Hyperbranched photo responsive and liquid crystalline azo-siloxane polymers synthesized by click chemistry

Article abstract of DOI:10.1002/pola.26042

Three new types of hyperbranched photoactive liquid crystalline siloxane polymers containing azo moieties were synthesized using click chemistry methodology. The polymers were soluble in most of the polar solvents like chloroform, tetrahydrofuran, dimethylformamide, dimethyl sulphoxide and dichloromethane. The molecular weights of the polymers were in the range of 9000-12,000 g mol^-1. The trans-cis photoisomerization of the polymer were studied both under UV radiation and dark. The isomerization rate constants were found to be in the range of 0.7-1.4 × 10^-2 sec ^-1 and 7.0 × -2.5 × 10^-5 sec^-1. The thermotropic behavior of the polymers was studied by using polarizing optical microscopy and differential scanning calorimetry, respectively. The polymers P1 and P2 showed liquid crystalline texture characteristic of nematic phase.

Full text of DOI:10.1002/pola.26042

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Hyperbranched Photo Responsive and Liquid Crystalline Azo-Siloxane
Polymers Synthesized by Click Chemistry
Someshwarnath Pandey, Sarada P. Mishra, Balakrishna Kolli, Tapan Kanai, Asit B. Samui
Polymer Science and Technology Centre, Naval Materials Research Laboratory, Shil-Badlapur Road, Ambernath 421506,
Maharashtra, India
Correspondence to: A. B. Samui (E-mail: absamui@gmail.com)
Received 14 February 2012; accepted 8 March 2012; published online 30 March 2012
DOI: 10.1002/pola.26042
ABSTRACT: Three new types of hyperbranched photoactive liq-
uid crystalline siloxane polymers containing azo moieties were
synthesized using click chemistry methodology. The polymers
were soluble in most of the polar solvents like chloroform, tet-
rahydrofuran, dimethylformamide, dimethyl sulphoxide and
dichloromethane. The molecular weights of the polymers were
in the range of 9000–12,000 g molꢀ1. The trans-cis photoisome-
rization of the polymer were studied both under UV radiation
and dark. The isomerization rate constants were found to be in
The thermotropic behavior of the polymers was studied by
using polarizing optical microscopy and differential scanning
calorimetry, respectively. The polymers P1 and P2 showed liq-
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uid crystalline texture characteristic of nematic phase.
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KEYWORDS: azo polymers; click chemistry; hyperbranched; liquid
crystalline; liquid-crystalline polymers (LCP); nematic; photo-
chemistry; photoswitching; polysiloxanes; UV–vis spectroscopy
the range of 0.7–1.4 ꢁ 10ꢀ2 secꢀ1 and 7.0 ꢁ ꢀ2.5 ꢁ 10ꢀ5 secꢀ1
.
INTRODUCTION A very active research field is represented
by the studies on azobenzene containing liquid crystalline
polymers.1–4 The reversible trans-cis photoisomerization of
the azo-benzene can be utilized to design new advanced
materials coupled with understanding of new phenomenon
and its underlying mechanisms. The resulting materials have
wide range of applications such as optical data storage and
other device applications.1–7 A recent discovery by Ikeda and
coworkers illustrates the light directed bending and unbend-
ing of thin films of crosslinked azobenzene liquid crystalline
polymer which can be used as an actuator for microdevice
applications.8 Surface relief grating (SRG) is an example
reported by two groups independently in 1995.9,10 Recently,
azo polymers are being synthesized with diverse architec-
tures such as dendrimers,11 proteins,12 micelles,13 and sol-
gel materials14 for studying the photoactivity directed
applications.
and photoactive polymer in particular. Synthesis and charac-
terization of branched LC polymers containing disc like meso-
gens16 and rod-like mesogens based on conformational iso-
merism17,18 were reported for the first time by Percec et al.
The electrophilic cyclotetramerization polymerization of
3,4-bis(n-alkyloxy) benzyl alcohol with an a,x-bis{[2-(alkoxy)-
5-(hydroxymethyl)phenyl]oxy} alkane16 resulted in hyper-
branched polymer with cyclotetraveratrylene units in the
branching points. Synthesis of hyperbranched 10-bromo-1-
(4-hydroxy-40-biphenylyl)-2-(4-hydroxyphenyl) decane (TPD-
b) based polymers demonstrating the dependence of phase
behavior on the nature of their chain end was also reported.17
Synthesis of dendrons up to fourth generation and their phase
behavior study were also reported.19 Viscoelastic properties
of dilute nematic mixtures containing cyclic and hyper-
branched liquid crystalline polymers dissolved in a nematic
solvent were studied.20 Rational design for the first nonspher-
ical dendrimers showing calamitic nematic and smectic
phases was reported by Percec et al.21 The dramatic stabiliza-
tion of hexagonal columnar mesophase was generated from
supramolecular and macromolecular columns, respectively.22
Design of first spherical supramolecular dendrimers which
self organize into cubic LC phase23 and controlling polymer
shape through self assembly of dendritic side group24
appeared in quick succession. The individual spherical and cy-
lindrical supramolecular dendrimers started getting confirmed
In our earlier work linear siloxane polymers having azo side
group have been synthesized and characterized.15 The poly-
mers were observed to undergo tran-cis isomerization at a
very high rate. The polymers were further analyzed for their
photomechanical properties. The rate was found to be quite
encouraging. Encouraged by the result, further study was
planned by synthesizing hyperbranched polymers containing
both siloxane and azo moieties. A good number of reports are
available in literature on hyperbranched polymer in general
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directly by using X-ray diffraction analysis.25 Synthesis and
structural analysis of two constitutional isomeric libraries of
AB2 based monodendrons and supramolecular dendrimers
were also reported.26 Regardless of repeat unit on their pe-
riphery, the first three or four generations of monodendrons
self assembled in to supramolecular dendrimers. Self-organi-
zation of supramolecular helical dendrimers in to complex
electronic material and amphiphilic dendritic dipeptides in to
helical pores were reported by Percec et al.27,28 Later, self-
assembled nanostructures obtained from natural and syn-
thetic amphiphiles were shown to mimic biological mem-
branes having stability and mechanical strength.29 Design
principles of self assembled dendrons via the primary struc-
tures generated form combinations of (AB)y-AB3 and (AB)y-
AB2 building blocks were reported to produce largest struc-
tural and dimensional diversity of supramolecular den-
drimers.30 Synthesis and retrostructural analysis of libraries
of AB3 and constitutional isomeric AB2 phenylpropyl ether
based supramolecular dendrimer was also reported there-
after.31 The dependence of shape of heat shrinking spherical
and columnar supramolecular dendrimers and their intercon-
version on molecular taper angle is also reported.32 Self-as-
sembly of semifluorinated tapered monodendrons containing
crown ethers into supramolecular columnar dendrimers
exhibiting a homeotropic hexagonal columnar liquid crystal-
line phase was also reported to be induced by fluorophobic
effect.33 Review on dendron-mediated self-assembly, disassem-
bly, and self-organization of complex systems is also reported
by Percec and coworkers.34 Most of the reported method of
polysiloxane synthesis only demonstrates hydrosilylation as
the most used polymerization method. Considering the advan-
tages of click chemistry such as low susceptibility to side
reactions, mild reaction conditions, nearly quantitative yields,
broad tolerance toward functional groups it has become one
of the most preferred route for synthesis of polymers. The
synthesis of a number of polymeric materials such as den-
drimers, block copolymers, complex macromolecular struc-
tures via click chemistry have been reported.35–38 The reports
on synthesis of hyperbranched polymers through click chem-
istry are still scarce and most of them were synthesized by
A2B3 approach39 where two monomers containing diazido
and diyne groups were used for the synthesis. The Cu(I)-cata-
lyzed click polymerizations of the AB2 monomers were found
to yield insoluble polymers due to self-oligomerization.40 Azo
moieties in the form of A2B type monomer self polymerized
to hyperbranch architecture,41,42 Click chemistry have also
been utilized for the synthesis of other hyperbranched poly-
mers using A2B approach.43,44 Similarly azo moieties can be
used as an A2 monomer which will polymerize with B3 mono-
mer to get A2B3 hyperbranched polymer through click chemis-
try route. In this report effort has been made to utilize click
chemistry to make hyperbranched siloxane azo polymers and
characterize them. To the best of our knowledge, for the first
time we have made azo triazide as a B3 moiety and polymer-
ized it with poly dimethyl siloxane-diyne (PDMS-diyne) which
is A2 moiety to get soluble liquid crystalline photoactive
hyperbranched polymers which fulfils the characterstics
required for both liquid crystallinity and photoactivity coupled
with good solubility in solvents.
EXPERIMENTAL
Materials
Propargyl bromide, hydride terminated polydimethyl siloxane
(Mn ¼ 580), 2% platinum(0)-1,3-divinyl-1,1,3,3-tetramethyl
disiloxane complex in xylene (karstedt’s catalyst), n,n-bis(die-
thanol)aniline, sodium-ascorbate, 4,4-dimethyl amino pyridine
(DMAP), dicyclohexyl carbodimide (DCC), allyl alcohol,
4-chloro-butan-1-ol, 6-chloro-hexan-1-ol, 8-chloro-octan-1-ol
were purchased from Sigma-Aldrich. p-Amino benzoic acid,
phosphorus oxychloride (POCl3) and all other chemicals were
purchased from S D fine chemicals, India. All solvents were
purchased from Merck India. THF and toluenes were dried
over sodium-ketyl radical. All chemicals were used as received
without further purifications unless otherwise specified.
Characterization
UV–visible spectra of the polymers were taken on a Cary 500
1
Scan UV–vis-NIR spectrophotometer. H and 13C NMR spectra
of the monomers and 1H NMR of polymers were recorded on ei-
ther a 400 or 500 MHz Bruker-FT NMR spectrometer. Heating /
cooling rate used for all DSC analysis was 10 ꢂC/min. For study-
ing the liquid crystalline behavior, a Leica DMLD, optical polar-
izing microscope with image analyzer equipped with LINKAM
TMS 94 hot stage and LINKAM LNP controlling unit was used.
For photoresponsive and photomechanical studies, the samples
were irradiated in the cabinet of a UV chamber (Spectrolinker
XL – 1500) fitted with 6 ꢁ 15 Watt medium pressure mercury
bulbs. The sample was placed at a distance of 15 cm from
source and irradiated for various time intervals. For prepara-
tion of sample for photomechanical study the silicone polymers
were blended with poly methyl methacrylate (PMMA) in solu-
tion phase and cast as a film on a rubbed glass plate
METHODS
The synthesis of PDMS-diyne and azo-triazide is shown in
scheme I. The synthesis of methyl 4-(prop-2-ynyloxy) benzo-
ate (1), 4-(prop-2-ynyloxy) benzoic acid. (2), N,N-bis(2-chlor-
oethyl)-aniline (5), 4-((4-bis(2-chloroethyl) amino) phenyl)
diazenyl) benzoic acid (6), was carried out by following the
procedure as reported earlier.15,45
PDMS-diol (3)
Allyl alcohol (11.0 g, 190 mmol) was mixed with anhydrous
toluene (25 mL) under inert condition in 100 mL R.B. flask
and maintained at 50 ꢂC. Karstedt’s catalyst (0.44 mL) was
added to the mixture through rubber septum using a syringe.
Hydride terminated PDMS (27.5 g, 47.4 mmol) was added
dropwise through syringe via rubber septum to the reaction
mixture. The temperature was raised further to 80 ꢂC and
continued until disappearance of SiAH peak at 2163 cmꢀ1 in
FTIR spectra. Excess of toluene and allyl alcohol was removed
by distillation under reduced pressure. The residue was
diluted with dichloromethane and passed over alumina to
remove the platinum catalyst. The filtrate was concentrated
by evaporating solvent in rotavapor.
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SCHEME 1 Synthesis of PDMS-diyne and azo-triazide monomers.
Yield: 32 g, (97 %) 1H NMR (400 MHz, CDCl3) d: 0.02 (bs,
56H), 0.51 (t, J ¼ 8.6 Hz, 4H), 1.55 (p, 7.5 Hz, 4H), 2.37 (bs,
2H), 3.54 (t, J ¼ 6.7 Hz, 4H); 13C NMR (125 MHz, CDCl3): 0.9,
1.8, 13.8, 26.3, 65.2.
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), 8.0 (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.
PDMS-diyne (4)
A THF solution (25 mL) of 4-(prop-2-ynyloxy) benzoic acid
(2) (10.11 g, 57.47 mmol), PDMS-diol (3) (10 g, 14.37 mmol),
and DMAP (0.26 g, 2.15 mmol) was taken in a 100 mL two
necked R.B.flask and nitrogen passed continuously. A solution
of DCC (4.45 g, 21.55 mmol) in 5 mL of THF was added to
the reaction mixture through septum using syringe. The mix-
ture was allowed to stir at room temperature. It was filtered
and the filtrate was evaporated. The crude compound was
purified by column chromatography by using a mixture of pe-
troleum ether and ethyl acetate (98 : 2, v/v) as eluant to
obtain the product as colorless oil.
Chlorobutyl 4-((4-bis(2-chloroethyl) amino) phenyl)
diazenyl) benzoate (7)
4-((4-bis(2-chloroethyl) amino) phenyl) diazenyl) benzoic
acid (2 g, 5.46 mmol), 4-chloro-1-butanol (3) (0.71 g, 6.58
mmol), DMAP (0.1 g, 0.81 mmol) and 25 mL of anhydrous
THF were taken in a 100 mL R.B.flask. A solution of DCC (1.6
g, 8.2 mmol in 5mL of THF) was added to the reaction flask
through septum via syringe. The mixture was allowed to stir
under nitrogen for 24 h. It was filtered and the filtrate was
evaporated. The crude material was purified by column chro-
matography using a mixture of petroleum ether and ethyl ace-
tate (95 : 5 v/v) as eluant to obtain the product as red col-
ored solid.
Yield: 12 g, (82.4%); 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
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SCHEME 2 Synthesis of azo containing siloxane hyperbranched polymers by click chemistry.
1
Yield: 2.28g (84%); H NMR (500 MHz, CDCl3) d: 8.14 (d, J ¼
8.5 Hz, 2H), 7.92 (d, J ¼ 9.0 Hz, 2H), 7.87 (d, J ¼ 8.5 Hz, 2H),
6.80 (d, J ¼ 9.0 Hz, 2H), 4.38 (t, J ¼ 6.5 Hz, 2H), 3.70 ( t, J ¼
6 Hz, 4H), 3.57 (t, J ¼ 6Hz, 4H), 3.38 (t, J ¼ 6.5 Hz,2H), 1.89
(p, J ¼ 6.5 Hz, 2H), 1.78 (p, J ¼ 7.0 Hz, 2H); 13C NMR (125
MHz, CDCl3) d: 25.64, 26.03, 48.77, 50.75, 51.02, 64.34,
111.87, 122.14, 125.69, 130.49, 130.60, 144.61, 149.30,
155.74, 166.13.
Chlorohexyl 4-((4-bis(2-chloroethyl) amino) phenyl)
diazenyl) benzoate (8)
The synthesis was carried out by similar procedure as men-
tioned for compound 7. In this case 4-chloro-1-butanol was
replaced by 6-chloro-1-hexanol.
1
Yield: 2.50g (91%); H NMR (500 MHz, CDCl3) d: 8.14 (d, J ¼
8.5 Hz, 2H), 7.92 (d, J ¼ 9.0 Hz, 2H), 7.87 (d, J ¼ 8.5 Hz, 2H),
FIGURE 1 1H NMR spectra of PDMS-diyne.
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FIGURE 2 1H NMR spectra of polymer P2.
6.80 (d, J ¼ 9.5 Hz, 2H), 4.34 (t, J ¼ 6.5 Hz, 2H), 3.69 ( t, J ¼
6 Hz, 4H), 3.57 (t, J ¼ 6.5 Hz, 4H), 3.28 (t, J ¼ 7.0 Hz, 2H),
1.81 (p, J ¼ 6.5 Hz, 2H), 1.64 (p, 7.0 Hz, 2H), 1.45–1.51 (m,
4H); 13C NMR (125 MHz, CDCl3) d:25.64, 26.40, 28.59, 28.74,
48.80, 50.76, 51.33, 111.87, 122.11, 125.67, 130.47, 130.85,
144.62, 149.27, 155.69, 166.23.
Azidobutyl 4-((4-bis(2-azidoethyl) amino) phenyl)
diazenyl) benzoate (10)
To a solution of (6) (0.9 g, 2.0 mmol) in DMSO (10 mL)
sodium azide (0.65 g, 10.0 mmol) was added. The reaction
mixture was heated at 90 ꢂC for 10 h and cooled to R.T. and
poured in to 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 column
chromatography using petroleum ether and ethyl acetate mix-
ture (95 : 5; v/v).
Yield: 0.91 g (97%); 1H NMR (500 MHz, CDCl3) d: 8.14 (d, J
¼ 8.5 Hz, 2H), 7.92 (d, J ¼ 9.0 Hz, 2H), 7.88 (d, J ¼ 8.5 Hz,
2H), 6.78 (d, J ¼ 9.0 Hz, 2H), 4.39 (t, J ¼ 5.0 Hz, 2H), 3.85 ( t,
J ¼ 7 Hz, 4H), 3.70 (t, 7 J ¼.0 Hz, 4H), 3.63 (t, J ¼ 5.0 Hz,
2H), 2.61 (s, 2H), 1.97 (p, J ¼ 2.5 Hz, 2H), 1.60 (p, J ¼ 7.0 Hz,
2H); 13C NMR (125 MHz, CDCl3) d:26.16, 29.24, 40.17, 44.76,
53.45, 64.24, 111.62, 122.16, 125.72, 130.50, 130.68, 144.68,
149.10, 155.70, 166.13.
Chlorooctyl 4-((4-bis(2-chloroethyl) amino) phenyl)
diazenyl) benzoate (9)
The synthesis was carried out by similar procedure as men-
tioned for compound 7. In this case 4-chloro-1-butanol was
replaced by 8-chloro-1-octanol.
Yield: 2.55g (88 %); 1H NMR (500 MHz, CDCl3) d: 8.14 (d, J
¼ 8.5 Hz, 2H), 7.91 (d, J ¼ 9.0 Hz, 2H), 7.87 (d, J ¼ 8.5 Hz,
2H), 6.79 (d, J ¼ 9.0 Hz, 2H), 4.33 (t, J ¼ 6.5 Hz, 2H), 3.69 ( t,
J ¼ 6 Hz, 4H), 3.56 (t, J ¼ 6.0 Hz, 4H), 3.25 (t J ¼, 7.0 Hz,
2H), 2.61 (s, 2H), 1.78 (p, J ¼ 7.0 Hz, 2H), 1.60 (p, J ¼ 7.0 Hz,
2H), 1.33–1.41 (m, 8H); 13C NMR (125 MHz, CDCl3) d: 25.91,
26.60, 28.65, 28.77, 28.99, 29.09, 40.95, 48.77, 50.75, 51.41,
65.15, 111.86, 122.09, 125.66, 130.46, 130.93, 144.60,
149.27, 155.65, 166.25.
Azidohexyl 4-((4-bis(2-azidoethyl) amino) phenyl)
diazenyl) benzoate (11)
The synthesis was carried out by using same procedure as
mentioned for product 10. In this case 4-chloro-1-butanol
was replaced by 6-chloro-1-hexanol.
1
Yield: 0.96 g (97 %); H NMR (500 MHz, CDCl3) d: 8.15 (d, J ¼
8.5 Hz, 2H), 7.92 (d, J ¼ 9.0 Hz, 2H), 7.87 (d, J ¼ 8.5 Hz, 2H), 6.78
(d, J ¼ 9.0 Hz, 2H), 4.35 (t, J ¼ 6.5 Hz, 2H), 3.85 ( t, J ¼ 7.5 Hz,
TABLE 1 Physico-Chemical Properties of Polymers
Thermal
Transition
(DSC)a
Polymer
POMb Ti
Mn
Mw
Mw/Mn
P1
P2
P3
a
4 N 63 I
2 N 66 I
–
70
69
–
11,300
9,900
9,800
20,400
14,400
14,900
1.80
1.47
1.49
Transition temperature identified from heating cycle with DSC (at a
heating rate of 10 ꢂC/min under N2 atm).
b
Transition temperature identified with POM at a heating rate of 5 ꢂC/min.
cTi ¼ isotropization temperature.
FIGURE 3 DSC thermogram of polymers P1 and P2.
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FIGURE 4 POM images showing nematic droplets texture for P1 at temperature 30 ꢂC (A) and 45 ꢂC (B).
4H), 3.70 (t, J ¼ 7.0 Hz, 4H), 3.55 (t, J ¼ 5.0 Hz, 2H), 1.79–1.84
(m, 4H), 1.49–1.56 (m, 4H); 13C NMR (125 MHz, CDCl3) d: 25.39,
26.54, 28.57, 32.43, 40.17, 44.90, 53.44, 111.62, 122.13, 125.70,
130.48, 130.91, 144.68, 149.07, 155.63, 166.22.
zoate (10) (0.052 g, 0.09 ꢁ 10ꢀ2 mmol) and sodium ascor-
bate (0.041 g, 0.20 mmol) was taken in 3 mL THF. The
mixture was allowed to stir under nitrogen atmosphere for
10 min. A solution of copper sulfate (0.027 g, 0.108 mmol) in
3 mL water was added to it. The reaction mixture was stirred
for additional 40 mins under N2 atmosphere and quenched
with methanol (20 mL). The ppt was filtered, dissolved in
THF and passed through alumina bed to remove the catalyst
and insoluble impurities. The THF solution was then evapo-
rated to obtain the polymer (P1) as red powder. Similarly
polymers P2 and P3 were synthesized by reacting (4) with
(11) and (12), respectively.
Azidooctyl 4-((4-bis(2-azidoethyl) amino) phenyl)
diazenyl) benzoate (12)
The synthesis was carried out by using same procedure as
mentioned for product 10. In this case 4-chloro-1-butanol
was replaced by 8-chloro-1-octanol.
1
Yield: 1.00g (96%); H NMR (500 MHz, CDCl3) d: 8.15 (d, J ¼
8.5 Hz, 2H), 7.92 (d, J ¼ 9.0 Hz, 2H), 7.87 (d, J ¼ 8.5 Hz, 2H),
7.47 (d, J ¼ 9.5 Hz, 2H), 4.34 (t, J ¼ 7.0 Hz, 2H), 3.85 (t, J ¼
7.0 Hz, 4H), 3.70 (t, J ¼ 7.0 Hz, 4H), 3.53 (t, J ¼ 6.5 Hz, 2H),
1.75–1.82 (m, 4H), 1.33–1.49 (m, 8H); 13C NMR (125 MHz,
CDCl3) d: 25.91, 26.75, 28.66, 28.73, 29.07, 32.55, 40.18,
45.07, 53.44, 65.17, 111.62, 122.12, 125.70, 130.47, 131.00,
144.68, 149.07, 155.61, 166.24.
P1
Yield: 0.12 g, (81.0%); 1H NMR (500 MHz, CDCl3) d: 0.00 -
0.07 (bs), 0.61 (bs, 4H), 0.88 (bs, 8H), 1.25–2.26 (br, 6H),
3.38 (bs, 4H), 3.59–3.64 (bs, 6H), 4.23 (bs, 2H), 4.39 (bs, 4H),
5.22 (bs, 4H), 6.68–6.97 (br, 10H); 7.45 (bs, 1H), 7.90–8.15
(br, 6H).
Synthesis of Polymers by Click-Polymerization (P1)
Scheme 2 shows the synthesis of azo-siloxane polymer by
click chemistry route.
P2
1
Yield: 0.11g, (79.3%); H NMR (500 MHz, CDCl3) d: 0.00 - 0.07
A mixture of PDMS-diyne (4) (0.1 g, 0.09 ꢁ 10ꢀ2 mmol), azi-
(bs), 0.61 (bs, 4H), 0.89–1.25 (bs, 8H), 1.26–1.77 (br, 10H),
3.28 (bs, 4H), 3.60–3.64 (bs, 6H), 4.23 (bs, 2H), 4.35–4.40 (bs,
dobutyl 4-((4-bis(2-azidoethyl) amino) phenyl) diazenyl) ben-
FIGURE 5 UV–visible absorption spectra in chloroform solution
FIGURE 6 UV–visible absorption spectra in chloroform solution
for trans-cis isomerization of Polymer P2.
for cis-trans isomerization of Polymer P2.
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5.25 (bs, 4H), 6.68–6.97 (br, 10H); 7.45 (bs, 1H), 7.87–8.15
(br, 6H).
RESULTS AND DISCUSSION
Synthesis and Characterizations
Hyperbranched siloxane polymers having azo moieties were
synthesized via click chemistry technique and thoroughly
characterized for chemical structure, liquid crystallinity, and
photoresponsive behavior. The design of the hyperbranched
polymer was done by reacting azo-triazide and siloxane-
diyne (PDMS-diyne). This study was done to find out the
variation in properties as a function of change of architecture
from linear to hyperbranched one. The synthesis of azo-tria-
zide is also targetted in the study which is not reported so
far. Keeping in view the polymer property requirement for
photoactuation, siloxane is used as a part of backbone for
maintaining flexibility of the polymer during photoactuation
process. Azo moieties acts as mesogens due to its rigid rod
like nature which is aided by flexible siloxane spacer to ena-
ble liquid crystalline phase generation. Further, the azo mole-
cule undergoes trans-cis isomerization on irradiation. The
mesogenic behavior and photoactivity coupled together ena-
ble the polymer to exhibit photoactuation on irradiation.
For the synthesis of polymers, reactions were carried out
between PDMS-diyne and three different types of azo-tria-
zido to give three polymers P1-P3. To attach third azide
group to azo benzene carboxylic acid (6), ACOOH group was
esterified with aliphatic chloroalcohol. The alkyl chain of
chloroalcohol was varied to make P1, P2, and P3. The LC
properties and photoswitching behavior of the polymers are
studied in this work. The synthesis of PDMS-diyne, was done
through four step synthetic procedure as shown in Scheme 1
similar to synthesis of trisiloxane-diol reported by us in our
earlier work.15 It was synthesized through esterification
between PDMS-diol (3) and 4-(prop-2-ynyloxy) benzoic acid
1
(2) using DCC/DMAP catalyst. Figure 1 shows H NMR spec-
tra of PDMS-diyne. The disappearance of hydroxyl peak cen-
tered at d ¼ 2.37 is observed in esterified compound. The
esterification is also confirmed by the shifting of SiACH2,
SiACH2ACH2, and SiACH2ACH2ACH2 peaks downfield in the
spectra.
For the synthesis of B3 (azo-triazide), the intermediate 4-((4-
bis(2-chloroethyl) amino) phenyl) diazenyl) benzoic acid (6)
was synthesized as described by Shen et al.45 The obtained
acid (6) was esterified separately with three different alco-
hols (4-chloro-1-butanol, 6-chloro-1-hexanol, 8-chloro-1-octa-
nol) to get three different trichloro-azo compound (7, 8, 9)
using DCC/DMAP catalyst in dry THF solvent. The product
FIGURE 7 Plot of lnA versus time for trans-cis isomerization of
Polymers.
1
was thoroughly characterized by H NMR and 13C NMR spec-
4H), 5.23 (bs, 4H), 6.68–6.97 (br, 10H); 7.45 (bs, 1H), 7.90–
8.15 (br, 6H).
troscopy. Further, the trichloro-azo moieties were converted
to azo-triazide moieties (B3) using sodium azide in DMSO.
The product was thoroughly characterized from the charac-
teristic peaks in 1H NMR and 13C NMR spectra. The conver-
sion of tri-chloro azo to azo triazide (10-12) is confirmed by
downfield shifting of the peak positions in 1H NMR and 13C
NMR spectra respectively.
P3
Yield: 0.12g, (78.6%); 1H NMR (500 MHz, CDCl3) d: 0.00 -
0.07 (bs), 0.60 (bs, 4H), 0.89–1.25 (bs, 8H), 1.37–2.26 (br,
14H), 3.26 (bs, 4H), 3.59 (bs, 6H), 4.22 (bs, 2H), 4.34 (bs, 4H)
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TABLE 2 The Rate Constants of the Polymers for Trans – Cis and Cis – Trans Isomerization
Polymers
kmax (nm)a p-p*
kmax (nm)a n-p*
Kx (sꢀ1 b
)
Ky (secꢀ1 c
)
Kz (minꢀ1 d
)
P1
P2
P3
a
405
406
406
526
525
525
0.7 ꢁ 10ꢀ2
1.1 ꢁ 10ꢀ2
1.4 ꢁ 10ꢀ2
1.8 ꢁ 10ꢀ3
2.8 ꢁ 10ꢀ3
5.8 ꢁ 10ꢀ3
7.0 ꢁ 10ꢀ5
5.0 ꢁ 10ꢀ5
2.5 ꢁ 10ꢀ5
Recorded in chloroform solution.
b
c
Rate constant calculated from the first slope of trans to cis isomerization graph (lnA vs. Time).
Rate constant calculated from the second slope of trans to cis isomerization graph (lnA vs. Time).
d
Rate constant calculated from the slope of cis to trans isomerization for initial 30 min of dark relaxation.(lnA vs. Time).
The polymerization was carried out by taking equimolar
amount of PDMS-diyne (4) and the azo-triazide (10-12),
along with copper-sulphate and sodium ascorbate in THF/
H2O (2 : 1; v/v). The polymerization reaction was completed
in less than 1 hour time and highly viscous polymer solution
resulted. The 1H NMR spectra shows the appearance of a
peak around 7.45–7.50 ppm which is attributed to the pro-
ton in triazole ring (Fig. 2).
of P1 and P2 can be attributed to isotropization. The isotrop-
ization temperatures of P1 and P2 lie at 63 ꢂC and 66 ꢂC,
respectively. This may be due to slightly higher orientation
originated from higher alkane tail in polymer P2 enabling LC
phase to exist up to higher temperature compared to poly-
mer P1. The polymer P3 does not show any LC transition.
This may be due to too much flexibility of the polymer
which is not conducive for LC ordering.
From the GPC results, the molecular weights (Mn) of the
polymers are found to be in the range of 9000–12,000 g
molꢀ1. The overall yield of the soluble polymer increases
from P1 to P3 to a maximum of 80–85%. All the polymers
are soluble in organic solvents like THF, DMF, DCM, and
DMSO.
POM Study
The mesogenic behavior was studied by POM. Thin layer of
the polymer was prepared on a glass slide by slow evapora-
tion of their chloroform solution and further drying the film
under vacuum. The thin film was kept on hot stage of the
POM and changes in texture with heating were observed.
The representative POM micrographs are shown in Figure 4
and the observation is summarized in Table 1. For polymer
P1 appearance of texture, characteristic of nematic phase, is
observed. For polymer P2 similar nematic texture appears.
The isotropiꢂzation temꢂperatures of P1 and P2 are observed
to be at 70 C and 69 C, respectively. This is similar to that
observed in DSC study.
DSC Study
Figure 3 shows the DSC thermogram of polymers P1-P2 and
the data are incorporated in Table 1. Both the polymers ex-
hibit two thermal transitions. The first thermal transition
can be attributed to glass transition temperature. The poly-
mer P1 exhibited slightly higher glass transition temperature
of 4 ꢂC compared to polymer P2 showing at 2 ꢂC. However,
the difference is very small. This could be attributed to
almost similar flexibility of the alkane chain (C4 and C6)
attached to the esters of the azo moiety in polymer P1 and
P2. In polymer P3 the glass transition temperature is found
to be at ꢀ2 ꢂC which has alkyl chain (C8) which is reason-
ably flexible. The second endotherm in the DSC thermogram
Photo Responsive Properties
Before carrying out photoirradiation study, the polymer solu-
tion was kept in dark for 3 days to assume 100% trans form
of the azobenzene. The kmax absorbance maxima was
recorded around 406 nm for the polymers. The absorbance
at 406 nm is due to p-p* transition of stable trans form. The
isomerization behavior was studied by irradiating the poly-
mer solutions with UV light (365 nm) for different time
intervals and the absorbance versus wavelength was plotted.
As described in our earlier report two absorbance peaks are
observed in UV–visible spectra for all the polymers (Fig. 5)
during irradiation. The peaks at around 400 nm and 500
nm, respectively are attributed as to p-p* and n-p* transi-
tions respectively. Further the n-p* transition is found to be
more intense as compared to the usually observed intensity.
TABLE 3 Energy Calculation of the Polymers from the
Molecular Modeling and Simulations Studies
Polymers
Total Energy (Kcal/mol)
P1
P2
P3
565.25
557.24
693.65
FIGURE 8 Three-dimensional structure of monodendron of
Polymer P2.
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P3 is the highest among the polymers while the energy of
P1 is the lowest. Therefore trans-cis photoisomerization rate
constant must be highest for P3 and lowest for P1. There is
not much difference between P1 and P2. This observation
supports the kinetic results.
FIGURE 9 Photomechanical bending of polymer P1 under irra-
diation with UV light (a) before irradiation (b) after irradiation.
Photomechanical Bending
During irradiation, the absorbance due to p-p* electronic
transition decreases and n-p* transition increases indicating
conversion of trans to cis form of the azo polymer. Figure 6
shows the change of absorbance with time for cis-trans
isomerisation in dark. The rate constants of isomerization of
the polymers are calculated from slope of the plot of ln A ver-
sus time (Fig. 7). The polymers are found to isomerize within
short time to cis form. Shouldering is observed in cis peak at
higher wave length during conversion of trans to cis form.
This can be attributed to solvatochromism.15,46 the polar
interaction of cis azo shifts the absorption spectra to higher
wave length.46 The isomerization process is also 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* electronic
transition is observed in UV–vis spectra. The solution was kept
in dark and periodic recording of UV–vis spectra was done.
lnA is plotted versus time for the polymers. Two slopes (Kx,
Ky) are observed for trans-cis isomerization (Fig. 7). The rate
constants for trans-cis isomerization increases with increase
in chain length attached to the ester group of azo moiety for
P1-P3. It may be due to ease in isomerization with increase in
the flexibility. As the alkyl chain length attached to the azo
group increases the flexibility which, in turn, enhances the
isomerization. Only one slope is observed for cis to trans isom-
erization. This may be due to very slow nature of the conver-
sion. Table 2 summarizes the values of rate constants for trans
to cis isomerization and back.
All the polymer P-P3 are found to be having glass transition
temperature around ꢀ4 to þ6 ꢂC therefore mechanical
strength of the free standing film was very poor and unsuit-
able for photomechanical study. To check its photomechanical
properties it was blended with PMMA and the blend film was
irradiated with UV light. Silicone Azo-PMMA blend film on
irradiation by 365 nm UV light has shown appreciable bend-
ing. The photoactuation of the polymers were studied by irra-
diation with 365 nm irradiation and typical irradiation inten-
sity of 5500–6500 lW/cm2. As can be observed from the
Figure 9 the flat film gets bend after 20 mins of irradiation.
CONCLUSIONS
Three types of hyperbranched siloxane polymers possessing
photoactive azo moiety, were synthesized using click chemis-
try route. The polymers have showed liquid crystalline char-
acteristic of nematic phase. All the three polymers showed
photoisomerization properties. The trans-cis conversion is
very fast. During the process one shoulder peak, at wave-
length higher than the cis peak, was observed due to solva-
tochromism. The rate constants for trans to cis isomerization
increases with increase in the aliphatic chain length attached
to ester of azo moieties. The rate constant for cis-trans con-
version is very slow. The rate constant pattern is exactly
reversed as compared to trans-cis conversion. This was con-
firmed by the energy calculation for the polymers; P3 show-
ing highest energy. Photomechanical bending was also
observed in polymer P1 on blending it with PMMA after irra-
diation with UV light.
Molecular Modeling Study of Model Compound
Due to the striking similarities of oligomers with polymers,
the monodendrons of the polymers have been constructed
using molecule-building tools in the Accelrys Materials Stu-
dio. The optimization of geometry and calculation of energies
were performed by using Material Studio 4.4 version
(Accelrys software). All the polymers structures were con-
structed in material studio and atom based, polymer consist-
ent force field (PCFF) forcite calculation was adopted to
carry out the simulation. The Figure 8 shows the three-
dimensional structure of monodendron of polymer.
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