Synthesis and characterization of amphiphilic poly(p phenylene) based nanostructured materials 4

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115

Renu, R.;

Ajikumar, P. K.;

Sheeja, B.;

Hanafiah,

N. B. M.; Baba, A.; Advincula, R. C.;

Knoll, W.; Valiyaveettil, S. Ultrathin Conjugated Polymer Network Films of Carbazole
Functionalized Poly(p-phenylene)s via Electropolymerization J. Phys. Chem. B (In
press).
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4


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116
4.1 Introduction

Ultrathin films of conjugated polymers have received tremendous interest during
the past few decades owing to their diverse applications and interesting physico-chemical
properties.
1-4
The intrinsic film forming abilities of polymers cast from solution using
convenient wet coating techniques are an attractive advantage for practical applications.
5

Polymers with a variety of tailored physico-chemical properties can be fabricated as
ultrathin films with many different methods such as spin coating, Langmuir-Blodgett
technique, layer-by-layer self-assembly, and surface-initiated polymerization.
6-7
Thin
films of conjugated polymers are expected to have wide range of applications in organic
light-emitting diodes (OLED), field-effect transistors (FET), and bio- and chemosensors,

and mostly fabricated by spin coating or electrochemistry through physisorption on the
substrate. Generally, the properties of conjugated polymers are the privileged domains of
chemists who can incorporate functional groups with specific electroactive properties.
8,9

It is well-known that a balanced and efficient charge injection/transport for both carrier
types (electron and hole) is essential for high device efficiency.
10
Polymers, however, are
rarely good conductors for both electrons and holes. In most cases, they transport holes
better than electrons. In order to facilitate the charge injection/transport, additional
electron injection/transport layer between the emitter and cathode or/and a hole-
transporting layer between the emitter and the anode needs to be introduced. Polymer
blends which contain a polymer matrix doped with the necessary components, usually
small molecules, facilitate electron/hole transporting properties.
11
In addition, a more
robust approach which minimizes the conventional problems involves the design of new
polymer containing both electron and hole transporting segments as well as emissive
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117
units.
10a,12
A hole transporting group such as oxadiazole or carbazole can be incorporated
either in the main chain or in the side chains to improve the hole transporting ability of
the polymer. Even when these requirements are achieved, it is necessary to optimize the
quality of the emitting layer by an appropriate deposition technique, to control the film
morphology, the carrier mobility, and the emission yield for the device development.
13
In
this respect, the Langmuir-Blodgett Kuhn (LBK) has been a most useful technique to
provide self-organized systems with good molecular order and molecular alignment.
14

The present study summarizes the preparation of two chemically distinct π-conjugated

polymers with a poly(p-phenylene) backbone and the incorporation of a hole transporting
polycarbazole as side chain. The development of highly crosslinked functional thin films
is delineated.


Poly(p-phenylene)s or PPPs are an interesting class of polymers which have
quantitative emission properties, interesting LC phases (anisotropic properties), and
enhanced ordering at interfaces.
15,16
Our group is focusing on the design and development
of homologous series of conjugated polymers

and fabrication of micro-/nano
architectures
17-18
to investigate the effectiveness of these polymers towards different film
deposition techniques which lead to interesting morphologies and improved properties.
Among the various polymers poly(p-phenylene) functionalized with six carbon alkoxy
chain and hydroxyl side-group (C
6
PPPOH) provided the desired amphiphilicity. It
displayed a three-phase region with interesting structural contrast along the polymer
backbone, which is directly observable in a Langmuir film.
18
The study of carbazole
based conjugated polymers have gained tremendous interest for the construction of
functional materials, such as photorefractive materials,
19
photoconductors,
20

nonlinear
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118
optical materials,
21

light-emitting,
22
and hole-transporting materials.
23
This is due to their
inherent electron-donating nature, excellent photoconductivity, and unique nonlinear
optical properties. Among the various carbazole incorporated polymers, poly(N-
vinylcarbazole), poly(3,6-N-vinylcarbazole) and polycarbazole have been extensively
studied and are of great interest for electrical conductivity and electrochemical device
applications.
24-25
Among these, poly(N-vinylcarbazole) exhibit interesting electrical and
optical properties as light emitting diode materials,
26
and photovoltaic materials.
27

Applications in various electrochromic devices and amperometric chemical sensors
should make carbazole based polymers attractive thin film materials.
28
Thus carbazole
incorporated polymers are potential candidates for tuning the optical and electrical
properties of light emitting and semiconducting organic materials.
29
The surface grafting
of carbazole-functionalized polyfluorenes to self-assembled monolayer (SAM) of
carbazole on indium tin oxide (ITO) surfaces has been demonstrated to form network
films.
30
Recently, electropolymerization of a substituted polyacetylene such as poly(N-

alkoxy-(p-ethynylphenyl)carbazole), with electropolymerizable carbazole resulted in the
formation of conjugated polymer network (CPN) films.
29a
In line with these previous
studies towards combining the physico-chemical properties of a soluble amphiphilic
poly(p-phenylene) and polycarbazole in functional thin films, a PPP derivative with
alkoxy carbazole group (-O(CH
2
)
5
Cb) incorporated on the polymer backbone
(C
6
PPPC
5
Cb) was synthesized and fully characterized. The polymer thin films were
prepared using the LBK and spin coating techniques and subsequently
electropolymerized for the preparation of mixed
π
-conjugated polymer network films.
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119

O
O
(CH
2
)
5
n
(CH
2
)
5
N

CH
3

Figure 4.1. Chemical structure of the polymer C
6
PPPC
5
Cb

4.2 Results and Discussion
4.2.1 Synthesis and Characterization of the polymer C
6
PPPC
5
Cb.
The polymer C
6
PPPOH was synthesized using Suzuki polycondensation of the
respective monomers and the details of the polymer synthesis and characterization is
described in the experimental section Chapter 6. The polymer C
6
PPPC
5
Cb was
characterized using NMR, FT-IR, and thermogravimetric analysis. Molecular weight of
the polymers were determined by gel permeation chromatography (GPC) with reference
to polystyrene standards using THF as eluent The number average molecular weight of
the hydroxyl protected precursor polymer C
6
PPPOBn was 10400 (Da) and that of the

polymer C
6
PPPC
5
Cb was 13100 (Da). The thermogravimetric analysis, of the polymer
showed good stability in nitrogen up to 325 °C, where the mass loss is less than 2 %
(Figure 4.2).
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120
Further, solution optical properties of the polymer was investigated and compared
with the parent polymer. The normalized UV-Vis and PL spectra of the polymer
C
6
PPPC
5
Cb and the parent polymer C
6
PPPOH are shown in Figure 4.3.

0 200 400 600 800 1000
0
10
20
30
40
50
60
70
80
90
100
110

C
6

PPPOH
C
6
PPPC
5
Cb
Weight (%)
Temperature (°C)
0 200 400 600 800 1000
0
10
20
30
40
50
60
70
80
90
100
110

C
6
PPPOH
C
6
PPPC
5
Cb

Weight (%)
Temperature (°C)

Figure 4.2. TGA traces of the polymer samples C
6
PPPC
5
Cb and C
6
PPPOH.

The absorption maxima at 332 nm for the C
6
PPPC
5
Cb is slightly blue shifted
compared to the C
6
PPPOH after the incorporation of alkoxy carbazole group. Similarly,
the onset is also slightly blue-shifted compared to parent polymer, indicating a change in
the conformation of polymer backbone owing to the presence of carbazole group. The
additional shoulder peaks below 300 nm was apparent which corresponds to the π-π*,
and n-π* transitions of the carbazole peak and were absent in the case of the absorption
spectra of the parent polymer C
6
PPPOH. The calculated electrooptical band gap, Eg, of
the polymer C
6
PPPC
5

Cb is 3.4 eV, slightly higher compared to the parent polymer (3.19
eV). Similar to the UV-Vis spectra, comparison of the PL spectra indicated that the
U
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121
C

6
PPPC
5
Cb emission maxima (λ
emis
= 400 nm) is blue shifted by 15 nm compared to the
parent polymer (λ
emis
= 415 nm) with a blue shift in the onset. It may be due to a
reduction in the persistence conjugation length of the PPP backbone due to the grafting of
the alkoxy carbazole moiety.
300 400 500 600
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
C
6
PPPOH (Abs)
C
6
PPPC
5
Cb (Abs)
C

6
PPPOH (emi)
C
6
PPPC
5
Cb (emi)
Normalized Absorbance
Normalized PL
Wavelength (nm)
300 400 500 600
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
C
6
PPPOH (Abs)
C
6
PPPC
5
Cb (Abs)
C
6

PPPOH (emi)
C
6
PPPC
5
Cb (emi)
Normalized Absorbance
Normalized PL
Wavelength (nm)

Figure 4.3. Absorbance and emission spectrum of the polymers C
6
PPPOH and
C
6
PPPCb in chloroform solution.
4.2.2 LB film deposition and characterization
In order to study the film deposition of the newly synthesized polymer, LBK technique
was used. This technique provides a way to fabricate self-organized systems with good
molecular order and molecular alignment. Previous studies about Langmuir-Schaefer
(LS) monolayer and LBK multilayer film of a newly designed conjugated polymer,
poly(p-phenylene)s (C
n
PPPOH) bearing amphiphilic side chains showed that the
U
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m
m
s
s


o
o
f
f


C
C
a
a

r
r
b
b
a
a
z
z
o
o
l
l
e
e


F
F
u
u
n
n
c
c
t
t
i
i
o
o

n
n
a
a
l
l
i
i
z
z
e
e
d
d


p
p
o
o
l
l
y
y
(
(
p
p
-
-

p
p
h
h
e
e
n
n
y
y
l
l
e
e
n
n
e
e
)
)
s
s


v
v

i
i
a
a


E
E
l
l
e
e
c
c
t
t
r
r
o
o
p
p
o
o
l
l
y
y
m
m

e
e
r
r
i
i
z
z
a
a
t
t
i
i
o
o
n
n


122
polymer with a short alkoxy group (C
6
PPPOH) forms a more uniform monolayer at the
air water interface and can be transferred to make multilayered polymeric films. The
isotherm of the polymer C
6
PPPOH, exhibited a liquid expanded region and similar
characteristic was observed for the C
6

PPPC
5
Cb. The isotherm of C
6
PPPC
5
Cb showed a
small shift to a more condensed solid-state phase (Figure 4.4). The addition of the
carbazole group probably increases the visco-elastic component of the film but at the
same time it retains amphiphilicity to form a good monolayer at the air water interface.
Both polymers, C
6
PPPOH and C
6
PPPC
5
Cb, have a collapse pressure of ~ 43 mN/m.
The calculated area per repeat unit for both polymers is 0.20 ± 0.02 nm². The
extrapolation of the solid region in the surface pressure-area isotherm to zero pressure,
resulted in the area per repeat unit (A) = 0.20 nm
2
,
which is close to the cross-sectional
area of an alkyl-chain. This confirms that the carbazole incorporated polymer,
C
6
PPPC
5
Cb, forms good monolayer at the air-water interface with close packed alkyl
chains.

0 1020304050
0
10
20
30
40
50

Mean Molecular Area (Å
2
)
Surface Pressure (mN/m)
C
6
PPPC
5
Cb
C
6
PPPOH
0 1020304050
0
10
20
30
40
50

Mean Molecular Area (Å

2
)
Surface Pressure (mN/m)
C
6
PPPC
5
Cb
C
6
PPPOH

Figure 4.4. Surface pressure-area (π-A) isotherm of C
6
PPPOH and C
6
PPPC
5
Cb.
U
U
l
l
t
t
r
r
a
a
t

t
h
h
i
i
n
n


C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d

P
P
o
o
l
l
y
y
m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k

k


F
F
i
i
l
l
m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b
a

a
z
z
o
o
l
l
e
e


F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l

l
i
i
z
z
e
e
d
d


p
p
o
o
l
l
y
y
(
(
p
p
-
-




p

p
h
h
e
e
n
n
y
y
l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a

E
E
l
l
e
e
c
c
t
t
r
r
o
o
p
p
o
o
l
l
y
y
m
m
e
e
r
r
i

i
z
z
a
a
t
t
i
i
o
o
n
n


123


Figure 4.5. Absorption spectra of LB films of C
6
PPPC
5
Cb with different number of
layers (A) and the dependence of the film absorption on the number of transferred layers
(B)
300 400 500 600
0.0
0.1
0.2
0.3

Absorbance (a.u)
Wavelength (nm)
5
10
15
20
300 400 500 600
0.0
0.1
0.2
0.3

Absorbance (a.u)
Wavelength (nm)
5
10
15
20
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20 25
Number of layers
Absorbance (a.u)
0
0.05

0.1
0.15
0.2
0.25
0 5 10 15 20 25
Number of layers
Absorbance (a.u)
Number of layers
Absorbance (a.u)
(A)
(B)
U
U
l
l
t
t
r
r
a
a
t
t
h
h
i
i
n
n

C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d


P
P
o
o
l
l
y

y
m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k
k


F
F
i
i
l

l
m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b
a
a
z
z
o
o
l
l
e

e


F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l
l
i
i
z
z
e
e
d

d


p
p
o
o
l
l
y
y
(
(
p
p
-
-




p
p
h
h
e
e
n
n
y

y
l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a


E
E
l
l
e
e
c

c
t
t
r
r
o
o
p
p
o
o
l
l
y
y
m
m
e
e
r
r
i
i
z
z
a
a
t
t
i

i
o
o
n
n


124
In order to study the deposition of multilayers of C
6
PPPC
5
Cb, the monolayers were
transferred to different hydrophilic substrates using Z-type deposition at a surface
pressure of 10 mN/m. Increase in absorbance from UV-Vis studies of LBK films of
C
6
PPPC
5
Cb transferred to quartz substrates was linear to the number of layers deposited
(Figure 4.5). A similar result was observed for the parent C
6
PPPOH polymer.
18a
The
peak-shifts (Δθ) of angular scans of the plasmon curves of LBK multilayer assemblies on
the Au surface relative to the bare gold increases linearly with the number of layers
(Figure 4.6A and B). This is also supported by our previous studies that the shorter
alkoxy chain polymer, C
6

PPPC
5
Cb is a better candidate for the preparation of LBK films
with layer-by-layer structure. Multilayers of up to 20 were deposited with a uniform
transfer and used for electropolymerization of the carbazole group for preparing a cross-
linked conducting polymer network film. The comparison of the solution (Figure 4.3)
and film state UV and PL indicated that there is blue shift in emission maxima for the
film with clear peak broadening at the higher wavelength region with appearance of a
shoulder around 530 nm (Figure 4.7). However there is no change in the observed UV
spectra in the solid-state film compared to the solution.
U
U
l
l
t
t
r
r
a
a
t
t
h
h
i
i
n
n

C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d


P
P
o
o
l
l
y
y

m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k
k


F
F
i
i
l
l

m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b
a
a
z
z
o
o
l
l
e
e

F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l
l
i
i
z
z
e
e
d
d

p
p
o
o
l
l
y
y
(
(
p
p
-
-




p
p
h
h
e
e
n
n
y
y

l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a


E
E
l
l
e
e
c
c

t
t
r
r
o
o
p
p
o
o
l
l
y
y
m
m
e
e
r
r
i
i
z
z
a
a
t
t
i
i

o
o
n
n


125

Figure 4.6. SPR curves of the multilayers of C
6
PPPC
5
Cb (A) and plot of the shift of
the resonance minimum for LBK films of C
6
PPPC
5
Cb obtained from the SPR angular
scan (B).

20 2
4
28 32 3
6
4
0
0.0
0.2
0.4
0.6

0.8
bare gold
5 layers
10 layers
15 layers
20 layers
Reflectivity (R)
θ/deg
20 2
4
28 32 3
6
4
0
0.0
0.2
0.4
0.6
0.8
bare gold
5 layers
10 layers
15 layers
20 layers
Reflectivity (R)
θ/deg
0
2
4
6

8
10
12
0 5 10 15 20 25
Number of layers
Δθ/deg
0
2
4
6
8
10
12
0 5 10 15 20 25
Number of layers
Δθ/deg
(A)
(B)
U
U
l
l
t
t
r
r
a
a
t
t

h
h
i
i
n
n


C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d

P
P
o
o
l
l
y
y
m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k
k

F
F
i
i
l
l
m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b
a
a

z
z
o
o
l
l
e
e


F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l
l

i
i
z
z
e
e
d
d


p
p
o
o
l
l
y
y
(
(
p
p
-
-




p
p

h
h
e
e
n
n
y
y
l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a

E
E
l
l
e
e
c
c
t
t
r
r
o
o
p
p
o
o
l
l
y
y
m
m
e
e
r
r
i
i

z
z
a
a
t
t
i
i
o
o
n
n


126

350 400 450 500 550 600 65
0
0.0
0.2
0.4
0.6
0.8
1.0

Solution

LB film
Spin coated film
Normalized Emission

Wavelength (nm)
350 400 450 500 550 600 65
0
0.0
0.2
0.4
0.6
0.8
1.0

Solution

LB film
Spin coated film
Normalized Emission
Wavelength (nm)

Figure 4.7. Comparison of the emission spectrum of polymer in CHCl
3
solution, 20
layers transferred to quartz at a surface pressure 10 mN/m and spin coated film.
4.2.3 Electropolymerization of the LB and spin coated films of
C
6
PPPC
5
Cb.
Recent studies about the electrochemical polymerization and cross-linking of poly(vinyl-
N-carbazole) (PVK), poly[9-[2-(4-vinylphenoxy)ethyl]-9H-carbazole] (PHC), and
poly(N-alkoxy-(p-ethynylphenyl)carbazole) (PAA-Cz-C6) through the carbazole units

demonstrated the ability to form thin films with unique optical and electrochemical
properties with different morphologies.
29c, 31
The cross-linked structures were formed
through a three-electron transfer process with dimerization of pendant carbazole ring
occurring via the 3,6-position leading to intermolecular cross-linking. The intermediate is
believed to be based on a carbazolylium radical cation which rapidly reacts via coupling-
deprotonation to form the dimer.
32
Subsequent cycles lead to higher oligomeric species
U
U
l
l
t
t
r
r
a
a
t
t
h
h
i
i
n
n

C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d


P
P
o
o
l
l
y
y

m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k
k


F
F
i
i
l
l

m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b
a
a
z
z
o
o
l
l
e
e

F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l
l
i
i
z
z
e
e
d
d

p
p
o
o
l
l
y
y
(
(
p
p
-
-




p
p
h
h
e
e
n
n
y
y

l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a


E
E
l
l
e
e
c
c

t
t
r
r
o
o
p
p
o
o
l
l
y
y
m
m
e
e
r
r
i
i
z
z
a
a
t
t
i
i

o
o
n
n


127
and further cross-linking as evidenced by a lowering of the oxidation potential and
increase in charge density with each succeeding cycle. Similarly, multilayer thin films of
carbazole incorporated polymer C
6
PPPC
5
Cb on ITO were prepared using Langmuir-
Blodgett technique and spin coating. The film was further electropolymerized to prepare
the crosslinked film. CV measurements were carried out using an electrolyte solution of
0.1 M tetrabutylammoniumpercholorate (Bu
4
NClO
4
) dissolved in acetonitrile, in which
the precursor polymer was not soluble. An undivided three electrode configuration cell
was used with the thin films of the polymer on ITO or gold coated LaSFN9 as the
working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference
electrode. Different scan rates such as 100 mV/s and 20 mV/s were used to study the
influence of scan rates on the stability of cross-linked polymers. The polymerization with
slow scan rate (20 mV/s) showed a reduction in the intensity of the oxidation and
reduction peaks after two cycles which may be due to degradation of the crosslinked film
after one cycle. Thus all further experiments were solely performed at a scan rate of 100
mV/s. This was further investigated using a combined SPR-cyclic voltammetry set up.

The oxidation and reduction potentials of C
6
PPPOH were not in the range of the applied
potentials for electropolymerization; therefore oxidation of only carbazole groups was
expected within this potential range. Cross linking of the carbazole monomer units occurs
during the electropolymerization without affecting the PPP conjugated polymer
backbone.
U
U
l
l
t
t
r
r
a
a
t
t
h
h
i
i
n
n


C
C
o

o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d


P
P
o
o
l
l
y
y
m
m
e

e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k
k


F
F
i
i
l
l
m
m
s

s


o
o
f
f


C
C
a
a
r
r
b
b
a
a
z
z
o
o
l
l
e
e


F

F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l
l
i
i
z
z
e
e
d
d


p

p
o
o
l
l
y
y
(
(
p
p
-
-




p
p
h
h
e
e
n
n
y
y
l
l
e

e
n
n
e
e
)
)
s
s


v
v
i
i
a
a


E
E
l
l
e
e
c
c
t
t
r

r
o
o
p
p
o
o
l
l
y
y
m
m
e
e
r
r
i
i
z
z
a
a
t
t
i
i
o
o
n

n


128




















Figure 4.8. CV for electrochemical cross-linking of 20 layers (A) 5 layers (B) of LB film
and spin coated (C) of C
6
PPPC
5
Cb at scan rate of 100 mv/s. (D), (E)and (F) are the

corresponding precursor polymer free scan.
Cyclic voltagram of the cross-linking of the LB multilayer and spin coated films
of C
6
PPPC
5
Cb deposited on ITO substrates with a scan rate of 100 mV/s is shown in
Figure 4.8. The oxidation onset for 20 layers of LB multilayers is 0.93 V and the
corresponding reduction peak is 0.78 V(vs Ag/AgCl) (Figure 4.8A). This oxidation peak

0.0 0.2 0.4 0.6 0.8 1.0 1.2
0.00000
0.00003
0.00006
0.00009
0.00012
Current (A)
Potential E Vs Ag/AgCl
1
st
cycle
4
th
cycle
10
th
cycle
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0.00000
0.00003

0.00006
0.00009
0.00012
Current (A)
Potential E Vs Ag/AgCl
1
st
cycle
4
th
cycle
10
th
cycle
0.0 0.2 0.4 0.6 0.8 1.0 1.
2
-0.00010
-0.00005
0.00000
0.00005
0.00010
0.00015
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.
2
-0.00010
-0.00005
0.00000
0.00005

0.00010
0.00015
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.00014
-0.00007
0.00000
0.00007
0.00014
0.00021
20
th
cycle
1
st
cycle
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.00014
-0.00007
0.00000
0.00007
0.00014
0.00021
20
th
cycle
1

st
cycle
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.00006
-0.00004
-0.00002
0.00000
0.00002
0.00004
0.00006
0.00008
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.00006
-0.00004
-0.00002
0.00000
0.00002
0.00004
0.00006
0.00008
Current (A)
Potential E Vs Ag/AgCl
0.
0
0.2 0.4 0.
6

0.8 1.
0
1.2
-0.00006
-0.00003
0.00000
0.00003
0.00006
0.00009
0.00012


Current (A)
Potential E Vs Ag/AgCl
1
st
cycle
20
th
cycle
0.
0
0.2 0.4 0.
6
0.8 1.
0
1.2
-0.00006
-0.00003
0.00000

0.00003
0.00006
0.00009
0.00012


Current (A)
Potential E Vs Ag/AgCl
1
st
cycle
20
th
cycle
A
B
C
D
F
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.000010
-0.000005
0.000000
0.000005
0.000010
0.000015
0.000020
Current (A)
Potential E Vs Ag/AgCl
1

st
cycle
4th
cycle
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.000010
-0.000005
0.000000
0.000005
0.000010
0.000015
0.000020
Current (A)
Potential E Vs Ag/AgCl
1
st
cycle
4th
cycle
E
U
U
l
l
t
t
r
r
a
a

t
t
h
h
i
i
n
n


C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d

P
P
o
o
l
l
y
y
m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r

k
k


F
F
i
i
l
l
m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b

a
a
z
z
o
o
l
l
e
e


F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a

l
l
i
i
z
z
e
e
d
d


p
p
o
o
l
l
y
y
(
(
p
p
-
-

p
p
h
h
e
e
n
n
y
y
l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a

E
E
l
l
e
e
c
c
t
t
r
r
o
o
p
p
o
o
l
l
y
y
m
m
e
e
r
r

i
i
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a
a
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n
n


129
was absent in the first cycle with an appearance of another peak at 1V, which indicated
that first cycle is different from the second cycle with the possibility of cross-linking of
carbazole units at about 1.0 V. Peaks due to doping and dedoping were found in the
second to the subsequent cycles with doping at 0.93 V and 0.80 V for dedoping. The
peaks due to dedoping were slightly shifted to 0.78 V in the subsequent cycles. The
observed peak value for doping and dedoping was slightly higher than previously
reported carbazole incorporated films. The current increased with the number of cycles.
For carbazole grafted poly(phenylacetylene) polymer, the oxidative doping was observed
at about 0.7-0.8 V, followed by another current increase at about 1.0 V during the anodic
scan of the cycle.
29a
In the case of the polyvinyl carbazole (PVK), the first cycle always
showed an oxidation onset at 0.9 V and the appearance of the oxidation doping peak in

the 0.6-0.7 V range in the subsequent cycles.
25
The slightly higher value in the doping
and dedoping for new polymer C
6
PPPC
5
Cb can be accounted for the rigid rod structure
of the polymer backbone with good chain-to-chain polymer packing in the film. This
generates a dense structure of the carbazole moiety compared to the previously reported
flexible polymers such as the copolymers of carbazole and thiophene, PVK or poly(N-
alkoxy-(p-ethynylphenyl)carbazole), but at the same time slows down the counter ion
transport properties due to a less porous structure.
29
The oxidation onset for 5 layers of
LB multilayers is at 0.93 V and the corresponding reduction peak is at 0.8 V. There was a
decrease in the peak area after four cycles which could indicate a slight degradation of
the film. In the case of the spin coated film the electrochemical behavior was almost the
same as the 20 layer LB film. The difference in behavior with the film thickness could be
U
U
l
l
t
t
r
r
a
a
t

t
h
h
i
i
n
n


C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d

P
P
o
o
l
l
y
y
m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k

k


F
F
i
i
l
l
m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b
a

a
z
z
o
o
l
l
e
e


F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l

l
i
i
z
z
e
e
d
d


p
p
o
o
l
l
y
y
(
(
p
p
-
-




p

p
h
h
e
e
n
n
y
y
l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a

E
E
l
l
e
e
c
c
t
t
r
r
o
o
p
p
o
o
l
l
y
y
m
m
e
e
r
r
i

i
z
z
a
a
t
t
i
i
o
o
n
n


130
due to the availability of more carbazole groups in a thick film which is crucial for the
formation of a stable cross linked network of C
6
PPPC
5
Cb.
A precursor polymer free scan was performed and showed a characteristic
oxidation peak at 0.93 V (vs Ag/ AgCl (0.01 M)) and corresponding reduction peak at
0.78 V (Figure 4.8D, E and F). The CV gives clear evidence of the
electropolymerization of the carbazole units. However at a slow scan rate 20 mV/s, the
peak area was reduced with each successive cycles indicating that it utilizes the species
that were left unpolymerized or crosslinked in the first few cycles. This can be correlated
with the carbazole groups tendency to dimerize first followed by higher orders of reaction
and the formation of higher orders of oligomers with possible 2,7 connectivity.

33

Interestingly the aforementioned behaviors are consistent with both LB films and spin
casted films (Figure 4.9 A and C).
U
U
l
l
t
t
r
r
a
a
t
t
h
h
i
i
n
n


C
C
o
o
n
n

j
j
u
u
g
g
a
a
t
t
e
e
d
d


P
P
o
o
l
l
y
y
m
m
e
e
r
r

N
N
e
e
t
t
w
w
o
o
r
r
k
k


F
F
i
i
l
l
m
m
s
s

o
o
f
f


C
C
a
a
r
r
b
b
a
a
z
z
o
o
l
l
e
e


F
F
u
u

n
n
c
c
t
t
i
i
o
o
n
n
a
a
l
l
i
i
z
z
e
e
d
d


p
p
o
o

l
l
y
y
(
(
p
p
-
-




p
p
h
h
e
e
n
n
y
y
l
l
e
e
n
n

e
e
)
)
s
s


v
v
i
i
a
a


E
E
l
l
e
e
c
c
t
t
r
r
o
o

p
p
o
o
l
l
y
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m
m
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r
r
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a
a
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n

131

Figure 4.9 (A) CV for electrochemical cross-linking of twenty layer LB film of C
6
PPPC
5
Cb at scan rate of 20 mv/s. (B) is the
corresponding polymer free scan. (C) and (D) are CV of five layer LB and spin casted film respectively at scan rate of 20 mv/s.
0.0 0.2 0.4 0.6 0.8 1.0 1.
2
-
0.00002
0.00000
0.00002
0.00004
0.00006
0.00008
0.00010
0.00012
20
th
scan
1
st
scan
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.
2
-

0.00002
0.00000
0.00002
0.00004
0.00006
0.00008
0.00010
0.00012
20
th
scan
1
st
scan
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.
2
-0.00006
-0.00003
0.00000
0.00003
0.00006
0.00009
0.00012
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.
2
-0.00006

-0.00003
0.00000
0.00003
0.00006
0.00009
0.00012
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.
2
-
0.000005
0.000000
0.000005
0.000010
0.000015
0.000020
0.000025
0.000030
Current (A)
Potential E Vs Ag/AgCl
1
st
scan
15
th
scan
0.0 0.2 0.4 0.6 0.8 1.0 1.
2
-

0.000005
0.000000
0.000005
0.000010
0.000015
0.000020
0.000025
0.000030
Current (A)
Potential E Vs Ag/AgCl
1
st
scan
15
th
scan
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.000006
-0.000003
0.000000
0.000003
0.000006
0.000009
0.000012
Current (A)
Potential E Vs Ag/AgCl
1
st
cycle
8

th
cycle
9
th
cycle
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.000006
-0.000003
0.000000
0.000003
0.000006
0.000009
0.000012
Current (A)
Potential E Vs Ag/AgCl
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.000006
-0.000003
0.000000
0.000003
0.000006
0.000009
0.000012
Current (A)
Potential E Vs Ag/AgCl
1
st
cycle
8
th

cycle
9
th
cycle
1
st
cycle
8
th
cycle
9
th
cycle
(A) (B)
(C) (D)
U
U
l
l
t
t
r
r
a
a
t
t
h
h
i

i
n
n


C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d


P
P
o

o
l
l
y
y
m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k
k


F

F
i
i
l
l
m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b
a
a
z
z
o

o
l
l
e
e


F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l
l
i
i
z

z
e
e
d
d


p
p
o
o
l
l
y
y
(
(
p
p
-
-




p
p
h
h
e

e
n
n
y
y
l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a


E
E
l

l
e
e
c
c
t
t
r
r
o
o
p
p
o
o
l
l
y
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m
m
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r
r
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a

a
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i
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n
n


132
The morphologies of the polymer films after electropolymerization were studied using
atomic force microscopy (AFM) in tapping mode. Figure 4.10 shows the height images
of the five (A) and twenty layers (B) of LB film and spin-coated film (C) after
electropolymerization and extensive washing with acetonitrile. The roughness of the film
measured in different areas in all three films is less than 5 nm with complete coverage. In
summary, the morphology of the film observed here is of good quality irrespective of the
deposition technique, LB film or spin-coated.



Figure 4.10. Morphology of the films after electropolymerization at scan rate of 100
mV/s. (A) five layer, (B) twenty layer and (C) spin coated films of C
6
PPPC
5
Cb.

(
A
)
(
B
)


(
C
)
U
U
l
l
t
t
r
r
a
a
t
t
h
h
i
i
n
n

C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d


P
P
o
o
l
l
y

y
m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k
k


F
F
i
i
l

l
m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b
a
a
z
z
o
o
l
l
e

e


F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l
l
i
i
z
z
e
e
d

d


p
p
o
o
l
l
y
y
(
(
p
p
-
-




p
p
h
h
e
e
n
n
y

y
l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a


E
E
l
l
e
e
c

c
t
t
r
r
o
o
p
p
o
o
l
l
y
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133
4.2.4 Electrochemical surface plasmon spectroscopy (ESPS) of LB
films.
In order to further observe the electropolymerization of carbazole group incorporated on
the PPP backbone, the LB films were deposited on gold coated LaSFN9 substrate and
simultaneous electrochemical surface plasmon spectroscopy (ESPS) measurements were
carried out. This technique allows the characterization of the change in dielectric constant
and thickness of a film during the in-situ electropolymerization process. Surface plasmon
spectroscopy (SPS) was used to investigate the electrochromic properties of C
6
PPPC
5
Cb
upon doping and dedoping and the effect on reflectivity was studied. The use of
electrochemical SPS for in situ characterization of the electropolymerization of
conjugated polymers has recently been described.
34
The change of SPS curves when the
polymer film was switched to different electrochromic states upon doping and dedoping
reveals important changes in dielectric constants and electrochromic behavior of the film.
U
U
l

l
t
t
r
r
a
a
t
t
h
h
i
i
n
n


C
C
o
o
n
n
j
j
u
u
g
g
a

a
t
t
e
e
d
d


P
P
o
o
l
l
y
y
m
m
e
e
r
r


N
N
e
e
t

t
w
w
o
o
r
r
k
k


F
F
i
i
l
l
m
m
s
s


o
o
f
f


C

C
a
a
r
r
b
b
a
a
z
z
o
o
l
l
e
e


F
F
u
u
n
n
c
c
t
t
i

i
o
o
n
n
a
a
l
l
i
i
z
z
e
e
d
d


p
p
o
o
l
l
y
y
(
(
p

p
-
-




p
p
h
h
e
e
n
n
y
y
l
l
e
e
n
n
e
e
)
)
s
s

v
v
i
i
a
a


E
E
l
l
e
e
c
c
t
t
r
r
o
o
p
p
o
o
l
l
y

y
m
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r
r
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134

0 700 1400
0.2
0.3
0.4
0.5

Time/sec
Reflectivity (R)
5 layers
Scan rate-20 mV/s
0 700 1400
0.2
0.3
0.4
0.5
Time/sec
Reflectivity (R)
5 layers
Scan rate-20 mV/s
4
5
50 5
5
6
0
6
5
7
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6

0.7
0.8
0.9
before
after
θ/deg
Reflectivity (R)
5 layers
4
5
50 5
5
6
0
6
5
7
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
before
after

θ/deg
Reflectivity (R)
5 layers
Time/sec
0 200 400
0.20
0.25
0.30
Reflectivity (R)
5 layers
Scan rate- 100 mV/s
Time/sec
0 200 400
0.20
0.25
0.30
Reflectivity (R)
5 layers
Scan rate- 100 mV/s
45 50 55 60 65 70 75
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
before cross-linking

after cross-linking
Reflectivity (R)
θ/deg
5 layers
45 50 55 60 65 70 75
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
before cross-linking
after cross-linking
Reflectivity (R)
θ/deg
5 layers
A
B
C
D

Figure 4.11. SPS (A and C) 5 layer film measured before and after electropolymerization at a scan rate of 100 mV/s and 20 mV/s
respectively. (B and D) difference in reflectivity with time at a scan rate of 100 mV/s and 20 mV/s respectively.
U
U
l
l

t
t
r
r
a
a
t
t
h
h
i
i
n
n


C
C
o
o
n
n
j
j
u
u
g
g
a
a

t
t
e
e
d
d


P
P
o
o
l
l
y
y
m
m
e
e
r
r


N
N
e
e
t
t

w
w
o
o
r
r
k
k


F
F
i
i
l
l
m
m
s
s


o
o
f
f


C
C

a
a
r
r
b
b
a
a
z
z
o
o
l
l
e
e


F
F
u
u
n
n
c
c
t
t
i
i

o
o
n
n
a
a
l
l
i
i
z
z
e
e
d
d


p
p
o
o
l
l
y
y
(
(
p
p

-
-




p
p
h
h
e
e
n
n
y
y
l
l
e
e
n
n
e
e
)
)
s
s

v
v
i
i
a
a


E
E
l
l
e
e
c
c
t
t
r
r
o
o
p
p
o
o
l
l
y
y

m
m
e
e
r
r
i
i
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a
a
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o
o
n
n


135
For ESPS measurement, the polymer was transferred to gold substrates with two different
film thicknesses (5 layer and 20 layers). Figure 4.11 shows the SPS curves, which were
measured with in-situ electropolymerization of the C
6
PPPC
5
Cb in acetonitrile solution

before and after electropolymerization. As shown in this figure, the minimum angle is
shifted to higher angles due to cross-linking, indicating an increase of dielectric constant
or thickness of the film. In the case of the scan rate at 100 mV/s., the amplitude of the
vibration increased as the number of cycling increased, which implies that the film
becomes more electroactive due to higher degree of cross-linking. On the other hand, in
the case of the scan rate at 20 mV/s., the reflectivity largely changed only in first scan,
and then the amplitude of the vibration decreased. This may be due to the completion of
cross-linking after the first scan followed by some degradation in subsequent cycling.
ESPS data are consistent with the results from the CV experiment. Similar trend was also
observed in the case of film with 20 layers as shown in Figure 4.12. The nature of the
cross-linking behavior is thus correlated with the film thickness of the LBK films and the
scan rate dependence is a reflection of the tighter chain-to-chain packing in higher order
films
U
U
l
l
t
t
r
r
a
a
t
t
h
h
i
i
n

n


C
C
o
o
n
n
j
j
u
u
g
g
a
a
t
t
e
e
d
d


P
P
o
o
l

l
y
y
m
m
e
e
r
r


N
N
e
e
t
t
w
w
o
o
r
r
k
k


F
F
i

i
l
l
m
m
s
s


o
o
f
f


C
C
a
a
r
r
b
b
a
a
z
z
o
o
l

l
e
e


F
F
u
u
n
n
c
c
t
t
i
i
o
o
n
n
a
a
l
l
i
i
z
z
e

e
d
d


p
p
o
o
l
l
y
y
(
(
p
p
-
-




p
p
h
h
e
e
n

n
y
y
l
l
e
e
n
n
e
e
)
)
s
s


v
v
i
i
a
a


E
E
l
l
e

e
c
c
t
t
r
r
o
o
p
p
o
o
l
l
y
y
m
m
e
e
r
r
i
i
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a
a
t

t
i
i
o
o
n
n


136
45 50 55 60 65 70 75
0.2
0.3
0.4
0.5
0.6
0.7
0.8
before
after cross-linking
20 layers
Reflectivity (R)
θ/deg
45 50 55 60 65 70 75
0.2
0.3
0.4
0.5
0.6
0.7

0.8
before
after cross-linking
20 layers
Reflectivity (R)
θ/deg
0 500 1000 1500
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
Time/sec
Reflectivity (R)
20 layers
Scan rate-20 mV/s
0 500 1000 1500
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60

Time/sec
Reflectivity (R)
20 layers
Scan rate-20 mV/s
A
B

Figure 4.12. SPS (A) 20 layer films measured before and after electropolymerization at a
scan rate of 20 mV/s.

4.3 Conclusion.
Detailed electrochemical cross-linking studies are reported towards the conjugated
polymer network (CPN) film formation for an alkoxy group (O(CH
2
)
5
-CH
3
) and alkoxy
carbazole group (O(CH
2
)
5
-Cb) functionalized poly(p-phenylene) (C
6
PPPC
5
Cb). This
chapter delineates the formation of CPN films using a precursor polymer where the
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137
carbazole moiety was separated by an alkoxy spacer from the polymer backbone. A thin
film from precursor polymer was deposited using LBK and spin coating techniques on
bare ITO and Au substrates. With a rigid rod structured poly(p-phenylene) backbone, the
ability to form a highly uniform and well packed thin film enabled efficient secondary

polymerization of the carbazole side groups leading to the formation of a “mixed
conjugated” polymer networks. Electropolymerization was facilitated without
decomposing the PPP backbone. The electrochemical data indicated the typical oxidation
and reduction peaks of carbazole cross-linking. However, the dependence on film
thickness and scan rate behavior highlighted possible parameters for control in CPN film
formation.
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138
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