Bistable mesomorphism and supramolecular stereomutation in chiral liquid crystal azopolymers

Article abstract of DOI:10.1039/b901384a

A series of side-chain liquid crystal polymers based on an azobenzene mesogenic unit bearing a terminal chiral chain has been synthesized. The thermal and mesomorphic properties of the homo- and copolymers have been investigated by polarizing optical micr

Full text of DOI:10.1039/b901384a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Bistable mesomorphism and supramolecular stereomutation in chiral liquid
crystal azopolymers†
a
a
a
b
a
Jesus del Barrio, Rosa M. Tejedor, Luiz S. Chinelatto, Carlos Sanchez, Milagros Pinol and Luis Oriol
a
*
ꢀ
ꢀ
~
Received 22nd January 2009, Accepted 24th April 2009
First published as an Advance Article on the web 4th June 2009
DOI: 10.1039/b901384a
A series of side-chain liquid crystal polymers based on an azobenzene mesogenic unit bearing a terminal
chiral chain has been synthesized. The thermal and mesomorphic properties of the homo- and
copolymers have been investigated by polarizing optical microscopy, DSC and X-ray diffraction.
Polymers having a chiral methylheptyloxy terminal chain (P8S and P8R) exhibit surprising
mesomorphic behavior, which depends on the thermal history of the sample. The influence of the
molecular chirality and thermal history on the supramolecular organization of the chromophores have
also been studied. Supramolecular chiral aggregation of the azobenzenes seems to be responsible for the
observed chiroptical properties of these materials, both in solution and solid state (polymeric films),
with the handedness controlled by the molecular chirality. Stereomutation of the chiral supramolecular
organization is detected when polymeric films of P8S and P8R experience a different thermal history.
reversible isomerization of azobenzenes upon irradiation with
UV and visible light. Furthermore, liquid crystallinity offers new
opportunities for these materials since the cooperative aniso-
tropic interactions of mesogenic azobenzenes can increase the
level and stability of the photo-orientation induced in the
material.7 Within this field, irradiation with CPL of liquid crys-
talline materials containing azobenzene derivatives may yield
supramolecular chiral assemblies with tuned and switchable
chiroptical properties. For example, Takezoe and co-workers
recently described the selective induction of an enantiomeric
excess of a chiral domain in a Bx phase of bent-core molecules by
irradiation with CPL (the chirality of the domain depended on
the handedness of CPL)8 and Sierra and co-workers described
columnar propeller-like complexes that have a chiral supramo-
lecular assembly of mesogens, the chirality of which depends on
the handedness of CPL.9 Nevertheless, most of the studies on the
photoinduction of chirality in liquid crystalline materials have
been carried out on azopolymers due to the easy preparation of
polymeric films and the stability of the induced properties below
the Tg. Nikolova and co-workers first reported the photoin-
duction of chiroptical properties (large circular birefringence and
circular dichroism) in films of liquid crystalline side-chain azo-
polymers.10 Natansohn, Rochon and co-workers also described
photoinduction of chirality in smectic azopolymers and proposed
a chiral helical structure whose helicity depends on the polari-
zation of light.11 Nikolova and co-workers also described similar
properties in amorphous polymers but in this case prior orien-
tation or irradiation with elliptically polarized light was
required.12 Recently, Cipparrone et al. have proposed the
photoinduction of a reversible chiral structure, using CPL, in
a non-oriented amorphous azopolymer, the structure of which is
closely related with mesogenic azopolymers.13 We have also
described the photoinduction and thermal stability of chiroptical
properties in liquid crystalline azopolymers.14 Based on CD
measurements we proposed that the origin of the chiroptical
properties in these materials may be attributed to chiral H-
aggregates. The supramolecular assembly of azobenzene units in
Introduction
Chirality in soft matter can be present at two different levels:
molecular and supramolecular. Chiral molecules have been
widely investigated and are of great interest in several research
fields, including pharmacy and biomedicine. Materials chemists
have used molecular chirality to produce chiral supramolecular
organizations, e.g. chiral mesophases.1 In recent years, the design
of new chiral supramolecular materials has received a great deal
of attention. For instance, the generation of asymmetry at the
supramolecular level starting from achiral molecular precursors
is an exciting challenge in Materials Science. Clearly, chirality
should appear at the supramolecular level in such achiral
compounds. Different physical forces have been studied as
sources of supramolecular chirality, e.g. breakdown of symmetry
in mesophases by vortex motions2 or by controlled spinning in
spin-coated films.3 Circular polarized light (CPL), as chiral
electromagnetic radiation, offers special advantages as a source
of chirality and this has been used widely, for instance, to induce
absolute asymmetric synthesis.4
Transfer of chirality from light to materials requires photo-
active groups. In this respect, photochromic polymers have been
widely studied and are attractive for many practical applications
because of the advantages of stability and processability. Among
these polymers, particular attention has been paid to polymers
containing azobenzene derivatives, also known as azopolymers.5
Applications described for these materials range from optical
storage5 to photomechanical actuators6 derived from the
a
´
Quımica Organica, Facultad de Ciencias-Instituto de Ciencia de
Materiales de Aragon, Universidad de Zaragoza-CSIC, Pedro Cerbuna
ꢀ
ꢀ
12, 50009 Zaragoza, Spain
b
´
Dpto. de Fısica de la Materia Condensada, Facultad de Ciencias-Instituto
de Ciencia de Materiales de Aragon, Universidad de Zaragoza-CSIC,
ꢀ
Pedro Cerbuna 12, 50009 Zaragoza, Spain
† Electronic supplementary information (ESI) available: Synthesis,
characterization and properties of intermediates and polymers. See
DOI: 10.1039/b901384a
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these aggregates is controlled by the handedness of the CPL used
as the chiral stimulus. Transfer of chirality could originate from
an asymmetric photoisomerization, which triggers the enrich-
ment of chiral conformations and, consequently, the enrichment
of chiral arrangements of azobenzene groups.15
In this paper we describe the synthesis of a series of azo
homopolymers with chiral terminal tails and also copolymers
derived from these materials. The structures of these polymers
are shown in Fig. 1. Optical, mesomorphic and thermal prop-
erties were investigated and, in particular, the influence of the
cooling process from the isotropic state on the structure of the
resulting mesomorphic glassy state was assessed. The influence of
molecular chirality as a chiral trigger for the chiral supramolec-
ular assembly of photochromic units was analyzed both in
solution and in polymeric films. The influence of thermal history
on the chiroptical properties of polymer films was also analyzed
along with the stereomutation of the chiral supramolecular
properties by means of the thermal treatment of polymeric films.
Nevertheless, less work has been done on chiral azopolymers
either in the field of liquid crystal azopolymers or in the study of
the supramolecular chirality in this kind of material. For
instance, Bobrovsky and co-workers recently reported
a comparative study of holographic recording in cholesteric (or
chiral nematic) and nematic azobenzene-containing polymers.16
Kozlovsky and co-workers reported a series of copolymers
having an azobenzene unit and a non-photochromic mesogenic
unit bearing a chiral terminal chain. In some of these copolymers,
complex thermal and mesomorphic behavior was described as
they exhibit two smectic phases that depend on the thermal
history and, in particular, on the cooling rate from the isotropic
state: a proper smectic A phase (SmA) and a phase that appears
as isotropic and transparent but with a hidden layered structure
of the smectic A type. This phase was designated as an ‘‘isotropic
smectic phase’’ and, according to Kozlovsky and co-workers, has
a TGBA* structure in which the helical pitch is shorter than the
visible wavelength.17–19 More recently, the same authors
described a homopolymer with an azobenzene containing
a chiral terminal chain, which also exhibits a bistable phase
behavior (SmA or TGBA*-like phase depending on the cooling
rate).20,21 The existence of this bistable phase behavior has
recently been related to two different isotropic states.22 As far as
the influence of irradiation on chiral properties is concerned,
Zhang and co-workers described the synthesis and character-
ization of chiral liquid crystalline azopolymers and studied the
CD response before and after irradiation with linearly polarized
light.23,24 Angiolini et al. reported the chiroptical properties of
non-liquid crystalline chiral azopolymers, in virgin and irradi-
ated films, both with linear and circular polarized light. It was
found that the macromolecular helical suprastructure of these
chiral azopolymers was switched by irradiation with circularly
polarized light.25–28
Experimental section
Synthesis of monomers
4-Cyano-40-(6-methacryloyloxyhexyloxy)azobenzene, which was
used for copolymerization, was synthesized according to
a previously described method.29 Synthesis of chiral monomers is
schematically represented in Fig. 2. Synthesis and characteriza-
tion data for intermediates are collected in the ESI.†
(R)-4-Methylpropyloxy-40-(600-methacryloylxyhexyl-100-oxy)-
azobenzene M4R. Methacryloyl chloride (0.81 g, 7.74 mmol) was
added to a solution of (R)-4-methylpropyloxy-40-(600-hydrox-
yhexyl-100-oxy)azobenzene (1.91 g, 5.16 mmol), triethylamine
(1.04 g, 10.30 mmol) and 2, 6-di-tert-butyl-4-methylphenol (3
mg) in THF (40.00 mL) under reflux under argon. After 24 h,
10% aqueous NH4Cl (40 mL) was added. The mixture was
extracted with CH2Cl2 and the combined organic extracts were
washed with water, dried over MgSO4 and evaporated to
dryness. The crude product was purified by flash chromatog-
raphy eluting with hexane/ethyl acetate (10:1) to yield the pure
monomer as a yellow solid. Finally, the product was recrystal-
1
lized from absolute ethanol. Yield: 76%. H NMR (400 MHz,
CDCl3, d): 7.86 (dd, J ¼ 9.0 Hz, J ¼ 2.1 Hz, 4H), 6.96 (dd, J ¼ 9.0
Hz, J ¼ 1.9 Hz, 4H), 6.08 (s, 1H), 5.51 (s, 1H), 4.40–4.29 (m, 1H),
4.13 (t, J ¼ 6.5 Hz, 2H), 3.94 (t, J ¼ 6.5 Hz, 2H), 1.92 (s, 3H),
1.89–1.35 (m, 10H), 1.32 (d, J ¼ 6.1 Hz, 3H), 0.99 (t, J ¼ 7.5 Hz,
3H). 13C NMR (100 MHz, CDCl3, d): 167.39, 161.05, 160.38,
146.90, 146.71, 136.52, 124.98, 124.34, 124.25, 115.71, 114.56,
75.19, 68.09, 64.64, 32.60, 29.12, 29.08, 25.81, 25.50, 19.16, 18.36,
9.70. IR (KBr, cmꢀ1): 2969, 2939, 2868, 1702, 1601, 1581, 1500,
1240, 1151, 840. Anal. Found (calcd): C 71.12% (71.21), H 7.83%
(7.81), N 6.34% (6.39).
(S)-4-Methylpropyloxy-40-(600-methacryloylxyhexyl-100-oxy)-
azobenzene M4S. Prepared from 7(S) according to the procedure
described for M4R. Yellow solid (72% yield). 1H NMR (400
MHz, CDCl3, d): 7.85 (dd, J ¼ 9.0 Hz, J ¼ 2.1 Hz, 4H), 6.96 (dd,
J ¼ 9.0 Hz, J ¼ 1.9 Hz, 4H), 6.08 (s, 1H), 5.51 (s, 1H), 4.38–4.29
(m, 1H), 4.13 (t, J ¼ 6.5 Hz, 2H), 3.94 (t, J ¼ 6.5 Hz, 2H), 1.92 (s,
3H), 1.89–1.35 (m, 10H), 1.33 (d, J ¼ 6.1 Hz, 3H), 0.99 (t, J ¼ 7.5
Hz, 3H). 13C NMR (100 MHz, CDCl3, d): 167.41, 161.05, 160.38,
146.91, 146.71, 136.52, 125.26, 124.34, 124.24, 115.71, 114.56,
75.19, 68.08, 64.64, 32.60, 29.12, 29.08, 25.81, 25.50, 19.16, 18.36,
9.70. IR (KBr, cmꢀ1): 2968, 2939, 2868, 1703, 1601, 1581, 1500,
1240, 1151, 840. Anal. Found (calcd): C 71.18% (71.21), H 7.80%
(7.81), N 6.45% (6.39).
Fig. 1 Chemical structure of the synthesized homo and copolymers
(configuration of stereogenic center in brackets).
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Fig. 2 Chemical structure of the synthesized homo and copolymers.
(R)-4-Methylheptyloxy-40-(600-methacryloylxyhexyl-100-oxy)-
azobenzene M8R. Prepared from 8(R) according to the proce-
dure described for M4R. Yellow solid (77% yield).1H NMR (400
MHz, CDCl3, d): 7.86 (dd, J ¼ 9.0 Hz, J ¼ 2.9 Hz, 4H), 6.97 (dd,
J ¼ 9.0 Hz, J ¼ 4.8 Hz, 4H), 6.10 (s, 1H), 5.55 (s, 1H), 4.49–4.40
(m, 1H), 4.17 (t, J ¼ 6.6 Hz, 2H), 4.04 (t, J ¼ 6.4 Hz, 2H), 1.95 (s,
3H), 1.88–1.23 (m, 21H), 0.88 (t, J ¼ 6.9 Hz, 3H). 13C NMR (100
MHz, CDCl3, d): 167.52, 161.01, 160.35, 146.98, 146.76, 136.48,
125.22, 124.32, 124.26, 115.76, 114.62, 74.18, 68.07, 64.62, 36.41,
31.77, 29.25, 29.10, 28.56, 25.80, 25.74, 25.47, 22.59, 19.72, 18.33,
14.07. IR (KBr, cmꢀ1): 2970, 2942, 2868, 1701, 1600, 1637, 1581,
1502, 1240, 1151, 843. Anal. Found (calcd): C 72.66% (72.84), H
8.60% (8.56), N 5.60% (5.66).
(S)-4-Methylheptyloxy-40-(600-methacryloylxyhexyl-100-oxy)-
azobenzene M8S. Prepared from 8(S) according to the procedure
described for M4R. Yellow solid (72% yield).1H NMR (400
MHz, CDCl3, d): 7.86 (dd, J ¼ 9.0 Hz, J ¼ 2.9 Hz, 4H), 6.97 (dd,
J ¼ 9.0 Hz, J ¼ 4.8 Hz, 4H), 6.10 (s, 1H), 5.55 (s, 1H), 4.50–4.40
(m, 1H), 4.17 (t, J ¼ 6.6 Hz, 2H), 4.04 (t, J ¼ 6.4 Hz, 2H), 1.95 (s,
3H), 1.88–1.22 (m, 21H), 0.88 (t, J ¼ 6.9 Hz, 3H). 13C NMR (100
MHz, CDCl3, d): 167.29, 161.06, 160.40, 146.85, 146.64, 136.48,
125.23, 124.38, 124.31, 115.77, 114.63, 74.19, 68.07, 64.62, 36.40,
31.77, 29.25, 29.10, 28.55, 25.80, 25.74, 25.47, 22.59, 19.72, 18.33,
14.07. IR (KBr, cmꢀ1): 2970, 2940, 2868, 1701, 1636, 1599, 1581,
1502, 1240, 1151, 843. Anal. Found (calcd): C 72.70% (72.84), H
8.58% (8.56), N 5.72% (5.66).
was dissolved in freshly distilled DMF (approximately 10% w/v)
under an argon atmosphere in a Schlenk tube. The sample was
degassed by several vacuum/argon cycles, the solution was heated
ꢁ
at 70 C and azabis(isobutyronitrile) (AIBN) (1% weight ratio)
was added. The solution was stirred at this temperature for 48 h
and the progress of the polymerization was monitored by thin
layer chromatography. The polymer was precipitated by pouring
the reaction mixture into in cold 96% ethanol and the product was
filtered off. Purification of the polymer was carried out by Soxhlet
extraction with ethanol for 48 h. The final products were obtained
as yellow powders and were dried at 50 ꢁC under vacuum to
constant weight (determined by thermogravimetry).
P4R. Yellow powder (71% yield). 1H NMR (400 MHz, CDCl3,
d): 7.88–7.76 (m, 4H), 7.01–6.84 (m, 4H), 4.43–4.28 (m, 1H),
4.06–3.82 (m, 4H), 2.00–1.17 (m, 16H), 1.14–0.78 (m, 5H). 13C
NMR (100 MHz, CDCl3, d): 177.76–176.89, 161.09, 160.40,
147.09, 146.95, 124.32, 124.29, 115.89, 114.70, 75.36, 68.15,
65.01, 54.30, 45.31–45.03, 30.25, 29.16, 27.31, 25.92, 25.74, 19.18,
18.76–16.84, 9.77. IR (KBr, cmꢀ1): 2974, 2935, 2860, 1727, 1599,
1580, 1499, 1452, 1246, 1146, 840. Anal. Found (calcd): C 71.04%
(71.21), H 7.87% (7.81), N 6.32% (6.39). [a]D ¼ ꢀ16.3 (c 1.19,
CHCl3). GPC (THF, PMMA standards): Mn ¼ 24000, Mw ¼
36200, IP ¼ 1.5.
P4S. Yellow powder (56% yield). 1H NMR (400 MHz, CDCl3,
d): 7.90–7.75 (m, 4H), 7.01–6.82 (m, 4H), 4.40–4.25 (m, 1H),
4.06–3.80 (m, 4H), 2.01–1.22 (m, 16H), 1.10–0.83 (m, 5H). 13C
NMR (100 MHz, CDCl3, d): 177.84–176.84, 161.08, 160.35,
146.96, 146.79, 124.41, 124.37, 115.80, 114.64, 75.25, 68.09,
65.02, 54.30 45.23–44.91, 30.29, 29.18, 27,35, 25.98, 25.81, 19.25,
Synthesis of polymers
Polymers and copolymers were prepared according to a standard
method: the corresponding monomer (or mixture of monomers)
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18.75–16.85, 9.79. IR (KBr, cmꢀ1): 2975, 2931, 2857, 1726, 1599,
1580, 1500, 1452, 1247, 1146, 839. Anal. Found (calcd): C 70.94%
(71.21), H 7.79% (7.81), N 6.31% (6.39). [a]D ¼ +19.0 (c 1.00,
CHCl3). GPC (THF, PMMA standards): Mn ¼ 23700, Mw ¼
34300, IP ¼ 1.4.
The molar compositions of the copolymers were determined
from 1H NMR spectra by comparing the integrated areas of the
signals at 4.35 and 3.95 ppm. The molar contents of the copol-
ymers reflect the feed composition, indicating similar reactivities
of the two monomers.
P8R. Yellow powder (76% yield). 1H NMR (400 MHz, CDCl3,
d): 7.88–7.70 (m, 4H), 7.00–6.79 (m, 4H), 4.46–4.28 (m, 1H),
4.10–3.74 (m, 4H), 1.85–0.98 (m, 26H), 0.97–0.74 (m, 3H). 13C
NMR (100 MHz, CDCl3, d): 177.49–176.56, 161.03, 160.29,
146.90, 146.71, 124.37, 124.32, 115.72, 114.58, 74.10, 68.10,
65.15–64.91, 54.60, 45.19–44.70, 36.40, 31.76, 29.30, 29.20, 28.13,
25.96, 25.79, 25.43, 22.58, 19.70, 18.76–16.84, 14.07. IR (KBr,
cmꢀ1): 2975, 2924, 2857, 1726, 1598, 1578, 1499, 1450, 1250,
1141, 842. Anal. Found (calcd): C 72.62% (72.84), H 8.60%
(8.56), N 5.72% (5.66). [a]D ¼ +9.5 (c 1.12, CHCl3). GPC (THF,
PMMA standards): Mn ¼ 36100, Mw ¼ 57300, IP ¼ 1.6.
General procedures
Elemental analyses were performed using a Perkin-Elmer 240C
microanalyzer. IR spectra were obtained on a Nicolet Avatar
1
360-FTIR using KBr discs. H NMR spectra were measured at
room temperature with a Bruker AV-4 spectrometer. Molecular
weights of polymers were determined by GPC using a Waters
2695 autosampler equipped with two in line Waters HR-4 and
HR-2 columns and a Waters 2420 ELSD detector, using THF as
the eluent at 35 ꢁC and calibrated with polystyrene narrow
standards. Measurements were performed in THF at room
temperature with 1 mL/min flow, using PMMA narrow molec-
ular weight standards. UV-Vis spectra were recorded with a UV-
Vis spectrophotometer UV4-200 from ATI-Unicam. Optical
rotations were measured at room temperature on chloroform
solutions (concentrations are given in g/100 mL) on a JASCO P-
1020 polarimeter, using a quartz cell with 1 cm optical path
length. CD spectra were registered using a Jasco J-810 spec-
tropolarimeter by using either polymer solutions of polymer
films. The CD spectra of the films were registered by rotating the
samples every 60 degrees around the light beam axis. In the case
of polymer solutions (see below) dichloromethane (DCM) or
DCM/hexane were used as solvents. DCM is a good solvent for
the polymers and transparent polymer solutions were easily
obtained in all cases. In the case of hexane solutions, the poly-
mers were first dissolved in the minimum amount of hot DCM
and warm hexane was added up to the required concentration.
Fresh and transparent solutions were only obtained in the case of
polymers having the longest terminal aliphatic chain and
measurements were made at room temperature (rt). Both in
DCM and DCM/hexane solutions the concentration was main-
tained at around 4 ꢂ 10ꢀ2 mg/mL. The polymer films were
obtained by casting from DCM solutions onto fused silica plates.
Thermogravimetric analyses (TGA) were performed using a TA
Q-5000 instrument at a heating rate of 10 ꢁC minꢀ1 under
a nitrogen atmosphere. Mesogenic behavior was evaluated by
polarized-light optical microscopy (POM) using an Olympus
BH- 2 polarizing microscope fitted with a Linkam THMS600 hot
stage and a CS196 cooling system and operating at different scan
rates in order to evaluate the influence of thermal history on the
liquid crystalline textures. Thermal transitions were determined
by differential scanning calorimetry (DSC) using a TA DSC Q-
2000 instrument under a nitrogen atmosphere. Glass transition
temperatures (Tg) were determined at the midpoint of the base-
line jump and the mesophase-isotropic phase transition temper-
ature was read at the maximum of the corresponding peaks using
powdered samples of about 4–6 mg. XRD measurements of
polymeric samples were performed with an evacuated Pinhole
camera (Anton–Paar) operating with a point-focused Ni-filtered
Cu-Ka beam. Samples were held in Lindemann glass capillaries
and the patterns were collected on flat photographic films
perpendicular to the X-ray beam.
P8S. Yellow powder (72% yield). 1H NMR (400 MHz, CDCl3,
d): 7.94–7.77 (m, 4H), 7.01–6.81 (m, 4H), 4.48–4.27 (m, 1H),
4.09–3.73 (m, 4H), 1.85–0.97 (m, 26H), 0.93–0.77 (m, 3H). 13C
NMR (100 MHz, CDCl3, d): 177.77–176.66, 161.26, 160.59,
146.40, 146.24, 124.65, 124.53, 115.80, 114.67, 74.20, 68.12,
65.08–64.87, 54.63, 45.19–44.70, 36.43, 31.81, 29.29, 29.15, 28.11,
25.96, 25.78, 25.47, 22.62, 19.70, 18.78–16.86, 14.11. IR (KBr,
cmꢀ1): 2976, 2925, 2856, 1727, 1600, 1578, 1500, 1453, 1251,
1139, 840. Anal. Found (calcd): C 72.70% (72.84), H 8.60%
(8.56), N 5.74% (5.66). [a]D ¼ ꢀ10.5 (c 1.04, CHCl3). GPC
(THF, PMMA standards): Mn ¼37700, Mw ¼ 67500, IP ¼ 1.8.
P4S-CN50. The molar composition of the feed in this case was
50% of 4-cyano-40-(6-methacryloyloxyhexyloxy)azobenzene and
1
50% of M4S. Orange powder (40% yield). H NMR (400 MHz,
CDCl3, d): 7.95–7.64 (m, 10H), 7.00–6.82 (m, 6H), 4.40–4.27 (m,
1H), 4.07–3.81 (m, 8H), 2.00–0.78 (m, 34H). 13C NMR (100
MHz, CDCl3, d): 177.46–176.34, 162.50, 162.34, 160.32, 154.56,
146.85, 146.72, 146.60, 133.05, 125.43, 124.33, 124.28, 124.03,
123.03, 118.58, 115.71, 114.72, 114.52, 75.20, 68.16, 67.98, 65.03–
64.82, 53.21–52.12, 45.23–44.82, 30.30, 29.09, 28.16, 28.05, 28.01,
25.91, 25.84, 25.74, 25.30, 19.17, 18.71–16.34, 9.73. IR (KBr,
cmꢀ1): 2975, 2926, 2855, 2225, 1726, 1599, 1580, 1499, 1451,
1250, 1140, 842. Anal. Found (calcd): C% 70.68 (70.89), H% 6.95
(7.12), N% 8.70 (8.56). GPC (THF, PMMA standards): Mn ¼
8800, Mw ¼ 10200, IP ¼ 1.2.
P4S-CN25. In this case, the molar composition of the feed was
25% of 4-cyano-40-(6-methacryloyloxyhexyloxy)azobenzene and
1
75% of M4S. Orange powder (45% yield). H NMR (400 MHz,
CDCl3, d): 7.95–7.63 (m, 9H), 7.03–6.82 (m, 7H), 4.43–4.27 (m,
1.5H), 4.09–3.80 (m, 8H), 1.98–0.77 (m, 38H). 13C NMR (100
MHz, CDCl3, d): 177.46–176.01, 162.42, 162.30, 160.45, 154.57,
146.95, 146.74, 146.58, 133.05, 125.41, 124.27, 124.25, 123.98,
123.03, 118.61, 115.67, 114.70, 114.52, 75.21, 68.21, 68.02, 65.10–
64.87, 53.19–52.03, 45.25–44.79, 30.26, 29.11, 28.16, 28.06, 27.96,
25.87, 25.82, 25.73, 25.29, 19.15, 18.75–16.31, 9.70. IR (KBr,
cmꢀ1): 2976, 2936, 2859, 2225, 1726, 1598, 1580, 1499, 1450,
1248, 1145, 841. Anal. Found (calcd): C% 70.96 (71.05), H% 7.38
(7.47), N% 7.51 (7.48). GPC (THF, PMMA standards): Mn ¼
10400, Mw ¼ 11600, IP ¼ 1.1.
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Results and discussion
Synthesis and characterization
The synthetic pathway to the target polymers is represented in
Fig. 2. Chiral terminal chains were introduced by etherification
of p-nitrophenol using a Mitsunobu reaction. Subsequent
reduction of the nitro group and azo-coupling of the derived
aniline with phenol gave rise to the desired azochromophores. A
flexible spacer was introduced by a Williamson reaction and
finally the methacroyl group was incorporated by esterification
with methacroyl chloride. Polymers were synthesized by
a conventional free-radical polymerization using AIBN as
a thermal initiator and DMF as the solvent. Average molecular
weights and polydispersities (of final purified samples) were
analyzed by GPC using PMMA standards as references. The
results are gathered in Table 1. The incorporation of cyanoazo-
benzene comonomer leads to a small decrease in the molecular
weight. Chemical structures were corroborated by elemental
analysis, NMR and spectroscopic methods (see Experimental
Part). A detailed analysis of the 13C NMR spectra (see ESI†)
provided evidence that the polymer chain has a tendency to
syndiotacticity, as widely reported for polymethacrylates under
similar conditions.30
Fig. 3 DSC traces corresponding to the second heating scans (10 ꢁC/
min) of P4R (up) and P4S (down).
relaxation. However, a small exotherm overlapping the glass
transition is also observed in the cooling process from the
isotropic state, which seems more likely to point to a phase
transition and could be related to an isotropic-mesophase tran-
sition. In fact, when the homopolymers P4R and P4S were
carefully studied under the polarizing optical microscope, a very
poorly defined grainy texture was detected on cooling from the
isotropic state. This result points to a short range mesophase that
in the case of P4R overlaps with the glass transition and,
consequently, only gives rise to a baseline jump. In order to
corroborate this mesomorphic behavior, samples with different
thermal histories were studied by X-ray diffraction. When
samples were cooled from approximately 100 ꢁC to rt at cooling
rates of about 10 ꢁC/min, X-ray diffraction patterns only showed
a diffuse and broad scattering peak at high angles, which roughly
Thermal and mesomorphic properties
The thermal stability of the target polymers was studied by
thermogravimetry (see Table 1) using powdered samples. All of
the studied polymers exhibit good thermal stability up to
approximately 270 ꢁC, with the onset of decomposition detected
at temperatures above 300 ꢁC. Mesomorphic properties were
analyzed by POM, DSC and X-ray diffraction and the results are
summarized in Table 1. Polymers with the shortest terminal
chain (P4R and P4S) seem to be amorphous according to the
DSC results. Thus, as can be observed in the second heating
scans corresponding to these polymers (Fig. 3), a glass transition
is detected for both polymers but in case of P4S a small endo-
therm can be more clearly seen in the final region of the glass
transition, which in principle might be associated to enthalpy
˚
corresponds to a distance of 4.5 A (related to the typical inter-
molecular distance of mesogenic units). However, when the
samples were annealed at temperatures just above the glass
ꢁ
transition (at around 55–60 C) and finally cooled to rt, a new
and relatively sharp scattering peak was clearly detected at low
angles. This new peak was detected after 5 h of annealing for P4S,
although longer annealing times (higher than 20 h) were needed
to observe a similar peak in the case of P4R. The appearance of
this peak can be associated with a lamellar phase with a measured
Table 1 Thermal and mesomorphic properties of the synthesized azo-
polymers
˚
layer spacing of 28.5 A. The molecular length of the repeating
Polymer
Mn/IP a
T1%/TONSET
Thermal transitionc
b
unit of the azobenzene chromophores with methylpropyl
˚
terminal chains is 28 A as estimated using Dreiding stereomodels
P4R
P4S
P4S-CN25
P4S-CN50
P8R
24000/1.5
23700/1.4
10400/1.1
8800/1.2
36100/1.6
37700/1.8
287/317
283/328
387/330
270/317
294/346
306/344
g 40 Id
g 51 SmA 66 Ie
with a fully extended conformation. This finding could be
indicative of a SmA phase with a monolayer structure.
g 49 SmA 94 (8.7) I
g 51 SmA 109 (6.8) I
g 50 SmA 78 (6.8) If
g 49 SmA 76 (6.3) If
Copolymers P4S-CN25 and P4S-CN50 combine this chiral
photochromic unit and the well-known smectogen 4-cyanoazo-
benzene. Mesomorphism was clearly observed for these copoly-
mers. Thus, when P4S-CN25 was cooled down from the isotropic
state, a turbid, polydomain mesomorphic phase was obtained
with a poorly defined, grainy texture. In the case of P4S-CN50,
typical fan-shaped and homeotropic textures of a SmA phase
were clearly detected. DSC traces of these polymers show a glass
transition and an endothermic peak corresponding to the iso-
tropization detected by optical microscopy. X-Ray diffraction
measurements also confirmed the SmA nature of the mesophase
P8S
a
b
Mn, weight average molecular mass; IP, polydispersity.
(ꢁC)
Decomposition detected in the derivative thermogravimetric curve;
T1%: temperature corresponding to 1% weight loss; TONSET: onset of
c
the decomposition peak. Transition temperatures are indicated in ꢁC
and enthalpy (in brackets) in J/g and correspond to the second heating
d
scan (scan rate: 10 ꢁC/min). Endotherm transition (mesophase-
isotropic, see text) overlapped with glass transition. Enthalpy cannot be
e
accurately calculated. Please see text for the discussion about the
dependence of the mesomorphic behaviour on the thermal history.
Glass transition was calculated in virgin samples.
f
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of both copolymers, with a monolayer structure with a measured
˚
layer thickness of about 29 A. In the case of P4S-CN50 a well-
oriented fiber could be obtained directly from the mesophase and
its diffractogram confirms the orthogonal arrangement of the
mesogenic units in the smectic phase.
In an attempt to obtain chiral azo homopolymers with
a broader range of mesomorphism, a longer terminal chain (P8R
and P8S) was introduced but with the stereogenic center main-
tained in the same position, i.e. adjacent to the aromatic core.
DSC traces of azopolymers containing 1-methylheptyl chiral
terminal chains (P8R and P8S) exhibit a peak both in the heating
and cooling processes and these correspond to the mesophase-
isotropic phase transition according to POM observations (see
below). The glass transition is only clearly detected in the first
heating scan, as can be seen in Fig. 4, which represents the first
heating and cooling scan as well as the second heating scan.
However, the glass transition is hardly detected on cooling or
successive scans.
Fig. 5 Microphotographs taken at room temperature of a sample of P8S
cooled from the isotropic state at 1 ꢁC/min (a) and 20 ꢁC/min (b). Insets:
photographs of a sample of P8S cooled at 1 ꢁC/min (translucent sample,
a) or 20 ꢁC/min (transparent sample, b).
The nature of the mesomorphic phase of these azopolymers
was studied by POM and X-ray diffraction. Firstly, it must be
pointed out that mesomorphic textures and the transmission of
films of chiral homopolymers P8R and P8S are strongly affected
by thermal history. Thus, when the polymers were slowly cooled
down in the absence of light from the isotropic state (at cooling
rates lower than 3 ꢁC/min) a birefringent, very poorly defined
sanded texture was observed and the sample appeared turbid by
the naked eye (Fig. 5a). However, on cooling at rates higher than
5
ꢁC/min the sample was more transparent and dark textures
Fig. 6 Dependence of the isotropization temperature of a sample of P8S
cooled at different rates from the isotropic state (isotropization temper-
ature was measured on heating at 10 ꢁC/min).
were detected by POM (Fig. 5b). These observations point to
kinetic and thermodynamic control of the mesomorphic orga-
nization of these materials.
Samples with a different thermal history were also studied by
DSC in order to investigate the influence of the cooling process
on the phase transitions of the resulting mesomorphic glasses. In
fact, the study of the mesophase-isotropic phase transition, as
well as the enthalpy of mesomorphic glasses obtained under
different cooling conditions, provided some evidence of
a thermal history-dependent mesomorphic organization. As an
example, the variation of the isotropization temperature
ꢁ
measured in the heating scan (fixed heating rate 10 C/min) of
a sample of P8S previously cooled down from the isotropic state
to the mesomorphic glass state at different cooling rates is rep-
resented in Fig. 6. As can be seen, the isotropization temperature
varies linearly with the cooling rate, but there are two different
regimes, with the change of slope of this dependence located at
around 3 ꢁC/min. This behavior is similar to the phase transition
behavior of bistable crystallizable polymers.31 In the field of
liquid crystalline polymers, this behavior has been assigned in
similar azopolymers to the existence of a bistable mesomorphism
and, consequently, to the appearance of two different meso-
phases depending on the thermal history.32
The nature of the mesophase of polymers P8R and P8S was
also studied by X-ray diffraction (see ESI†). Samples of these
polymers were cooled down from the isotropic state up to rt at
different cooling rates and finally studied at rt (mesomorphic
glass). All of the materials (even quenched samples) exhibit
typical X-ray diffraction patterns compatible with a smectic
phase, with a peak at low angles corresponding to a layer spacing
˚
of about 54.8 A (the molecular length of the monomeric unit is 33
˚
A, as estimated with Dreiding stereomodels for a fully extended
conformation). Significant differences were not observed in the
layer thickness for samples cooled at different cooling rates and
only a more reinforced peak at low angle was detected for
samples cooled at low cooling rates. Furthermore, the layer
thickness did not vary with the temperature. These facts point to
Fig. 4 DSC traces corresponding to first heating scan (up), first cooling
scan (down) and second heating scan (middle) of P8S (10 ꢁC/min).
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an orthogonal smectic A mesophase that has a bilayer structure
with interdigitation of the terminal chains.
Bearing these results in mind, the mesophase obtained by slow
cooling (slower than 3 ꢁC/min) can be assigned as a SmA phase.
Furthermore, the phase obtained by a fast cooling process must
be structurally related with an orthogonal layered phase
according to the X-ray diffraction data. Small domains of SmA
phase might partially explain these results, however, as
mentioned above, Kozlovski and co-workers previously
described similar bistable mesomorphic behavior in chiral homo-
and co-azopolymers having a similar chemical structure. They
described a ‘‘transparent mesophase’’33 with a hidden layered
structure obtained by fast cooling, which was assigned to
a TGBA* mesophase, i.e. smectic A domains rotated in a regular
way to form a helical superstructure.34 According to these
authors the bistable mesomorphism of P8R and P8S may be
tentatively assigned to SmA or TGBA* phases, and these phases
have a thermodynamic or kinetic control imposed by the thermal
history.
Fig. 7 UV-vis (down) and CD spectra (up) of P8S in DCM solution (—),
and P8S and P8R in DCM/hexane solution (–C–) and (– –) respectively.
polar solvent, which forces non-covalent interactions between
the azo units and results in a supramolecular arrangement.
The DCM solutions of P8S (Fig. 7) and P8R (ESI) are CD-
silent. However, these polymers in DCM/hexane solutions
exhibit significant CD signals of opposite sign: the spectrum of
P8S displays a band that can be associated to a negative exciton
couplet while P8R exhibits the inverted band corresponding to
a positive couplet. Angiolini et al. studied amorphous chiral
azopolymethacrylates with a chiral spacer between the main
chain and the azobenzene units, which was consequently close to
the methacrylate group.26,37,38 These materials exhibit similar CD
responses in different organic solvents, e.g. CHCl3. Angiolini
et al. proposed that chiral properties are due to a chiral helical
macromolecular conformation of the polymeric backbone as
a consequence of the influence of the stereogenic center on the
polymer chain growth. However, the stereogenic center in
monomers M8R and M8S is located at the terminal alkyl chain
and as a consequence it has a low influence on the polymerization
of the methacrylate group. Taking this fact into account, as well
as the CD properties of DCM (CD-silent) and DCM/hexane
solutions, we propose that the chiral properties are more prob-
ably due to a chiral supramolecular organization of chromo-
phores whose handedness is imposed by the molecular chirality.
Chiroptical properties were also evaluated on polymeric films
of these polymers. As mentioned above, the mesomorphic
behavior of P8S and P8R seems to depend strongly on the
thermal history of the sample. For this reason, spectra of these
polymers were registered on as-prepared cast films and on ther-
mally treated films—after a slow (1.2 ꢁC/min) and a fast (10 ꢁC/
min) cooling process from the isotropic state. UV-Vis spectra of
P8S and P8R films were similar to those registered in DCM/
hexane solution, which points to similar aggregation behavior
(see ESI†). Furthermore, marked differences in the absorption
spectra were not observed for polymeric films with different
thermal histories. The registered CD spectra of P8S films are
presented in Fig. 8 (The ESI includes the CD data for P8R, which
are similar to those for P8S but are of opposite sign). These CD
spectra are essentially invariable by rotation around the light
beam axis, which rules out the existence of linear dichroism in the
polymeric films. The CD spectrum of an as-prepared film of P8S
Supramolecular chirality
As reported previously, circular polarized light (CPL) can be
used as a chiral stimulus to generate asymmetry in liquid crys-
tals.35 Indeed, CPL is capable of photoinducing chiral aggrega-
tion of azobenzene chromophores of side-chain liquid crystalline
azopolymers.14 In this respect, we recently proposed that
supramolecular chiral aggregation could begin at the molecular
level with an asymmetric chemical modification photoinduced by
CPL.15 In a similar way, molecular chirality can induce a chiral
aggregation of azobenzene units and give rise to a chiral azo-
material that combines molecular and supramolecular chirality.
In order to assess the formation of chiral aggregation due to
molecular chirality, solutions of the synthesized chiral homo-
polymers were prepared both in DCM and DCM/hexane
mixtures in order to study the relationship between the solvent
polarity and the aggregation of azobenzene moieties. Unfortu-
nately, P4R and P4S precipitate on adding even a small amount
of hexane and, consequently, these studies were only carried out
with P8S and P8R using fresh samples (eventually a partial
precipitation of the polymers was observed on storing the solu-
tions for several days).
UV-Vis and circular dichroism (CD) spectra of P8S and P8R
in DCM and DCM/hexane mixtures are gathered in Fig. 7. The
UV-Vis spectrum of P8S in DCM solution (or P8R, see ESI†) is
similar to the spectrum of the monomer and exhibits a strong p–
p* transition at 362 nm and a weak n–p* transition at around
450 nm, as corresponds to E-azobenzene chromophores. Taking
into account that DCM is a good solvent for P8S and P8R, along
with the symmetry of the main band of the absorption spectrum
(the band is very similar to the exhibited by low concentrated
monomeric solutions), it can be concluded that the chromo-
phores are essentially isolated. The UV-Vis spectra of P8S and
P8R in a low polarity solvent such as DCM/hexane mixtures
(minimum amount of DCM) display a significant blue shift of the
p–p* transition compared to those exhibited by homopolymers
in DCM solutions (crossover from 362 to 342 nm). This blue shift
is associated with the formation of H-aggregates36 as a conse-
quence of the low interaction of azochromophores with the non-
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Fig. 8 CD spectra of P8S as prepared film (—), and annealed at
isotropic temperature and after cooling down at 10 ꢁC/min (– –) and 1.2
ꢁC/min (–C–).
is very similar to that measured in DCM/hexane solution for this
polymer (a band corresponding to a negative exciton couplet
with a crossover at around 340 nm). However, CD spectra
obtained by cooling from the isotropic state to room temperature
at different cooling rates depend on the thermal cooling process.
The rapidly cooled films exhibit a negative exciton couplet, which
is red-shifted with respect to the as-prepared film. Surprisingly,
a slow cooling rate (1.2 ꢁC/min) gave rise to a mirror image CD
spectrum (a band associated to a positive exciton couplet). This
complete stereomutation of the chiral supramolecular organi-
zation is caused by the thermal history. Most of the reported
examples of stereomutation are due to a change in the
surrounding conditions such as solvents, temperature or sample
preparation—including cooling rate.39–46 In addition, the inver-
sion of chirality in synthetic materials has been found in the field
of liquid crystals, specifically those exhibiting chiral meso-
phases.47–49 The observed inversion of the CD spectrum could be
due to kinetically and thermodynamically controlled chiral self-
organization of azobenzene units with opposite chirality. Taking
into account the fact that the configuration of the chiral center in
the alkoxy terminal chain is not altered during the stereo-
mutation process, it is required that the two chiral supramolec-
ular forms obtained by slow and fast cooling processes have
a diastereomeric relation. Thus, the slow cooling process gives
rise to the thermodynamically most stable form, while the fast
cooling process freezes the kinetically favored form.
Fig. 9 CD spectra of P8S (a) and P4S/PCN (b): as prepared film (—),
and annealed at isotropic temperature after cooling down at 10 ꢁC/min
(– –); 5 ꢁC/min (–C–); 2.5 ꢁC/min (–O–) and 1.2 ꢁC/min (–:–). Insets:
ellipticity values at 310 nm of P8S (up) and P4S/PCN-25 (down) as
prepared film and annealed at isotropic temperature after cooling down
at 10 ꢁC/min, 5 ꢁC/min, 2.5 ꢁC/min and 1.2 ꢁC/min.
predominantly obtained. When the cooling rate is 2.5 ꢁC/min the
CD signal can correspond to an intermediate of both forms with
a slight excess of the thermodynamically controlled form, which
ꢁ
clearly prevails at cooling rates equal to or lower than 1.2 C/
min. Furthermore, the stereomutation is detected at a cooling
rate similar to that used in the DSC study, which points to
a relation between the chiroptical properties and the bistable
mesomorphic thermal behavior detected for these polymers.
The CD spectra of copolymers P4S/PCN were also investi-
gated. For these copolymers, as-prepared cast films exhibited an
exciton splitting similar to that in P8S with the same thermal
history. Annealing of this copolymer in the isotropic state led to
an increase in the CD signal, which increases as the cooling rate
increases (Fig. 9b). However, stereromutation was not detected
for these copolymers, a finding that seems to be in accordance
with the observed thermal properties.
In order to investigate the evolution of the CD response upon
thermal treatment, a film of P8S was cooled from the isotropic
state to rt at different rates (Fig. 9a). At cooling rates higher or
ꢁ
equal to 5 C/min a negative exciton couplet was detected, the
intensity of which decreases with the cooling rate. However, after
cooling at 2.5 ꢁC/min the exciton couplet disappears and only
a negative Cotton effect at around 340 nm was detected. The
inversion of the exciton couplet from negative to positive is
clearly induced when the iꢁsotropic state is cooled to room
temperature at a rate of 1.2 C/min. According to these results,
both diastereomeric supramolecular organizations are probably
present in the rapidly cooled materials, but at high cooling rates
($5 ꢁC/min) the kinetically controlled organization is
Conclusions
A new series of azopolymers having a chiral terminal alkyl chain
has been synthesized and characterized. Polymers having a chiral
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~ ꢀ
15 R. M. Tejedor, L. Oriol, J. L. Serrano, F. Partal Urena and J. J. Lopez
ꢀ
methylpropyloxy terminal chain exhibit a monolayer smectic A
phase but in a very short range of temperature that increases by
copolymerization with a cyanoazobenzene monomer. The most
interesting results have been obtained for polymers bearing
a chiral methylheptyloxy terminal chain (P8S and P8R), which
show a bistable mesomorphism governed by the cooling rate
from the isotropic state: low cooling rates determine a standard
SmA phase and high cooling rates impose a transparent and
lamellar mesophase which has been assigned by Kozlovsky and
co-workers to a TGBA* phase. Azobenzene units in these azo-
polymers tend to aggregate both in low polar solvents or in films,
which results in chiral supramolecular organizations. The chi-
roptical properties of the polymeric films strongly depend on the
thermal history. Mirror-image CD spectra of P8S (or P8R) are
obtained by cooling the films from the isotropic state to rt at
different rates. This thermal stereomutation is associated to the
bistable mesomorphism. The cooling rate at whicꢁh mesomorphic
and chiroptical properties change, is about 2–5 C/min. Conse-
quently, these materials have a thermal and reversible switching
both on mesomorphic and chiroptical properties.
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