Carbazole-containing conjugated copolymers as colorimetric/fluorimetric sensor for iodide anion

Article abstract of DOI:10.1021/ma0613537

A series of conjugated poly(p-phenylene carbazole) copolymers, poly(N-hexyl-3,6-carbazole-alt-2,5-bisdodecyloxyphenylene) [P1], poly(N-ethylhexyl-3,6-carbazole-alt-2,5-bisdodecyloxyphenylene) [P2], and poly(N-dodecyl-3,6-carbazole-alt-2,5-bisdodecyloxyphenylene) [P3] were prepared by using palladium-catalyzed Suzuki coupling reactions. All copolymers were characterized by means of FT-IR, GPC, ¹H and ¹³C NMR, UV-vis, fluorescence spectroscopy, and X-ray powder diffraction patterns. All copolymers were soluble in common organic solvents, exhibited good thermal stability up to 390 °C, and showed UV-vis absorbance and fluorescence maxima around 328 and 394 nm, respectively. The polymers P1-P3 differed in N-alkyl substitution, i.e., n-hexyl (for P1), n-ethylhexyl (for P2), and n-dodecyl (for P3) groups. In addition, a third monomer, 2,5-substituted pyridine for poly(N-ethylhexyl-3,6-carbazole-co-2,5-didodecyloxy phenylene-co-2,5-pyridine) (P4) and dialkylated fluorene for poly(N-ethylhexyl-3,6-carbazole-co-2,5- didodecyloxy phenylene-co-9,9′-dihexylfluorene) (P5) was incorporated into the copolymer P2 to investigate the influence on their optical properties. The UV-vis and fluorescence maxima of P4 and P5 showed significant red shift in λ_max by 29 and 19 nm and Δλ_em of 38 and 21 nm, respectively as compared to the polymers P1-P3. The quantum efficiencies of these polymers (52-78%) were relatively higher and may have good applications in the fabrication of LEDs. The UV-vis and fluorescence spectra of the polymers were significantly affected by the addition of tetrabutylammonium iodide and other metal iodides. A colorless polymer solution turns to deep yellow color on the addition of iodide salts. This change is, however, not observed in the presence of other anions such as fluoride, chloride and bromide salts. Therefore, the polymers P1-P5 may be useful in fabricating selective iodide sensors.

Full text of DOI:10.1021/ma0613537

Macromolecules 2006, 39, 8303-8310
8303
Carbazole-Containing Conjugated Copolymers as Colorimetric/
Fluorimetric Sensor for Iodide Anion
Muthalagu Vetrichelvan,† Rajagopal Nagarajan,† and Suresh Valiyaveettil*,†,‡
Singapore-MIT Alliance, E4-04-10, 4 Engineering DriVe 3, Singapore-117576, and Department of
Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore-117543
ReceiVed June 17, 2006; ReVised Manuscript ReceiVed September 8, 2006
ABSTRACT: A series of conjugated poly(p-phenylene carbazole) copolymers, poly(N-hexyl-3,6-carbazole-alt-
2,5-bisdodecyloxyphenylene) [P1], poly(N-ethylhexyl-3,6-carbazole-alt-2,5-bisdodecyloxyphenylene) [P2], and
poly(N-dodecyl-3,6-carbazole-alt-2,5-bisdodecyloxyphenylene) [P3] were prepared by using palladium-catalyzed
Suzuki coupling reactions. All copolymers were characterized by means of FT-IR, GPC, 1H and 13C NMR, UV-
vis, fluorescence spectroscopy, and X-ray powder diffraction patterns. All copolymers were soluble in common
organic solvents, exhibited good thermal stability up to 390 °C, and showed UV-vis absorbance and fluorescence
maxima around 328 and 394 nm, respectively. The polymers P1-P3 differed in N-alkyl substitution, i.e., n-hexyl
(for P1), n-ethylhexyl (for P2), and n-dodecyl (for P3) groups. In addition, a third monomer, 2,5-substituted
pyridine for poly(N-ethylhexyl-3,6-carbazole-co-2,5-didodecyloxy phenylene-co-2,5-pyridine) (P4) and dialkylated
fluorene for poly(N-ethylhexyl-3,6-carbazole-co-2,5-didodecyloxy phenylene-co-9,9′-dihexylfluorene) (P5) was
incorporated into the copolymer P2 to investigate the influence on their optical properties. The UV-vis and
fluorescence maxima of P4 and P5 showed significant red shift in λmax by 29 and 19 nm and ∆λem of 38 and 21
nm, respectively as compared to the polymers P1-P3. The quantum efficiencies of these polymers (52-78%)
were relatively higher and may have good applications in the fabrication of LEDs. The UV-vis and fluorescence
spectra of the polymers were significantly affected by the addition of tetrabutylammonium iodide and other metal
iodides. A colorless polymer solution turns to deep yellow color on the addition of iodide salts. This change is,
however, not observed in the presence of other anions such as fluoride, chloride and bromide salts. Therefore,
the polymers P1-P5 may be useful in fabricating selective iodide sensors.
Introduction
The synthesis of alternating copolymers containing electron-
rich n-alkyl-3,6-substituted carbazoles and dialkoxybenzene
systems are described with full details. In addition, a third
monomer, using pyridine and alkylated fluorene moieties, was
also introduced on the copolymer backbone in order to enhance
electron affinity and reduce the band gap of these copolymers.
The syntheses, characterization and optical properties of poly-
(N-hexyl-3,6-carbazole-alt-2,5-bisdodecyloxy phenylene) [P1],
poly(N-ethylhexyl-3,6-carbazole-alt-2,5-bisdodecyloxy phen-
ylene) [P2], poly(N-dodecyl-3,6-carbazole-alt-2,5-bisdodecyloxy
phenylene) [P3], poly(N-ethylhexyl-3,6-carbazole-co-2,5-bis-
dodecyloxy phenylene-co-2,5-pyridine) [P4], and poly(N-eth-
ylhexyl-3,6-carbazole-co-2,5-bisdodecyloxy phenylene-co-9,9′-
dihexylfluorene) [P5] were investigated in detail (Scheme 1).
The N-alkyl groups and dodecyloxy groups incorporated on
polymer backbone to increase the solubility and ordered packing
through alkyl chain crystallization.
Conjugated polymers have received much attention due to
their extended π-electron systems. Their potential applications
includes light emitting diodes,1-7 thin film transistors,1,2 chemi-
cal sensors,8 and electronic and photonic devices.9-11 The
optoelectronic properties of conjugated polymers vary signifi-
cantly based on the degree of extended conjugation between
the consecutive repeating units and the inherent electron
densities on the polymer backbone.12-14 Soft organic materials
possess several advantages such as ease of processing and the
tunability of their properties through chemical modifications.15
Poly(vinylcarbazole) is known to be a good hole-transporting
material.16 Homopolymers of n-alkyl substituted 3,6-car-
bazole17,18a-c and its copolymers with thiophene,18d bithiophene,18d
benzothiadazole,18d,e dialkylated fluorene,18f-g and pyridine18h
have been reported as interesting optical materials. The optical
properties and solvatochromism of homopolymers and copoly-
mers of n-alkyl-substituted 2,7-carbazole were extensively
investigated by the Leclerc group.16,19
The carbazole is an interesting functional group for incor-
poration on to polymer backbone owing to its higher thermal
stability, solubility, extended glassy state and moderately high
oxidation potential.20-24 In addition, it is well-known that the
carbazole containing oligomers and conjugated polymers are
good hole-transport materials.18g,25-27 Our research efforts are
focused on the synthesis and structure-property investigations
of asymmetrically functionalized, planar conjugated polymers.28-33
Experimental Section
Chemicals and Instrumentation: All reagents were purchased
from commercial sources and used without further purification
unless otherwise stated. All reactions were carried out under inert
atmosphere (N2 or Ar) using freshly distilled anhydrous solvents.
Tetrahydrofuran (THF) was purified by distillation over sodium
1
under nitrogen atmosphere. The H and 13C NMR spectra were
collected on a Bruker ACF 300 spectrometer operating at 300 and
75.5 MHz, respectively. Chloroform-d was used as solvent and
tetramethylsilane as internal standard. FT-IR spectra were recorded
on a Bio-Rad FTS 165 spectrophotometer using KBr as matrix. A
UV-vis spectra was recorded using a Shimadzu 3101 PC spec-
trophotometer and fluorescence measurements were made on a RF-
5301PC Shimadzu spectrofluorophotometer. All spectra were
recorded in chloroform. Thermogravimetric analyses were done
using TA Instruments SDT 2960 with a heating rate of 10 °C/min
* To whom correspondence should be addressed at the Department of
Chemistry, National University of Singapore. Telephone: (65) 6516 4327.
Fax: (65) 6779 1691. E-mail: chmsv@nus.edu.sg.
† Singapore-MIT Alliance.
‡ Department of Chemistry, National University of Singapore.
10.1021/ma0613537 CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/04/2006

8304 Vetrichelvan et al.
Macromolecules, Vol. 39, No. 24, 2006
Scheme 2. Synthesis of Polymers P1-P3a
Scheme 1. Molecular Structures of Polymers, P1-P5
under nitrogen atmosphere. Differential scanning calorimetry (DSC)
measurements were done on TA Instruments DSC 2960 at a heating
rate of 20 °C/min. Gel permeation chromatography (GPC) was used
to obtain the molecular weight of the polymers with reference to
polystyrene standards with THF as eluant. X-ray powder patterns
were obtained using a D5005 Siemens X-ray diffractometer with
Cu KR (1.54 Å) radiation (40 kV, 40 mA). Samples were mounted
on a sample holder and scanned between 2θ ) 1.5 and 40° with a
step size of 0.01°.
a Key: (i) Br2/AcOH, 80%; (ii) NaOH, CH3(CH2)11Br, 70 °C, 12 h,
75%; (iii) n-BuLi, THF, -78°C, triisopropyl borate, RT, 10 h, 70%;
(iv) NBS/DMF, 80%; (v) KOH, RBr, THF, reflux, 2h; 85% (vi) K2CO3,
THF, 3.0 mol % Pd(PPh3)4, CTAB, reflux, 4 d.
P2: Glassy black crystalline solid. Yield: 75%. 1H NMR (CDCl3,
δ, ppm): 8.41 (s, 2H, Car-H), 7.79 (d, 2H, Car-H), 7.45 (d, 2H,
Car-H), 7.26 (m, 2H, Ar-H), 4.24 (b, 2H, Car-NCH2-), 3.99
(b, 4H, ArOCH2-), 1.72 (b, 4H, ArOCH2CH2-), 1.36-1.16 (b,
45H, -CH2(CH2)9CH3 and -N(CH2CH(CH3)(CH2)4CH3), 0.91-
0.80 (m, 12H, -CH3). 13C NMR (CDCl3, δ, ppm): 150.6, 140.4,
131.2, 129.3, 127.8, 127.5, 123.0, 121.1, 117.2, 108.2, 69.9, 47.6,
39.5, 31.8, 31.0, 29.3, 28.8, 26.1, 24.4, 23.0, 22.6, 14.0, 10.8. FT-
IR (KBr, cm-1): 3424, 3223, 3053, 2923, 2852, 1601, 1512, 1477,
1381, 1291, 1195, 1152, 1043, 866, 802, 720, 695, 637. Anal. Calcd
for (C50H75NO2)n: C, 83.16; H, 10.47; N, 1.94. Found: C, 83.10;
H, 11.08; N, 1.92.
Synthesis: Synthetic scheme for polymers P1-P3 and P4-P5
are shown in Schemes 2 and 3, respectively. 2,5-Dibromohydro-
quinone (2), 2,5-dibromo-1,4-bis(dodecyloxybenzene) (3), and 1,4-
bis(dodecyloxy phenyl)-2,5-diboronic acid (4) were synthesized
using the standard procedure reported in the literature.28,34
General Procedure for the Synthesis of Polymers P1, P2, and
P3. Compound 4 (3.05 g, 5.7 mmol) and the N-alkylated dibro-
mocarbazole 7 (5.7 mmol) were dissolved in dry THF (70 mL)
under nitrogen atmosphere. After the addition of 2 M solution of
K2CO3 (100 mL), the catalyst tetrakis(triphenylphosphino)pal-
ladium(0) (0.20 g, 3 mol %) and cetyltriethylammonium bromide
(CTAB) (0.9 g, 40 mol %) were added and the mixture stirred
vigorously for 4 days at 70-80 °C, cooled to room temperature
and poured into 1 L of methanol. The black residue separated from
the aqueous solution was extracted with large excess of chloroform,
washed with water, dried with MgSO4, filtered, and concentrated.
The polymer was then redissolved in chloroform and precipitated
from a large excess of methanol. The black glassy polymers
obtained were washed with water followed by methanol (2 × 25
mL) and acetone (3 × 25 mL).
1
P3: Black solid. Yield: 67%. H NMR (CDCl3, δ, ppm): 8.41
(s, 2H, Car-H), 7.78 (d, 2H, Car-H), 7.48 (d, 2H, Car-H), 7.26
(m, 2H, Ar-H), 4.35 (b, 2H, Car-NCH2-), 3.98 (b, 4H,
ArOCH2-), 1.94 (b, 2H, Car-NCH2CH2-), 1.71 (b, 4H,
ArOCH2CH2-), 1.30-1.16 (b, 54H, -CH2(CH2)9CH3 and -N-
(CH2)2(CH2)9CH3), 0.90 (b, 6H, -CH3). 13C NMR (CDCl3, δ,
ppm): 150.6, 139.8, 132.1, 128.4, 128.2, 122.9, 117.1, 110.9, 107.9,
69.9, 31.8, 29.5, 29.2, 27.3, 26.1, 22.6, 14.0. FT-IR (KBr, cm-1):
3426, 3055, 2925, 2851, 1597, 1512, 1477, 1379, 1292, 1154, 1045,
866, 804, 719, 695, 637. Anal. Calcd for (C54H83NO2)n: C, 83.34;
H, 10.75; N, 1.80. Found: C, 81.03; H, 10.30; N, 1.90.
P1: Glassy black crystalline solid. Yield: 60%. 1H NMR (CDCl3,
δ, ppm): 8.42 (s, 2H, Car-H), 7.78 (d, 2H, Car-H), 7.48 (d, 2H,
Car-H), 7.26 (m, 2H, Ar-H), 4.37 (b, 2H, Car-NCH2-), 3.99
(b, 4H, ArOCH2-), 1.96 (b, 2H, CarNCH2CH2-), 1.71 (b, 4H,
ArOCH2CH2-), 1.36-1.16 (b, 42H, -CH2(CH2)9CH3 and -N-
(CH2)2(CH2)3CH3), 0.90 (b, 9H, -CH3). 13C NMR (CDCl3, δ,
ppm): 150.6, 139.8, 132.1, 131.2, 129.3, 128.2, 127.5, 123.0, 121.2,
117.2, 107.9, 69.9, 43.3, 31.8, 31.6, 30.8, 29.5, 29.3, 29.0, 27.0,
26.1, 22.6, 14.0. FT-IR (KBr, cm-1): 3436, 3229, 3053, 2921, 2852,
2031, 1603, 1512, 1479, 1379, 1294, 1200, 1153, 1043, 866, 804,
719, 695, 637. Anal. Calcd for (C48H71NO2)n: C, 83.00; H, 10.31;
N, 2.01. Found: C, 82.63; H, 10.06; N, 2.12.
General Procedure for the Synthesis of Polymers, P4 and
P5. Compound 4 (6 mmol), N-alkylated dibromo carbazole (7b)
(3 mmol) and 2,5-dibromo pyridine (8) or alkylated dibromofluo-
rene 9 (3 mmol) were dissolved in dry THF (70 mL) under nitrogen
atmosphere. The remaining synthetic procedure was same as for
polymers P1-P3 (see above).
1
P4: Pale greenish-black solid. Yield: 70%. H NMR (CDCl3,
δ, ppm): 8.98 (b, 1H, py-H), 8.40 (s, 2H, Car-H), 8.10 (b, 2H,
py-H), 7.76 (d, 2H, Car-H), 7.47 (d, 2H, Car-H), 7.11 (m, 2H,

Macromolecules, Vol. 39, No. 24, 2006
Carbazole Containing Conjugated Copolymers 8305
Scheme 3. Synthesis of Polymers P4 and P5a
a Key: (i) K2CO3, THF, 3.0 Mol % Pd(PPh3)4, CTAB, reflux, 4 d.
Table 1. Molecular Weight, Polydispersity Index, and Thermal
Ar-H), 4.30-3.99 (m, 6H, Car-NCH2- and ArOCH2-), 2.10
(b, 2H, CarNCH2CH2-), 1.78 (b, 4H, ArOCH2CH2-), 1.50-1.16
(b, 79H, -CH2(CH2)9CH3, -NCH2CH(CH3)(CH2)3CH3), 0.87 (b,
9H, -CH3). 13C NMR (CDCl3, δ, ppm): 150.6, 149.4, 140.4, 136.3,
132.2, 131.6, 129.2, 128.4, 128.2, 124.4, 124.2, 123.0, 121.1, 115.6,
115.3, 69.8, 69.4, 31.8, 31.6, 30.9, 29.5, 29.3, 28.8, 26.1, 24.4, 23.0,
14.0, 10.8. FT-IR (KBr, cm-1): 3424, 3055, 2921, 2851, 2361,
2028, 1548, 1510, 1465, 1378, 1293, 1200, 1152, 1028, 869, 866,
804, 720, 694, 635. Anal. Calcd for (C85H130N2O4)n: C, 82.07; H,
10.53; N, 2.25. Found: C, 81.84; H, 10.41; N, 2.26.
P5: Black rubbery solid. Yield: 65%. 1H NMR (CDCl3, δ,
ppm): 8.42 (s, 2H, Car), 7.79-7.48 (m, 4H, Car-H), 7.23 (m,
6H, Flu-H), 7.16 (m, 4H, Ar-H), 4.26 (b, 2H, Car-NCH2-),
4.00 (b, 8H, ArOCH2-), 1.73 (b, 8H, ArOCH2CH2-), 1.38-1.18
(b, 99H, -CH2(CH2)9CH3, -N(CH2CH(CH3)(CH2)4CH3), Flu-
(CH2)5CH3), 0.91-0.80 (m, 18H, -CH3). 13C NMR (CDCl3, δ,
ppm): 150.5, 140.4, 139.9, 137.1, 132.1, 131.9, 131.8, 131.2, 129.3,
128.4, 128.3, 127.9, 127.5, 124.4, 123.0, 121.1, 119.1, 117.0, 116.7,
115.3, 108.2, 69.9, 55.0, 40.6, 39.5, 31.8, 31.6, 31.0, 30.1, 29.9,
29.5, 29.2, 28.8, 26.1, 24.4, 24.0, 23, 22.6, 14.0, 13.9, 10.9. FT-IR
(KBr, cm-1): 3464, 3054, 2924, 2853, 1865, 1606, 1511, 1465,
1378, 1294, 1200, 1120, 1045, 867, 805, 721, 695. Anal. Calcd
for (C106H163NO2)n: C, 84.01; H, 10.84; N, 0.92. Found: C, 84.02;
H, 10.59; N, 1.00.
Properties of the Polymers, P1-P5
polymer
color
Mn
Mw
PDI
onset, Td
Tg
P1
P2
P3
P4
P5
black
black
black
pale black
grey black
9850
23117
18581
11816
21965
14688
32412
24437
14879
30943
1.4
1.4
1.3
1.2
1.4
376, 486
372, 504
382
390
370
112
116
114
96
98
alkanes in the presence of KOH in THF to afford the N-alkylated
dibromocarbazoles (7a-7c).17 All polymerizations were done
using Suzuki polycondensation reactions using a mixture (2:3
v/v) of THF and aqueous potassium carbonate solution (2 M),
containing 3.0 mol % Pd(PPh3)4 and stirred for 4 days at 70-
80 °C under nitrogen atmosphere. About 40 mol % of CTAB
was added to the reaction mixture as a phase transfer catalyst.
For the polymers P1-P3, compound 4 and N-alkylated dibro-
mocarbazoles (7a-7c) were used for the polymerization in the
ratio of 1:1 while for P4-P5, the boronic acid 4, N-alkylated
dibromo carbazole (7b), and either 2,5-dibromopyridine (8) or
alkylated dibromofluorene (9)36 were used in a ratio of 1:0.5:
0.5. After completion of the reaction, the polymers were
precipitated from methanol. The resultant solid was suspended
in acetone to remove impurities including oligomers and catalyst.
Purified polymers were then filtered and reprecipitated from
methanol.
Results and Discussion
Synthesis and Characterization: The general synthetic route
for the monomers and polymers P1-P3 and P4-P5 are outlined
in Schemes 2 and 3, respectively. The monomer 1,4-bis
(dodecyloxyphenyl)-2,5-diboronic acid (4) was synthesized from
hydroquinone using a reported procedure.34 2,5-dibromohy-
droquinone (2) was obtained via bromination of hydroquinone
with bromine in acetic acid.35 Alkylation of 2 with dodecyl
bromide in the presence of NaOH/EtOH gave compound 3,
which was reacted with n-butyllithium followed by quenching
with triisopropyl borate and hydrolyzed with hydrochloric acid
to afford bis(boronic acid) 4. N-alkylated dibromocarbazoles
(7a-7c) were synthesized from carbazole 5. The carbazole was
brominated with N-bromo succinimide (NBS) in DMF at room
temperature to give the 3,6-dibromo carbazole (6).27 The
obtained dibromo compound was then alkylated with bromo-
The molecular weights of the polymers were measured using
gel permeation chromatography (GPC) with THF as eluant and
polystyrene as the standard (Table 1). The obtained molecular
weights were in the range of 15-33 kDa. Polymers, P1-P5
were soluble in common organic solvents, such as THF,
dichloromethane, chloroform, toluene and hexane and insoluble
in polar solvents such as acetone and methanol. Full charac-
terizations of the polymers (P1-P5) were achieved using FTIR,
1H NMR, 13C NMR, GPC, elemental analysis, TGA, and X-ray
diffraction.
The assignments of the 1H and 13C NMR peaks are given in
the Experimental Section. For polymer P1, the peaks at 8.42
(singlet), 7.78 (doublet) and 7.48 ppm (doublet) can be assigned
to the carbazole protons and the one around 7.26 ppm could be

8306 Vetrichelvan et al.
Macromolecules, Vol. 39, No. 24, 2006
Table 2. Absorption and Emission Wavelength for the Polymers
P1-P5 in THF
solution (THF)
λmax λem
polymer (nm) (nm) (nm)
solid (film)
λmax
λem
(nm)
band
ꢀmax
gapa QE(φ)b
P1
P2
P3
P4
P5
329
329
328
357
347
393
394
395
432
415
338 399, 424
338 397, 421
3.55 × 104 3.28 0.58
337 395, 418(s) 3.32 × 104 3.26 0.61
382 440, 464(s) 4.98 × 104 2.96 0.78
359 421, 437(s) 6.26 × 104 3.09 0.72
3.55 × 104 3.27 0.62
a Calculated from the onset wavelength. b Measured using quinine sulfate
as standards in THF.
Figure 1. (A) TGA and (B) DSC traces of the polymers, P1-P5.
due to the aromatic protons. The peak at 4.37 ppm is due to the
-CH2- protons of the N-hexyl group. The -OCH2- groups
of the dialkyl benzene units appeared at 3.99 ppm, -NCH2CH2
protons of the carbazole unit appeared at 1.96 ppm and the
remaining peaks centered at 1.71, 1.36, and 1.15 ppm are
assigned to the hydrogen atoms on the aliphatic dodecyloxy
groups and the remaining -CH2 protons of the N-hexyl group.
The peaks at 0.9-0.8 ppm could be due to the -CH3 groups
of the alkyl chains. Data for polymers P2-P5 were also in good
agreement with their molecular structures (Scheme 1). From
the 1H NMR spectrum of P4, the integration of aromatic protons
of pyridine, carbazole and benzene are 3, 6, and 4, respectively.
It confirmed that the distribution of pyridine and carbazole is
about 25% each in the polymer backbone and the remaining
50% from the dialkoxy phenylene units. A similar result was
obtained for the polymer P5. In addition to NMR spectra, the
elemental analysis results of polymers P1-P5 supported the
proposed structures (please see the Supporting Information for
the 1H NMR spectra with assignments). Thus, we confirm that
the polymers (P4 and P5) have the composition 0.5:0.5:1 of
alkylated carbazole, pyridine (for P4) or dialkyl fluorene (for
P5) and dialkoxybenzene, respectively.
Thermal Properties. Thermal properties of the polymers
were investigated using thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) at a heating rate of 10
°C/min under nitrogen atmosphere. The polymers showed a
small weight loss (<5%) below 300 °C which may be due to
the trapped solvent in the polymer matrix. The degradation
temperatures and Tg values were tabulated in Table 1. The
thermogram indicates that the polymers P1-P5 are thermally
stable up to 390 °C.
Figure 2. Absorption (a) and emission (b) spectra of the polymers
P1-P5 in THF; The concentration of the polymers P1-P5 were 5.75
× 10-5, 5.52 × 10-5, 5.13 × 10-5, 3.21 × 10-5 and 3.99 × 10-5 M,
respectively (shoulders are indicated by arrows).
observed by incorporating pyridine or dialkylated fluorene on
the polymer backbone. DSC thermograms of the polymers
showed (Figure 1B) that the glass transition temperatures ranged
from 96 to 116 °C. Higher the amount of carbazole content on
the polymer backbone (P1-P3), the observed Tg values were
higher.
Optical Properties
Absorption and Emission Spectra. The optical properties
of the polymers P1-P5 were measured both in solution and in
thin films (Table 2). As expected, the optical properties of the
polymers in tetrahydrofuran (THF) solution are dependent on
the structure of the polymer backbone. The representative UV-
vis absorption and fluorescence spectra of the polymers P1-
P5 in THF are given in Figure 2. Absorption spectra of these
polymers showed two maxima (λmax), one in the range 290-
300 nm and another above 300 nm. For copolymers P1-P3,
the absorption maxima were observed at 329, 328, and 329 nm,
respectively, (Figure 2, Table 2). There were no significant
changes in the λmax with changes in the length of alkyl chains.
These values lie between those for homopolymers of 3,6-
substituted carbazoles (308 nm) and dialkoxybenzene (370
nm).18,37 Introduction of electron rich dialkoxybenzene units into
the homopolymer of carbazole, increase the electron density of
the polymer backbone as compared to polycarbazoles. This is
consistent with the observed red shift in the λmax for P4 (357
nm) and P5 (347 nm) with decreasing carbazole contents. By
introduction of 0.25 equiv of pyridine (for P4) or fluorene (for
P5), red-shifts in λmax of 28 (P4) and 18 nm (P5) were observed,
implying the extension of conjugation and incorporation of
donor/acceptor system along the polymer backbone.
The first onset degradation temperatures were in the range
of 380-400 °C (Figure 1A). The subsequent onset degradation
temperature and weight loss of these materials were beyond
400 °C was proportional to the mass of their corresponding alkyl
substituents as shown in Figure 1A. Degradation pathways
depend on the number of alkyl chain substituents, which were
decomposed very rapidly. In the case of polymers P1-P2, there
was a second decomposition starting at 490 °C, which may due
to the complete decomposition of the polymer backbone.
However, in the case of polymers P3-P5, no further degradation
or weight loss was observed up to 1000 °C. No significant
changes in the decomposition temperature of the polymers were
The band gap of the polymers P1-P3 (3.28, 3.27, and 3.26
eV) are comparable with the corresponding homopolymer of
3,6-disubstituted carbazole (3.2 eV). Introduction of pyridine
and fluorene units on the polymer backbone results in the
reduction of band gap to 2.96 and 3.09 eV, for P4 and P5,
respectively. The presence of the fluorene unit in P5 is
responsible for the enhanced steric effect between dialkoxy-

Macromolecules, Vol. 39, No. 24, 2006
Carbazole Containing Conjugated Copolymers 8307
Figure 3. (a) Absorption and (b) Emission spectra of the polymers
P1-P5 in dip-casted thin film state.
benzene and fluorene backbone, which may lower the effective
conjugation length in P5 as compared to polymer P4.
The fluorescence spectra of the polymers were recorded with
the excitation wavelength corresponding to their maximum
absorption wavelength. The λem of the polymers, P1-P3, P4,
and P5 are 338, 432, and 415 nm, respectively. The λem values
for the copolymers P1-P3 were comparable with the polycar-
bazole as well as the alkoxylated poly(p-phenylene)s. There was
a red shift observed when a third monomer (e.g., pyridine or
fluorene) was incorporated on the polymer backbone. In the
emission spectrum of P4 and P5 in dilute solution, there was a
shoulder peak at around 449 and 434 nm (see arrows in Figure
2b). These shoulder peaks could be due to the emission of
aggregates in dilute solution or localized conjugated segments
along the polymer backbone.38
Figure 4. XRD powder patterns of the polymers P1-P5. The powder
spectra were taken using powdered polymer samples placed on the
sample pan.
The fluorescence quantum efficiencies (φfl) of the polymers
were also measured in dilute THF solution using quinine sulfate
(φfl ) 0.55 in 0.1 N H2SO4) as standard.39,40 The quantum yields
of the polymers P1-P3 were in the range of 0.58-0.78, which
was significantly higher than the polycarbazoles.18a Also for
polymers P4 and P5, the quantum yields were increased to 0.78
and 0.72, respectively. Incorporation of pyridine and fluorene
resulted in the extension of conjugation of the backbone. As
the conjugation length increases, the nonradiative decay rate
decreases,41 which result in the observed trend in quantum yield.
Thus, the polymers are highly fluorescent which may be used
for application in LED. The quantum yields of the polymers
are listed in Table 2.
The transparent polymer thin films of all polymers (P1-P5)
were fabricated on quartz plates by dip casting their solutions
in THF (5 mg/mL) at room temperature. These films were
subjected to both absorption and emission measurements (Figure
3) and showed bathochromic shifts of a few nanometers and
broadening of the emission bands in comparison to their solution
spectra. For the polymers P1-P3, a red shift of about 10 nm
in the UV-vis spectra, 5 nm red shift in the emission spectra
and significant shoulder peaks around 420 nm were observed.
Similar red shifts were also observed for the polymers P4 and
P5. The significant shoulder peaks may due to the π-π stacking
or aggregation of the polymer chains. All spectral parameters
are summarized in Table 2.
Figure 5. Absorption spectrum of P1, P1+Iodides. Concentration of
P1 is 5.75 × 10-5 M.
understand their self-assembly in the solid state. The X-ray
patterns for the polymer powders (P1-P5) are given in Figure
4. For polymer P2, the peak appearing at a small angle region
(2θ ) 4.5°) is assigned to the intermolecular distance between
two main chains separated by the long alkyl chains. A similar
lattice was also observed for the alkoxy-substituted π-conjugated
polymers.27,42-45 The observed distance of about 20.0 Å supports
the interdigitation of alkyl chains.42 In the case of polymers P1
and P3-P5, no peaks were observed around 5° or lower. The
peak around 21-22° (4.2 Å) for all the polymers is assigned to
the face-to-face distance of inter polymer chain in the parallel
packed lattice. Unfortunately, our machine does not allow us
to measure the diffraction pattern below a value of 2θ ) 1.5°
and small angle diffraction peaks are not visible in the X-ray
diffraction pattern.
Iodide Sensing Studies. Research on conjugated polymer-
based sensors has attracted considerable attention in the recent
years.8 Selective chemosensors of anions such as fluoride,46-50
nitrate,51 and cyanide52 based on small molecules and a few
conjugated polymers53-56 have been reported in the literature.
Among the range of biologically important anions, iodide is of
particular interest due to its essential role for thyroid gland
function.54 The sensing properties of the polymers P1-P5 were
studied in THF solutions using aliquots of methanolic/aqueous
Powder Diffraction Studies. The XRD patterns of the
powdered samples of polymers P1-P5 were recorded to
Table 3. Absorption and Emission Responses of P1-P5 upon Addition of Iodides
∆λmax/ ∆λem in the presence of iodides (nm)
λ
max/λem (nm)
Ksv upon
addition of (TBA)I
polymer
(I- free)
(TBA)I
35/25
35/26
40/44
16/20
17/26
LiI
NaI
KI
P1
P2
P3
P4
P5
329/393
329/394
328/395
357/432
347/415
31/25
32/33
31/26
4/8
31/27
33/33
31/28
4/7
33/27
35/30
31/28
4/8
2.74 × 104
3.17 × 104
3.34 × 104
1.43 × 104
1.60 × 105
12/19
14/25
13/20

8308 Vetrichelvan et al.
Macromolecules, Vol. 39, No. 24, 2006
intermolecular charge-transfer complex between the carbazole
moieties of the polymer backbone with the iodide ions. Among
the polymers, P1-P3 show larger red-shift in the absorption
compared to P4-P5, which may be due to the lesser amount
of carbazole moieties present in the polymers P4-P5.
Figure 6 displays the fluorescence titration spectra of the
polymers P1-P5 with the addition of tetrabutylammonium
iodides. Upon addition of iodide salts, the emission intensity
decreased significantly with a red shift of 44 nm (Table 3). In
consistent with the UV spectra, the fluorescence measurements
also showed a similar observation that the shift and quenching
of the emission (∆λem) upon addition of (TBA)I for the polymers
P1-P3 and P5 is larger than that of polymer P4.
Fluorescence Quenching. In general, fluorescence quenching
happened through variety of mechanisms, like ground-state
complexation,57a charge-transfer phenomena,57b electronic en-
ergy transfer,57c,d fluorescence resonance energy transfer
(FRET),57e-g heavy atom effect,57h magnetic perturbations,57i,j
etc. In this case, the charge-transfer complex via a “heavy-atom”
effect is proposed. The “heavy-atom” interaction between the
excited state of the polymer and the inorganic anion (iodides),
lead to an enhancement of the spin-orbit coupling.58,59 Here,
the “heavy-atom” effect is caused by the formation of a weak,
transient, probably charge-transfer complex between the car-
bazole moieties in the polymer backbone and the iodide ions.60
In addition, there are two types of quenching, one is static
quenching through the formation of a complex and the other is
the dynamic quenching due to the random collisions between
the emitter and the quencher. In both quenching mechanisms,
the electron/energy transfer is involved from the emitter to the
quencher and each can be quantitatively described by the Stern-
Volmer studies.58 Dynamic quenching involves the deactivation
of the polymer exciton through collisions with quencher
molecules or ions in solution. The rate of quenching can be
determined by using the slope of the Stern-Volmer plot and
lifetime measurements. Conversely, the static quenching results
from complexation of the analyte to the receptor sites, creating
a new absorption, relaxation or energy transfer processes that
favor nonradiative decay or through a new excitation/emission
behavior. Using Stern-Volmer plots, the equilibrium constant
for static quenching (Ksv) could be calculated and considered
as the binding constants for the quencher-acceptor system.57e
The quenching effects of the polymer fluorescence in the
presence of (TBA)I salts and other metal iodides have been
investigated in detail. The addition of iodide salts into the
polymer solution quenched the fluorescence intensities of the
polymer. The fluorescence intensities are decreased much with
increase in time and are constant after some particular time
interval. For every addition of iodides, the intensities were
Figure 6. Changes in the emission spectra of P1-P5 at different
concentrations of (TBA)I (methanolic solution): For P1-P4, (1) 0 µM,
(2) 0.4 µM (0 mts), (3) 0.4 µM (20 mts), (4) 0.4 µM (30 mts), (5) 0.8
µM (5 mts), (6) 0.8 µM (20 mts), (7) 1.2 µM, (8) 1.6 µM, (9) 2.0 µM,
(10) 2.0 µM (30 mts). For P5, concentration of (TBA)I: (1) 0 µM, (2)
0.4 µM, (3) 0.4 µM (10 mts), (4) 0.8 µM, (5) 1.2 µM, (6) 1.6 µM.
Inset shows the Stern-Volmer plots for the respective iodides. Please
see Supporting Information for spectral data of other iodides.
solutions of the tetrabutylammonium halides or metal halides
(Figure 5, Table 3). Interestingly, these polymers detect only
iodides among other common anions such as fluorides, chlorides,
bromides, perchlorates, nitrates, hydrogen phosphates, and
hydrogen sulfates. The initial colorless solutions of the polymers
change to yellow color on the addition of iodide salts. With
increase in time, the intensity of the yellow color increases. The
nature of cations such as tetrabutylammonium (TBA) to lithium/
sodium/potassium ions does not influence the color changes.
The addition of iodide salt to the polymers produced a red shift
in the absorption and emission maxima of the polymers and
observed ∆λmax and ∆λem are given in Table 3. The difference
in λmax is larger for all polymers (∼31-40 nm) except P4 (∼4-
17 nm). Figure 5 shows the influence of the addition of different
iodides on the absorption spectra of polymer P1. The red-shift
in the absorption and emission maxima can be explained through
Figure 7. Color changes of polymer P1 in THF upon addition of anion F- ((TBA)F), Cl- ((TBA)Cl), Br- ((TBA)Br), I- ((TBA)I), ClO4- ((TBA)-
PC), and hydrogen sulfate ((TBA)HS), H2PO4- ((TBA)DP). Concentration of P1 and salt concentrations are 5.75 × 10-5 M and 10 mM, respectively.

Macromolecules, Vol. 39, No. 24, 2006
Carbazole Containing Conjugated Copolymers 8309
spectrum of polymer P5, with a and b ) Car-H, c ) Flu-H, d )
phenyl protons, e ) Car-N-CH2-, f ) Ar-OCH2-, and g )
remaining aliphatic protons), (2) absorbance changes of the
polymers, P1-P5 by the addition of (TBA)I (Figure 2a, absorption
spectrum of P2, P2 + (TBA)I, where the concentration of P2 was
5.52 × 10-5 M; Figure 2b, absorption spectrum of P3, P3 +
(TBA)I, where the concentration of P3 was 5.13 × 10-5 M; Figure
2c, absorption spectrum of P4, P4 + (TBA)I, where the concentra-
tion of P4 was 3.21 × 10-5 M; Figure 2d, absorption spectrum of
P5, P5 + (TBA)I, where the concentration of P5 was 3.99 × 10-5
M), (3) absorbance changes of the polymers, P1-P5, by the
addition of LiI, NaI, and KI (Figure 3a, UV-vis absorption
spectrum of P2, P2 + iodides, where the concentration of P2 was
5.52 × 10-5 M; Figure 3b, UV-vis absorption spectrum of P3,
P3 + iodides, where the concentration of P3 was 5.13 × 10-5 M;
Figure 3c, UV-vis absorption spectrum of P4, P4 + iodides, where
the concentration of P4 was 3.21 × 10-5 M; Figure 3d, UV-vis
absorption spectrum of P5, P5 + iodides, where the concentration
of P5 was 3.99 × 10-5 M), and (4) fluorescence changes of the
polymers, P1-P5 by the addition of iodides (Figure 4a, changes
in the emission spectra of P1 at different concentrations of LiI, (1)
0, (2) 4, (3) 8, (4) 12, (5) 16, and (6) 24 mM, where the
concentration of P1 is 5.75 × 10-5 M; Figure 4b, changes in the
emission spectra of P1 at different concentrations of NaI, (1) 0,
(2) 4, (3) 8, (4) 16, and (5) 24 mM, where the concentration of P1
is 5.75 × 10-5 M; Figure 4c, changes in the emission spectra of
P1 at different concentrations of KI, (1) 0, (2) 4, (3) 8, and (4) 16
mM, where the concentration of P1 is 5.75 × 10-5 M; Figure 4d,
changes in the emission spectra of P2 at different concentrations
of LiI (methanolic solution), (1) 0, (2) 4, (3) 8, (4) 12, (5) 16, and
(6) 24 mM, where the concentration of P2 was 5.52 × 10-5 M;
Figure 4e, changes in the emission spectra of P2 at different
concentrations of NaI (methanolic solution), (1) 0, (2) 4, (3) 8, (4)
16, and (5) 24 mM, where the concentration of P2 was 5.52 ×
10-5 M; Figure 4f, changes in the emission spectra of P2 at different
concentrations of KI (methanolic solution), (1) 0, (2) 4, (3) 12,
and (4) 16 mM, where the concentration of P2 was 5.52 × 10-5
M; Figure 4g, changes in the emission spectra of P3 at different
concentrations of LiI (methanolic solution), (1) 0, (2) 4, (3) 8, (4)
12, (5) 16, (6) 24, (7) 28, and (8) 32 mM, where the concentration
of P2 was 5.13 × 10-5 M; Figure 4 h, changes in the emission
spectra of P3 at different concentrations of NaI (methanolic
solution), (1) 0, (2) 4, (3) 8, (4) 12, (5) 16, and (6) 24 mM, where
the concentration of P3 was 5.13 × 10-5 M; Figure 4i, changes in
the emission spectra of P3 at different concentrations of KI
(methanolic solution), (1) 0, (2) 4, (3) 8, (4) 12, (5) 16, (6) 24, and
(7) 28 mM, where the concentration of P3 was 5.13 × 10-5 M;
Figure 4j, changes in the emission spectra of P4 at different
concentrations of LiI (methanolic solution), (1) 0, (2) 4, (3) 8, (4)
12, and (5) 16 mM, where the concentration of P4 was 3.21 ×
10-5 M; Figure 4K, changes in the emission spectra of P4 at
different concentrations of NaI (methanolic solution), (1) 0, (2) 4,
(3) 8, (4) 12, and (5) 16 mM, where the concentration of P4 was
3.21 × 10-5 M; Figure 4l, changes in the emission spectra of P4
at different concentrations of KI (methanolic solution), (1) 0, (2)
4, (3) 8, (4) 12, and (5) 16 mM, where the concentration of P4
was 3.21 × 10-5 M; Figure 4m, changes in the emission spectra
of P5 at different concentrations of LiI (methanolic solution), (1)
0, (2) 4, (3) 8, (4) 12, (5) 16, (6) 20, and (7) 24 mM, where the
concentration of P5 was 3.99 × 10-5 M; Figure 4n, changes in the
emission spectra of P5 at different concentrations of NaI (metha-
nolic solution), (1) 0, (2) 4, (3) 8, (4) 12, (5) 16, and (6) 20 mM,
where the concentration of P5 was 3.99 × 10-5 M; Figure 4o,
changes in the emission spectra of P5 at different concentrations
of KI (methanolic solution), (1) 0, (2) 4, (3) 8, (4) 12, (5) 16, and
(6) 20 mM, where the concentration of P5 was 3.99 × 10-5 M.
This material is available free of charge via the Internet at http://
pubs.acs.org.
recorded for every 5 min until there was no change in intensities
observed. The Stern-Volmer equation states that I0/I ) 1 +
Ksv[quencher], where I0 is the intensity of the fluorescence from
the polymer in the absence of quencher and I is the intensity in
the presence of the quencher. Ksv is the Stern-Volmer constant
which provides the quantitative measure of the quenching. At
low concentration of quenchers ((TBA)I), the quenching ef-
ficiency was more efficient and I0/I vs the concentration of the
quencher gives a linear plot,61 indicating a static quenching in
which complex formation between polymer and quencher was
observed. The Stern-Volmer constant Ksv for all the polymers
are tabulated in Table 3. Among the polymers P1-P5, the
highest Ksv values were obtained for P5 by the addition of
(TBA)I (1.6 × 105) followed by the polymers, P1-P3 (Table
3). The least Ksv value is obtained for the polymer, P4 (1.4 ×
104) which may be due to the presence of electron deficient
pyridine moiety on the backbone.
Figure 7 shows a photograph of THF solutions of polymer
P1 containing anions F- , Cl-, Br-, I-, ClO4-, hydrogen sulfate
(HSO3-), and H2PO42- ((TBA)DP). Tetrabutylammonium was
used as cation in all case. The solutions remained colorless upon
addition of all the above anions except for I-, where color
changed to yellow. Hence, these polymers may be useful as
colorimetric and fluorimetric sensors of iodide anions.
Conclusion
A new series of copolymers consisting N-alkylated carbazole
and dialkoxybenzene was successfully synthesized (P1-P3).
The optical properties of these copolymers can be changed by
changing the composition of the polymer backbone. Introduction
of the pyridine and alkylated fluorene units on the polymer
backbone of P2 resulted in a red shift of 28 and 18 nm for P4
and P5, respectively. There was a reduction in band gaps of
polymers P4 and P5 (2.96 and 3.09 eV) compared to the
carbazole-bis(dodecyloxy) benzene copolymers (3.26-3.28
eV). Thus, decrease in the amount of carbazole content along
the polymer backbone increases the λmax and reduces band gaps.
The molecular weights of these polymers were in the range of
15-33 kDa. These conjugated polymers were found to be
thermally stable up to 390 °C and possessed good solubility,
film-forming quality, thermal stability, optical properties and
moderate quantum efficiencies. The sensing studies of these
polymers were investigated by using the fluorescence quenching
experiment by adding iodide salts and confirmed that these
polymers may find useful applications in selective iodide
identification colorimetrically as well as fluorimetrically. Such
properties make these conjugated polymers promising candidates
for advanced material such as sensors.
Acknowledgment. S.V. acknowledges the funding support
from the National University of Singapore and the Agency for
Science, Technology, and Research (ASTAR), Singapore. We
thank the Singapore-MIT Alliance for research fellowships. The
technical support from the Department of Chemistry, National
University of Singapore, is also acknowledged. We thank the
reviewers for their constructive comments and suggestions.
1
Supporting Information Available: Figures showing (1) H
1
NMR spectra of the polymers (Figure 1a, H NMR spectra of the
polymers P1-P3, with a and b ) carbazole protons, c and d )
aromatic protons, and e ) Car-N-CH2-, f ) Ar-OCH2-, and
g ) remaining aliphatic protons; Figure 1b, 1H NMR spectrum of
polymer P4, with a ) py-H, b ) Car-H, c ) py-H, d and e )
Car-H, f ) phenyl protons, g and h ) Car-N-CH2- and Ar-
OCH2-, i ) remaining aliphatic protons; Figure 1c, 1H NMR

8310 Vetrichelvan et al.
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