to assign by HMBC, but by means of steric and in-
ductive effects of their corresponding neighbors in the alpha
and beta positions.[35]
Figure 1. 1H NMR spectrum (A) and 13C NMR spectrum (B) of the mesogen
Mes6.
2.3. Thermal Properties
All synthesized monomers and mesogens were first analyzed
by thermogravimetric analysis (TGA) and the obtained results
are shown in Table 1. It can be noticed that for monomers the
initial decomposition temperature (T5%) is located between 313
and 3248C, whereas for mesogens it is between 254 and
2608C. The thermal stability of mesogens is much lower than
that of monomers, probably due to the low dissociation
energy of the BrÀC bond.[36] The T5% is well above the clearing
point for all molecules, thus assuring that no decomposition
occurs during their thermotropic examination.
to two oxygen and bromine atoms showed first-order pattern
coupling as described below.
1
The fairly good resolution of the H NMR spectrum of Mes6
(Figure 1A) let us distinguish seven signals in the aromatic
region, which correspond to seven chemically different protons
out of a total of 11 aromatic hydrogen atoms. The two dou-
3
blet-like signals centered at d=7.95 (d, JH20–H19 =8.5 Hz) and
3
7.92 ppm (d, JH15–H16 =8.8 Hz) correspond to H19 and H16 lo-
cated in the ortho position to the azo group. The signal cen-
4
tered at d=7.82 ppm was assigned to H5 (d, JH5–H3 =2.2 Hz).
The thermotropic behavior of monomers and mesogens was
first studied by DSC in both heating and cooling scans, and
thermograms showed at least two first-order thermal transi-
tions. The DSC curves of both mesogens and monomers are
grouped in Figure 2. It can be seen that each monomer dis-
plays a similar polymorphic behavior to its mesogen counter-
part. The only difference is the temperature at which thermal
transitions occur, which is in general lower for monomers. In
both monomers and mesogens the transition from the LC to
the crystal state is lacking, which suggests long induction peri-
ods for crystallization. To see the melting endotherm in the
DSC traces, samples were run (second heating) for several
hours after the first cooling.
The doublet of doublets centered at d=7.77 ppm and coupled
3
4
with H2 and H5 (dd, JH3–H2 =8.8, JH3–H5 =2.4 Hz) was attributed
to H3. The doublet centered at d=7.62 ppm and coupled with
3
H16 (d, JH15–H16 =8.5 Hz) was assigned to two equivalent H15
3
hydrogen atoms. Finally, signals at d=7.02 (d, JH2–H3 =8.8 Hz)
and 7.00 ppm (d, 3JH20–H19 =9.0 Hz) correspond to H2 and to
two equivalent H20 hydrogen atoms, respectively. H2 and H20
are shifted towards the lowest frequencies of the aromatic
region due to mesomeric effects by oxygen atoms. Note that
all experimentally observed chemical shifts concur with those
1
calculated by a theoretical simulation of the H NMR spectrum
1
(ACD-Labs software). In the middle part of the H NMR spec-
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D. Navarro-Rodrꢀguez et al.
DSC traces remained unchanged
at such temperatures. Undetect-
able or almost imperceptible
DSC enthalpies have been re-
ported in some other works on
liquid crystals. For instance, in
Table 1. Thermal properties and structural parameters for monomers and mesogens in the mesophase.
Compound
L
[ꢅ]
T
[8C]
d
[ꢅ]
d/L
Tilt angle[a]
[8]
T5%
[8C]
Mesophase[b]
(enthalpy)
[8C (kJmolÀ1)]
Mes6
Mon6
34.2
36.6
140
120
32.2
34.9
0.94
0.95
19.7
17.5
254
313
SmA147 (0.29)
N160 (0.70) I
SmA140 (0.47)
N144 (0.48) I
SmA150 (3.68) I
SmC95
SmA132 (3.75) I
SmA151 (3.50) I
SmC105
studying
semi-perfluorinated
alkyl chain-substituted terphenyl
molecules, Ishida et al. reported
a SmA to SmC transition from
optical observations at variable
Mes10
Mon10
39.2
41.6
140
80
120
120
80
36.0
36.8
37.8
38.7
40.7
39.9
0.92
0.89
0.91
0.93
0.92
0.91
23.3
27.8
24.7
21.9
22.6
25.2
256
318
Mes12
Mon12
41.7
44.1
260
324
temperature;
the
enthalpy
change for this transformation
was too small to be detected in
DSC thermograms.[38]
120
SmA133 (3.02) I
[a] The tilt angle in SmA phases corresponds to the angle of molecules randomly oriented. [b] N=nematic, I=
isotropic.
The nature of the observed LC
phases was determined by XRD
analysis. Only nematic and nonordered smectic phases were
found by this technique. The nematic phase was determined
Figure 2. DSC thermograms (heating and cooling scans) of monomers and
mesogens.
2.4. Structure of Mesophases
Figure 3. Focal-conic fan texture of Mes10 at 1498C (A) and Mes6 at
1208C (B), homeotropic texture of Mon6 at 1358C (C), and gray schlieren tex-
ture of Mon6 at 141.68C (D).
Monomers and mesogens were studied by polarizing optical
microscopy (POM) at variable temperature, first to corroborate
the DSC results and second to obtain and analyze the optical
texture of each mesophase. From all the micrographs captured
upon cooling, only a few of them were selected as representa-
tives of these materials (Figure 3).
All samples exhibited focal-conic fan (Figure 3A,B) and ho-
meotropic (Figure 3C) textures, typical of smectic phases. Only
the shortest ones (Mon6 and Mes6) develop an additional low-
viscosity schlieren texture of a nematic phase (Figure 3D). The
presence of a nematic phase is common in liquid crystals con-
taining short flexible chains, as was reported for many homolo-
gous series including those having an azobenzene group.[37] It
is worth mentioning that an unexpected optical transition was
observed for the monomers with a longer tail. For Mon10, a ho-
meotropic to a sanded texture transition was detected around
958C (Figure 4A,B), whereas for Mon12 the smooth fan-
shaped texture developed at high temperature was trans-
formed around 1008C into a broken fan-shaped texture with
a sanded appearance (Figure 4C,D). This change suggests
a phase transition despite the fact that in both monomers the
Figure 4. Homeotropic texture with residual “bꢆtonnets” at 1338C (A) and
sanded texture at 958C of Mon10 (B). Focal-conic fan texture at 1208C (C)
and broken fan texture at 1008C of Mon12 (D).
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Azo Monomers Exhibiting Low Layer Shrinkage
for the monomer Mon6 and the mesogen Mes6, which dis-
played no sharp reflection while being birefringent at high
temperature (schlieren texture by POM). On the other hand,
the smectic phases were determined from the sharp Bragg re-
flection appearing in the low-angles region of the XRD pat-
terns (Figure 5). Such reflection is related to the interlayer dis-
tance (d) of the lamellar structure.
Figure 6. XRD patterns of Mon12 captured at different temperatures upon
cooling.
The exact nature of the mesophases of each of the mono-
mers and mesogens was determined from the combination of
DSC, POM, and XRD results. Nematic and single-layer SmA and
SmC phases were the only observed mesophases (Table 1). As
is known, molecules in a SmA phase are on average perpendic-
ular to the smectic plane, although a close inspection of the
calculated d/L ratios indicates that molecules are somewhat
tilted (cosÀ1 d/L=17.5–25.28). On the other hand, molecules in
a SmC phase are tilted, but for Mon10 and Mon12 the tilt
angle is rather weak (cosÀ1 d/L=27.8 and 22.68, respectively).
In terms of tilt angle there is not a sharp borderline between
SmA and SmC phases, but an undefined region. For the SmA
and SmC phases of these monomers the measured tilt angle is
close to (or within) such a region. Therefore, on cooling from
one phase to the other one might expect a minor change in
the interlayer distance (by XRD) and a low enthalpic transition
(by DSC), as actually observed for the monomers Mon10 and
Mon12. For the vast majority of liquid crystals the SmA–SmC
phase transition is accompanied by a significant contraction of
the interlayer distance (around 7–10%).[18] For de Vries-like
liquid crystals the minor layer contraction (less than 1%) is nor-
mally explained through an idealized diffuse cone model
(Figure 7) consisting in the ordering of the random azimuthal
distribution of tilted molecules within the smectic layers that
results in no or low layer contraction.[22] Molecules showing
contractions between 1 and 5% at the SmA–SmC transition
were also classified as de Vries materials. Roberts et al. reported
1.6–4.2% for pyrimidinol-trisiloxane derivatives,[22] whereas
Ishida et al. reported values of around 2% for amphiphilic
liquid crystals bearing semi-perfluorinated chains.[38] Podoliak
Figure 5. XRD patterns of monomers and mesogens captured at 1208C
upon cooling.
To determine the way the molecules are arranged within the
lamellar structure, the experimental interlayer distance was
compared with the length of the molecule in its most extend-
ed conformation (L) calculated by molecular modeling soft-
ware (Spartan 2010) and the resulting d/L ratios are presented
in Table 1. For both monomers and mesogens the calculated
d/L values are close to unity, which suggests a single-layer
packing with molecules oriented on average perpendicular to
the smectic plane.
The smectic phases were definitely recognized from the
analysis of the wide-angle region of the X-ray patterns
(Figure 5). In this region, all molecules developed only one
broad Bragg reflection centered around 228, which is indicative
of a liquid-like ordering of molecules within the smectic layers
as occurs in SmA or SmC phases. As mentioned above, none
of the molecules showed a crystallization exotherm by DSC
(Figure 3); however, a clear spherulitic growth was perceived
by POM in some of them, proba-
bly promoted by
a shear-in-
duced flow orientation occurring
between parallel glass plates.
The wide-angle region of the X-
ray patterns registered at room
temperature also confirms the
crystallization of molecules, as
shown for Mon12 in Figure 6.
Figure 7. Model for a SmA–SmC transition with a minor change in the interlayer distance (d).
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D. Navarro-Rodrꢀguez et al.
et al. reported a chiral liquid crystal bearing an azobenzene
group in which the layer contraction is about 4%.[39]
alkyl chain) are not highly segregating elements, but they are
incompatible enough with one another to weaken the cou-
pling between neighboring layers. Our molecules share some
structural characteristics with the already mentioned azo mon-
omers, and therefore their low layer-shrinkage behavior may
be explained in similar terms. The smectic order may arise
from the incompatibility between the (phenylene)azobenzene
core and the alkyl chains, but it seems to be reinforced by
bond dipole–bond dipole interactions that may occur between
CN groups. It is noteworthy that among the bond dipole mo-
ments one of the largest is that of the cyano group.[40] The
(phenylene)azobenzene is a long rigid core that might develop
strong p–p interactions, which in turn may induce a high
smectic order.[41] Sandhya et al. have reported a variety of
de Vries-like molecules with not only chiral and highly segre-
gating groups but also a long rigid core, as indeed is the case
of many other compounds showing such a behavior.[42] The
effect of the long rigid rodlike core was not examined in such
compounds, but considering the large number of de Vries-like
liquid crystals with a long aromatic group, one may perhaps
consider it as a reinforcing element for a de Vries-like behavior.
Up to now, all reported de Vries-like materials have displayed
upon cooling an isotropic (I)–SmA–SmC sequence. The ab-
sence of a nematic (N) phase is in fact considered as one of
the essential conditions for de Vries behavior.[20] The monomers
Mon10 and Mon12, which show a low layer shrinkage, dis-
played upon cooling an I–SmA–SmC sequence, whereas Mon6,
which shows no de Vries-like character, displayed an I–N–SmA
sequence. Thus, our results corroborate once again this essen-
tial condition.
To assess the de Vries character of Mon10 and Mon12, the
d/dAC ratios were determined each 58C in the reduced temper-
ature of TÀTAC =À808C (dAC and TAC are the interlayer distance
and the temperature at the SmA–SmC transition, respectively).
The d/dAC ratios were then plotted against TÀTAC (Figure 8A) to
Figure 8. Relative smectic layer spacing d/dAC versus reduced temperature
&
*
TÀTAC for Mon10 ( ) and Mon12 ( ) (A), and interlayer distance versus tem-
~
perature for Mon6 ( ) measured from small-angle X-ray scattering analy-
sis (B).
characterize the interlayer contraction in the vicinity of the
SmA–SmC transition. Both the overall feature of the resulting
curves and the d/dAC ratios higher than 0.95 confirm the
de Vries-like behavior of Mon10 and Mon12. The gradual de-
crease in d/dAC, which occurs from TAC to TÀTAC =À308C, indi-
cates that the SmA–SmC transition is second order. The maxi-
mum layer contraction measured at TÀTAC =À308C is 3.3 and
4.8% for Mon10 and Mon12, respectively. A strong negative
thermal expansion is observed in both SmA and SmC phases.
This effect is simply associated with an increase of both the
orientational ordering and the effective length of molecules
that continuously stiffens as the temperature decreases. The
monomer Mon6 showed a typical behavior of a smectic phase
whose interlayer distance steadily increased upon cooling due
to such a negative thermal expansion effect (Figure 8B).
Much work has been devoted to de Vries-like materials
having different chemical structure. However, the structural
characteristics of molecules inducing low layer shrinkage at
the SmA–SmC transition are not yet fully understood. We hope
this work will draw academic interest to design de Vries-like
materials without highly segregating groups, which up to now
have been scarcely explored.
2.5. Optical Properties
The light-induced properties of azo chromophores depend pri-
marily on their capacity to photoisomerize under the effect of
a specific light stimulus. In this section we present preliminary
results on the light absorption characteristics as well as on the
photochemical trans–cis isomerization kinetics of synthesized
monomers and mesogens, irradiated in solution using spectro-
metric-grade THF as solvent.
To our knowledge there is only one report on methacrylic
azo monomers showing low layer-shrinkage behavior.[31] Such
monomers have an azobenzene unit 4,4’-substituted with alky-
loxy chains and a lateral OH group, the methacrylic monomer
being attached to one of the chain ends. These monomers
have no chiral or highly segregating elements as should be
normal for typical de Vries-like materials, which according to
theory might have a high lamellar order with low correlation
between layers. These authors do not discuss the origin of the
low layer shrinkage, but it can be argued that the lamellar or-
dering may arise from dispersing forces between the rigid rod-
like aromatic groups (azobenzenes) and the alkyl chains. Such
order seems to be reinforced by hydrogen-bonding interac-
tions between the OH groups substituted in the central core.
The end groups of these monomers (methacrylic group and
The UV/Vis absorption spectra of monomers are similar to
those of the corresponding mesogens. Also, the chain length
has no effect on the UV/Vis absorption characteristics. For
these two reasons only the spectrum of Mon12 was selected
for discussion (Figure 9). Before irradiation (t=0 s), the trans
isomer displays one strong absorption band centered at
368 nm and one weak band at 460 nm, associated with the
p–p* and n–p* electronic transitions, respectively.[41] It can be
seen that after irradiation at definite time intervals the intensi-
ty of the strong absorption band decreases to reach a photo-
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Azo Monomers Exhibiting Low Layer Shrinkage
groups. On the other hand, monomers photoisomerize in rela-
tively short times and reach high cis-isomer contents (ꢀ96%).
Experimental Section
Materials: Methacrylic acid, potassium hydrogen carbonate, and
hydroquinone were purchased from Aldrich and used as received
unless otherwise noted. Reactive grade solvents such as dimethyl-
formamide (DMF), acetone, and chloroform were purchased from
J. T. Baker or Aldrich and used without further purification. Mes6,
Mes10, and Mes12 were prepared using the materials and proce-
dures reported previously.[32]
Characterization: All intermediates and final molecules were char-
acterized by 1H, 13C, COSY, HMQC, and HMBC NMR techniques,
with CDCl3 as solvent at 258C. H and COSY spectra were obtained
Figure 9. UV/Vis absorption spectra of Mon12 obtained by photoisomeriza-
tion at intervals of 10 s up to the photostationary state.
1
in a 300 MHz Jeol Eclipse spectrometer (using a 5 mm inverse
broad band probe (BBI)), and 13C, HMQC, and HMBC spectra in
a 500 MHz Bruker Avance III instrument (using a 5 mm direct broad
band with Z-grad (PABBO-1H/D Z-GRAD) probe, optimized for
1JC–H =145 Hz and long range JC–H =8 Hz). The chemical shifts in
stationary state (PSS). This simply indicates that the more
stable trans isomers are progressively transformed into the less
stable cis isomers. For Mon12 the PSS was reached in only
40 s, thus showing high cis-isomer contents (ꢀ96%). The pres-
ence of neat isosbestic points in superposed UV spectra con-
firms that distinct absorbing species were in equilibrium with
each other and that no degradation or side reactions took
place during the photochemical isomerization experiments.
For the other compounds similar results were obtained as
shown in Table 2. Both the short PSS and the high cis-isomer
contents obtained for all these molecules are promising for
further studies on photoinduced properties.
1
both H and 13C NMR spectra were referenced to residual nondeu-
terated solvent. The thermal stability of vacuum-dried samples was
examined in a thermal analyzer from DuPont Instruments (TGA
951) connected to a nitrogen gas cylinder and with heating at
108CminÀ1 from 30 to 8008C. DSC traces were obtained in a differ-
ential scanning calorimeter from Mettler Toledo (FP84HT). Heating
and cooling rates were 108CminÀ1 and ranged from room temper-
ature to 1708C. Optical textures were captured in a polarized opti-
cal microscope from Olympus (BX60) coupled with a temperature-
controlling hot stage from Mettler (FP82HT) and a digital camera
from Hitachi. XRD analysis was performed in a small- and wide-
angle X-ray scattering instrument from Anton Paar (SAXSess mc2)
equipped with a sample holder unit (TCS 300-C), an image plate
detector, and a temperature control unit (TCU50). Each sample was
sealed in a Lindeman glass capillary with an outer diameter of
1.0 mm and a wall thickness of 0.01 mm. The X-rays (CuKa radia-
tion; l=0.1542 nm) were generated at 40 kV and 50 mA. XRD pat-
terns were captured sequentially at determined temperatures on
cooling from the isotropic state. The UV/Vis spectra were recorded
in a spectrophotometer from Varian (Cary 5000) using a standard
quartz cell. The trans–cis photoisomerization was induced in solu-
tion (0.05 mgmLÀ1 using spectrophotometric grade THF as solvent)
by irradiation with a 366 nm nonpolarized handheld UV lamp
(UVGL-58). All solutions were irradiated at time intervals up to
a PSS.
Table 2. UV/Vis optical properties for mesogens and monomers.
[a]
Compound
etrans
[LmolÀ1 cmÀ1
lmax
PSS[b]
[s]
% cis[c]
]
[nm]
Mes6
3.26ꢇ104
3.98ꢇ104
3.46ꢇ104
3.82ꢇ104
4.19ꢇ104
4.23ꢇ104
368
368
368
368
368
368
45
50
40
50
40
40
98
97
98
98
97
96
Mon6
Mes10
Mon10
Mes12
Mon12
[a] Calculated from A=elc at different concentrations. [b] Irradiation at
366 nm until no change in absorbance is observed. [c] Calculated as in
ref. [32].
Acknowledgements
3. Conclusions
This research was sponsored by CONACYT-Mꢃxico (Project ref.:
61773-R and PhD grant to T.G. ref.: 164848). We are grateful to
G. Mꢃndez and J. Zamora for technical assistance in the DSC and
POM experiments, respectively.
Methacrylic monomers bearing a (phenylene)azobenzene core
substituted with a lateral CN group were synthesized and char-
acterized. These highly anisotropic molecules exhibit nematic
and smectic-type LC phases in wide temperature ranges. The
smectic phases were all single layered, and in some of the
monomers low layer shrinkage in the SmA to SmC transition
was observed, typical of de Vries-like liquid crystals. This behav-
ior is probably associated with strong p–p and bond dipole–
bond dipole interactions between the cyano-substituted cen-
tral cores that may induce a high smectic order, and also with
dispersive forces of the methacrylic and alkyl chain end
Keywords: azo compounds · isomerization · liquid crystals ·
mesophases · phase transitions
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Received: July 19, 2012
Revised: August 23, 2012
Published online on && &&, 2012
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ChemPhysChem 0000, 00, 1 – 9
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ARTICLES
Liquid assets: Methacrylic monomers
bearing a long azo chromophore core
(see picture) show low layer shrinkage
at the smectic A to smectic C transition.
This unexpected de Vries-like behavior
in molecules without highly segregating
elements is attractive for applications in
optical devices. These monomers also
show high cis-isomer contents (>96%)
after photoirradiation.
T. Garcꢀa, L. Larios-Lꢁpez,
R. J. Rodrꢀguez-Gonzꢂlez,
J. R. Torres-Lubiꢂn, D. Navarro-Rodrꢀguez*
&& – &&
Azo Monomers Exhibiting Low Layer
Shrinkage at the SmA–SmC Transition
and trans–cis Light-Induced
Isomerization
ChemPhysChem 0000, 00, 1 – 9
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
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ÞÞ
These are not the final page numbers!