Characterization of reaction intermediate aggregates in aniline oxidative polymerization at low proton concentration

Article abstract of DOI:10.1021/jp102623z

Aggregates of reaction intermediates form during the early stages of aniline oxidative polymerization whenever the initial mole ratio of proton concentration to aniline monomer concentration is low ([H⁺] ₀/[An]₀ 1.0). Detailed characterization is carried out on those aggregates. The intermediate aggregates show a UV-Vis absorption peak at around 410 nm when dispersed in aqueous solution, whereas the peak is centered on 370 nm when dissolved in an organic solvent such as N-methylpyrrolidone. The electronic band gap decreases when the intermediates aggregate to form a solid, and thus, the absorption peak is red-shifted. Gel permeation chromatography (GPC) shows the aggregates contain a major low molecular weight peak with a long tail. The oligoanilines with low molecular weights consistently show a UV-Vis absorption peak at around 370 nm. Mass spectrometry confirms that the intermediate aggregates contain mainly a component with mass number 363 (M + H⁺), likely a tetramer. UV-Vis, GPC, mass spectrometry, NMR, FTIR, and XRD characterization results are presented and chemical structures for the tetramer are proposed. The major components of the intermediate aggregates are likely highly symmetric phenazine- and dihydrophenazine-containing structures. These particular organic compounds have not been identified before as intermediates. The aggregation and precipitation of the tetramers apparently stabilizes these intermediates. The aggregates are highly crystalline, as evidenced by powder X-ray diffraction. A new reaction mechanism for the formation of these intermediates is proposed.

Full text of DOI:10.1021/jp102623z

J. Phys. Chem. B 2010, 114, 10337–10346
10337
Characterization of Reaction Intermediate Aggregates in Aniline Oxidative Polymerization
at Low Proton Concentration
Zhongfen Ding,*,† Timothy Sanchez,‡ Andrea Labouriau,§ Srinivas Iyer,‡ Toti Larson,|
Robert Currier,⊥ Yusheng Zhao,# and Dali Yang*,§
Materials Physics and Applications DiVision, Bioscience DiVision, Materials Science and Technology DiVision,
Earth and EnVironmental Science DiVision, Chemistry DiVision, and Los Alamos Neutron Scattering Center,
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
ReceiVed: March 23, 2010; ReVised Manuscript ReceiVed: June 30, 2010
Aggregates of reaction intermediates form during the early stages of aniline oxidative polymerization whenever
the initial mole ratio of proton concentration to aniline monomer concentration is low ([H+]0/[An]0 e 1.0).
Detailed characterization is carried out on those aggregates. The intermediate aggregates show a UV-Vis
absorption peak at around 410 nm when dispersed in aqueous solution, whereas the peak is centered on 370
nm when dissolved in an organic solvent such as N-methylpyrrolidone. The electronic band gap decreases
when the intermediates aggregate to form a solid, and thus, the absorption peak is red-shifted. Gel permeation
chromatography (GPC) shows the aggregates contain a major low molecular weight peak with a long tail.
The oligoanilines with low molecular weights consistently show a UV-Vis absorption peak at around 370
nm. Mass spectrometry confirms that the intermediate aggregates contain mainly a component with mass
number 363 (M + H+), likely a tetramer. UV-Vis, GPC, mass spectrometry, NMR, FTIR, and XRD
characterization results are presented and chemical structures for the tetramer are proposed. The major
components of the intermediate aggregates are likely highly symmetric phenazine- and dihydrophenazine-
containing structures. These particular organic compounds have not been identified before as intermediates.
The aggregation and precipitation of the tetramers apparently stabilizes these intermediates. The aggregates
are highly crystalline, as evidenced by powder X-ray diffraction. A new reaction mechanism for the formation
of these intermediates is proposed.
Introduction
others.21 We found that the [H+]0/[An]0 ratio is important in
the polymerization kinetics due to aniline protonation (its
conjugated acid has a pKa of 4.87).22 Whether the aniline
monomer is fully protonated appears to determine the initial
reaction pathway. Protonated aniline is slow to lose an electron
to form a radical cation, whereas free aniline loses the electron
easily.
When [H+]0/[An]0 > 1.0, the intermediate aggregates were
not detectable by UV-Vis, as was the case for the traditional
PANI synthesis in strong acids.23 When [H+]0/[An]0 e 1.0,
unprotonated aniline was oxidized quickly to form the inter-
mediate aggregates that precipitated from the reaction suspen-
sion, thus slowing down further reaction.20 These intermediate
aggregates can form tetragonal rods when aggregating slowly
or platelets when aggregation is fast.20 By tuning the initial
[H+]0/[An]0 ratio, we slowed aggregation of the intermediates
and demonstrated synthesis of nanotubes with rectangular cross
sections.24
Structured materials made from the conducting polymer
polyaniline (PANI) have unique properties associated with their
high surface area and ease of processing. The applications
include antistatic and anticorrosion coatings,1-3 sensors,4,5
supercapacitors,6,7 microelectronics,8,9 batteries, and gas storage.10,11
Thus, the link between synthesis conditions and PANI morphol-
ogy has been investigated intensely in recent years. Factors
known to affect PANI morphology include reaction temperature,
aniline concentration, acid concentration, reactant ratios, me-
chanical disturbances, seeding strategy, and dopants.12-15
Aniline oxidative polymerization reaction can be carried out
at different acid conditions, ranging from pH ) 0 to pH )
11.16,17 In recent years, it was discovered that aniline polym-
erization in aqueous solutions of a variety of weak acids or in
water leads to PANI nanotube formation.13,14,18,19 We recently
showed that the initial proton-to-aniline mole ratio ([H+]0/[An]0)
affects PANI nanomorphologies and found that the formation
of detectable intermediate aggregates is directly related to PANI
nanotube formation,20 consistent with the observations by
Aniline polymerization in pH-static conditions25 differs from
our reaction system in which pH naturally decreases as reaction
proceeds. At the initial stage, the pH drops as aniline is oxidized
in our system. When [H+]0/[An]0 < 1.0, there is a very fast step
during which intermediates form, followed by a decrease in the
rate of intermediates formation. Under the pH-static condition,
formation of the intermediates remains constant for a significant
amount of time. Nevertheless, the pH condition for both the
pH-static method and our simple synthesis system correlates
well.25 Specifically, [H+]0/[An]0 ) 1.0 corresponds to a point
between pH ) 2 and pH ) 3 regarding the initial free aniline
* To whom correspondence should be addressed. (Z.D.) Address: MPA-
11, MS D429, Los Alamos National Laboratory, Los Alamos, NM 87545.
Fax: +1 505 665 4292. E-mail: zding@lanl.gov. (D.Y.) Address: MST-7,
MS E549, Los Alamos National Laboratory, Los Alamos, NM 87545. Fax:
+1 505 667 8109. E-mail: dyang@lanl.gov.
† Materials Physics and Applications Division.
‡ Bioscience Division.
§ Materials Science and Technology Division.
| Earth and Environmental Science Division.
⊥ Chemistry Division.
# Los Alamos Neutron Scattering Center.
10.1021/jp102623z  2010 American Chemical Society
Published on Web 07/26/2010

10338 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Ding et al.
SCHEME 1: The Chemical Structures of Some Organic
Molecules Discussed in the Text
polymerization products (dissolved in NMP) that absorb at ∼370
nm have been synthesized and reported by different groups with
quite different interpretation as to what the products are, ranging
from linear tetramers,30 to “azane” type structures,31 to “N-phenyl
phenazine and pseudomauveine” type structures,14 and to
macromolecules with “Michael-type addition on benzoquinone
monoamine” structures.32 Recently, Zujovic et al. isolated a
reaction intermediate after 1 h into reaction, when the reaction
suspension had a pH of 3.5. The intermediate was diluted and
settled-out in water for 5 days. When dissolved in NMP, the
intermediates also showed a UV-Vis absorption peak at 372
nm.33 Alanine was added in the reaction, and as a result, the
initial pH was close to neutral for their reaction. Many possible
structures were eliminated, on the basis of 15N NMR results,
and an oxidized trimer structure (C18H13N3O), with molecular
weight of 287 was proposed.33 Zujovic et al. expressed reserva-
tions on this proposed structure, noting the complexities in
characterizing the various reaction products.33
Elucidating chemical structures of the intermediates is central
to fully understanding aniline polymerization mechanisms. Due
to multiple concurrent reactions, intermediate aggregates are
expected to be a mixture of various species. However, the
consistent UV-Vis absorption peak20 suggests certain relatively
stable chromophores exist in intermediate aggregates formed
at the early stages of polymerization when the [H+]0/[An]0 is
less than 1.0. To probe these intermediates, their aggregates were
separated after only 10 min of reaction (to avoid complexities
associated with further reactions). Combined UV-Vis, GPC,
GC/MS, ESI-MS, MALDI-TOF MS, NMR, FTIR, and XRD
results for the intermediate aggregates are reported. The
aggregates clearly consist of several major components although
multiple other species (reaction products) are also present.
Chemical structures of the major components of the intermediate
aggregates and a formation mechanism are proposed on the basis
of our analyses. The proposed tetramer structures are different
from the related results in the literature.14,30-33 The intermediate
aggregates are quite stable as a solid phase and, thus, may be
asignificantpartofmanyPANIproductsstudiedpreviously.14,30-33
They may also be responsible for the specific properties of those
PANI products, such as absorbing at ∼370 nm when dissolved
in NMP, specific FTIR peaks (∼1414 cm-1), or having
crystalline X-ray diffraction peaks.34 Structures for the major
components of the intermediate aggregates are proposed without
ruling out other possible structures (e.g., larger molecules,
hydrolysis products, oxidized products etc) as the minor
components.
concentration; [H+]0/[An]0 < 1 corresponds to the condition of
pH ) 4, 5, and 6; whereas [H+]0/[An]0 > 1 corresponds to pH
) 1.
The intermediate aggregates absorb at ∼410 nm in situ.20 This
∼410 nm absorption peak is a single peak in the 300-1000
nm range, which is different from the broad band (∼400-440
nm) observed together with the broad band at >600 nm for
typical PANI emeraldine salt (ES) spectra.26 This ∼410 nm
absorption was observed before and was considered to be
N-phenylquinonediimine (PQDI, Scheme 1b).17,27 This assign-
ment was based on the results that PQDI can be synthesized by
oxidizing aminodiphenylamine (ADPA, Scheme 1a) using
freshly synthesized silver oxide,28 and it has a UV-Vis
absorption peak at ∼428 nm.29 We repeated the PQDI synthesis
and characterized the ADPA and PQDI using both GC/MS and
MALDI-TOF mass spectrometry as a reference. We found that
the dimer ADPA (mass 184) was detectable by GC/MS, and
Experimental Section
both ADPA (mass M+ ) 184) and PQDI (mass M + H+
)
Reagents. Aniline (An), ammonium persulfate (APS) (ACS
regent, 98+%), aminodiphenylamine (ADPA), HPLC grade
N-methyl-2-pyrrolidone (NMP), and 60 Å silica gel for column
chromatography were purchased from Sigma Aldrich. 1 N
hydrochloric acid (HCl), ammonia, silicone gel flexible-backed
thin layer chromatography (TLC) plates with fluorescent indica-
tor, HPLC grade tetrahydrofuran (THF), methanol, acetonitrile,
and cyclohexane were obtained from Fisher Scientific. Deionized
(DI) water was used for all reactions and washing purposes.
Aniline was vacuum-distilled prior to use.
Synthesis and Separation. Aniline polymerization at low
[H+]0/[An]0 (e1.0) led to the formation of intermediate ag-
gregates at the beginning of the reaction.20 The goal of the
synthesis was to control the concurrent reactions so that the
intermediate aggregates formed while limiting subsequent
reactions. The reaction intermediates were synthesized at three
183) were detectable by MALDI-TOF MS. However, in the
intermediate aggregates from the aniline polymerization, no
dimers were detected by either GC/MS or MALDI-TOF MS,
which confirmed that the intermediate aggregates that absorbed
at ∼410 nm in aqueous reaction mixture were not likely to be
PQDI (or ADPA).
The intermediate aggregates can be recovered by either
centrifugation or filtration. We further found that the intermedi-
ate aggregates absorbed at ∼370 nm when fully dissolved in
an organic solvent, such as N-methylpyrrolidone (NMP),
acetonitrile (ACN), or tetrahydrofuran (THF). Therefore, both
the ∼410 nm absorption in aqueous dispersion and the ∼370
nm absorption in organic solvents are from the same intermedi-
ate aggregates. This finding is significant and may shed new
light on previously reported results. For example, aniline

Structure Characterization of Oligoanilines
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10339
different conditions: (a) [H+]0/[An]0 ) 1, specifically, 0.2 M
aniline reacting with 0.05 M APS in 0.2 M HCl; (b) [H+]0/
[An]0 ) 0, corresponding to 0.2 M aniline reacting with 0.05
M APS in DI water; (c) 0.2 M aniline reacting with 0.05 M
APS in 0.1 M ammonia aqueous solution. After 10 min of
reaction, 10 times by volume DI water was added into the
reaction mixture to quench the reaction. Intermediate aggregates
were then collected either through centrifugation or filtration.
The intermediate aggregates were washed three times using DI
water and vacuum-dried.
Instrumentation and Characterization. 1. UV-Vis. A Cary
5000 spectrometer (Varian) was used for UV-Vis measure-
ments. After washing, the intermediate aggregates were dis-
persed in DI water for UV-Vis measurements using DI water
as the blank. The dehydrated aggregates were dissolved in
various organic solvents for UV-Vis using the corresponding
organic solvents as blanks. For the measurements in water/NMP
mixtures, the corresponding solvent mixtures were used as
blanks.
2. Gel Permeation Chromatography (GPC). The GPC
consisted of an Alliance 2690 pump equipped with a Wyatt Rex
differential refractive index detector and Waters 996 PDA
detectors and two Polymer Laboratories PL Mixed B GPC
columns. The temperature of the column was held at 60 °C.
The injection volume was 100 µL, and the effluent flow rate
was 1.0 mL/min. Molecular weights were estimated using
retention times of polystyrene standards (Polymer Laboratories,
Inc.) and Waters Corporation’s Empower software. Washed and
vacuum-dried samples were dissolved in NMP (with 0.01 M
LiBF4)35 for GPC studies. The solution was filtered through a
0.45 µm Teflon filter prior to injection.
3. Gas Chromatography Mass Spectrometry (GC/MS). Analy-
ses were performed using a Hewlett-Packard 6890 gas chro-
matograph (GC) interfaced with an Agilent 5975 inert source
mass spectrometer detector (MS). The GC separation was
performed on an Agilent J&W DB-WAXetr column (30 m ×
0.32 mm, 0.50 µm film thickness) using high-purity helium as
a carrier gas (average velocity, 49 cm/s; flow rate, 1.8 mL/min).
The following GC temperature program was used: injector
temperature, 250 °C; initial oven temperature, 100 °C; initial
time, 5 min; temperature ramp, 10 °C/min up to 250 °C with a
40 min final holding time. The mass spectrometer ionization
temperature was 250 °C, and the ionization energy was 40 eV.
The column materials were observed to leach when NMP was
used as solvent. Intermediate aggregates were thus dissolved
in THF instead for the measurements. Aniline and ADPA were
also dissolved in THF and measured by GC/MS as a reference
and to set up the experimental conditions.
Figure 1. UV-Vis spectra of the intermediate aggregates dissolved
in different H2O/NMP mixtures. The solid lines start from a dispersion
in water (line 1) with more and more NMP added into the dispersion
(line 2 f 5). The dashed lines start from a solution in NMP (line 6)
with more and more H2O added in. The ratio are volumetric, for
example, 0.9H2O means 0.9/0.1 v/v of H2O/NMP.
d6) was used as the NMR solvent and as an internal reference
1
for the H and 13C chemical shifts.
6. Fourier Transform Infrared (FTIR) spectroscopy. FTIR
was directly measured on powder samples using a Thermo
Nicolet FTIR instrument powered by OMNIC software.
7. X-ray Powder Diffraction (XRD). Data were obtained
using a Siemens D5000 diffractometer. The diffractometer was
equipped with a Huber Ge (220) cut curved crystal incident
beam monochromator and operated at 50 kV accelerating
voltage and 40 mA of beam current using Cu-KR1 radiation.
8. Elemental Analysis. Elemental analysis was performed
in Atlantic Microlab, Inc. (Norcross, GA). The H, C, N, and S
measurements were accomplished by combustion analysis using
automatic analyzers. Oxygen analysis was performed by py-
rolysis. All analyses were percent by weight determinations.
Results and Discussion
1. UV-Vis Characterization. We recently reported that
whenever the initial proton-to-aniline mole ratio is less than
1.0, the intermediate aggregates form at the early stage of aniline
oxidative polymerization and have an absorption peak at ∼410
nm.20 Due to its poor solubility in water, the intermediate
aggregates form a turbid dispersion in water that can easily be
separated by centrifugation and filtration. On the other hand,
the intermediates can completely dissolve in aprotic polar
solvents (e.g., NMP, DMSO, THF, etc) and show an absorption
peak at ∼370 nm. We studied the UV-Vis absorption of the
intermediate aggregates in solvents of different H2O/NMP
volumetric ratios and found that the absorption peak changes
with solvent composition as shown in Figure 1. The solid curves
were generated by adding increasing amounts of NMP to the
aqueous dispersion. Since intermediate aggregates have poor
solubility in water, the UV-Vis absorbance of the peak is low
(line 1). The absorption peak does not change significantly when
the water content is high (lines 1-3). As more NMP is added
to the solution, more aggregates are dissolved, and finally, a
transparent yellow-brownish solution is formed. The peak
absorbance increases with increasing NMP concentration, and
4. Electro Spray Ionization Mass Spectrometry (ESI-MS)
and Matrix Assisted Desorption Laser Ionization Time-of-
Flight Mass Spectrometry (MALDI-TOF MS). ESI-MS was
performed on a Thermo LCQ-DECA. Samples were prepared
in methanol/H2O (1:1) and injected at a flow rate of 10 µL/
min. The following conditions were used: capillary temp., 280
°C; spray voltage, 3 kV; sheath gas (N2), 60 psi. MALDI-TOF
MS was performed on a 4800 Plus MALDI-TOF TOF (Applied
Biosystems, Framingham, MA) in positive ion, reflector mode.
To avoid interference from matrix peaks, 1 µL of sample was
directly spotted on 16 × 16 stainless steel sample plate, allowed
to air-dry, and analyzed at varying laser intensities.
5. Liquid 1H and 13C Nuclear Magnetic Resonance (NMR).
Experiments were performed at room temperature using a
Bruker Avance NMR spectrometer operating at 300.13 MHz
for 1H and at 75.47 MHz for 13C. Dimethylsulfoxide-d6 (DMSO-

10340 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Ding et al.
Figure 2. GPC result for the intermediate aggregates separated after 10 min of reaction. The major eluent is a small molecular peak (Mp ∼ 320
using polystyrene standards). It has an absorption peak centered at ∼370 nm. The eluent at position 4 (Mp ∼ 1700) also has the 370 nm absorption
peak, with the regular EB peaks emerging.
the peak blue-shifted to 370 nm (line 5). When we reversed
this process, adding water to a concentrated NMP solution of
the intermediate aggregates (dashed lines), the UV-Vis spectra
showed that the peak red-shifted from 370 (line 6) to 420 nm
(line 8).
2. GPC Characterization. The typical molecular weight
distribution for the intermediate aggregates is shown in Figure
2. The elution curve contains a low molecular weight peak (Mp
∼320 relative to the polystyrene standard) with a long tail, which
corresponds to the higher molecular weight components. All
the UV-Vis spectra of the lower molecular weight components
show the same UV-Vis absorption peaks centered at ∼370 nm
(curves 1-4) between 300 and 800 nm. For the higher molecular
weight components, the UV-Vis spectra show a typical PANI
EB spectrum with the absorption peaks centered at ∼310 and
∼630 nm. Although the molecular weight obtained from GPC
is based on the polystyrene standards rather than an absolute
number, the GPC results still provide insight into the molecular
weight distribution. The intermediate aggregates mainly consist
of small molecules, that is, oligoanilines. The long tail reflects
the complications associated with multiple simultaneous reac-
tions in the aniline polymerization process (including spontane-
ous reactions for further chain growth in NMP solution in air).
This results in the presence of species with varying chain
lengths.
All elution curves show a major low molecular weight peak
at the same elution time with a long tail, although the tail part
shows slight differences between samples synthesized at the
three different acid conditions. The GPC results suggest a major
component (∼70 wt %) with a low molecular weight. This
component has a UV-Vis absorption at ∼370 nm. In addition,
the oligoanilines possibly contain the same stable functional
group (chromophore) that absorbs at ∼370 nm even when the
molecular weights are slightly higher (<1700 nominally from
GPC). When the oligoanilines increase in chain length, a broad
absorption at >600 nm appears due to the formation of longer
conjugated structures.
These experiments suggest that the same intermediate ag-
gregates absorb at ∼370 nm in organic solvents but absorb at
∼410 nm in aqueous dispersion. The 410 nm absorption peak
was reported previously for aniline polymerization and was
associated with aniline dimer PQDI.16,27,29 Other research groups
argue that the brown aniline polymerization product with
UV-Vis absorption at around 420 nm is due primarily to the
formation of branched structures.36,37 Aniline polymerization
products that dissolve in organic solvent and absorb at ∼370
nm have also been reported with different assigned structures
ranging from linear tetramer,30 to azane type structure,31 to
macromolecules based on benzoquinoimine structure.32 In
Nalwa’s study, the product absorbed at 370 nm in dimethyl
formamide solution and absorbed at 430 nm when deposited
on a glass slide.30 The reaction was carried out in sulphanilic
acid, a weak acid; thus, it is expected that a significant amount
of the intermediate aggregates existed.20
Comparing the two cases when ∼50 vol% of water is mixed
with ∼50 vol% of NMP, when the mixture starts from the water
dispersion (line 4), it is not as well dissolved as when the mixture
starts from the NMP solution (line 7). Since the measurement
was carried out immediately after mixing, there may not have
been enough time for the NMP molecules to break apart the
aggregates in water to fully solvate them, thus making the peak
slowly blue-shift on line 4. On the other hand, when the
intermediates were first well dissolved in NMP, the absence of
precipitation suggests that the intermediates have strong interac-
tions with NMP solvent.
3. Mass Spectrometry Characterization. The intermediate
aggregates are a mixture of components because there are
multiple concurrent reactions occurring. For such a mixture,
elemental analysis is not a definitive method to determine
composition. The measurements typically result in an average
of all components in the mixture. Thus, we employed a mass
spectrometry technique to probe the major components.
The intermediate aggregates formed at the three different acid
conditions (as described in the synthesis part) have different
morphologies, presumably due to different reaction kinetics.20,24
Nevertheless, the intermediate aggregates show the same
UV-Vis spectra whether dissolved in organic solvents such as
NMP and THF or measured in situ in the reaction systems.

Structure Characterization of Oligoanilines
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10341
3.1. GC/MS Characterization. GC/MS conditions were
optimized for the separation and detection of aniline and ADPA
reference solutions. Aniline had a retention time of 15.211 min
and preserved a primary molecular ion with a mass-to-charge
ratio (m/z) value of 93 (M+). In addition to aniline, a small peak
was observed at a retention time of 15.784 min, the mass spectra
of which matched that of the THF stabilizer, butylated hydroxy-
ltoluene (BHT). For the ADPA in THF solution, ADPA eluted
at a retention time of 43.799 min and preserved a primary
molecular ion with a m/z value of 184 (M+). BHT was also
observed in this sample with a retention time of 15.784 min. In
addition to BHT and the primary ADPA peaks, several small
peaks that are impurities were observed including an eluent with
a retention time of 20.068 min and a m/z value of 169 (M+),
which was identified as diphenylamine (Scheme 1c) and another
eluent with a retention time of 42.204 min, with preserved
molecular fragments with m/z values of 210, 195, 184, and 167,
matching diaminophenazine molecules (Scheme 1g or i).
Figure 3. ESI-MS results for the intermediate aggregates dissolved
in MeOH/H2O (1:1). The major component has a mass number of 363,
corresponding to a tetramer. There are minor impurities in the
intermediate aggregates.
GC/MS with similar experimental conditions has been used
to detect other related small molecules, such as benzoquinone
(Scheme 1h), azobenzene (Scheme 1e), and substituted azoben-
zene etc.38 Similar methods were used to conduct GC/MS
analysis of our intermediate aggregates in THF solution. The
eluent peaks appeared at identical times for the three different
intermediate aggregates formed at (1) low acid, (2) aqueous (no
acid), and (3) ammonia reaction conditions, which suggest that
these intermediate aggregates appear to have similar chemical
structures, compositions, and properties. There were a total of
15 eluent peaks from the intermediate aggregates solutions. The
eluent with a retention time of 15.784 min matches BHT.
Interestingly, the other eluents with different retention times
have similar mass fragments, with each fragment having
different abundances. For example, the eluent at 10.388 min
consisted primarily of fragments with the m/z values of 355
and 267, whereas the eluent at 12.027 min was dominated by
fragments with m/z values of 341 and 429, and the eluent at
12.948 min was dominated by fragments with m/z values of
281, 327, and 147. The major masses in all of these eluents
included 73, 147, 221, 281, 327, 341, 355, and 429. The reason
for formation of these fragments is most probably due to the
cross-linking of the oligoanilines. It is well documented that
the quinoneimine groups are highly reactive. The imine nitrogen
cross-links with benzene ring to form a networked structure
containing mainly tertiary amines at 140 °C in a diluted EB
solution in NMP.39 Although the cross-linking during the
analysis prohibited the usage of the GC/MS characterization to
study the main component of the intermediate aggregates, the
GC/MS analysis ruled out some related small molecules as major
components, such as benzoquinone, azobenzene, ADPA, phena-
zine (Scheme 1d), and N-phenyliminocyclohexa-2,5-dienone
(Scheme 1 f). These molecules should have easily been detected
if they were present.
Figure 4. MALDI-TOF results for the intermediate aggregates that
dissolved in THF. No matrix is added for the measurement to eliminate
interference peaks. The major peak is also at mass number 363.
solvent, and thus, longer oligoanilines might not have dissolved.
However, our primary interest is the major component corre-
sponding to the low molecular weight GPC peak.
The MALDI-TOF MS measurements were performed without
matrix to eliminate the interferences from common matrix
molecules. The resolved MALDI-TOF MS spectrum appears
to be more complicated than that of ESI-MS, as shown in Figure
4. Multiple peaks were detected with high mass resolution.
Consistent with ESI-MS, the major peak also had the m/z value
of 363, although other peaks with the m/z values of 290, 454,
545, 635, 725, and 816 were observed with significant intensity,
as well. Control experiments showed that the MALDI-TOF
results using freshly prepared solution were quite different from
that obtained using a sample aged 1 day. The aged sample had
many more peaks without a predominant peak. We suspect that
cross-linking or chain growth when the intermediate aggregates
were dissolved and then dried up led to the increase in the
number of peaks and higher molecular weight components.
On the basis of the mass spectrometry and GPC results, we
propose that the major components in the intermediate ag-
gregates (>70 wt % from GPC results) are tetramers with a
molecular weight of ∼363. However, the intermediate ag-
gregates also appear to contain multiple oligoanilines with
3.2. ESI-MS and MALDI-TOF Characterization. Unlike the
GC/MS, both ESI-MS and MALDI-TOF MS use soft ionization
techniques at low temperature and, thus, can prohibit the
reactions between the intermediate molecules. Figure 3 shows
the ESI-MS spectra of the intermediate aggregates dissolved in
1:1 methanol/H2O solvent. The major component of the
intermediate aggregates had a m/z value of 363. There were
also minor peaks with the m/z values such as 273, 289, 335,
379, 454, and 470. These minor components may be due to
mass fragmentations or other species formed by further reaction
of the intermediates. It is worth noting that the intermediate
aggregates did not completely dissolve in 1:1 methanol/H2O

10342 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Ding et al.
Figure 5. 1H NMR spectra of (a) the intermediate aggregates and (b) a separated minor component.
1
SCHEME 2: Characteristic H and 13C [in brackets]
NMR Data for the Tetramersa
different chain lengths. Dimers were not detected in the
intermediate aggregates, suggesting that PQDI or two-ring
molecules are not major components in the intermediate
aggregates.
1
1
4. Liquid H NMR and 13C NMR Characterization. H
NMR spectra of the aggregates in DMSO-d6 solution are shown
in Figure 5a. The major peaks of the intermediate aggregates
have chemical shifts at 7.4-7.3 (10H), 7.25-7.1 (4H), 6.27
1
(1H), and 5.72 ppm (1H). The H signals with chemical shifts
at 7.0-7.5 ppm (14H) are associated with protons directly
bonded to aromatic rings. Since the observed chemical shifts
are all <7.5 ppm, each aromatic ring may be directly bonded to
amines. The sharp peaks at 6.27 and 5.72 ppm are characteristic
of the intermediate aggregates. These sharp singlets appear to
1
be a pair with a 1:1 ratio. A similar set of H NMR peaks
between 5.5 and 6.5 ppm was reported independently by
Venancio et al. and assigned to substituted quinoneimine units.31
Kriz and co-workers deduced that these were not N-H protons
and also assigned these peaks to similar substructure “oxygen-
containing substituted quinoneimine structure”.40 On the other
hand, Cotarelo et al. assigned similar peaks (5.16 and 5.57 ppm
singlets) to amine protons in their study of 2-ADPA polymer-
ization.41 It is not certain that these peaks can be readily assigned
to N-H. The primary amine groups (-NH2) easily form
hydrogen bonding and, thus, usually have broad peaks. The
secondary amine groups (-NH-), when connected to two
aromatic rings, usually have a high chemical shift (>8 ppm). It
is not clear that the two peaks observed in this work at 6.27
and 5.72 ppm can unambiguously be assigned to substituted
quinoneimine units or to branched structures.
a The C-H protons not labeled have chemical shifts at ∼7.0-7.5
ppm.
A series of organic molecules that contain quite similar
structures have characteristic 1H chemical shifts at 5.5-6.5 ppm.
For instance, the C-H protons in diaryldihydrophenazines have
chemical shifts at 5.8-6.3 ppm for the C-H protons next to
the C-N on dihydrophenazine rings.42 C-H groups in Mau-
veine dyes have chemical shifts of 5.9-6.3 ppm at similar
positions, even though the detailed structure is considered to
two singlet peaks at 6.27 and 5.72 ppm belong to the tetramer,
5,10-diphenyl-5,10-dihydrophenazine structure, as shown in
Scheme 2a.
The 13C NMR (Figure 6) spectra are surprisingly clean,
considering that the aggregates are a mixture of different
components with the major component having a molecular
weight of 363 (possibly a tetramer with 24 carbon atoms). The
simplicity of the carbon lines suggests that the major components
(tetramers) have symmetric structures. The 13C lines at 95.49
and 95.72 ppm correspond to carbons connecting to the protons
with chemical shifts of 5.72 and 6.27 ppm, respectively. The
13C peaks at 120-130 ppm correspond to the C atoms in
aromatic benzene rings that connected to other C atoms. The
peaks around 140 ppm likely correspond to the C atoms on
1
be N-phenylphenazinium-based.43-45 The H NMR study of
safranine derivatives also shows that the neighboring amino
groups and the N-phenyl substitution on the phenazinium ring
lead to the C-H proton in between to resonate at 5.5-6.0 ppm.46
N-Substitution appears to be critical for the C-H adjacent to
the N-C to have a chemical shift reduced by ∼0.6-0.7 units.46
Combining the mass spectroscopic results, we propose that these

Structure Characterization of Oligoanilines
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10343
Figure 6. 13C NMR for the intermediate aggregates.
peaks at ∼300 and ∼600 nm,48-51 which are different from the
∼370 nm UV-Vis absorption consistently observed for the
intermediate aggregates. In addition, linear tetramers have a 1H
NMR chemical shift at 7-8 ppm for both benzoid and quinoid
groups and have secondary amine (-N-H) protons at ∼8.3 ppm
and primary amine (-NH2) protons at ∼5.5 ppm,15,51 which
are also different from our NMR data. Therefore, we do not
presently believe that the tetramers have linear structures, as
suggested by Nalwa’s study.30 For the azane type structures, as
proposed by Venancio and co-workers,31 and for the Michael
type addition products based on benzoquinone monoamine as
proposed by Surwade and co-workers,32 there should obviously
be strong FTIR absorption peak for the carbonyl group at
1650-1800 cm-1. However, no absorption peak was observed
at the 1650-1800 cm-1 region in the FTIR spectra of the
intermediate aggregates. In addition, the 13C NMR is far too
simple for the polymer or macromolecular structures as proposed
in these articles because those structures should have several
dozens of 13C lines. The wide variety of structures that often
contain “N-phenyl phenazine and pseudomauveine” subunits as
proposed by Stejskal and co-workers14 are the closest to the
structures that we present here. However, all the pseudomau-
veine, or phenasafranin, or N-phenyl phenazine are well-known
purple dyes, which have UV-Vis absorption at 520-540
nm43-45,52 but not at ∼370 nm, as we have observed in the
UV-Vis characterization of the intermediate aggregates in
organic solutions.
Figure 7. FTIR results of the powder of intermediate aggregates
measured directly.
phenazine or hydrophenazine structures directly connected to
N atoms. The peak at 150 ppm suggests carbon on a benzene
ring directly connected to a nitrogen atom. The two peaks with
very high chemical shifts may correspond to carbon atoms
connected to the phenazinium N because the positively charged
N atoms can attract electron clouds similar to what is seen with
oxygen atom. However, these lines may be due to some larger
oligomers with protonated quinoneimine structures.
5. FTIR Characterization. The FTIR spectra are shown in
Figure 7. The plot includes the peaks typically seen in PANI
EB structure:47 (1) the 1582 cm-1 peak corresponding to the
quinoid imine structure, (2) the 1502 cm-1 peak corresponding
to the benzenoid amine structure, (3) the 1297 cm-1 peak due
to C-N-C stretch vibration, and (4) the 1175 cm-1 for benzene
C-H bending vibration. These peaks are similar to those found
in the conventionally synthesized PANI EB (synthesized in
strong acid),47 which have much broader peaks at the similar
wavenumbers. In addition, there are peaks at 1444 and 1413
cm-1. The 1444 cm-1 is likely due to aromatic C-N vibration,
and the 1413 cm-1 is possibly due to totally symmetric ring
stretch of phenazine structure.47 The relatively small 1363 cm-1
peak is likely due to a -CdN-C stretch vibration, and the
1208 cm-1 can be associated with aromatic C-H bending
vibration. The peak at 862 cm-1 is due to a 1,2,4-substituted
benzene ring, and 737 and 696 cm-1 are due to monosubstituted
benzene structure. The peaks at 3264 and 3196 cm-1 are due to
N-H asymmetric and symmetric stretching vibrations. All the
FTIR peaks are accountable from the structures in Scheme 2.
On the basis of the presented UV-Vis, GPC, MS, NMR,
and FTIR characterizations of the intermediate aggregates, we
point out that the proposed structures in the literature conflict
with one or more of our characterization results. For example,
the phenyl capped linear tetramers have UV-Vis absorption
We suggest that the proposed tetramers have two main
structures, as shown in Scheme 2a and b. Phenazine has a
UV-Vis absorption at 368 nm.53,54 The 2,7-diaminophenazine
also has absorption at 370 nm.55 In addition, when phenazine
is mixed with dihydrophenazine, phenazhydrins form, which
absorb similarly to phenazine.56 This electronic absorption is
due to π f π* transitions. Substitution to the benzene rings on
the phenazine structure does not strongly affect the absorption.54
However, substitution on the N atoms forming the phenazinium
structure leads to characteristic absorption at 500-540 nm, as
for the mauveine type dyes.44,54 The structures a and b thus
match the UV-Vis results and can also account for the
consistent ∼370 nm peak. These two structures are both
symmetric, and as a result, the characteristic totally symmetric
phenazine ring stretching vibration peak (1413 cm-1) measured
by FTIR is quite strong. All the FTIR peaks and major NMR
peaks can also be accounted for in these structures. On the basis
1
of the H NMR peak integration, the ratio of structures a to b
in the intermediate aggregates is approximately ∼1:1. The
compound diphenyldiaminophenazine (b) has 16 C-H protons
with a chemical shift of 7-7.5 ppm, whereas the compound
diphenyldiaminodihydrophenazine (a) has 12 C-H protons with
a chemical shift of 7-7.5 ppm, 2 C-H protons of 5.72 ppm,
and 2 C-H protons of 6.27 ppm. A 1:1 ratio of the two

10344 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Ding et al.
compounds matches the integrated results of 14H/1H/1H of the
main peaks in H NMR, as shown in Figure 5a. Zujovic and
co-workers found that the intermediates should have only one
type of imine nitrogen, on the basis of their 15N NMR study,
which is consistent with our proposed structures (b).33
SCHEME 3: Formation Mechanism of the Intermediate
Aggregates at Low Initial Proton Concentrations ([H+]0/
[An]0 e1.0)
1
In addition to these two major compounds, we propose two
minor components shown as Scheme 2c and d. These two
1
structures contribute to the small peaks detected in H NMR
(Figure 5a).
Chemical reaction mechanisms consistent with the observa-
tions were formulated. Scheme 3 shows several options for
aniline polymerization and intermediate formation at low proton
concentrations. The chemical reasoning behind these proposed
mechanisms is as follows: Due to low proton concentration,
free aniline molecules are quite easily oxidized into dimers in
the oxidized PQDI form. Therefore, an appreciable amount of
PQDI accumulates in the reaction system. Since the PQDI is
quite reactive with APS as oxidant, the accumulation promotes
rapid cross-linking to form tetramers. Whether a tetramer is in
either the oxidized or reduced form depends on the stability of
the tetramers. We suggest that there are at least three plausible
cross-linking routes for forming tetramer structures: namely, a,
b, and c, as shown in Scheme 3. In addition to the PQDI cross-
linking, the PQDI can certainly react with free aniline molecules
to form yet another tetramer structure (d). However, structure
d was found to exist only in small amounts, according to the
NMR analysis. Specifically, there were only minor components
in 1H NMR spectra corresponding to the C-H chemical shifts
at ∼7.6 and ∼7.9 ppm as assigned in Scheme 2d. Additional
work will be required to determine more precisely which
mechanisms are dominant.
The tetramer molecules have strong π-π interactions with
poor solubility in water and, thus, form aggregates rapidly. The
aggregation causes phase separation. The aggregates are rela-
tively stable in aqueous solution or in anhydrate form. However,
the aggregates are highly reactive when they dissolve in organic
solvents. The presence of oxygen will promote oxidation and
further reaction of the oligoanilines to form larger molecules,
as well.
The complete separation of the intermediate contents proves
to be very difficult due to the reactivity as observed during mass
spectrometry measurements. TLC was tried for the separation,
and different components were seen to be separated by forming
separated lines. Yet, each line was not pure and showed higher
molecular weight components with the m/z values of 545, 725
etc., indicating that further chemical reaction had been concur-
rent during the separation process. Silica gel column was also
used for separating the intermediate aggregates for multiple
times. Different components could be separated, but the
complication of further reaction remained during the separation
process or other handling processes (e.g., solvent evaporation,
deposition). Therefore, the separated components always con-
tained higher molecular weight peaks. Correspondingly, the 13
C
NMR results show many more lines for the separated compo-
nents than for the original intermediate aggregates collected from
the aqueous reaction mixture. It was found that the oligoanilines
were easily oxidized or react with each other when well-
dissolved in organic solvents.
in Scheme 2c. The fact that structure c forms only in a small
amount may be due to the bulky benzene rings repelling each
other at this position (Scheme 3c) and, thus, inhibiting cross-
linking reactions. Clearly, to obtain pure intermediate aggregates,
extreme care is needed. An improved separation technique (e.g.,
handling the separation under inert atmosphere) is definitely
needed.
One component was separated and purified (after multiple
runs of column chromatography) which appeared to be pure by
1
MALDI-TOF measurement. The H NMR for this component
is shown as Figure 5b. Interestingly, the NMR data matches
1
the set of small peaks when compared to Figure 5a. The H
6. XRD Characterization. The powder XRD pattern of the
intermediate aggregates powder is shown in Figure 8. The
NMR suggests that the structure of this component is as shown

Structure Characterization of Oligoanilines
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10345
Figure 8. Powder X-ray diffraction patterns of the intermediate aggregates.
aggregates formed highly crystalline structures. The XRD
pattern shows a major peak with a 2θ value of 6.28°,
corresponding to a d value of 14.07 Å. This indicates a preferred
orientation, possibly a one-dimensional preference due to strong
π-π interaction of the chromophore functional group. XRD
peaks at 6.28° have been observed before in PANI products.34
The intermediates should be the origin of the detected crystal-
linity in PANI products.
7. Elemental Analysis. The samples were filtered, washed,
and dried under ambient conditions after isolation. A full
elemental analysis of H, C, N, O, and S indicated that the
formula for the intermediates mixture should be
C6.18H5.09N1O0.30, with S at 0.02 at. %. The S is most likely
from SO42- ion adsorption. Thus, we propose the intermediate
tetramers have the atomic ratios (formulas) C6H6N (C24H20N4)
or C6H4.5N (C24H18N4), which corroborates our comprehensive
study results. The small amount of oxygen detected in
the mixture could well be due to hydrolysis of quinoid
matography because the separated components appear to have
a wider range of distinct species (reaction products) than the
original material. Further efforts will concentrate on purifying
the major components to provide additional supporting evidence
for the proposed intermediate structures and formation mechanism.
The formation of intermediate aggregates appears to be a
general phenomenon for reactions at low initial proton to aniline
ratio (<1.0). A reaction mechanism is proposed for the formation
of these molecules. We speculate that tetramers are formed
through cross-linking between oxidized dimers, N-phenyl quino-
nediimine (PQDI). Some minor components are also separated
and identified, supporting the proposed mechanism. We believe
that this work provides some new scientific insights into the
chemical structures of the intermediates. The proposed reaction
mechanism may explain the reaction pathways of the formation
of intermediates. Clearly, additional work is needed to com-
pletely verify our proposed structures and reaction mechanism.
We hope this study will spur additional investigation of this
challenging reactive system.
(
(
)
structures into benzoquinone imine
Acknowledgment. We thank Dr. Ross E. Muenchausen
(LANL, MST-8) for UV-Vis access and Dr. Debra Wrobleski
(LANL, MST-7) for GPC access. We thank Dr. Weizhong Chen
(LANL, C-IIAC) for advice on column chromatography and
thin layer chromatography. We also thank Prof. Richard B.
Kaner (UCLA) for thoughtful discussions. This work was mainly
supported by the Laboratory Directed Research and Develop-
ment Program at Los Alamos National Laboratory, and some
supported by DOE EERE-ITP Program.
) structures.
Conclusion
The intermediate aggregates formed at the early stage of
aniline chemical oxidative polymerization at low initial proton
concentrations were characterized using UV-Vis, GPC, ESI-
MS, MALDI-TOF, 1H and 13C NMR, FTIR, and XRD. It
appears that the intermediate aggregates contain tetramers as
major components. The tetramers consist of mainly symmetric
diphenyldiaminophenazine and diphenyldiaminodihydrophena-
zine structures with a ratio of ∼1:1. These chromophores led
to a consistent UV-Vis absorption at ∼370 nm when dissolved
in organic solvents. The symmetric planar structures of these
molecules also led to strong π-π interactions, accounting for
their poor solubility in water, phase segregation, and packing
into a solid crystalline structure.
The aggregates are relatively stable in aqueous dispersion or
after drying. However, the aggregates are highly reactive when
well dissolved in organic solvents. Oxygen in air promotes
further oxidation of the oligoanilines to form larger molecules.
Therefore, separation and purification of each component
remains a challenge using traditional TLC and column chro-
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