Synthesis and phase transitions of monomers carrying a biphenyleneazobenzene or an azotolane group. Precursors of photo-responsive liquid crystal polymers

Article abstract of DOI:10.1016/j.molliq.2016.11.040

Four functional methacrylate monomers, two of them carrying a biphenyleneazobenzene group, and the other two an azotolane group, were synthesized. The synthetic route involved several reactions, among which are the Suzuki-Miyaura and Sonogashira cross-coupling reactions that allowed us to prepare long rigid rodlike azobenzene cores. The chemical structure of these new azo-monomers was confirmed by proton nuclear magnetic resonance (¹H NMR) spectroscopy, and the thermotropic liquid-crystalline behavior was determined from differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) analyses. The four monomers showed a mesomorphic behavior that extends over a broad temperature range. The phase transitions of the monomers carrying a biphenyleneazobenzene core were much broader and weaker than those of monomers carrying an azotolane group. Also, the former showed no crystallization in cooling to room temperature. The four studied monomers displayed liquid crystal phases with lamellar order (SmA and SmC), but only those carrying an azotolane core developed an additional nematic phase. The lamellar stacking of monomers is discussed in terms of lateral interactions between the rigid rodlike cores. Both types of monomers showed a trans to cis photo-conversion of around 90% when irradiated with UV–light. These new monomers are precursors of photo-responsive liquid crystal polymers.

Full text of DOI:10.1016/j.molliq.2016.11.040

Synthesis and phase transitions of monomers carrying a biphenyleneazoben-
zene or an azotolane group. Precursors of photo-responsive liquid crystal
polymers
Olga Lidia Torres-Rocha, Leticia Larios-Lo´pez, Isaura Felix-Serrano,
Da´maso Navarro-Rodr´ıguez
PII:
DOI:
Reference:
S0167-7322(16)32521-1
doi:10.1016/j.molliq.2016.11.040
MOLLIQ 6587
To appear in:
Journal of Molecular Liquids
Received date:
Accepted date:
30 August 2016
1 November 2016
Please cite this article as: Olga Lidia Torres-Rocha, Leticia Larios-Lo´pez, Isaura Felix-
Serrano, D´amaso Navarro-Rodr´ıguez, Synthesis and phase transitions of monomers carry-
ing a biphenyleneazobenzene or an azotolane group. Precursors of photo-responsive liquid
crystal polymers, Journal of Molecular Liquids (2016), doi:10.1016/j.molliq.2016.11.040
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ACCEPTED MANUSCRIPT
Synthesis and phase transitions of monomers carrying a biphenyleneazobenzene or an
azotolane group. Precursors of photo-responsive liquid crystal polymers
Olga Lidia Torres-Rocha, Leticia Larios-López, Rosa Julia Rodríguez-González, Isaura Felix-
Serrano, Dámaso Navarro-Rodríguez*
Departamento de Materiales Avanzados. Centro de Investigación en Química Aplicada (CIQA),
Blvd. Enrique Reyna 140, 25294, Saltillo, Coahuila, Mexico
Olga Lidia Torres-Rocha: tantrol@hotmail.com
Leticia Larios-López: leticia.larios@ciqa.edu.mx
Rosa Julia Rodríguez-González: julia.rodriguez@ciqa,edu.mx
Isaura Felix Serrano: fesisaura@gmail.com
*Corresponding author: Dámaso Navarro-Rodríguez: damaso.navarro@ciqa.edu.mx
Departamento de Materiales Avanzados. Centro de Investigación en
Química Aplicada (CIQA), Blvd. Enrique Reyna 140, 25294, Saltillo,
Coahuila, Mexico. Phone Number: 52 844 4389846
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ABSTRACT
Four functional methacrylate monomers, two of them carrying a biphenyleneazobenzene group,
and the other two an azotolane group, were synthesized. The synthetic route involved several
reactions, among which are the Suzuki-Miyaura and Sonogashira cross-coupling reactions that
allowed us to prepare long rigid rodlike azobenzene cores. The chemical structure of these new
azo-monomers was confirmed by proton nuclear magnetic resonance (1H NMR) spectroscopy,
and the thermotropic liquid-crystalline behavior was determined from differential scanning
calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) analyses.
The four monomers showed a mesomorphic behavior that extends over a broad temperature
range. The phase transitions of the monomers carrying a biphenyleneazobenzene core were much
broader and weaker than those of monomers carrying an azotolane group. Also, the former
showed no crystallization in cooling to room temperature. The four studied monomers displayed
liquid crystal phases with lamellar order (SmA and SmC), but only those carrying an azotolane
core developed an additional nematic phase. The lamellar stacking of monomers is discussed in
terms of lateral interactions between the rigid rodlike cores. Both types of monomers showed a
trans to cis photo-conversion of around 90% when irradiated with UV–light. These new
monomers are precursors of photo-responsive liquid crystal polymers.
Key words: azobenzene, azotolane, liquid crystal, monomer
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1. Introduction
The current increasing demand for digital storage devices of high capacity has motivated the
development of new functional materials with specific properties [1,2]. This has been the case for
the azobenzene-based liquid crystal (LC) polymers, which combine the unique molecular self-
organization characteristic of liquids crystals with the exceptional light-induced response
(molecular photo-alignment) of the azobenzene groups [3], and that have found potential
application in high density (volume or 3D) optical data storage devices [4,5]. The preparation of
such photo-responsive LC polymers requires the design and synthesis of functional monomers
with a specific chemical structure. Such monomers can homopolymerize or co-polymerize with
other monomers, either functional or non-functional, leading to an almost unlimited possibility of
polymeric structures [6-9]. LC polymers bearing long or highly anistotropic “azo-mesogenic”
groups have attracted great attention because they have shown to exhibit a mesomorphic behavior
over broad temperature ranges and/or high response (photo-alignment) to light stimuli at the UV-
vis wavelengths [10,11]. Examples of highly anisotropic azo-mesogens are the biphenylene-
azobenzene and azotolane groups substituted with terminal alkyl chains. The former develop
mesophases of high thermal stability [11], whereas the latter reach high photo-induced
birefringence (Δn) values [12-14]. It is worth to mention that the highest photo-induced
birefringence values so far reported for azo-compounds belong to LC polymers bearing azotolane
groups (Δn > 0.5) [15]. In spite of such outstanding properties, there are relatively few reports on
the synthesis of photo-responsive monomers carrying a highly anisotropic azo-mesogenic group.
Their poor solubility is undoubtedly the main drawback as it restrains their own synthesis as well
as that of their corresponding polymers. Common practices to improve the solubility of long
aromatic cores are the substitution of their two ends with long alkyl chains and the introduction
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of one or more short lateral groups; the former improve the mobility of molecules, whereas the
latter impede their close packing (aggregates with tightly held molecules) [16]. Also, the lateral
groups depress the melting and clearing temperatures as compared with those of their
homologous molecules without such groups [17].
Recently, we reported the synthesis and thermotropic liquid crystal properties of monomers
carrying a phenyleneazobenzene group substituted with two terminal alkyl chains and a lateral
cyano (CN) group [18]. These monomers displayed nematic, smectic A, and smectic C phases in
a broad temperature range. They also showed long induction periods for crystallization in such a
way that, upon cooling, the lowest temperature mesophase remained temporarily unchanged up to
room temperature. In this paper we report on the synthesis procedures and the thermotropic
characterization of new methacrylate monomers carrying a highly anisotropic azobenzene
derivative substituted with a lateral cyano group. Such a derivative is either, a biphenylene-
azobenzene core having three successive phenyl groups, or an azotolane core possessing two
phenyls linked through an ethynylene group; the former is longer (extra phenyl group) than that
reported by us in a recent paper [18], whereas the latter shares some chemical characteristics with
those reported by Okano et al. and that showed outstanding photo-induced optical properties [15].
2. Experimental section
2.1. Materials
4-Bromoaniline, n-bromodecane, n-bromohexane, 1,6-dibromohexane, 5-bromo-2-hydroxy-
benzonitrile, 4-bromo-phenylboronic acid, 1.6 M butyllithium in hexanes (BuLi), phenol,
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bis(triphenylphosphine)palladium(II) dichloride (PdCl2(PPh3)2), trimethylsilylacetylene (TMSA),
tetrakis(triphenylphosphine)palladium(0) (Pd[0]), hydroquinone (HQ), sodium carbonate, sodium
hydroxide, sodium nitrite, potassium carbonate, potassium hydrogen carbonate, copper iodide,
potassium iodide, methacrylic acid (MA), and triisopropyl borate (TIPBO), were purchased from
Sigma-Aldrich and used as received. Chloroform (CHCl3), deuterated chloroform (CDCl3),
dimethylformamide (DMF), 1,4-dioxane, carbon tetrachloride (CCl4), methylene chloride
(CH2Cl2), tetrahydrofuran (THF), deuterated tetrahydrofuran (THF-d8), triethylamine (TEA)
acetone, ethanol, methanol, ethyl ether and hexanes were purchased from J.T. Baker. TEA (over
KOH) and THF (over a Na/benzophenone complex) were always freshly distilled before use.
2.2. Synthesis
The route for the synthesis of the new biphenyleneazobenzene and azotolane–functionalized
monomers is outlined in Scheme 1. For the sake of brevity, these azo-monomers were named B6,
B10, A6, and A10, where B and A refer to the biphenyleneazobenzene and azotolane
chromophores, respectively. The numbers 6 and 10 indicate the carbon atoms in the terminal
alkyl chain. The spacer (chain of 6 carbon atoms between the methacrylic group and the
azobenzene core) was not considered in these encoded names because it is identical for the four
studied monomers. Intermediates 1, 2, and 4 to 7 were prepared as described in previous reports
for similar compounds [11,16,19]. Procedures for the synthesis of the intermediates 3 and 8 to 13,
as well as those for the preparation of the monomers B6, B10, A6, and A10, are detailed below.
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Scheme 1. Route for the synthesis of monomers A6, A10, B6 and B10
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(E)-1-(4-((6-bromohexyl)oxy)phenyl)-2-(4-bromophenyl)diazene (3)
In a 250 mL two-neck round-bottom flask, under inert atmosphere (Argon) and magnetic
stirring, 4 g (9.09 mmol) of 2, 0.15g (0.13 mmol) of tetrakistriphenylphosphine palladium[0], 60
mL of THF, and 29.5 mL of Na2CO3 2M (aqueous solution), were introduced. The reaction
mixture was heated to 60 °C, and then, a solution (THF solvent) containing 0.91 g (4.54 mmol)
of 4-bromophenyl boronic acid and tetrakistriphenylphosphine palladium(0) (catalytic amount)
was added dropwise very slowly during 48 h. The reaction was held at 60 °C for 70 h under
argon atmosphere and stirring. After this reaction period, the solution was allowed to cool down
to room temperature and 40 mL of CHCl3 were added. Next, the solution was introduced into a
separation funnel and the organic phase was washed with water (until pH 7) and evaporated.
Then, the residual solid was introduced into a silica-gel chromatographic column (CHCl3 as
eluent) to remove the catalyst. Finally, the solution was evaporated and the intermediate 3
(orange crystals, yield 65 %) was recrystallized from ethyl acetate. 1H NMR (CDCl3,  = ppm): 
= 1.43-1.61 (m, CH2, 4H); 1.74-1.94 (m, CH2, 4H); 3.45 (t, CH2, 2H, J = 6.9 Hz); 4.07 (t, CH2,
2H, J = 6.3 Hz); 7.02 (d, CH, 2H, J = 9.1 Hz); 7.53 (d, CH, 2H, J = 8.5 Hz); 7.59 (d, CH, 2H, J =
8.5 Hz); 7.70 (d, CH, 2H, J = 8.5 Hz); 7.89-7.98 (m, CH, 4H).
(E)-1-(4 -((6-bromohexyl)oxy)phenyl)-2-(4-((trimethylsilyl)ethynyl)phenyl)diazene (8)
In a 250 mL two-neck round-bottom flask, under inert atmosphere and stirring, 2 g (4.54
mmol) of 2, 0.14g (0.77 mmol) of CuI, 0.15 g (0.23 mmol) of PdCl2 (PPh3)2, 0.29g (1.13 mmol)
of PPh3, 60 mL of TEA and 60 mL of THF, were introduced. The reaction mixture was heated to
60 °C and maintained under vigorous stirring for 30 min. Next, 1.28 mL (0.89 g, 9.08 mmol) of
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trimethylsilylacetylene was added, and the reaction was heated to reflux for 24 h under argon
atmosphere and stirring. After this reaction period, the solution was allowed to cool down to
room temperature before being transferred to a separation funnel where diethyl ether was added
for solids extraction. Then, the recuperated solid was introduced into a short chromatographic
column (silica-gel and CHCl3 as eluent) to remove the catalyst. Finally, the compound 8 (orange
powder, yield 62 %) was purified by column chromatography using CHCl3:hexane 8:2 as eluent.
1H NMR (CDCl3,= ppm): .26 (s, CH3, 9H); 1.52-1.56 (m, CH2, 4H); 1.83-1.95 (m, CH2,
4H); 3.45 (t, CH2, 2H, J = 6.7 Hz); 4.06 (t, CH2, 2H, J = 6.3 Hz); 7.01 (d, CH, 2H, J = 7.99 Hz);
7.58 (d, CH, 2H, J = 8.3 Hz); 7.81 (d, CH, 2H, J = 8.5 Hz); 7.91 (d, CH, 2H, J = 9.1 Hz).
(E)-1-(4-((6-bromohexyl)oxy)phenyl)-2-(4-ethynylphenyl)diazene (9)
In a 250 mL one-neck round-bottom flask, 0.7 g (1.59 mmol) of compound 8, 0.015 g (0.63
mmol) of K2CO3, 8 mL of THF and 35 mL of methanol, were introduced. Next, the reaction
mixture was stirred for 3 h at room temperature. The solvent was then evaporated and the solid
was introduced into a silica-gel chromatographic column (CH2Cl2:hexane 1:1 as eluent) to
recuperate the compound 9 as an orange solid (yield 90 %). 1H NMR (CDCl3,= ppm):  = 1.37-
1.64 (m, CH2, 4H); 1.84-1.95 (m, CH2, 4H); 3.22 (s, CH, 1H); 3.45 (t, CH2, 2H, J = 6.7 Hz); 4.06
(t, CH2, 2H, J = 6.3 Hz); 7.01 (d, CH, 2H, J = 8.5 Hz); 7.65 (d, CH, 2H, J = 8.3 Hz); 7.85 (d, CH,
2H, J = 8.3 Hz); 7.93(d, CH, 2H, J = 8.3 Hz).
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(E)-4”-((4-((6-bromohexyl)oxy)phenyl)diazenyl)-4-(hexyloxy)-[1,1':4’,1”-terphenyl]-3-
carbonitrile (10)
In a 250 mL two-neck round-bottom flask, under inert atmosphere and magnetic stirring, 0.5 g
(0.97 mmol) of 3, 0.239 g (0.97 mmol) of 6, 30 mL of THF, 6.3 mL of Na2CO3 2M (aqueous
solution), and 0.033g (0.03 mmol) of tetrakistriphenylphosphine palladium(0), were introduced.
Next, the reaction mixture was heated to 60 °C and during 8 h catalytic amounts of
tetrakistriphenylphosphine palladium[0] were added at intervals of 2 h. The reaction was held at
60 °C for 24 h under argon atmosphere and stirring. After this reaction period, the solution was
allowed to cool down to room temperature and 30 mL of CHCl3 were added. The organic phase
was then separated from the aqueous phase, and it was subsequently washed with water until pH
7, dried with sodium sulfate, and evaporated. The recuperated solid was introduced into a silica-
gel chromatographic column (CHCl3 as eluent) to remove the catalyst. Finally, the compound 10
(orange solid; yield 74 %) was purified by column chromatography using CHCl3:hexane 1:1 as
eluent. 1H NMR (CDCl3,  = ppm): 0.92 (t, CH3, 3H, J = 6.5 Hz); 1.26-1.65 (m, CH2, 12H);
1.87-1.92 (m, CH2, 4H); 3.45 (t, CH2, 2H, J = 6.7 Hz); 4.07 (t, CH2, 2H, J = 6.1 Hz); 4.13 (t,
CH2, 2H, J = 6.1 Hz); 6.94-7.09 (m, CH, 3H); 7.63 (d, CH, 2H, J = 8.4 Hz); 7.70-7.84 (m, CH,
6H); 7.90-8.01 (m, CH, 4H).
(E)-4”-((4-((6-bromohexyl)oxy)phenyl)diazenyl)-4-(decyloxy)-[1,1':4’,1”-terphenyl]-3-
carbonitrile (11)
Orange powder (yield 57 %).1H NMR (300 MHz, CDCl3,  = ppm): 0.89 (t, CH3, 3H, J
= 6.5 Hz); 1.26-1.55 (m, CH2, 20H); 1.85-1.93 (m, CH2, 4H); 3.45 (t, CH2, 2H, J = 6.7 Hz); 4.07
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(t, CH2, 2H, J = 6.3 Hz); 4.13 (t, CH2, 2H, J = 6.5 Hz); 6.96-7.09 (m, CH, 3H); 7.63 (d, CH, 2H,
J = 8.5 Hz); 7.71-7.86 (m, CH, 6H); 7.90-8.07 (m, CH, 4H).
(E)-5-((4-((4-((6-bromohexyl)oxy)phenyl)diazenyl)phenyl)ethynyl)-2-(hexyloxy)benzonitrile (12)
In a 250 mL two-neck round-bottom flask, under argon atmosphere and stirring, 0.46 g (1.66
mmol) of 4, 0.034 g (0.18 mmol) of Cul, 0.048 g (0.07 mmol) of PdCl2 (PPh3)2, 0.09 g (0.35
mmol) of PPh3, 30 mL of TEA and 30 mL of THF, were introduced. The reaction mixture was
heated to 60 °C and stirred for 30 min. Then, 0.493 g (1.38 mmol) of 9 were added and the
reaction was held for 24 h under reflux. After this reaction period, the solution was allowed to
cool down to room temperature to be further transferred into a separation funnel for solid
extraction with THF. The extracted solid was then introduced into a short silica-gel
chromatographic column (CHCl3 as eluent) to remove the catalyst. Finally, the compound 12
1
(orange powder, yield 28 %) was recrystallized twice from hexane:ethyl acetate 8:2. H NMR
(CDCl3, = ppm): = 0.93 (t, CH3, 3H, J = 6.9 Hz); 1.36-1.56 (m, CH2, 12H), 1.85-1.95 (m,
CH2, 4H); 3.44 (t, CH2, 2H, J = 6.3 Hz); 4.07 (t, CH2, 2H, J = 6.3 Hz); 4.11 (t, CH2, 2H, J = 6.3
Hz); 6.90-7.05 (m, CH, 3H); 7.59-7.71 (m, CH, 3H); 7.75 (d, CH, 1H, J = 1.9 Hz); 7.88 (d, CH,
2H, J = 8.5 Hz); 7.93 (d, CH, 2H, J = 9.1 Hz).
(E)-5-((4-((4-((6-bromohexyl)oxy)phenyl)diazenyl)phenyl)ethynyl)-2-(decyloxy)benzonitrile (13)
This compound was synthesized from 9 and 5 following a procedure similar to that described
for the synthesis of 12. The compound 13 (orange powder, 27 %) was recrystallized from
1
acetone. H NMR (CDCl3,  = ppm):  = 0.93 (t, CH3, 3H, J = 6.9 Hz); 1.36-1.56 (m, CH2,
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12H), 1.85-1.95 (m, CH2, 4H); 3.44 (t, CH2, 2H, J = 6.3 Hz); 4.07 (t, CH2, 2H, J = 6.3 Hz); 4.11
(t, CH2, 2H, J = 6.3 Hz); 6.90-7.05 (m, CH, 3H); 7.59-7.71 (m, CH, 3H); 7.75 (d, CH, 1H, J = 1.9
Hz); 7.88 (d, CH, 2H, J = 8.5 Hz); 7.93 (d, CH, 2H, J = 9.1 Hz).
(E)-6-(4-((3''-cyano-4”-(hexyloxy)-[1,1':4',1''-terphenyl]-4-yl)diazenyl)phenoxy)hexyl
methacrylate (B6)
In a 250 mL round-bottom flask, under argon atmosphere and stirring, 0.15 g (1.55 mmol) of
KHCO3 and 0.13 mL (1.55 mmol) of methacrylic acid, were introduced. Next, 0.0062 g (0.06
mmol) of hydroquinone, 0.365 g (0.55 mmol) of 10, and 40 mL of DMF were added to the
reaction flask. The reaction mixture was heated to 80 °C and held under stirring and argon
atmosphere for 24 h. After this reaction period, the solution was allowed to cool down to room
temperature to be further transferred into a separation funnel where 30 mL of CHCl3 and 30 mL
of H2O were added. The organic phase then subsequently washed with HCL 5% (aqueous
solution), K2CO3 5% (aqueous solution), and water until pH 7. Then, it was dried with sodium
sulfate, filtered and evaporated. Finally, B6 (orange powder, yield 72 %) was purified by silica-
1
gel column chromatography using CHCl3:hexane 9:1 as eluent. H NMR (CDCl3, = ppm):  =
0.94 (t, CH3, 3H, J = 6.6 Hz); 1.49-1.58 (m, CH2, 10H), 1.70-1.94 (m, CH2, 6H); 1.96 (s, CH3,
3H); 4.07 (t, CH2, 2H, J = 6.3 Hz); 4.12-4.21 (m, CH2, 4H); 5.56 (s, =CH, 1H); 6.12 (s, =CH,
1H); 6.97-7.14 (m, CH, 3H); 7.63 (d, CH, 2H, J = 8.3 Hz); 7.70-7.86 (m, CH, 6H); 7.89-8.02 (m,
CH, 4H).
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(E)-6-(4-((3”-cyano-4”-(decyloxy)-[1,1´:4´,1”-terphenyl]-4-yl)diazenyl)phenoxy)hexyl
methacrylate (B10)
Orange powder (yield 75 %). 1H NMR (CDCl3,  = ppm):  = 0.88 (t, CH3, 3H, J = 6.9 Hz);
1.27-1.57 (m, CH2, 18H), 1.68-1.89 (m, CH2, 6H); 1.94 (s, CH3, 3H); 4.05 (t, CH2, 2H, J = 6.3
Hz); 4.06-4.18 (m, CH2, 4H); 5.53 (s, =CH, 1H); 6.10 (s, =CH, 1H); 6.92-7.13 (m, CH, 3H); 7.64
(d, CH, 2H, J = 8.3 Hz); 7.70-7.86 (m, CH, 6H); 7.89-8.02 (m, CH, 4H).
(E)-6-(4-((4-((3-cyano-4-(hexyloxy)phenyl)ethynyl)phenyl)diazenyl)phenoxy)hexyl methacrylate
(A6)
Orange powder (yield 76 %). 1H NMR (CDCl3,  = ppm):  = 0.92 (t, CH3, 3H, J = 6.9 Hz);
1.34-1.57 (m, CH2, 10H), 1.69-1.89 (m, CH2, 6H); 1.95 (s, CH3, 3H); 4.04-4.20 (m, CH2, 6H);
5.56 (s, =CH, 1H); 6.12 (s, =CH, 1H); 6.90-7.05 (m, CH, 3H); 7.58-7.71 (m, CH, 3H); 7.75 (d,
CH, 1H, J = 1.9 Hz); 7.87 (d, CH, 2H, J = 8.5 Hz); 7.92 (d, CH, 2H, J = 8.8 Hz).
(E)-6-(4-((4-((3-cyano-4-(decyloxy)phenyl)ethynyl)phenyl)diazenyl)phenoxy)hexyl methacrylate
(A10)
1
Orange powder (yield 90 %). H NMR (CDCl3, = ppm):  = 0.91 (t, CH3, 3H, J = 6.9 Hz);
1.35-1.57 (m, CH2, 18H), 1.68-1.89 (m, CH2, 6H); 1.94 (s, CH3, 3H); 4.04-4.20 (m, CH2, 6H);
5.55 (s, =CH, 1H); 6.10 (s, =CH, 1H); 6.90-7.05 (m, CH, 3H); 7.58-7.71 (m, CH, 3H); 7.74 (d,
CH, 1H, J = 1.9 Hz); 7.87 (d, CH, 2H, J = 8.5 Hz); 7.92 (d, CH, 2H, J = 8.8 Hz).
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2.3. Instruments
The chemical structure of intermediates and monomers was confirmed by proton nuclear
magnetic resonance (1H NMR) spectroscopy using a Jeol 300 MHz spectrometer (model JNM-
ECO300) and THF-d8 or CDCl3 as solvents. The thermal stability of monomers was determined
by thermogravimetry using a TGA 951 thermal analyzer from DuPont Instruments connected to a
N2 vector gas and heating at 10 °C min-1 from 30 to 800 °C. The transition temperatures were
measured in a MDSC 2920 differential scanning calorimeter from TA Instruments at
heating/cooling rates of 10 °C min-1. The optical textures were obtained from samples confined
between two glass cover slides and observed between crossed polarizers in an Olympus BX53
optical microscope coupled to a Linkam LTS420E hot stage and to a T95-LinkSys controller.
The presented textures were captured with a Micropublisher 3.3 RTV digital camera. The layer
spacing of the smectic phases was determined from the X-ray diffraction patterns registered at
different temperatures on cooling from the isotropic liquid using a SAXSess mc2 from Anton
Paar GmbH, equipped with a sealed cooper tube (CuKα 0.154 nm), a sample holder unit (TCS
300-C), an image plate detector, and a temperature controller (TCU50). XRD experiments were
conducted with samples encapsulated in glass capillaries (Hampton Research) having an outer
diameter and a wall thickness of 1.0 and 0.01 mm, respectively. The photo-isomerization
experiments were performed in solution (THF as solvent) using a laboratory UV lamp (UVGL-
58; λ = 365nm). A CARY 5000 spectrophotometer was used to record the spectrum just after
each irradiation interval (5 seconds).
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3. Results and Discussion
3.1. Synthesis and characterization of monomers
The monomers B6, B10, A6 and A10 were prepared through the intermediates 1 to 13 as
shown in Scheme 1. The intermediate 1 was obtained from 4-bromoaniline, phenol and NaNO2
via the in situ formation of a diazonium salt [19]. This intermediate was next reacted with 1,6-
dibromohexane through a Williamson reaction to get the intermediate 2, which was in turn
coupled with the 4-bromophenyl boronic acid to produce 3, using the Suzuki-Miyaura cross-
coupling reaction mediated by Pd[0] [20]. In a parallel way, the intermediates 4 and 5 were
obtained by reacting 5-bromo-2-hydroxybenzonitrile with bromohexane and bromodecane
(Williamson reaction), respectively. These two intermediates were then reacted with butillitium
and triisopropyl borate to prepare the corresponding alkyloxy-cyanophenyl boronic acids 6 and 7,
which were subsequently coupled with 3 to get the long bromo end-functionalized biphenylene-
azobenzene precursors 10 and 11, respectively [21]. The Suzuki-Miyaura cross-coupling reaction
is perhaps the most popular method for the aryl-aryl coupling. It proceeds under mild conditions,
tolerate a variety of functional groups, and uses boronic acids (laboratory-made or commercially
available), which are fairly stable and easy to handle [22]. The relatively low temperature (often
less than ‒ 40 °C) required for the preparation of boronic acids is the only significant drawback,
particularly when their preparation involves long rigid rodlike intermediates (oligoaryls), which
become fairly insoluble at such temperatures [23]. For our intermediates the Suzuki-Miyaura
reaction resulted in relatively good yields (> 50 %). The general catalytic cycle of this coupling
reaction is known and has been described elsewhere [24].
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On the other hand, the intermediate 2 was coupled with trimethylsilylacetylene to obtain 8
using a Sonogashira coupling reaction mediated by PdCl2(PPh3)2/CuI/PPh3 [25]. This C‒C
coupling method has become a practical and useful tool not only for the synthesis of compounds
with terminal alkynes, but also of those with internal alkynes like tolanes or longer conjugated
molecules like the oligo- and poly-phenyleneethynylenes [26,27]. The further elimination of the
trimethylsilyl group from 8 using K2CO3 gave the intermediate 9, which was later coupled
(Sonogashira-coupling reaction) with 4 and 5 to produce the bromo end-functionalized azotolane
precursors 12 and 13, respectively. Finally, the monomers B6, B10, A6 and A10 were prepared
by reacting the bromo end-functionalized precursors (10, 11, 12 and 13, respectively) with
methacrylic acid [16]. All the synthesized compounds were obtained in good yields (> 60%),
except for some of the long final intermediates whose conversion was rather low (yield ~26%).
The 1H NMR spectra of 1, 2, 3, 10 and B6 are grouped in Figure 1. The chemical modification
can be simply followed by comparing the spectra of each one of these compounds. The alkylation
of 1 with 1,6–dibromohexane to obtain 2 was confirmed by the presence of two triplets at 4.05
(Ar‒O‒CH2) and 3.44 ppm (CH2‒Br), and also by various multiplets between 1.5 and 2.0 ppm
(rest of the alkyl chain). The compound 2 was then coupled with the 4-bromophenyl boronic
acid, which can also react with itself to produce oligo and poly(p-phenylene)s [28]. In order to
minimize this polycondensation reaction, the bromophenyl boronic acid was added dropwise very
slowly over 48 h. Under this condition, the intermediate 3 was obtained and isolated in a
relatively good yield (~65%). The spectrum of the intermediate 3 displayed additional signals at
low fields (around 7.6 ppm) as compared with that of the intermediate 2. Such signals were
attributed to the protons of the extra phenyl group in the intermediate 3, supporting the
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accomplishment of the C‒C coupling reaction. To further increase the conjugation length of the
azobenzene core, the intermediate 3 was next reacted with the 4-hexyloxy-(3-cyano)phenyl
boronic acid (6) in the presence of Pd[0], producing 10 at a relatively good yield (74%). The
inserted hexyloxyphenyl group can be readily identified in the spectrum of 10 by new signals at
low (doublet at 6.9 ppm, CH o- to the oxygen atom), medium (triplet at 4.1 ppm, ‒CH2‒O), and
high fields (triplet at 0.9 ppm, CH3) of the spectrum. Finally, the precursor 10 was reacted with
methacrylic acid to obtain the azo monomer B6. The spectrum of B6 shows new signals between
5.5 and 6.15 ppm, and around 4.1 ppm (overlapped), corresponding to protons of the vinyl
(‒CH2=C‒) and ester (‒CH2‒O‒CO‒) groups, respectively. These new signals and the absence of
the triplet at 3.5 ppm (CH2‒Br) indicate that the functionalization of the acrylic monomer has
proceeded as expected. The 1H NMR spectra of 11 and B10 are not shown here because they are
similar to those of 10 and B6, respectively. The only change is the relative higher intensity of the
mehylene signal at high fields (between 1.0 and 1.6 ppm) resulting from the difference in the
terminal alkyl chain length (decyloxy for 11 and B10, and hexyloxy for 10 and B6).
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Figure 1. 1H.NMR spectra of the precursors 1, 2, 3, 10 and monomer B6.
1
The H NMR spectra of 8, 9, 12 and A6 are depicted in Figure 2. The intermediate 8 was
synthesized from 2 and trimethylsilylacetylene (Sonogashira coupling reaction) under rigorous
inert conditions [29]. Compared with the spectrum of the intermediate 2, the spectrum of 8
displays one additional singlet at 0.26 ppm, which was assigned to the protons of the
trimethylsilyl group. It also shows signals with different multiplicity at low fields, indicating that
the electronic environment produced by the trimethysilylacetylene group (non-polar group) in 8,
is some different to that produced by the bromine atom (electron attracting group) in 2. The
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purification of the trimethylsilyl-functionalized azocompound (8) by column chromatography
was particularly difficult because the elusion rate of both 2 and 8 was similar in most tested
solvents (pure and mixed solvents), finding that only CCl4 allowed a relatively good separation
(yield of 47.3 %) in long silica-gel columns. The trimethylsilyl group of the intermediate was
then removed from 8 using K2CO3 [30]. The spectrum of the resulting product (compound 9)
shows a singlet at 3.2 ppm associated to the proton of the remaining terminal acetylene group
[31]. This singlet and the absence of the singlet close to 0 ppm provide evidence of the
elimination of the trimethylsilyl group from 8. In a further step, the intermediates 4 and 9 were
coupled to each other through a Sonogashira reaction to form the compound 12. This later
reaction gave low yields (around 25%) due to purification problems associated to the separation
of this later compound from byproducts (non-identified). The hexyloxyphenyl group of 12 can be
readily identified by new signals at low (doublet at 6.9 ppm, CH o- to the oxygen atom), medium
(triplet at 4.1 ppm, CH2‒O), and high fields (triplet at 0.9 ppm, ‒CH3) in the spectrum. Finally,
the cyano azotolane-functionalized methacrylate monomer (A6) was obtained by reacting 12 with
methacrylic acid. The spectrum of A6 clearly shows the proton signals of the vinyl group
between 5 and 6 ppm, as well as that of the ester group at 4.1 ppm. These signals and the absence
1
of the triplet at 3.5 ppm (CH2‒Br) confirm the right synthesis of the monomer A6. The H NMR
spectra of 13 and A10 (not shown here) are similar to those of 12 and A6, respectively. The only
difference is the relative higher intensity of methylene signal at high fields (between 1.0 and 1.6
ppm).
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Figure 2. 1H.NMR spectra of the precursors 8, 9, 12 and monomer A6.
3.2. Thermal behavior
The azo-monomers B6, B10, A6, and A10 start to decompose above 260 °C, as determined by
TGA (not shown here). This temperature was fixed as the limiting temperature for their further
thermotropic examination by DSC, POM and XRD. By DSC, B6 and B10 displayed only broad
and weak transitions in both heating and cooling runs (Figure 3). Despite the lack of sharp DSC
transitions, clear optical changes between typical liquid crystal textures were observed by POM
for both monomers (Figure 4). Such changes correspond to transitions from the isotropic to the
focal conic fan / homeotropic texture (Figure 4A, around 200 °C); from the focal conic fan /
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homeotropic texture to the broken focal conic fan / gray schlieren texture (Figure 4B; around
184°C); and from the broken focal conic fan / gray schlieren texture to a paramorphotic texture
(Figure 4C; around 80°C). As the optical textures were quite similar for both monomers, only
those of B10 are shown here. The focal conic fan / homeotropic texture is characteristic of a
smectic A phase, whereas the broken focal conic fan / gray schlieren texture is typical of the
smectic C phase [32]. It is to point out that the paramorphotic texture remained unchanged up to
room temperature, indicating that B6 and B10 do not crystallize. These monomers were then
analyzed by X-ray diffraction at room temperature (Figure 5). In both cases, the X-ray patterns
showed two sharp equidistant Bragg diffractions peaks (001 and 002) confirming the smectic
order (lamellar stacking).
Figure 3. DSC thermograms of the monomers B6 and B10 obtained on heating and cooling runs
at 10 °C min-1.
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Figure 4. Optical textures of the monomer B10 observed on cooling from the isotropic state.
A) 200 °C, B) 184 °C, and C) 80 °C.
Figure 5. X-ray diffraction patterns of the monomers B6 and B10 captured at room temperature.
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On the other hand, the DSC traces of the azotolane monomers (A6 and A10) showed various
thermal transitions at both heating and cooling runs, indicating the presence of enantiotropic
liquid crystal phases (Figure 6). Only the transition from the liquid-crystalline to the crystalline
state showed a marked hysteresis that can be attributed to the long induction period for the
crystallization process. Monomers might crystallize at low rate upon cooling since some
exotherms were registered under heating. The expected decrease in the clearing temperature, by
increasing the alkyl chain length, can be readily seen by comparing the highest temperature
endotherm of A6 (155 °C) with that of A10 (138 °C) [17]. The phase transitions detected by
DSC for A6 and A10 were next corroborated by polarizing optical microscopy (POM) at variable
temperature. By this technique, typical optical textures of liquid crystal phases were observed. As
both A6 and A10 displayed similar optical textures, only those of the monomer A6 are shown
here (Figure 7). Upon cooling, this monomer displayed a schlieren texture (Figure 7A)
characteristic of a nematic phase, followed by a focal conic fan texture surrounded with a large
homeotropic zones (Figure 7B), which is typical for a smectic A phase [33]. An additional
transition (non-observed by DSC) was detected on cooling to around 100 °C. At this temperature
the focal-conic fan texture gradually turned into a broken focal conic fan / gray schlieren texture,
which is typical of the tilted smectic C phase (Figure 7C).
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Figure 6. DSC thermograms of the monomers A6 and A10 obtained on heating and cooling runs
(10 °C min-1).
Figure 7. Optical textures of the monomer A10 observed on cooling from the isotropic state. A)
138 °C, B) 130 °C and C) 100 °C.
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To determine the exact nature of the liquid crystals phases, A6 and A10 were next studied by
XRD at variable temperature. Several XRD patterns were captured upon cooling, but only those
obtained from a phase transition are here presented for discussion (Figure 8). At high temperature
(just below the clearing temperature according to DSC and POM analyses), the absence of sharp
diffraction peaks at both low and wide angles in the XRD patterns corroborates the nematic
phase. On cooling from this mesophase, the development of one diffraction peak (001) at low
angles (the broad peak at wide angles remains unchanged) confirms the presence of a smectic
phase of low order. Around 100°C this sharp peak vanishes whereas other diffraction peak
appears at higher angles (the broad peak at wide angles still remaining unchanged), suggesting
the occurrence of a phase transition between two smectic phases of low order, the former more
expanded than the later. The experimental smectic period (d001) of these two smectic phases was
compared with the theoretical length of molecules (L) in their most extended conformation
calculated by molecular modelling (Spartan 10). For A6, the calculated d001/L ratios were 0.9 at
110 °C and 0.81 at 90 °C. These values are close to unity, indicating that mesophases are single
layered with molecules orthogonal (at 110°C) and tilted (at 90°C) with respect to the normal of
the smectic planes. The average tilt angle (cos-1 0.81) of molecules in the tilted mesophase is
35.9°. These results corroborate the presence of a smectic A and C phases, as it was anticipated
by POM. For A10, the calculated d001/L ratios were 0.89 at 110 °C and 0.8 at 190 °C (tilt angle =
36.9°), also indicating the presence of single layer smectic A and C phases, respectively. A
summary of the thermal and mesomorphic behavior of both A and B monomers is gathered in
Table 1.
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Table 1. Thermal transitions (T), mesophases, and decomposition temperature (Tid) of
the monomers A6, A10, B6, and B10.
Monomer
T °C (mesophase)
Tid, °C
Crystal –3.1 (SmC) 96.8 (SmA) 138 (N)155 Isotropic
Crystal 28 (SmC) 106.7 (SmA)133 (N) 138 Isotropic
SmX 120 (SmC) ? SmA (277) Isotropic/Degradation
SmX 128 (SmC) 192 SmA (255) Isotropic
293
277
266
267
A6
A10
B6 a
B10 a
a) Determined by POM as no sharp transitions were detected by DSC.
Figure 8. X-ray diffraction patterns of the monomers A6 and A10 captured at different
temperatures.
The observed lamellar stacking of monomers may be determined by the strong π -interaction
occurring between their long rigid rodlike aromatic cores (biphenylene-azobenzene or azotolane).
Such interaction may also induce a segregation of the two different parts of the monomers
(aromatic cores and aliphatic chains), which are fairly incompatible to one another and may lead
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to the formation two distinct domains (sublayers) within the lamellae [11]. For the biphenylene-
azobenzene such interactions might be stronger than those for the azotolane core. This was
reflected on the poor solubility and high clearing temperature (overlaps with the initial
decomposition temperature) of the monomers B6 and B10.
3.3 Light-induced isomerization
The azobenzene derivatives are able to isomerize from the trans to the cis isomer when they are
irradiated with UV light [3]. This photo-isomerization process is helpful to align the
chromophore units into a preferential direction leading to birefringence and dichroic properties
[16]. The photo-isomerization is also useful to record surface reliefs gratings of a specific
topological pattern. These light-induced properties and patterns are being explored for a number
of practical applications [34].
In this work the ability of the monomers A6 and B6 to photo-isomerize was tested in solution
using THF as solvent. The obtained UV-Vis absorption spectra for the initial and irradiated
solutions are shown in Figure 9. As can be noticed, the initial spectrum (0 s) exhibits a strong
band around 374 nm, which is due to the  –  electronic transition of the trans isomer [35]. On
irradiating the solution at time intervals of 5 seconds this strong band decreases whereas a weak
band centered around 455 nm (n –  electronic transition of the cis isomer) increases. Both
monomers attained a trans–to–cis photo-conversion of around 90 % in only 20 seconds. These
values are slightly smaller than those reported for shorter azo-monomers whose corresponding
polymers have shown a good photo response [16, 18].
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Figure 9. UV-Vis spectra of A6 and B6 (in THF) before and after irradiation with a UV lamp (λ=
365 nm) at time intervals of 5 seconds.
The four new functionalized methacrylate monomers (A6, A10, B6 and B10) are precursors of
liquid crystal homopolymers and copolymers whose highly anisotropic azo-mesogenic pendant
groups may be favorable for the development of liquid crystal phases in broad temperature
ranges. A liquid crystal behavior is in turn favorable for high photo-induced alignments (large
birefringence), particularly for highly anisotropic azobenzene chromophores, like reported for
side chain LC polymers bearing long azotolane pendant moieties [15], whose -electrons are
delocalized all along this aromatic core [10].
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4. Conclusion
Four new functional methacrylate monomers carrying a biphenyleneazobenzene or an
azotolane group, substituted at both ends with alkyl chains (terminal chain and spacer) and a
lateral cyano group, were synthesized through several reactions, including the Suzuki-Miyaura
and Sonogashira reactions that allowed the coupling of aryl-aryl and aryl-ethynyl groups,
respectively. Most intermediates were obtained at relatively good yields (> 60 %), except for
some of the long final intermediates (yields of 25-30 %) whose purification was difficult. The 1H
NMR analysis confirmed the right synthesis of each one of the intermediates and final azo-
monomers. The combined DSC, POM and XRD results indicated that the new azo-monomers
develop nematic and/or smectic crystal phases, which are low viscous and low ordered liquid-like
mesophases. The smectic phases appeared in wide temperature intervals due to the strong -
interactions between the long azobenzene cores, particularly for monomers carrying a
biphenylene-azobenzene chromophore. The monomers here studied are precursors of functional
LC azopolymers that may show good photo-induced response.
Acknowledgements
We thank CONACYT – Mexico for the financial support provided under the projects 130902 and
242232, and the PhD grant (276107) given to OLTR. We also thank G. Méndez and M. Mata for
their helpful assistance in the DSC and XRD experiments, respectively.
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References
1. F. M Raymo, Adv. Mater. 14 (2002) 401.
2. L. Dhar, MRS Bulletin 31 (2006) 324.
3. A. Natansohn, P. Rochon, Chem Rev 102 (2002) 4139.
4. S. Hvilsted, C. Sánchez, R. Alcalá, J. Mater. Chem. 19 (2009) 6641.
5. A. S. Matharu, S. Jeeva, P. S. Ramanujan, Chem. Soc. Rev. 36 (2007) 1868.
6. R. Hagen, T. Bieringer, Adv. Mater. 13 (2001) 1805.
7. P. Forcén, L. Oriol, C. Sánchez, F. J. Rodríguez, R. Alcalá, S. Hvilsted, K Jankova, Eur.
Polym. J. 43 (2007) 3292.
8. O. L. Torres-Rocha, R. J. Rodríguez-González, L. Larios-López, G. Martínez-Ponce, C.
Solano, D. Navarro Rodríguez, Polym. Int. 63 (2014) 652.
9. C. Cojocariu, P. Rochon, Pure Appl. Chem. 76 (2004) 1479.
10. A. Shishido, Polym. J. 42 (2010) 525.
11. R. J. Rodríguez-González, L. Larios-López, D. Navarro-Rodríguez, Liq. Cryst. 38 (2011)
831.
12. S. Yoneyama, T. Yamamoto, O. Tsutsumi, A. Kanazawa, T. Shiono, T. Ikeda,
Macromolecules 35 (2002) 8751.
13. Z-C Miao, Y-M Zhang, Y-Z Zhao, D. Wang, Mol. Cryst. Liq. Cryst. 582 (2013) 98.
14. K. Okano, A. Shishido, O. Tsutsumi, T. Shiono, T. Ikeda, J. Mater. Chem. 15 (2005) 3395.
15. K. Okano, O. Tsutsumi, A. Shishido, T. Ikeda T, J. Am. Chem. Soc. 128 (2006) 15368.
16. T. García, L. Larios-López, R. J. Rodríguez-González, G. Martínez-Ponce, C. Solano, D.
Navarro-Rodríguez, Polymer 53 (2012) 2049.
17. J. Dave, C. B. Upasani, P. D. Patel, Mol. Cryst. Liq. Cryst. 533 (2010) 73.
29