Conjugated polymers with carbazole, fluorene, and ethylene dioxythiophene in the main chain and a pendant cyano group: Synthesis, photophysical, and electrochemical studies

Article abstract of DOI:10.1002/pola.28160

Six new conjugated polymers comprising of carbazole, fluorene, and ethylene dioxythiophene (EDOT) moieties along the backbone with a pendant cyano group attached to the ethylene moiety have been designed and synthesized via Sonogashira coupling polymerization reaction. Optical and electrochemical characterizations have shown that the energy band gaps lie within the range of 2.35–2.44 eV. Additionally, the presence of carbazole and EDOT makes these polymers better hole transporting materials, which is reflected from their low oxidation potential peaks (0.55–1.11 V) in cyclic voltammograms. Furthermore, the aggregation enhanced emission (AEE) phenomenon resulted in a 2.6-fold increase in fluorescence intensity in a 90:10 THF/water mixture in comparison to pristine THF. The AEE properties were further verified by DLS (dynamic light scattering) experiment and SEM (scanning electron microscopy) studies. Polymers in solution as well as in polystyrene matrix emit in the green region (quantum yield in solution state Φ_f =41–43%) with CIE values (0.25–0.36, 0.52–0.57). Excellent thermal stability is observed for the new polymers.

Full text of DOI:10.1002/pola.28160

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ARTICLE
Conjugated Polymers with Carbazole, Fluorene, and Ethylene
Dioxythiophene in the Main Chain and a Pendant Cyano Group:
Synthesis, Photophysical, and Electrochemical Studies
Vivek Anand, Elumalai Ramachandran, Raghavachari Dhamodharan
Department of Chemistry, IIT Madras, Chennai 600036, Tamil Nadu, India
Correspondence to: R. Dhamodharan (E-mail: damo@iitm.ac.in)
Received 1 April 2016; accepted 27 April 2016; published online 00 Month 2016
DOI: 10.1002/pola.28160
ABSTRACT: Six new conjugated polymers comprising of carba-
zole, fluorene, and ethylene dioxythiophene (EDOT) moieties
along the backbone with a pendant cyano group attached to
the ethylene moiety have been designed and synthesized via
Sonogashira coupling polymerization reaction. Optical and
electrochemical characterizations have shown that the energy
band gaps lie within the range of 2.35–2.44 eV. Additionally,
the presence of carbazole and EDOT makes these polymers
better hole transporting materials, which is reflected from their
low oxidation potential peaks (0.55–1.11 V) in cyclic voltammo-
grams. Furthermore, the aggregation enhanced emission (AEE)
phenomenon resulted in a 2.6-fold increase in fluorescence
intensity in a 90:10 THF/water mixture in comparison to pris-
tine THF. The AEE properties were further verified by DLS
(dynamic light scattering) experiment and SEM (scanning elec-
tron microscopy) studies. Polymers in solution as well as in
polystyrene matrix emit in the green region (quantum yield in
solution state U
0.57). Excellent thermal stability is observed for the new poly-
2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
f
541–43%) with CIE values (0.25–0.36, 0.52–
mers.
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Polym. Chem. 2016, 00, 000–000
KEYWORDS: conjugated polymers; carbazole; fluorene; ethylene
dioxythiophene; Sonogashira polymerization; AEE; light-emit-
ting diodes (LED); copolymerization; luminescence
1
(a,b)
INTRODUCTION Conducting polymers (CPs)
have been
is found to be the restriction of intramolecular rotation
(RIR) in aggregated state, which leads to enhancement in flu-
orescence emission. Since its discovery, increasing atten-
applied immensely in the field of organic light emitting
2
–4
5–7
16
diodes (OLEDs),
sensors,
field effect transistors (FETs),
and organic photovoltaics (OPDs)
blend the electrical properties of metals with the benefits of
polymers. Among these, polymers with tricyclic fused aro-
matic compounds such as carbazole and fluorene, in the
main chain have been well reported to possess excellent
properties as materials for OLEDs. Conventionally, it has
been found that CPs are highly emissive in the dilute solu-
tions but become nonemissive or weakly fluorescent in the
thin solid films, which is a severe problem hindering the
chemical
as they
8
(a,b)
9,10
tion has been paid in the synthesis and development of AIE
active conjugated polymers as they are rare, and can find
1
1
17
intense application as PLED materials.
Many groups have incorporated EDOT and fluorene in the
backbone of numerous CPs. EDOT exhibits very high molar
absorption coefficient (E) and efficiently donates electrons
1
2
18–20(c)
owing to the presence of heteroatoms.
Fluorene, con-
versely, apart from being a good hole transporter, has excel-
lent chemical and thermal robustness and good solubility as
well as high fluorescence quantum yield, high photolumines-
1
3
development of efficient PLEDs.
21–24
Since the discovery of aggregation-induced emission (AIE) in
001 by Tang et al., increasing attention has been paid in
the synthesis of AIE molecules. AIE is a phenomenon in
which a luminogen, nonemissive in good solvents becomes
highly emissive when aggregated in poor solvents or in thin
films. If a CP shows aggregation induced emission (AIE)
property then the notorious fluorescence quenching in film
form can be overcome. The physical phenomenon for AIE
cence efficiency, and good photostability.
Polyfluorene is
2
in general used as a blue emitter and by fusing it with other
1
4
25
systems such as carbazole, the conjugation can be extended.
The carbazole moiety, apart from extending the conjugation,
imparts excellent hole transporting property, high triplet
energy, amorphous nature, and can be easily functionalized by
reaction at the nitrogen atom. Moreover, on incorporation of
carbazole, thermal and photochemical stability, and durability
1
5
1
6
Additional Supporting Information may be found in the online version of this article.
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of the polymers were found to be significantly
product was purified by silica gel column chromatography
(hexane/DCM 3/2) to give the dialdehyde (18). Yield: 88%,
yellow solid. mp: 152 8C.
1
9,26,27
improved.
The presence of bulky cyano group in the
CPs hinders p-p stacking and maintains coplanarity, which are
essential factors for the formation of J-type aggregation lead-
ing to aggregation enhanced emission (AEE).
1
2
8,29
H NMR (400 MHz, CDCl , d, ppm): 9.92 (s, 2H), 7.66 (d,
3
J 5 8 Hz, 2H), 7.53 (d, J 5 8Hz, 2H), 7.45 (s, 2H), 4.40 (s, 8H),
1.94–1.98 (m, 4H), 1.03–1.22 (m, 20H), 0.80 (t, J 5 8 Hz,
6H), 0.55 (s, 4H). C NMR (125 MHz, CDCl , d, ppm): 179.5,
151.3, 147.5, 143.5, 141.3, 131.0, 126.0, 121.0, 120.2, 118.1,
109.3, 101.7, 79.8, 65.1, 64.9, 55.4, 55.4, 40.3, 31.8, 31.8,
30.0, 29.3, 29.2, 23.7, 22.6, 22.5, 14.1, 14.1. IR (KBr, cm ):
2943, 2861, 2199, 1654, 1468, 1364, 1271, 1155, 1073, 957,
829, 678. UV–Vis (DCM) kmax, nm: 392. MS (MALDI) m/z:
The fusion of carbazole-fluorene or EDOT-fluorene in one mol-
ecule towards the enhancement of AEE in the solid state is
1
3
3
2
3,30
well known.
However, the simultaneous incorporation of
carbazole, fluorene, and EDOT in the same main chain along
with a pendant cyano (CN) group toward developing an AEE
chromophore has not been reported so far. This strategy can
be very promising as the resulting polymers can exhibit tre-
mendous potential toward development of PLED materials.
21
1
775.16 (100, M ). Anal. Calcd for C H O S : C, 72.84; H,
4
7 50 6 2
6
.50. Found: C, 73.17; H, 6.34.
In this work, we report the synthesis and characterization of
six polymers that have been designed to contain carbazole,
fluorene, and EDOT repeat units in the main chain as well as
a pendant cyano group with the specific aim of introducing
AEE. To the best of our knowledge, this series of compounds
has not been reported till date and the synthetic design, in
principle, should enable potential toward PLED applications.
The polymers were characterized in detail using spectro-
scopic tools and their properties were studied by UV–Visi-
ble/fluorescence spectroscopy and cyclic voltammetry.
Theoretical studies were carried out using gas phase density
functional theory (DFT) to get insight into the molecular
arrangements and their energy levels.
Compound (19) (new) also a dialdehyde was synthesized by
the same procedure as 18. In this case, compound (6) was
taken in place of (5). Yield: 77%, yellow solid. mp: 156 8C.
1
H NMR (400 MHz, CDCl , d, ppm): 9.92 (s, 2H), 7.66 (d, J5
3
8
4
Hz, 2H), 7.53 (dd, J 5 8 Hz, J 5 1.2 Hz, 2H), 7.50 (s, 2H),
1 2
.33 (s, 8H), 1.94–1.98 (m, 4H), 1.03–1.24 (m, 28H), 0.80 (t,
1
3
J 5 8 Hz, 6H), 0.55 (s, 4H). C NMR (125 MHz, CDCl , d,
ppm): 179.6, 151.4, 147.6, 143.6, 141.5, 131.1, 126.1, 121.1,
3
1
3
20.3, 118.2, 109.5, 101.8, 80.0, 65.3, 65.0, 55.6, 40.4, 32.0,
0.1, 30.1, 29.7, 29.6, 29.6, 29.4, 29.3, 23.9, 22.7, 14.2. IR
2
1
(KBr, cm ): 2931, 2850, 2211, 1654, 1503, 1457, 1364,
1259, 1155, 1085, 969, 806, 678. UV–Vis (DCM) kmax, nm:
1
392. MS (MALDI) m/z: 831.30 (100, M ). Anal. Calcd for
EXPERIMENTAL
C H O S : C, 73.70; H, 7.03. Found: C, 73.81; H, 7.18.
5
1 58 6 2
Chemicals
Palladium (II) chloride which was used to synthesize the cat-
alyst PdCl (PPh ) and CuI were purchased from Sigma
2 3 2
The monomer (M1) (new) was synthesized using standard
Knoevenagel condensation reaction. The procedure is as fol-
lows: In a round bottom flask containing the dialdehyde
(18) (5.6 mmol, 1 eq.), THF (100 mL) was added followed
by the addition of 4-bromophenyl acetonitrile (20) (12.3
mmol, 2.2 eq.) and aqueous solution of NaOH (22.5 mmol
Aldrich and Finar & Co., respectively. The compound 3,4-eth-
ylene dioxythiophene (15, CAS No. 126213-50-1) was pur-
chased from Sigma Aldrich while 4,4’-dibromobiphenyl (7,
CAS No. 92-86-4) and fluorene (1, CAS No. 86-73-7) were
purchased from Alfa Aesar. All these chemicals were used as
such without further purification. The solvents: DIPA, THF,
DCM, and toluene were purchased from Rankem & Co. were
distilled, where necessary, using standard procedures. The
deuterated solvents were purchased from Merck & Co.
in 10 mL H O, 4 eq.). The resulting mixture was stirred at
2
70 8C for 18 h. After the reaction, the solvent was evapo-
rated under vacuum and the residue was extracted with
DCM and water, washed with brine solution, dried with
Na
product was purified by silica gel column chromatography
hexane/DCM 3/7) to give monomer (M1). Yield: 83%, red
SO and filtered. After removing the solvent, the crude
2
4
Synthesis of Precursors and Polymers
(
The dialdehyde (18) (new) was prepared using standard
Sonogashira coupling reaction. The procedure is as follows:
In a two-necked round bottom flask, the bromo compound
solid. mp: 274 8C.
1
3
H NMR (400 MHz, CDCl , d, ppm): 7.72 (s, 2H), 7.66 (d,
(
%
(
17) (6.83 mmol, 1 eq.), Pd(II) (5 mol %), and CuI (5 mol
) were dissolved in a 1:1 mixture of solvents (DIPA:THF)
total 50 mL) and degassed for 15 min using argon. Then, a
J 5 8 Hz, 2H), 7.47–7.54 (m, 12H), 4.36 (s, 8H), 1.96–1.99
(m, 4H), 1.05–1.22 (m, 20H), 0.81–0.86 (m, 6H), 0.58 (s, 4H).
1
3
C NMR (125 MHz, CDCl , d, ppm): 151.2, 143.8, 143.1,
3
degassed solution of the alkyne (5) (15.04 mmol, 2.2 eq.) in
THF was added through a syringe to the reaction mixture at
room temperature (final solvent ratio 5 1:1). Then, the tem-
perature was raised to 70 8C and maintained for 12 h. After
the reaction, the solvents were evaporated under vacuum
and the residue was extracted with DCM and water, washed
with brine solution, dried with Na SO and filtered. The sol-
141.1, 133.1, 132.2, 132.1, 132.1, 130.7, 129.1, 126.9, 126.9,
126.8, 125.9, 122.7, 121.3, 120.1, 118.3, 114.4, 104.4, 104.0,
101.5, 80.3, 65.0, 65.0, 55.3, 40.3, 31.7, 30.0, 29.3, 29.2, 23.8,
21
22.6, 14.0. IR (KBr, cm ): 2943, 2850, 2211, 1584, 1457,
1387, 1317, 1073, 1003, 957, 829. UV–Vis (DCM) kmax, nm:
1
444. MS (MALDI) m/z: 1130.15 (100, M ). Anal. Calcd for
C H Br N O S : C, 66.90; H, 5.17; N, 2.48. Found: C, 66.95;
2
4
63 58
2 2 4 2
vent was removed under vacuum and the resulting crude
H, 4.50; N, 2.32
2
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1
Monomer (M2) (new) was synthesized following the same
procedure as (M1) only compound (19) was taken in place
of (18). Yield: 34%, red solid. Mp: 230 8C.
P4: Yield: 45%, red solid. H NMR (400 MHz, CDCl , d,
3
ppm): 8.02–8.05 (m), 7.24–7.79 (m), 4.34–4.40 (m), 1.91–
1.99 (m), 1.59 (s), 1.00–1.34 (m), 0.81–0.91 (m), 0.53–0.58
m). IR (KBr, cm ): 2915, 2839, 2217, 1580, 1489, 1352,
292, 1246, 1079, 973, 806. UV–Vis (DCM) kmax, nm: 453.
Monomer ratio in the polymer by H NMR: 1.0:1.0 (M2:12).
P5: Yield: 49%, a red solid. H NMR (400 MHz, CDCl , d,
ppm): 7.73 (s), 7.67 (d), 7.48–7.55 (m), 4.36–4.39 (m), 4.33
s), 1.96–2.00 (m), 1.06–1.25 (m), 0.84 (t), 0.59 (s). IR (KBr,
cm ): 2943, 2850, 2211, 1584, 1468, 1352, 1259, 1096,
003, 945, 829. UV–Vis (DCM) kmax, nm: 451. Monomer ratio
in the polymer by H NMR: 2.0:1.0 (M2:13).
21
(
1
H NMR (400 MHz, CDCl , d, ppm): 7.73 (s, 2H), 7.67 (d,
3
1
J 5 8 Hz, 2H), 7.48–7.55 (m, 12H), 4.39–4.36 (m, 8H), 1.96–
1
2
0
1
1
1
3
.00 (m, 4H), 1.06–1.25 (m, 28H), 0.84 (t, J 5 6.8 Hz, 6H),
1
3
1
3
.59 (s, 4H). C NMR (125 MHz, CDCl , d, ppm): 151.2,
3
43.8, 143.1, 141.1, 133.1, 133.0, 132.2, 132.1, 132.1, 130.7,
29.1, 126.9, 126.9, 126.8, 125.9, 122.7, 121.3, 120.1, 118.3,
14.4, 104.4, 104.0, 101.5, 80.3, 65.0, 65.0, 55.3, 40.3, 31.7,
0.0, 29.7, 29.6, 29.6, 29.5, 29.4, 29.3, 29.2, 23.8, 22.6, 14.0.
(
21
1
1
21
IR (KBr, cm ): 2924, 2852, 2204, 1556, 1500, 1447, 1407,
403, 1361, 1307, 1255, 1087, 1005, 950, 820. UV–Vis
1
P6: Yield: 56%, a red solid. H NMR (400 MHz, CDCl , d,
ppm): 8.02–8.05 (m), 7.34–7.79 (m), 4.34–4.39 (m), 1.88–
3
1
1
(DCM) kmax, nm: 445. MS (MALDI) m/z: 1186.24 (100, M ).
1
0
1
.99 (m), 1.59 (s), 1.62 (s), 1.00–1.26 (m), 0.80–0.88 (m),
Anal. Calcd for C H Br N O S : C, 67.78; H, 5.60; N, 2.36.
6
7
66
2
2
4
2
21
.52–0.58 (m). IR (KBr, cm ): 2931, 2850, 2199, 1596,
Found: C, 67.89; H, 4.71; N, 2.22.
561, 1468, 1375, 1329, 1271, 1085, 1015, 945. UV–Vis
DCM) kmax, nm: 425. Monomer ratio in the polymer by H
1
(
General Procedure for the Synthesis of Polymers P1–P6
The polymers (P1–P6) were prepared using standard Sono-
gashira coupling reaction. The general procedure is as fol-
lows: In a two-necked round bottom flask, the monomers
NMR: 1.8:1.0 (M2:14).
RESULTS AND DISCUSSION
(
(
M1 or M2 as the case may be) (0.67 mmol, 1 eq.), Pd(II)
5 mol %), and CuI (5 mol %) were dissolved in a 1:1 mix-
Synthesis and Characterization
Toward the synthesis of the CPs, dialdehydes 18, 19 were
synthesized as shown in Scheme 1. The dialdehydes in turn
were synthesized from the dialkynedialkylfluorenes 5, 6 that
in turn were obtained via Sonogashira reaction from 2,7-
dibromodialkylfluorene 3. The dibromodialkylfluorene 3 was
synthesized by the alkylation of dibromofluorene 2, which in
turn was synthesized from fluorene 1. The alkyl groups were
attached to increase the solubility of the polymers in com-
mon organic solvents. The bromoaldehyde 17 was obtained
by the bromination of 16, which in turn, was obtained by
the formylation of EDOT 15. Then, bromoaldehyde 17 was
reacted with dialkynedialkylfluorenes 5, 6 to give the dialde-
hydes 18 and 19 via Sonogashira coupling reactions, which
were further converted to the dibromomonomers M1, M2
via Knoevenagel condensation reaction between 4-
bromophenylacetonitrile 20 and the dialdehydes 18, 19. The
dialdehydes were obtained in 88% 18 and 77% 19 yields,
whereas the yields for monomers M1 and M2 were 83%
and 34%, respectively. All these compounds were thoroughly
characterized by H NMR, C NMR, MALDI, and CHN analy-
sis as detailed in the Supporting Information. The structures
of the starting compounds and the dialdehydes 18, 19 were
characterized by spectroscopic tools and found to be consist-
ent with that expected for the given structures (please see
characterization data shown in Supporting Information Figs.
S1–S12 along with the peak assignments). Monomers M1
and M2 were also synthesized as shown in Scheme 1. The
structure of the monomers M1, M2 were characterized by
spectroscopic tools and they were found to be consistent
with that expected for the given structures (please see char-
acterization data shown in Supporting Information Figs.
S13–S24). The compounds 12, 13, 14 were synthesized
from 4, 4’-dibromobiphenyl 7 through standard protocols as
shown in Scheme 1.
ture of solvents (DIPA:Toulene) (total 60 mL) and degassed
for 15 min using argon. Then, a degassed solution of the
dibromocarbazoles (12 or 13 or 14) (0.67 mmol, 1 eq.) in
THF was added to the reaction mixture at room temperature
(
final solvent ratio 5 1:1). Then, the temperature was raised
to 70 8C and maintained for 24 h. After the reaction, the sol-
vents were evaporated under vacuum and the residue was
extracted with DCM and water, washed with brine solution,
dried with Na SO and filtered. The solvent was removed
2
4
under vacuum and the resulting crude product was purified
by soxhlet extraction using hexane, methanol and chloroform
to give the polymers (P1–P6).
1
P1: Yield: 69%, red solid. H NMR (400 MHz, CDCl , d,
3
ppm): 7.65–7.66 (d), 7.52–7.60 (m), 7.34–7.46 (m), 7.25–
7
.28 (m), 4.23–4.32 (m), 1.82–1.92 (m), 1.56 (s), 1.34–1.38
2
1
(m), 0.89–1.26 (m), 0.72–0.76 (m). IR (KBr, cm ): 2931,
2861, 2222, 1596, 1491, 1364, 1259, 1085, 1015, 945, 818.
UV–Vis (DCM) kmax, nm: 447. Monomer ratio in the polymer
1
13
1
by H NMR: 1.5:1.0 (M1:12).
P2: Yield: 49%, red solid. H NMR (400 MHz, CDCl , d,
1
3
ppm): 7.66 (s), 7.54–7.60 (m), 7.41–7.47 (m), 7.36 (s), 7.26–
7
.27 (d), 4.26–4.32 (m), 1.85–1.92 (m), 1.57 (s), 0.95–1.20
2
1
(m), 0.71–0.75 (m), 0.46–0.51 (m). IR (KBr, cm ): 2943,
2
850, 2199, 1596, 1468, 1375, 1096, 1003, 945, 818. UV–Vis
1
(DCM) kmax, nm: 447. Monomer ratio in the polymer by H
NMR: 1.5:1.0 (M1:13).
P3: Yield: 62%, red solid. H NMR (400 MHz, CDCl , d,
1
3
ppm): 7.65–7.66 (d), 7.55–7.61 (m), 7.36–7.48 (m), 7.26–
7
.28 (m), 4.27–4.33 (m), 1.86–1.92 (m), 1.50 (s), 0.96–1.20
21
(m) 0.73–0.76 (m), 0.46–0.51 (m). IR (KBr, cm ): 2931,
2
850, 2211, 1608, 1457, 1375, 1271, 1096, 1027, 969,
21
829 cm . UV–Vis (DCM) kmax, nm: 445. Monomer ratio in
1
the polymer by H NMR: 2.0:1.0 (M1:14).
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SCHEME 1 Synthetic route for monomers M1 and M2.
Polymers P1–P6 were synthesized by the Sonogashira cou-
pling as shown in Scheme 2. The low yield of the polymers
the solvent as the polymerization proceeded. Thus, a yield of
69% for polymer P1 has been obtained under these condi-
tions, when the polymerization was carried out for 5 days.
The H NMR and C NMR of the polymers are shown in
Supporting Information Figures S25–S30. The peak
(
47–62%) in all the cases suggested that the coupling reac-
1
13
tion was not efficient from the polymerization point of view.
The yield of the polymers could be improved by driving out
4
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SCHEME 2 Synthetic route for polymers P1–P6.
assignments are also shown in the corresponding figures.
The disappearance of the acetylenic proton (peak at 3.2 ppm
in H NMR) of the monomers suggests the formation of
respectively, which could be attributed to p-p* transition
(Supporting Information Fig. S32). A red shift of around
100 nm was observed in the kmax of the monomers, which
resulted due to the extension of the conjugation through the
attachment of EDOT and carbazole moieties to fluorene (kma
value for pure fluorene was observed at 320 nm, while that
1
polymers.
The number average molecular weight (M ) and polydisper-
n
3
1
sity index (PDI) values of the polymers as ascertained by gel
permeation chromatography (GPC) using narrow molecular
weight polystyrene standards are given in Table 1. The ratio
of the monomer composition in the polymers, as ascertained
for EDOT is 260 nm).
monomers 18 and 19, the monomers showed a red shift of
3 nm. This implies that the extent of conjugation is
When compared to the pre-
5
increased by attaching 4-bromophenylacetonitrile, as
expected. In polystyrene film, the monomers M1 and M2
showed absorption maxima at 445 nm and 439 nm, respec-
tively (Supporting Information Fig. S32); they were not sig-
nificantly different from what was observed in solution. The
emission maxima of the monomers in DCM solution (for
excitation wavelength of 444 nm and 445 nm, respectively)
were 510 nm and 511 nm for M1 and M2, respectively.
Apart from the prominent peaks, shoulder peaks were
observed at 539 nm and 581 nm. In polystyrene matrix, the
emission maxima of the monomers were 504 nm and
1
by H NMR is also given in Table 1. The method used to cal-
culate the monomer ratios is discussed in Supporting Infor-
1
mation Fig. S31. For the purpose of clarity, H NMR of
polymer P6 along with specific peak assignments is also
included in Supporting Information Fig. S31.
Photophysical Studies
The absorption spectra of the monomers (M1 and M2 in
DCM) showed absorption maxima at 444 nm and 445 nm,
n
TABLE 1 Number Average Molecular Weight (M ),
513 nm, respectively (Supporting Information Fig. S32).
Polydispersity Index (PDI) of the Polymers and Molar
Ratios of the Comonomers in the Polymers
Apart from this, prominent shoulder peaks at 524 nm, 574
nm for M1 and 520 nm and 580 nm for M2 were also
observed. However, in the pristine powder form, the emis-
sion maxima of the monomers were 627 nm and 604 nm for
M1 and M2, respectively, (Supporting Information Fig. S33).
This may be attributed to their aggregation which restricts
the rotation of the polymer backbone and improves the
effective conjugation leading to the red shifted absorption
and emission maxima. The absorption and emission spectra
of all the polymers in DCM solution are shown in Figures 1
1
Cmp
M
n
(Daltons)
PDI
Composition ( H NMR)
P1
P2
P3
P4
P5
P6
6000
5900
9800
5700
6800
6000
NA
1.5(M1):1.0(12)
1.5(M1):1.0(13)
2.0(M1):1.0(14)
1.0(M2):1.0(12)
2.0(M2):1.0(13)
1.8(M2):1.0(14)
1.39
1.65
2.34
3.25
1.75
(
P1–P3) and 2 (P4–P6). The polymers, P1–P6, exhibited
broad absorption band with maxima between 447 nm to
53 nm. The absorption maxima of the polymers remained
Note: For P2 and P3 the concentration of the higher molecular weight
polymer fragment was lower than the concentrations of the oligomers
as inferred from GPC chromatograms.
4
the same as the monomers, although broadening of peaks
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The emission maxima for the polymers P1, P2, and P3 were
observed between 492 and 510 nm, while those for the poly-
mers P4, P5, and P6 were observed between 506 and
578 nm, respectively, in DCM solution (Figs. 1 and 2). These
values correspond to green emission as can be seen in the
photographs [Fig. 3(a)]. The polymers also exhibited signifi-
cant shoulder peaks similar to those of the monomers, which
might be attributed to the aggregation. Since minimal over-
lap is observed between the absorption and emission spectra
of both the monomers and polymers, insignificant Forster
energy transfer is expected under some perturbation (pH
change, doping, etc.), which is desirable for PLED applica-
3
2
tions. The fluorescence quantum yields in the solution
state (Fluorescein standard) for polymers P2 and P5 were
found to be 41% and 43%, respectively.
FIGURE 1 UV–Visible absorption and emission (kexc5kmax
)
3
3
spectra of P1–P3 in DCM. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
In PS matrix, the emission maxima for polymers P1, P2, and
P3 in PS matrix were centred on 529 nm with a prominent
shoulder peak around 580 nm (Supporting Information Fig.
S32). Those for P4, P5, and P6 were observed between 502
and 533 nm with prominent shoulder peak at around
was observed. Moreover, P1–P6 showed multiple absorp-
tions (five peaks) below kmax in DCM. This was more promi-
nent in P4–P6. These broadening can be due to the
absorption from different segments of the polymers, which is
a general phenomenon in case of CPs. The optical band gap
580 nm (Supporting Information Fig. S32). Photographs of
the emission in the thin film form are shown in Figure 3(b)
for P1–P6 in PS matrix. The emission maximum is signifi-
cantly enhanced and red shifted from 5 to 37 nm in film
state, owing to the efficient p-p stacking and increase in
effective conjugation in the film state due to freezing of the
segmental motion in PS matrix (with a T of 95 to 1008C).
g
This is another evidence for the chromophores exhibiting
excellent AEE.
(Eg) of the polymers (P1–P6) was determined from the
onset of absorbance using the formula Egap 5 1242/konset (1)
and found to be ranging between 2.35 and 2.42 eV, which
was essentially independent of the nature of the side chain.
The polymers P4–P6 showed lower energy band gap (0.08
eV) than those of P1–P3. This indicates that the variation of
R-group may have detrimental effect on the band gap
energy.
The photographs of P1–P6 under UV light irradiation in the
solid state are shown in Figure 3(c). The emission maxima
in powder form for P1, P2, P3 polymers were 599 nm, 617
nm, and 623 nm (Supporting Information Fig. S33), while
those for P4, P5, and P6, were 603 nm, 594 nm, and
In polystyrene film, the polymers P1–P3 showed absorption
maxima at 420 nm, 419 nm, 439 nm, while polymers P4–P6
showed absorption maxima at 447 nm, 419 nm, 421 nm,
respectively (Supporting Information Fig. S32). Thus, there is
not much change as we compare the absorption maxima of
molecules in DCM solution to those in the polystyrene matrix
although the kmax decreases slightly in polymer matrix.
594 nm (Supporting Information Fig. S33), respectively. In
addition to the main emission peak, the three polymers
exhibited shoulder peaks. There was remarkable red shift of
117 nm, 93 nm, 89 nm, 125 nm, 129 nm, 97 nm, 16 nm,
and 68 nm for M1, M2, P1, P2, P3, P4, P5, and P6,
FIGURE 3 Photos of P1–P6 taken in (a) solution state (b) PS
matrix and (c) solid state under UV lamp. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
FIGURE 2 UV–Visible absorption and emission (kexc5kmax
)
spectra of P4–P6 in DCM. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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TABLE 2 Absorption and Emission Spectral Data for M1, M2,
and P1–P6
a
b
k
max (abs.)
kmax (em.)
Cmp
DCM
PS Matrix
DCM
PS Matrix
Solid
M1
M2
P1
P2
P3
P4
P5
P6
444
445
447
447
445
453
451
425
445
439
420
419
439
447
419
421
510
511
510
492
494
506
578
526
504
513
529
529
529
502
533
531
627
604
599
617
623
603
594
594
FIGURE 5 Variation in the integrated PL intensity of P1 with
increasing water fractions in THF. Inset depicts snapshots of P1
taken under UV light in the THF–water mixture with different
water volume fractions 0–90%. Left to right, increase in water
fraction. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
a
Absorption maximum at the longest wavelength.
Emission maximum on photoexcitation.
b
respectively, in the kmax in the solid state compared to their
solution maxima. The bathochromic shift in the powder form
was even more remarkable. This substantial red shift
observed in the powder form when compared to solution
and polystyrene matrix shows that in polystyrene matrix the
stacking and intermolecular interaction are not as pro-
nounced as in the powder form, where the molecules are at
close proximity to each other. The photophysical data is sum-
marized in Table 2.
fraction was 90%. This indicates that the polymer P1 shows
aggregation enhanced emission (AEE) on addition of a non-
solvent, such as water, which increases the polarity of the
solution. The emission intensity increased 2.6 fold (Fig. 5)
when water fraction was 90% as compared to that in pure
THF solution. The emission color shifted from green to
orange. The red shifted emission may be attributed to the
enhanced planarization of the polymer backbone, which
increases the effective p-conjugation.
AEE Studies
The aggregation enhanced emission (AEE) may be caused
To explore the AEE properties of these polymers, we carried
out the aggregation studies of the polymers. The photolumi-
nescence (PL) spectra of polymer P1 in THF/water mixtures
with 0 to 90% water fractions is shown in Figure 4. It can
be seen that the emission remains the same till 50% water
fraction, as in THF, which means the solute resists the forma-
tion of aggregate. But when the water fraction was increased
beyond 50%, (from 60% to 90%), the intensity of fluores-
cence increased significantly along with considerable red
shift and reached the maximum intensity when the water
3
4,35
due to restriction of intramolecular rotation (RIR).
Thus,
the RIR is effectively restricting the nonradiative decay chan-
nels leading to enhanced emission in THF/water mixture.
From the emission spectra and the AEE studies, it can be
concluded that the polymer P1 shows enhanced fluorescence
in aggregated state and its emission lies in the green to yel-
low region of the electromagnetic spectrum. The enhance-
ment in fluorescence with increase in water fraction is
shown in inset of Figure 5.
The aggregation of the polymer particles in water-THF mix-
ture was further confirmed by DLS (dynamic light scattering)
(
Supporting Information Fig. S35) and SEM (scanning elec-
tron microscopy) (Fig. 6) analyses. The average radius of the
aggregates decreased progressively from 751 nm, 292 nm,
292 nm, 327 nm, 226 nm to 157 nm as the vol % of water
FIGURE 4 Changes in the emission spectrum of P1 in THF/
water mixture with different water volume fractions 0–90%.
[
Color figure can be viewed in the online issue, which is avail-
FIGURE 6 SEM images of P1 in THF–water mixture (a) 50:50 v/
v and (b) 20:80 v/v.
able at wileyonlinelibrary.com.]
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was increased from 40%, 50%, 60%, 70%, 80% to 90%,
respectively. The size of the aggregates decreases with
increasing water fraction because of the rapid reduction in
the solvating power of the solvent mixture that is supported
by SEM images [Figs. 6(a) for 50% water and 6(b) for 80%
2
9,36(a,b)
water]. This observation has been well reported.
The AEE studies for other polymers P2–P6 were also carried
out and found to show similar patterns. The increase in emis-
sion intensities are 2.3, 2.7, 1.7, 1.5, and 1.3 folds for polymers
P2, P3, P4, P5, and P6, respectively, when water fraction is
90% as compared to that in pure THF solution. The details
can be found in Supporting Information Figure S34.
Electrochemical Properties
The electrochemical behavior of the monomers and polymers
was investigated both in DCM solution (Supporting Informa-
tion Fig. S36) and in film form (Fig. 7 for polymers and Sup-
porting Information Fig. S37 for monomers) from the cyclic
voltammograms (CV). The films were formed by drop casting
monomer and polymer films from DCM solution on the
4 6
working electrode containing 0.1 M n-Bu NPF as a support-
ing electrolyte. The HOMO-LUMO energy gaps and the
energy levels of the compounds were determined by using
optical and electrochemical methods, which are summarized
in Table 3 for film form and in Supporting Information Table
S1 in DCM solution. The CV of monomers and polymers
envisaged an irreversible oxidation process with Eonset oxida-
tion in the range 0.55–1.11 V in the film form and 0.86–1.38
V in DCM solution. The onset of oxidation peak can be attrib-
uted to the formation of mono radical cation of the mono-
mers and the polymers. The lower oxidation potential can be
due the presence of carbazole, which is known to form radi-
cal cation easily (because of the presence of nitrogen atom).
These low oxidation values also support that these polymers
may serve as good hole transporting materials in
TABLE 3 The Optical Band Gap and Electrochemical Data of
Monomers M1, M2, and Polymers P1–P6
a
gap
b
c
d
Cmp
E
HOMO
LUMO
E
gap
M1
M2
P1
P2
P3
P4
P5
P6
2.44
2.44
2.42
2.43
2.43
2.35
2.35
2.35
25.56
25.50
25.43
25.01
24.94
25.19
25.15
24.99
23.08
23.14
23.16
23.08
23.06
23.14
23.09
23.09
2.51
2.36
2.27
1.93
1.88
2.05
2.06
2.05
a
Energy band gap calculated from konset in eV;.
b
HOMO
energy
level
calculated
using
the
the
equation
equation
OX
E
HOMO 5 2(E 1 4.39) eV (2);.
c
LUMO
energy
level
calculated
using
red
E
LUMO 5 2(E 1 4.39) eV (3);.
FIGURE 7 Cyclic voltammograms of P1–P6 in film form
d
Energy band gap calculated from CV in eV by drop casting films of
(
cathodic region) by drop casting polymers from DCM solution
monomers and polymers films from DCM solution on the working elec-
trode, the HOMO and LUMO were also calculated for the same.
on working electrode (0.1 M Bu NPF as supporting electrolyte.
4
6
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FIGURE 8 B3LYP/6-31G* DFT calculated HOMO (bottom) and LUMO (top) contours of P2 (left) and P5 (right). [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
photovoltaics. The onset potentials for polymers were 0.41
eV less than those of monomers in the film form, which sig-
nify the importance of incorporating carbazole in the poly-
meric backbone. The irreversible reduction process with
design point of view. Moreover, the HOMO energy levels of
the polymers were approximately 0.41 eV in film form and
0.25 eV in DCM solution, lower than those of monomers.
Similarly, LUMO energy levels of the polymers were also
found to be lower than those of monomers. Thus, polymer-
ization has effectively increased the conjugation which is
altering the energy levels.
Eonset reduction in the range 1.23–1.33 V in film form and
1.11–1.34 V in DCM solution can be due to the formation of
radical anions. The higher values for reduction onsets as
compared to oxidation onsets reveal that it is easier to oxi-
dize these molecules and hence they can function better as
hole transporters than electron transporters. The redox
Computational Studies
Gas phase density functional theory (DFT) calculations
1
potential of Fc/Fc (standard value 5 4.8 eV with respect to
(
B3LYP/6-31G* level) were carried out to get insight into the
vacuum) was observed at 0.41 V in 0.10 M tetrabutyl ammo-
nium hexaflurophosphate/DCM solution. Based on this the
HOMO and LUMO energy levels of the monomers and poly-
electronic distribution in the polymers. The DFT generated
HOMO and LUMO levels were also calculated and compared
with the experimentally determined values. The HOMO, LUMO
patterns of polymers P1 and P5 are shown in Figure 8.
3
7
mers were estimated using the following equation:
À
Á
EHOMO5 2 EOX1 4:39 eV
(2)
(3)
In all the polymers, the HOMO is mainly located on the fluo-
rene and carbazole rings and the LUMO is primarily located
on the oxygen atoms of EDOT. In both P2 and P5 the HOMO
energy level is 5.52 eV while LUMO energy level is 3.31 eV
and the energy band gap is 2.1 eV. These values are compa-
rable (with slight difference) to the experimentally deter-
mined values.
À
Á
ELUMO5 2 Ered1 4:39 eV
OX
red
where E and E are the onset potentials of oxidation and
reduction, respectively, as measured against Ag/AgCl refer-
ence. The HOMO energy levels of the monomers and poly-
mers were found to be in the range 24.94 eV to 5.56 eV in
film form and 25.25 to 25.77 eV in DCM solution and the
LUMO energy levels ranged from 23.06 eV to 23.16 eV in
film form and 23.05 to 3.28 eV in DCM solution. Thus, the
energy band gaps calculated in film form are less than those
in DCM solution. Moreover, in film form the difference in the
band gaps of monomers was 0.39 eV more than the band
gaps in the polymers. This indicates the increase in conjuga-
tion on polymerization. The electrochemical band gaps of the
monomers and polymers, calculated from the onset poten-
tials of the oxidation and reduction, were lower than the val-
ues obtained from the optical band gap. This difference can
be due to the interface between the electrodes and the mole-
cules in the electrochemical system, which is absent in the
optical system. Since the PLED configuration is very similar
to the electrochemical system, the data calculated from elec-
trochemical method are more acceptable from the device
Thermal Properties
The thermal properties of the target compounds were deter-
mined by thermogravimetric analysis (TGA) (Supporting
Information Figs. S38, S39; Tables S2 and S3). The mono-
mers as well as all the polymers showed very high thermal
stability with a decomposition temperature of 390 and
397 8C, respectively, at 5% weight loss for monomers M1
and M2 while the decomposition temperatures observed for
polymers P1, P2, P3, P4, P5, and P6 were 365, 391, 402,
389, 410, and 405 8C, at 5% weight loss, respectively. The
thermal decomposition peak for pure PF is 448 8C. The high
thermal stability observed is due to the fused aromatic sys-
tem. Such high thermal robustness is essential when PLEDs
are fabricated, and for the device lifespan.
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PXRD of Monomers and Polymers
2 A. Kraft, A. C. Grimsdale, A. B. Holmes, Angew. Chem. Int.
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The powder XRD (PXRD) pattern of the monomers M1 and
M2 (Supporting Information Fig. S40) and the polymers P1
to P6 (Supporting Information Fig. S41) showed that the
polymers were highly amorphous in the solid state suggest-
ing that dissolution as well as film forming tendency (espe-
cially in combination with high molecular weight) could be
expected. These amorphous polymers can be advantageously
used in the fabrication of light emitting diodes as crystalline
materials are not so preferred for the same. In contrast, the
PXRD pattern of the monomers (M1 and M2) showed that
both the monomers are crystalline in nature as can be seen
by the sharp peaks with narrow peak width.
3
4
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CONCLUSIONS
1
0 C. Winder, N. S. Saricftci, J. Mater. Chem. 2004, 14, 1077–
1
086.
We have synthesized six new conjugated polymers with car-
bazole, fluorene, EDOT, acetylene, phenylene, and ethylene
moieties along the main chain and a cyano pendant group
attached to the ethylene moiety. Palladium catalysed Sonoga-
shira coupling reaction was used as the tool and the poly-
mers were characterized thoroughly using various
spectroscopic analyses. The polymers P1–P6 in solution and
in PS matrix show emission in green region while it is red in
the solid state. Aggregation enhanced emission (AEE) prop-
erties were also observed as expected from the synthetic
design considerations. Furthermore, the AEE studies showed
that a 2.6 fold increase in fluorescence intensity was
observed in 90:10 THF/water mixture as compared to pris-
tine THF solution of polymer P1. The AEE property was fur-
ther verified by DLS and SEM studies. The band gaps of the
polymers were assessed by optical, electrochemical, and
computational studies (gas phase density functional theory
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This work was supported by Indian Institute of Technology,
Madras (IITM), India. We thank the Department of Chemistry
and especially Prof. Prasad for the support in doing DLS studies.
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