Synthesis and property studies of linear and kinked poly(pyreneethynylene)s

Article abstract of DOI:10.1016/j.polymer.2010.09.012

A series of fluorescent conjugated polymers, poly(pyreneethynylene)s, have been designed and synthesized to investigate the effect of shape of polymer backbone on physical properties. Polymers with linear and kinked backbone were synthesized using 1,6- and 1,8-disubstituted pyrene. The target copolymers were designed to incorporate various spacer units, such as, alkoxyphenyl, carbazole and fluorene on the polymer backbone. Characterization of the target compounds was achieved by NMR, IR, GPC and MALDI-TOF mass spectrometry. Detailed investigation on their optical, electrochemical and thermal properties revealed significant contribution of the geometry of polymer backbone towards physical properties. Kinked backbone of cisoid-polymers was found to result in lower optical band gap, less negative E_HOMO and higher thermal stability as compared to their linear analogues, most probably due to the coiling of polymer chains. Comparison of the physical properties of the polymers with those of the model compounds suggested similar extent of conjugation through 1,6- and 1,8-position of pyrene.

Full text of DOI:10.1016/j.polymer.2010.09.012

Polymer 51 (2010) 5078e5086
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and property studies of linear and kinked poly(pyreneethynylene)s
*
Jhinuk Gupta, Sajini Vadukumpully, Suresh Valiyaveettil
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of fluorescent conjugated polymers, poly(pyreneethynylene)s, have been designed and synthe-
sized to investigate the effect of shape of polymer backbone on physical properties. Polymers with linear
and kinked backbone were synthesized using 1,6- and 1,8-disubstituted pyrene. The target copolymers
were designed to incorporate various spacer units, such as, alkoxyphenyl, carbazole and fluorene on the
polymer backbone. Characterization of the target compounds was achieved by NMR, IR, GPC and MALDI-
TOF mass spectrometry. Detailed investigation on their optical, electrochemical and thermal properties
revealed significant contribution of the geometry of polymer backbone towards physical properties.
Kinked backbone of cisoid-polymers was found to result in lower optical band gap, less negative EHOMO and
higher thermal stability as compared to their linear analogues, most probably due to the coiling of polymer
chains. Comparison of the physical properties of the polymers with those of the model compounds sug-
gested similar extent of conjugation through 1,6- and 1,8-position of pyrene.
Received 6 June 2010
Received in revised form
18 August 2010
Accepted 3 September 2010
Available online 15 September 2010
Keywords:
Conjugated polymer
Pyrene
Polymer synthesis
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
solid state. Moreover, pyrene offers the possibility of incorporating
interesting structural features to the polymer backbone due to easy
Synthesis of structurally versatile
p
-conjugated polymers is
functionalization at various positions. It is conceivable that selective
functionalization of pyrene at 1,8-positions (cis) introduces a kink
of 63ꢀ on the polymer backbone whereas, functionalization at
1,6-positions (trans) gives a linear polymer. Polymers with pendant
pyrene moieties are common [20e22], but, only a few polymers and
oligomers with pyrene incorporated on the polymer backbone are
known in literature [23e25].
In this work, synthesis and characterization of a series of
conjugated poly(pyreneethynylene)s are reported, where dieth-
ynylpyrene is copolymerized with various spacer units, such as
alkoxybenzene, carbazole and fluorene (Fig. 1). The shapes of the
polymer backbones are controlled by using 1,6- and 1,8-dis-
ubstitution on the pyrene moieties. Synthesized polymers are
named as cisoid-(1, 3, 5) or transoid-(2, 4, 6) depending on different
mode of functionalization (1,8- vs. 1,6-) on pyrene. Unlike the
reported phenylene based systems [16,18], both cisoid and transoid
interesting owing to their extensive use in technologies such as LED
displays [1e6], molecular electronics [7e11], sensors [12,13] and
lasers [14]. Developing new fluorescent polymers with interesting
properties, such as, higher quantum efficiency, charge transfer
mobility, thermal stability, solubility and ease of processiblityallows
us to explore interesting applications. All these physical properties
can be manipulated through proper molecular design. Poly
(phenyleneethynylene) is one of the well explored class of fluores-
cent conjugated polymers [15]. Structureeproperty relationship of
homopolymers [16] and copolymers [17,18] of poly(p-phenyl-
eneethynylene) is reported in literature. Chemical nature and shape
of backbone influence the physical properties of polymers. It is
found that introduction of kinks or twists on the polymer chain
reduces the stiffness and enhances the processibility. For example,
incorporation of 2,5-thienylene [19] and m-phenylene [18] groups
in poly(p-phenyleneethynylene) systems enhances the flexibility
and processibility of the resulting polymers by introducing a twist of
143ꢀ and 120ꢀ to the polymer backbone. Pyrene may be considered
as a rigid unit to influence the backbone and enhance the fluores-
cence properties. Pyrene is one of the interesting fluorophores to
show high quantum yield, longer fluorescence life time, high
have strong p-conjugation. Thus the system allows us to investigate
solely the effect of shape on the properties of the polymers. In
addition, two structurally related model compounds (7 and 8) are
designed for comparison purpose.
2. Experimental section
thermal stability and significant
p-stacking ability in solution or
2.1. Materials and methods
All reagents were purchased from commercial sources and used
withoutfurtherpurificationunlessotherwisestated. Tetrahydrofuran
* Corresponding author. Tel.: þ65 65164327; fax: þ65 67791691.
E-mail address: chmsv@nus.edu.sg (S. Valiyaveettil).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.09.012

J. Gupta et al. / Polymer 51 (2010) 5078e5086
5079
Fig. 1. Structure of the target compounds.
(THF) was purified by distillation over sodium under nitrogen
atmosphere. The 1H and 13C NMR spectra were collected on a Bruker
ACF 300 spectrometer operating at 300 and 75.5 MHz, respectively.
CDCl3 was used as solvent with tetramethylsilane as internal stan-
dard. FT-IR spectra were recorded using BIO-RAD FT-IR spectro-
photometer. MALDI-TOF mass spectra of the compounds were
recorded using Bruker Deltronix Autoflex __ instrument. Solution of
1,8,9-trihydroxyanthracene (0.1 M) and 2,5-dihydroxybenzoic acid
(0.1 M) in THF was used to prepare the matrix. Molecular weight of
the polymers was determined using Shimadzu LC vt 10AT gel
permeation chromatography (GPC) instrument equipped with UV
and refractive index (RI) detectors connected in series, polystyrene
as standard and THF as an eluent at a flow rate of 0.3 mL/min.
UVevis spectra were recorded using a Shimadzu 3101 PC spectro-
photometer and fluorescence measurements were made on an RF-
5301PC Shimadzu spectrofluorophotometer. The quantum yields of
the synthesized compounds in dichloromethane (DCM) were
measured using quinine sulfate (0.1 M H2SO4) as a reference [26].
The chromaticity of the compounds was measured using chroma-
meter CS 100 A. The electrochemical behavior of the compounds
was investigated with cyclic voltammetry (CV). CV experiments
were performed using an Autolab potentiostat (model PGSTAT30)
by Echochimie and data recorded in acetonitrile with 0.1 M tetra-
butylammonium hexafluorophosphate as supporting electrolyte
(scan rate of 100 mV sꢁ1). The experiments were performed at room
temperature with a conventional three-electrode configuration
consisting of an indium tin oxide (ITO) working electrode, a plat-
inum counter electrode, and Hg/Hg2Cl2 in 3 M KCl as reference
electrode. Thermogravimetric analyses (TGA) were done on
a TA-SDT 2960 at a heating rate of 10 ꢀC/min under nitrogen
atmosphere. Differential scanning calorimetry (DSC) thermograms
were recorded using a TA-DSC 2920 at a heating rate of 10 ꢀC/min
under nitrogen atmosphere. Scanning electron microscopic (SEM)
images were recorded in JEOL JEM-6010 F field emission scanning
electron microscope. Atomic force microscopy (AFM) experiment
was performed at room temperature using a commercial AFM
Nanoscope IV (Dimension 3100, Digital Instruments) in the tapping
mode. The samples for SEM and AFM experiments were prepared by
evaporating a few drops of toluene solution of the polymers on glass
plates and clean ITO substrates, respectively.
2.2. Synthesis
Compounds 11 [27,28], 13 [29], 15 [30], 16a, 16b [31], 17a, 17b,
18a and 18b [32,33] were prepared using reported procedures.
2.2.1. General synthetic procedure for Sonogashira polymerization
To a degassed solution of freshly distilled THF and triethylamine,
dihalocompound (11 or 13 or 15), CuI, PPh3 and Pd(PPh3)2Cl2 were
added under nitrogen atmosphere. The terminal acetylene was
added to the stirred solution at 55 ꢀC (for 11) or 80 ꢀC (for 13 and
15) and the heating was continued for 5 days under nitrogen
atmosphere. After cooling, the reaction mixture was added drop-
wise to a methanol/water mixture (98:2). The precipitated polymer
was filtered and purified by reprecipitation from methanol.

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J. Gupta et al. / Polymer 51 (2010) 5078e5086
2.2.1.1. Synthesis of polymer 1. Compound 11 (560 mg, 0.80 mmol),
Pd(PPh3)2Cl2 (28 mg, 0.04 mmol), CuI (16 mg, 0.08 mmol), PPh3
(42 mg, 0.16 mmol), 18a (200 mg, 0.80 mmol), triethylamine
(15 mL), THF (30 mL). Yield ¼ 300 mg, 40%.
2.2.1.6. Synthesis of polymer 6. Compound 15 (423 mg, 0.64 mmol),
Pd(PPh3)2Cl2 (23 mg, 0.03 mmol), CuI (13 mg, 0.06 mmol), PPh3
(34 mg, 0.13 mmol), 18b (160 mg, 0.64 mmol), triethylamine
(25 mL), THF (50 mL). Yield ¼ 310 mg, 53%.
1H NMR (300 MHz, CDCl3, ppm)
d
: 8.98e8.08 (pyrene and
1H NMR (300 MHz, CDCl3, ppm)
d: 8.80e7.43 (pyrene and flu-
phenyl Hs), 4.22 (eOeCH2e), 4.10 (eOeCH2e), 2.08e0.77 (alkyl
orene Hs), 2.03e0.66 (alkyl Hs). 13C NMR (75.5 MHz, CDCl3, ppm)
d:
Hs). 13C NMR (75.5 MHz, CDCl3, ppm)
d
: 153.8, 132.3, 131.4, 130.7,
153.3, 151.4, 150.6, 141.0, 139.5, 132.0, 131.9, 131.7, 131.2, 131.0, 130.1,
130.0, 128.5, 128.4, 128.2, 126.3, 126.0, 125.6, 125.2, 124.3, 122.1,
121.7, 121.4, 120.2, 119.9, 118.7 (pyrene and fluorene eCHe), 96.8,
89.1 (eC^Ce), 55.6, 55.5, 53.4, 40.3, 40.1, 31.9, 30.1, 30.0, 29.6, 29.3,
23.8, 22.7, 14.1 (eCH2). FT-IR (KBr, cmꢁ1): 3426, 3037, 2916, 2855,
2191, 1887, 1605, 1466, 1252, 1187, 1065, 1002, 895, 834, 726.
129.7, 128.8, 127.8, 126.7, 125.1, 124.3, 118.8, 116.4, 115.8, 114.2
(pyrene and phenyl eCHe), 94.6, 92.8 (eC^Ce), 69.6, 68.1, 38.9,
32.0, 29.8, 29.4, 26.4, 23.8, 22.7, 14.2, 10.7. FT-IR (KBr, cmꢁ1): 2923,
2849, 2364, 2200, 1596, 1562, 1511, 1465, 1380, 1278, 1215, 1032,
850, 724.
2.2.1.2. Synthesis of polymer 2. Compound 11 (392 mg, 0.56 mmol),
Pd(PPh3)2Cl2 (20 mg, 0.03 mmol), CuI (12 mg, 0.06 mmol), PPh3
(30 mg, 0.12 mmol), 18b (138 mg, 0.56 mmol), triethylamine
(10 mL), THF (20 mL). Yield ¼ 350 mg, 66%.
2.2.2. Synthesis of compound 10
Compound 9 (380 mg, 0.85 mmol), KIO3 (36 mg, 0.17 mmol) and
I2 (119 mg, 0.47 mmol) were added to a mixture of acetic acid
(10 mL), H2SO4 (0.1 mL) and water (1 mL). The mixture was grad-
ually brought to reflux and continued refluxing for 6 h. After
cooling the reaction mixture to room temperature, the unreacted
iodine was quenched using 20% aqueous Na2S2O3. The precipitate
was collected and washed with water. The air-dried solid was
purified by column chromatography using hexane as the eluent.
The pure compound was obtained as colorless semisolid with 32%
yield (156 mg).
1H NMR (300 MHz, CDCl3, ppm)
d: 8.89e7.87 (pyrene and
phenyl Hs), 4.22e3.92 (eOeCH2e), 1.58e0.86 (alkyl Hs). 13C NMR
(75.5 MHz, CDCl3, ppm) : 154.0, 132.0, 130.9, 129.6, 128.8, 127.9,
d
126.8, 125.0, 124.2, 118.9, 116.3, 115.8, 114.2, 113.8 (pyrene and
phenyl eCHe), 94.6, 92.6 (eC^Ce), 69.6, 68.2, 38.8, 31.9, 29.7, 29.3,
26.4, 26.3, 26.1, 22.6, 14.1, 10.7. FT-IR (KBr, cmꢁ1): 2974, 2923, 2854,
2354, 2205, 1727, 1465, 1271, 1209, 845.
1H NMR (300 MHz, CDCl3, ppm)
d: 7.32 (1H, d, J ¼ 2.94 Hz,
2.2.1.3. Synthesis of polymer 3. Compound 13 (986 mg, 2.00 mmol),
Pd(PPh3)2Cl2 (70 mg, 0.10 mmol), CuI (38 mg, 0.20 mmol), PPh3
(105 mg, 0.40 mmol), 18a (500 mg, 2.00 mmol), triethylamine
(30 mL), THF (60 mL). Yield ¼ 680 mg, 30%.
IeCeCH), 6.83 (1H, dd, J ¼ 2.94 Hz and 8.88 Hz, IeC-CeCOeCHe),
6.72 (1H, d, J ¼ 8.88 Hz, IeCeCOeCHe), 3.93 (2H, t, eOeCH2e),
3.87 (2H, t, eOeCH2e), 1.84e0.90 (40H, m, alkyl Hs), 0.88 (6H,
t, eCH3). 13C NMR (75.5 MHz, CDCl3, ppm)
d: 153.8 (eOeCeCHe),
1H NMR (300 MHz, CDCl3, ppm)
d: 8.86e7.39 (pyrene and
152.2 (eOeCeCHe), 125.4 (eCHe), 115.4 (eCHe), 113.1 (eCHe),
87.0 (IeCeCHe), 70.2 (eOeCH2e), 68.8 (eOeCH2e), 31.9, 29.6,
29.5, 29.3, 29.2, 26.1, 26.0, 22.7, 14.1 (alkyl Cs).
carbazole Hs), 4.24e4.19 (eNeCH2e), 1.83e0.87 (alkyl Hs). 13C
NMR (75.5 MHz, CDCl3, ppm) : 140.4, 139.3, 131.7, 130.0, 129.7,
d
129.0, 128.8, 128.6, 128.5, 128.4, 127.7, 127.6, 126.4, 126.3, 125.0,
124.3,124.2,123.4,123.3,123.2,122.0,121.9,119.0,114.0,112.2,111.9,
110.4, 109.1 (pyrene and carbazole eCHe), 96.9, 87.2 (eC^Ce),
43.3, 31.9, 29.6, 29.51, 29.47, 29.43, 29.3, 28.8, 27.2, 27.1, 22.7, 14.1.
FT-IR (KBr, cmꢁ1): 3488, 3427, 2916, 2854, 2191, 1878, 1589, 1466,
1357, 1281, 1217, 1141, 1048, 849, 801, 711.
2.2.3. General synthetic procedure for Sonogashira coupling
reaction
To a degassed solution of freshly distilled THF and triethylamine,
10, CuI, PPh3 and Pd(PPh3)2Cl2 were added under a continuous
nitrogen flow. Diethynylpyrene (18a or 18b) was added to the
stirred solution at 55 ꢀC and the heating was continued for 24 h
under nitrogen atmosphere. The solvent was removed completely
under vacuum to get the crude product which was purified through
column chromatography.
2.2.1.4. Synthesis of polymer 4. Compound 13 (315 mg, 0.64 mmol),
Pd(PPh3)2Cl2 (23 mg, 0.03 mmol), CuI (13 mg, 0.06 mmol), PPh3
(34 mg, 0.13 mmol), 18b (160 mg, 0.64 mmol), triethylamine
(25 mL), THF (50 mL). Yield ¼ 230 mg, 48%.
1H NMR (300 MHz, CDCl3, ppm)
d: 8.79e7.43 (pyrene and
2.2.3.1. Synthesisofcompound7. Compound 10 (189 mg, 0.33 mmol),
Pd(PPh3)2Cl2 (5 mg, 0.01 mmol), CuI (3 mg, 0.01 mmol), PPh3
(8 mg, 0.03 mmol), triethylamine (10 mL), THF (20 mL), 18a (33 mg,
0.13 mmol). Column chromatography using 4:1 hexane/DCM.
Yield ¼ 92 mg, 61%.
carbazole Hs), 4.30e4.21 (eNeCH2e), 1.82e0.85 (alkyl Hs). 13C
NMR (75.5 MHz, CDCl3, ppm) : 139.4, 139.3, 131.7, 131.1, 130.0,
d
129.8, 129.0, 128.8, 128.5, 128.4, 127.6, 126.4, 125.0, 124.2, 123.4,
123.2, 122.0, 119.0, 114.0, 112.2, 111.9, 110.4, 109.1 (pyrene and
carbazole eCHe), 96.9, 87.2 (eC^Ce), 43.3, 31.9, 29.6, 29.51, 29.47,
29.4, 29.3, 28.9, 28.8, 27.21, 27.19, 22.6, 14.1. FT-IR (KBr, cmꢁ1): 3443,
3038, 2918, 2855, 2191, 1887, 1589, 1481, 1358, 1280, 1220, 1142,
1064, 956, 849, 724.
1H NMR (300 MHz, CDCl3, ppm)
d: 8.93 (2H, s, pyrene eCHe),
8.23e8.04 (6H, m, pyrene eCHe), 7.22 (2H, s, phenyl eCHe),
6.90 (4H, s, phenyl eCHe), 4.11 (4H, t, eOeCH2e), 3.98 (4H,
t, eOeCH2e), 2.03e1.07 (80H, m, alkyl Hs), 0.92e0.83 (12H,
m, eCH3). 13C NMR (75.5 MHz, CDCl3, ppm)
d: 154.4 (eOeCeCHe),
2.2.1.5. Synthesis of polymer 5. Compound 15 (660 mg, 1.00 mmol),
Pd(PPh3)2Cl2 (35 mg, 0.05 mmol), CuI (19 mg, 0.10 mmol), PPh3
(52 mg, 0.20 mmol), 18a (250 mg, 1.00 mmol), triethylamine
(30 mL), THF (60 mL). Yield ¼ 530 mg, 58%.
152.9 (eOeCeCHe), 132.0, 131.2, 129.7, 127.8, 126.7, 124.9, 124.2,
119.0, 118.4, 116.5, 113.8, 113.6 (pyrene and phenyl eCHe), 92.6,
92.4 (eC^Ce), 69.7, 68.8 (eOeCH2e) 31.9, 31.8, 29.69, 29.64,
29.55, 29.53, 29.46, 29.42, 29.36, 29.3, 26.3, 26.1, 22.7, 22.6, 14.1
(alkyl Cs). FT-IR (KBr, cmꢁ1): 2955, 2921, 2854, 2363, 2336, 2196,
1644, 1488, 1466, 1377, 1304, 1276, 1220, 1159, 1026, 853, 797, 758,
724. MALDI-TOF MS: Calcd. for C80H114O4: 1140. Found: 1141
[M þ H]þ.
1H NMR (300 MHz, CDCl3, ppm)
d: 8.91e7.44 (pyrene and fluo-
rene Hs), 2.12e0.82 (alkyl Hs). 13C NMR (75.5 MHz, CDCl3, ppm)
d:
153.3, 152.6, 151.4, 150.6, 141.0, 140.6, 139.4, 139.0, 131.9, 131.8, 131.4,
131.1, 130.2, 130.0, 128.5, 128.0, 126.5, 126.2, 126.0, 125.2, 124.3,
122.2, 121.7, 121.4, 121.1, 120.2, 119.9, 118.7 (pyrene and fluorene
eCHe), 96.7, 89.0 (eC^Ce), 55.6, 53.4, 40.3, 40.1, 31.9, 30.0, 29.6,
29.3, 23.8, 23.6, 22.6, 14.1. FT-IR (KBr, cmꢁ1): 3429, 3037, 2917, 2855,
2191, 1878, 1589, 1451, 1249, 1189, 1110, 1065, 1003, 833, 710.
2.2.3.2. Synthesisofcompound8. Compound 10 (189 mg, 0.33 mmol),
Pd(PPh3)2Cl2 (5 mg, 0.01 mmol), CuI (3 mg, 0.01 mmol), PPh3 (8 mg,
0.03 mmol), triethylamine (10 mL), THF (20 mL), 18b (33 mg,

J. Gupta et al. / Polymer 51 (2010) 5078e5086
5081
OC12H25
I
(10)
OC12H25
OH
b
OC12H25
a
OC12H25
I
c
OC12H25
OH
(11)
I
(9)
OC12H25
Br
Br
d
e
N
H
N
N
C12H25
C12H25
(12)
(13)
f
Br
Br
Br
Br
C12H25
C12H25
OH
OH
(14)
(15)
I
I
g
h
I
OH OH
I
(17b)
10, j
(17a)
(16a)
(16b)
10, j
11,k
(7)
(1)
(8)
11, k
13, l
15, l
(2)
(4)
(6)
i
i
17b
17a
13, l
15, l
(3)
(5)
(18b)
(18a)
Scheme 1. Synthesis of the target compounds: (a) K2CO3, acetone,1-dodecylbromide, N2, reflux, 3 d; (b) I2/KIO3, HOAc, H2SO4, H2O, room temperature to reflux, 6 h; (c) I2/KIO3, HOAc,
H2SO4, H2O, reflux, 6 h; (d) Tetrabutylammonium bromide, 50% NaOH, 1-bromododecane, toluene, 70 ꢀC, 24 h; (e) NBS, dichloromethane, 0 ꢀC to room temperature, 6 h; (f) Trie-
thylbenzylammonium chloride,1-bromododecane, 50% NaOH, 60 ꢀC,12 h; (g) I2/KIO3, HOAc, H2SO4, H2O, 40 ꢀC, 4 h; (h) 2-Methylbut-3-yn-2-ol, Et3N, Pd(PPh3)2Cl2, CuI, 50 ꢀC, 20 h; (i)
NaOH, toluene, reflux, 3 h; (j) THF, Et3N, Pd(PPh3)2Cl2, CuI, PPh3, 55 ꢀC, 24 h; (k) THF, Et3N, Pd(PPh3)2Cl2, CuI, PPh3, 55 ꢀC, 5 d; (l) THF, Et3N, Pd(PPh3)2Cl2, CuI, PPh3, 80 ꢀC, 5 d.
0.13 mmol). Column chromatography using 82:18 hexane/DCM.
reported procedures with or without modifications. Diiodination of
pyrene, followed by coupling with 2-methylbut-3-yn-2-ol gener-
ated the mixture of 17a and 17b. These two isomers were separated
owing to high degree of intermolecular H-bonding in 17b, which
allowed it to precipitate out immediately from a DCM solution.
Terminal acetylene groups were deprotected using sodium
hydroxide in boiling toluene. Coupling of 18a and 18b to 11, 13 and
15 generated the series of target polymers. Compounds 18a and
18b were coupled to the monoiododerivative 10 to synthesize
two model compounds 7 and 8. All polymers were red in
colour, whereas, oligomers were yellow in colour. Solubility of
the synthesized compounds were checked in common organic
solvents, e.g. cyclohexane, DCM, chloroform, THF, acetonitrile and
dimethylsulfoxide, with the compounds showing maximum solu-
bility in DCM, chloroform and THF. In general, cisoid-polymers (1, 3
and 5) were more soluble as compared to their transoid-counter
parts (2, 4 and 6).
Yield ¼ 106 mg, 70%.
1H NMR (300 MHz, CDCl3, ppm)
d
: 8.86 (2H, d, J ¼ 9.03 Hz,
pyrene eCHe), 8.22e8.11 (6H, m, pyrene eCHe), 7.20 (2H, s, phenyl
eCHe), 6.90 (4H, d, J ¼ 1.32, phenyl eCHe), 4.12 (4H, t, eOeCH2e),
3.98 (4H, t, eOeCH2e), 2.05e1.17 (80H, m, alkyl Hs), 0.90e0.82
(12H, m, eCH3). 13C NMR (75.5 MHz, CDCl3, ppm)
d: 154.4
(eOeCeCHe), 152.8 (eOeCeCHe), 132.2, 131.1, 129.6, 127.9, 126.7,
125.0, 124.3, 118.9, 118.4, 116.6, 113.6, 113.4 (pyrene and phenyl
eCHe), 92.6, 92.3 (eC^Ce), 69.6, 68.8, 31.9, 31.8, 29.6, 29.43,
29.40, 29.35, 29.32, 26.2, 26.1, 22.68,22.66, 14.1 (alkyl Cs). FT-IR
(KBr, cmꢁ1): 2957, 2917, 2849, 2359, 2335, 2205, 1602, 1505,
1471,1408, 1397, 1323, 1289, 1272, 1221, 1164, 1146, 1044, 1026, 850,
816, 776, 719. MALDI-TOF MS: Calcd. for C80H114O4: 1140. Found:
1141 [M þ H]þ.
3. Results
Full characterization of the compounds was accomplished with
NMR, IR and MALDI-TOF mass spectrometry and GPC. Though 1H
NMR spectra of the polymers were not well resolved as compared
to the model compounds, the regions for characteristic peaks could
be identified. NMR signals of aromatic protons of pyrene and
associated phenyl/carbazole/fluorene units appeared between 8.9
and 7.4 ppm. Signal for alkyl protons were observed between 2.0
3.1. Synthesis and characterization
Syntheses of the target compounds are illustrated in Scheme 1.
1,8- or 1,6-Diethynylpyrene (18) was polymerized with dihaloder-
ivatives of phenyl, carbazole and fluorene under Sonogashira
coupling conditions. Dihaloderivatives were synthesized following

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J. Gupta et al. / Polymer 51 (2010) 5078e5086
Table 1
property is quite similar with poly(phenyleneethynylene)s [15] and
Molecular weight and TGA data of the target compounds.
can be attributed to the curing reaction between acetylene groups
on the polymer backbone during heating [34]. DSC trace (Fig. 2c) of
the model compound 7 showed tꢁh1ree phase transitions at 45 ꢀC
(12.6 kJ molꢁ1), 75 ꢀC (42.1 kJ mol ) and 100 ꢀC (49.5 kJ molꢁ1).
Compound 7 melts to a discotic phase at 75 ꢀC, followed by iso-
tropization at 100 ꢀC. The cooling trace exhibited isotropicediscotic
transition at 63 ꢀC followed by crystallization at 42 ꢀC [35]. The
liquid crystalline nature of 7 indicated by DSC trace was further
examined using polarized optical microscope (Supplementary
material Fig. S13). Compound 8, with a single phase transition
(112 ꢀC, 161.2 kJ molꢁ1) between crystal and isotropic phases, did
not show any liquid crystalline phase (Fig. 2d).
PDIa
T5 (ꢀC)
a
a
b
Polymer
Mw
Mn
1
2
3
4
5
6
7
8
18,700
12,400
12,600
11,600
11,900
12,300
e
11,900
9800
9000
8400
8600
8200
1141c
1141c
1.6
1.3
1.4
1.4
1.3
1.5
e
234
222
300
277
329
310
338
344
e
e
a
Determined by GPC using polystyrene standard.
Temperature corresponding to 5% weight loss.
b
c
[M þ H]þ determined by MALDI-TOF mass spectrometry.
3.3. Optical properties
and 0.8 ppm, whereas, alkyl protons connected to oxygen or
nitrogen atoms shifted to a lower field (4.3e3.9 ppm). GPC was
used to estimate the molecular weight of the polymers. Weight
average (Mw) and number average (Mn) molecular weights along
with polydispersity index (PDI) are listed in Table 1.
Optical property is useful to gain information regarding struc-
tureeproperty relationship of compounds. Detailed study of optical
properties of all target compounds was accomplished in solution
and in thin film (Figs. 3 and 4), the results are summarized in
Table 2. It is observed that all polymers and oligomers have more
than one strong absorption and emission peaks, which is a charac-
teristic feature of a pyrene moiety [36].
3.2. Thermal properties
Thermogravimetric analyses were doneforall targetcompounds.
The detailed findings are listed in Table 1. All polymers showed
similar trend in thermal properties with one sharp decomposition
followed by gradual decay (Fig. 2a). Some residual weight remained
even beyond 900 ꢀC indicating char formation. The first decompo-
sition of polymers signifies partial or complete breaking of alkyl
chains from polymer backbone. Model compounds (7 and 8) showed
sharp single step decomposition (Fig. 2b).
Enhanced conjugation offered by ethynyl bonds shift the
absorption wavelengths of both polymers and model compounds to
red region (labs ¼ 450e500) as compared to pyrene (labs ¼ 335 nm)
and the previously reported poly(pyrenevinyl) system (labs
¼
395 nm) [24]. Optical properties of target compounds were exam-
ined in different solvents and no significant polarity dependance
was detected. Similar absorptioneemission spectra were observed
in cyclohexane, DCM and acetonitrile. All synthesized compounds
showed Stokes shift in the range of 15e30 nm which could be due
to mechanisms such as intersystem crossing, migration of excitons
along the polymer chains and configurational relaxation of excited
states, as reported by other research groups working on similar
Thermal properties of polymers were examined between ꢁ40 ꢀC
and decomposition temperature (w220e300 ꢀC) using a differen-
tial scanning calorimetry (DSC). DSC traces of the polymers did not
show any glass transition or melting before degradation. This
q
6) 3 (-) 4 (,) 5 (C) 6 (B) 7 (+) 8 ( ). DSC traces of (c) 7 and (d) 8.
Fig. 2. Thermograms of polymers (a) and model compounds (b). 1 (:) 2 (

J. Gupta et al. / Polymer 51 (2010) 5078e5086
5083
Fig. 3. Absorption (a, c) and emission (b, d) spectra of the polymers in DCM (0.1 mg/mL, 28 ꢀC) (a, b) and thin film (c, d). 1 (:) 2 (
6) 3 (-) 4 (,) 5 (C) 6 (B).
polymers [16,37,38]. Identification of the exact mechanism is
currently in progress and will be reported elsewhere.
Fluorescence quantum yields were measured for the compounds
using quinine sulfate as standard. Autoquenching of fluorescence
results in much lower quantum yield for the polymers as compared
to the model compounds (7 and 8). Amongst various polymers,
transoid-series (2, 4, 6) showed higher quantum yield as compared
to their cisoid-(1, 3, 5) analogues. Thin films of polymers and model
Fig. 4. Absorption (a, c) and emission (b, d) spectra of model compound 7 ( ) and 8 ( ) in DCM (10ꢁ5 M, 28 ꢀC) (a, b) and thin film (c, d).
7
;

5084
J. Gupta et al. / Polymer 51 (2010) 5078e5086
Table 2
Optical properties of the target compounds.
Compound
Solutiona
Filmb
labs (nm)
lem (nm)
Band gapc
(eV)
Quantum
yield
Stokes
shift (nm)
CIE coordinatesd
labs (nm)
lem (nm)
1
2
3
4
5
6
7
8
484, 446, 313
450, 407, 295
473, 418, 294
441, 407, 306
453, 430, 299
452, 419, 353
436, 412, 303
436, 412, 303
544, 508
493, 467
494, 470
494, 467
493, 470
497, 470
477, 452
477, 452
2.30
2.43
2.44
2.58
2.53
2.54
2.74
2.74
0.18
0.20
0.14
0.20
0.17
0.20
0.70
0.75
24
17
21
26
17
18
16
16
0.420, 0.437
0.404, 0.420
0.413, 0.430
0.407, 0.422
0.408, 0.431
0.400, 0.417
0.396, 0.416
0.395, 0.413
502, 460, 322
477, 419, 299
505, 434, 306
500, 430, 307
450
444, 301
492, 451, 306
460, 274
554, 518
546
571
555
571
551
538
522
a
Solutions were made in DCM with a concentration of 0.1 mg/mL (polymers 1e6) and 10ꢁ5 M (compounds 7 and 8).
Thin films were prepared by dropcasting concentrated solution of the compounds on quartz plate at room temperature.
Bandgap, Eg ¼ hc/lonset, where h ¼ Plank’s constant, c ¼ velocity of light.
b
c
d
Colour coordinates were measured using chromameter CS 100 A, calibrated with white light.
compounds showed bathochromic shifts, possibly due to close
packing of polymer chains and molecules in the solid state
[15,39,40].
For these compounds, reduction waves were not detectable within
the scan range of ꢁ2 to þ2 V. A completely different scenario was
observed for polymers 3 and 4, where, weak anodic peak was
accompanied with strong cathodic peaks. It is conceivable that
carbazole, being more electron rich compared to pyrene, donates
electron to pyrene [42] and this leads to oxidation of carbazole and
reduction of pyrene during the redox process. Both oxidation and
reduction processes appeared to be irreversible in nature.
3.4. Electrochemical properties
Electrochemical properties of the target compounds were
investigated using cyclic voltammetry (Fig. 5). Acquired data are
summarized in Table 3. Energy levels of highest occupied molecular
orbitals (EHOMO) were calculated from the half-wave potentials of
oxidation peaks. Cyclic voltammograms of polymer 1, 2, 5, 6 and
model compounds 7, 8 showed two irreversible oxidation peaks.
Pyrene, being more electron rich compared to phenyl and fluorene
moieties, acts as electron donor. Hence, the observed anodic peaks
may be assigned to electrochemical oxidation of pyrene units [41].
3.5. Self-assembly
Self-assembly of the synthesized compounds (1e8) were studied
to obtain further insight about several interchain or intermolecular
interactions. Toluene was selected as the solvent medium with the
virtue of its high boiling point and solubilizing effect. Thin films of
6
), (b) 3 (-) and 4 (,), (c) 5 (C) and 6 (B), (d) 7 (+) and 8 ( ). The data were collected at a scan rate of 100 mV sꢁ1 using spin
q
Fig. 5. Cyclic voltammograms of (a) 1 (:) and 2 (
coated films (thickness w50 nm) of the target compounds on ITO substrate and n-Bu4NPF6 in acetonitrile (0.1 M) as electrolyte.

J. Gupta et al. / Polymer 51 (2010) 5078e5086
5085
Table 3
between several non-adjacent units, which is not possible for the
transoid-polymers with rigid rod backbone. Enhanced -stacking
Electrochemical properties of the target compounds.
p
a
1/2b
c
between two non-adjacent pyrene in coil like cisoid-polymers is
expected to show lower optical band gap. Similarly, higher thermal
stability of cisoid-polymers can be attributed to the enhanced bond
strength imparted by extra intra- and interchain interactions
offered by coil morphology. The lower oxidation potentials of
cisoid-polymers again can be attributed to the enhanced intra- and
interchain interactions because, enhanced conjugation is known to
result in lower oxidation potential [45]. SEM images of the drop-
casted films of polymer solution (Fig. 6) showed coiled morphology
of the cisoid-polymers.
Compound
Eox (V)
E
(V)
EHOMO (eV)
Ered (V)
ox
1
2
3
4
5
6
7
8
0.44, 0.95
0.67, 1.14
0.24
0.78, 1.19
0.79, 0.95
0.76, 1.05
0.72, 1.10
0.78, 1.17
0.05
0.42
0.12
0.57
0.54
0.56
0.51
0.56
ꢁ4.35
ꢁ4.72
ꢁ4.42
ꢁ4.87
ꢁ4.84
ꢁ4.86
ꢁ4.81
ꢁ4.86
ꢁ0.51, ꢁ1.35
ꢁ1.33
a
Peak value of the oxidation wave.
Half-wave oxidation potential.
EHOMO ¼ ꢁ(4.3 þ E1ox/2) eV [43,44].
b
c
Similar optical properties of the model compounds (7 and 8)
suggested similar
p-conjugation through 1,8- and 1,6-linkages of
pyrene and difference in physical properties between cisoid- and
transoid-polymers could be due to the rigid shape of the polymer
backbone. Lower optical band gap of polymers 1 and 2 compared to
the corresponding model compounds (7 and 8) may be attributed
to the extended conjugation over the polymer backbone or
secondary interaction between several polymer chains.
The observed effect of spacer groups on optical properties can
also be explained with respect to their role in controlling the shape
of the polymer backbone. Comparison between polymers with
different spacer groups (1, 2 and 3, 4 and 5, 6) revealed the order of
band gap, fluorene > carbazole > phenyl (for 1, 3, 5) or carbazo-
le > fluorene > phenyl (for 2, 4, 6), which may be due to the overall
shape of polymer backbone. The reason for phenyl spacer to show
significant difference in optical properties between cisoid- and
transoid-polymers (1 and 2) may be due to small size and rigid
1,4-linkage, which facilitates more effective interaction between
two pyrenes in comparison with 3,6-disubstituted carbazole and
2,7-disubstituted fluorene.
The observed low band gap for the cisoid-polymers was in
contrast with the results observed for homopolymer of p-phenyl-
eneethynylene(PpPE, [16]) and copolymer of p-phenyleneethynylene
and m-phenyleneethynylene (PmPE, [18]). Absorption wavelength
of PpPE is known to be bathochromically shifted as compared to
PmPE, which may be due to the lack of conjugation in 1,3-linkage
of phenylene as compared to 1,4-linkages along the polymer
backbone.
the target compounds were made by dropcasting 50 mL of the
solution (0.2 mg/mL) on glass plate and allowing to slowly evapo-
rate inside a desiccator. The SEM images of the dropcasted films
of cisoid-polymers showed coiled fiber structure (Fig. 6 and
Supplementary material Fig. S14), whereas, no well defined
morphologies were found for the transoid-analogues and the model
compounds. AFM image of similarly prepared film on ITO surface
(Supplementary material Fig. S15) provided evidence for the thick-
ness of the fiber as w50e200 nm.
4. Discussion
The results obtained from thermal, optical, electrochemical and
self-assembly studies of the cisoid-, transoid-polymers and the
model compounds suggest direct correlation between the shape of
polymer backbone and physical properties. Cisoid-polymers (1, 3, 5)
were found to be thermally more stable as compared to their
transoid-analogues (2, 4, 6) (Table 1). Decomposition temperature
corresponding to 5% weight loss (T5, above 200 ꢀC) was considered
for the comparison purpose. A careful examination on the optical
data revealed that absorption wavelength of cisoid-polymers is
significantly red shifted as compared to their transoid-analogues.
However, the model compounds (7 and 8) showed similar optical
properties (Table 2). Results of cyclic voltammetry studies found
EHOMO of cisoid-polymers to be less negative as compared to trans-
oid-polymers (Table 3). Although, the difference between model
compounds is marginal.
5. Conclusion
The twisted backbone of cisoid-polymers forces the polymer
chain to coil and facilitates intra- and interchain interactions
A new family of fluorescent conjugated polymers, poly(pyr-
eneethynylene)s, have been synthesized by Sonogashira coupling
of 1,8- or 1,6-diethynyl pyrene with dihaloderivatives of alkox-
ybenzene, carbazole and fluorene. The resultant polymers with
differently shaped backbone were characterized using NMR, IR,
MALDI-TOF and GPC. Detailed investigation about their optical,
electrochemical and thermal properties revealed useful insight
regarding structureeproperty relationship. Significant influence of
shape of polymer backbone was found on the physical properties of
the polymers. Kinked backbone of cisoid-polymers appeared to
contribute towards lower band gap, less negative EHOMO and higher
thermal stability. A plausible explanation for such behaviour has
been hypothesized with the formation of coil and rod structures by
cisoid- and transoid-polymers, respectively. Added evidence of
coiling of cisoid-polymers was obtained from SEM images of the
dropcasted polymer films. Two model compounds were synthe-
sized to check the extent of conjugation over 1,8- and 1,6-position
of pyrene. The observed differences in physical properties between
cisoid- and transoid-polymers is solely due to the shape of polymer
backbone. These findings are significant as they enrich the
conceptual knowledge required for designing performance specific
polymers.
Fig. 6. SEM image of polymer 1 showed coiled fiber structure.

5086
J. Gupta et al. / Polymer 51 (2010) 5078e5086
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Authors acknowledge funding support from Agency for Science,
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from Department of Chemistry, National University of Singapore.
We also acknowledge the help and suggestions from Professor
S. Sankararaman, Indian Institute of Technology Madras, India. JG
thanks National University of Singapore and Ministry of Education,
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