Synthesis, characterization, and structure–property investigation of conformationally rigid regioisomers of poly(p-phenylene ethynylene)s

Article abstract of DOI:10.1002/pola.28255

A series of rigid poly(p-phenylene ethynylene)s (PPE1–PPE4) with biphenyl- (M1–M3) and phenyl- (M4) side groups is prepared from appropriately functionalized monomers. Herein, the solution and solid state absorption studies show the polymers have adopted

Full text of DOI:10.1002/pola.28255

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Synthesis, Characterization, and Structure–Property Investigation of
Conformationally Rigid Regioisomers of Poly(p-phenylene ethynylene)s
Choong Ping Sen, Suresh Valiyaveettil
Department of Chemistry, National University of Singapore, 117543 Singapore, Singapore
Correspondence to: S. Valiyaveettil (E-mail: chmsv@nus.edu.sg)
Received 25 June 2016; accepted 2 August 2016; published online 00 Month 2016
DOI: 10.1002/pola.28255
ABSTRACT: A series of rigid poly(p-phenylene ethynylene)s
PPE1–PPE4) with biphenyl- (M1–M3) and phenyl- (M4) side
examined using scanning electron microscopy. The fibers are
not observed in PPE4 with short phenyl side group, suggesting
the important role of the interplay between rigidity, position,
and size of the side chains toward the formation of fibers. VC
2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2016, 00, 000–000
(
groups is prepared from appropriately functionalized mono-
mers. Herein, the solution and solid state absorption studies
show the polymers have adopted twisted and rigid conforma-
tions, as supported by deep HOMO energy levels (25.76 to
25.81 eV). The absorption maxima of PPE1–PPE3 are shifted to
KEYWORDS: conformational analysis; conjugated polymers;
nonplanar; poly(p-phenylene ethynylene)s; rigid; self-assembly;
synthesis
shorter wavelength (kmax 5 375–381 nm) as compared to linear
poly(p-phenylene ethynylene)s (446 nm), implying a nonplanar
conformation. The self-assembly of polymers into fibers is
2
1,22
INTRODUCTION Chiral-conjugated polymers have been con-
sidered as a useful material owing to interesting electrical
and electronic properties, flexibility and processability, ability
to form supramolecular structures and unique chiroptical
properties. For example, they could be used as circular polar-
structural rigidity, and solvophobic effects.
On the other
hand, the conformation of a rigid-conjugated polymer back-
bone can be frozen in helix architecture by incorporating bulky
substituents on every repeating units, which prevents relaxa-
2
3,24
tion or free rotation along the chain.
1
,2
ized luminescent materials, chiral catalyst for asymmetric
3
,4
5
6
Herein, we report the synthesis and characterization of a
series of conformationally rigid poly(p-phenylene ethyny-
lene)s (PPEs) and comparison of their properties in detail
synthesis, chiral sensor, and nonlinear optics. The general
approaches for inducing chirality in conjugated polymers
7
–9
include introducing a chiral group on the side chain,
building blocks incorporated on the polymer backbone,
chiral
(
Fig. 1). The PPE backbone is designed to investigate the
1
0,11
or
role of steric hindrance of the biphenyl side chains incorpo-
rated on every repeating units on polymer properties. More-
over, the rotations along polymer backbone are restricted
depending on the position of the alkoxy biphenyl side chains.
The effect of regioregularity of the polymer backbone on chi-
roptical properties is investigated by synthesizing regioirre-
gular PPE1, PPE2, and regioregular PPE3. Symmetrical
PPE4 with shorter phenyl side groups is also synthesized
and properties are compared with unsymmetrical polymers,
PPE1–PPE3. Incorporation of alkyl chain on the biphenyl
group helps to improve solubility of the target polymers.
helical macrostructure assembly induced by interaction
1
2,13
between functionalized polymers with metal ions.
Majority
of the studies were focused on polythiophenes, and poly(p-phe-
nylene ethynylene)s incorporated with chiral functional
1
4
groups. Supramolecular helical structures are induced inside
the polymer lattice by chiral alkoxy side group on polymer
1
4
backbone, or by the “sergeant-and-soldiers” co-assembly
1
5–18
method,
where the chiral moieties (sergeant) decide the
overall chirality of the polymer conformation mediated through
nonchiral (soldiers) moieties. Other approaches were used to
enhance the overall chirality of poly(phenylene ethynylene)s
via structural modifications, including polymerization through
1
9
EXPERIMENTAL
meta-positions of benzene rings, or introduction of branching
2
0
on polymer backbone. It has been demonstrated that poly(m-
phenylene ethynylene)s or oligo(m-phenylene ethynylene)s
thermodynamically fold into helical conformation due to
Materials
All chemicals and reagents were purchased from commercial
sources (Sigma Aldrich, Strem Chemical, Alfa Aesar and TCI)
Additional Supporting Information may be found in the online version of this article.
2016 Wiley Periodicals, Inc.
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C
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were acquired on a Jasco J-810 CD spectropolarimeter using
a quartz cuvette with 1 mm light path.
Synthesis of Monomers and Polymers
1-(2-Ethylhexyloxy)24-Iodobenzene (3a)
A stirred solution of 4-iodophenol (2.56 g, 11.6 mmol), 2-
ethylhexyl bromide (2.70 g, 14.0 mmol), and potassium car-
bonate (3.30 g, 23.9 mmol) in DMF (80 mL) were added and
2
5
heated to 90 8C for 18 h. The reaction mixture was cooled,
diluted with water (50 mL) and extracted with diethyl ether
(
2 3 100 mL). The combined organic fractions were washed
with water (2 3 50 mL), followed by brine solution and
water, dried over anhydrous sodium sulfate, filtered, and
organic filtrate was concentrated under reduced pressure.
The crude liquid was purified on a silica gel column using a
mixture of 5% ethyl acetate in hexane as eluent to afford a
FIGURE 1 Schematic representation and molecular structures
of different poly(p-phenylene ethynylene) (PPE1–PPE4). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
1
colorless liquid. Yield 5 2.78 g (72%), H NMR (300 MHz,
CDCl , d ppm) 7.54 (d, 2H, J 5 6.81 Hz), 6.67 (d, 2H, J 5 6.81
3
and used without further purification. All reactions were
monitored by thin-layer chromatography (TLC) carried out
on silica gel plates. Preparative separations were performed
by column chromatography on a silica gel grade 60 (0.040–
Hz), 3.79 (d, 2H, J 5 5.71 Hz), 1.70 (m, 1H), 1.51–1.28 (m,
1
3
8
1
2
3
H), 0.94–0.88 (m, 6H). C NMR (75 MHz, CDCl , d ppm)
59.26, 138.10, 116.96, 82.26, 70.61, 39.27, 30.46, 29.03,
3.80, 23.01, 14.05, 11.06. MS (EI): M (C H IO) Calculat-
1
0.063 mm) from Silicycle.
14 21
ed m/z 5332.0, Found m/z 5 332.1.
Instrumentation
H NMR and C NMR spectra were recorded on a Bruker
0
1
13
4-Bromo-4 -(2-Ethylhexyloxy)Biphenyl (4a)
Compound 3a (1.70 g, 5.11 mmol), 4-bromophenylboronic
Avance AV300 (300 MHz), a Bruker Avance AV400 (400
MHz), or a Bruker Avance AV500 (500 MHz) NMR instru-
ments using appropriate deuterated solvents from Cam-
bridge Isotope Laboratories. The chemical shifts were
reported in part per million or ppm and referenced to the
residual solvent peak: s 5 singlet, d 5 doublet, t 5 triplet,
m 5 multiplet, and br 5 broad. Electron Impact mass spec-
troscopy (EI–MS) mass spectra were obtained on a Finnigan
TSQ7000. Atmospheric-pressure chemical ionization (APCI)
mass spectra (APCI-MS) were obtained on Bruker MicroTOF-
QII. UV–visible spectra were measured on a UV-1800 Shi-
madzu UV–VIS spectrophotometer with an optical filter that
is calibrated at a bandwidth of 1 nm using a quartz cuvette
with 10 mm light path. Emission spectra were measured on
an Agilent Cary Eclipse Fluorescence Spectrophotometer
with respective analytical grade solvents. Cyclic voltammo-
grams were recorded with a computer controlled CHI elec-
trochemical analyzer at a constant scan rate of 100 mV/s.
The potentials were calibrated using ferrocene/ferocenium
ion redox couple as internal reference. The onset of oxida-
acid (1.24 g, 5.60 mmol), and PdCl (PPh ) (0.18 g, 0.25
mmol) were added to a predegassed solution mixture of THF
2
3 2
(
12 mL)/ethanol (3 mL)/2M aqueous
K
2
CO
3
solution
(
10 mL) under nitrogen atmosphere. The reaction mixture
was heated to reflux for 20 h, cooled, concentrated, diluted
with water (25 mL), and extracted with ethyl acetate (2 3
50 mL). The combined organic fractions were washed with
water (2 3 15 mL), followed by brine solution and water,
dried over anhydrous sodium sulfate, filtered, and organic fil-
trate was concentrated under reduced pressure. The crude
product was purified on silica gel column using a mixture of
5
% ethyl acetate in hexane as eluent to afford a colorless liq-
1
uid. Yield 5 1.16 g (63%), H NMR (300 MHz, CDCl , d ppm)
3
7
.52 (d, 2H, J 5 8.68 Hz), 7.47 (d, 2H, J 5 8.85 Hz), 7.40 (d,
H, J 5 8.68 Hz), 6.96 (d, 2H, J 5 8.85 Hz), 3.88 (d, 2H,
J 5 5.78 Hz), 1.75 (m, 1H), 1.50–1.30 (m, 8H), 0.96–0.85 (m,
2
13
6
1
3
H). C NMR (75 MHz, CDCl , d ppm) 159.25, 139.84,
3
32.16, 131.75, 128.26, 127.89, 120.67, 114.90, 70.59, 39.37,
0.52, 29.69, 29.07, 23.85, 23.05, 14.08, 11.10. HR-MS (EI):
1
M
(C H BrO) Calculated m/z 5360.1083, Found m/
20 25
onset
onset
tion (Eox ) and reduction (Ered ) were used to estimate
z 5 360.1090. Anal. Calcd. for C20H25BrO: C, 66.48; H, 6.97.
Found: C, 66.37; H, 7.01%.
HOMO (EHOMO) and LUMO (ELUMO) energy levels of polymers
onset
using
the
equation
EHOMO 5 2(4.8 1 Eox
)
and
onset
0
ELUMO 5 2(4.8 1 Ered ). Thermogravimetric analysis (TGA)
2-(4 -(2-Ethylhexyloxy)-Biphenyl-4-Yl)24,4,5,5-Tetramethyl-
1,3,2-Dioxaborolane (5a)
was recorded under nitrogen atmosphere at heating rate of
10 8C/min on a TA Instruments 2960. All samples (3 mg)
Compound 4a (1.56 g, 4.31 mmol), bis(pinacolato)diboron
were dried at 50 8C under vacuum for 24 h to remove resid-
ual moisture prior to thermal analysis. Scanning electron
micrograph was recorded on a JEOL JSM-6701F field emis-
sion scanning electron microscope. Optical rotations were
acquired on a ADP 440 Polarimeter using a 10-cm polarime-
ter tube at 589 nm at 20 8C. Circular dichroism (CD) spectra
(3.30 g, 13.00 mmol), Pd(dppf)Cl (0.15 g, 0.18 mmol), and
2
KOAc (2.55 g, 25.96 mmol) were added to a round bottom
flask under nitrogen atmosphere. 1,4-Dioxane (35 mL) was
added to the mixture via syringe and stirred at 80 8C for
20 h, cooled, concentrated, diluted with water (25 mL) and
extracted with ethyl acetate (2 3 50 mL). The combined
2
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organic fractions were washed with water (2 3 20 mL), fol-
lowed by brine solution and water, dried over anhydrous
sodium sulfate, filtered, and organic filtrate was concentrated
under reduced pressure. The crude product was purified on
a silica gel column using a mixture of 5% ethyl acetate in
1.70 (s, 3H), 1.65 (m, 1H), 1.62 (s, 3H), 1.40 (m, 1H), 1.26
1
3
(m, 2H), 0.98 (d, 3H, J 5 6.45 Hz). C NMR (75 MHz, CDCl3,
d ppm) 158.98, 139.81, 132.26, 131.75, 131.30, 128.25,
127.91, 124.65, 120.69, 114.91, 66.41, 37.12, 36.12, 29.55,
1
25.69, 25.45, 19.55, 17.65. HR-MS (EI): M (C H BrO) Cal-
2
2 27
hexane as eluent to afford a colorless sticky liquid. (1.29 g,
culated m/z 5386.1245, Found m/z 5 386.1244. Anal. Calcd.
for C H BrO: C, 68.22; H, 7.03. Found: C, 68.32; H, 6.93%.
22 27
1
yield 73%) H NMR (500 MHz, CDCl , d ppm) 7.85 (d, 2H,
3
J 5 8.20 Hz), 7.57 (d, 2H, J 5 8.21 Hz), 7.54 (d, 2H, J 5 8.77
Hz), 6.97 (d, 2H, J 5 8.77 Hz), 3.89 (d, 2H, J 5 5.64 Hz), 1,75
0
2
-(4 -[(S)23,7-Dimethyl-Oct-6-Enyloxy]-Biphenyl-4-
Yl)24,4,5,5-Tetramethyl-1,3,2-Dioxaborolane (5b)
Compound 4b (1.33 g, 3.43 mmol), bis(pinacolato)diboron
3.50 g, 13.78 mmol), Pd(dppf)Cl (0.17 g, 0.21 mmol) and
2
KOAc (2.00 g, 20.37 mmol) were added to a round bottom
flask under nitrogen atmosphere. 1,4-Dioxane (12 mL) was
added via syringe and the reaction mixture was stirred at 80
(
m, 1H), 1.51–1.26 (m, 8H), 1.36 (s, 12H), 0.96–0.90 (m, 6H).
1
3
C NMR (75 MHz, CDCl , d ppm) 159.19, 143.51, 135.16,
3
(
1
33.11, 128.09, 125.87, 114.74, 83.66, 70.52, 39.33, 30.47,
1
29.01, 24.80, 23.81, 22.98, 14.00, 11.04. HR-MS (EI): M
(
C H BO ) Calculated m/z 5408.2830, Found m/
26 37 3
3
z 5 408.2836. Anal. Calcd. for C26H37BO : C, 76.47; H, 9.13.
Found: C, 75.99; H, 9.03%.
8
C for 20 h. The reaction mixture was cooled, concentrated,
diluted with water (50 mL) and extracted with ethyl acetate
(50 mL 3 2). The combined organic layers were washed
with water twice (25 mL 3 2), followed by brine solution
and water, dried over anhydrous sodium sulfate, filtered, and
organic filtrate was concentrated under reduced pressure.
The crude product was purified on a silica gel column using
a mixture of 5% ethyl acetate in hexane as eluent to afford a
1
-[(S)23,7-Dimethyl-Oct-6-Enyloxy]-4-Iodobenzene (3b)
A stirred solution of 4-iodophenol (4.86 g, 22.1 mmol), (S)-
1)-citronellyl bromide (5.80 g, 26.5 mmol) and potassium
(
carbonate (6.10 g, 44.1 mmol) in DMF (40 mL) was heated
to 90 8C for 18 h, cooled, concentrated, diluted with water
(
50 mL) and extracted with diethyl ether (50 mL x 2). The
1
combined organic fractions were washed with water (20 mL
x 2), followed by brine solution and water, dried over anhy-
drous sodium sulfate, filtered and organic filtrate was con-
centrated under reduced pressure. The crude liquid was
purified on a silica gel column using a mixture of 5% ethyl
pale-yellowish sticky liquid. Yield 5 0.98 g (66%), H NMR
(300 MHz, CDCl , d ppm) 7.86 (d, 2H, J 5 8.28 Hz), 7.56 (d,
3
2H, J 5 8.28 Hz), 7.55 (d, 2H, J 58.83 Hz), 6.97 (d, 2H,
J 5 8.83 Hz), 5.12 (t, 1H, J 5 7.11 Hz), 4.03 (t, 2H, J 5 6.83
Hz), 2.00 (m, 2H), 1.85 (m, 1H), 1.69 (s, 3H), 1.65 (m, 1H),
acetate in hexane as eluent to give a colorless liquid.
1
.61 (s, 3H), 1.42 (m, 1H), 1.35 (s, 12H), 1.26 (m, 2H), 0.97
1
1
3
Yield 5 5.86 g (74%), H NMR (300 MHz, CDCl , d ppm)
3
(d, 3H, J 5 6.45 Hz). C NMR (75 MHz, CDCl , d ppm)
3
7
1
.53 (d, 2H, J 5 8.99 Hz), 6.68 (d, 2H, J 5 8.99 Hz), 5.10 (t,
H, J 5 7.11 Hz), 3.94 (t, 2H, J 5 6.81 Hz), 2.03 (m, 2H), 1.81
m, 1H), 1.69 (s, 3H), 1.68–1.65 (m, 1H), 1.60 (s, 3H), 1.37
1
1
2
58.96, 143.53, 135.22, 133.26, 131.28, 128.18, 125.93,
24.66, 114.80, 83.73, 66.36, 37.12, 36.13, 29.53, 25.70,
1
(
(
(
5.45, 24.85, 19.55, 17.65. HR-MS (EI): M (C H BO ) Cal-
2
8
39
3
1
3
m, 1H), 1.23 (m, 2H), 0.95 (d, 3H, J 5 6.46 Hz). C NMR
75 MHz, CDCl , d ppm) 158.97, 138.11, 131.31, 124.58,
culated m/z 5434.2992, Found m/z 5 434.3006. Anal. Calcd.
for C H BO : C, 77.41; H, 9.05. Found: C, 77.46; H, 8.94%.
3
2
8
39
3
116.92, 82.37, 66.38, 37.07, 35.96, 29.46, 25.69, 25.41,
1
19.50, 17.64. HR-MS (EI): M (C H IO) Calculated m/z
1-(Octyloxy)24-Iodobenzene (6)
1
6 23
5
358.0794, Found m/z 5 358.0788. Anal. Calcd. for
A stirred solution of 4-iodophenol (1.32 g, 6.0 mmol), 1-
bromooctane (1.61 g, 8.3 mmol), and potassium carbonate
C
16
H
23IO: C, 53.64; H, 6.47. Found: C, 53.73; H, 6.41%.
(
1
1.65 g, 11.9 mmol) in DMF (15 mL) was heated to 90 8C for
8 h. The reaction mixture was cooled, concentrated, dilut-
0
2
6
4
-Bromo-4 -[(S)23,7-Dimethyl-Oct-6-Enyloxy]Biphenyl (4b)
Compound 3b (3.84 g, 10.72 mmol), 4-bromophenylboronic
acid (2.80 g, 13.94 mmol) and PdCl (PPh ) (0.49 g, 0.70
mmol) were added to a predegassed solution of THF
ed with water (25 mL), and extracted with diethyl ether
(30 mL 3 2). The combined organic layers were washed
with water (20 mL 3 2), followed by brine solution and
water, dried over anhydrous sodium sulfate, filtered, and
organic filtrate was concentrated under reduced pressure.
The crude liquid was purified on a silica gel column using
2
3 2
(
(
20 mL)/ethanol (5 mL)/2M aqueous K2CO solution
15 mL) under nitrogen atmosphere. The reaction mixture
3
was refluxed for 20 h, cooled, concentrated, diluted with
water (25 mL), and extracted with ethyl acetate (50 mL 3
hexane as eluent to afford a colorless liquid. Yield 5 1.45 g,
2). The combined organic fractions were washed with water
1
(
73%), H NMR (300 MHz, CDCl
3
) d 7.54 (d, 2H, J 5 9.00
(
50 mL), followed by brine solution and water, dried over
Hz), 6.66 (d, 2H, J 5 9.00 Hz), 3.91 (t, 2H, J 5 6.58 Hz), 1.74
m, 2H), 1.43 (m, 2H), 1.30 (m, 8H), 0.89 (t, 3H, J 5 6.80 Hz).
anhydrous sodium sulfate, filtered and organic filtrate was
concentrated under reduced pressure. The crude product
was purified on a silica gel column using a mixture of 5%
(
2-(4-Octyloxyphenyl)24,4,5,5-Tetramethyl-1,3,2-
Dioxaborolane (7)
Compound 6 (0.89 g, 2.68 mmol), bis(pinacolato)diboron
ethyl acetate in hexane as eluent to afford a white solid.
1
Yield 5 2.11 g (51%), H NMR (300 MHz, CDCl , d ppm)
3
7.53 (d, 2H, J 5 8.62 Hz), 7.48 (d, 2H, J 5 8.81 Hz), 7.42 (d,
(1.02 g, 4.00 mmol), Pd(dppf)Cl (0.10 g, 0.12 mmol), and
2
2
H, J 5 8.62 Hz), 6.97 (d, 2H, J 5 8.81 Hz), 5.12 (t, 1H, J 57.1
KOAc (1.57 g, 15.99 mmol) were added to a round bottom
flask under nitrogen atmosphere. 1,4-Dioxane (20 mL) was
Hz), 4.03 (t, 2H, J 5 6.83 Hz), 2.03 (m, 2H), 1.85 (m, 1H),
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added via syringe and the reaction mixture was stirred at 80
Hz), 2.03 (m, 2H), 1.86 (m, 1H), 1.70 (s, 3H), 1.66 (m, 1H),
8
C for 20 h. The reaction mixture was cooled, concentrated,
diluted with water (15 mL) and extracted with ethyl acetate
25 mL 3 2). The combined organic layers were washed
1.62 (s, 3H), 1.42 (m, 1H), 1.26 (m, 1H), 0.98 (d, 3H, J 5 6.44
1
3
Hz). C NMR (75 MHz, CDCl , d ppm) 158.95, 144.13,
3
(
140.61, 138.00, 134.49, 133.99, 132.74, 131.57, 131.29,
129.59, 128.10, 126.30, 124.67, 121.34, 121.18, 114.88,
66.40, 37.14, 36.16, 29.57, 25.70, 25.46, 19.57, 17.66. HR-MS
with water (15 mL 3 2), followed by brine solution, dried
over anhydrous sodium sulfate, filtered and organic filtrate
was concentrated under reduced pressure. The crude prod-
uct was purified on a silica gel column using a mixture of
1
(EI): M (C H Br O) Calculated m/z 5540.0663, Found
2
8
30
2
m/z 5 540.0667. Anal. Calcd. for C H Br O: C, 62.01; H,
2
8
30
2
1
5% DCM in hexane as eluent to afford a pale-yellowish
5.58. Found: C, 61.93; H, 5.51%.
1
sticky liquid. Yield 5 0.71 g (80%), H NMR (300 MHz,
CDCl , d ppm) 7.73 (d, 2H, J 5 8.68 Hz), 6.88 (d, 2H, J 5 8.68
Compound 8c
3
Hz), 3.97 (t, 2H, J 5 6.61 Hz), 1.78 (m, 2H), 1.55 (m, 2H),
Compound 7 (0.38 g, 1.14 mmol), compound 2 (0.24 g, 0.49
V
R
1
.33 (s, 12H), 1.31 (m, 8H), 0.88 (t, 3H, J 5 6.95 Hz). HR-MS
mmol), Aliquat 336 (2 mL), and Pd(PPh
mmol) were added to a predegassed mixture of THF (8 mL)
and 2 M aqueous K CO solution (3 mL) under nitrogen
3 4
) (0.03 g, 0.025
1
(
EI): M (C H BO ) Calculated m/z 5332.2522, Found m/
20 33 3
z 5 332.2531. Anal. Calcd. for C20
H
33BO
3
: C, 72.29; H, 10.01.
2
3
Found: C, 72.25; H, 9.94%.
atmosphere. The reaction mixture was refluxed for 24 h. The
work up procedure is same as described for 8a. The crude
product was purified on a silica gel column using hexane as
Compound 8a
Compound 5a (0.79 g, 1.93 mmol), compound 1 (0.70 g,
eluent to afford a colorless sticky liquid. Yield 5 0.45 g
1
0
.93 mmol), AliquatVR 336 (3 mL), and Pd(PPh3)4 (0.06 g,
.05 mmol) were added to a predegassed mixture of THF
1
(
61%), H NMR (300 MHz, CDCl ) d 7.61 (s, 2H), 7.36 (d,
3
4H, J 5 8.79 Hz), 6.96 (d, 4H, J 5 8.79 Hz), 4.01 (t, 4H,
(
22 mL) and 2M aqueous K CO solution (18 mL) under
2 3
J 5 6.55 Hz), 1.82 (m, 4H), 1.48 (m, 4H), 1.33 (m, 16H), 0.90
nitrogen atmosphere. The reaction mixture was refluxed for
4 h, cooled, concentrated, diluted with water (15 mL) and
13
(
t, 6H, J 5 6.91 Hz). C NMR (75 MHz, CDCl ) d 159.05,
3
2
1
2
42.24, 135.19, 131.66, 130.48, 121.53, 114.05, 68.07, 31.83,
extracted with ethyl acetate (20 mL 3 2). The combined
organic fractions were washed with water (15 mL), followed
by brine solution, dried over anhydrous sodium sulfate, fil-
tered and organic filtrate was concentrated under reduced
pressure. The crude product was purified on a silica gel col-
1
9.37, 29.25, 26.08, 22.67, 14.11. HR-MS (EI):
M
(
C
34
H
44Br
2
O
2
)
Calculated m/z 5 642.1708, Found m/
2 2
z 5 642.1703. Anal. Calcd. for C34H44Br O : C, 63.36; H, 6.88.
Found: C, 63.38; H, 6.82%.
umn using hexane as eluent to afford a colorless sticky liq-
Monomer M1
1
uid. Yield 5 0.71 g (72%), H NMR (300 MHz, CDCl , d ppm)
3
3 4
Compound 8a (0.33 g, 0.64 mmol), Pd(PPh ) (0.06 g, 0.05
mmol), and CuI (0.01 g, 0.06 mmol) were dissolved in a pre-
degassed solution of THF (3 mL) and diisopropylamine
7.62 (d, 2H, J 5 8.48 Hz), 7.56 (d, 2H, J 5 8.9 Hz), 7.54 (d,
1H, J 5 8.51 Hz), 7.52 (d, 1H, J 5 2.41 Hz), 7.43 (d, 2H,
J 5 8.48 Hz), 7.33 (dd, 1H, J 5 8.51, 2.45 Hz), 7.00 (d, 2H,
J 5 8.9 Hz), 3.90 (d, 2H, J 5 5.68 Hz), 1.76 (m, 1H), 1.51–1.28
(
3 mL) under nitrogen atmosphere, followed by addition of
ethynyltrimethylsilane (0.31 g, 3.19 mmol) via syringe. The
reaction mixture was stirred at 80 8C for 24 h, cooled and
concentrated under reduced pressure. The crude product
was purified on a silica gel column using a mixture of 5%
DCM in hexane as eluent to afford a colorless sticky liquid
1
3
(
m, 8H), 0.98–0.92 (m, 6H). C NMR (75 MHz, CDCl , d
3
ppm) 159.23, 144.12, 140.64, 137.96, 134.49, 133.99,
1
1
2
32.65, 131.57, 129.59, 128.08, 126.31, 121.34, 121.18,
14.87, 77.42, 77.00, 76.58, 70.60, 39.41, 30.55, 29.09,
1
3.89, 23.05, 14.08, 11.12. HR-MS (EI): M (C26
H
28Br
2
O)
(
(
0.31 g, yield 89%), which was dissolved in DCM/MeOH
10 1 10 mL) followed by addition of K CO3 (0.31 g, 2.25
Calculated m/z 5514.0501, Found m/z 5 514.0502. Anal.
2
Calcd. for C H Br O: C, 60.48; H, 5.47. Found: C, 60.19; H,
2
6
28
2
mmol). The reaction mixture was stirred overnight at room
temperature, diluted with DCM (20 mL), washed with water
5.53%.
(
15 mL), followed by brine solution and water, dried over
Compound 8b
Compound 5b (0.31 g, 0.71 mmol), compound 1 (0.23 g,
anhydrous sodium sulfate, filtered and organic filtrate was
concentrated under reduced pressure. The crude product
was purified on a silica gel column using 5% DCM in hexane
V
R
0.64 mmol), Aliquat 336 (1 mL), and Pd(PPh3)4 (0.02 g,
.017 mmol) were added to a predegassed mixture of THF
0
as eluent to afford a colorless sticky liquid. Yield 5 0.21 g
(
10 mL) and 2M aqueous K
nitrogen atmosphere. The reaction mixture was refluxed for
4 h. The work up procedure is same as described for 8a.
2 3
CO solution (5 mL) under
1
(92%), H NMR (400 MHz, CDCl
, d ppm) 7.67 (d, J 5 1.3 Hz,
3
2
4H), 7.64–7.59 (m, 4H), 7.45 (dd, J 5 7.9, 1.7 Hz, 1H), 7.04
(d, J 5 8.7 Hz, 2H), 3.94 (d, J 5 5.8, 2H), 3.22 (s, 1H), 3.20 (s,
The crude product was purified on a silica gel column using
a mixture of 5% DCM in hexane as eluent to afford a color-
less sticky liquid. Yield 5 0.24 g, (63%), H NMR (300 MHz,
1
3
1H), 1.81 (m, 1H), 1.63–1.36 (m, 8H), 0.99 (m, 6H). C NMR
(101 MHz, CDCl , d ppm) 159.22, 144.18, 140.46, 137.50,
1
3
CDCl , d ppm) 7.62 (d, 2H, J 5 8.49 Hz), 7.58 (d, 2H, J 5 8.82
Hz), 7.54 (d, 1H, J 5 8.48 Hz), 7.52 (d, 1H, J 5 2.45 Hz), 7.44
133.99, 133.19, 132.87, 130.43, 129.57, 128.12, 126.36,
122.85, 120.96, 114.93, 83.08, 82.76, 82.15, 79.23, 70.65,
39.49, 30.63, 29.18, 23.97, 23.14, 14.18, 11.22. MS (APCI,
3
(d, 2H, J 5 8.49 Hz), 7.33 (dd, 1H, J 5 8.48, 2.45 Hz), 7.00 (d,
1
2H, 8.82 Hz), 5.12 (t, 1H, J 5 7.11 Hz), 4.05 (t, 2H, J 5 6.83
1ve): [M 1 H] (C H O) Calculated m/z 5 407.24, Found
3
0 31
4
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m/z 5 407.33. Anal. Calcd. for C H O: C, 88.63; H, 7.44.
J 5 8.25, 2.07 Hz), 7.00 (d, 2H, J 5 8.81 Hz), 3.91 (d, 2H),
3
0 30
Found: C, 85.49; H, 7.21%.
3.14 (s, 1H), 1.79–1.73 (m, 1H), 1.58–1.34 (m, 8H), 0.99–
1
3
0
1
1
.89 (m, 6H). C NMR (75 MHz, CDCl , d ppm) 159.17,
42.49, 140.45, 138.31, 134.68, 133.26, 132.71, 131.91,
29.64, 128.06, 126.25, 123.39, 121.53, 114.85, 82.46, 78.39,
3
Monomer M2
Compound 8b (0.19 g, 0.35 mmol), Pd(PPh ) (0.03 g, 0.028
mmol), and CuI (6.7 mg, 0.035 mmol) were dissolved in a
predegassed solution of THF (3 mL) and diisopropylamine
3
4
7
0.58, 39.40, 30.54, 29.68, 29.08, 23.87, 23.05, 14.08, 11.11.
1
HR-MS (EI): M (C H BrO) Calculated m/z 5460.1396,
2
8 29
(
3 mL) under nitrogen atmosphere, followed by addition of
Found m/z 5 460.1402. Anal. Calcd. for C H BrO: C, 72.88;
2
8 29
ethynyltrimethylsilane (0.17 g, 1.75 mmol) via syringe. The
reaction mixture was stirred at 80 8C for 24 h, cooled and
concentrated under reduced pressure. The crude product
was purified on a silica gel column using a mixture of 10%
DCM in hexane as eluent to afford a colorless sticky liquid
H, 6.33. Found: C, 72.59; H, 6.30%.
Monomer M4
Compound 8c (0.56 g, 0.87 mmol), Pd(PPh3)4 (0.08 g, 0.07
mmol), and CuI (0.016 g, 0.087 mmol) were dissolved in a
predegassed solution of THF (8 mL) and diisopropylamine
(8 mL) under nitrogen atmosphere, followed by addition of
ethynyltrimethylsilane (0.43 g, 4.35 mmol) via syringe. The
reaction mixture was stirred at 80 8C for 24 h, cooled and
concentrated under reduced pressure. The crude product
was purified on a silica gel column using a mixture of 5%
DCM in hexane as eluent to afford a white solid (0.49 g, yield
84%). The white solid (0.16 g, 0.24 mmol) was dissolved in
(
(
0.14 g, yield 71%), which was dissolved in DCM/MeOH
5 1 5 mL) followed by addition of K CO (0.13 g, 0.94
2
3
mmol). The reaction mixture was stirred overnight at room
temperature, diluted with DCM (10 mL), washed with water
(
15 mL), followed by brine solution and water, dried over
anhydrous sodium sulfate, filtered and organic filtrate was
concentrated under reduced pressure. The crude product
was purified on a silica gel column using 10% DCM in hex-
ane as eluent to afford
Yield 5 0.09 g (86%), H NMR (400 MHz, CDCl , d ppm)
a
colorless sticky liquid.
DCM/MeOH (5 1 5 mL) followed by addition of K CO3
2
1
3
(0.13 g, 0.94 mmol). The reaction mixture was stirred over-
night at room temperature, diluted with DCM (10 mL),
washed with water (15 mL), followed by brine solution and
water, dried over anhydrous sodium sulfate, filtered and
organic filtrate was concentrated under reduced pressure.
The crude product was purified on a silica gel column using
7
.66 (m, 4H), 7.63–7.58 (m, 4H), 7.44 (dd, J 5 7.9, 1.7 Hz,
H), 7.02 (d, J 5 8.7 Hz, 2H), 5.17 (t, J 5 7.1 Hz, 1H), 4.08 (m,
J 5 9.3 Hz, 2H), 3.21 (s, 1H), 3.19 (s, 1H), 2.15–2.01 (m, 2H),
1
1.95–1.86 (m, 2H), 1.74 (s, 3H), 1.68 (s, 3H), 1.51–1.42 (m,
1
3
1H), 1.34–1.24 (m, 2H), 1.02 (d, J 5 6.6 Hz, 3H). C NMR
(
1
1
101 MHz, CDCl , d ppm) 158.84, 144.07, 140.32, 137.43,
33.88, 133.08, 132.85, 131.24, 130.33, 129.47, 128.03,
26.25, 124.67, 122.74, 120.86, 114.82, 82.98, 82.65, 82.03,
3
hexane as eluent to afford a white solid. Yield 5 0.10 g
1
(
79%), H NMR (300 MHz, CDCl , d ppm) 7.60 (s, 2H), 7.55
3
(d, J 5 8.7 Hz, 4H), 6.96 (d, J 5 8.7 Hz, 4H), 4.01 (t, J 5 6.5
7
9.12, 66.33, 37.12, 36.14, 29.53, 25.70, 25.45, 19.55, 17.65.
Hz, 4H), 3.14 (s, 2H), 1.87–1.77 (m, 4H), 1.47 (m, 4H), 1.32
(m, 16H), 0.90 (t, J 5 6.7 Hz, 6H). C NMR (75 MHz, CDCl3,
d ppm) 158.99, 142.09, 134.78, 131.11, 130.25, 120.85,
1
1
3
MS (APCI, 1ve): [M 1 H]
5
(C H O) Calculated m/z
32 33
433.25, Found m/z 5 433.42. Anal. Calcd. for C H O: C,
3
2 32
88.85; H, 7.46. Found: C, 89.92; H, 7.19%.
1
2
14.05, 82.89, 81.48, 68.01, 31.82, 29.37, 29.30, 29.25,
6.08, 22.66, 14.10. HR-MS (EI): M (C38H O ) Calculated
46 2
1
Monomer M3
m/z 5534.3498, Found m/z 5 534.3496. Anal. Calcd. for
3 4
Compound 8a (0.30 g, 0.58 mmol), Pd(PPh ) (0.03 g, 0.02
mmol), and CuI (9 mg, 0.05 mmol) were dissolved in a pre-
degassed solution of THF (6 mL) and diisopropylamine
C H O : C, 85.35; H, 8.67. Found: C, 85.46; H, 8.71%.
3
8 46 2
General Polymerization Procedures for PPEs
Sonogashira polymerization
(
6 mL) under nitrogen atmosphere, followed by addition of
ethynyltrimethylsilane (0.06 g, 0.61 mmol) via syringe. The
reaction mixture was stirred at 80 8C for 24 h, cooled and
concentrated under reduced pressure. The crude product
was purified on a silica gel column using a mixture of 5%
DCM in hexane as eluent to afford a colorless sticky liquid
3 4
Monomers, Pd(PPh ) and CuI were added into a sealed
2
7
Schlenk tube under nitrogen atmosphere. A solution of tet-
rahydrofuran/triethylamine (2:1) was degassed with nitro-
gen for 30 min, transferred into the sealed Schlenk tube
under nitrogen atmosphere. The reaction mixture was
refluxed for 48 h under darkness, cooled and concentrated
under reduced pressure. The resulting sticky solid was dis-
solved in minimal amount of chloroform and precipitated
from excess MeOH to afford the crude polymer.
(
(
0.16 g, yield 52%), which was dissolved in DCM/MeOH
5 1 5 mL) followed by addition of K CO3 (0.12 g, 0.86
2
mmol). The reaction mixture was stirred overnight at room
temperature, diluted with DCM (10 mL), washed with water
(
15 mL), followed by brine solution and water, dried over
anhydrous sodium sulfate, filtered, and organic filtrate was
concentrated under reduced pressure. The crude product
Purification of Polymers
The crude polymer was purified using Soxhlet extraction
with various organic solvents. MeOH was used to remove
unreacted monomers and catalyst. Acetone was used to
remove small molecules and oligomers. Chloroform was used
to extract the polymers, concentrated, and precipitated from
excess MeOH to yield the target polymer.
was purified on a silica gel column using hexane as eluent to
1
afford a colorless sticky liquid. Yield 5 0.11 g (80%),
H
NMR (300 MHz, CDCl , d ppm) 7.65 (d, 1H, J 5 8.20 Hz),
3
7.63 (d, 2H, J 5 8.46 Hz), 7.58 (d, 2H, J 5 8.81 Hz), 7.50 (d,
1H, J 5 2.07 Hz), 7.46 (d, 2H, J 5 8.46 Hz), 7.32 (dd, 1H,
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Polymer PPE1
The polymerization of M1 (0.06 g, 0.15 mmol), and com-
pound 8a (0.08 g, 0.15 mmol) in presence of Pd(PPh3)4
(
(
17 mg, 0.015 mmol) and CuI (4 mg, 0.021 mmol) in THF
6 mL) and triethylamine (3 mL) was carried out according
to the Sonogashira polymerization procedure. The purifica-
tion of polymer was done according to the general procedure
1
to yield the yellow powder. Yield 5 0.08 g (72%), H NMR
(
400 MHz, CDCl , d ppm) 7.65 (br, 2H), 7.61–7.51 (br, 3H),
3
7.51–7.44 (br, 2H), 7.44–7.31 (br, 2H), 7.03–6.87 (br, 2H),
3
0
1
.94–3.73 (br, 2H), 1.81–1.66 (br, 1H), 1.63–1.23 (br, 8H),
.95 (br, 6H). IR (KBr, cm ): 3030, 2957, 2926, 2858, 1606,
501, 1467, 1382, 1286, 1246, 1178, 1113, 1036, 900, 824,
2
1
7
93, 691. GPC (THF, polystyrene standard), M : 7500 g/mol;
n
w
M : 13,600 g/mol; PDI: 1.81.
SCHEME 1 Synthetic routes for borylated phenyl compounds
5
a, 5b, and 7. (i) R-Br,
PdCl (PPh , THF/EtOH/2M aqueous K
0 h; (iii) bis(pinacolato)diboron, Pd(dppf)Cl
ane, 80 8C, 20 h.
2 3
K CO , DMF, 90 8C, 18 h; (ii)
Polymer PPE2
2
3
)
2
2
CO
3
solution, reflux,
2
, KOAc, 1,4-diox-
The polymerization of M2 (0.06 g, 0.14 mmol), and com-
pound 8b (0.076 g, 0.14 mmol) in presence of Pd(PPh3)4
2
(
(
16 mg, 0.014 mmol) and CuI (4 mg, 0.021 mmol) in THF
6 mL) and triethylamine (3 mL) was carried out according
1
250, 1179, 1094, 1029, 832, 803. GPC (THF, polystyrene
to the Sonogashira polymerization procedure. The purifica-
tion of polymer was done according to the general procedure
to yield the yellow powder. Yield 5 0.075 g (66%), H NMR
standard), M : 3800 g/mol; M : 4800 g/mol; PDI: 1.24.
n
w
1
RESULTS AND DISCUSSION
(400 MHz, CDCl , d ppm) 7.76–7.66 (br, 1H), 7.66–7.52 (br,
3
4
5
1
1
2
1
8
H), 7.51–7.44 (br, 2H), 7.38 (br, 2H), 7.02–6.88 (br, 2H),
.16–5.05 (m, 1H), 4.09–3.87 (br, 2H), 2.02 (br, 2H), 1.93–
.73 (br, 2H), 1.70 (br, 3H), 1.62 (br, 3H), 1.48–1.36 (br, 1H),
.24 (br, 2H), 1.02–0.87 (br, 3H). IR (KBr, cm ): 3036,
923, 2853, 1608, 1519, 1468, 1390, 1290, 1246, 1176,
094, 1024, 831, 804. GPC (THF, polystyrene standard), Mn:
Synthesis and Characterization
The syntheses of functionalized phenyl and biphenyl starting
materials are shown in Scheme 1.
2
1
Commercially available 4-iodophenol was alkylated with
three different alkyl bromides (2-ethylhexyl bromide, (S)-
(1)-citronellyl bromide, and octyl bromide) using potassium
carbonate as base to afford 3a (72%), 3b (74%), and 6
650 g/mol; M : 13,300 g/mol; PDI: 1.54.
w
Polymer PPE3
(
73%) in good yields. The iodo groups on 3a and 3b were
The polymerization of M3 (0.21 g, 0.45 mmol) in presence
of Pd(PPh3) (0.04 g, 0.03 mmol) and CuI (0.01 g, 0.05
28
used for selective Suzuki-Miyaura coupling
bromophenylboronic acid in presence of PdCl (PPh ) as cat-
with 4-
4
2
3 2
mmol) in THF (15 mL) and triethylamine (7 mL) was carried
out according to the Sonogashira polymerization procedure.
The purification of polymer was done according to the gen-
alyst to afford biphenyl derivatives, 4a and 4b in 63 and
29
51% yields. Subsequently, Miyaura borylation of 4a, 4b,
and 6 with bis(pinacolato)diboron using Pd(dppf)Cl2 as the
catalyst and KOAc as the base gave borylated compounds, 5a
(73%), 5b (66%), and 7 (80%), which were used for further
coupling as described in Scheme 2.
eral procedure to yield the yellow powder. Yield 5 0.12 g
1
(
68%), H NMR (500 MHz, CDCl , d ppm) 7.79–7.30 (br, 6H),
3
7
.24–6.54 (br, 5H), 3.97–3.53 (br, 2H), 1.83–1.25 (br, 9H),
2
1
1
1
5
8
.04–0.69 (br, 6H). IR (KBr, cm ): 3031, 2957, 2926, 2858,
607, 1498, 1463, 1379, 1287, 1246, 1175, 1033, 821, 692,
18. GPC (THF, polystyrene standard), M : 7950 g/mol; M :
The Suzuki-Miyaura coupling of 1,4-dibromo-2-iodobenzene
with one equivalent of 5a or 5b gave the corresponding
compounds 8a or 8b with achiral and chiral alkyl groups in
n
w
660 g/mol; PDI: 1.09.
7
2 and 63% yields, respectively. Similarly, the symmetric
Polymer PPE4
compound 8c was synthesized using Suzuki coupling of 1,4-
diiodo-2,5-dibromobenzene (2) with 7 in 61% yield. All com-
pounds were characterized fully using NMR spectroscopy,
mass spectrometry and elemental analysis (Supporting Infor-
mation, Figs. S1–S22).
The polymerization of M4 (0.034 g, 0.06 mmol), and com-
pound 8c (0.041 g, 0.06 mmol) in presence of Pd(PPh3)4
(
(
6.9 mg, 0.006 mmol) and CuI (1.7 mg, 0.009 mmol) in THF
3 mL) and triethylamine (1.5 mL) was carried out according
to the Sonogashira polymerization procedure. The purifica-
tion of polymer was done according to the general procedure
to yield the yellow powder. Yield 5 0.049 g (75%), H NMR
The bi-functionalized monomers M1, M2, and M4 were pre-
pared by using Sonogashira coupling reactions of compounds
8a, 8b, and 8c with ethynyltrimethylsilane, followed by
deprotection of trimethylsilyl group with potassium carbon-
ate in DCM/MeOH (Scheme 2, yields 61–81%). The
1
(
(
(
500 MHz, CDCl
3
, d ppm) 7.53 (br, 6H), 6.96 (br, 4H), 4.01
br, 4H), 1.81 (br, 4H), 1.48–1.30 (br, 20 H), 0.89 (br, 6H). IR
2
1
KBr, cm ): 3035, 2924, 2853, 1607, 1519, 1467, 1393,
6
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All aromatic protons of M3 were assigned based on their
chemical shifts and couplings (splitting and coupling con-
stant) (Supporting Information, Fig. S29). It is observed that
chemical shift of proton 1 shifts to downfield (from 7.54 to
7.65 ppm) upon substitution with acetylene group. On the
other hand, the chemical shifts of proton 2 and proton 3
showed little changes. Similar changes were also observed
for model compounds (Supporting Information, Scheme
3
0,31
S2).
Hence we propose that M3-isomer 1 is obtained
based on the NMR spectrum of the molecule.
The monomers were polymerized via Sonogashira polymeri-
zation to afford PPEs in good yields (66–75%, Scheme 3).
The crude polymers were purified by using Soxhlet extrac-
tion with a few organic solvents. Methanol extraction (12 h)
was used to remove any unreacted monomers and catalysts.
Acetone extraction (24 h) was used to remove low molecular
weight oligomers. Chloroform extraction (24 h) was used to
extract the target polymers, followed by precipitation from
excess methanol to afford yellow polymers. All polymers are
soluble in common organic solvents such as chloroform, THF,
and toluene. The structures of PPE1–PPE4 were confirmed
1
using H NMR (Supporting Information, Fig. S32–S35) and
molecular weights were estimated using gel permeation
chromatography (GPC) with polystyrene as standards
SCHEME 2 Synthetic approaches to the monomers (M1–M4);
V
R
(
i) Pd(PPh
C, 24 h; (ii) (a) ethynyltrimethylsilane, Pd(PPh
CO , DCM/MeOH, r.t, 18 h.
3
)
4
, THF/2M aqueous K
2
CO
3
solution, Aliquat 336, 80
8
3 4
)
, CuI, THF/
DIPA, 80 8C, 24 h; (b) K
2
3
copolymerization of bifunctionalized monomers, such as, M1
and 8a, gave regioirregular polymer PPE1 as a result of non-
regioselective step growth polymerization (Scheme 3).
In order to prepare regioregular polymer, the monoacetylene
functionalized monomer (M3) was synthesized using Sono-
gashira coupling reaction of 8a with one equivalent of ethy-
nyltrimethylsilane, followed by deprotection of trimethylsilyl
group in an overall yield of 42%. The polymerization of M3
is expected to proceed in a regioselective manner compared
to the nonregioselective polymerization of bi-functionalized
monomers (M1 and M2).
1
H NMR spectrum of isolated product was used to establish
the regioisomer of M3. It is conceivable that the Sonogashira
reaction between compound 8a and ethynyltrimethylsilane
will result in two regioisomers of M3 as shown in Scheme
1
S1. The H NMR data of M3 showed that it is not a mixture
of both regioisomers. If it is a mixture of both regioisomers,
the two different acetylene protons (f) peaks on NMR spec-
trum (Supporting Information, Fig. S27) should be observed
owing to the nonsymmetrical structures between isomer 1
and isomer 2. In addition, proton 1, 2, and 3 of the benzene
ring would have different chemical shifts for both isomer 1
and isomer 2, resulting in a complex NMR spectrum. How-
ever, the NMR spectrum of isolated product showed relative-
ly simple and clear peaks.
SCHEME 3 Polymerization route to afford PPE1–PPE4. (i)
3 4
Pd(PPh ) , CuI, THF/TEA, 80 8C, 48 h.
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TABLE 1 Summary of molecular weights and decomposition temperatures of PPEs
a
b
Polymer
M
n
a (g/mol)
w
M a (g/mol)
Polydispersity Index
Decomposition Temperature (8C)
PPE1
PPE2
PPE3
PPE4
7 500
8 650
7 950
3 800
13 600
13 300
8 660
1.81
1.54
1.09
1.24
403
390
360
340
4 800
a
b
n w
M , M , and polydispersity indexes were determined using GPC with polystyrene as standard in THF. Measured at 5% weight loss of polymer
under nitrogen atmosphere with a heating rate of 10 8C/min from TGA thermograms.
(
Table 1). TGAs were done using a heating rate of 10 8C/min
biphenyl side groups on the PPEs induces high degree of
twisting and freezing of conformation of the polymer back-
bone, resulting in poor p-orbital overlapping and blue-shift
in absorption maxima. In general, red-shifts in absorption
maxima are explained by better conjugation length along
polymer backbone with higher molecular weight. However,
the red-shifts in absorption maxima are limited according to
the concept of effective conjugation length. The theoretical
calculation showed that the effective conjugation length of
linear poly(p-phenylene ethynylene)s is around 10 repeating
units, however, this number decreases to less than 5 for the
up to 700 8C under nitrogen atmosphere (Fig. 2). All poly-
mers showed good thermal stability up to 340 8C.
Photophysical Properties of PPEs
The solution state absorption spectra of PPEs showed an
absorption maximum at 290 nm in chloroform (Fig. 3),
which corresponds to the absorption of terphenyl group, as
compared to the spectra of monomers in chloroform (Sup-
porting Information, Fig. S48). In addition, PPE1, PPE2, and
PPE3 showed shoulder peaks at 380 nm, which are attribut-
ed to the absorption of poly(p-phenylene ethynylene)s poly-
mer backbone. PPE1 and PPE2 exhibited similar absorption
3
4
twisted poly(p-phenylene ethynylene)s backbones.
Such
blue-shift in absorption maximum was reported for similar
(
k
max 5 292 nm; 378–380 nm) and emission maxima
poly(p-phenylene ethynylene)s with bulky and extended side
groups.
3
5–37
(kems 5 443–451 nm), indicating a negligible effect of differ-
ent alkoxy chains on photophysical property. PPE3 showed
an absorption maximum at 380 nm, and emission maximum
at 448 nm in chloroform. In contrast, PPE4 showed absorp-
tion and emission maxima at longer wavelength
It is known that planarization and p-stacking of poly(p-phe-
nylene ethynylene) chains lead to strong red shift of absorp-
3
8
tion maximum in solid state. All polymer thin films were
examined by spectroscopies on quartz plates (Supporting
Information, Fig. S49). PPE1 and PPE2 thin films showed
red shifts in absorption peak edges by 25–30 nm but with
no significant red shift in absorption maxima as compared to
solution states. On the other hand, thin film of regioregular
polymer, PPE3 showed larger red shift (40 nm) in absorp-
tion edge than the regioirregular polymers, PPE1 and PPE2.
(kmax 5 460 nm, kems 5 480 nm) in chloroform. It is conceiv-
able that the observed red-shifts in absorption and emission
maxima of PPE4 are originated from a better p-conjugation
along the polymer backbone of PPE4.
In comparison to the maximum wavelengths of 446 nm and
3
90 nm reported in poly(p-phenylene ethynylene)s with alk-
3
2
33
oxy and alkyl substituents, PPEs showed blue-shifts in
The higher red shift in PPE3 is attributed to its regioregular
arrangement of biphenyl arms along the polymer backbone
which allows better packing and planarization. PPE4 did not
show significant red shift in absorption edge in thin film,
the absorption maxima. This suggests that presence of
FIGURE 2 Thermogravimetric analysis (TGA) curves of PPE1,
PPE2, PPE3, and PPE4 at a heating rate of 10 8C/min under
nitrogen atmosphere. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
FIGURE 3 Absorption spectra (a) and photoluminescence spec-
tra (b) of PPE1 (-᭿-), PPE2 ( ), PPE3 (
), and PPE4 (
) in
chloroform. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
8
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TABLE 2 Summary of optical properties of PPEs in chloroform and thin films
Absorption
Fluorescence
CHCl
3
Thin Film
CHCl
3
Thin Film
a
4
21
21
b
c
Polymer
PPE1
k
max (nm)
e 10 (cm
50, 20
M
)
k
max (nm)
k
onset (nm)
E
solid (eV)
k (nm)
443
k (nm)
501
g
292, 378
290
465
2.66
2.69
2.64
2.56
PPE2
292, 380
67, 28
290
460
470
485
451
504
PPE3
290, 381
39, 7
300
448
518
PPE4
280, 420, 460
13, 8, 8
278, 420, 450
450, 480
520
a
b
e extinction coefficient was calculated by dividing absorbance with concentration (M) and cuvette path length (1 cm).
konset was calculated from
c
the intersection of the tangent lines drawn to the lowest energy absorption edge to the baseline.
E
g
5 1240/konset. The concentrations (molarity on
monomer basis) of PPE1, PPE2, PPE3, and PPE4 are 2.4, 2.0, 2.3, and 3.1 mM, respectively.
1
which suggests that the PPE4 has adopted similar conforma-
tion in solution and in solid state. The effect of interchain p-
stacking in PPE4 is minimized with two phenyl side groups.
On the other hand, the interchain p-interaction is more pro-
nounced in PPE1-PPE3. All photophysical properties of
polymers are summarized in (Table 2).
level (0.45 V vs. Fc/Fc , thus a 25.25 eV for HOMO)
4
0
reported for alkoxy poly(p-phenylene ethynylene)s. No sig-
nificant differences in HOMO energy levels of PPE1–PPE4
were observed. It is conceivable that the highly twisted and
rigid polymer conformations lead to poorly overlapped p-
electron wave function along PPEs backbone, which results
in the deep HOMO energy level.
Electrochemical Properties of PPEs
Cyclic voltammetry was carried out to probe the electrochemi-
cal properties of PPEs thin films. The HOMO energy levels of
PPEs were estimated from the onset oxidation potential as
summarized in Figure 4 and Table 3. In our studies, the HOMO
energy levels of PPE1–PPE4 were estimated under same con-
ditions (solvent, electrolyte, and electrodes) for comparison.
All PPEs showed irreversible oxidation waves at a significantly
Chiroptical Properties of PPEs
The effects of bulky substituents on the chiral properties of
the PPEs were studied. Optical rotation and CD spectroscopy
has been widely used to study the chiroptical properties of
polypyrroles, polythiophenes, and poly(aryleneethynylene)s to
provide useful information about the variations in polymer
conformation under different environments.
reported that the conjugated polymers such as polythiophene,
showed chiroptical CD signals when their backbone aggregates
into chiral conformation in the presence of a poor solvent.
1
higher potential of 1.59–1.70 V (vs. Ag /Ag) as compared to
that reported in poly(p-phenylene ethynylene) (1.10 V vs.
14,24,41
It is
1
39
Ag /Ag) with alkoxy side chains.
The phenyl substituents and low degree of conjugation in
our polymers are responsible for high oxidation potentials as
compared to alkoxy PPEs. The high onset oxidation potential
is accompanied with a deep HOMO energy level (25.77 to
8
,42
The optical activities of the PPEs in chloroform were first
examined using a polarimeter at 589 nm under ambient condi-
tions. PPE1 and PPE3 showed no specific rotations (0.5 mg/
mL, chloroform). On the other hand, the PPE2 with chiral (S)-
25.81 eV) in PPEs, which is lower than the HOMO energy
(1)-citronellyl group showed a specific rotation [a]D of 123.
Similarly, no specific optical rotation was observed from sym-
metric PPE4 with a shorter phenyl side group in chloroform.
TABLE 3 Summary of electrochemical properties of PPEs
b
c
E
ox
a
E
HOMO
(eV)
oxonset
Polymer
(V vs. Ag/AgCl)
(V vs. Ag/AgCl)
PPE1
PPE2
PPE3
PPE4
1.65
1.60
1.59
1.70
1.40
1.39
1.42
1.43
25.77
25.76
25.80
25.81
FIGURE 4 Cyclic voltammograms of PPE1 (-᭿-), PPE2 (-•-),
PPE3 (-᭡-), and PPE4 (-᭢-) thin films measured with 0.1 M n-
a
21
Determined from the potential where maximum oxidation occurred
Bu
4
NPF
6
in acetonitrile at the scan rate of 100 mV s
using
b
c
Determined from the onset oxidation potential. Electrochemical
Ag/AgCl as reference electrode. The PPE films were prepared
by drop casting chloroform solution of PPEs on platinum disk
electrode.
ox calibrated
ox calibrated
HOMO 5 2(E
14.8) eV, where E
were determined
from the onset potentials of first oxidation peak calibrated with ferro-
cene/ferocenium ion redox couple.
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FIGURE 5 SEM images of fibers obtained by dropcasting the chloroform/methanol (2:1) solution of PPEs on glass substrates. The
solvent was allowed to evaporate slowly inside a desiccator over 24 h at room temperature. The concentration of all polymer solu-
tions was kept at 0.05 mg/mL.
In order to investigate the effect of substituents on chiral
conformation of PPEs, the CD spectra of the polymers were
recorded in chloroform and chloroform/methanol solution.
Only weak CD signals were observed for all PPEs (Support-
ing Information, Figs. S50–S53). This suggests the formation
of single-handed helical macrostructures for PPEs in pres-
ence of poor solvent may not be strong with a small side
chain. It is known that the interaction of chiral alkyl chains
between adjacent polymer backbones leads to highly ordered
CONCLUSIONS
In summary, a series of rigid poly(p-phenylene ethynylene)s
(PPE1-PPE3) with biphenyl side chains was synthesized.
Photophysical studies in solution and solid states showed
that PPE1-PPE3 adopted a twisted conformation of the
backbone with significant blue shifts in absorption maxima,
and supported by the deep HOMO energy levels (25.76 to
25.81 eV). Helical aggregates were not confirmed for PPEs
8
with short biphenyl side groups, which may not be sufficient
to stabilize chiral aggregations. Scanning electron micro-
graphs showed the formation of twisted fibers after slow
evaporation of PPE1–PPE3 polymer solutions on glass sub-
strates. Overall, this article highlights the substituent induced
structural and property changes on the PPE backbone.
helical aggregates. However, PPE2 incorporated with chiral
alkyl substituents showed no recognizable CD signals in
presence of methanol. The absence of helical aggregates for
PPE2 is attributed to the short biphenyl side groups, which
may not facilitate chiral p-stacking of polymer chains. We are
currently developing other substituents to test such a
hypothesis.
Morphology Studies of PPEs
ACKNOWLEDGMENTS
The morphology of the PPEs film was examined by field-
emission scanning electron microscopy (SEM). The polymer
solution in chloroform/methanol (0.05 mg/mL) was dropcasted
on a glass substrate, which was kept inside a desiccator. The
solvent was allowed to evaporate slowly over 24 h at room tem-
perature prior to SEM examination. The scanning electron
micrographs of PPE1 and PPE3 dropcasted from chloroform/
methanol (2:1) solution showed fibers with a diameter of 150–
The authors would like to thank Ministry of Education, Singa-
pore for funding support (R-143-000-570-112), Department of
Chemistry, National University of Singapore (NUS) for technical
supports. CPS thanks the NUS for a research scholarship for his
PhD program.
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