Conjugated polymer network films from precursor polymers: Electrocopolymerization of a binary electroactive monomer composition

Article abstract of DOI:10.1021/ma050001h

We report the synthesis and electropolymerization of a precursor polymer with a binary molecular composition of thiophene and carbazole electroactive groups to form ultrathin films of conjugated polymer networks (CPN) on flat indium tin oxide (ITO) substrates. In the past we have demonstrated the precursor polymer approach based on a single pendant electroactive group. In this work, we describe the interesting electrocopolymerization mechanism and properties of precursor polymers prepared with two different types of pendant electroactive groups (statistical copolymer) and compared behavior to their respective homopolymers. The presence of a smaller amount of carbazole induces the electropolymerization of the higher oxidation potential thiophene units via the reaction of a radical cation and a neutral molecule pathway. These electrochemically generated thin films gave unique optical, electrochemical, and morphological properties as a function of composition. The film properties were investigated by cyclic voltammetry (CV), spectroelectrochemistry, EQCM, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS).

Full text of DOI:10.1021/ma050001h

Macromolecules 2005, 38, 3679-3687
3679
Conjugated Polymer Network Films from Precursor Polymers:
Electrocopolymerization of a Binary Electroactive Monomer Composition
Prasad Taranekar, Akira Baba, Timothy M. Fulghum, and Rigoberto Advincula*
Department of Chemistry, University of Houston, Houston, Texas 77204-5003
Received January 1, 2005; Revised Manuscript Received February 28, 2005
ABSTRACT: We report the synthesis and electropolymerization of a precursor polymer with a binary
molecular composition of thiophene and carbazole electroactive groups to form ultrathin films of conjugated
polymer networks (CPN) on flat indium tin oxide (ITO) substrates. In the past we have demonstrated
the precursor polymer approach based on a single pendant electroactive group. In this work, we describe
the interesting electrocopolymerization mechanism and properties of precursor polymers prepared with
two different types of pendant electroactive groups (statistical copolymer) and compared behavior to their
respective homopolymers. The presence of a smaller amount of carbazole induces the electropolymerization
of the higher oxidation potential thiophene units via the reaction of a radical cation and a neutral molecule
pathway. These electrochemically generated thin films gave unique optical, electrochemical, and
morphological properties as a function of composition. The film properties were investigated by cyclic
voltammetry (CV), spectroelectrochemistry, EQCM, atomic force microscopy (AFM), and X-ray photo-
electron spectroscopy (XPS).
Scheme 1. Synthetic Scheme for the Monomers Used
in Precursor Polymer Synthesis
Introduction
Conjugated polymers are widely studied polymer
systems. These materials have received much attention
due to their potential applications in light-emitting
devices, field effect transistors, charge storage devices,
photodiodes, sensors, resist materials, etc.1 Substituted
polythiophenes and poly(N-alkyl-3,6-carbazoles) are
materials of great interest as they can act as hole
transport materials when utilized in organic light-
emitting diode (OLED) applications.2 They can be
synthesized both chemically and electrochemically.3 A
number of groups have in fact reported the copolymer-
ization of carbazole and thiophenes and their application
as OLED materials.3,16,22 Their application to electro-
grafting methods have been reported but with ill-defined
electrochemical mechanisms.3,27 It is well-known that
introducing substituents to an electropolymerizable
monomer increases the overall oxidation potential such
that it can become difficult to electrochemically deposit
without decomposing the polymer at the same time. In
the past this problem was overcome by extending the
conjugation length of the monomer to a dimer, trimer,
etc., resulting in a significant reduction of the overall
oxidation potential.4 In the present work, the presence
of a very small amount of carbazole triggers the electro-
polymerization of the higher oxidation potential thio-
phene units and thus opening an alternative pathway
to polymerize higher oxidation potential electroactive
monomers without having to do extensive synthesis of
dimers, trimers, etc.
none has been reported for binary tethered monomer
compositions pendant on a polymer backbone.
Here we report the formation of ultrathin films of a
CPN based on a precursor polymer with thiophene and
carbazole electroactive monomers grafted onto a poly-
(4-vinylphenol) backbone (Scheme 2). The poly(4-vinyl-
phenol) backbone was chosen because of its relatively
high glass transition temperature, chemical inertness,
and optical transparency. Changes in the optical prop-
erties of conjugated polymers can be driven by a delicate
balance between repulsive steric interactions and at-
tractive interchain and (or, interchain, due to chain
folding) interactions with its alkyl substituent (or non-
conjugated component).7 In addition, poly(N-vinylcar-
bazole) and other N-substituted carbazole with PVP-
like backbones such as poly(9-(4-ethynylphenyl)carbazole)
and poly(N-(p-ethynylbenzoyl)carbazole) polymers are
materials of interest in terms of their redox activity,
porosity, and doping-dedoping reversibility.8 We have
used this doping-dedoping reversibility to tune the
work function of a cross-linked poly(vinylcarbazole)
(PVK) film on ITO in a multilayer OLED device in order
to improve hole-injection/transport properties.6b
In our previous work, we have demonstrated the
viability of the “precursor polymer” approach based on
single pendant electroactive monomer group using vari-
ous polymer backbones.5 This involved the tethering of
electroactive monomers to a polymer backbone which
results in the formation of electrooptically active con-
jugated polymer network (CPN) or cross-linked films
after electropolymerization.1d,5 These materials have
been reported to be useful for device applications.6 While
a number of homopolymers and copolymers have been
reported which contain one type of tethered monomer,
It is interesting to study both thiophene and carbazole
monomers because they can form unique copolymer
compositions, polymerization mechanisms, and electro-
10.1021/ma050001h CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/08/2005

3680 Taranekar et al.
Macromolecules, Vol. 38, No. 9, 2005
Scheme 2. Synthetic Scheme for the Cross-Linked
Homopolymers and Copolymer from Their Respective
Precursors
performed using a Viscotek 270 quad detector equipped with
VE3210 UV/vis detector and VE3580 RI detector. The columns
used for finding size exclusion chromatography (SEC) number-
average molecular weight (Mn,sec) type are G3000HHR and
GHMHR-M viscogel. All FTIR measurements were done using
a Digilab FTS 7000 step scan spectrometer. Cyclic voltamme-
try was performed on an Amel 2049 potentiostat and power
lab/4SP system with a three-electrode cell. In all the measure-
ments the counter electrode was platinum wire, and the
indium tin oxide (ITO) was used as a working electrode and
was pretreated with RCA recipe (H2O/H2O2/NH3::15.1 g/26.6
g/8.57 g).11 The QCM apparatus, probe, and crystals are
available from MAXTEK Inc. The diameter of the polished AT-
cut QCM crystals (5 MHz) was 13 mm. The data acquisition
was done using a RQCM (Research Quartz Crystal Microbal-
ance, MAXTEK, Inc.) system equipped with an inbuilt phase
lock oscillator and the RQCM Data-Log Software. The gold
electrodes were cleaned with a plasma etcher (Plasmod,
March). To measure the polymer adsorption, the inert probe
was first immersed in methylene chloride until a stable
frequency was obtained. Atomic force microscopy (AFM) imag-
ing was examined in ambient conditions with a PicoSPM II
(PicoPlus, Molecular Imaging) in the Magnetic AC mode (MAC
mode). MAC mode uses a magnetic field to drive a magnetically
coated cantilever in the top-down configuration. Type II MAC
levers with a spring constant of 2.8 nN/M with about 10 nm
tip radius were used for all scans. A contact mode was used
for surface profilometry measurements using a cantilever
(Nanosensor) of spring constant 0.07-0.4 nN/m and resonance
frequency of 10-17 kHz. X-ray photoelectron spectroscopy
(XPS) was carried out on a Physical Electronics 5700 instru-
ment with photoelectrons generated by the nonmonochromatic
Al KR irradiation (1486.6 eV). Photoelectrons were collected
at a takeoff angle of 45° using a hemispherical analyzer
operated in the fixed retard ratio mode with an energy
resolution setting of 11.75 eV. The binding energy scale was
calibrated prior to analysis using the Cu 2p3/2 and Ag 3d5/2
lines. Charge neutralization was ensured through cobombard-
ment of the irradiated area with an electron beam and the
use of the nonmonochromated Al KR source. This placed the
adventitious C 1s peak at a binding energy of 284.6 ( 0.2 eV.
optical applications.3,16,22,27 The copolymer bearing a
statistical binary electroactive monomer composition
will have properties in between that of polythiophenes
and polycarbazoles. The ability of one monomer to
polymerize in the presence of another monomer has also
been reported in the past.9 Both N-ethylcarbazole and
N-phenylcarbazole produce noncoherent electropoly-
merized films,10 which can be overcome by the introduc-
tion of thiophene into the copolymer structure, resulting
in stable, optically clear, and coherent conducting
polymer films.
In this work, we investigated the precursor polymer
route for these two monomers by depositing films using
anodic electropolymerization methods. We describe the
synthesis, film preparation, and characterization in
terms of in-situ mass change, electrooptical properties
during and after electropolymerization, and surface
morphological behavior for the copolymers. An interest-
ing electrocopolymerization mechanism was observed
along with tunable electrooptical properties for these
monomers. The electrochemical copolymerization be-
havior was compared to their individual homopolymer
analogues. Studies were also made involving their
mixtures and comonomer deposition (monomer directly
introduced in solution). The possibilities of intra-
molecular “template” electropolymerization vs inter-
molecular cross-linking reactions were discussed.
Reagents. Poly(4-vinylphenol) (Mw ) 11 430) was pur-
chased from Aldrich, and all other chemicals used were from
Fischer Chemical Co. The ITO was used as a working electrode
and was pretreated with RCA recipe (H2O/H2O2/NH3::15.1
g/26.6 g/8.57 g). All solvents were distilled and dried properly
before use.
Synthesis of 3-(2-Bromoethyl)thiophene (T-Br) (Scheme
1). A solution of 4.15 g (12.5 mmol) of CBr4 in 15 mL THF
was added dropwise to an ice cold magnetically stirred solution
containing 1.28 g (10.0 mmol) 2-thiophen-3-ylethanol and 3.92
g (15.0 mmol) of PPh3 dissolved in 10 mL THF. After addition
was complete, the mixture was stirred for an additional 6 h,
whereupon the solvent was removed under vacuum. The
concentrate was then dissolved in methylene chloride and
washed with aqueous NaOH solution. The organic layer was
then extracted again thoroughly washed with water and was
dried over Na2SO4. The residue was purified by flash column
chromatography with hexane as an eluent to give 1.6 g (83.7%)
1
of the bromide as a colorless oil. H NMR (δ ppm in CDCl3):
7.16 (q, 1H), 7.084-6.99 (m, 2H), 3.58 (t, 2H), 3.21 (t, 2H).
Synthesis of 9-(2-Bromoethyl)-9H-carbazole (C-Br)
(Scheme 1). A solution of 4.15 g (12.5 mmol) of CBr4 in 15 mL
THF was added dropwise to an ice cold magnetically stirred
solution containing 2.11 g (10.0 mmol) 2-carbazol-9-ylethanol
and 3.92 g (15.0 mmol) of PPh3 dissolved in 10 mL THF. After
addition was complete, the mixture was stirred for an ad-
ditional 10 h, whereupon the solvent was removed under
vacuum. The concentrate was then dissolved in methylene
chloride and washed with aqueous NaOH solution. The organic
layer was then extracted again thoroughly washed with water
and was dried over Na2SO4. The residue was purified by flash
column chromatography with hexane as an eluent to give 2.1
g (76.6%) of the bromide as white crystals. 1H NMR (δ ppm in
Experimental Section
Instrumentation. NMR was done on a General Electric
QE 300 spectrometer (1H 300 MHz). UV-vis was recorded
using Agilent 8453 spectrometer. The SEC analysis was

Macromolecules, Vol. 38, No. 9, 2005
CPN Films from Precursor Polymers 3681
CDCl3): 8.20 (d, 2H), 7.62-7.52 (m, 4H), 7.39 (m, 2H), 4.81 (t,
g (0.25 mmol) of 18C6, and 0.7 g (2.55 mmol) of C-Br (9-(2-
bromoethyl)-9H-carbazole) were all mixed at once and refluxed
under nitrogen for 72 h. The solvent was removed under
vacuum, and the reaction mixture was then poured in water
and extracted with ethyl acetate. The organic extract was dried
over Na2SO4. After filtration the organic extract was concen-
trated. The concentrate was then added dropwise in hexane
to give a white precipitate, which was then filtered and washed
thoroughly again with diethyl ether. The precipitate was dried
under vacuum and the product was confirmed by NMR. The
NMR showed more then 95% of grafting. SEC analysis failed
on this polymer even after repeated trials. 1H NMR (δ ppm in
CDCl3): 8.14 (b, 2H), 7.33 (b, 6H), 6.54 (b, 4H), 4.51-4.04 (b,
2H), 3.78 (t, 2H).
Synthesis of Poly([3-{2-(4-vinylphenoxy)ethyl}thio-
phene]-co-4-vinylphenol) (PVP1) (Scheme 2). A 250 mL
round-bottom flask was charged with poly(4-vinylphenol) (0.75
g), 2-thiophen-3-ylethanol (0.66 g), and triphenylphosphine
(2.09 g) in 20 mL of distilled THF under nitrogen. At 0 °C a 5
mL THF solution of diisopropylazodicarboxylate (DIAD, 1.21
g) was poured in the reaction mixture. After 24 h water was
added to quench the reaction, and then solvent was subjected
to vacuum. The reaction mixture was redissolved in methylene
chloride and extracted from water. The solvent was then
removed under vacuum, and the resulting reaction mixture
was dropwise added to cold diethyl ether under vigorous
stirring. The polymer was obtained as a white precipitate. The
obtained polymer was redissolved in methylene chloride and
was reprecipitated in cold methanol. This process was repeated
several times until no other impurities were observed. Proton
NMR was done on the vacuum-dried product, and in 5%
confidence limit based on the integration we propose 68%
grafting of thiophene units. 1H NMR (δ ppm in CDCl3): 7.20-
7.00 (b, 1.99H), 6.54 (b, 4H), 5.33 (b, 0.6H), 4.04 (b, 1.33H),
3.029 (b, 1.30H), 1.98-0.63 (b, 6H).
4H), 1.98-0.63 (b, 6H). The FTIR (KBr) in wavenumber (cm-1
)
shows12 (Ph-O-C) 810-850, (Ph-O-C) 1210 (br, s), aromatic
(C-H) 1250-1270 (s), (C-N)ring 1250-1380, aromatic (C-H)
1470-1525 (s), aromatic (C-H) 1590-1625, carbazole (Cd
C)ring 1650 (s), υs(CH2) 2860 (br), υ(CH) 2910 (br), υas(CH2) 2932
(br), carbazole (C-N)ring 3000 (br, s), (Ph-O-H) 3400 (br, w).
Electrochemical Synthesis of Cross-Linked Polymers
(CPHC/CPHT/CPMTC). The precursor polymers were elec-
tropolymerized using cyclic voltammetry techniques. In a
three-electrode cell 0.1 M tetrabutylammonium hexafluoro-
phosphate (TBAP) as supporting electrolyte and 10 mM of each
polymer were dissolved in 5 mL of methylene chloride sepa-
rately. The electropolymerization of the each precursor poly-
mer was performed by sweeping the voltage at a scan rate of
20 mV/s from 0 to 1.1 V in the case of CPHC, 1.5 V for CPMTC,
and up to 1.6 V was used for CPHT against Ag/AgCl as a
reference electrode and platinum as a counter electrode. The
ITO was used as a working electrode and also as a substrate.
Synthesis of Poly([3-{2-(4-vinylphenoxy)ethyl}thio-
phene]-co-9-[2-(4-vinylphenoxy)ethyl]-9H-carbazole)
(PMTC) (Scheme 2). In a solution of 15 mL of distilled acetone,
0.255 g (1.70 mmol) of PVP1, 1.38 g (10 mmol) of K2CO3, 0.066
g (0.25 mmol) of 18C6, and 0.246 g (0.9 mmol) of C-Br (9-(2-
bromoethyl)-9H-carbazole) were all mixed at once and refluxed
under nitrogen for 48 h. The solvent was removed under
vacuum, and the reaction mixture was then poured in water
and extracted with methylene chloride. The solvent was
removed under vacuum, and the resulting reaction mixture
was dropwise added to cold solution of hexane under vigorous
stirring. A white precipitate was recovered dried under
vacuum and was confirmed by NMR. The SEC analysis showed
Mn ) 34 742 Da as the number-average molecular weight. The
Results and Discussion
Electropolymerization. To first test their ability to
form thin films, the precursor polymers (homopolymer
and copolymer) were electropolymerized and deposited
on the ITO substrate. Their chemical structures are
outlined in Scheme 2. Before investigating the electro-
chemical behavior of the cross-linked copolymer CP-
MTC, we first synthesized and studied the cross-linked
CPHT and CPHC homopolymers. The CPHT homopoly-
mer is poorly electropolymerized despite several trials
attempted within a range of applied potentials. Under
visual observation, it started to form a thick white layer
from the first cycle, and if the potential is exceeded
beyond 1.6 V, the film started to degrade as seen from
the appearance of the film turning brown in color. We
believe this happens mainly because of two reasons: (1)
The first is the low grafting density (∼80%) of thiophene
and the remaining phenolic groups, which rapidly
physisorbs and pacifies the ITO surface. (2) Another
reason is that the thiophene monomer oxidation is very
high compared to its polymer. In this case, the introduc-
tion of the nonconducting polymer backbone further
increases the oxidation potential; therefore, high applied
potentials causes degradation of the polymer.13 This is
in contrast to the CPHC homopolymer, which electro-
polymerized readily at a lower onset of oxidation
potential of 0.7 V (Figure 1a).6b This should occur
primarily at the carbazole 3,6-position vs the lower
electron density 2,7-position which is accessible only via
metal-mediated coupling reactions.3,8
1
NMR showed more then 95% of overall grafting. H NMR (δ
ppm in CDCl3): 8.135-7.00 (b, 5.5H), 6.54 (b, 4H), 4.51-4.04
(b, 3H), 3.029 (b, 1.3H), 1.98-0.63 (b, 6H). The FTIR (KBr) in
wavenumber (cm-1) shows12 (Ph-O-C) 810-850, (C-S)ring
950, (Ph-O-C) 1210 (br, s), aromatic (C-H) 1250-1270 (s),
(C-N)ring 1250-1380, aromatic (C-H) 1470-1525 (s), thiophene
(C-C)ring 1580 (s), aromatic (C-H) 1590-1625, carbazole (Cd
C)ring 1650 (s), thiophene (CdC)ring 1746 (s), υs(CH2) 2860 (br),
υ(CH) 2910 (br), υas(CH2) 2932 (br), carbazole (C-N)ring 3000
(br, s), thiophene (C-C)ring 3310 (w), (Ph-O-H) 3400 (br, w).
Synthesis of Poly[3-{2-(4-vinylphenoxy)ethyl}thio-
phene] (PHT) (Scheme 2). In a solution of 20 mL of distilled
acetone, 0.255 g (1.70 mmol) of PVP1, 1.38 g (10 mmol) of K2-
CO3, 0.066 g (0.25 mmol) of 18C6, and 0.4 g (2.55 mmol) of
T-Br (3-(2-bromoethyl)thiophene) were all mixed at once and
refluxed under nitrogen for 48 h. The solvent was removed
under vacuum, and the reaction mixture was then poured in
water and extracted with ethyl acetate. The organic extract
was dried over Na2SO4. After filtration the organic extract was
concentrated. The concentrate was then added dropwise in
hexane to give a white precipitate, which was then filtered
and washed thoroughly again with diethyl ether. The precipi-
tate was dried under vacuum, and the product was confirmed
by NMR. The NMR showed more then 80% grafting. The SEC
analysis showed Mn ) 16 257 Da as the number-average
1
molecular weight. H NMR (δ ppm in CDCl3): 7.20-7.00 (b,
2.6H), 6.54 (b, 4H), 5.33 (b, 0.2H), 4.04 (b, 1.60H), 3.029 (b,
1.60H), 1.98-0.63 (b, 6H). The FTIR (KBr) in wavenumber
(cm-1) shows12 (Ph-O-C) 810-850, (C-S)ring 950, (Ph-O-
C) 1210 (br, s), aromatic (C-H) 1250-1270 (s), aromatic (C-
H) 1470-1525 (s), thiophene(C-C)ring 1580 (s), aromatic (C-
H) 1590-1625, thiophene (CdC)ring 1746 (s), υs(CH2) 2860 (br),
υ(CH) 2910 (br), υas(CH2) 2932 (br), thiophene (C-C)ring 3310
(w), (Ph-O-H) 3400 (br, w).
Thus, for the CPHT precursor polymer, it is of high
interest to determine electropolymerizability when an-
other monomer, carbazole, is present in the same
polymer backbone.3,9,16 By introducing the carbazole
monomer with a lower oxidation potential and ensuring
high grafting density close to 100%, copolymerization
can potentially extend the conjugation of the thiophene
Synthesis of Poly[9-[2-(4-vinylphenoxy)ethyl]-9H-car-
bazole] (PHC). In a solution of 20 mL of distilled acetone,
0.255 g (1.70 mmol) of PVP1, 1.38 g (10 mmol) of K2CO3, 0.066

3682 Taranekar et al.
Macromolecules, Vol. 38, No. 9, 2005
In the past Sarac et al.16 have demonstrated that a
high feed ratio of thiophene to carbazole is required to
enhance effective copolymerization. Therefore, we in-
corporated more thiophene units than the carbazole in
the precursor copolymer to facilitate electrocopolymer-
ization. The CV curves are shown in Figure 1. In
CPMTC (Figure 1b) after 15 cycles we observed two
anodic oxidation peaks Epa1 and Epa2 at 1.14 and 1.37
V, respectively, with their corresponding reduction peak
potentials Epc1 and Epc2 occurring at 0.89 and 1.19 V. It
is interesting to observe that after five cycles the Epc1
value does not increase; instead, we see a constant
growth in Epc2 values only, suggesting the formation of
polythiophene.16 Thus, on an electrode surface, although
the percentage of incorporated carbazole was low, the
polymerization of thiophene only becomes facile due to
the presence of carbazole. The fact that we observe a
broad oxidation peak in the range of the oxidation
potentials of the two monomers suggests that the
electrocopolymerization process has taken place. A
smaller amount of the carbazole triggers the electro-
polymerization of the thiophene units. This fact is
highlighted since the statistical ratio of thiophene to
carbazole in the precursor polymer is almost 2:1. Note
that the possibility of R-R, R-â, and â-â reaction on
the thiophene units cannot be easily distinguished by
CV methods alone, although on the basis of electron
density of the C centers, the R-R coupling is expected
to predominate.3 Similarly, the 3,6-position coupling is
expected to predominate vs the 2,7-positions of the
carbazole.8
To further understand the CPMTC polymer behavior,
we studied the copolymerization of blends formed by
mixing two homopolymers with different feed ratios, as
seen in Figure 2. The cross-linked copolymer 27-(cbz-
co-th) comprised a mixing feed ratio of 25:75 PHC:PHT.
As shown in Figure 2a, two cathodic peaks Epc1 and Epc2
at 0.73 and 1.1 V, respectively, were observed. The
absence of the 1.1 V in the case of CPHC-1.6 polymer
(Figure 2a) but present in the copolymer 27-(cbz-co-th)
suggests that this peak is primarily due to poly-
thiophene formation. This observation supports our
argument of polythiophene formation as stated in the
case of CPMTC. In the case of copolymer 72-(cbz-co-th)
formed upon mixing the two homopolymers PHC:PHT
in the ratio of 75:25 (Figure 2b), we observed similar
cathodic and anodic peak potentials to that of copolymer
27-(cbz-co-th) but with different peak current ratios. The
peak cathodic current Ipc1 was found to be twice that of
Ipc2 in the case of copolymer 72-(cbz-co-th), but both of
the Ipc values were found to be equal for that of
copolymer 27-(cbz-co-th). Both these copolymers were
found to be optically opaque as compared to the CPMTC
copolymer, which was optically clear. This is perhaps
due to the influence on film morphology with higher
polymer backbone (polystyrene) content on the mixture.
It should be noted that the copolymer formed from the
mixture of the two individual homopolymers bear twice
the molar content of the “nonconducting” PVP backbone
as compared to that of CPMTC. It should be possible to
distinguish synergistic, nonsynergistic, or additive elec-
trochemical behavior by investigating more composition
ratios.
Figure 1. Cyclic voltammetry on ITO for precursor poly-
mers: (a) CPHC, (b) CPMTC, and (c) CPHT.
monomer, resulting in a significant reduction in the
overall oxidation potential of the precursor polymer.
Increased planarity along the polymer chain caused by
the coupling of thiophene and carbazole units can also
lower the oxidation potential.14
Spectroelectrochemical Studies. Figure 3a shows
the in-situ spectroelectrochemical studies of the CPHC
homopolymer cross-linked film analyzed after stepwise

Macromolecules, Vol. 38, No. 9, 2005
CPN Films from Precursor Polymers 3683
Figure 2. Cyclic voltammetry on ITO for copolymers formed
upon mixing the two homopolymers up to three cycles: (a)
copolymer 27-(cbz-co-th) and CPHC polymerized up to 1.6 V;
(b) copolymer 72-(cbz-co-th).
Figure 3. Spectroelectrochemical analysis performed in 0.1
M TBAP/CH2Cl2 on (a) PHC polymer with (a) 0, (b) 0.8, (c)
0.9, (d) 1.0, and (e) 1.1 V vs Ag/AgCl and on (b) PMTC polymer
with (a) 0, (b) 0.8, (c) 1.0, (d) 1.1, (e) 1.2, and (f) 1.4 V vs Ag/
AgCl.
oxidation up to 1.1 V and then brought to its neutral
state at 0 V. As the applied potential is increased, the
absorption in the visible due to the π-π* transition
bleaches.17 In CPHC, which contains carbazole units,
an absorption peak is visible at 310 nm and a shoulder
at 335 nm is observed, which are typically due to the
π-π* transition in polycarbazole.18,19 In the case of
spectroelectrochemical observation done under mono-
mer free conditions, we observed that with increasing
voltage the peak at 310 nm bleaches and the film starts
to turn green giving rise to two new peaks at 418 and
699 nm during the stepwise oxidation of the cross-linked
polymer film. The peaks at 310, 418, and 699 nm have
been understood to arise from the valence band to
conduction band, polaron bonding level to π* conduction
band, and bonding level to antibonding state of polaron
transitions, respectively.20,21
677 nm. This peak around 677 nm is attributed to the
formation of a copolymer of thiophene and carbazole.22
The absence of the absorbance at 418 nm in the case of
CPMTC compared to CPHC indicates that individual
carbazole polaronic transitions are not occurring, and
this optical behavior can therefore be attributed to the
carbazole intervention in the thiophene to form a true
copolymer. Nevertheless, the possibility of carbazole-
carbazole oligomerization cannot be discounted.9 In
general, with copolymerization, the reaction of two
radical cation species results in dimer formation9
R1H f R1H•+ + e-
R2H f R2H•+ + e-
In the case of cross-linked CPMTC the polymer peak
due to the π-π* transition appears at 366 nm (Figure
3b). After stepwise variation in the potentials from
0-1.5 V and back to 0 V, we observe that the π-π*
transition bleaches at 1.5 V and a new peak appears at
R1H•+ + R2H•+ f R1-R2 + 2H+
while the reaction of a radical cation with the neutral

3684 Taranekar et al.
Macromolecules, Vol. 38, No. 9, 2005
comonomer results in copolymer formation:
R1H f R1H•+ + e-
R1H•+ + R2H f R1H-R2H•+
R1H-R2H•+ f R1-R2 + 2H+ + e-
In this case, this latter mechanism is predominant in
the presence of the lower oxidation potential carbazole
with thiophene. The neutral state (dedoped) of both
cross-linked CPHC and CPMTC polymers are colorless.
It should be noted that the combination of two
monomers in the precursor copolymers or the blend of
the homopolymers results in the statistical possibilities
of thiophene-thiophene, carbazole-carbazole, and
thiophene-carbazole electropolymerizations. The case
can also be added for two other limiting cases: intra-
molecular reaction (with the same chain) which can lead
to a ladder-type structure or intermolecular reactions
(with another chain), which leads to a highly cross-
linked or network architecture. The former gives way
to a “templating” of the electrocopolymerization process
where the monomers are already connected to the same
polymer backbone. The latter results in network cross-
linking of individual polymer chains through the pen-
dant monomers. These possibilities are not easily dis-
tinguished in the current system although the higher
probability of intermolecular cross-linking is more prob-
able because of (a) an insoluble film is formed, (b) the
appearance of electropolymerized CPHT species only in
the presence of carbazole monomers, and (c) the blends
of the homopolymers resulted in electropolymerization,
which is not possible for the PHT alone. Future studies
can be made using labeled monomers or even a pre-
formed polymer backbone template that favor intra-
chain ladder-type reactions, e.g., stiff polymer chains.
EQCM. The mass deposition per cycle and ion trans-
port properties were studied by electrochemical quartz
crystal microbalance (EQCM) during the electrochemi-
cal polymerization. While it is true that viscoelastic
effects do play a role in frequency changes when
characterizing films by EQCM, it has been shown that
EQCM is an effective tool for characterizing sufficiently
thin films.23 For thin films (where the film thickness is
much less than the wavelength of the piezoelectrically
launched shear waves) a simplified form of the mass-
frequency relation can be used. From the well-known
Sauerbrey equation, the frequency shift ∆F (Hz) is a
function of several parameters in the QCM setup.24
Upon application of potential (see Supporting Infor-
mation Figure 1a,b), the deposited mass increases due
to the oxidative CPHC or CPMTC deposition, in addition
to the mass associated with hexafluorophosphate coun-
terions. After sweeping the potential back to 0 V, the
polymer is reduced to the neutral form, and a loss of
mass on the crystal occurs due to predominant transport
of anions out of the polymer. It is interesting to note
that the mass loss in the region corresponding to anion
departure from the film upon reduction is identical for
each scan in the case of CPMTC as compared to CPHC
polymer. Both films as seen from Figure 4a show almost
uniform rate of growth with time as observed by the
periodic changes in their frequency values. Figure 4b
shows the change of mass per cycle. Since the applied
potentials are different for each case, it is hard to make
conclusions regarding any autoacceleration effects from
Figure 4. EQCM studies for the polymers PHC and PMTC
during electropolymerization.
the carbazole during the electropolymerization. How-
ever, it should be noted that both CPMTC and CPHC
polymers behave ideally in terms of the mass deposition/
cycle, giving a homogeneous film growth.
Band Gap/Scan Rate Studies. The redox behavior
of the CPMTC film was investigated by cyclic voltam-
metry with films prepared by electropolymerization up
to five cycles. The polymer film showed a reversible
p-doping and a pseudo-reversible n-doping. The anodi-
cally scanned cyclic voltammograms (see Supporting
Information, Figure 2a) showed an oxidation onset of
0.65 V and gave a peak at 0.89 V and a corresponding
reduction peak at 0.68 V. The cathodic wave during
n-doping had an onset of -1.36 V and gave a peak at
-2.22 V. The electrochemical energy gap, ∆E° (∆E° )
∆E°ox - ∆E°red), was found to be 3.12 V, which is slightly
lower than the optical energy band gap of 3.38 eV (366
nm).
At this point, it is interesting to investigate the
kinetics of the redox reaction of the adsorbed layer,
which was performed by varying the scan rate from 20
up to 100 mV/s (Supporting Information, Figure 2b). The
electrochemical conditions were kept similar as de-
scribed earlier for the band gap studies. An analysis of

Macromolecules, Vol. 38, No. 9, 2005
CPN Films from Precursor Polymers 3685
the CV peak current as a function of scan rate in
monomer-free solution shows a linear response to ion
transport (inset, Figure 2b). This indicates a diffusion-
controlled process for ion transport on the adhered
polymer to the electrode surface. In this case, if the mass
transfer is diffusion-limited, Ipa is proportional to the
scan rate1/2
.
XPS Studies. Figure 5 shows the high-resolution
XPS spectra CPHT, CPHC, and CPMTC cross-linked
polymers on ITO. The absence of fluorine (F 1s) and
phosphorus (P 2p) indicates that the films were com-
pletely reduced, and therefore no counterions were
detected. The CPHT cross-linked polymer primarily
physisorbs on the surface of ITO because of the phenolic
group, and therefore we observed very poor sulfur
intensity. Instead, we observed a broad band with a very
high intensity of O 1s (Figure 5d). The CPHC and
CPMTC cross-linked polymers gave a single peak
around 532.8 eV, which can be attributed to C-O-C
bonds. The observation of only nitrogen peak in the case
of CPHC and both nitrogen and sulfur peaks in CPMTC
confirms the presence of the cross-linked films on the
electrode. Table 1 summarizes the presence of carbon,
nitrogen, and sulfur elements, their binding energies
(BE), and their relative atomic concentrations (AC)
present in the films. In all the quantitative analysis the
theoretical value is assumed on the basis of 100%
grafting and then compared to the experimental value.
As an example for CPHC, the theoretical values are O/N
) 1.14 and C/N ) 20.57 and the experimental values
are 1.68 and 22.2, respectively, which showed close
agreement. In CPMTC assuming the grafting density
for thiophene as 65% and 35% for carbazole, the
calculated theoretical value is S/N ) 4.59, but the
experimental value was found to be S/N ) 3.92. It is
possible that more carbazole containing precursor poly-
mers were incorporated in the film. However, XPS alone
cannot reveal the extent of cross-linking in the system,
and therefore detailed studies are underway to under-
stand and possibly distinguish more complicated be-
havior in terms of their inter- vs intramolecular cross-
linking.
AFM Analysis. Interesting features of these films
also relate to the morphology of the electropolymerized
films at different cycles. The cross-linked film average
thickness was characterized using AFM profilometry
measurements (see Supporting Information, Figure 2).
The CPHC and CPMTC cross-linked polymer average
film thicknesses after 15 CV cycles were found to be 110
and 95 nm, respectively. Figure 6a,b shows the CPHC,
CPMTC, and CPHT cross-linked films after five CV
cycles. The CPHC after five cycles showed a relatively
rough morphology with rms roughness of 5.7 nm as
compared to that of CPMTC, which showed more porous
morphological features. Last, the CPHT cross-linked
homopolymer shows (Figure 6c) a very rough and patchy
film which was nonuniform macroscopically. These
observations also correlated with the optical transpar-
ency of the CPHC and CPMTC films as compared to
the opaque CPHT film. Since these measurements were
taken on the initial stages of the film formation after
five CV cycles, the observed morphologies can be related
to the nucleation process as well as the effects of mass
ion transport. The fact that the CPHC has a lower
oxidation potential and linear CV deposition behavior
suggests a rapid 3-D nucleation growth. For CPMTC
Figure 5. High-resolution XPS spectra of the cross-linked
polymers: (A) C 1s, (B) N 1s, (C) S 2p, and (D) O 1s.
the porous structure is believed to be due to presence
of more linear thiophene, facilitating better ion trans-
port. In previous studies done on thiophene copolymers,
similar trends in terms of morphologies were observed27

3686 Taranekar et al.
Macromolecules, Vol. 38, No. 9, 2005
Figure 6. (3 × 3 µm2) 3D atomic force microscopy of the cross-linked precursor polymer after five CV cycles: [A] CPHC, [B]
CPMTC, and [C] CPHT.
Table 1. High-Resolution XPS Spectra of the Cross-Linked Precursor Polymers
25
25
26
C1s
N1s
S2p
precursor polymers
PHT
BE (eV)
AC
BE (eV)
AC
BE (eV)
AC
284.8 (C-C, C-H)
32.62
163.6 (2p3/2
164.8 (2p1/2
)
)
1.04
PHC
284.8 (C-C)
88.55
86.90
3.98
1.25
286.4 (C-N, C-O)
284.8 (C-C)
398.8 (N-C)
398.8 (N-C)
PMTC
163.6 (2p3/2
164.8 (2p1/2
)
)
4.90
286.4 (C-N, C-O)
and were related to 3D disordered structures. Since the
CPHT was hardly polymerized electrochemically, it was
not surprising to see a patchy morphology resulting from
incoherent deposition of the homopolymer. Last, cor-
relation with the CV and electro-QCM studies and the
XPS results confirms this copolymer behavior. The fact
that the CPMTC was well-behaved in terms of linear
film growth, uniform ion transport, and high incorpora-
tion of copolymer units results into films with high
uniformity and optical transparency. Further studies
will be made on these polymers as it relates to poten-
tiostatic deposition methods. OLED applications studies
toward ITO hole-transport layers are underway.6b
Conclusion
In this investigation, two different pendent electro-
active monomers (carbazole and thiophene) were grafted
onto a polymer to form precursor copolymer with a
binary composition. Homopolymers were also synthe-
sized for comparative studies. The precursor polymers
were electrochemically polymerized anodically by CV to
study the cross-linking behavior and CPN film forma-
tion. The homoprecursor polymers showed good elec-
tropolymerizability for the carbazole but not for the
thiophene. However, the copolymer showed very good
electropolymerizability which showed incorporation of
thiophene and carbazole units, and the films were

Macromolecules, Vol. 38, No. 9, 2005
CPN Films from Precursor Polymers 3687
Advincula, R. C. J. Phys. Chem. B 2004, 108, 18949-18955.
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opaque films and required higher potentials to deposit.
The mechanism involved a “trigger” effect for the
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zole units in a radical cation to neutral monomer
reaction pathway. Both copolymer and homopolymer
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molecular and intermolecular reaction between indi-
vidual polymer chains, resulting in templating and
cross-linking. This was verified by CV, spectroelectro-
chemistry, and XPS measurements. The morphology of
the films correlated well with the deposition behavior.
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Acknowledgment. The authors of this paper thank
the partial funding from the Robert Welch Foundation
(E-1551), NSF-DMR (99-8820110), NSF-DMR (0315565),
and NSF-CTS (0330127). We also thank Dr. Paul A. W.
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Supporting Information Available: EQCM studies,
band gap/scan rate, and AFM profilometery measurements.
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