Liquid-crystalline polymers bearing phenylene(azobenzene) moieties substituted with an electron-donor or electron-acceptor lateral group. Synthesis, mesomorphic behavior and photo-induced isomerization

Article abstract of DOI:10.1016/j.polymer.2012.03.024

Methacrylic polymers bearing phenylene(azobenzene) moieties substituted with a OCH₃ (electron-donor) or a CN (electron-acceptor) lateral group were synthesized and their liquid-crystalline (LC) and photo-induced isomerization properties were studied. The thermotropic LC properties were analyzed by DSC, POM, and XRD. All polymers showed a mesomorphic behavior that extends over wide temperature ranges (>100 °C) displaying smectic type mesophases. The effect of the lateral group was reflected in the packing of mesogens in the smectic phases; the polar interactions of the CN groups seem to counterbalance the hindering effects of this lateral group. On the other hand, the photoisomerization of all polymers was induced in dilute solution and films (spin coated and casted). All polymers photoisomerize relatively fast with the CN-substituted polymers reaching high cis-isomer contents (>95%) at the photostationary state. Light induced birefringence experiments indicated a tight interplay between the photo-induced orientation and the LC intrinsic order. Homogeneous sinusoidal surface relief gratings were obtained.

Full text of DOI:10.1016/j.polymer.2012.03.024

Polymer 53 (2012) 2049e2061
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Liquid-crystalline polymers bearing phenylene(azobenzene) moieties substituted
with an electron-donor or electron-acceptor lateral group. Synthesis,
mesomorphic behavior and photo-induced isomerization
Tonatiuh García a, Leticia Larios-López a, Rosa Julia Rodríguez-González a, Geminiano Martínez-Ponce b,
,
Cristina Solano b, Dámaso Navarro-Rodríguez a
*
a Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna H. 140, 25253 Saltillo Coahuila, México
b Centro de Investigaciones en Óptica A. C. Apdo. Postal 1-948, 37000 León Guanajuato, México
a r t i c l e i n f o
a b s t r a c t
Article history:
Methacrylic polymers bearing phenylene(azobenzene) moieties substituted with a OCH3 (electron-
donor) or a CN (electron-acceptor) lateral group were synthesized and their liquid-crystalline (LC) and
photo-induced isomerization properties were studied. The thermotropic LC properties were analyzed by
DSC, POM, and XRD. All polymers showed a mesomorphic behavior that extends over wide temperature
ranges (>100 ꢀC) displaying smectic type mesophases. The effect of the lateral group was reflected in the
packing of mesogens in the smectic phases; the polar interactions of the CN groups seem to counter-
balance the hindering effects of this lateral group. On the other hand, the photoisomerization of all
polymers was induced in dilute solution and films (spin coated and casted). All polymers photoisomerize
relatively fast with the CN-substituted polymers reaching high cis-isomer contents (>95%) at the pho-
tostationary state. Light induced birefringence experiments indicated a tight interplay between the
photo-induced orientation and the LC intrinsic order. Homogeneous sinusoidal surface relief gratings
were obtained.
Received 20 January 2012
Received in revised form
7 March 2012
Accepted 10 March 2012
Available online 16 March 2012
Keywords:
Azobenzene
Liquid crystal polymers
Photo-induced isomerization
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
In 1987, Wendorff et al. synthesized LC polymers bearing azo-
benzene mesogens whose cooperative motion (nanometric level)
Over the last 25 years, liquid-crystalline polymers (LCPs)
bearing azobenzene moieties have attracted much attention
because of synergistic effects produced by coupling the sponta-
neous molecular order of liquid crystals with the photo-sensitive
response of azocompounds [1]. The interplay between these two
physical properties has been of great interest for applications in
optics [2], photonics [3], photo-mechanics [4,5], among others
[6,7]. One of the main characteristics of azobenzenes is their clean
photoisomerization produced under a specific UVeVis irradiation
[8]. Depending on the material and irradiation conditions, the
trans 4 cis cyclic photoisomerization can induce motions at
molecular, nanometric (domain), and micrometric (mass motion)
levels [9].
lead to a significant molecular photo-alignment, which turned out
to be stable once the irradiation was stopped [10,11]. Moreover, the
attachment of azobenzene moieties to a polymer backbone gives
rise to a photo-induced mass migration, which at that time was
unknown and became a promising tool for the fabrication of
regular micro scale surface relief gratings (SRG) [12e14]. More
recently, several groups have synthesized new LCPs aiming to
improve the photo-induced properties taking advantage of the
intrinsic molecular order of liquid crystals [15e19]. It is noteworthy
that the higher birefringence values so far reported for azo-
compounds belong to LCPs. Ikeda et al reported in 2006 remarkably
high birefringence values (
chain azotolane moieties [20]. Such outstanding birefringence
values were associated not only with the long -conjugation of the
D
n ¼ 0.48e0.76) for LCPs bearing side
p
azotolanes but also to their LC order [11]. In these moieties, the
additional rigid group, which is directly linked to the azobenzene
moiety, maintains the linearity of the mesogen and increases the
lateral cohesive interaction between molecules in the mesophase.
This increased molecular anisotropy may induce a richer poly-
morphism, and higher thermal and mesophase stabilities. These
* Corresponding author. Tel.: þ52 844 4389846; fax: þ52 844 4389839.
E-mail addresses: tonatiuhgarcia@gmail.com (T. García), llarios@ciqa.mx
(L. Larios-López), rjrodrig@ciqa.mx (R.J. Rodríguez-González), geminis@cio.mx
(G. Martínez-Ponce), csolano@cio.mx (C. Solano), damaso@ciqa.mx (D. Navarro-
Rodríguez).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.03.024

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T. García et al. / Polymer 53 (2012) 2049e2061
effects were also demonstrated in azobenene derivatives
substituted with a biphenylene group, which display a mesomor-
phic behavior that extends over 200 ꢀC [21]. In spite of such
remarkable results, there are few reports on side-chain liquid-
crystalline polymers (SCLCPs) bearing highly anisotropic azo-
benzene mesogens and this is probably due to solubility (or pro-
cessing) problems. Rigid rodlike molecules are insoluble in most
purification. Tetrahydrofuran (THF) was dried over a sodium/
benzophenone complex and distilled prior to use.
2.2. Synthesis
Two groups of azopolymers were prepared from monomers
obtained following the synthetic route outlined in Scheme 1.
The first group corresponds to polymers having phenyl-
ene(azobenzene) moieties substituted with a lateral OCH3 group. In
these polymers the length of the spacer is 0, 4 and 6 methylene
groups, whereas the length of the tail (terminal chain) is kept
constant (12 carbon atoms). These polymers were named
pOCH312,0, pOCH312,4 and pOCH312,6, respectively. The second
group corresponds to polymers bearing phenylene(azobenzene)
moieties substituted with a lateral CN group. In these polymers the
length of the tail is 6, 10 or 12 carbon atoms, whereas the flexible
spacer is kept constant (6 methylene groups). These polymers were
named pCN6,6, pCN10,6 and pCN12,6, respectively.
common solvents and this is normally associated to the strong pep
lateral interactions that maintain the molecules close together. A
common practice to reduce such interactions is the introduction of
relatively small lateral groups like Cl, Br, OCH3, among others [22].
The presence of such lateral groups not only improves the solubility
of the rigid rodlike azobenzenes but also influences their meso-
morphic behavior [23]. Moreover, if such groups are polar in nature
they may have important effects on the spectroscopic properties [8]
as well as on the photoisomerization process [24]. In the present
work, the synthesis, the liquid-crystalline properties and the pho-
toisomerization of new side-chain liquid-crystalline polymers
bearing phenylene(azobenzene) moieties substituted with a OCH3
or a CN group is presented and discussed.
Intermediates 1aed, 3, 4a and 4b were synthesized according to
known procedures [21,24e26]. For the other precursors typical
procedures are presented below.
2. Experimental
2.2.1. 3-Cyano-4-(decyloxy)phenylboronic acid. (2b)
2.1. Materials
In a 250 mL three-neck round-bottom flask, equipped with two
addition funnels (100 mL) and septa, 3 g (8.87 mmol) of 1b were
dissolved in about 120 mL of freshly distilled THF. The solution was
cooled down to ꢁ78 ꢀC and then 6.2 mL (9.92 mmol) of BuLi in
hexanes 1.6 M was added dropwise keeping the temperature
below ꢁ70 ꢀC. The reaction mixture was stirred for 90 min and then
4.5 mL (19.57 mmol) of triisopropyl borate (TIPBO) were added
All reagents were purchased from Aldrich and used without
further purification unless otherwise noted. Azobisisobutyronitrile
(AIBN) was recrystallized from methanol (MeOH). Reactive grade
acetone, methanol, chloroform (CHCl3), and hexanes were
purchased from J. T. Baker and were used without further
Scheme 1. Synthesis route for the preparation of the phenylene(azobenzene) monomers.

T. García et al. / Polymer 53 (2012) 2049e2061
2051
dropwise. The reaction mixture was stirred overnight allowing the
2.2.4. 5-Bromo-2-(hexyloxy)benzonitrile. (1a)
temperature to increase up to room temperature. Next, 30.5 mL of
HCl 2 N were added and the solution was stirred for 90 min. The
solution was then transferred to a separation funnel where diethyl
ether was added. The organic layer was separated and washed with
water until neutral pH. The organic solvent was evaporated and the
product was suspended in cold diethyl ether. The suspension was
stirred for 1 h and then filtered and washed with diethyl ether. The
solid (2b) was obtained as a white powder (Yield: 67%). 1H NMR
White solid (Yield: 89%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
0.90 (t, CH3, 3H, J ¼ 6.77 Hz); 1.54e1.22 (m, CH2, 6H); 1.83 (p, CH2,
2H, J ¼ 6.6 Hz); 4.04 (t, CH2, 2H, J ¼ 6.5 Hz); 6.84 (d, CH, 1H,
J ¼ 9.06 Hz); 7.59 (dd, CH, 1H, J ¼ 2.8, 9.2 Hz) and 7.61 (d, CH, 1H,
J ¼ 9.5 Hz).
2.2.5. 5-bromo-2-(decyloxy)benzonitrile. (1b)
White solid (Yield: 80%). m.p. 41.4 ꢀC 1H NMR (300 MHz, CDCl3,
(300 MHz, THFed8,
d
¼ ppm): 0.88 (t, CH3, 3H, J ¼ 6.56 Hz);
d
¼ ppm): 0.87 (t, CH3, 3H, J ¼ 6.7 Hz); 1.47e1.26 (m, CH2, 14H); 1.84
1.54e1.29 (m, CH2, 12H); 1.83 (m, CH2, 2H); 4.11 (t, CH2, 2H,
J ¼ 6.47 Hz); 7.06 (d, CH, 1H, J ¼ 8.91 Hz); 7.28 (d, OH, 2H,
J ¼ 0.86 Hz) and 7.95 (m, CH, 2H).
(p, CH2, 2H J ¼ 7.1 Hz); 4.04 (t, CH2, 2H, J ¼ 6.5 Hz); 6.84 (d, CH, 1H,
J ¼ 8.95 Hz); 7.59 (dd, CH, 1H, J ¼ 2.5, 8.9 Hz) and 7.64 (d, CH, 1H,
J ¼ 2.5 Hz).
2.2.2. (E)-40-((4-(6-bromohexyloxy)phenyl)diazenyl)-4-(decyloxy)
biphenyl-3-carbonitrile. (CN10,6)
2.2.6. 5-Bromo-2-(dodecyloxy)benzonitrile. (1c)
White solid (Yield: 80%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
In a 250 mL two-neck round-bottom flask, 1.03 g (3.40 mmol) of
2b, 1.60 g (3.63 mmol) of 4b and 50 mL of THF (freshly distilled)
were introduced and the mixture was stirred to complete disso-
lution. Then, 0.1 g (0.09 mmol) of tetrakis(triphenylphosphine)
palladium(0) and 21.5 mL of Na2CO3 2 M were added to the solution
and the reaction was heated to 65 ꢀC and stirred overnight. Next,
the reaction was cooled to room temperature and the organic phase
was separated from the aqueous phase; the organic layer was then
washed with water until neutral pH. The crude product was passed
through a column chromatography on silica gel using CHCl3 as
eluent in order to remove the catalyst and some other residual
solids. The mesogen CN10,6 (recrystallized from acetone) was
obtained as an orange solid (Yield: 62%). 1H NMR (300 MHz, CDCl3,
0.87 (t, CH3, 3H, J ¼ 6.7 Hz); 1.53e1.18 (m, CH2, 18H); 1.84 (p, CH2,
2H, J ¼ 6.6 Hz); 4.04 (t, CH2, 2H, J ¼ 6.5 Hz); 6.84 (d, CH, 1H,
J ¼ 8.95 Hz); 7.59 (dd, CH, 1H, J ¼ 2.5, 8.9 Hz) and 7.64 (d, CH, 1H,
J ¼ 2.4 Hz).
2.2.7. 4-bromo-1-(dodecyloxy)-2-methoxybenzene. (1d)
White solid (Yield: 87%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm)
0.88 (t, CH3, 3H, J ¼ 6.4 Hz); 1.45e1.19 (m, CH2, 18H); 1.82 (p, CH2,
2H, J ¼ 6.9 Hz); 3.84 (s, CH3, 3H); 3.96 (t, CH2, 2H, J ¼ 6.9 Hz); 6.73
(d, CH, 1H, J ¼ 8.4 Hz), 6.98 (dd, CH, 1H, J ¼ 2.21, 4.68 Hz) and 7.01 (d,
CH, 1H, J ¼ 2.29 Hz).
2.2.8. 3-cyano-4-(hexyloxy)phenylboronic acid. (2a)
d
¼ ppm): 0.88 (t, CH3, 3H, J ¼ 6.71 Hz); 1.56e1.28 (m, CH2, 18H);
White powder (Yield: 70%). 1H NMR (300 MHz, THFed8,
1.94e1.80 (m, CH2, 6H); 3.44 (t, CH2, 2H, J ¼ 6.75 Hz); 4.06 (t, CH2,
2H, J ¼ 6.40 Hz); 4.12 (t, CH2, 2H, J ¼ 6.53 Hz); 7.02 (m, CH, 3H); 7.64
(m, CH, 2H); 7.84e7.77 (m, CH, 2H) and 7.97e7.91 (m, CH, 4H).
d
¼ ppm) 0.92 (t, CH3, 3H, J ¼ 7.11 Hz); 1.57e1.30 (m, CH2, 6H);
1.83 (p, CH2, 2H, J ¼ 6.74 Hz); 4.11 (t, CH2, 2H, J ¼ 6.48 Hz); 7.07 (d,
CH, 1H, J ¼ 8.93 Hz); 7.29 (s, OH, 2H) and 7.96 (m, CH, 2H).
2.2.3. (E)-6-(4-((30-cyano-40-(decyloxy)biphenyl-4-yl)diazenyl)
phenoxy)hexylmethacrylate. (MCN10,6)
2.2.9. 3-cyano-4-(dodecyloxy)phenylboronic acid. (2c)
White powder (Yield: 34%). 1H NMR (300 MHz, MeOHed4,
In a 250 mL three-neck round-bottom flask, 0.38 g (3.80 mmol)
of KHCO3, 0.37 mL/0.38 g (4.41 mmol) of methacrylic acid and
15 mL of DMF were introduced and stirred to complete dissolution.
Then, 1 g (1.62 mmol) of CN10,6 and 0.02 g (0.18 mmol) of hydro-
quinone were added to the solution. The reaction mixture was
heated to 80 ꢀC and stirred overnight. Next, the solution was cooled
down to room temperature and then 50 mL of H2O and 50 mL of
CHCl3 were added. The organic/aqueous phases were separated and
the organic layer was subsequently washed with aqueous solutions
of HCl 5%, K2CO3 5% and finally with water until neutral pH. The
organic layer was dried with Na2SO4 and the solvent was evapo-
rated. The monomer MCN10,6 was obtained as an orange solid
d
¼ ppm): 0.88 (t, CH3, 3H, J ¼ 6.75 Hz); 1.56e1.16 (m, CH2, 18H);
1.81 (m, CH2, 2H); 4.11 (t, CH2, 2H, J ¼ 6.36 Hz); 7.08 (d, CH, 1H,
J ¼ 8.41 Hz) and 7.92 (m, CH, 2H).
2.2.10. 3-methoxy-4-(dodecyloxy)phenylboronic acid. (2d)
White powder (Yield: 76%). 1H NMR (300 MHz, THFed8,
¼ ppm) 0.89 (t, CH3, 3H, J ¼ 6.67 Hz); 1.56e1.22 (m, CH2, 18H);
d
1.77 (m, CH2, 2H, J ¼ 6.70 Hz); 3.78 (s, CH3, 3H), 3.95 (t, CH2, 2H,
J ¼ 6.47 Hz); 6.83 (d, CH, 1H, J ¼ 7.89 Hz); 6.93 (d, OH, 2H); 7.30 (d,
CH, 1H, J ¼ 1.49 Hz) and 7.33 (dd, CH, 1H, J ¼ 1.43, 3.92 Hz).
2.2.11. (E)-4-((4-bromophenyl)diazenyl)phenol. (3)
(Yield: 69%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm) 0.88 (t, CH3, 3H,
Reddish solid (Yield: 74%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
J ¼ 6.53 Hz); 1.6e1.16 (m, 18H); 1.9e1.66 (m, 6H), 1.95 (s, CH3, 3H);
4.06 (t, CH2, 2H, J ¼ 6.74 Hz); 4.12 (t, CH2, 2H, J ¼ 6.02 Hz); 4.17 (t,
CH2, 2H, J ¼ 6.62 Hz); 5.55 (s, 1H, ]CH); 6.10 (s, 1H, ]CH); 7.03 (m,
CH, 3H); 7.64 (m, 2H), 7.84e7.77 (m, 2H); 7.94 (m, CH, 4H). FTeIR
(KBr, cmꢁ1) 2923 (s, nas CH2), 2854 (m, ns CH2), 2227 (w, CN
stretch), 1718 (m, C]O stretch), 1602, 1490, 1472 (m, C]C ring
stretch), 1290 (m, Ce(C]O)eO stretch), 1252 (m, CeOeC asym.
stretch), 1173 (m, OeCeC asym. stretch), 1011 (m, CeOeC sym.
stretch), 852, 817 (m, CeH out of plane bending). 13C NMR (75 MHz,
5.39 (s, OH, 1H); 6.95 (d, CH, 2H, J ¼ 8.82 Hz); 7.62 (d, CH, 2H,
J ¼ 8.71 Hz); 7.75 (d, CH, 2H, J ¼ 8.69 Hz), and 7.87 (d, CH, 2H,
J ¼ 8.82 Hz).
2.2.12. (E)-1-(4-(4-bromobutoxy)phenyl)-2-(4-bromophenyl)
diazene. (4a)
Orange solid (Yield: 79%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
2.15e1.94 (m, CH2, 4H); 3.51 (t, CH2, 2H, J ¼ 6.45 Hz); 4.09 (t, CH2,
2H, J ¼ 5.92 Hz); 6.99 (d, CH, 2H, J ¼ 9.10 Hz); 7.62 (d, CH, 2H,
J ¼ 8.88 Hz); 7.76 (d, CH, 2H, J ¼ 8.86 Hz) and 7.90 (d, CH, 2H,
J ¼ 9.08 Hz).
CDCl3,
d
¼ ppm): 167.6, 161.9, 160.6, 152, 147, 140.5, 136.6, 133.1,
132.9, 132.2, 127.3, 125.3, 125, 123.4, 116.4, 114.9, 112.8, 102.7, 69.5,
68.3, 64.7, 32, 29.6, 29.6, 29.4, 29.2, 29, 28.7, 26, 25.9, 25.8, 22.8,
18.4, 14.2.
2.2.13. (E)-1-(4-(6-bromohexyloxy)phenyl)-2-(4-bromophenyl)
The other intermediates and monomers were obtained by
similar methods. They were all characterized by 1H NMR as
described below.
diazene. (4b)
Orange solid (Yield: 68%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
1.53 (m, CH2, 4H); 1.91e1.82 (m, CH2, 4H); 3.44 (t, CH2, 2H,

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T. García et al. / Polymer 53 (2012) 2049e2061
J ¼ 6.75 Hz); 4.05 (t, CH2, 2H, J ¼ 6.38 Hz); 6.99 (d, CH, 2H,
J ¼ 9.05 Hz); 7.62 (d, CH, 2H, J ¼ 8.83 Hz); 7.76 (d, CH, 2H,
J ¼ 8.81 Hz) and 7.90 (d, CH, 2H, J ¼ 9.05 Hz).
1719 (m, C]O stretch), 1601, 1491, 1468 (m, C]C ring stretch), 1287
(m, Ce(C]O)eO stretch), 1254 (m, CeOeC asym. stretch), 1164 (m,
OeCeC asym. stretch), 1012 (m, CeOeC sym. stretch), 851, 816 (m,
CeH out of plane bending). 13C NMR (75 MHz, CDCl3,
d
¼ ppm):
2.2.14. (E)-40-((4-(6-bromohexyloxy)phenyl)diazenyl)-4-(hexyloxy)
167.6, 161.9, 160.6, 152.1, 147, 140.5, 136.6, 133.1, 132.9, 132.2, 127.3,
125.3, 125, 123.4, 116.4, 114.8, 112.8, 102.7, 69.5, 68.3, 64.7, 32,
29.8, 29.7, 29.7, 29.6, 29.5, 29.4, 29.2, 29, 28.7, 26, 25.9, 25.8, 22.8,
18.4, 14.2.
biphenyl-3-carbonitrile. (CN6,6)
Orange solid (Yield: 83%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
0.92 (t, CH3, 3H, J ¼ 7.03 Hz); 1.56e1.30 (m, CH2, 10H); 1.96e1.80 (m,
CH2, 6H); 3.44 (t, CH2, 2H, J ¼ 6.75 Hz); 4.05 (t, CH2, 2H, J ¼ 6.40 Hz);
4.12 (t, CH2, 2H, J ¼ 6.54 Hz); 7.02 (m, CH, 3H); 7.64 (m, CH, 2H); 7.80
(m, CH, 2H) and 7.94 (m, CH, 4H).
2.2.20. (E)-4-((40-(dodecyloxy)-30-methoxy-[1,10-biphenyl]-4-yl)
diazenyl)phenol. (OCH312,0)
This compound was prepared from 3 and 2d following the same
procedure as for compound CN10,6. The mesogen OCH312,0,
recrystallized from acetone, was obtained as an orange solid (Yield:
2.2.15. (E)-40-((4-(6-bromohexyloxy)phenyl)diazenyl)-4-
(dodecyloxy)biphenyl-3-carbonitrile. (CN12,6)
Orange solid (Yield: 70%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
64%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm): 0.83 (t, CH3, 3H,
0.88 (t, CH3, 3H, J ¼ 6.70 Hz); 1.58e1.20 (m, CH2, 22H); 1.96e1.80
(m, CH2, 6H); 3.44 (t, CH2, 2H, J ¼ 6.75 Hz); 4.06 (t, CH2, 2H,
J ¼ 6.38 Hz); 4.12 (t, CH2, 2H, J ¼ 6.55 Hz); 7.02 (m, CH, 3H); 7.64 (m,
CH, 2H); 7.80 (m, CH, 2H) and 7.94 (m, CH, 4H).
J ¼ 6.70 Hz); 1.5e1.10 (m, CH2, 18H); 1.82 (m, CH2, 2H); 3.91 (s, CH3,
3H); 4.02 (t, CH2, 2H, J ¼ 6.90 Hz); 6.98 (m, CH, 3H); 7.14 (m, CH,
2H); 7.64 (m, CH, 2H) and 7.84 (m, CH, 4H).
2.2.21. (E)-(4-((40-dodecyloxy)-30-methoxybiphenyl-4-yl)diazenyl)
phenyl)methacrylate. (MOCH312,0)
2.2.16. (E)-1-(4-(4-bromobutoxy)phenyl)-2-(40-(dodecyloxy)-30-
methoxybiphenyl-4-yl)diazene. (OCH312,4)
In a 100 mL two-neck round-bottom flask, 0.87 g (1.78 mmol) of
OCH312,0 and 40 mL of THF (freshly distilled) were introduced and
the mixture was cooled to T ¼ 0e5 ꢀC. Then 0.36 g (3.56 mmol) of
triethylamine and 0.24 g (2.30 mmol) of methacryloyl chloride
were added dropwise. The reaction temperature was allowed to
increase gradually up to room temperature and stirred overnight.
Then, 30 mL of CH2Cl2 and 30 mL of H2O were added and the
organic phase was separated from the aqueous phase; the organic
layer was subsequently washed with aqueous solutions of HCl 5%,
K2CO3 5% and finally with water until neutral pH. The organic layer
was dried with Na2SO4 and the solvent was evaporated. The
monomer MOCH312,0 (recrystallized from acetone) was obtained
Orange solid (Yield: 73%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
0.88 (t, CH3, 3H, J ¼ 6.70 Hz); 1.56e1.16 (m, CH2, 18H); 2.2e1.80 (m,
CH2, 6H); 3.51 (t, CH2, 2H, J ¼ 6.49 Hz); 3.96 (s, CH3, 3H); 4.07 (m,
CH2, 4H); 6.98 (m, CH, 3H); 7.21 (m, CH, 2H); 7.69 (d, CH, 2H,
J ¼ 8.43 Hz) and 7.93 (m, CH, 4H).
2.2.17. (E)-1-(4-(6-bromohexyloxy)phenyl)-2-(40-(dodecyloxy)-30-
methoxybiphenyl-4-yl)diazene. (OCH312,6)
Orange solid (Yield: 86%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
0.88 (t, CH3, 3H, J ¼ 6.74 Hz); 1.57e1.20 (m, CH2, 22H); 1.96e1.79 (m,
CH2, 6H); 3.44 (t, CH2, 2H, J ¼ 6.75 Hz); 3.96 (s, CH3, 3H); 4.05 (t,
CH2, 2H, J ¼ 6.32 Hz); 4.07 (t, CH2, 2H, J ¼ 6.88 Hz); 6.99 (m, CH, 3H);
7.20 (m, CH, 2H); 7.69 (d, CH, 2H, J ¼ 8.6 Hz) and 7.93 (m, CH, 4H).
as an orange solid (Yield: 77%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
0.88 (t, CH3, 3H, J ¼ 6.71 Hz); 1.53e1.18 (m, 18H); 1.88 (m, 2H); 2.09
(dd, CH3, 3H, J ¼ 0.95, 1.47 Hz); 4.04 (s, CH3, 3H); 4.07 (t, CH2, 2H,
J ¼ 6.9 Hz); 5.8 (m, 1H, ¼CH); 6.40 (m, 1H, ¼CH); 6.97 (d, CH, 1H,
J ¼ 8.31 Hz); 7.18 (d, CH, 1H, J ¼ 2.08 Hz); 7.21 (dd, CH, 2H, J ¼ 2.14,
8.24 Hz); 7.29 (d, CH, 2H, J ¼ 9.03 Hz); 7.71 (d, CH, 2H, J ¼ 8.82 Hz)
and 7.98 (m, CH, 4H). FTeIR (KBr, cmꢁ1) 2919 (s, nas CH2), 2852 (m,
ns CH2), 1742 (m, C]O stretch), 1592, 1493, 1467 (m, C]C ring
stretch), 1257 (m, CeOeC asym. stretch), 1202 (m, Ce(C]O)eO
stretch), 1151 (m, OeCeC asym. stretching), 1122 (m, OeCeC
asym. stretch), 1024 (m, CeOeC sym. stretch), 844, 802 (m, CeH
2.2.18. (E)-6-(4-((30-cyano-40-(hexyloxy)biphenyl-4-yl)diazenyl)
phenoxy)hexyl methacrylate. (MCN6,6)
Orange solid (Yield: 84%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
0.92 (t, CH3, 3H, J ¼ 7.04 Hz); 1.60e1.3 (m, 10H); 1.93e1.68 (m, 6H);
1.94 (dd, CH3, 3H, J ¼ 0.71, 1.75 Hz); 4.05 (t, CH2, 2H, J ¼ 6.43 Hz);
4.12 (t, CH2, 2H, J ¼ 6.54 Hz); 4.17 (t, CH2, 2H, J ¼ 6.61 Hz); 5.55 (p,
1H, ]CH, J ¼ 1.61 Hz); 6.10 (m, 1H, ]CH); 7.00 (d, CH, 2H,
J ¼ 8.98 Hz); 7.04 (d, CH, 1H, J ¼ 8.87 Hz); 7.64 (d, 2H, J ¼ 8.44 Hz),
7.79 (dd, CH, 1H, J ¼ 2.38, 8.75 Hz), 7.84 (d, CH, 1H, J ¼ 2.35 Hz); 7.94
(dd, CH, 4H, J ¼ 6.94, 8.67 Hz). FTeIR (KBr, cmꢁ1) 2939 (s, nas CH2),
2869 (m, ns CH2), 2229 (w, CN stretch),1703 (m, C]O stretch),1603,
1490, 1475 (m, C]C ring stretch), 1278 (m, Ce(C]O)eO stretch),
1260 (m, CeOeC asym. stretch), 1174 (m, OeCeC asym. stretch),
1018 (m, CeOeC sym. stretch), 849, 818, 802 (m, CeH out of plane
out of plane bending). 13C NMR (75 MHz, CDCl3,
153, 151.4, 150.4, 149.8, 149, 143.8, 135.8, 133, 127.8, 127.4, 124.1,
123.5, 122.4, 119.8, 113.2, 110.9, 69.2, 55.3, 32, 29.8, 29.7, 29.7, 29.7,
29.5, 29.5, 29.3, 26.1, 22.8, 18.5, 14.2.
d
¼ ppm): 165.6,
2.2.22. (E)-4-(4-((40-(dodecyloxy)-30-methoxybiphenyl-4-yl)
bending). 13C NMR (75 MHz, CDCl3,
152.1, 147, 140.5, 136.6, 133.1, 132.9, 132.2, 127.3, 125.34, 125, 123.4,
116.4, 114.8, 112.8, 102.7, 69.5, 68.3, 64.7, 31.6, 29.2, 29, 28.7, 25.9,
25.8, 25.6, 22.6, 18.4, 14.1.
d
¼ ppm): 167.6, 161.9, 160.6,
diazenyl)phenoxy)butylmethacrylate. (MOCH312,4)
Orange solid (Yield: 89%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
0.88 (t, CH3, 3H, J ¼ 6.65 Hz); 1.52e1.2 (m, 18H); 1.94e1.81 (m, 6H),
1.95 (m, 3H); 4.07 (t, CH2, 2H, J ¼ 6.95 Hz); 4.09 (t, CH2, 2H,
J ¼ 5.58 Hz); 4.25 (t, CH2, 2H, J ¼ 5.91 Hz); 5.56 (p, 1H, ]CH,
J ¼ 1.61 Hz); 6.12 (m, 1H, ]CH); 6.96 (d, CH, 1H, J ¼ 8.36 Hz); 7.01 (d,
CH, 2H, J ¼ 9.06 Hz); 7.19 (m, CH, 3H); 7.69 (d, 2H, J ¼ 8.74 Hz), 7.93
(dd, CH, 4H, J ¼ 3.68, 8.85 Hz). FTeIR (KBr, cmꢁ1) 2919 (s, nas CH2),
2852 (m, ns CH2), 1718 (m, C]O stretch), 1594, 1497, 1468 (m, C]C
ring stretch), 1255 (m, CeOeC asym. stretch), 1155 (m, OeCeC
asym. stretch), 1024 (m, CeOeC sym. stretch), 846, 805 (m, CeH
2.2.19. (E)-6-(4-((30-cyano-40-(dodecyloxy)biphenyl-4-yl)diazenyl)
phenoxy)hexyl methacrylate. (MCN12,6)
Orange solid (Yield: 62%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm):
0.88 (t, CH3, 3H, J ¼ 6.5 Hz); 1.56e1.2 (m, 22H); 1.93e1.68 (m, 6H);
1.95 (dd, CH3, 3H, J ¼ 0.99, 1.58 Hz); 4.05 (t, CH2, 2H, J ¼ 6.42 Hz);
4.12 (t, CH2, 2H, J ¼ 6.55 Hz); 4.17 (t, CH2, 2H, J ¼ 6.6 Hz); 5.55 (p, 1H,
]CH, J ¼ 1.59, 1.60 Hz); 6.10 (qd, 1H, ]CH, J ¼ 0.96, 1.95 Hz); 7.02
(m, CH, 3H); 7.64 (m, 2H), 7.79 (dd, CH, 1H, J ¼ 2.40, 8.76 Hz), 7.84 (d,
CH, 1H, J ¼ 2.23 Hz) and 7.94 (dd, CH, 4H, J ¼ 6.97, 8.90 Hz). FTeIR
(KBr, cmꢁ1) 2924 (s, nas CH2), 2853 (m, ns CH2), 2227 (w, CN stretch),
out of plane bending). 13C NMR (75 MHz, CDCl3,
161.6, 151.5, 149.8, 148.9, 147.1, 143.1, 136.5, 133.1, 127.4, 125.5, 124.9,
123.2, 119.8, 114.8, 113.2, 110.9, 100, 69.2, 67.5, 64.4, 56.3, 32.0, 29.8,
29.7, 29.7, 29.7, 29.5, 29.5, 29.3, 26.1, 26, 25.5, 22.8, 18.4, 14.2.
d
¼ ppm): 167.5,

T. García et al. / Polymer 53 (2012) 2049e2061
2053
2.2.23. (E)-6-(4-((40-dodecyloxy)-30-methoxybiphenyl-4-yl)
intervals until the photostationary state was reached. For irradia-
tion experiments, a 366 nm non-polarized handheld UV lamp
(UVGL-58) was used.
diazenyl)phenoxy)hexylmethacrylate. (MOCH312,6)
Orange solid (Yield: 83%). 1H NMR (300 MHz, CDCl3,
d
¼ ppm)
0.88 (t, CH3, 3H J ¼ 6.68 Hz); 1.92e1.18 (m, 28H); 3.96 (s, CH2, 2H);
4.06 (m, CH2, 4H); 4.17 (t, CH2, 2H, J ¼ 6.59 Hz); 5.55 (s, 1H, ]CH);
6.10 (s, 1H, ]CH); 6.98 (m, CH, 3H); 7.19 (m, 2H); 7.69 (m, 2H) and
7.93 (m, CH, 4H). FTeIR (KBr, cmꢁ1) 2923 (s, nas CH2), 2853 (m, ns
CH2), 1719 (m, C]O stretch),1594,1497,1467 (m, C]C ring stretch),
1260 (s, CeOeC asym. stretch), 1152 (s, OeCeC asym. stretch), 1024
(m, CeOeC sym. stretch), 846, 804 (m, CeH out of plane bending)
2.3.1. Films preparation
Thin films (300e450 nm) were prepared by spin-coating as
follows: 50
mL of a polymer solution (40 mg/mL in CHCl3) was
deposited onto a clean glass substrate at a spinning rate of
800e2000 rpm. The film was spun for 60 s and then dried over-
night in a vacuum oven at 35 ꢀC. For films obtained by casting
13C NMR (75 MHz, CDCl3,
d
¼ ppm): 167.6, 161.7, 151.5, 149.8, 148.8,
(3e15 mm), 80e120 mL of a polymer solution (40 mg/mL in CHCl3)
147, 143.1, 136.6, 133.2, 127.4, 125.3, 124.9, 123.2, 119.7, 114.8, 113.2,
110.9, 100, 69.2, 68.2, 64.7, 55.3, 32, 29.8, 29.7, 29.7, 29.7, 29.5, 29.5,
29.3, 29.2, 28.7, 26.1, 25.9, 25.8, 22.8, 18.4, 14.2.
were deposited onto a glass substrate and then dried overnight at
35 ꢀC in a vacuum oven. The film thickness of both spin coated and
casted films was measured by using an AFM-3100 (DI-Veeco) and
a Surfanalyzer 4000 (Federal), respectively.
All polymers were obtained via free-radical polymerization
using AIBN as initiator. A typical procedure is described below.
2.3.2. Photo-induced birefringence
2.2.24. pCN10,6
Spin coated films were irradiated with a linearly polarized
In a Schlenk tube 0.49 g (0.79 mmol) of the monomer MCN10,6,
0.013 g (0.08 mmol) of AIBN and 4 mL of freshly distilled THF were
introduced and the solution was degassed three times by freeze-
pump-thaw cycles. The Schlenk tube was closed and heated to
65 ꢀC for 48 h. The obtained polymer was purified by subsequent
precipitation with methanol, re-dissolved in CHCl3 and re-
precipitated in hexanes. The polymer pCN10,6 was obtained as
a yellow powder (Yield: 72%).
semiconductor laser beam (
l
¼ 413 nm, 20 mW). At the same time
a laser beam at 633 nm (25 mW) was used as a probe and the
change in the polarization state was measured with a Stokes
polarimeter using the equation:
ꢀ
ꢁ
l
p
Vo
Uo
D
n ¼
tanꢁ1
(1)
2
d
where
l
is the wavelength of the laser probe (633 nm), d is the film
Some polymers were re-dissolved in CHCl3 and re-precipitated
with hexanes, acetone and/or methanol. All final polymers
showed good purity with yields ranging between 68 and 79%.
thickness, Vo and Uo are the third and fourth Stokes parameters,
which represent the components of linear polarized light at ꢂ45ꢀ
and the left/right circular polarized light, respectively.
2.3. Characterization
2.3.3. Surface relief gratings (SRG)
Spin casted films were irradiated with an interference pattern
employing a Lloyd arrangement. A 413 nm laser was split into two
equal intensity beams but with a different polarization state (right/
left circularly polarized light). The two beams hit the sample in the
same spot with an incidence angle of 4ꢀ. The irradiation time for all
samples was 30 min. The diffraction efficiency was calculated as the
ratio between the incident light (633 nm) and the first order dif-
fracted beam. AFM images were captured in an AFM-3100 from DI-
Veeco.
The average molecular weights (Mn and Mw) and polydispersity
(Mw/Mn) of polymers were determined by size exclusion chroma-
tography using a HewlettePackard instrument (HPLC 1100 series)
equipped with a refractive index detector. A series of PLGel columns
covering a wide range of molecular weights (103e107) were used.
Tetrahydrofuran HPLC-grade (Aldrich) was used as solvent and as
eluent. Monodisperse polystyrene standards were used for cali-
bration. The chemical structure of synthesized products was veri-
fied by 1H NMR and 13C NMR using a 300 MHz Jeol spectrometer
(JNM-ECO300) and CDCl3, MeOHed4, or THFed8 as solvents. The
thermal stability of compounds was determined on vacuum dried
samples and using a thermal analyzer from Dupont instruments
(TGA 951) connected to a nitrogen vector gas and heating at a rate
of 10 ꢀC/min from 30 to 800 ꢀC. DSC traces were obtained in
a differential scanning calorimeter from TA instruments (MDSC
2920). The scanning rate was 10 ꢀC/min and heatingecooling cycles
range from ꢁ20e250 ꢀC; reported data correspond to the second
heating scans. Optical textures of mesophases were captured upon
cooling using a polarized optical microscope from Olympus (BX60)
coupled with a temperature controlling hot stage from Mettler
(FP82HT) and a digital camera from Hitachi. X-ray diffraction
analysis was performed in an 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). 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. X-rays (Cu ka
radiation; lmax ¼ 0.1542 nm) were generated at 40 kV and 50 mA.
X-ray patterns were captured at different temperatures upon
cooling from the isotropization temperature. UVeVis spectra were
recorded in a spectrophotometer from Shimadzu (UV-2401PC)
using a standard quartz cell. All polymers were dissolved in spec-
trophotometric grade THF (0.05 mg/mL). The photoisomerization
was studied by irradiating the polymer solutions and films at time
3. Results and discussion
3.1. Synthesis
All azopolymers were synthesized by free-radical polymeriza-
tion using AIBN as initiator. The number (Mn) and weight (Mw)
average molecular weights, the Mw/Mn, the temperature of 5 wt%
loss (T5%), and the glass transition temperature (Tg) of each azo-
polymer are summarized in Table 1. The Mn ranged between 12,000
and 24,000 g/mol and the Mw/Mn between 2.11 and 2.61, which are
Table 1
Molecular weights and thermal behavior for the azopolymers.
a
b
Polymer
Mn (g/mol)
Mw/Mn
T5% (ꢀC)
Tg (ꢀC)
pCN6,6
pCN10,6
pCN12,6
pOCH312,6
pOCH312,4
pOCH312,0
23,677
23,790
21,726
12,160
15,459
20,562
2.23
2.40
2.39
2.11
2.61
2.52
328
335
329
337
331
325
60
65
62
57
69
e
a
Determined by gel permeation chromatography (GPC) in THF on the basis of
a polystyrene calibration.
b
Measured in the second heating cycle.

2054
T. García et al. / Polymer 53 (2012) 2049e2061
typical values for this kind of polymers synthesized via free-radical
polymerization [27]. These relatively low molecular weights are in
fact desirable for a good solubility, which is necessary for the
preparation of high quality thin films from solution. The thermal
stability (T5%) for all azopolymers was above 300 ꢀC, which is much
higher than that reported for acrylic SCLCPs bearing azobenzene
moieties (around 200 ꢀC) [28]. This increase in the thermal stability
is due to the presence of one extra phenylene ring in the azo-
compound as reported by Rodríguez-González et al [21]. The high
T5% values allowed us to study the thermotropic behavior of azo-
polymers at relatively high temperatures.
shown in Figs. 2 and 3, respectively. In viscous polymer liquids it is
sometimes difficult to observe identifiable typical textures, partic-
ularly for some smectic type mesophases; rather, a fine granulated
(or sanded-like) texture of birefringent domains is frequently seen
between crossed polars [29]. Focal-conic fan textures were
preceded by the presence of bâtonnets that appear at the isotropic
to the liquid-crystalline transition (not shown here) and that grow
up to coalesce. For pOCH312,6, the expected transition around
120 ꢀC (according to DSC) was not observed by POM; only a smooth
change in color (Fig. 3) was perceived probably associated to
a phase transition, although such change in color could be simply
due to a variation in the lamellar spacing as reported by Goodby
and Gray for liquid crystals with tilted molecules within the mes-
ophase [30].
3.2. Thermotropic behavior
The nature of mesophases for all the polymers was confirmed by
X-ray diffraction (XRD) analysis. X-ray diffraction patterns,
captured upon cooling, showed in the small angle region one or
two equidistant sharp Bragg reflections (d001 and d002), indicating
a lamellar arrangement, which is typical for smectic phases (Fig. 4).
Also, all polymers displayed only a single broad diffraction peak
(centered around 4.3 Å) in the wide angles region of the X-ray
patterns confirming the presence of a non-ordered smectic phase;
the X-ray patterns of the polymer pOCH312,6 were selected as
examples (Fig. 5). It is noteworthy that for all the polymers such
broad peak remained unchanged up to room temperature meaning
that the smectic order is maintained below Tg. Fixed mesophases in
glassy polymers is desirable, particularly for liquid crystals bearing
azo groups whose intrinsic order can amplify the light induced
effects.
To elucidate the packing of mesogens in the mesophase, the
experimental interlayer distance (or smectic period) d001 was
compared to the lamellar spacing L calculated using molecular
modeling software from Spartan 2010. L is the distance between an
H-atom of the end-methyl group of the mesogen (full extended)
and the carbon atom to which such mesogen is linked to the
polymer chain. The d/L ratios for all smectic phases are presented in
Table 2. For pCN6,6, the d/L ratio is close to unity indicating a single
layer packing of mesogens whereas for the rest of polymers such
ratio is around 1.5, indicating either a double layer packing with
mesogens tilted with respect to the normal of the smectic plane or
with mesogens perpendicular to the smectic plane but partially
interdigitated the ones to the others. On the other hand, the
dependence of the interlayer distance with temperature provides
a deep insight into the structure of the smectic phase. This struc-
tural parameter may remain constant within the mesophase range
or it can change for instance as a result of variations in the tilt angle
[30]. The dependence of the interlayer distance with temperature
for all the polymers is presented in Fig. 6. This figure shows that the
interlayer distance for polymers bearing CN groups remains almost
constant whereas for polymers bearing OCH3 groups such distance
decreases upon cooling. Taking into account these differences, and
considering some related works, three models for molecular
packing are proposed.
Fig. 1 shows the DSC thermograms for all synthesized azopol-
ymers. In some of the curves a second order thermal transition,
indicative of a glass transition temperature (Tg), is clearly seen. For
some other curves (transition barely perceived) the Tg is indicated
between dashed lines in Fig. 1; its corresponding value is shown in
Table 1. For pOCH312,0 it was not possible to detect a Tg even if it is
brittle at room temperature. It can also be noted that all polymers
(except pOCH312,0) show a first order thermal transition above
220 ꢀC corresponding to the liquid-crystalline to the isotropic liquid
transition (Ti), as confirmed by POM. Upon cooling to room
temperature no crystallization was observed (DSC traces not shown
here) meaning that the mesophase remained fixed below Tg. On the
other hand, polymers bearing OCH3 groups showed differences in
the mesomorphic behavior depending on the length of the flexible
spacer: pOCH312,0 (no spacer) showed no thermal transitions,
pOCH312,4 displayed one first-order thermal transition around
230 ꢀC corresponding to the Ti, and pOCH312,6 developed two first
order thermal transitions, one at 120 ꢀC and the other around
220 ꢀC (Ti). These results confirm the decoupling effect exerted by
the flexible spacer allowing the mesogens to self-organize in
a liquid crystalline state [29].
3.3. Nature and structure of mesophases
All polymers were studied by POM to get a first approach of the
possible nature of mesophases. Polymers bearing CN groups
exhibited focal-conic fan and sanded-like textures, whereas those
bearing OCH3 groups displayed only focal-conic fan textures as
For the polymer pCN6,6, a d/L z 1 suggests a single layer
packing with mesogens fully interdigitated and oriented on average
perpendicular to the smectic plane (SmA1). Such an arrangement
assures an optimum interaction between different moieties (Fig. 7A
and B). In this antiparallel packing not only the alkylealkyl and
aromaticearomatic lateral interactions are satisfied but also the
molecular dipole moment interaction between rigid cores (polar-
ized by the CN group) is energetically favorable, in such a way that
the dipolar energy is minimized. A similar fully interdigitated
monolayer structure with antiparallel arrangement of mesogens
was proposed by Laus et al for SCLCPs containing alkyl sulfide
substituents, for which the smectic interlayer spacing (d) was
Fig. 1. DSC thermograms (second heating scan) of azopolymers at a scan rate of 10 ꢀC/
min g ¼ glassy state, Tg ¼ glass transition temperature, LC ¼ liquid-crystalline region,
I ¼ isotropic state.

T. García et al. / Polymer 53 (2012) 2049e2061
2055
Fig. 2. Mesomorphic textures for the polymers bearing CN groups (A) pCN6,6 (200 ꢀC) and (B) pCN10,6 (223 ꢀC) observed on cooling from their isotropic state.
constant in the all temperature range, and in close agreement with
the length of the respective side chains [31].
The obtained experimental d value for the polymer pCN10,6 is
barely 59 Å, suggesting a compactness of the smectic layers
(Fig. 9B). For an orthogonal smectic mesophase, a 7 Å difference
may be solely explained as a consequence of either a deeper
interpenetration of mesogens or a coiling of both the spacer and the
terminal chain (tail). The former seems more plausible under the
following arguments. It was reported that molecular dipoles play
a significant role in crystal packing but they are not decisive if other
types of interactions like bond dipole-bond dipole interactions are
involved (a bond dipole is a dipole moment associated with a given
bond). In this polymer the bond dipole-bond dipole interaction
occurring between CN groups is thought to be the driving force for
a deeper interdigitation of mesogens even if it implies a partial
overlay of alkyl chains and aromatic groups. It is noteworthy that
among the bond dipole moments one of the largest is that of the
cyano group [34]. Although unusual, molecular arrangements with
alkyl chains and aromatic groups facing each other can occur; the
only condition is to minimize the repulsions between all different
parts of the mesogens [35,36]. The involved forces for a smectic
stacking are the mutual effective repulsions between alkyl chains
and aromatic cores, and the bond dipole-bond dipole interaction
For the polymers pOCH312,4 and pOCH312,6, a d/L z 1.5 along
with an important variation in the interlayer distance (Fig. 6)
implies a bilayer smectic packing with tilted molecules (SmC2) as is
schematically represented in Fig. 8. This significant change in d or in
the tilt angle appears to concur with the color change previously
observed by POM (Fig. 3). A similar variation in the interlayer
distance was observed in liquid-crystalline methacrylic polymers
with
u-hexyloxysalicylaldimine side-groups carrying p-alkyl
chains, for which an SmC2 structure was also proposed [32,33].
Finally, for the polymers pCN10,6 and pCN12,6 a d/L z 1.5
together with an interlayer distance practically invariable in all the
temperature range (250e80 ꢀC) suggests a bilayer smectic packing
with mesogens partially interdigitated and orthogonal to the
smectic plane (SmAd), as represented in Fig. 9. The existence of an
interdigitated mesophase deserves an explanation in more detail.
Let us explain the case of the azopolymer pCN10,6. For a partial
interdigitation of mesogens one would expect a full interaction
between adjacent terminal alkyl chains, however such an
arrangement implies an interlayer distance of around 66 Å (Fig. 9A).
Fig. 3. Mesomorphic textures for the polymers bearing OCH3 groups (A) pOCH312,6 (180 ꢀC) (B) pOCH312,6 (89 ꢀC) (C) pOCH312,4 (200 ꢀC) (D) pOCH312,4 (120 ꢀC) observed on
cooling from their isotropic state.

2056
T. García et al. / Polymer 53 (2012) 2049e2061
Table 2
Structural parameters for the azopolymers determined from SWAXS.
Compound. L (Å)a T (ꢀC). d001 (Å). d001/L. Tilt angleb Mesophase.c
pCN6,6
pCN10,6
pCN12,6
35.2
39.4
42.3
200
200
200
100
200
160
200
33.3
58.8
65.3
55.2
60.3
55.1
59.5
0.946
1.492
1.544
1.311 49.0
1.432 44.3
1.402 45.5
1.514 40.8
e
e
e
g 60 SmA1 244 I
g 65 SmAd 228 I
g 62 SmAd 224 I
g 57 SmC2 220 I
pOCH312,6 42.1
pOCH312,4 39.3
g 77 SmC2 233 I
a
Measured from molecular modeling using Spartan 2010.
b
c
Calculated from
q
¼ cosꢁ1(d/2L).
g ¼ glassy state, SmA1 ¼ Smectic A phase monolayer, SmAd ¼ Smectic A phase
interdigitated, SmC2 ¼ Smectic C phase bilayer, I ¼ isotropic state.
3.4.1. UVeVis absorption
The optical properties obtained from the UVeVis spectra (no
shown here) are summarized in Table 3. The reported unsub-
stituted phenylenazobenzene displays one strong (lmax ¼ 372 nm)
Fig. 4. XRD patterns for the azopolymers in the small angle region. Polymers were
heated near the isotropization temperature and patterns were taken at 200 ꢀC on
cooling.
and one weak (lmax ¼ 460 nm) absorption bands due to the
pe
p*
and nep* electronic transitions, respectively [21]. For polymers
bearing CN groups the corresponding bands appear at 368 and
460 nm whereas for those with OCH3 groups both absorption bands
overlap to each other and only one lmax at 375 nm is observed. As
noted, for polymers bearing CN groups the strong absorption band
is blue shifted with respect to the unsubstituted phenyl-
eneazobenzene whereas for polymers bearing OCH3 groups it is red
shifted. This different behaviors come out from the opposite elec-
tronic affinity of the CN and OCH3 groups, the former being an
electron-withdrawing and the latter an electron-donor group.
The peak shift for both types of polymers was relatively small
probably due to the fact that the lateral group is substituted into
the second phenyl ring far apart from the azo group. Only the
polymer pOCH312,0 showed an opposite behavior (blue shift,
lmax ¼ 368 nm) compared to the other polymers of the same series
and it is certainly related to the electron withdrawal character of
the methacrylate group, which is directly linked to the azobenzene.
On the other hand, all polymers showed a molar absorptivity
coefficient for the trans isomer (εtrans) falling in the range of
between cyano groups (antiparallel pairs). In this case, the strong
interactions between CN groups seem to retain them relatively
close to each other, reducing in this way the length of the smectic
period. Interdigitated smectic phases have been reported in poly-
mers bearing mesogens substituted with polar groups, where the
antiparallel pairs seem to dominate the lamellar stacking [36e39].
To conclude this structural analysis, pOCH312,6 is compared to
its counterpart pCN12,6. The only difference between these two
polymers is the nature of the lateral group, one being electron-
donor and the other electron-withdrawing group. This difference
in electronic properties seems to be at the origin of the observed
dissimilar molecular arrangement. The CN groups induce dipolar
interactions between mesogens leading to interdigitated smectic
layers, whereas the OCH3 groups push out each other resulting in
a segregation of mesogens from one smectic layer to the other,
which in this case has conducted to tilted mesophases.
105e106
L
molꢁ1cmꢁ1 with values for the CN-substituted
3.4. UVeVis optical properties
polymers doubling those of the OCH3 substituted ones. It should
be said that these values are much higher than those reported
(104 L molꢁ1cmꢁ1) for polymers bearing highly anisotropic azoto-
lane mesogens for which high photo-induced birefringence values
Azobenzene derivatives are well known by their light-induced
properties, which essentially depend on their ability to photo-
isomerize under the effect of light irradiation. Before irradiating
the synthesized azopolymers, it is important to explore their light
absorption characteristics.
Fig. 5. XRD patterns for the azopolymer pOCH312,6 at different temperatures on
Fig. 6. Interlayer distance e temperature dependence for azopolymers in the smectic
cooling from the isotropic state.
phase on cooling from their isotropic state. The solid lines are drawn as a guide.

T. García et al. / Polymer 53 (2012) 2049e2061
2057
Fig. 7. Schematic representation for the monolayer stacking (SmA1) of pCN6,6. (A) 3-D view. (B) Lateral view (solid circles represent the polymer chains viewed end-on).
progressive decrease in the intensity of the strong
pep* absorption
band and a slow increase of the weak ne * absorption band up to
p
a photostationary state (pss). The presence of isosbestic points
confirms that distinct absorbing species are in equilibrium with
each other and indicate that no degradation or side reactions take
place during the photoisomerization experiments. Polymers
bearing CN groups reached the pss in shorter time intervals
compared to those bearing OCH3 groups (Table 3). In this kind of
polymers one would intuitively expect a decrease in the photo-
isomerization rate as the length of the terminal chain increases,
however this was not the case as the rate of the photoisomerization
for pCN(6,6) and pCN(12,6) was practically the same, both reaching
cis isomer contents of around 95%. Conversely, the effect of the
flexible spacer was clearly evidenced by the strong differences
found not only in the photoisomerization rate, but also in the cis
isomer contents (%cis) at the pss. For the polymer with no spacer
(pOCH312,0) the elapsed time to attain the pss was 2.5 times that
corresponding to the polymer with the longest spacer (pOCH312,6),
the latter reaching twice the content of the cis isomer. This con-
trasting result arises from the fact that the azobenzene moiety in
the polymer pOCH312,0 photoisomerizes under the structural
constraints imposed by the backbone.
Fig. 8. Schematic representation for the bilayer structure (SmC2) of pOCH312,4 and
pOCH312,6. (Solid circles represent the polymer chains viewed end-on).
were observed [20]. High ε values are desirable as more photons are
absorbed by the chromophores and that may be reflected in the
photoisomerization levels.
3.4.2. Photoisomerization of polymers in solution
The photoisomerization kinetics for the azopolymers was
first measured in solution using THF as solvent (Fig. 10). Under
UVeVis irradiation all polymers photoisomerize, showing a marked
An azocompound under illumination will eventually achieve
a photostationary state, with a steady-state trans-cis ratio that
Fig. 9. Schematic representation of the interdigitated bilayer packing (SmAd) for pCN10,6 and pCN12,6. (A) Expected packing with a full interaction of terminal chains. (B) Proposed
model with deep interdigitation and bond dipole-bond dipole interactions of CN groups. Solid circles represent the polymer chains viewed end-on.

2058
T. García et al. / Polymer 53 (2012) 2049e2061
Table 3
Calculated quantum yields ratios (Table 3) for the polymers ranged
from 0.3 to 0.52. Those polymers with the highest quantum yields
ratio showed a high content of the cis isomer, emphasizing the
direct correspondence between these two parameters. On the other
hand, the first order rate constant (kct) of the thermal back relax-
ation (cis-trans isomerization) was determined by fitting the
experimental UVeVis spectroscopy data to the equation
UVeVis absorption and photo-induced properties of the azopolymers in solution
(THF).
pssb %cisc
(s)
(fcis/ftrans)
kct ꢃ 105d
a
c
Polymer
lmax
(nm) (L molꢁ1cmꢁ1
εtrans
)
(sꢁ1
)
pCN6,6
pCN10,6
pCN12,6
pOCH312,6 375
pOCH312,4 375
pOCH312,0 368
368
368
368
1.44 ꢃ 106
1.47 ꢃ 106
1.19 ꢃ 106
6.30 ꢃ 105
8.29 ꢃ 105
9.59 ꢃ 105
66
77
65
80
96
94.5
94.6
94.8
82.5
78.1
40.8
0.45
0.37
0.52
0.34
0.30
e
3.94
3.84
3.02
4.98
3.60
3.16
ꢀ
ꢁ
AN ꢁ At
AN ꢁ A0
ln
¼ ꢁkctt
(3)
204
a
Calculated from A ¼ εlc at different concentrations.
where Aa, At, and A0 denote the absorbance at infinite time, at time
t, and at time zero, respectively [41]. These data are plotted in Fig.11
for the two polymer series and the calculated kct values are listed in
Table 3. For all polymers Eq. (2) gave a good fit to the thermal cis-
trans isomerization. Reconversion times from the cis into the trans
state depend on the type of substituted groups. For azobenzenes,
aminoazobenzenes, and pseudostilbenes the reconversion times
are in general on the order of days, hours and seconds, respectively.
kct values of our polymers range between 2.9 and 5.24 ꢃ 10ꢁ5 sꢁ1
indicating that relaxation times are in the order of hours, therefore
corresponding to the aminoazobenzene type [8].
b
c
Irradiation at 365 nm until no change in absorbance is observed.
Calculated from Eq. (2) in the text.
d
Calculated from Eq. (3) in the text.
depends on competing effects namely the photoisomerization to
the cis state, the thermal relaxation back to the trans state, and the
cis reconversion upon irradiation [1]. The trans-cis ratio at the
steady state is characteristic to each system, as it depends on the
quantum yields for the two processes and the thermal relaxation
rate constant. Therefore, it is important to determine the quantum
yield ratio and the thermal back relaxation for each synthesized
polymer.
3.4.3. Photoisomerization of polymers in film
For the calculation of the quantum yield ratio the Fischer
equation was used [40]
It was demonstrated that the reversible light induced trans-cis
isomerization of thin films of azocompounds gives rise to specific
molecular arrangements that can be manipulated by light to obtain
interesting macroscopic properties. Compared to small molecules,
the mobility of chromophores in azopolymers is markedly reduced
in a way that photoisomerization creates an alignment quasi-
ꢀ
ꢁ
ꢀ
ꢁ ꢀ
ꢁ
Atrans
Bcis
εcis
εtrans
fcis
ftrans
¼
(2)
l
l
l
where Atrans and Bcis denote the content of the trans and cis species
at the pss, εtrans and εcis are the molar absorptivity coefficients, and
ftrans and fcis are the quantum yields for each isomer as labeled.
permanent, useful for instance as
a writing-erasing optical
memory. Before analyzing the photo-induced properties it is also
Fig. 11. Thermal back relaxation for the azopolymers bearing (A) CN and (B) OCH3
groups, measured at 25 ꢀC. Irradiated polymers (365 nm) were stored in the dark and
the cis-trans isomerization was measured at time intervals.
Fig. 10. Photoisomerization kinetics for (A) pCN12,6 and (B) pOCH312,6 in solution
using THF as solvent and irradiating at 365 nm.

T. García et al. / Polymer 53 (2012) 2049e2061
2059
Table 4
necessary to explore the spectroscopic features of polymers, which
UVeVis absorption and photo-induced properties for the azopolymers in film.
might surely change with respect to those observed in solution
where chromophores are much less constrained. The UVeVis
spectra of two distinct azopolymer films are shown in Fig. 12.
From the spectra it is clear that some other phenomena occur in
films that were not seen in solution. In dilute regime there are no
interchain interactions, and if the polymeresolvent interaction is
good enough, the chromophores will respond to irradiation as
independent units. On the contrary, for polymers deposited as thin
films, inter and intrachain interactions between chromophores will
produce aggregates as deduced from the observed marked peak
c
Polymer
lmax (nm)
pssa (s)
%cisb
lmax
pCN6,6
pCN10,6
pCN12,6
pCN10,6T
pOCH312,6
pOCH312,4
pOCH312,0
349
350
352
341
357
359
365
600
600
1000
2000
2000
e
61.9
50.0
56.1
38.3
71.4
e
ꢁ19
ꢁ18
ꢁ16
ꢁ27
ꢁ18
ꢁ16
ꢁ7
5000
55.2
a
Irradiation at 365 nm until no change in absorbance is observed.
Amount of cis-isomer in the photostationary state.
Calculated as the difference between lmax in solution and film.
b
c
shifts (Table 4). For almost all polymers the peak shift Dlmax (lmax in
film minus lmax in solution) is of around ꢁ18 nm indicating the
formation of H-aggregates [42]. It is interesting to note that for the
polymer with no flexible spacer (pOCH312,0) the hypsochromic
shift is much less significant (ꢁ7 nm) than for all others due to
motion restrictions the polymer chain imposes to the directly
linked chromophores. Taking into account that polymers display
liquid crystalline properties it was of our interest to test the effect of
the self-induced order in the spectroscopic characteristics. To look
for such effect a polymer film (pCN10,6) was heated above the
clearing temperature and then it was cooled-down to room
temperature in order to freeze a liquid-crystalline phase. This
polymer was named pCN10,6T to differentiate it from the non-
thermally treated pCN10,6. The consequence of such heating was
an important hypsochromic peak shift of ꢁ27 nm indicating
a stronger interaction between chromophores than that produced
in the H-aggregates of the pCN10,6 and that may arise from the
higher molecular order fixed upon cooling from the liquid-
crystalline phase. It was reported that peak shifts higher
than ꢁ25 nm might correspond to the aggregation over extended
occur upon cooling from a liquid crystal [42]. It may be reminded
that this polymer showed no crystallization upon cooling from the
mesophase; the molecular order was then preserved at room
temperature as a glassy liquid crystalline state.
Once the spectroscopic features were analyzed, it was impor-
tant to study the light-induced behavior of polymer films in
which chromophores are densely packed. The photoisomerization
rate is much slower for polymers in film than for polymers in
solution, reaching the pss state after longer irradiation times
ranging from 600 to 5000 s, depending basically upon the lateral
substituent group (CN or OCH3) in the chromophore. Polymers
bearing CN-substituted chromophores reached the pss state
much faster than those bearing OCH3-substituted chromophores,
a difference not observed in solution in such an extent. It should
be said that these latter polymers showed no isosbestic points
(Fig. 12B) indicating that not only the photoisomerization is
taking place, but some other phenomena like photo-bleaching or
degradation. On the other hand, the extremely large time to reach
the pss state for the pOCH312,0 is evidently due to the lack of
a flexible spacer.
domains of parallel packed
p-conjugated chromophores as it may
3.4.4. Birefringence
One of the advantages of highly anisotropic azobenzenes is the
fact that large changes in birefringence can be induced in thin films
by photoisomerization [20]. Moreover, high stability of the light-
induced properties along with good mechanical properties makes
these films excellent candidates for high-density optical storage.
Fig. 13. Photoinduced birefringence for the spin coated polymer films determined by
Eq. (1). The polymers were irradiated at 413 nm up to saturation and the change in
birefringence was measured at 633 nm.
Fig. 12. Photoisomerization kinetics for the azopolymers (A) pCN12,6 and (B)
pOCH312,6 in film, irradiated at 365 nm at time intervals up to photostationary state.

2060
T. García et al. / Polymer 53 (2012) 2049e2061
Table 5
performed in films prepared by casting showing similar trends,
although the saturation times were much longer (>10 min)
which is normal taking into account the optical path length of
thick films.
Birefringence values for spin coated thin and casted polymer films.
Polymer
D
na
D
na
Diffraction
(spin coating)
(casting)
efficiencyb (%)
pCN6,6
pCN10,6
pCN12,6
pOCH312,6
pOCH312,4
pOCH312,0
0.066
0.063
0.078
0.068
0.065
0.051
0.030
0.028
0.07
0.057
0.030
0.049
4.74
0.20
1.4
0.82
0.24
0.17
In order to test the influence of the liquid-crystalline order on
the relaxation kinetics of light-induced effects, one of the films
pCN12,6 was subjected to pump-offepump-on cycles (Fig. 14).
Initially there is a rapid increase in the birefringence value up to
saturation (
D
n ¼ 0.08). At this point the irradiation was switched-
off and contrary to the expected relaxation process, the birefrin-
gence continues to increase monotonically. It is also noticeable
that, once the irradiation was switched-on again a slight decrease
in the birefringence value is produced restoring the initial satura-
tion value. In subsequent switch-offeswitch-on cycles the same
behavior is reproduced. To explain this atypical behavior one
should remember that in liquid crystals there is an intrinsic ther-
modynamic tendency to self-organize into ordered phases. So,
when the azo groups are subjected to a polarized light they move in
concert following the direction given by the polarization direction
reaching certain directional order, although once the pump is
switch-off the liquid-crystalline domains do not relax, but in some
way they continue to improve the molecular order in a minor
extent [43]. The contrary effect produced in switching-on may be
justified by the fact that light produces cis isomers that tend to
disturb the liquid-crystalline order [44]. Taking into account that
the polymer is in a glassy state, the loss in birefringence is not
significant. In a second experiment, a film of pCN12,6 was heated
up to the isotropization temperature (Ti) and then it was quenched
in order to fix the liquid-crystalline phase at room temperature
(glassy state). This thermally treated film (pCN12,6T) was then
subjected to several pump-offepump-on cycles as shown in Fig. 14.
In this figure also a rapid increase in the birefringence value was
observed but it was much limited compared to that observed for
the untreated film. In this case the switch-off produced a slight
decrease in the birefringence level whereas a switch-on again
restores the birefringence to its highest previous value. Inducing
molecular order in a smectic phase, however, involves moving the
azobenzene groups from a thermodynamically stable state of
domains with various director distributions to a specific unidirec-
tional oriented state. This requires longer times of irradiation and
the level of orientation is limited as it was observed in this exper-
iment. Conversely, in amorphous polymers one would see in
switch-offeswitch-on cycles important relaxation-reorientation
processes [45]. Results shown here indicate that there is a tight
interplay between the photo-induced order and the LC intrinsic
order present in films.
a
Calculated from Eq. (1).
Determined as the percentage ratio between the incident beam and the first
b
order diffracted beam.
Photo-induced birefringence (Dn) experiments were performed in
films prepared by spin coating (300e450 nm), all of them showing
good optical quality. The n versus time plots (Fig. 13) showed clear
differences in the saturation value and in the time to reach it. The
polymer showing the highest n saturation value (0.078) was the
pCN12,6 while the one showing the lowest value (0.051) was the
polymer pOCH312,0 (Table 5). In all polymers the saturation value
was reached in less than 30 s, except for the polymer pOCH312,0
that took 60 s. Once again the proximity of chromophores to the
polymer backbone in the polymer pOCH312,0 perturbs to some
extent the photo-induced response. Some experiments were
D
D
Fig. 14. Switch-on e switch-off cycles for untreated pCN12,6 and thermally treated
pCN12,6T.
Fig. 15. AFM micrographs of surface relief gratings of azopolymers (A) pCN12,6 and (B) pOCH312,6 obtained by irradiation with an interference pattern at 413 nm.

T. García et al. / Polymer 53 (2012) 2049e2061
2061
3.4.5. Surface relief gratings
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Acknowledgments
The authors thank the CONACYT México for the grant given to
T. García (164848) during his PhD studies. The authors also thank
G. Méndez (DSC and TGA), J. Zamora (POM) and H. Maldonado
(GPC) for their helpful technical assistance in the polymer analysis.
This research project was supported by CONACYT (61773-R)
México.
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.polymer.2012.03.024.