Cyanopyridine based conjugated polymer-synthesis and characterization

Article abstract of DOI:10.1016/j.polymer.2015.09.053

Two novel D-A polymers containing pyridine with vinylene spacer, P1 and P2 were synthesised through Heck polymerisation. In both the polymers, pyridine is linked with two donor phenylene at position-4 and -6, and electron withdrawing cyano group at positi

Full text of DOI:10.1016/j.polymer.2015.09.053

Accepted Manuscript
Cyanopyridine based conjugated polymer-synthesis and characterization
B. Hemavathi, T.N. Ahipa, Saju Pillai, Ranjith Krishna Pai
PII:
S0032-3861(15)30258-5
10.1016/j.polymer.2015.09.053
JPOL 18139
DOI:
Reference:
To appear in: Polymer
Received Date: 10 August 2015
Revised Date: 18 September 2015
Accepted Date: 21 September 2015
Please cite this article as: Hemavathi B, Ahipa TN, Pillai S, Pai RK, Cyanopyridine based conjugated
polymer-synthesis and characterization, Polymer (2015), doi: 10.1016/j.polymer.2015.09.053.
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ACCEPTED MANUSCRIPT
Cyanopyridine based Conjugated Polymer-Synthesis and Characterization
ab
Hemavathi B.,a Ahipa T.N., a Saju Pillai c and Ranjith Krishna Pai *
a
Nanostructured Hybrid Functional Materials and Devices, Center for Nano and Material Sciences,
Jain University, Jain Global Campus, Bangalore – 562112, India.
bDepartment of Science & Technology, Technology Bhavan, New Mehrauli Road, New Delhi 110016,
India.
c
Materials Science and Technology Division & Academy of Scientific and Innovative Research
(AcSIR), CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST),
Thiruvananthapuram, Kerala - 695019, India.
* Corresponding author E-mail addresses: ranjith.krishnapai@gmail.com
ABSTRACT: Two novel D-A polymers containing pyridine with vinylene spacer, P1 and P2
were synthesised through Heck polymerisation. In both the polymers, pyridine is linked with
two donor phenylene at position-4 and -6, and electron withdrawing cyano group at position-
3. Additionally, P1 possess methoxy side chain at position-2 of 3-cyanopyridine and P2 is
flanked with both methoxy and octyloxy side chain at alternate pyridine. The polymers are
1
well characterised with H-NMR, UV-Vis absorption, PL, GPC, TGA, cyclic voltammetry
and AFM. TGA data reveal that the polymers P1 and P2 are thermally stable up to 350-
400oC. The UV-Vis absorption spectra for solid thin films envisage efficient molecular
stacking with red shifting the absorption maxima to 30 nm when compared to solution. The
obtained polymers have a band gap of 1.8-1.93 eV with low laying HOMO and LUMO
energy level. The polymers displayed very good blue fluorescence in CHCl3 upon irradiated
with UV light (254 nm) and Atomic force microscopy (AFM) applications to polymer P1 and
P2 surfaces are providing interesting images of nanometre-scale polymer structures.
Keywords: Cyanopyridine, Heck reaction, Vinylene spacer, fluorescence.
Introduction
With ever increasing demand for clean energy, the need to develop new technology to
harness renewable energy has become very important. In this aspect, polymer solar cells
(PSCs) and electroluminescence have been extensively researched due to their inherent
advantages viz., ease of processibility, flexibility and cost effectiveness.[1-3] PSCs also have
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the advantage of tunability of electronic property with regard to the donor and acceptor unit
in the polymer leading to extension in the absorption range of solar spectrum.[4, 5] Polymers
with narrow band gap are highly crucial for the performance of the PSC [6, 7] and this can be
successfully achieved with molecular designing of the polymer having an electron rich donor
(D) and electron deficient acceptor (A) which facilitates intra-molecular charge transfer
(ICT).[8-13] Further, introduction of π-spacers enables the stacking of the polymer chain
facilitating intermolecular charge transfer.[14, 15]
Pyridine is a known electron deficient molecule and it is used as an acceptor owing to
its very good electron withdrawing property.[16] Generally, low laying lowest unoccupied
molecular orbital (LUMO) reduces the environmental oxidation of the polymer leading to a
stable polymer.[17] Hence, the quest for new polymeric designs to reduce both the highest
occupied molecular orbital (HOMO) and LUMO is essential. Further, the presence of
electron withdrawing cyano group in the polymer reduces both HOMO and LUMO energy
level, thus facilitates higher open circuit voltage (VOC).[18, 19] Cyano group also facilitates
the self-assembly of the polymer leading to a better morphology.[19] Seri et.al synthesised
pyridine based polymer with vinylene spacer containing 3,4-ethylenedioxythiophene as donor
moiety but found that the polymer obtained is having lower molecular weight and less
solubility.[20] It has been reported that the presence of vinylene spacer reduces the band gap,
[21-24] increases charge transfer with increased self-assembly [25, 26] and also enhances the
delocalisation of the electron leading to increased π conjugation than compared to the simple
D-A copolymers.
On this premises, we have designed a cyanopyridine acceptor with enhanced electron
withdrawing capacity thus to lower the HOMO and LUMO. Two D-A-D copolymers were
synthesised with two arylene as donor and 3-cyanopyridine as acceptor. Further, the repeating
D-A-D units are linked with vinylene spacer to extend the π- conjugation in the polymer main
chain. The polymer P1 carries methoxy side chain as substituent at position-2 of pyridine,
whereas, P2 bears the alternating methoxy as well as octyloxy side chain as substituents at
position-2 of pyridine (Scheme 1). The present design is intended to facilitate the
intramolecular charge transfer and flanking different side chain is to enables polymer
processibility. The copolymers were synthesised with Heck polymerisation technique with
very narrow polydispersity index.
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OC8H17
CN
OCH3
H3CO
N
CN
H3CO
N
CN
N
A
N
CN
D
D
A
A
A
D
D
D
D
D
D
P2
P1
n
n
Scheme 1: Structure of polymers P1 and P2
The pyridine is linked at ortho and para positions with the phenylene donor. Pyridine
is an electron deficient and the resonance structure of the pyridine creates electron deficiency
at ortho and para site of the pyridine. The resonance will be stabilised with the linking at
ortho and para position of pyridine with phenylene donor and the resonance as shown in
Scheme 2. This structure allows effective conjugation and leads to intramolecular charge
transfer (ICT).[20] Together with this, the availability of non-bonding electron on the
nitrogen allows the n→π* transition and can also facilitate intersystem crossing (ISC).[17]
These criterion makes the pyridine- based polymer a good candidate for light emitting
material.
Scheme 2: Electron delocalization in D-A-D conjugated polymers (P1 and P2)
Experimental section
Materials: The reagents and organic solvents were procured from Spectrochem, and used as
received without much purification. Compound 2 and 3 were procured from Spectrochem and
Compound 1 was synthesised according to the literature.[27]
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Instrumentation: IR spectra were collected using Bruker ALPHA eco-ATRIR on ZnSe
crystal, UV-Vis spectra were obtained using a Shimadzu UV-1800 spectrophotometer and
1
photoluminescence spectra were collected with Shimadzu RF-5301PC. The H-NMR spectra
were collected on a Jeol AV-III400 (L) spectrometer with TMS as reference and deuterated
chloroform (CDCl3) or DMSO-d6 as solvents operating at 400 MHz. Mass spectra were
recorded with Turbo Ion spray (ESI), AB Sciex/ API 4000 instrument with samples dissolved
in DMSO and diluted with Methanol. Melting point of the monomers and intermediate
compounds were estimated with differential scanning calorimetric (DSC) analysis using DSC
o
851 (Mettler Toledo) under a nitrogen atmosphere at heating rates of 10 C min-1. The
stability of the polymer were analysed with thermo gravimetric analysis using Mettler Toledo
TGA/ SDTA851e instrument. Cyclic voltammetry (CV) measurements for the polymers were
carried out with CHI66OD potentiostat CH instrument using an Ag/AgNO3 as reference
electrode, a Pt wire as counter electrode and a glassy carbon as working electrode in
tetrabutylammoniumhexafluorophosphate as electrolyte (TBAPF6) solution (0.1 M) in
acetonitrile. Ferrocene/ferrocenium redox couple (Fc/Fc+) was used as external calibration
reference. The molecular weight and polydispersity index were analysed by gel permeation
chromatography (GPC) on Agilent 1260 Infinity system at 30 °C with anhydrous
tetrahydrofuran (THF) as eluent and polystyrene as the reference standard. Atomic force
microscopy (AFM) imaging was performed under dry conditions at room temperature (22 ±2
oC) using MultiMode 8 AFM equipped with NanoScope V controller (Bruker, Santa Barbara,
CA, USA). Si cantilevers (NSG 01, NT-MDT) with a typical radius of curvature of
approximately 10 nm were used. The force constants of AFM probe in the range of 2.5-10
N/m and with resonance frequency in the range of 120-180 kHz. The scan rate used was 1
Hz. The samples for AFM measurements were prepared by drop-casting 30 µL of MST
samples onto a fleshly cleaved mica sheet and dried overnight under vacuum. Raw data were
processed offline using Bruker’s NanoScope Analysis software. Surface roughness was
reported both in average roughness values.
Synthesis of monomers and polymers. Synthesis of monomers are as shown in Scheme 3.
Polymers are synthesised via Heck reaction and are shown in Scheme 4. The detailed
synthesis of monomers and copolymers are described as follows.
1,3-Bis(4-bromophenyl)prop-2-en-1-one (4). Compound 2, 4-bromobenzaldehyde (2g,
10.8 mmol) and compound 3, 4-bromoacetophenone (2.15g, 10.8 mmol) were dissolved in
15mL ethanol. Aqueous solution of potassium hydroxide (3.24 mmol) was added with
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constant stirring, and kept the reaction on for 8 hrs. The obtained precipitate was filtered and
washed with 10 mL ethanol and recrystallized from 1:1 ratio of ethanol and chloroform
mixture to obtain a pale yellow product 4 (3.722g, 94%). Melting point, Tm=186.57 oC. ATR-
1
IR (cm-1): 810, 998, 1180, 1595, 1654. H NMR (400 MHz, CDCl3, d/ppm):7.43(S, 1H),
7.49(d, J=1.6 Hz, 1H), 7.51(d, J=2 Hz, 1H), 7.55(d, J=2 Hz, 1H), 7.57(d, J=2 Hz, 1H),
7.64(d, J=2 Hz, 1H), 7.66(d, J=2 Hz, 1H), 7.72(s, 1H), 7.88(d, J=2 Hz, 1H), 7.89(d, J=2 Hz,
1H). ESI-MS/MS (C15H10Br2O), [M+Na]+: calcd, 389.05; found, 389.2.
4,6-Bis(4-bromophenyl)-2-hydroxypyridine-3-carbonitrile (5). Compound 2, 4-
bromobenzaldehyde (2g, 10.8 mmol) and compound 3, 4-bromoacetophenone (2.15g, 10.8
mmol) were dissolved in 35 mL of 1,4-dioxane. Ammonium acetate (6.6g, 85.7 mmol) was
added followed with 1.5 mL of ethyl cyanoacetate (12.9 mmol) with constant stirring and
kept the reaction at 80oC for 24 hrs. The reaction mixture was cooled and poured into 100 mL
of distilled water and stirred for 10 min. The obtained precipitate was filtered and the filtrate
was thoroughly washed with 1, 4- dioxane and dried to afford a white product 5 (1.66g,
o
36%). Melting point, Tm=324.89 C. ATR-IR (cm-1): 810, 1004, 1230, 1495, 1635, 2219 (–
1
CN). H NMR (400 MHz, CDCl3, d/ppm): 3.56 (s, 1H), 7.68 (d, J=2 Hz, 2H), 7.70(d, J=2
Hz, 2H), 7.73(d, J=2 Hz, 1H), 7.75(d, J=2 Hz, 2H), 7.77(t, J=2 Hz, 2H), 7.79(d, J=2 Hz,
1H). ESI-MS/MS (C18H10Br2N2O), [M+H] +: calcd, 431.09; found, 431.2.
2-Methoxy-4, 6-bis (4-vinylphenyl) pyridine-3-carbonitrile (M1). Compound 1 (1g, 1.003
mmol) was dissolved in 8 mL N,N-dimethylfarmamide (DMF) and 67.90 mg of
paraformaldehyde (2.204 mmol) was added and stirred the mixture. Further, sodium
methoxide solution (prepared by adding 103 mg of sodium (4.904 mmol) in 4 mL of
methanol) was added slowly into it with constant stirring and allowed to stand for 2 days at
room temperature. The reaction mixture was poured into 30 mL of distilled water and the
precipitate was filtered. The filtrate was washed with 15 mL of methanol and dried to afford
o
bright yellow product M1 (250 mg, 73%). Melting point, Tm=196.87 C. ATR-IR (cm-1):
1
1106, 1436, 1571, 1659, 2218 (–CN). H NMR (400 MHz, CDCl3, d/ppm): 4.16 (m, J=5.2
Hz, 3H), 7.41 (m, J=7.6 Hz, 2H), 7.53(m, J=12Hz, 2H), 7.65(m, J=8 Hz, 2H), 7.82(m, J=8.4
Hz, 5H), 7.91(m, J=12 Hz, 2H), 8.33(m, J=4.8 Hz, 2H). ESI-MS/MS (C23H18N2O), [M+H] +:
calcd, 339.40; found, 339.2.
4,6-Bis(4-bromophenyl)-2-methoxypyridine-3-carbonitrile (M2). Compound 4 (3.5g,
9.560 mmol) was dissolved in 5 mL of methanol and 15 mL of sodiummethoxide (190 mmol)
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solution (prepared by dissolving 4g of sodium in 15 mL methanol) was added with constant
stirring and allowed the reaction for 8 hours at room temperature. The precipitate was filtered
and washed with methanol and dried to afford creamy yellow coloured product M2 (2.008g,
47%). Melting point, Tm=174.13 oC. ATR-IR (cm-1): 817, 1005, 1366, 1539, 2217 (–CN). 1H
NMR (400 MHz, CDCl3, d/ppm): 3.30 (s, 3H), 7.52 (m, J=1.6 Hz, 1H), 7.53(m, J=2.8Hz,
2H), 7.55(m, J=2.8 Hz, 2H), 7.60(m, J=1.6 Hz, 2H), 7.63(m, J=1.2 Hz, 2H). ESI-MS/MS
(C19H12 Br2N2O), [M+CH3OH]+: calcd, 473.9; found, 473.0
4, 6-Bis (4-bromophenyl)-2-(octyloxy) pyridine-3-carbonitrile (M3). Compound 5 (0.33g,
0.767 mmol) was dissolved in 8 mL of DMF. 1-Bromooctane (0.15mL, 0.777 mmol) was
then added, followed by 148 mg of potassium carbonate (1.072 mmol) and the reaction
o
mixture was stirred at 90 C for overnight. The reaction mixture was cooled and poured into
ice cold water with constant stirring. The obtained precipitate was filtered, washed with water
o
and dried to obtain creamy yellow product M3 (280 mg, 75%). Melting point, Tm=79.16 C.
1
ATR-IR (cm-1): 820, 998, 1348, 1540, 2219 (–CN), 2918. H NMR (400 MHz, CDCl3,
d/ppm): 0.88 (t, J=4.8 Hz, 3H), 1.29 (m, J=6.4 Hz, 4H), 1.39(m, J=4Hz, 4H), 1.519(m, J=7.2
Hz, 4H), 1.89(m, J=6.8 Hz, 2H), 4.576(t, J=6.4 Hz, 2H), 7.26(s, 1H) 7.52(d, J=6.8 Hz, 2H),
7.61(d, J=2 Hz, 1H), 7.63(d, J=2 Hz, 1H), 7.66(d, J=2 Hz, 1H), 7.68(d, J=2 Hz, 1H), 7.92(d,
J=2 Hz, 1H), 7.94(d, J=2 Hz, 1H). ESI-MS/MS (C26H26Br2N2O), [M+H] +: calcd, 543.31;
found, 543.1
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OCH3
CN
OCH3
CN
N
N
(HCHO)n
NaOCH3, DMF
BrPh3P
PPh3Br
RT, 1 day
M1
1
OCH3
CN
O
CHO
Br
Br
N
KOH
CH2(CN)2
C2H5OH
RT, 8 hrs
NaOCH3, CH3OH
RT, 8 hrs
O
4
Br
Br
Br
2
Br
3
M2
CNCH2COOC2H5
CH3COONH4, 1,4-dioxane
80oC, 1 day
OH
OC8H17
CN
CN
N
N
1-bromooctane
K2CO3, DMF
Br
Br 80oC, 9 hrs
Br
Br
M3
5
Scheme 3: Synthesis of monomers M1, M2 and M3
Synthesis of polymers P1 and P2. Synthetic route for the preparation of copolymer P1 and
P2 is shown in Scheme 2.
Polymer P1 was synthesised from the reaction of the monomer M2 with monomer M1
under nitrogen atmosphere via Heck coupling reaction. The monomers, M1 (0.1g, 0.295
mmol) and M2 (0.1312g, 0.297 mmol) were dissolved in 8 mL DMF and 7 mL triethylamine
(TEA) solvents and stirred under nitrogen atmosphere for 15 min. To this, triphenylphosphine
(250 mg, 0.953 mmol) was added and again purged with nitrogen gas for 10 min. Further,
catalytic amount (0.1 eq.) of palladium (II) acetate (Pd(OAc)2) was added to the reaction
mixture under inert atmosphere and stirred at 120oC for 2 days. The reaction mixture was
cooled and poured into a beaker containing 40 mL methanol and the obtained precipitate was
filtered. Finally, the crude polymer was purified by dissolving it in chloroform and re-
precipitated with methanol and the obtained copolymer precipitate was filtered and washed
with acetone to afford dark yellow product P1 (0.099g, 53%). ATR-IR (cm-1): 696, 826, 963
(trans-vinylene), 1008, 1360, 1589, 2222 (–CN). 1H NMR (400 MHz, CDCl3, d/ppm):
4.24(b, 3H), 7.26(b, 2H), 7.54(b, 4H), 7.69(b, 5H), 8.13(b, 2H).
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Scheme 4: Synthesis of polymers P1 and P2
Copolymer P2 was synthesised from the reaction of the monomer M3 with monomer
M1 under nitrogen atmosphere via Heck coupling reaction. The monomers, M1 (0.1g, 0.295
mmol) and M3 (0.16g, 0.295 mmol) were dissolved in 8 mL DMF and 7 mL triethylamine
(TEA) solvents and stirred under nitrogen atmosphere for 15 min. To this, triphenylphosphine
(250 mg, 0.953 mmol) was added and again purged with nitrogen gas for 10 min. Further,
catalytic amount (0.1 eq.) of palladium (II) acetate (Pd(OAc)2) was added to the reaction
mixture under inert atmosphere and stirred at 120oC for 2 days. The reaction mixture was
cooled and poured into a beaker containing 40 mL methanol and the obtained precipitate was
filtered. Finally, the crude polymer was purified by dissolving it in chloroform and re-
precipitated with methanol and the obtained copolymer precipitate was filtered and washed
with acetone to afford brownish yellow product (0.125g, 57%). ATR-IR (cm-1): 832, 966
1
(trans-vinylene), 1004, 1147, 1358, 1576, 2222 (–CN), 2946. H NMR (400 MHz, CDCl3,
d/ppm): 0.88 (b, 3H), 1.31 (b, 10H), 1.91(b, 2H), 4.24(b, 3H), 4.61(b, 2H), 7.27(b, 6H),
7.54(b, 4H), 7.71(b, 8H), 8.15(b, 4H). Mn=841,401 g/mol, Mw= 941,465 g/mol, PDI= 1.12.
The copolymer P2 has very good solubility in chloroform and sparingly soluble in
chlorobenzene. Whereas, P1 has poor solubility in chloroform or in any other organic solvent
due to non-availability of bulky alkyl side chain.[28] In fact, very good solubility of P2 is
attributed to the presence of octyloxy side chain on the polymer main chain.[29]
Results and discussion
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Monomer M1 was synthesised in good yield (73%) from compound 1 and
paraformaldehyde through Wittig reaction. M1 is a dark yellow compound with solubility in
DMSO. Compound 4 was synthesised from 2 and 3 using Claisen–Schmidth reaction.
Further, the compound 4 was cyclised with malononitrile to get comonomer M2 (yield:
47%). On the other hand, one pot synthesis from 2 and 3 in presence of ethylcyanoacetate and
excess ammonium acetate afforded compound 5. By O-alkylation of 5 with 1-bromooctane
in presence of alkali affords comonomer M3 with 75% yield. Both the monomers, M2 and
M3 are soluble in chloroform. Polymers P1 and P2 were synthesised via Heck
polymerisation reaction. P1 synthesised from M1 and M2 and P2 from M1 and M3 with
yield being 53% and 57%, respectively. The obtained crude polymers were purified by re-
precipitation technique. Both the polymers are soluble in chloroform but the polymer P1 is
not completely soluble in chloroform whereas P2 is having very good solubility. The poor
solubility of P1 is attributed to non-availability of bulky side chain. However, P2 with
octyloxy side chain is endowed with very good solubility. The molecular structures of the
monomers and polymers were characterized by 1H NMR (Figure 1) and ATR-IR techniques.
The molecular weight of polymer was determined by GPC using THF as eluent and
polystyrene as standard at 30oC. The polymer P1 possess poor solubility in THF and
remained as insoluble particles. Hence, the polymer P1 was unable subject for GPC study and
to ascertain its actual molecular weight. But, P2 possess excellent solubility in THF. Its GPC
study shows that Mn=841,401 g/mol and Mw=941,465 g/mol with a very narrow
polydispersity index (PDI) of 1.12. This reiterates that the Heck polymerisation is a very
good technique to obtain a high molecular weight polymers.
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Figure 1: 1H NMR spectra of P1 and P2
Thermal Property. Thermal stability of the polymer were investigated with thermo
gravimetric analysis (TGA) under nitrogen atmosphere. The thermal plots obtained for P1
and P2 were as shown in Figure 2. Both the polymers showed good thermal stability with 5
% weight loss for P1 at 468oC and 336oC for P2. P1 with no bulky side chain shows better
thermal stability compared to P2. Further, P2 showed a loss of 17 % at 400oC correspond to
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detachment of octyloxy side chain from the polymer main chain which is particularly absent
in P1 as it bears only methoxy side chain.
Figure 2: TGA curve for P1 and P2 obtained under nitrogen atmosphere
Optical property. Photophysical properties of P1 and P2 were investigated in chloroform
using UV-Vis and photoluminescence spectroscopy. The UV-Vis spectra of polymers P1 and
P2 in solution and film is depicted in Figure 3. Their optical data is summarised in Table 1.
The polymer P1 shows two peaks: higher energy peak with absorption maxima at 273 nm
corresponds to π-π* of the donor moiety, lower energy peak appears at 374 nm indicating the
intramolecular charge transfer (ICT) from the donor to the acceptor group.[30] Also, P2
shows similar trend with high energy peak at 274 nm representing the donor π - π* transition
which is similar as P1 indicating the presence of same donor in the P2 as in P1. The lower
energy transition is also observed at 376 nm which is very close to the value observed in P1
infers the presence of same a kind donor and acceptor charge transfer in P2.
It is noteworthy that, the absorption spectra of P1 and P2 thin films are broader
compared to its solution indicating the molecular stacking leading to intermolecular charge
transfer. This large π-stacking of the molecule is indicated by the red shift for both the
polymers with absorption maxima of about 33 nm for P1 and 23 nm for P2. Red shift is more
pronounced for P1 reiterates a much efficient stacking in case of P1 having methoxy as side
group than in case of P2 with octyloxy side chain prevent efficient π- π stacking.[31] The
absorption band edges for P1 and P2 were 664 and 640 nm (drawn with tangent extending to
opt
x-axis) with optical band gap calculated with these onset values with equation Eg
=
1240/λedge were found to be 1.86 eV for P1 and 1.93 eV for P2 (Table 1).
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Figure 3: UV-Vis absorption spectra of polymers in (a) solution (CHCl3) and (b) as-cast film
Table 1 Photophysical and electrochemical properties of the polymers
Polymer
Solutiona
Filmb
λmax
(nm)
c
red
Eg
λem
Eonset
LUMO
(eV)
HOMOd
(eV)
onset
onset
λmax
λmax
(nm)
λmax
(nm)
407
(nm)
(eV)
(eV)
(nm)
273,374
274,377
441
443
664
640
1.86
1.93
435
433
-0.622
-0.577
-3.707
-3.753
-5.63
-5.68
P1
P2
400
a
b
Measured with dilute solution of polymer in chloroform. Measured with polymers film as cast from
opt
chloroform solution. c Calculated from the onset wavelength of the absorption spectra. d Calculated from Eg
Photoluminescence (PL) studies of the polymers P1 and P2 were carried out in chloroform
(CHCl3) solvent. PL spectra were obtained by irradiative excitation at the wavelength of the
absorption maxima. The obtained PL spectra is shown in Figure 4 (a). The shape of emission
spectra is almost identical for the polymers P1 and P2 with an intense emission band at 435
and 433 nm, respectively. In both the cases, polymer shows well-resolved vibronic shoulder
peak at 451 and 454 nm for P1 and P2, respectively, which infers the rigidity of the core
structure. The vibronic behaviour suggest a highly ordered regions with an extended chain
conformation which favours enhanced π- π stacking.[32-34] These results are also consistent
with the AFM results in Figure 7 and 8. But, a slightly decreased intensity in vibronic
shoulder of P2 was seen compared P1 and this can be attributed to the less structural rigidity
of the polymer due to introduction of octyloxy side chains. They displayed very good blue
fluorescence in CHCl3 upon irradiated with UV light (254 nm), as shown in Figure 4 (b).
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Figure 4: (a) Photoluminescence spectra of P1 and P2 in solution (CHCl3): (b) Polymer solutions in
CHCl3 under UV light irradiation (365 nm).
Electrochemical Property. The electrochemical properties were analysed with cyclic
voltammetry (CV) measurement to elucidate the HOMO and LUMO energy level which are
very crucial to design a polymer for OPVs. CV was recorded for polymer coated on glassy
carbon from chloroform solution (2mg/ml) with Ag/AgCl as reference electrode and
calibrated against Fc/Fc+ redox couple for which ionization energy is taken to be 4.80 eV.[35]
For both polymers, only reduction potential was observed (Figure 5) and unfortunately, there
was no oxidation potential observed. Hence, LUMO of the polymers were calculated with
ELUMO = −e (Ered + 4.33) (ionisation potential for Fc/Fc+ measured was 0.47 V vs. Ag/AgCl).
Measured LUMO energy levels for P1 and P2 were -3.707 and -3.753 eV, respectively. The
lower LUMO for P2 is attributed octyloxy side chain. Since, oxidation potential was not
obtained for the polymers, HOMO was calculated from the optical band gap obtained from
UV-Vis spectra for polymer films.[36] The HOMO for P1 and P2 were -5.63 and -5.68 eV.
The energy levels are tabulated in Table 1.
a)
b)
c)
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Figure 5: Cyclic voltammograms of a) P1 and b) P2. Solution in 0.1M TBAPF6– acetonitrileat scan
rate of 10 mV/s with c) potential calculated versus ferrocene.
Figure 6: LUMO calculated from CV and HOMO calculated with optical band gap and LUMO of
polymers were compared with energy levels of PPV[37] polymer and PC61BM acceptor.
The energy level diagram envisaged the HOMO and LUMO energy level for the
polymers P1 and P2 and the comparison is made with a PPV polymer (Figure 6). The energy
level of an acceptor (PCBM) is compared with that of the polymers P1 and P2 which are
treated as acceptor. To obtain high Voc, the HOMO of the polymer should be lower which can
lead to higher offset energy level between the donor and acceptor which is evident from the
diagram. To have a higher Jsc, the offset energy between the LUMO of the donor and the
acceptor should be more than 0.3 eV[38] which also evident in the diagram. These
parameters suggest that the polymers synthesised can be successfully used to fabricate OPVs.
Morphological study: Morphology of the polymer is very crucial to design a device for OPV
since morphology influence the device parameter and hence affects the efficiency. To
investigate the surface morphology of the polymers film (as-cast) with thickness of 100-200
nm, AFM analysis under tapping mode was done and the images are as shown in Figure 7
and 8. Surface features of P1 and P2 polymers from molecular scale lattice to large scale
morphology patterns can be examined with AFM. The imaging of polymer film nanostructure
is a primary goal of AFM study of oriented polymers. Using AFM technique, one can
observe nanoscale structural features near the surface of a thick sample that are not accessible
by TEM or SEM.
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Figure 7: AFM images of P1 (a) Height image, (b) phase image (c) corresponds to higher-
magnification image and figure (d) is corresponding vertical height images with position of vertical
height is marked in (c)
Figure 8: AFM images of P2 (a) Height image, (b) phase image (c) corresponds to higher-
magnification image and figure (d) is corresponding vertical height images with position of vertical
height is marked in (c)
In Figure 8(a), for P2, circular- like patterns (size 180-593 nm) were perceived and in
Figure 7(a) of P1, a more fully developed circular patterns (size 300-464 nm) decorated
with spherules on the apex of the circle was evident. The regular ordering of these polymers
into circular- like patterns likely resemble the assembly of polymer molecule into supra-
molecular aggregates which was previously observed with AFM. These examples
demonstrate that AFM can produce a data about ordering of polymers P1 and P2, a new class
of materials in which supra-molecular aggregations are formed through repeated chemical
bonding which is assumed to be a intramolecular hydrogen bonding associated with N-of
pyridine. Figure 7(c) and Figure 8(c) corresponds to higher- magnification images and
Figure 7(d) and Figure 8 (d) are the corresponding vertical height images with position of
vertical height is marked in Figure 7(c) and Figure 8(c) as the lines are guides for the eye. It
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can be seen from the image (1d), that the surface of P1 is smoother with Rmax of 179 nm.
However, for P2 with octyloxy side chain, in Figure 8(c) and 8(d), has enabled us to
visualize in-situ the interaction of side group resulting in a circular like structure with surface
roughness of (Rmax)-408 nm.
Distinctive features are seen in phase image, where, the bright area correspond to stiff
substrate region and the dark area corresponds to amorphous polymer. Arrays of nano
spherules found at the apex of circles (Figure 7(b) of P1) have a contrast that is closer to that
of harder region.
Conclusion.
In summary, we report the design, synthesis and characterisation of two novel 3-
cyanopyridine based conjugated polymers P1 and P2 with methoxy and octyloxy side chain
along with vinylene linkage. The presence of octyloxy side chain renders P2 more soluble
than P1. However, P1 possess lower band gap compared to P2 which is attributed to different
side chain. This also reiterated in UV-Vis spectra of the polymer films where, P1 with
methoxy is endowed with much efficient π-stacking compared to P2 with octyloxy side chain.
However, the polymer P2 has excellent solubility with a band gap of 1.93 eV and very low
HOMO and LUMO of -5.68 and -3.75 eV, respectively. Also, P2 possesses very good
thermal stability up to 336oC and displayed blue fluorescence in CHCl3 upon irradiated with
UV light. Hence, it is endowed with all the necessary property required to be used as
electroluminescence and PSC material.
Acknowledgement: This research was supported by the Solar Energy Research Initiative
(Project code: DST/TMC/SERI/FR/201), Department of Science and Technology (DST),
Govt. of India, New Delhi. The work of Ahipa T.N. was supported by the Jain University,
through the postdoctoral fellowship and Dr. Saju Pillai thank for the support of CSIR network
projects (NWP IntelCoat and M2D) and DAE-BRNS grants. The authors are grateful to the
NMR research centre and Solid State Chemistry Unit, IISc Bangalore for providing
instrumental analyses.
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HIGHLIGHTS
•
Two new cyanopyridine based conjugated polymers, P1 and P2 with vinylene spacer
were synthesised via Heck reaction.
•
P1 and P2 have band gap of 1.83 and 1.93 eV, respectively and have very lower
HOMO and LUMO energy level -5.6 to -3.7 eV and are endowed with good thermal
stability of 468 oC and 336 oC for P1 and P2, respectively.
•
•
Both polymers exhibit fluorescence with emission of blue light when irradiated with
UV light.
The photophysical and electrochemical data thus conclude that the polymers P1 and
P2 are a promising candidate for the application in photoluminascent and solar cell.