Synthesis of vinyl-functionalized azobenzene mesogens and study of their liquid-crystalline behavior

Article abstract of DOI:10.1080/15421406.2017.1289607

Four vinyl-functionalized azobenzene-based liquid crystals (LCs) were synthesized and characterized by ¹H and ¹³C nuclear magnetic resonance, and Fourier transform infrared spectroscopies. Their thermal stability was measured by thermogravimetric analysis and their thermotropic liquid-crystalline properties were determined by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD). The azobenzene-based LCs displayed a nematic phase whereas those mesogens having an extra phenyl group developed lamellar mesophases of the SmA and SmC types. Combined with non-photoactive monomers, these vinyl-functionalized azobenzene-based mesogens can be used to prepare photo-active side-chain liquid crystal elastomers having potential use in optical actuators.

Full text of DOI:10.1080/15421406.2017.1289607

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis of vinyl-functionalized azobenzene
mesogens and study of their liquid-crystalline
behavior
Marco De Jesús Téllez, Dámaso Navarro-Rodríguez & Leticia Larios-Lopez
To cite this article: Marco De Jesús Téllez, Dámaso Navarro-Rodríguez & Leticia Larios-
Lopez (2017) Synthesis of vinyl-functionalized azobenzene mesogens and study of their
liquid-crystalline behavior, Molecular Crystals and Liquid Crystals, 647:1, 269-278, DOI:
10.1080/15421406.2017.1289607
To link to this article: http://dx.doi.org/10.1080/15421406.2017.1289607
Published online: 23 May 2017.
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Date: 24 May 2017, At: 02:42

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Synthesis of vinyl-functionalized azobenzene mesogens and
study of their liquid-crystalline behavior
Marco De Jesús Téllez, Dámaso Navarro-Rodríguez, and Leticia Larios-Lopez
Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada. Blvd. Enrique Reyna,
Saltillo Coahuila, México
ABSTRACT
KEYWORDS
Azobenzene-based
mesogen;
vinyl-functionalized;
Four vinyl-functionalized azobenzene-based liquid crystals (LCs) were
1
synthesized and characterized by H and 13C nuclear magnetic reso-
nance, and Fourier transform infrared spectroscopies. Their thermal
stability was measured by thermogravimetric analysis and their ther-
motropic liquid-crystalline properties were determined by differential
scanning calorimetry (DSC), polarized optical microscopy (POM), and X-
ray diffraction (XRD). The azobenzene-based LCs displayed a nematic
phase whereas those mesogens having an extra phenyl group devel-
oped lamellar mesophases of the SmA and SmC types. Combined with
non-photoactive monomers, these vinyl-functionalized azobenzene-
based mesogens can be used to prepare photo-active side-chain liquid
crystal elastomers having potential use in optical actuators.
photosensitive liquid crystal
1. Introduction
Liquid crystals (LCs) are mesomorphic materials possessing orientational and/or long-range
positional order like solids, and fluidity like liquids. The combination of these two char-
acteristics in LCs makes them unique and suitable for specific technological applications,
some of them considered revolutionary. Most liquid crystals are anisotropic organic com-
pounds (known as mesogens) combining a rigid core that is often composed of aromatic
rings, and one or more alkyl chains; the former is able to interact through strong inter-
molecular π-bonds whereas the latter is flexible and can disrupt to some extent such inter-
actions [1,3]. Among the wide variety of mesogens, the alkyl-substituted azobenzenes have
attracted great attention due to their unique and remarkable response (photo-isomerization
and photo-orientation) to light stimuli in the UV-vis wavelengths, and that find use in a variety
of applications, including optical data storage memories, artificial muscles, molecular rotary
motors, micro-pumps, autonomous valves, among others [4,5]. Azobenzene groups can be
easily modified with specific terminal and lateral groups to induce either small structural
changes to finely adjust the mesomorphic and optical response, or big changes to produce
a significant modification in such properties or to induce new and sometimes unexpected
properties [6].
CONTACT Leticia Larios-Lopez
leticia.larios@ciqa.edu.mx
tigación en Química Aplicada. Blvd. Enrique Reyna , C.P. , Saltillo Coahuila, México.
Departamento de Materiales Avanzados, Centro de Inves-
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

270/[538]
M. DE JESÚS TÉLLEZ ET AL.
Azobenzene liquid crystals carrying a vinyl group are “reactive mesogens” that can be used
to prepare polymers and copolymers also showing liquid crystal and photo-responsive prop-
erties [7]. Lee and White employed vinyl-functionalized azobenzene mesogens to synthesize
liquid crystal networks (LCN), which were evaluated in their photo-mechanical, photochemi-
cal and photothermal responses [8]. García-Amoros et al. used vinyl cyano-azobenzene meso-
gens (5 wt%) to prepare side-chain liquid crystal elastomers for which the photo-actuation and
thermal isomerization mechanisms were studied [9]. Despite the relative low content of azo-
mesogens, these elastomers were reported to exhibit a contraction along de director direction
as an effect of the trans-cis photo-conversion of azobenzene groups. Herein, the synthesis and
the liquid-crystalline behavior of four vinyl-functionalized liquid crystals are studied. These
materials will be further used for the preparation of side-chain liquid crystal elastomers that
are able of exhibiting photo and thermal-actuation.
2. Experimental
2.1. Materials
Reagents and solvents were purchased from Aldrich and/or J. T. Baker, and used as
received, except for tetrahydrofurane (THF), which was dried and distilled from a
sodium/benzophenone complex immediately prior to use.
2.2. Synthesis
Four vinyl-functionalized azobenzene mesogens were synthesized following the gen-
eral route outlined in Figure 1. For the sake of brevity, these mesogens were named
u-Az, p-Az, u-phAz, and p-phAz, where u and p refer to the undecenyloxy and
pentenoate flexible chain, respectively. Az and phAz correspond to the azobenzene and
phenylenazobenzene rigid unit, respectively. The precursors were prepared through several
reactions such as azo-coupling (I-1 and I-5), Williamson (I-2), aryl-boronation (I-3) and
Suzuki-Miyaura cross-coupling (I-4); some of them reported in previous works [10–12]. u-
phAz and u-Az were obtained through a Williamson reaction by reacting 11-bromoundecene
with I-4 and I-5, respectively. p-phAz and p-Az were prepared through an esterification reac-
tion by combining pentenoyl chloride with I-4 and I-5, respectively.
-decyloxy-ꢀ-(-hydroxyphenyl)-azo)-[,ꢀ-biphenyl]--carbonitrile (I-)
In a 250 mL two-neck round-bottom flask, equipped with stirring and Argon atmosphere,
1.51 g (5.45 mmol) of I-1, 1.5 g (4.95 mmol) of I-3 and 100 mL of THF were introduced.
The mixture was stirred and heated to 65°C. Next, 0.11 g (0.10 mmol) of tetrakis (triphenyl
phosphine) palladium [0] and 37.5 mL (75 mmol) of Na2CO3 2M were added, and the reaction
proceeded for about 84 h. Then, the reaction was allowed to cool to room temperature and
the organic phase was subsequently dispersed in HCl (10 wt%), washed until neutral pH,
and dried overnight. The crude product was purified in a chromatographic column using
two different eluent solutions: in a first stage hexanes-ethyl acetate [80:20] and in a second
stage THF-hexanes [40:60]. Finally, the brown-yellow powder (I-4) was dried overnight (yield
84.8%). 1H NMR (500 MHz, CDCl3, δ = ppm): 0.89 (t, J = 6.78 Hz, 3H, CH3); 1.21–1.59 (m,
–
=
26H, CH2); 1.85 (m, 4H, CH2, α CH2 Oph); 2.05 (m, 2H, CH2, α C C) 4.12 (t, J = 6.47 Hz,
2H, CH2, α O-ph); 5.07 (s, 1H, OH); 6.96 (d, J = 9.14 Hz, 2H, Ar-H, o-OH); 7.05 (d, J =
–
–
=
8.83 Hz, 1H, Ar H, o-OCH2, m-CN); 7.63 (d, J = 8.51 Hz, 2H, Ar H, o-ph, m-N N); 7.78

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[539]/271
Figure . Synthesis route for the preparation of the vinyl-functionalized mesogens u-Az, p-Az, u-phAz,
and p-phAz. (i) NaNO, HCl; NaOH, (ii) KCO, KI, acetone, °C, (iii) BuLi/THF; B[OCH(CH)]; HCl M, (iv)
Pd(PPh)/THF/NaCO M, °C, (v) CHCl/TEA, °C.
–
–
(d, J = 8.83 Hz, 1H, Ar H, o-ph, p-CN); 7.83 (s, 1H, Ar H, o-CN, m-OCH2); 7.93 (d, J =
–
=
–
=
8.51 Hz, 2H, Ar H, o-N N, m-OH); 7.95 (t, J = 8.83 Hz, 2H, Ar H, o-N N, m-ph). FT-IR
−1
≡
(KBr, cm ) 3329 (s, νas OH), 2923 (s, νas CH2); 2851 (m, νs CH2); 2240 (m, ν C N); 1606,
– –
1586, 1490, 1465 (m, νs ph); 1285 (s, νas C-O-ph); 1138 (m, νas O C C); 846, 816 (s, δ CH2).
(-((ꢀ-cyano-ꢀ-decyloxy)-[,ꢀ-biphenyl]--azo-)-phenyl pent--enoate (p-phAz)
In a 25 mL Schlenk tube 1.0 g (2.2 mmol) of I-4, 5 mL of chloroform, and 2 mL of triethy-
lamine were introduced. The solution was stirred and heated at 40°C before adding 0.286 g
(2.42 mmol) of 4-pentenoyl chloride, and then, the reaction was allowed to proceed for about
48 h. Next, all solvents were evaporated, and the solid was dried and thoroughly washed with
ethanol. Finally, the mesogen p-phAz obtained as a brown-yellow powder was dried overnight

272/[540]
M. DE JESÚS TÉLLEZ ET AL.
(Yield: 86.4%). 1H NMR (500 MHz, CDCl3, δ = ppm): 0.82 (t, J = 6.78 Hz, 3H, CH3); 1.15–
=
1.55 (m, 14H, CH2); 1.81 (q, J = 7.01 Hz, 2H, CH2); 2.47 (m, 2H, CH2, α C C); 2.65 (t, J =
7.26 Hz, 2H, CH2, α COO); 4.06 (t, J = 6.62 Hz, 2H, CH2, α OAr); 5.06 (m, 2H, CH2, α (CH
=
=
–
)); 5.85 (m, 1H, CH, α (CH2 )); 6.98 (d, J = 8.83 Hz, 1H, Ar H, o-OCH2, m-CN); 7.18
–
–
=
(d, J = 8.82 Hz, 2H, Ar H, o-OCO); 7.58 (d, J = 8.2 Hz, 2H, Ar H, o-ph, m-N N); 7.72 (d, J
–
–
= 8.83 Hz, 1H, Ar H, o-ph, p-CN); 7.77 (s, 1H, Ar H, o-CN, m-OCH2); 7.91 (d, J = 5.67 Hz,
–
=
–
=
2H−, A1 r H, o-N N, m-ph); 7.93 (t, J = 6.31 Hz, 2H, Ar H, o-N N, m-OCO). FT-IR (KBr,
= –
≡
cm ) 3071 (w, νas C C H); 2923 (s, νas CH2); 2850 (m, νs CH2); 2225 (m, ν C N); 1757
=
–
= –
(s, νas (C O) O); 1639 (w, νas C C H); 1599 y 1490 (s, νs ph); 1274 (s, νas C-O-ph); 1132
(s, νas O C C); 851 (s, δ CH2).
– –
((ꢀ-cyano-phenyl)-azo)-phenyl pent--enoate (p-Az)
This compound was synthesized using a similar procedure (reaction and purification condi-
tions) to p-phAz. 1.0 g (4.5 mmol) of I-5, 5 mL of chloroform, 2 mL of triethylamine and
0.586 g (4.95 mmol) of 4-pentenoyl chloride. The product was obtained like an orange pow-
1
=
der (Yield: 65.8%). H NMR (500 MHz, CDCl3, δ = ppm): 2.47 (m, 2H, CH2, α C C); 2.65
=
=
(t, J = 7.25 Hz, 2H, CH2, α COO); 5.07 (m, 2H, CH2, α (CH )); 5.85 (m, 1H, CH, α (CH2
–
–
)); 7.20 (d, J = 9.14 Hz, 2H, Ar H, o-OCO); 7.75 (d, J = 8.51 Hz, 1H, Ar H, o-ArH, p-CN);
–
=
–
=
7.91 (d, J = 5.99 Hz, 2H, Ar H, o-N N, m-CN); 7.93 (d, J = 6.31 Hz, 2H, Ar H, o-N N,
−1
=
–
m-OCO). FT-IR (KBr, cm ) 3081 (w, νas C C H); 2924 (m, νas CH2); 2853 (m, νs CH2);
≡
=
–
= –
2224 (m, ν C N); 1771 (s, νas (C O) O); 1643 (w, νas C C H); 1586 y 1494 (m, νs ph);
1200 (s, νas C-O-ph); 1105 (m, νas O C C); 851 (s, δ CH2).
– –
-(decyloxy)-ꢀ-((undec--enyl--oxy)-phenyl)-azo)-[,ꢀ-biphenyl]--carbonitrile
(u-phAz)
In a 250 mL two-neck round-bottom flask 1.0 g (2.2 mmol) of I-4, 0.36 g (2.64 mmol) of
K2CO3, 0.04 g (0.3 mmol) of KI and 80 mL of acetone were introduced. Next, the mixture
was stirred and heated to 65°C and then 0.58 mL (2.64 mmol) of 11-bromo-1-undecene were
added dropwise. The reaction was allowed to proceed during 84 h. Once this period elapsed
the solution was filtrated and the solid was dried. Afterwards, the product was separated in
a chromatographic column (250 mL-silica gel) using hexanes-ethyl acetate [80:20] as eluent
and the recuperated solid was dried. The product u-phAz was obtained like a yellow-brown
powder (Yield: 72.3%). 1H NMR (500 MHz, CDCl3, δ = ppm): 0.89 (t, J = 6.78 Hz, 3H, CH3);
=
1.21–1.59 (m, 26H, CH2); 1.85 (m, 4H, CH2, α CH2-Oph); 2.05 (m, 2H, CH2, α C C); 4.04
=
and 4.12 (t, J = 6.47 Hz, 4H, CH2, α O-ph); 4.97 (m, 2H, CH2, α (CH )); 5.81 (m, 1H,
–
–
CH, α (CH2 = )); 6.99 (d, J = 9.14 Hz, 2H, Ar H, o-OCH2); 7.05 (d, J = 8.83 Hz, 1H, Ar H,
–
=
o-OCH2, m-CN); 7.63 (d, J = 8.51 Hz, 2H, Ar H, o-ph, m-N N); 7.78 (d, J = 8.83 Hz, 1H,
–
–
–
Ar H, o-ph, p-CN); 7.83 (s, 1H, Ar H, o-CN, m-OCH2); 7.93 (d, J = 8.51 Hz, 2H, Ar H, o-N
−1
=
–
=
N, m-OCH2); 7.95 (t, J = 8.83 Hz, 2H, Ar H, o-N N, m-ph). FT-IR (KBr, cm ) 3054
=
–
≡
=
(w, νas
C
C H); 2923 (s, νas CH2); 2850 (m, νs CH2); 2223 (w, ν C N); 1636 (w, νas C
–
– –
C H); 1601 y 1494 (s, νs ph); 1247 (s, νas C-O-ph); 1135 (m, νas O C C); 849 (s, δ CH2).
-((undec--enyl--oxy)-phenyl)-azo)-benzonitrile (u-Az)
The mesogen u-Az was synthesized following the same procedure as for u-phAz. 5.0 g
(22.4 mmol) of I-5, 5.9 mL (26.9 mmol) of 11-bromo-1-undecene, 3.7 g (26.9 mmol) of
K2CO3, 0.5 g (3 mmol) of KI and 80 mL of acetone. The purification process consisted in
separate the liquid phase by filtration to be later concentrated and precipitated in methanol.
The final product was recuperated by filtration to be further dried in a vacuum oven. The
product was obtained like a yellow powder (Yield: 89.6%). 1H NMR (500 MHz, CDCl3, δ =

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[541]/273
=
ppm): 1.21–1.59 (m, 12H, CH2); 1.83 (m, 2H, CH2, α CH2-Oph); 2.05 (m, 2H, CH2, α C
=
C); 4.05 (t, J = 6.6 Hz, 4H, CH2, α O-ph); 4.96 (m, 2H, CH2, α (CH )); 5.81 (m, 1H, CH, α
–
–
(CH2 = )); 7.01 (d, J = 9.05 Hz, 2H, Ar H, o-OCH2); 7.78 (d, J = 8.56 Hz, 2H, Ar H, o-CN);
–
=
–
=
7.92 (d, J = 1.5 Hz, 2H, Ar H, o-N N, m-OCH2); 7.95 (d, J = 2.2 Hz, 2H, Ar H, o-N N,
−1
= –
m-CN). FT-IR (KBr, cm ) 3074 (w, νas C C H); 2919 (s, νas CH2); 2853 (m, νs CH2); 2222
≡
=
–
C H); 1600 y 1498 (s, νs ph); 1250 (s, νas C-O-ph); 1141 (m,
(w, ν C N); 1639 (w, νas
C
– –
νas O C C); 841 (s, δ CH2).
2.3. Instruments
The chemical structure of mesogens was characterized by infrared spectroscopy using a Nico-
let Magna IR-550 spectrophotometer and by nuclear magnetic resonance with a Bruker Ultra-
shield 500 MHz spectrometer. The thermal stability was evaluated in a thermogravimet-
ric analyzer (TGA-Q500) of TA-Instruments employing a heat rate of 10°C/min from 30
to 800°C. The thermal transitions were determined with a differential scanning calorimeter
(Discovery Series) from TA-Instruments, at heating-cooling rates of 10°C/min under nitrogen
atmosphere (high purity). Optical textures of these compounds were observed in an Olym-
pus BX53 microscope equipped with a temperature controlling hot stage from Linkam and
a Micropublisher 3.3 RTV Q-Imaging digital camera. The molecular stacking of mesogens
was determined from the X-ray diffraction analysis. The diffraction patterns were acquired
at different temperatures upon cooling from the isotropic phase, using a SWAXS from Anton
Paar (SAXSess mc2) equipped with a sample holder unit (TCS 300-C), an image plate detector,
and a temperature control unit (TCU50). For the XRD analysis, each mesogen was sealed in
a Hampton Research glass capillary having an outer diameter of 1.0 mm and a wall thickness
of 0.01 mm. The X-rays (Cu kα radiation; λmax = 0.1542 nm) were generated at 40 kV and
50 mA.
3. Results and discussion
3.1. Synthesis and chemical structures
The chemical structure of the vinyl-functionalized mesogens was characterized by 1H NMR
and 13C NMR spectroscopies. Figure 2 shows the 1H NMR spectra of u-phAz, p-phAz, u-Az,
and p-Az. The four spectra show two signals at 5.0 and 5.8 ppm corresponding to the protons
of the vinyl group. The spectra of p-phAz and p-Az display a signal at 2.65 ppm which was
assigned to the protons of the CH2 group in α position to the ester group. The spectra of u-
phAz and u-Az show a signal at 4.05 ppm which was associated to protons of the CH2 group
in α position to the phenoxy group. Figure 3 shows the 13C NMR spectra of p-Az and u-Az,
–
=
where it is possible to observe the characteristic signals of the vinyl group at 118 ( HC CH2)
–
=
and 137 ppm ( HC CH2). Other signals of functional groups are that of C-O at 69 for u-Az
– =
and that of O C O at 171 ppm for p-Az.
The presence of characteristic functional groups was corroborated by infrared spec-
troscopy as observed in the spectra depicted in Figure 4. The absorption bands were assigned
−1
as follows: 3080 and 1640 cm−1 stretching of vinyl groups (C C H); 2920 and 2850 cm
=
–
stretching of methyl and methylene groups (C-H); 2222 cm−1 stretching of the nitrile group
(CN); 1770 cm−1 stretching of carbonyl group (C O) presenting a bathochromic effect
=
arising from its close vicinity to the nucleophilic substituent (O-ph); 1600 cm−1 stretching
of aromatics groups (C C H), 1250 cm stretching of ether groups C-O-C; 1200 cm−1
−1
=
–
stretching of esters with phenyl substituent (ph O C O); 1140 cm−1 stretching in phenyl-
– – =
alkyl ethers (C O ph), and 850 cm−1 flexion of the C-H group [13].
– –

274/[542]
M. DE JESÚS TÉLLEZ ET AL.

Figure . H NMR spectra of vinyl-functionalized mesogens.
3.2. Thermotropic behavior
The initial decomposition temperature (2wt% lose weight) of the vinyl-functionalized meso-
gens is around 210°C. This temperature was taken into account for the further analysis by dif-
ferential scanning calorimetry (DSC) as well as by polarizing optical microscopy and X-ray
diffraction performed at variable temperature. The heating and cooling DSC thermograms
Figure . C NMR spectra of the mesogens u-Az and p-Az.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[543]/275
Figure . FT-IR spectra of vinyl-functionalized mesogens.
for mesogens are depicted in Figure 5. As it can be noted, the four mesogens display a pair
of transitions (endotherms-exotherms) indicating the presence of liquid-crystalline phases.
The effect of the length and rigidity of the molecules on the clearing temperature can be read-
ily seen by comparing the short mesogens p-Az (Ti = 119°C) and u-Az (Ti = 99°C) with the
long ones u-phAz (Ti = 152°C) and p-phAz (Ti = 188°C). The high temperature exotherm of
u-Az and p-Az is comparatively small (ꢀH ࣘ 3.7 J/mol) and can be attributed to an isotropic-
nematic transition (inset Figure 5) [14,15].
The mesomorphic behavior of the vinyl-functionalized mesogens was corroborated by
polarized optical microscopy. On cooling from the isotropic state, focal-conic fan and
homeotropic textures were observed for u-phAz (Figure 6a) and p-phAz (Figure 6c). For
the former, a clear homeotropic to Schlieren transition was also detected (Figure 6a-b). These
kind of textures and transitions are characteristics of smectic phases possessing positional and
orientational order[16]. On the other hand, p-Az developed a Schlieren texture (Figure 6d)
which was associated with a low ordered phase (nematic) while u-Az exhibited a dark field
typical of a homeotropic behavior (molecules aligned in a perpendicular direction to the glass
slides) [17].
Figure . DSC thermograms of vinyl-functionalized mesogens obtained on heating and cooling runs.

276/[544]
M. DE JESÚS TÉLLEZ ET AL.
Figure . Optical textures for the vinyl-functionalized mesogens.
3.3. Nature and structure of mesophases
The four vinyl-functionalized mesogens were analyzed by X-ray diffraction at variable tem-
perature, capturing diffraction patterns during the cooling process (Figure 7). The XRD pat-
terns confirmed the presence of nematic or smectic liquid-crystalline phases for the studied
mesogens. The diffractograms of p-Az and u-Az displayed only broad and weak peaks at both
Figure . XRD patterns of mesogens obtained at different temperatures.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[545]/277
Table . Mesophases and their arrangement observed for the vinyl-functionalized mesogens.
Mesophaseb and transition temperature (°C)
˚
˚
Mesogen Temperature (°C) L (A)
d(A)
d/L
p-Az
u-Az
p-phAz



.
.
.
—
—
.
—
—
Cr  N  I
Cr  N  I
.
Cr  SmA  I
u-phAz

.
.
.a
Cr  SmCa  SmAC  I
atilt angle = °.
bNematic (N), Smectic A (SmA), Smectic C (SmC).
Cdetected only by POM.
˚
low and wide-angle regions (2θ = 20.5°, 4.4 A), which corroborate the presence of a nematic
(N) phase. The other two mesogens u-phAz and p-phAz displayed a sharp Bragg diffraction
peak (001) at 2.47 and 2.50°, respectively. This sharp peak corresponds to the layer spacing of
˚
the smectic stacking. In the wide-angle region, the broad diffraction peak (4.5A) indicates an
in plane random order of molecules, which is characteristic of fluidic or low viscous smectic
phases (SmA or SmC) [18].
To determine the way the mesogens are arranged in the smectic phase, the experimental
˚
˚
distance (d001) of u-phAz (35.8 A) and p-phAz (35.3 A) was compared with their theoretical
length in their minimal energy conformation (L), calculated by molecular modelling soft-
ware (Spartan 14, equilibrium geometry / Hartree Fock/6–31G∗). The obtained d/L ratios
are gathered in Table 1. For p-phAz a d/L ratio close to unity corresponds to a monolayer
arrangement, where the strong interactions between the phenylene groups may promote the
molecular order inside the smectic layers. For u-phAz the d/L ratio is around 0.85, also indi-
cating a monolayer arrangement but with mesogens tilted with respect to the smectic plane,
characteristic of a Smectic C (SmC) phase [19]. POM results indicated that u-phAz first devel-
ops a Smectic A phase from the isotropic state, which then undergoes a transition towards a
paramorphotic Smectic C phase. Such transition, registered by POM in a short temperature
interval, was no detected neither by DSC nor by XRD. The mesomorphic sequence for each
one of the mesogens can be seen in Table 1.
The vinyl-functionalized liquid crystals synthesized in this work are materials possessing a
chromophore azobenzene group that can be photo-isomerized with linearly polarized light to
achieve aligned molecular domains [20]. These monomeric mesogens can be polymerized via
free radical polymerization or grafted onto poly-hydrosiloxanes in order to obtain side-chain
liquid crystal polymers (SCLCP) and elastomers (SCLCE) [21]. The photo-triggerable liquid
crystal elastomers are being intensely explored for photo-mechanical and thermo-mechanical
actuation [22].
4. Conclusions
The four synthesized vinyl-functionalized azobenzene mesogens showed a mesomorphic
behavior. These mesogens exhibit a relatively high thermal stability and a mesomorphism
that extends over a broad temperature range, particularly those having an extra phenyl group.
The mesogens having a CN-azobenzene core (p-Az and u-Az) develop a nematic phase with
a clearing temperature that decreases by increasing the length of the flexible segment. The
molecules carrying the long CN-phenyleneazobenzene core (p-phAz and u-phAz) exhibit a
higher clearing temperature and develop smectic phases. The mesogen p-phAz showed only
an orthogonal smectic phase (SmA) where the long rigid cores may have strong lateral inter-
actions. Respect to u-phAz, this developed orthogonal (SmA) and tilted (SmC) monolayer

278/[546]
M. DE JESÚS TÉLLEZ ET AL.
arrangements. The vinyl-functionalized mesogens here studied can be used for the synthesis
of side-chain liquid crystal elastomers with potential application in photo-mechanical or
thermo-mechanical actuation systems.
Acknowledgments
The authors thank Guadalupe Méndez, Dinorah Rodríguez, Jorge L. Robles, and Jorge F. Espinoza for
their helpful technical assistance.
Funding
This research project was supported by CONACYT (157652-Y and 258195) México.
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