Intramolecular transport of charge carriers in trimeric aniline upon a three-step acid doping process

Article abstract of DOI:10.1021/jp3027258

The acid doping of a methyl-capped aniline trimer, N-[4-(dimethylamino)phenyl]-N-(4-{[4-(dimethylamino)phenyl]imino}-2,5- cyclohexadien-1-ylidene)-amine (TANI), was performed stoichiometrically to study the nature of charge carriers induced by the acid protonation process. The redox centers in TANI were found to undergo a reversible three-step protonation with 1 equiv, 2 equiv and a large molar excess of dodecylbenzenesulfonic acid (DBSA) in chloroform, as evidenced by three different chromophores (doping levels I, II and III) observed using UV-vis-NIR. Acidity of the dopants and solvent polarity were controlling factors. As revealed by electron paramagnetic resonance spectroscopy (EPR), the doping levels I, II, and III achieved by doping 0.1 mM TANI/chloroform solutions with different amounts of DBSA exhibited relative spin densities of 1:1.2:2.2. Since the expected maximum spin population of TANI through acid doping is two spins per molecule, the reduced paramagnetism given by the doubly protonated TANI (doping level II) indicated partially coupled unpaired spins. The third protonation step (doping level III) produced almost double the unpaired spin concentration compared to the lower doping levels and a multiline EPR spectrum likely comprising two overlapping signals of similar overall line width. The hyperfine couplings contributing to the splittings in this signal were estimated by simulation incorporating 6-H and 1-N nuclei most likely from one highly localized unpaired spin on a terminal dimethylamino group, with an underlying apparent singlet arising from a delocalized unpaired spin; the diradical proposed as the species exhibiting the multiplet EPR signal is isolated by the bridging ammonium cation created by the third doping step. The phenomena suggested the stepwise evolution of partly formed diamagnetic bipolarons from polaron interactions at doping level II and the transformation to the more isolated unsymmetrical system we label two polarons on a chain in a triplet state at doping level III. The results provide the characterization of novel doping behaviors for a trimeric aniline molecule in organic solution.

Full text of DOI:10.1021/jp3027258

Article
pubs.acs.org/JPCA
Intramolecular Transport of Charge Carriers in Trimeric Aniline upon
a Three-Step Acid Doping Process
Qi Zhang,† Abdelahad Khajo,‡ Tsunehiro Sai,† Ian de Albuquerque,† Richard S. Magliozzo,‡
and Kalle Levon*,†
†Department of Chemical and Biological Sciences, Polytechnic Institute of New York University, Six MetroTech Center, Brooklyn,
New York 11201, United States
‡Department of Chemistry, Brooklyn College and the Graduate Center of the City University of New York, 2900 Bedford Avenue,
Brooklyn, New York 11210, United States
ABSTRACT: The “acid doping” of a methyl-capped aniline trimer, N-[4-
(dimethylamino)phenyl]-N-(4-{[4-(dimethylamino)phenyl]imino}-2,5- cyclohex-
adien-1-ylidene)-amine (TANI), was performed stoichiometrically to study the
nature of charge carriers induced by the acid protonation process. The redox
centers in TANI were found to undergo a reversible three-step protonation with 1
equiv, 2 equiv and a large molar excess of dodecylbenzenesulfonic acid (DBSA) in
chloroform, as evidenced by three different chromophores (doping levels I, II and
III) observed using UV−vis−NIR. Acidity of the dopants and solvent polarity were
controlling factors. As revealed by electron paramagnetic resonance spectroscopy
(EPR), the doping levels I, II, and III achieved by doping 0.1 mM TANI/
chloroform solutions with different amounts of DBSA exhibited relative spin
densities of 1:1.2:2.2. Since the expected maximum spin population of TANI
through acid doping is two spins per molecule, the reduced paramagnetism given
by the doubly protonated TANI (doping level II) indicated partially coupled unpaired spins. The third protonation step (doping
level III) produced almost double the unpaired spin concentration compared to the lower doping levels and a multiline EPR
spectrum likely comprising two overlapping signals of similar overall line width. The hyperfine couplings contributing to the
splittings in this signal were estimated by simulation incorporating 6-H and 1-N nuclei most likely from one highly localized
unpaired spin on a terminal dimethylamino group, with an underlying apparent singlet arising from a delocalized unpaired spin;
the diradical proposed as the species exhibiting the multiplet EPR signal is isolated by the bridging ammonium cation created by
the third doping step. The phenomena suggested the stepwise evolution of partly formed diamagnetic bipolarons from polaron
interactions at doping level II and the transformation to the more isolated unsymmetrical system we label “two polarons on a
chain” in a triplet state at doping level III. The results provide the characterization of novel doping behaviors for a trimeric aniline
molecule in organic solution.
“two polarons on a chain” structure is predominant because of
reduced coupling between two sites, while the bipolaron
structure (S = 0) tends to form in oligomers with shorter length
of the conjugated system, evidenced by the loss of EPR
signals.9,10
Oligomeric PANI analogues have been synthesized with a
variety of well-defined conjugational lengths for the past
decade, mostly because of their good solubility in common
solvents, ease of crystallization, and chemical manipula-
tion.11−15 While oligomers of the thiophene family have
frequently been used to study the electronic properties evolved
upon oxidation in conjugated structures both experimentally
and theoretically, the systematic study of stoichiometric acid−
base doping−dedoping of aniline oligomers has rarely been
conducted.16−18 For example, Janssen et al. synthesized and
INTRODUCTION
■
Polyaniline (PANI), as one of the most studied conducting
polymers, has attracted considerable attention during the past
20 years due to its adjustable conductivity, good stability and
mechanical flexibility. It has been widely used in sensors, solar
cells, rechargeable batteries, light emitting diodes and electro-
chromic materials.1−6 Unlike most conducting polymers
depending on oxidative and reductive doping, PANI changes
into its conducting form, known as the emeraldine salt form
(ES), through the so-called nonredox or acid doping of its
insulating emeraldine base form (EB).7 Two types of charge
carriers created in that process have been described: the
delocalized paramagnetic singly charged polaron with spin = 1/
2, and the localized doubly charged diamagnetic bipolaron. The
bipolaron structure is widely accepted as being formed by the
energetically favorable coalescence of two polarons into a
nonparamagnetic dication.8 In such a bipolaron structure, the
two unpaired spins are coupled and form a unique deformation
segment. It was postulated that in relatively longer chains, the
Received: March 21, 2012
Revised: June 19, 2012
© XXXX American Chemical Society
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studied the doping and redox behavior of a series of meta−para
aniline oligomers. However, the relationship between chemical
structure evolved through quantitative protonation and the
resulting EPR signals and properties was not addressed in
detail.19,20
product was collected by Buchner vacuum filtration using
̈
Nylon 6-6 filter paper with 0.2 μm pore size and washed with
40 mL of 1 M HCl aqueous solution followed by 80 mL of
distilled water. The product was then treated with 40 mL of 1
M aqueous solution of ammonium hydroxide for 2 h. The solid
was collected and the precipitate cake was washed with distilled
water until the filtrate gave a neutral pH. The remaining solid
was then dried overnight in a vacuum oven at 60 °C. The yield
was 30%. Further purification was performed using flash
chromatography with silica gel (40−200 mesh). The eluant was
hexane:chloroform = 60%:40% (v/v) with an additional 2% of
triethylamine. Thin layer choromatography (TLC) was used to
In this contribution, TANI, a trimeric aniline with 4 redox
centers including 2 imino nitrogens atoms and 2 amino
nitrogens, similar to PANI, was synthesized and studied upon
stepwise acid doping. It exhibited three distinct doped states
(levels I, II, and III) with increasing amount of organic acid
added in chloroform solution, behavior more complex than the
single doped state ES of PANI achieved with 2 equiv of acid.
The follow-up titration with tetrabutylammonium hydroxide
(TBAH), a Brønsted base, proved the reversibility of the three-
step doping process. Although the concept of “secondary
doping” beyond the ES species was proposed by MacDiarmid
and Epstein, the phenomenon was attributed to the change in
PANI-ES conformation from “compact coil” to “expanded coil”
induced by m-cresol instead of a newly formed chemical
structure caused by additional acid doping.21 Here, we confirm
and characterize the three-step acid doping of TANI’s redox
centers through a polaron, two partially coupled polarons, and a
triplet “two polarons on a string” transformation process,
providing a new view of the transport properties of the different
charge carriers.
1
verify the purity of the product. H NMR in DMSO-d6 was
then used to analyze the compound.
1H NMR (500 MHz, DMSO-d6): δ (ppm) 7.00 (s, 4H, Ar−
H ortho to =N)), 6.94 (d, 4H, Ar−H ortho to −N), 6.78 (d,
4H, Ar−H ortho to N-(CH3)2), 2.96 (s, 12H, N-(CH3)2).
“Acid Doping”. TANI solutions (0.1 mM in 10 mL
solvents) were titrated at ambient temperature with the
appropriate amounts of DBSA, MSA, and HCl in toluene,
chloroform, and DMF, respectively, to achieve molar ratios
given in the text and figures. At high DBSA concentration, and
for dedoping with TBAH, DBSA and TBAH (when used to
dedope after addition of DBSA) were mixed into 5 mL of 0.2
mM TANI solution in sequence, and the resulting solution was
diluted to 10 mL. Thus, TANI in all solutions was always 0.1
mM. Very small amounts of methanol were added in the
titrations with HCl/MeOH and TBAH/MeOH, so no solvent
effect from methanol is expected in our experiments.
Moreover, the three-level doping that produces a wide range
of optical absorbance phenomena compared to PANI makes
the TANI molecule a more versatile and unique candidate for
sensors and electrochromic materials worthy of further study.
Spectroscopic Measurements. Measurements of ultra-
violet−visible−near-infrared (UV−vis−NIR) and electron para-
magnetic resonance (EPR) spectra were carried out at room
temperature immediately after the solutions were prepared.
UV−vis−NIR absorption spectra (250−1200 nm) were
recorded using a Shimazu UV-3101PC spectrophotometer with
samples held in 1 mm optical path length quartz cells. EPR
spectra were recorded on a Bruker E500 ElexSys EPR
spectrometer operating at X-band using an ER4122SHQE
resonator cavity. Data acquisition and manipulation were
performed using XeprView and WinEPR software (Bruker).
Samples of the TANI/DBSA mixtures were held in 4 mm o.d.
(3 mm i.d.) precision bore quartz EPR tubes. Experimental
parameters used to acquire the EPR spectra were as follows:
modulation amplitude, 1 G (or 0.1 G for the experimental
spectrum that was simulated); microwave power, 1 mW;
modulation frequency, 100 kHz; microwave frequency, 9.86
GHz; sweep time, 327 s; conversion time, 335 ms; time
constant, 1310 ms; number of scans, 4. All spectra were
recorded at ambient temperature under identical instrumental
conditions. Spin concentration was calculated by double
integration of EPR signals assigning a value of unity to the
spectrum of TANI at doping level I. Since the EPR samples
varied in ionic composition and could be lossy to different
extents, we present the spin concentrations only in relative
terms and note that intensities corresponded to less than one
spin per molecule for all doping levels. Simulation of EPR
spectra was performed using Simfonia software (Bruker).
EXPERIMENTAL SECTION
■
Materials. Dodecylbenzene sulfonic acid (DBSA) (70% in
heptanol), methanesulfonic acid (MSA) (99.5%), HCl (1 M in
methanol) and TBAH (1 M in methanol) were purchased from
Sigma Aldrich. Toluene and chloroform were anhydrous grade
from Sigma Aldrich. Dimethylformamide (DMF) was molec-
ular biology grade from Aldrich. Dimethyl sulfoxide-d6
(DMSO-d6) was purchased from Sigma Aldrich. Chemicals
used during the synthesis of N-[4-(dimethylamino)phenyl]-N-
(4-{[4-(dimethylamino)phenyl]imino}-2,5-cyclohexadien-1-yli-
dene)-amine (TANI), including p-phenylenediamine (99%),
ammonium persulfate (98%), and N,N-dimethylaniline
(99.5%), were also purchased from Sigma Aldrich.
Synthesis of Aniline Trimer. TANI in its undoped or EB
form (Scheme 1) was prepared by a one-step method reported
Scheme 1. TANI
in our previous contribution.22 A 0.86 g (8 mmol) sample of p-
phenylenediamine was dissolved in the mixed solvent of 100
mL 1 M HCl aqueous solution and 40 mL ethanol. The
solution was constantly stirred as it was chilled to −5 °C using
an ice-salt bath. 1.8 g (8 mmol) of ammonium persulfate was
dissolved in 30 mL of distilled water and added to the p-
phenylenediamine solution. After about 5 min, a color change
from light yellow to dark brown was observed. At this time 2.0
mL (16 mmol) of N,N′-dimethylaniline was quickly added. A
blue suspension immediately formed, and the reaction mixture
was maintained between −5 and 0 °C for 2 h. The solid
RESULTS AND DISCUSSION
■
The stepwise protonation of TANI was achieved by titrating
the 0.1 mM TANI/chloroform solution with DBSA as shown in
Figure 1. In Figure 1a, the absorption spectrum of TANI in its
undoped or EB form exhibits a maximum at 597 nm
B
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The shifts demonstrate the similarity between the PANI-EB
to PANI-ES transition and the TANI-EB transition to doping
level I. However, in the case of PANI, the polaron peak at
around 800 nm reaches its maximum intensity only after the
addition of 2 equiv of acid, while 1 equiv of acid gives the
superposition of the exciton peak together with the polaron
peak.22 This indicates that only half of the total iminoquinone
groups (2 imine nitrogens) are doped in PANI under such
conditions.
Here, we found that 1 equiv of acid was sufficient to fully
dope TANI to produce the first polaron, as evidenced by the
complete exciton (597 nm) to polaron peak (806 nm)
transition. The behavior observed for TANI had been explained
using a disproportionation proton−electron exchange model.22
However, the previously proposed model now seems to be
unlikely for the behavior of TANI because it fails to explain the
complete elimination of iminoquinone character revealed by
UV−vis−NIR. The complete doping of TANI with the first
equiv of DBSA will decrease its basicity, making further doping
of the same molecule more difficult than the first doping step.
The electronic change arises from elimination of the
iminoquinone group, as the protonation of one of the two
imine nitrogens leads to resonance structures with a delocalized
charge/unpaired spin. (Scheme 2)
Scheme 2. The Protonation of TANI by First 1 equiv of
DBSA (Level I)
Figure 1. UV−vis-NIR spectra during the stepwise protonation (acid
doping) of 0.1 mM TANI with DBSA in chloroform at ambient
temperature: (a) TANI-EB titrated to doping level I, molar ratio
DBSA/TANI from 0 to 1 (b) doping level I to II, DBSA/TANI molar
ratio from 1 to 2 (c) Doping level II to III, DBSA/TANI molar ratio
from 2 to 700.
Interestingly, a second equivalent of DBSA produced a new
strong absorbance maximum at 957 nm together with several
weak absorptions in the visible region and a loss of the 806 nm
polaron peak, with the solution color changing to yellow-green.
At this point (doping level II), each TANI molecule is doubly
doped (protonated at two sites), and the iminoquinone
nitrogens are assumed to have been converted into secondary
corresponding to the exciton transition from the iminoquinone
group, as in PANI-EB.22 During the stepwise addition of DBSA,
this peak gradually disappeared, and the band at 806 nm grew
in intensity; this red-shifted band in PANI is defined as a π-to-
polaron band. The solution turned from purple-blue to
emeraldine green.
C
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amine groups as in PANI (Scheme 3), and similar to the
double-doping of other aniline oligomers.19,23
Scheme 3. The Protonation of TANI by First 2 equiv of
DBSA (Level II)
Figure 2. UV−vis spectra of the follow-up stepwise deprotonation of
0.1 mM TANI and 700 equiv of DBSA mixture (doping level III)
using TBAH in chloroform at ambient temperature.
Acid doping is considered more favorable in more nonpolar
solvents due to increasing acid−solvent interactions in more
polar solvents.27 Here, 2.7 and 100 equiv of DBSA were
required to achieve doping levels I and II in the most polar
solvent DMF, which was in excess relative to the theoretical
molar equivalents effective in toluene and chloroform. Addi-
tionally, further protonation was not achievable with large
excesses of DBSA. Apparently, DMF possesses too high a
polarity for the doping level II to level III transition to occur in
it. Also, in strong acid the carbonyl oxygen atom of the DMF
molecule could become the target of protonation, consuming
acid equivalents. The same trend in achieving level III doping
could be seen when HCl and MSA were used as dopant in
DMF.
The acidity and solvent polarity effects are now considered
the reasons why doping levels II and III were not seen in the
previous study of TANI.22 More dilute TANI/chloroform
solutions had been used, in which the acidity was too low to
trigger protonation even with the same DBSA/TANI molar
ratios. Also, the more polar solvent (N-methyl-2-pyrrolidone)
used previously also disfavored protonation of TANI.
The observations made using various dopant/solvent systems
confirmed that the transitions observed in the optical spectra
were from protonation-induced electronic changes, as other
more complex behaviors would not be expected to be
reproduced under the variety of conditions.
EPR measurements were undertaken to monitor the polaron
evolution as a result of acid doping and for comparison to the
optical experiments. Spin densities at different DBSA/TANI
ratios were normalized to the spin density exhibited by the
DBSA/TANI at 1:1 ratio (level I).
More surprisingly, with the addition of a large excess of
DBSA (700 equiv), the 957 nm peak disappeared with a new
peak appearing at 742 nm (doping level III; Figure 1c). The
solution turned from yellow-green back to emeraldine green.
This absorbance shift back to higher energy is likely from
further protonation of one of the two middle amine nitrogens
in level II-doped TANI. The dication species in the oligomer at
doping level II is very difficult to protonate beyond this form
such that very high acid concentrations were required for the
third conversion. This result coincides with the descriptions of
the protonation of amine nitrogen sites in PANI at high acid
concentrations reported elsewhere.24−26
In order to exclude the possibility of irreversible chemical
changes due to the large amount of acid, a dedoping process
was initiated to neutralize the excess DBSA. A Brønsted base,
TBAH, was used to deprotonate the TANI/DBSA complex
starting from doping level III. TBAH was selected due to its
solubility in chloroform. The protonation process was shown to
be fully reversible such that the molar equivalents of TBAH
required to recover the starting optical spectrum of TANI-EB
was close to the equivalents of DBSA added (Figure 2).
Therefore, it can be concluded that the shifts in UV−vis−NIR
correlate with acid−base equilibria and not irreversible chemical
reactions.
To further confirm these observations, other acids and
aprotic solvents with different polarities were employed to
study the doping of TANI (Table 1). We found the three-step
protonation exists in other acid/solvent systems. HCl in MeOH
was used due to the limited miscibility of aqueous HCl and
chloroform.
Acids with different dissociation constants were employed,
with a trend in pKa values in water as follows: HCl < MSA <
DBSA. In chloroform, doping level I and II can be readily
achieved using stoichiometric equivalents of each of these acids.
However, much more DBSA (700 equiv) and MSA (500 equiv)
relative to HCl (200 equiv) was required to achieve doping
level III, as expected. Similarly, in DMF, doping level II was
achieved with fewer equivalents of HCl relative to MSA and
DBSA.
When 1 equiv of DBSA is added to 0.1 mM TANI in
chloroform, an EPR singlet is found, characterized by a line
width of ∼21 G (peak to trough) and a g-factor of 2.0026
(Figure 3). This spectrum lacks resolved hyperfine splittings
and is assigned to a single structure with two unpaired
electrons, one of which is a cation radical and the second is
either a radical in the central ring or a neutral radical on the
other bridging nitrogen (resonance forms given in Scheme 2).
Both the neutral radical and the polaron (radical cation) of level
D
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Table 1. The Acid/TANI Ratios for Doping Levels I, II, and III in 0.1 mM TANI Solution in Different Acid/Solvent Systems
Acidity
←⎯⎯⎯⎯⎯⎯
HCl
MSA
II
DBSA
II
doping levels
I
II
III
I
III
I
III
20
c
a
a
a
a
toluene (κ = 2.38)
1
1
2
2
1
1
2
2
d
chloroform (κ = 4.81)
1
2
200
b
500
b
700
b
c
DMF (κ = 38)
2.7
50
2.7
100
2.7
100
a
b
c
Can not be achieved due to the limited solubility of methanolic-HCl and MSA in toluene. Doping level III does not occur. Dielectric constant at
d
25 °C. Dielectric constant at 20 °C.
doping level I (Figure 4 and Table 2). The level III doping is
assumed to occur at one of the two central secondary amine
Figure 3. Room-temperature EPR spectra of chloroform solutions of
TANI (0.1 mM) with various molar equivalents of DBSA. All signals
were centered at g = 2.0026 + 0.00005.
I-doped TANI (Scheme 2) are assumed to be delocalized such
that no hyperfine structure was seen in the EPR spectrum.
Surprisingly, when an additional equivalent of DBSA was
added, the spin concentration increased only marginally, while
in the optical spectra, a complete conversion to a new species
was apparent under the same conditions (Figure 1). Therefore,
the EPR signal must also arise from a single new species. The
EPR line shape remained an apparent singlet without hyperfine
splittings again consistent with a delocalized spin system for
two unpaired electrons in a diradical dication shown in one
resonance form in Scheme 3. In this structure, a polaron pair
(two weakly or noninteractings spins) and a bipolaron (coupled
spins) can be in resonance. The potential resonance forms in
the dication radical (Scheme 3) can place unpaired electron
spin into the terminal rings and reduce the spin−spin
interaction that would lead to complete loss of paramagnetism
if the polaron (radical) centers were constrained close together.
As the chain length of TANI is short, spins in partially coupled
states are reasonable for the doubly doped species. This result is
consistent with what Barlow et al. reported. They found the
extent to which dication radicals couple with each other
depends on the geometry and chain length of the molecules
and can vary from 0 to 2.0 (which would correspond to a range
of couplings from strong to completely noninteracting spins).10
The fact that only a single prominent polaron peak was seen
at all three doping levels suggests the absence of any
noteworthy intermolecular interactions such as dimerization,
since the spin−spin interaction in dimers gives rise to energy
level splittings and thus multiple π-to-polaron absorption peaks
would be expected, which we do not observe.28,29
Figure 4. EPR signal intensity (expressed as spin per molecule relative
to doping level I) of chloroform solutions of TANI (0.1 mM) with
various molar equivalents of DBSA.
Table 2. Unpaired Spin Per TANI Molecule Based on EPR
Spectra of Chloroform Solutions of TANI (0.1 mM) with
Various Molar Equivalents of DBSA
TANI/
DBSA
unpaired spin/TANI molecule normalized to that of doping
level I
a
1:1
1:2
1.00
1.16
1.11
1.19
1.70
2.16
2.24
1:4
1:10
1:50
1:100
1:700
a
All EPR signals were centered at g = 2.0026 0.00005.
groups, which are available in resonance forms of the level-II
doped molecule. As a result, the ammonium cation −(C6H4)−
+
NH2 −(C6H4)− segregates the polarons and prevents strong
coupling of the unpaired spins (Scheme 4).
Interestingly, an apparent seven-line hyperfine pattern is
resolved in the EPR signal with the addition of excess DBSA
(Figure 3). This phenomenon could arise from the formation
of a structure in which hyperfine interactions at a terminal
dimethylamino group contribute to the EPR active species
(Scheme 4 bottom structure). The newly formed ammonium
cation at a bridging nitrogen limits delocalization within a
After a large excess of acid was added, the spin concentration
increased to approximately twice the value for the structure at
E
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The polaron, partially coupled polarons, and two non-
interacting polaron structures formed at different stages of
doping gave deeper insight into the property of the
intramolecular transport of various charge carriers in TANI.
The chemical and electronic properties of the novel and
interesting structural motifs may be useful in the development
of new conjugated materials with applications in organic
electronics.
Scheme 4. Doping of Secondary Amine Group in TANI with
Large Excess of DBSA (level III)
a
AUTHOR INFORMATION
Corresponding Author
■
*Tel: 718-260-4039; Fax: 718-260-3136; E-mail address: kalle.
levon@gmail.com.
a
Radical cations formed at 2 equiv of acid were more stable at both
ends, which left the secondary amine groups in the middle more
susceptible to doping.
Notes
The authors declare no competing financial interest.
shortened conjugational length structure such that one
unpaired spin now resides on a terminal dimethylamino
nitrogen in the level III-doped molecule. A radical on the
dimethylamino group was supported by simulating the
multiline EPR spectrum using hyperfine splitting parameters
for six equivalent hydrogens (A = 5.1 G) and one nitrogen (A =
6.0 G). These parameters are closely related to those reported
for an isolated dimethylaniline radical (Figure 5b).30 The
ACKNOWLEDGMENTS
■
The authors want to thank Dr. Eduard Nasybulin and Dr. Alok
Prabhu for their helpful discussion and assistance.
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Figure 5. (a) Experimental EPR spectrum of chloroform solutions of
TANI (2.5 mM) at doping level III. (b) Simulation assuming
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structure we propose (Scheme 4) consistent with the EPR and
optical observations.31
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■
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The UV−vis−NIR and EPR spectroscopic measurements
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charge carriers as a function of protonation states of the two
internal nitrogens and according to reasonable resonance forms
of the doped species. The doping levels were proven to exist in
different dopant and/or solvent systems with the acidity of
dopants and polarity of solvents as controlling factors.
F
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The Journal of Physical Chemistry A
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(31) While other sets of hyperfine interactions could produce similar
splitting patterns (for example, 4 equiv H (5.1 G) and 1-N (5.1 G)),
we cannot draw a reasonable chemical structure for a species at doping
level III that would be expected to yield those couplings. Other
possible sets of hyperfine interactions to include more sites (2-N and
6-H, or 2-N, 4-H (5 G) and 2-H (2.0 G)) yielded poor simulation of
the experimental spectrum.
G
dx.doi.org/10.1021/jp3027258 | J. Phys. Chem. A XXXX, XXX, XXX−XXX