Photophysical and photochemical investigations on triazole ring linked chalcone containing polymethacrylates

Article abstract of DOI:10.1016/j.polymer.2012.06.037

A series of polymethacrylates containing triazole ring linked chalcone were designed and synthesized. These conjugated chalcone polymers are modulate under the light depending on the pendant substituents (-N(CH₃) ₂, CH₃, O

Full text of DOI:10.1016/j.polymer.2012.06.037

Polymer 53 (2012) 4104e4111
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Polymer
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Photophysical and photochemical investigations on triazole ring linked chalcone
containing polymethacrylates
,
S. Balamurugan a, S. Nithyanandan b, C. Selvarasu b, G.Y. Yeap a, P. Kannan b
*
a Liquid crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia
b Department of Chemistry, Anna University, Chennai 600-025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of polymethacrylates containing triazole ring linked chalcone were designed and synthesized.
These conjugated chalcone polymers are modulate under the light depending on the pendant substit-
uents (eN(CH3)2, CH3, OCH3, Br, H & NO2). The steady-state absorption and emission spectroscopic
techniques have been used to investigate intramolecular charge transfer (ICT) behaviour of the polymers.
Absorption spectra in different organic solvents demonstrate the presence of ICT in polymer 5a. On the
other hand, its excited singlet state exhibits high ICT characters as manifested by polarity of solvents.
Interestingly, ICT emission maximum is strongly red shifted (53 nm). The emission intensities of fluo-
rophore are compared with those measured after crosslinking, suggesting that the statically quenched
fluorophores are entirely non-emissive. The emission decay of polymer 5a displays bi-exponentially with
life time of 0.52 and 1.62 ns ascribed to presence of locally excited ICT state. Cyclic voltammogram
demonstrates irreversible oxidation potential at 0.9 V indicates formation mono-radical cation.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 16 April 2012
Received in revised form
14 June 2012
Accepted 24 June 2012
Available online 7 July 2012
Keywords:
Click-reaction
Polymethacrylates
Pullepush effect
1. Introdution
Main-chain chalcone containing polymers exhibit solubility
defect attributed to its rigid-rod nature, a recent emphasis has been
Polymers fabricated with pushepull effect have a potential
application in both fundamental and applied field of physics,
chemistry and biology [1e3]. In particular, optimizing the amount
observed on polymers with side-chain chalcone units. In the past
few years, “click reactions”, as termed by Sharpless et al. [23] have
gained a great deal of attention due to their high specificity,
quantitative yields, and near-perfect fidelity in the presence of
most functional groups [24]. The potential of azide-alkyne coupling
procedure for construction of well-defined polymers, bio-
conjugated polymers, and polymers with complex topologies has
been quickly recognized [25].
of charge transfer (CT) in a
p-conjugated pushepull system is
a critical issue for practical application such as design of nonlinear
optical (NLO) materials [4]. To realize the potential for these
applications, substantial efforts have been directed to understand
molecular design with large
p-conjugated pushepull systems [5].
Materials exhibiting NLO response are of great interest for opto-
electronic [6] and photonic applications [7]. The photo-
crosslinkable polymers [8,9] have been extensively studied as
photoresists to use in macro and microlithography. In addition,
chalcones are employed for various optical applications including
second harmonic generation materials in non-linear optics [10],
photorefractive polymers [11], holographic recording materials [12]
and fluorescent probes for sensing of metal ions [13e15], biological
macromolecules [16e20] and microenvironment in micelles [21].
Thus, functionalized polymers can be considered as building blocks
for a variety of nano-technological applications [22], ranging from
vectors for drug and DNA delivery systems.
We have synthesized a series of polymers with “pushepull” type
donor and acceptor substituents and examined their photo-
luminescent properties. Polymer with large
p-conjugated system
provides aneffectiveintramolecularchargetransfer(ICT). Thissystem
is good models for the study of photoinduced electron transfer (PET).
A detailed study was made on ultraviolet (UV) induced 2
p
þ 2
p
cycloaddition of chalcone. The photoluminescence (PL) of polymers
prior to and after crosslink, especially the effect of terminal substit-
uent and photo-crosslinking process were discussed.
2. Experimental
2.1. Materials
Benzaldehyde, p-nitrobenzaldehyde, p-bromobenzaldehyde,
* Corresponding author. Tel.: þ91 44 22358666; fax: þ91 44 2220 0660.
E-mail address: pakannan@annauniv.edu (P. Kannan).
p-methylbenzaldehyde,
p-methoxybenzaldehyde,
N,
N0-
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.06.037

S. Balamurugan et al. / Polymer 53 (2012) 4104e4111
4105
dimethylaminobenzaldehyde, p-aminoacetophenone, propargyl
alcohol, methacrylic acid, sodium hydroxide, sodium nitrite,
hydrochloric acid, sodium azide, triethylamine, copper sulphate
pentahydride, sodium ascorbate, and 2, 20-azobisisobutyronitrile
(AIBN) (Merck, Germany) were used as received. Acetone, ethanol,
t-butanol, THF and methanol (SRL, India) solvents were purified by
usual procedures [26]. Silica gel (100e200 mesh) (SRL, India) was
dried in a hot air oven at 100 ꢀC for 2 h and cooled before use.
149.2 (CeN3), 135.9, 129.6, 128.9 (Aromatic carbons), 26.8 (aliphatic
carbon).
2.4. Synthesis of 1-(4-azidophenyl)-3-(4-(dimethylamino)phenyl)
prop-2-en-1-one (2a)
N,N0-Dimethylaminobenzaldehyde (2 g, 0.013 mol) and 4-
azidoacetophenone (2 g, 0.013 mol) were added to a solution of
NaOH in ethanol (0.1 M, 10 mL). The reaction mixture was stirred at
2.2. Measurements
25 ꢀC for 24 h. The reaction mixture was poured in 150 mL of water
and resulting precipitate was filtered and dried. The crude product
FT-IR spectra were recorded in the 4000e400 cmꢁ1 range on
aShimadzuFTIR-8400susingKBrpellets,1HNMRand13CNMRspectra
were recorded on a Bruker-AVANCE-III 500 MHz spectrometer. The 1H
NMR and 13C NMR chemical shifts are reported as parts per million
(ppm) downfield from TMS (Me4Si) used as an internal standard.
Elemental analyses (C, H, N) were done on a CHN rapid analyser. All the
new compounds gave C, H and N analysis within ꢂ0.03% of the theo-
retical values. Thermogravimetric analysis (TGA) was performed on
a Mettler 851e TGA under an argon atmosphere at a heating rate of
10 ꢀC/min. The molecular weights and its corresponding distributions
were determined by gel permeation chromatography (GPC) with
tetrahydrofuran as an eluent and mono disperse polystyrene stan-
dards used for calibration. UV absorption spectra were recorded on
was
purified
by
column
chromatography
(silica,
hexane:EtOAc ¼ 5:1) and recrystallized from cyclohexane to yield
pure compound 2a as a yellow powder (2.8 g, 74% yield). Similar
procedure was adopted for other substituted compounds (2be2f).
Anal. Calcd for C17H16N4O: C, 69.85%; H, 5.52%; N, 19.17%; O, 5.47.
Found: C, 69.75%; H, 5.46%; N, 19.33%; O, 5.36%. FTIR (KBr pellet,
cmꢁ1): 2090(eN3, azide), 1649 (C]O), 1615 (aliphatic C]C
stretching), 1565 (Ar C]C), 1050 (CeN, eN (CH3)2), 815 (CeH plane
bending). 1H NMR (CDCl3,
d
ppm): 8.18 (s, 1H, C¼CHeAr), 7.99 (d,
2H, AreH), 7.61 (s, 1H; eCOeCH ¼ Ce), 7.83(d, 2H, AreH), 7.64 (d,
2H, AreH), 6.83(d, 2H, AreH), 3.18 (s, 6H, eCH3). 13C NMR (CDCl3,
d
ppm): 190.0(C]O), 150.1(CeN3), 145.4, 121.6 (C]C, aliphatic),
150.6 (CeN (CH3)2, aromatic), 138.2, 130.2, 129.0, 129.6, 125.0, 112.0
a
UV-1650PC UVeVIS spectrophotometer (Shimadzu). Photo-
(Aromatic carbons), 41.6(CH3).
crosslinking studies were carried out using 6 W high-pressure Hg
lamp (Sankyo, 365 nm). Fluorescence measurements were performed
using an LS-55 fluorescence spectrophotometer (Perkin Elmer)
equipped with a plotter unit and a quartz cell (1 cm ꢃ 1 cm). Time-
resolved fluorescence measurements were carried out using (IBH)
time-correlated single photon counting techniques by exciting the
sample at 420 nm. Fluorescence decay was measured at 520 nm. The
instrumentresponsetime is 52 ps. Thedata analysis wascarried out by
the software provided by IBH (DAS-6), which is based on de-
convolution techniques using nonlinear least square method, and
quality of the fit is normally identified by the value c2 < 1.2 and
weighted residual. Cyclic voltammetry (CV) was carried out in
nitrogen-purged acetonitrile at room temperature with a CHI Elec-
trochemical analyser (600D). Tetrabutylammonium perchlorate
2.5. Synthesis of 3-(4-(dimethylamino) phenyl)-1-(4-(4-
(hydroxymethyl)-1H-1,2,3-triazol-1-yl)phenyl)prop-2-en-1-one
(3a)
A mixture of propargyl alcohol (0.24 g, 0.004 mol) and azide
(1.2 g, 0.004 mol) in t-butanol-H2O (4:1, 20 mL) in the presence of
5 mol% CuSO4.5H2O with 10 mol% sodium ascorbate was stirred at
room temperature for 6 h. The reaction mixture was poured into
water (100 mL) and the resulting solution was extracted with ethyl
acetate. The combined organic phase was dried over sodium
sulphate. Solvent was removed under vacuum and purified by
column chromatography (3:1, EtOAc/methanol) to afford the
desired product as yellow coloured solid with yield: 1.1 g (78%).
Similar procedure has been adopted for preparation of remaining
substituted compounds.
(TBAP) (0.1 M) was used as the supporting electrolyte. The conventional
three-electrode configuration consists of a Glossy carbon as the
working electrode, a platinum foil used as the auxiliary electrode, and
an Ag/AgCl as the reference electrode and potential scanning from
0 to þ1.0 V with scan rate of 50 mV sꢁ1 were carried out.
Anal. Calcd for C20H20N4O2: C, 68.95%; H, 5.79%; N, 16.08%; O,
9.18. Found: C, 68.90%; H, 5.75%; N, 16.18%; O, 9.17. FTIR (KBr pellet,
cmꢁ1): 3628 (eOH), 1652 (C]O), 1624 (aliphatic C]C stretching),
1600 (triazole), 1569 (Ar C]C), 1052 (CeN, eN(CH3)2), 816 (CeH
2.3. Synthesis
plane bending). 1H NMR (CDCl3,
d ppm): 8.24 (s, 1H, AreH), 8.19
(s, 1H, Chalcone), 8.02 (d, 2H, AreH), 7.62 (s, 1H; Chalcone), 7.84 (d,
2H, AreH), 7.94 (d, 2H, AreH), 6.84 (d, 2H, AreH), 5.11 (s, 2H, eCH2),
2.3.1. Synthesis of 1-(4-azidophenyl)ethanone (1)
To a solution of 4-aminoacetophenone (0.1 mol,13.5 g), conc. HCl
(28 mL) and water (28 mL) weremaintained at 0 ꢀC and cold aqueous
solution of sodium nitrite (0.1 mol, 6.9 g) added drop wise under
constant stirring. Thenthe solution ofsodium azide(0.1 mol, 6.5 g) in
water (13 mL) was added slowly. Theresultant mixturewas stirred at
0 ꢀC for half an hour, and the mixture was slowly warmed to room
temperature by keeping the reaction with constant stirring for 6 h.
The reaction mixture was diluted with excess of water and extracted
with ethyl ether, washed with water (3 ꢃ 200 mL), saturated brine
solution (3 ꢃ 200 mL), and dried over anhydrous sodium sulphate.
The solvent was removed by rotatory evaporator, and purified by
column chromatography hexane used as eluent to afford 1-(4-
azidophenyl)ethanone as yellow-colour liquid (14.8 g, 92%).
FTIR (KBr plates, cmꢁ1): 2090(eN3, azide), 1651 (C]O, carbonyl
stretching), 1565 (C]C, aromatic), 815 (CeH plane bending). 1H
3.77 (s, 1H, eOH), 3.19 (s, 6H, eCH3). 13C NMR (CDCl3,
d ppm): 190.0
(C]O), 145.4, 121.5 (C]C, aliphatic), 150.4 (CeN (CH3)2, aromatic),
144.9, 119.8 (C-triazole), 138.2, 130.0, 129.0, 126.1, 125.0, 112.0
(Aromatic carbons), 54.0 (eCH2), 41.6 (CH3).
2.6. Synthesis of monomers
In a 250 mL double-necked flask, compound 3a (2 g, 0.006 mol)
and triethylamine (0.84 mL, 0.006 mol) were dissolved in 40 mL of
THF and cooled to ꢁ5 ꢀC. Methacryloyl chloride (0.3 mL, 0.003 mol)
in 25 mL of THF was added drop wise with stirring, keeping the
temperature in the range 0ꢁ5 ꢀC. Temperature of the reaction
mixture was allowed to rise slowly to room temperature and the
content was stirred for 1 h. The quaternary ammonium salt was
filtered and the solvent removed by rotary evaporator. The mono-
mer was purified by column chromatography (silica,
EtOAc:hexane ¼ 1:1) to afford the desired product as yellow
NMR (
d
ppm, CDCl3, 500 MHz): 8.04 (d, 2H; AreH), 7.43 (d, 2H;
ppm, CDCl3): 197.1 (C]O),
AreH), 2.50 (s, 3H; CH3), 13C NMR (
d

4106
S. Balamurugan et al. / Polymer 53 (2012) 4104e4111
coloured solid with yield 1.66 g (66%). Similar procedure was
adopted for remaining substituted compounds.
viscosity, which typically took around 48 h. After the desired
reaction time, the contents of the vials were allowed to cool room
temperature, diluted with an additional 5 mL of THF to reduce
viscosity, and precipitated into cold methanol 3e5 times to remove
unreacted monomers. The polymers were then allowed to dry
overnight at 60 ꢀC in a vacuum oven (yield 0.6 g, 75%). The similar
procedure was adapted to the synthesis of other substituted poly-
mers (5be5f; Scheme 1).
Anal. Calcd for C24H24N4O3: C, 69.21%; H, 5.81%; N, 13.45%; O,
11.52. Found: C, 69.26%; H, 5.83%; N, 13.41%; O, 11.50%. FTIR (KBr
pellet, cmꢁ1): 1739 (C]O, ester), 1652 (C]O, ketone), 1624
(aliphatic C]C stretching), 1600 (triazole), 1569 (Ar C]C), 1220
(CeO, ester), 1052 (CeN, eN(CH3)2), 816 (CeH plane bending). 1H
NMR (CDCl3, d ppm): 8.22 (s,1H, Chalcone-H), 7.33 (s,1H, Chalcone),
8.19 (d, 2H, AreH), 7.89 (s, 1H; -triazole), 7.88 (d, 2H, AreH), 7.56 (d,
2H, AreH), 6.70 (d, 2H, AreH), 5.61, 6.17 (m, H, methacrylic-H), 5.40
(s, H, eCH2), 3.05 (s, 6H, eCH3), 1.96 (t, 3H, eCH3) (Fig. S1). 13C NMR
FTIR (KBr pellet, cmꢁ1): 1736 (C]O, ester), 1649 (C]O, ketone),
1621 (aliphatic C]C stretching), 1601 (triazole), 1569 (C]C, Ar),
1223 (CeO, ester),1050 (CeN, eN(CH3)2), 815 (CeH plane bending).
(CDCl3,
d
ppm): 188.2 (C]O), 167.3 (C]O, ester), 146.8, 122.0 (C]C,
1H NMR (CDCl3,
d ppm): 8.22 (s, 1H, AreH), 8.19 (s, 1H, Chalcone),
aliphatic), 152.3 (CeN (CH3)2, aromatic), 144.0, 120.1 (C-triazole),
135.8, 122.3 (methacrylic), 139.2, 130.7, 130.7, 126.5, 116.0, 111.8
(Aromatic carbons), 57.7 (eCH2), 40.1 (CH3), 18.2 (eCH3) (Fig. S2).
8.02 (d, 2H, AreH), 7.62 (s, 1H; eCOeCH ¼ Ce), 7.84(d, 2H, AreH),
7.94 (d, 2H, AreH), 6.84 (d, 2H, AreH), 5.75 (s, 2H, eCH2), 3.19 (s,
6H, eCH3), 1.77 (s, 2H, eCH2), 1.27 (t, 3H, eCH3). 13C NMR (CDCl3,
d
ppm): 190.0 (C]O), 175.7 (C]O, ester), 145.4, 121.5 (C]C,
aliphatic), 150.4 (CeN (CH3)2, aromatic), 144.9, 119.8 (C-triazole),
138.2, 130.0, 129.0, 126.1, 125.0, 112.0 (Aromatic carbons), 61.1
(eCH2), 44.6 (eCH), 41.6 (CH3), 34.5 (eCH2), 24.9 (eCH3).
2.7. Synthesis of polymers (5a)
Polymerisation was carried out in rubber septum sealed test
tubes immersed in an oil bath at 50e60 ꢀC. The content of the test
tubes was under constant stirring during polymerization period
and typically included monomer 4a (0.8 g) and 2 wt% AIBN, diluted
to 5 mL in dry HPLC grade THF. Reaction mixtures were purged with
nitrogen to remove oxygen from the reaction vessel for 10 min prior
to heating. The majority of the polymerization reactions were
allowed to proceed until the mixture exhibited an elevated
3. Results and discussion
3.1. Thermal analysis
The thermal properties of polymers were evaluated by ther-
mogravimetric analysis under N2 atmosphere at the heating rate of
Scheme 1. Synthesis of various substituted polymers.

S. Balamurugan et al. / Polymer 53 (2012) 4104e4111
4107
has absorption band at 282 and 427 nm (Fig. 2). Among the poly-
mers, polymer 5a has the longest absorption band attributed to the
presence of strong electron donor eN(CH3)2 group; it facilitates the
pushepull character of ICT [27]. The absorption bands of remaining
polymers are significantly blue shifted. This blue shift is associated
with decreases in the electron donating ability of substituents. A
pushepull effect is more prominent between terminal donor
substituent and an acceptor carbonyl group of polymer. Further
Fayed [27] confirmed this concept through charge density calcu-
lations. The N(CH3)2 group containing polymers led to strong
intramolecular charge transfer, which act as a better donor group
compared to other substituents. Braun et al. [28] deliberated
various substituted amide derivatives and explored that steric
hindrance dramatically affects the charge transfer reaction. In this
case there is no finding of steric effect of substituents in all the
polymers 5(aef). The charge transfer transition overcomes the
steric effect of the polymers.
3.3. Photo-crosslinking studies of polymers
The photo-crosslinking studies of polymers were carried out
upon illumination with 365 nm light, and monitored by UVevisible
spectroscopy. A medium pressure mercury vapour lamp (365 nm)
equipped with a filter to get a single emission was used for illu-
mination of polymer in solution. The quartz cell containing poly-
mer solution was kept at a distance of 10 cm from the UV lamp for
different time intervals. Immediately after each exposure of
interval, the UV spectrum of polymer solution was recorded. The
absorption spectrum under UV irradiation of polymers 5a, 5b and
5c was shown in Figs. 3e5 respectively. The time taken for total
photo-crosslinking depends on nature of the terminal substituents,
which varies the electron withdrawing/donating nature and thus
alters mesomeric or resonance effect. Among the polymers, the
electron donating group present in the terminal substituent took
longer time for completion of photo-crosslinking process compare
to electron withdrawing group in the terminal substituents. This is
due to electron donating group at the terminal end strengthen the
olefin electron density through an extended conjugation, therefore
the photo-crosslinking time was increased; whereas electron
withdrawing group reduces electron density of the olefin moiety,
which leads to decrease in photo-crosslinking time [29]. The
presence of strong electron donating group increases the
pushepull transition and thus increases photo-crosslinking time.
The photo-crosslinking time of unsubstituted (5e) polymer is
higher compared to all other substituents and reveals that meso-
meric effect of H on chalcone moiety was nullified. Based on the
above observations, the saturation of photo-crosslinking duration
for polymers was depicted in the following order:
5f < 5b < 5c < 5d < 5a < 5e. Fig. 3 shows the decrease in absor-
bance of polymer 5a at its maximum (427 nm). The disappearance
of absorption peak at 427 nm is attributed to the loss of conjugation
Fig. 1. TGA thermogram traces of polymers 5(aef).
10 ꢀC/min up to 750 ꢀC. Thermogram of polymers is depicted in
Fig. 1. The glass transition temperature (Tg) of polymers 5ae5f
possess 131e149 ꢀC and listed in Table 1. Decomposition temper-
atures (Td, defined as the temperature at which a polymer loses 5%
of its original weight) are 253 ꢀC, 255 ꢀC, 225 ꢀC, 248 ꢀC, 271 ꢀC &
280 ꢀC of polymers 5ae5f respectively. All polymers show
reasonably good thermal stability as shown in Table 1. The degra-
dation of polymers occurs in two-step manner. The rate of
decomposition is dependent on tacticity and molecular weight of
polymer. On the basis of reported thermolysis, all the six polymers
underwent polymeric chain scission to small fragments and char
yield of the entire polymer measured at 750 ꢀC. TGA thermogram
reveals that two distinct behaviours dependent on the substituents.
The unsubstituted and methyl substituted polymers have least char
yield as compared with polar heteroatom [N(CH3)2, NO2 & OCH3]
substituents containing polymers. Moreover, all polymers have
high char yield ascribed to triazole ring containing hetero atom in
the polymers.
3.2. Photophysical properties
Absorption spectra of polymers 5(aef) were obtained using
chloroform and its absorption band are summarized in Table 2. The
absorption spectra of all the polymers are shown in Fig. 2. Polymers
5ae5d demonstrate two well segregated absorption bands, while
polymers 5e and 5f exhibited strong absorption band in the blue
shift and a shoulder peak was observed in the red shift. All poly-
mers show an absorption band at <350 nm except polymer 5a, it
from the donor group. Formation of cyclobutane ring by 2
p
þ 2
p
Table 1
TGA data of polymers 5(aef).
Polymers
Tg (ꢀC)
Decomposition temperature (Td)
I stage decomposition temperature (ꢀC)
II stage decomposition temperature (ꢀC)
Char yield (%)
5%
50%
Min
Max
Min
Max
5a
5b
5c
5d
5e
5f
135
138
131
147
141
149
253
255
225
248
271
280
467
442
462
459
465
476
260
275
249
262
283
297
346
325
335
348
351
370
380
349
358
375
388
405
510
495
525
500
519
560
29
20
23
27
18
33

4108
S. Balamurugan et al. / Polymer 53 (2012) 4104e4111
Table 2
Absorbance, emission band and GPC data of polymers 5(aef).
max
em
Polymer
lmax (nm)
l
(nm)
Molecular weight
PDI
(CHCl3)
CHCl3
Mw
Mn
5a
5b
5c
5d
5e
5f
282, 421
282, 350
285, 328
295, 322
290, 320a
302, 324a
524
e
63,000
56,400
33,600
45,200
65,800
75,900
42,600
44,300
28,100
33,500
54,800
62,100
1.47
1.27
1.19
1.34
1.20
1.22
e
e
e
e
a
This peak observed as a shoulder.
cycloaddition reaction between two chromophores vanish the
charge transfer state.
3.4. Fluorescence studies of polymers
The emission spectra of polymers 5(aef) have been recorded by
exciting the molecule its corresponding absorption band in chlo-
roform. Interestingly, among all the polymers, polymer 5a shows an
emission band at 524 nm (Fig. 6) but other derivatives of the
polymer did not exhibit emission band. The presence of eN (CH3)2
group allows ICT emission in longer wavelength region [30,31].
The photo-crosslinking of polymers 5a was carried out by steady
state fluorescence spectroscopy using similar procedure as dis-
cussed earlier. Upon illumination of 365 nm light, the emission
intensity of polymer 5a solution decreased (Fig. 6). Moreover, the
colour diminution was observed by naked eye, which indicates the
Fig. 3. Ultravioletevisible spectral changes of polymer 5a in chloroform upon UV
illumination at different time intervals.
and compiled in Table 3. The visual emission changes of 5a with
different solvent are shown in Fig. 7b. Polymer 5a exhibits an
intense absorption band within the range 413e428 nm and emis-
sion band within the range of 471e524 nm. The absorption and
emission spectra of 5a are strongly depend upon the polarity of
solvent. Since polarities of the ground and excited state of a chro-
mophore are different, a change in polarity solvent will lead to
differential stabilization of ground and excited states. Conse-
quently, variations in the position, intensity, and shape of absorp-
tion spectra can be direct measures of specific interactions between
solute and solvent molecules. The solvent is varied from dioxane to
chloroform, these results suggest the presence of intramolecular
charge transfer of the electronic transition with eN(CH3)2 group as
donor and carbonyl group as acceptor and hence differences ICT
efficiency [32]. The protonation of carbonyl group takes place in
chloroform solvent, which enhances ICT and thus shifting the
formation of cyclobutane ring from 2
p
þ 2
p cycloaddition reaction
[28,29]. It can be concluded that olefinic moiety (eC¼Ce) is dis-
appeared during the formation of cyclobutane ring (Scheme 2).
However static quenching is explained in terms of cyclobutane ring
formation. Hence there is no interaction between acceptor and
donor moiety; this leads to disappearance of ICT emissive state. The
ICT behaviour of 4-(N, N0-dimethyl amino) with related systems has
been reported earlier [30,31]. Static quenching typically resulted
from the formation of non-fluorescent cyclobutane ring between
two fluorophore with increasing illumination time.
3.5. ICT (solvatochromic) behaviour of polymer 5a
The electronic absorption and emission spectral properties of
polymer 5a have been studied in different organic solvents (Fig. 7a)
Fig. 4. Ultravioletevisible spectral changes of polymer 5b in chloroform upon UV
Fig. 2. Absorption spectra of polymers 5ae5f in chloroform.
illumination at different time intervals.

S. Balamurugan et al. / Polymer 53 (2012) 4104e4111
4109
Fig. 5. Ultravioletevisible spectral changes of polymer 5c in chloroform upon UV
illumination at different time intervals.
absorption maximum to longer wavelength region. Similarly,
fluorescence spectra of polymer 5a in different solvents are pre-
sented in Fig. 7b. Polymer 5a in acetonitrile shows an emission
maximum at 500 nm which can be assigned to intramolecular
charge transfer emissive state as reported earlier [33].
3.6. Fluorescence quantum yield of polymer 5a
The acetonitrile solution of rhodamine B (ff ¼ 0.69) was used as
reference for determination of quantum yield of polymer 5a. Upon
illumination of 365 nm UV lamp, fluorescence quantum yield of
polymer 5a decreases from 0.10 to 0.02. The decrease in fluores-
cence quantum yield is attributed to photo-dimerization of
pendant chalcone group leading to vanishing of ICT emissive state.
This is due to inter-conversion of non-radioactive transitions of the
polymer.
Scheme 2. Cyclobutane ring formation upon illumination with UV light.
3.7. Lifetime measurement (TCSPC)
The fluorescence decay of polymer 5a is recorded using time-
correlated single photon counting technique (TCSPC) in acetoni-
trile upon excitation at 420 nm laser as shown in Fig. 8. The fluo-
rescence lifetime decay of polymer 5a fitted satisfactorily to bi-
exponential function with lifetime of 0.52 (90.8%) and 1.62
Fig. 6. Emission spectral changes of polymer 5a in chloroform upon UV illumination
Fig. 7. a) Normalized emission spectral changes of polymer 5a in different solvent. b)
different time intervals.
Visual emission colour changes of 5a in different solvents.

4110
S. Balamurugan et al. / Polymer 53 (2012) 4104e4111
Table 3
quenching of the locally excited ICT state associated non-
radioactive decay.
Absorption and emission band of polymer 5a in different solvents.
max
em
Solvents
lmax (nm)
l
(nm)
Dioxane
THF
CH3CN
CH2Cl2
CHCl3
413
422
419
428
427
471
500
500
510
524
3.8. Electrochemical properties
Cyclic voltammetry (CV) was performed to investigate electro-
chemical properties of polymers 5ae5f in acetonitrile. Polymer 5a
was found to exhibit a single irreversible oxidation wave at 0.9 V vs
Ag/AgCl, which could be assigned to single electron transfer
through the formation of mono radical cation (Fig. 9) [34]. The
solution used for cyclic voltammetry studies was further subjected
to photo-crosslinking studies. Interestingly, no change in the
absorption and emission is observed, which indicates the absence
of [2
p
þ 2
p] cycloaddition. This is evident that presence of mono
radical cation hinders the formation of cyclobutane ring. While rest
of polymers 5(bef) exhibit no reduction or oxidation wave owing to
the absence of strong electron donating group.
4. Conclusion
In this work, we described various donor and acceptor
substituted triazole ring linked pendant chalcone containing pol-
ymethacrylates. All polymers were thermally stable and tend to
decompose in two stage manner. We have speculated that polymer
5a hardly produces an intramolecular charge transfer absorption
and emission maxima at 427 nm and 524 nm respectively. Char-
acteristic ICT of absorption and fluorescence properties were
recorded in different solvents. Increasing the polarity of solvents
increases the quantum yield. The polymer 5a did not exhibit ICT
emission after photo-crosslinking which was observed in naked
eye. Although remaining substituents of polymer 5(bef) not dis-
played intramolecular charge transfer, they could be cross linked by
photo dimerization of the chalcone pendant group upon UV-
Fig. 8. Fluorescence decay profile of polymer 5a.
Table 4
Fluorescence decay profile of polymer 5a at different time intervals of illumination
under 365 nm UV light.
Time (s)
c2
s1 (ns)
s2 (ns)
B1
B2
9.17
10.18
9.75
illumination at
l
¼ 365 nm to form cyclobutane ring. The fluores-
0
90
300
1.3
1.1
1.4
0.52
0.51
0.50
1.62
1.62
1.70
90.83
89.82
90.25
cence decay was fitted bi-exponentially with lifetime of 0.52 ns and
1.62 ns associated with locally excited ICT emissive state. The
irreversible oxidation potential of polymer 5a was observed at
0.9 V. These results widen the scope of pullepush effect on photo-
luminescent polymer synthesis and it will serve as a valuable
guideline for the synthesis of fluorescent probe of polymers, metal
ion sensors and biological sensor with click reactions.
(9.2%) ns. The observation of bi-exponential decay of polymer 5a
indicated the presence locally excited state and ICT emission. As
shown in Table 4, the lifetime of polymer 5a were measured after
photolysis, which do not show any significant change. From this
result, polymer 5a does not undergo complete crosslinking and
exhibited weak emission intensity. It might be ascribed to static
Acknowledgement
P.K. gratefully acknowledges the University Grants Commission,
New Delhi, Government of India [39-691/2010 (SR)] for the finan-
cial support. S.B. sincerely acknowledges the University of Science
Malaysia, sponsored Post-doctoral Fellowship. Measurement of life
time studies carried out at National center for ultrafast processes,
University of Madras has gratefully acknowledged. Instrumentation
facility provided under FIST-DST and DRS-UGC to department of
chemistry, Anna University were sincerely acknowledged.
Appendix A. Supplementary material
Supplementary data related to this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.polymer.2012.06.037.
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