Siloxane polymers containing azo moieties synthesized by click chemistry for photo responsive and liquid crystalline applications

Article abstract of DOI:10.1002/pola.25885

Three new types of siloxane-based photoactive liquid crystalline polymers containing azo side groups were synthesized through the click chemistry route. The polymers having molecular weight range of 14,000-34,000 g mol^-1 were soluble in most of the polar solvents like chloroform, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, and dichloromethane. The photoresponsive trans-cis photoisomerization under UV radiation and cis-trans relaxation process in dark for the polymers were studied. The isomerization rate constants were found to be 0.01-0.04 sec^-1 and 1.16*10^-4-4. 67*10^-4 sec^-1, respectively. It has been noted that the polymers showed high intensity absorption for n-π* in chloroform. Both trans and cis forms of azide monomers having azo moiety exhibited molar extinction coefficient (Iμ_max) in the range of 22,000-33,000 L mol^-1 cm^-1. The thermotropic behavior of the polymers was studied by polarizing optical microscope (POM) and differential scanning calorimetry (DSC) experiments. Polymer P1 showed liquid crystalline textures of nematic droplets, whereas P2 showed smectic focal conic texture and nematic droplets. Polymer P1 was also studied for photomechanical bending on exposure to UV radiation. The polymers showed initial degradation temperature in the range of 210-275°C.

Full text of DOI:10.1002/pola.25885

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Siloxane Polymers Containing Azo Moieties Synthesized by Click
Chemistry for Photo Responsive and Liquid Crystalline Applications
Someshwarnath Pandey, Balakrishna Kolli, Sarada P. Mishra, Asit B. Samui
Naval Materials Research Laboratory, Polymer Science and Technology Centre, Ambernath-421506, India
Correspondence to: A. B. Samui (E-mail: absamui@gmail.com)
Received 2 November 2011; accepted 29 November 2011; published online 29 December 2011
DOI: 10.1002/pola.25885
ABSTRACT: Three new types of siloxane-based photoactive liq-
uid crystalline polymers containing azo side groups were syn-
thesized through the click chemistry route. The polymers
having molecular weight range of 14,000–34,000 g molꢀ1 were
soluble in most of the polar solvents like chloroform, tetrahy-
drofuran, dimethylformamide, dimethyl sulfoxide, and
dichloromethane. The photoresponsive trans–cis photoisomeri-
zation under UV radiation and cis–trans relaxation process in
dark for the polymers were studied. The isomerization rate
22,000–33,000 L molꢀ1 cmꢀ1. The thermotropic behavior of the
polymers was studied by polarizing optical microscope (POM)
and differential scanning calorimetry (DSC) experiments. Poly-
mer P1 showed liquid crystalline textures of nematic droplets,
whereas P2 showed smectic focal conic texture and nematic
droplets. Polymer P1 was also studied for photomechanical
bending on exposure to UV radiation. The polymers
showed initial degradation temperature in the range of 210–
ꢁ
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275 C. 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
constants were found to be 0.01–0.04 secꢀ1 and 1.16*10ꢀ4
–
Chem 50: 1205–1215, 2012
4.67*10ꢀ4 secꢀ1, respectively. It has been noted that the poly-
mers showed high intensity absorption for n-p* in chloroform.
Both trans and cis forms of azide monomers having azo moiety
exhibited molar extinction coefficient (fimax) in the range of
KEYWORDS: azo polymers; click chemistry; differential scanning
calorimetry (DSC); liquid crystalline; nematic; photoswitching;
polysiloxanes; smectic; UV–vis spectroscopy
INTRODUCTION Azo polymers are well known for their
trans–cis isomerization. The two isomers can switch between
them with specific irradiations.1 The photo-isomerization of
azobenzene is also designated by a form of light-induced mo-
lecular motion.2–4 Further as research has progressed, more
and more applications started emerging such as, photo-
switching, information storage, holography, etc.5 The azo
polymers are also found to have LC properties due to the
rod-like nature of azo benzene moiety.6 The basic properties
of side-chain liquid crystal polymers have been extensively
studied because of their potential applications in nonlinear
optics, information storage, and display devices, etc.7
drons.11 Stabilization of a hexagonal columnar mesophase,
generated from supramolecular and macromolecular col-
umns, was also reported.12 Isomerization of helical dendron-
ized polyphenylacetylenes was reported in the series.13 One
interesting review article by the same author appeared very
recently, which deals with the pattern of self organization in
conventional functional polymers, where the functional
group was replaced with a dendron.14
While the implication of structural parameters affecting the
properties were given importance, a new area emerged,
which was concerned with the application of various design
so that it was very close to mimicking biological systems. A
library of amphiphilic dendritic dipeptides that self-assemble
in solution and in bulk through a complex recognition pro-
cess into helical pores was described by Percec et al.15 The
molecular recognition and self-assembly process was found
to be sufficiently robust to tolerate a series of modifications
to the amphiphile structure. Attaching conducting organic
donor or acceptor groups to the apex of a dendron leads to
supramolecular nanometer-scale columns that contain p-
stacks of donors, acceptors, or donor–acceptor complexes in
their cores which imparted them high charge carrier
mobilities.16
Molecular engineering of side chain liquid crystalline poly-
mer, by living cationic polymerization, was published by Per-
cec et al.8 Flexible backbone was selected to tailor the LC
transition temperatures. Side chain branched polymers were
beginning to show their usefulness in a big way due to their
property of self assembly which made them eligible for
showing excellent optical properties.9 Percec et al. investi-
gated the changes in structure with temperature for the
supramolecular structure formed by a poly(methacrylate)
with tapered side chains.10 Cylindrical and spherical assem-
bly was developed further by starting with amphiphilic den-
C
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The synthesis and calorimetric properties of azo methacry-
late was reported by Angeloni et al.17 The strategy of synthe-
sizing side chain liquid crystalline polymers depends mostly
on methacrylates and siloxane having pendant LC molecules.
The strategies adopted are very important as it affects the
final shape and properties of the polymer. There are a num-
ber of reports that encompass a variety of synthetic strat-
egies while considering chain conformation, association,
degree of polymerization, and other related parameters.18–24
The complexation of small molecules with salts can also
impart specific properties to the associated species such as
stabilization of the mesophase.25 Molecular weight plays also
a major role in exhibiting the LC nature of the polymers.26
meric liquid crystalline materials with azobenzene moieties
could be promising materials for both optical switching and
image storage as the orientation of mesogens in the thin
films of these materials can be modified by light.
In this article we report the synthesis of various siloxane-
based polymers, containing azo as pendent group, by using
the click chemistry route. Although there are several reports
in the literature regarding the synthesis of azo polymers by
click chemistry, few of them suffered from solubility and
processability problems.42 In this work we have incorporated
a siloxane unit in the main chain as a flexible spacer by the
click chemistry route. The polymers were characterized by
electronic spectroscopy, thermogravimetry, differential scan-
ning calorimetry, and hot stage optical microscopy. The
kinetic study on isomerization has also been studied.
The first report of both photoexpanded and photocontracted
surface patterns in the same material, at different tempera-
tures, was published by Yager et al.27 Later it was identified
as photomechanical effect. The photoisomerization of LC
moieties to impart actuator effect in materials was first pro-
posed by de Gennes et al.28
EXPERIMENTAL
Materials
Propargyl bromide, 1,1,3,3,5,5-hexamethyl trisiloxane, 2%
platinum(0)-1,3-divinyl-1,1,3,3-tetramethyl disiloxane com-
plex in xylene (karstedt’s catalyst), allyl alcohol, aniline, n,n-
bis(diethanol)aniline, aniline, sodium-ascorbate, 4,4-dimethyl
amino pyridine (DMAP), dicyclohexyl carbodimide (DCC),
and phosphorus oxychloride (POCl3) were purchased from
Sigma-Aldrich. All other chemicals were purchased from SD
Fine Chemicals, India. All solvents were purchased from
Merck India Pvt. Ltd. THF and toluene were dried over
sodium-ketyl radical. All the chemicals were used as received
without further purifications unless otherwise specified.
Thus, the theoretical aspects combined with potential uses in
devices affected increasing interest in their synthesis for var-
ious applications.29 A lot of work has been done to realize
the above effects by designing mainly side chain liquid crys-
talline azo polymer. Encompassing all the efforts, these poly-
mers have been the subject of intense study for quite a long
time.30 The proportion and positional role of the photosensi-
tive groups in the crosslinked polymer network influence
different types and magnitudes of responses.31 Photoisomeri-
zation-induced phase transition of liquid-crystalline azo-
benzene chromophore and its influence on the phase dia-
grams of its mixtures with reactive mesogenic diacrylate
monomer has also been investigated both theoretically and
experimentally.32
Characterization
UV–Visible spectra of the polymers were taken on a Cary
500 Scan UV-Vis-NIR spectrophotometer. 1H and 13C NMR
spectra of the monomers and the polymers were recorded
on either a 500-MHz Bruker-FT NMR spectrometer or a 300-
MHz JEOL-FT NMR spectrometer. Thermogravimetric analysis
(TGA) was performed with a TA instruments His Res TGA
2950 with a heating rate of 20ꢁC/min under N2 atmosphere.
Heating/cooling rate used for all DSC analysis was 10ꢁC/
min. For studying the liquid crystalline behavior, a Leica
DMLD, an optical polarizing microscope with image analyzer
equipped with LINKAM TMS 94 hot stage, and a LINKAM
LNP controlling unit were used. The molar extinction coeffi-
cients (fimax) of diazide azo monomers were calculated by
following the procedure described by Zhu et al.23 For photo-
responsive studies the samples were irradiated in the cabi-
net of a UV chamber (Spectrolinker XL – 1500) with 6 ꢂ 15
Watt medium pressure mercury bulbs. The sample was
placed at a distance of 15 cm from the irradiation source for
various time intervals.
Siloxane based side-chain azo polymers can be considered as
a class of photoactive material due to the chain flexibility
which will help in rapid photoactuation. Their other proper-
ties are easy processablity, high thermoxidative stability, low
surface energy, high chemical resistance, and good film form-
ing properties, etc. In addition, as siloxane units are highly
flexible, they are expected to show a lower liquid crystalline
transition temperature as compared to the carbon ana-
logues.33 A great number of studies have been reported on
liquid crystalline polymers incorporating siloxane as a flexi-
ble spacer in the main chain that includes polysiloxane liquid
crystalline polymer, which was first synthesized by Finkel-
mann et al. through hydrosilylation reaction between hydro-
gen containing silicone-oil and vinyl liquid crystalline mono-
mers.34 Recently, Mather et al. have also reported the
synthesis of main-chain siloxane-based liquid crystalline
polymer for soft actuation devices.35 A wide range of liquid
crystalline polysiloxanes possessing half-disk and rod-like
moieties,36 methyl stilbene groups,37 macroheterocyclic
ligands,38 have been reported. Percec et al. reported a micro-
phase-separated morphology in siloxane-based LC polymer
consisting of main chain and side chain domains.39,40 Racles
et al. reported the synthesis of siloxane-based liquid crystal-
line photoactive polyethers.41 Therefore, siloxane-based poly-
Methods
Synthesis of Methyl 4-(prop-2-ynyloxy) Benzoate (1)
Scheme 1 shows the synthesis of diyne and diazide mono-
mers. To a solution of methyl 4-hydroxy benzoate (5 g, 33
mmol) in DMF (40 mL), K2CO3 (16.67 g, 120 mmol) was
added. The mixture was heated to 80ꢁC for 30 min followed
by dropwise addition of propargyl bromide (80% in toluene)
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(7.4 mL, 50 mmol). The reaction mixture was stirred at the
same temperature for additional 12 h, quenched in cold
water, and extracted with chloroform. The organic layer was
washed with water, dried over anhydrous Na2SO4 and evapo-
rated under vacuum. The product was purified by column
chromatography using ethyl acetate and pet ether (5:95 v/v)
to obtain off-white colored waxy solid.
ether and ethyl acetate (98:2) eluant to obtain the product
as colorless oil.
Yield: 3.5 g, (77 %); 1H NMR (500 MHz, CDCl3) d: 0.03 (s,
6H), 0.09 (s, 12H), 0.61 (t, J ¼ 8.5 Hz, 4H), 1.76 (q, J ¼ 7.0
Hz, 4H), 2.53 (t, J ¼ 2.0 Hz, 2H), 4.23 (t, J ¼ 6.5 Hz, 4H),
4.72 (d, J ¼ 4.0 Hz, 4H), 6.98 (d, J ¼ 9.0 Hz, 4H), 8.00 (d, J
¼ 9.0 Hz, 4H); 13C NMR (125 MHz, CDCl3) d: 1.14, 1.24,
14.13, 22.74, 55.77, 67.13, 76.00, 77.83, 114.41, 123.81,
131.45, 161.04, 166.14
Yield: 4.6 g (74.6%); 1H NMR (500 MHz, CDCl3) d: 2.54 (t, J
¼ 2.5 Hz, 1H), 3.88 (s, 3H), 4.74 (d, J ¼ 2.5 Hz, 2H), 6.99 (d,
J ¼ 8.5 Hz, 2H), 8.00 (d, J ¼ 9.0 Hz, 2H); 13C NMR (125
MHz, CDCl3) d: 51.86, 55.78, 76.05, 77.79, 114.51, 123.42,
131.50, 161.11, 166.64.
N,N-Bis-(2-chloroethyl)-aniline (5)
The synthesis was carried out by following the procedure as
reported by Shen et al.44 N,N-diethanol aniline was con-
verted to N,N-dichlorodiethyl aniline using POCl3 as chlori-
nating agent.
4-(Prop-2-ynyloxy) Benzoic Acid (2)
4-(prop-2-ynyloxy) benzoate (1) (3.5g, 18.4 mmol) was dis-
solved in a mixed solvent of tetrahydrofuran (THF) (20 mL)
and methanol (10 mL). To the solution, an aqueous solution
of sodium hydroxide (40%, 10 mL) was added. The reaction
mixture was allowed to reflux for 2 h. The organic solvent
was distilled off under vacuum and water was added to
make a clear solution. The solution was acidified by addition
of conc. HCl acid. The precipitate obtained was filtered,
washed with water, and dried under vacuum to obtain a
white solid.
Synthesis of Azo Compound (6)
The synthesis of azo compound was carried out as reported
by Shen et al.44 In a typical recipe aniline (2 g, 21.5 mmol)
was added to 40 mL of 5M HCl at 0ꢁC. To the mixture, ice-
cooled aqueous saturated solution of sodium nitrite (1.78 g,
25.8 mmol) was added maintaining the temperature between
0 and 5ꢁC. This mixture was then added dropwise to another
round bottom flask containing N,N-bis(2-chloroethyl)-aniline
(5) (4.68 g, 21.5 mmol) dissolved in ethanol and maintained
at 0ꢁC (3% solution w/v). During addition, the temperature
was not allowed to rise above 10ꢁC. The stirring was contin-
ued for 4 h at the same temperature and then warmed to
room temperature. The reaction mixture was filtered,
washed several times with water, dried, and recrystallized
from 2-methoxyethanol to obtain the product as a red solid.
Yield: 2.6 g (80.2 %); 1H NMR (500 MHz, CDCl3) d: 3.59 (s,
1H), 4.86 (d, J ¼ 2.0 Hz, 2H), 7.04 (d, J ¼ 9.0 Hz, 2H), 7.88
(d, J ¼ 9.0 Hz, 2H); 13C NMR (125 MHz, CDCl3) d: 39.91,
56.05, 79.02, 79.14, 115.04, 124.11, 131.67, 161.13.
Trisiloxane-Diol (3)43
To a solution of allyl-alcohol (11.2 g, 193 mmol) in anhydrous
toluene (25 mL), Karstedt’s catalyst (0.45 mL) was added. It
was followed by dropwise addition of 1, 1, 3, 3, 5, 5-hexam-
ethyl trisiloxane (10 g, 48 mmol) over half an hour at 50ꢁC.
After the addition was complete, the temperature was raised
to 70ꢁC and the reaction continued up to 48 h. Progress of
the reaction was monitored through disappearance of SiAH
peak in IR spectra at 2163 cmꢀ1. Toluene and excess allyl
alcohol were removed by distillation. The crude product was
dissolved in 30 mL dichloromethane (DCM) and purified by
passing through a bed of alumina followed by evaporation of
solvent under vacuum to yield a colorless viscous liquid.
Yield: 5.1 g, (71 %); 1H NMR (500MHz, CDCl3,) d: 3.69 (t, J
¼ 7.0 Hz, 4H), 3.84 (t, J ¼ 7.0 Hz, 4H), 6.77 (d, J ¼ 9.0 Hz,
2H), 7.39–7.42 (m, 1H), 7.47–7.50 (m, 2H), 7.85 (d, J ¼ 7.5
Hz, 2H), 7.90 (d, J ¼ 7.0 Hz, 2H); 13C NMR (125 MHz, CDCl3)
d: 40.23, 53.46 ,111.59, 122.35, 125.23, 128.97, 129.87,
144.64, 148.45, 152.97.
4-((4-(Bis-(2-chloroethyl) amino) phenyl)
diazenyl) Benzonitrile (7)
The synthesis was carried out by using same procedure as
mentioned for product 6 except the use of aniline. In this
case ACN (cyano) group was present at the para position
and the product was recrystallized from a water–ethanol
mixture.
1
Yield: 14.5 g, (93.1 %); H NMR (300 MHz, CDCl3) d: 0.01 (s,
18H), 0.50 (s, 4H), 1.52 (s, 4H), 2.60 (bs, 2H), 3.50 (s, 4H);
13C NMR (125 MHz, CDCl3): 0.92, 1.87, 13.87, 26.38, 65.25.
Yield: 6.58 g, (85 %); 1H NMR (500 MHz, CDCl3) d : 3.71 (t,
J ¼ 6.5 Hz, 4H), 3.86 (t, J ¼ 7.0 Hz, 4H), 6.78 (d, J ¼ 9.0 Hz,
2H), 7.77 (d, J ¼ 8.0 Hz, 2H), 7.91 (d, J ¼ 8.0 Hz, 2H), 7.92
(d, J ¼ 9.0 Hz, 2H); 13C NMR (125 MHz, CDCl3) d: 40.45,
53.75, 111.97, 112.81, 119.09, 123.17, 126.30, 133.39,
144.87, 149.85, 155.40.
Trisiloxane-Diyne (4)
4-(prop-2-ynyloxy) benzoic acid (2) (3g, 17.1 mmol), trisilox-
ane-diol (3) (2.3g, 7.1 mmol), DMAP (0.19 g, 1.5 mmol) and
25 mL of anhydrous THF were taken in a 100-mL round
bottom flask. The flask was made air tight by using a rubber
septum. Nitrogen gas was continuously passed through
schlenk tube. A solution of DCC (3.07 g, 15 mmol) in 5 mL
THF was added to the round bottom flask through the sep-
tum via a syringe. The mixture was allowed to stir under
nitrogen for 48 h. The reaction mixture was filtered and the
organic phase was evaporated. The residue was purified by
column chromatography by using a mixture of petroleum
4-((4-Bis-(2-chloroethyl) amino) phenyl) diazenyl)
Benzoic Acid (8)
The synthesis was carried out by using the same procedure
as mentioned for product 6, except the use of aniline. Para-
amino benzoic acid was used in this case and the product
was recrystallized from a water–ethanol mixture.
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SCHEME 1 Synthesis of siloxane-diyne and diazide monomers.
1
Yield: 7.2 g (89%); H NMR (500 MHz, DMSO–d6) d: 3.81 (t,
J ¼ 4.5 Hz, 4H,), 3.88 (t, J ¼ 4.5 Hz, 4H), 6.95 (d, J ¼ 6.5 Hz,
2H), 7.84–8.09 (m, 6H); 13C NMR (125 MHz, DMSO-d6) d:
41.40, 52.36, 112.43, 122.30, 125.80, 130.93, 131.71, 143.91,
150.49, 155.35, 167.27.
The purification was done by column chromatography using
a mixture of ethyl acetate and petroleum ether (20:80 v/v).
An orange solid product was obtained.
Yield: 1.6 g, (77 %); 1H NMR (500 MHz, CDCl3) d: 3.70 (t, J
¼ 7.0 Hz, 4H), 3.85 (t, J ¼ 6.5 Hz, 4H), 3.94 (s, 3H), 6.78 (d,
J ¼ 9.0 Hz, 2H), 7.87 (d, J ¼ 8.5 Hz, 2H), 7.92 (d, J ¼ 8.5 Hz,
2H), 8.15 (d, J ¼ 8.0 Hz, 2H); 13C NMR (125 MHz, CDCl3) d:
40.17, 52.18, 53.44, 111.62, 122.14, 125.70, 130.52, 130.64,
144.67, 149.08, 155.64, 166.68.
Synthesis of Methyl 4-((4-Bis(2-chloroethyl) amino)
phenyl) diazenyl) Benzoate (9)
To a solution of 4-((4-(bis(2-chloroethyl) amino) phenyl) dia-
zenyl) benzoic acid (8) (2 g, 5.27 mmol) in methanol (100
mL), two drops of conc. H2SO4 was added. The solution was
refluxed for 24 h. The reaction mixture was cooled to room
temperature and added to ice cold water. The product was
extracted in chloroform, washed with saturated solution of
NaHCO3, dried over Na2SO4, and evaporated under vacuum.
Representative Example of Synthesis
of Azo-Diazide (10)44
To a solution of N,N-bis(2-chloroethyl)-4-(phenyldiazenyl)
aniline (6) (2 g, 6.2 mmol) in DMSO (10 mL), sodium azide
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SCHEME 2 Synthesis of azo containing siloxane polymers by click chemistry.
1
(1.1 g, 16.9 mmol) was added. The reaction mixture was
heated at 90ꢁC for 5 h and cooled to room temperature and
poured into dilute HCl (5%, 15 mL). The compound was
extracted with dichloromethane. The organic layer was
washed with water, dried over Na2SO4, and evaporated
under vacuum. Purification of the product was done by col-
umn chromatography using petroleum ether and ethyl ace-
tate mixture (95:5; v/v).
Yield: 2.07 g, (85 %); H NMR (500 MHz, CDCl3) d: 3.58 (t, J
¼ 6.0 Hz, 4H), 3.70 (t, J ¼ 6 .0 Hz, 4H), 3.94 (s, 3H), 6.80 (d,
J ¼ 9.0 Hz, 2H), 7.87 (d, J ¼ 8.5 Hz, 2H), 7.92 (d, J ¼ 9.0 Hz,
2H), 8.15 (d, J ¼ 8.5 Hz, 2H); 13C NMR (125 MHz, CDCl3) d:
48.77, 50.75, 52.17, 111.86, 122.11, 125.67, 130.52, 130.58,
144.61, 149.27, 155.69, 166.70.
Synthesis of Polymers by Click-polymerization (P1)
Scheme 2 shows the synthesis of azo-siloxane polymer by
click chemistry route.
Yield: 1.97 g, (95%); 1H NMR (500 MHz, CDCl3) d: 3.56 (t, J
¼ 6.0 Hz, 4H), 3.69 (t, J ¼ 6.0 Hz, 4H), 6.79 (d, J ¼ 9.0 Hz,
2H), 7.39–7.50 (m, 3H), 7.47–7.50 (m, 2H), 7.85 (d, J ¼ 8.0
Hz, 2H), 7.89 (d, J ¼ 7.0 Hz, 2H); 13C NMR (125 MHz, CDCl3)
d: 48.76, 50.77, 111.90, 122.33, 125.19, 128.98, 129.83,
144.58,148.65, 153.01.
A mixture of trisiloxane-diyne (4) (0.20 g, 0.312 mmol), N,N-
bis(2-azidoethyl)-4-(phenyldiazenyl) aniline (9) (0.10 g,
0.312 mmol), and sodium ascorbate (0.130 g, 0.656 mmol)
was taken in 3 mL of THF. The mixture was allowed to stir
under nitrogen atmosphere for 10 min. A solution of copper
sulfate (0.086 g, 0.34 mmol) in 3 mL of water was added to
it. The reaction mixture was allowed to stir for additional 40
min under N2 atmosphere and quenched with methanol (20
mL). The precipitate was filtered, dissolved in THF, and
passed through an alumina bed to remove the catalyst and
insoluble impurities. The THF solution was then evaporated
to obtain the polymer (P1) as a red powder. Similarly, poly-
mers P2 and P3 were synthesized by reacting (4) with (10)
and (11), respectively.
4-((4-Bis(2-azidoethyl) amino) phenyl) diazenyl)
Benzonitrile (11)
The synthesis was carried out by using (7) as starting mate-
rial following the method as used for synthesis of (10).
1
Yield: 2.02 g, (91 %); H NMR (500 MHz, CDCl3,) d: 3.59 (t, J
¼ 6.0 Hz, 4H), 3.71 (t, J ¼ 6.0 Hz, 4H), 6.82 (d, J ¼ 3.0 Hz,
2H), 7.76 (d, J ¼ 8.0 Hz, 2H), 7.90 (d, J ¼ 8.5 Hz, 2H), 7.92
(d, J ¼ 9.0 Hz, 2H); 13C NMR (125 MHz, CDCl3) d: 48.76,
50.74, 111.87,112.35,118.91, 122.83,125.96, 133.07, 144.46,
149.69, 155.17.
Yield: 0.22 g, (73 %); 1H NMR (500 MHz, CDCl3) d: 0.06–
0.08 (bs, 18H), 0.59 (bs, 4H), 1.24 (bs, 4H), 3.53–3.60 (bs,
4H), 4.20 (bs, 4H), 4.38 (bs, 4H), 5.17 (bs, 4H), 6.64–7.95
(bs, 17H), 7.46 (bs, 1H); 13C (125 MHz, CDCl3) d: 0.95, 1.23,
14.10, 22.71, 47.31, 51.46, 61.71, 67.19, 112.19, 114.35,
Methyl 4-((4-(Bis(2-azideoethyl) amino) phenyl)
diazenyl) Benzoate (12)
The synthesis was carried out by using (9) as starting mate-
rial following the method as used for the synthesis of (10).
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FIGURE 1 1H NMR spectra of polymer P2.
1
122.42, 123.95, 125.31, 129.00,130.16, 131.52, 143.74,
145.06, 147.49, 152.80, 161.57, 166.14.
The hydroxyl peak of H NMR spectra of siloxane-diol (3) [d
2.6] is found to be absent in trisiloxane-diyne (4), thus indi-
cating the completion of the reaction. The other peak posi-
tions of trisiloxane-diol (3) are also found to be shifted
downfield in trisiloxane-diyne (4).The respective 1H NMR
peak positions of 10, 11, and 12 also identify the targeted
structure.
P2
Yield: 0.22 g, (71.2 %); 1H NMR (500 MHz, CDCl3) d: 0.02–
0.09 (br, 18H), 0.59 (bs, 4H), 1.25 (bs, 4H), 3.65 (bs, 4H),
4.21 (bs, 4H), 4.41 (bs, 4H), 5.19 (bs, 4H), 6.65–7.90 (br,
16H), 7.25 (bs, 1H); 13C NMR (125 MHz, CDCl3) d: 0.10,
1.25, 14.11, 22.73, 47.33, 51.40, 61.73, 67.22, 112.07,
114.37,115.12, 118.68, 122.95, 126.06, 131.53,133.10,
143.93, 144.86, 148.10, 154.30, 161.58, 166.15.
The polymerizations were carried out by taking equimolar ra-
tio of the trisiloxane-diyne (4) and one of the azo diazides
(10–12) in the presence of copper sulfate and sodium ascor-
bate in an aqueous solution of THF. The reaction was found to
be very fast with a completion time of approximately 1 h to
obtain a highly viscous polymer solution. The H NMR spectra
shows the appearance of a peak around 7.46–7.48 ppm which
can be attributed to the proton in the triazole ring (Fig. 1).
P3
Yield: 0.25 g, (77 %); 1H NMR (500 MHz, CDCl3) d: 0.02–
0.09 (bs, 18H), 0.59 (bs, 4H), 1.10 (bs, 4H), 3.64 (bs, 4H),
3.93 (bs, 3H) 4.21(bs, 4H), 4.40 (bs, 4H), 5.18 (bs, 4H), ,
6.60–8.20 (br, 16H), 7.48 (bs, 2H); 13C NMR (300 MHz,
CDCl3) d: 0.10, 14.09, 22.71, 47.31, 51.41, 52.21, 61.71,
67.20, 112.08, 114.34, 122.21, 123.57, 123.97, 125.78,
130.54, 131.52, 143.82, 145.12, 155.44, 161.54, 166.13.
1
From the GPC results, the molecular weights (Mn) of the
polymers are found to be in the range of 14,000–35,000
gmolꢀ1. It can be noted that although the reaction parame-
ters remained same the molecular weight of the polymer
varies with the substituents of the azo groups. Polymer P3
has the highest molecular weight whereas polymer P1 has
the lowest. All the polymers are soluble in most of the
chlorinated solvents and THF. The film-forming properties
are also found to be very good.
RESULTS AND DISCUSSION
Synthesis and Characterizations
Synthesis of silicone polymers, having azo side groups, by
click chemistry route and their characterization are pre-
sented in this article. The design of polymers is planned for
their use as liquid crystalline polymers as well as photoac-
tuators. The siloxane backbone is chosen with an idea of
allowing a higher degree of freedom so that azo may work
both as a chromophore as well as a mesogen.
The synthesis of trisiloxane-diyne, was done through a four-
step synthetic procedure as shown in Scheme 1. The overall
yield of the monomer trisiloxane-diyne (4) is found to be
77%. The trisiloxane-diol (3) was successfully synthesized in
high yield (95%) using platinum catalyzed (karstedt’s cata-
lyst) hydrosilylation reaction at moderate temperature. Vari-
ous diazide containing azo monomers were prepared by
using procedure as shown in Scheme 1. The final azo mono-
mers with different substitutions at the para position are
synthesized and purified successfully. All the monomers are
1
characterized by both H and 13C NMR spectra.
FIGURE 2 DSC Thermogram of polymers P1 (A) and P2 (B).
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TABLE 1 Physico-Chemical Properties of Polymers
Thermal
Transition
(DSC)a
Polymer
POMb Ti
Mn
Mw
29,200
Mw/Mn
Td
P1
P2
P3
a
C 73 N 165 I
C 53 Sm 142 I
Tg 44
170
149
14,550
17,140
33,750
2.00
2.36
3.06
210
230
275
40,500
c
_
103,600
Transition temperature identified from heating cycle with DSC (at a heating rate of 10ꢁC/min under N2
atm).
b
Transition temperature identified with POM at a heating rate of 5ꢁC/min.
c
Not identified.Td ¼ onset degradation temperature of polymers; Ti ¼ isotropization temperature.
Thermogravimetric analysis (TGA) of the synthesized poly-
mers shows that the polymers possess reasonable stability;
only 5% wt loss is observed at 210–275ꢁC. The initial degra-
dation temperature increases from polymer P1 to polymer
P3. This may be due to the increase in molecular weight in
the order P1 < P2 < P3.
showing two and polymer P2 showing three, respectively.
The first one can be attributed to the crystalline (Cr) and liq-
uid crystalline (LC) phase transition. The transition of crys-
talline phase (Cr) to liquid crystalline phase (LC) for P1 is
higher at 73ꢁC whereas for polymer P2 it is 53ꢁC and for P3
the liquid crystallinity is absent. This could be due to the
highest ordering/packing efficiency of polymer P1 as the
size of the substitution on azo is lowest. In polymer P3, the
glass transition temperature is found to be at 44ꢁC. The ab-
sence of the LC phase indicates poor ordering of the methyl
ester substituent on the azo group. The second endotherm in
the DSC thermogram of P1 can be attributed to
Liquid Crystalline Property
DSC Study
Figure 2 shows the DSC thermogram of polymers P1 and P2
and the data are incorporated in Table 1. It is noted that
both the polymers exhibit endothermic peaks; polymer P1
FIGURE 3 POM images showing nematic droplets texture for P1 at temperature 150ꢁC (A) and 160ꢁC (B) and focal conic texture
for P2 at 100ꢁC (C) and threaded type nematic texture at 140ꢁC (D).
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TABLE 2 The Molar Extinction Coefficient of the Diazide
Monomers in CHCl3
fimax
fimax
(p ꢀ p*)
L molꢀ1
mꢀ1
(n ꢀ p*)
L molꢀ1
cmꢀ1
Diazide
N,N-bis(2-azidoethyl)-4-(phenyldiazenyl)
aniline (10)
21,519
24,810
32,000
22,456
25,783
33,594
4-((4-Bis(2-azidoethyl) amino)
phenyl) diazenyl) benzonitrile (11)
Methyl 4-((4-(bis(2-azideoethyl)
amino) phenyl) diazenyl) benzoate (12)
isotropization. For P2, apart from isotropization one more
endotherm is observed which appears just before isotropiza-
tion. This may be due to the appearance of the second LC
phase after the first one disappears. The isotropization tem-
peratures of P1 and P2 lie at 165ꢁC and 142ꢁC, respectively.
FIGURE 4 UV–Visible absorption spectra in chloroform solu-
tion for trans–cis isomerization of polymer P3.
POM Study
ꢀ p* transition in chloroform solution, the color of the
solution changes from yellow to orange on UV irradiation
indicating rapid transition to cis state.
The mesogenic behaviors were studied by POM. The POM
micrographs are shown in Figure 3 and the observation is
summarized in Table 1. The appearance of the droplet tex-
ture of P1 is observed at 79ꢁC, indicating the presence of
nematic phase. At 179ꢁC, the polymer undergoes isotropiza-
tion. Similarly, polymer P2 exhibits the appearance of a
texture characteristic of broken focal conic smectic (SmA)
phase [Fig. 3(c)]. A similar texture was also reported by
Chao et al. for liquid crystalline polysiloxane containing fluo-
rinated mesogens.45 On further heating the polymer, the
texture changes to droplets type, which is characteristic of
nematic state [Fig. 3(d)]. The isotropization temperatures of
P1 and P2 are observed to be 170ꢁC and 149ꢁC, respectively.
This is near to that observed in the DSC study. Thus, the LC
phase extends over a broad temperature range for P1 and
P2. The isotropization displays a broad transition for both
the polymers P1 and P2. This can be due to the fact that the
azo side chain is directly linked through a short spacer to
the main chain having siloxane segments in the backbone. In
general, broad isotropization temperature is common to side
chain liquid crystalline polymers having short spacers.46
For studying the photoresponsive properties, the chloroform
solution of the polymer was kept in dark for three days to
assume 100% trans form of the azobenzene. Any small
amount of cis form present is also converted to stable trans
form. Before irradiation the kmax absorbance maxima was
recorded at 387, 415, and 406 nm for polymers P1, P2, and
P3, respectively. This is due to p ꢀ p* transition for stable
trans form. The polymer solutions were then irradiated with
UV light (365 nm) for different time intervals and the ab-
sorbance vs. wavelength was plotted. Two absorbance peaks
are observed in UV–Visible spectra for P3 (Fig. 4). The peaks
around 400 and 500 nm can be due to p ꢀ p* and n ꢀ p*
transitions, respectively. It is found that the absorbance of
Photo Responsive Properties
Having azo side groups, the polymers are photoresponsive in
nature. The molar extinction coefficient of the diazide mono-
mers (11–12) for both p ꢀ p* and n ꢀ p* transitions were
calculated. During irradiation the diazide monomer shows
gradual increase of n ꢀ p* transition peak for trans–cis isom-
erization. The molar extinction coefficient for their n ꢀ p*
transition ranges in the order of 22,456 (10), 24,810 (11),
and 33,600 (12) L molꢀ1 cmꢀ1, respectively (Table 2). Usu-
ally molar extinction coefficient for n ꢀ p* absorption is
very low as compared to p ꢀ p* absorption. The diazide
monomers described here are found to have very high molar
extinction coefficient for the n ꢀ p* transition due to the
hyperchromic effect of the nitrogen atom attached to the
azobenzene group.43 Due to high intensity absorption for n
FIGURE 5 UV–Visible absorption spectra in chloroform solu-
tion for cis–trans isomerization of polymer P3.
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FIGURE 6 Plot of lnA vs. time for trans–cis isomerization of P1, P2, and P3.
the p ꢀ p* electronic transition decreases and n ꢀ p* transi-
tion increases with time of irradiation. This phenomenon
indicates the conversion of trans to cis form of the azo poly-
mer. The rate constants of isomerization of the polymers are
calculated from slope of the plot of ln A vs. time (Fig. 6). All
the polymers are found to isomerize within few minutes to
the cis form. It is observed that shouldering in cis peak
occurs at a higher wave length during conversion of trans to
cis form. This can be due to solvatochromism; the polar
interaction of cis azo shifts the absorption spectra to higher
wave length.47 The isomerization process was studied for cis
to trans interconversion for all the three polymers. For this
process, the chloroform solution of polymers was kept under
UV irradiation until complete disappearance of p ꢀ p* elec-
tronic transition as monitored by UV spectra. The samples
were then kept in dark and its absorption spectra recorded
at different time intervals. It is found that the cis–trans isom-
erization (Fig. 5) of the polymer samples is slow as com-
pared to trans–cis photoisomerization (Fig. 4). Ln A is plot-
ted vs. time for all the polymers. Two slopes (Kx, Ky) are
observed for all the polymers for trans–cis isomerization
(Fig. 6). The rate constants for trans–cis isomerization
decrease with increase in molecular weight. It may be due to
sluggishness in isomerization. Further, only one slope is
TABLE 3 The Rate Constants of the Polymers for trans–cis and cis–trans Isomerization
k
max(nm)a
k
max(nm)a
Kx
Ky
Kz
Polymers
p ꢀ p*
n ꢀ p*
(secꢀ1 b
)
(secꢀ1 c
)
(minꢀ1 d
)
P1
P2
P3
a
387
415
406
525
525
523
3.41 ꢂ 10ꢀ2
2.21 ꢂ 10ꢀ2
1.41 ꢂ 10ꢀ2
5.03 ꢂ 10ꢀ3
3.80 ꢂ 10ꢀ3
2.85 ꢂ 10ꢀ3
1.18 ꢂ 10ꢀ4
2.33 ꢂ 10ꢀ4
4.67 ꢂ 10ꢀ4
Recorded in chloroform solution.
b
c
Rate constant calculated from the first slope of trans to cis isomerization graph (ln A vs. time).
Rate constant calculated from the second slope of trans to cis isomerization graph (ln A vs. time).
Rate constant calculated from the slope of cis to trans isomerization (ln A vs. time).
d
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FIGURE 7 Photomechanical bending of polymer P1 under irradiation with UV light (a) before irradiation (b) after irradiation.
Chem. Phys. 2001, 202, 1649–1657; (e) Turkey, G.; Schonhals,
A. Polymer 2004, 45, 255–262; (f) Wang, H.; He, Y.; Tuo, X.;
Wang, X. Macromolecules 2004, 37, 135–146.
observed for cis–trans isomerization. This is due to slower
process of the latter conversion. Table 3 shows the rate con-
stants for trans–cis isomerization (Kx, Ky) and cis–trans isom-
erization (Kz).
6 Alazaroaie, S.; Toader, V.; Carlescu, I.; Kazmierski, K.; Scu-
taru, D.; Hurduc, N.; Simionescu, I. Eur. Polym. J. 2003, 39,
1333–1339.
Photomechanical Properties
7 McArdle, C. B. In Side Chain Liquid Crystal Polymers; McAr-
dle, C. B, Ed.; Blackie and Sons: Glasgow, 1989; Chapter 13.
For photomechanical study, the free standing film was pre-
pared as a bilayer. Polyvinyl alcohol (PVA) was first cast on a
glass slide and rubbed with silk cloth in one direction. The
azo polymer solution in THF was later cast on the PVA film
to make a bilayer. The bilayer was made as the azo polymer
free standing film which was very fragile.
8 Percec, V.; Tomazos, D. Adv. Mater. 1992, 4, 548–561.
9 Donald, A. M.; Windle, A. H.; Hanna, S. In Liquid Crystalline
Polymers; Cambridge University Press: Cambridge, UK, 2006;
Part 1, pp. 8.
10 Kwon, Y. K.; Chvalun, S. N.; Blackwell, J.; Percec, V.; Heck,
J. A. Macromolecules 1996, 28, 1552–1558.
The photoactuation of the polymers was studied by irradia-
tion with 365-nm irradiation. The PVA-Azo polymer P1 was
subjected to irradiation for its photomechanical effect. The
onset of bending of PVA-Azo bilayer is observed during a
typical irradiation intensity of 5500–6500 lW/cm2. The
bending increases as the isomerization of trans–cis azo goes
further. After 15 min, the bending angle is observed around
45ꢁ (Fig. 7). The bent film on being kept in the dark for a
long period of time regains the original shape, indicating the
transformation from cis to trans form.
11 Rudick, J. G.; Perec, V. Acc. Chem. Res. 2008, 41,
1641–1652.
12 Percec, V.; Schlueter, D.; Kwon, Y. K.; Blackwell, J. Macro-
molecules 1996, 28, 8807–8818.
13 Percec, V.; Rudick, J. G.; Peterca, M.; Martin Wagner, M.;
Obata, M.; Mitchell, C. M.; Cho, W.-D.; Venkatachalapathy, B. S.
K.; Heiney, P. A. J. Am. Chem. Soc. 2005, 127, 15257–15264.
14 Rosen, B. M.; Wilson, C. J.; Wilson, D. A.; Peterca, M.;
Imam, M. R.; Percec, V. Chem. Rev. 2009, 109, 6275–6540.
15 Percec, V.; Dulcey, A. E.; Venkatachalapathy, B. S. K.; Miura,
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CONCLUSIONS
Three types of siloxane polymers possessing both photoac-
tive and liquid crystalline properties have been successfully
prepared using click chemistry route. The polymers P1 and
P2 showed liquid crystalline characteristics existing at a
broad temperature range and all the three polymers showed
photoactive properties. The rate constants for trans–cis isom-
erization decreases with an increase in the molecular weight
of the polymers. The rate of trans–cis isomerization was
found to be higher than that of cis–trans isomerization. The
trans–cis isomerization has led to bending of bilayer film,
thus showing photomechanical effect.
16 Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanov-
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