The different electronic natures displayed by the alkylthio groups in simple and higher conjugated aniline systems

Article abstract of DOI:10.1039/b609506b

Systematic studies based on ¹H NMR and ¹³C NMR indicated that the alkylthio group behaves as a weak electron-withdrawing group in a simple aniline system like 2-butylthioaniline, while the same alkylthio group clearly acted as a resonance electron-donating group in higher conjugated aniline trimer systems, like butylthio-substituted PDA (mono-PDA) and dibutylthio-substituted PDA (2,6-diPDA). The formation of 2,6-diPDA as the major byproduct during the preparation of mono-PDA from PDI and butane-1-thiol provided additional support for the resonance electron donating nature of the butylthio group in these aniline trimer systems. Furthermore, CV studies also clearly indicated that the redox potential E° (vs. SCE) of the aniline trimer systems decreased with the increase in the number of butylthio groups, further confirming the electron-donating nature of the butylthio group in these higher conjugated trimer systems. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b609506b

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The different electronic natures displayed by the alkylthio groups in simple
and higher conjugated aniline systems†
Chien-Chung Han,* R. Balakumar, D. Thirumalai and Ming-Tsu Chung
Received 4th July 2006, Accepted 12th July 2006
First published as an Advance Article on the web 8th August 2006
DOI: 10.1039/b609506b
Systematic studies based on 1H NMR and 13C NMR indicated that the alkylthio group behaves as a
weak electron-withdrawing group in a simple aniline system like 2-butylthioaniline, while the same
alkylthio group clearly acted as a resonance electron-donating group in higher conjugated aniline
trimer systems, like butylthio-substituted PDA (mono-PDA) and dibutylthio-substituted PDA
(2,6-diPDA). The formation of 2,6-diPDA as the major byproduct during the preparation of
mono-PDA from PDI and butane-1-thiol provided additional support for the resonance electron
donating nature of the butylthio group in these aniline trimer systems. Furthermore, CV studies also
clearly indicated that the redox potential E◦ (vs. SCE) of the aniline trimer systems decreased with the
increase in the number of butylthio groups, further confirming the electron-donating nature of the
butylthio group in these higher conjugated trimer systems.
this new CRS method to Pan did not lower the conductivity
as it would otherwise be expected; instead these Pan-SR gained
Introduction
even higher conductivity (7–10 S cm−1)5 than the parent Pan (1–
5 S cm−1).6 Since both Pan-SR and Pan have exactly the same
backbone structure, we had attributed the improved conducting
nature of Pan-SR (vs. Pan) to the electron-donating contribution
of the alkylthio groups, which helps increase the electron density
of the backbone and results in higher conductivity. The increased
backbone electron density in this case has been partly supported by
our previous CV and UV studies, showing the lowered oxidation
potentials and red-shifted absorption wavelength in Pan-SBu
as compared with its parent unsubstituted Pan. Therefore, to
clearly understand the electronic behaviour of the SBu group
in the aromatic aniline systems, we carried out a systematic
investigation with a series of simple and conjugated aniline model
compounds and tried to analyse them with unambiguous spectral
characterisations using various NMR experiments.
Among the conducting polymers, polyaniline (Pan) has gained
widespread attention due to its excellent stability in air, low
cost, and great opportunities for application.1–3 However, just
like other conducting polymers, the application potential of
Pan is seriously limited due to its poor solution processibility.
Although the solubility problem can be eased by incorporating
alkyl and alkoxy groups on polymer backbones based on modified
aniline monomers, the alkyl- and alkoxy-substituted polyanilines
obtained however suffered a severe loss in conductivity by ca. 1
to 7 orders of magnitude,4 which has long been attributed by the
community to arise from the steric effects associated with the sub-
stituent group. On the contrary, we believe it is probably caused by
the increased numbers of nonconjugated backbone linkage defects
(1,3-ring linkage) due to the electronic directing influences of the
-R and -OR groups on the formation of backbone linkage.4c Such
an adverse electronic influence was even more severe in the case of
2-butylthioaniline, and seemed to even prevent it from forming its
own homopolymer. The copolymers prepared from aniline and 2-
butylthioaniline, via the conventional oxidative copolymerization
(OCP) method, containing only ∼32 mol% of the butylthio group
already showed a much poorer conductivity (10−5 S cm−1)4c than
the homopolymer of butoxyaniline (10−3 S cm−1).4d On the other
hand, the butylthio-substituted polyaniline (Pan-SBu) prepared
via our previously reported concurrent reduction and substitution
(CRS) method (via post-functionalization directly on the existing
Pan backbones) had not only gained greatly improved solution
processibility but also retained the high conducting nature of the
unsubstituted Pan.4a We further found out that the introduction
of even bulkier groups, such as octylthio and dodecylthio, via
Results and discussion
In our continuous efforts to strive for even more rigorous
spectroscopic evidence to show that the electron density of the
phenyl rings is indeed enhanced by the butylthio substituent, we
have prepared the butylthio-substituted aniline monomer, i.e., 2-
butylthioaniline, via selective S-alkylation of 2-aminobenzenethiol
by butyl bromide in a THF medium using sodium hydride as
1
the base and used it for detailed NMR studies. Based on its H
NMR spectrum and an NOE experiment, we have unambiguously
assigned the NMR peaks and the results are summarized in Fig. 1.
The results however indicated that the incorporation of the
butylthio group into aniline does not help to increase the electron
density of the phenyl ring, instead it make its ortho proton (i.e.,
one of the meta protons to the amino group) shift downfield
from d 7.00 to d 7.21; whereas all the four phenyl protons of its
alkoxy analogues, e.g., 2-methoxyaniline, were found to be shifted
upfield to the range of d 6.7–6.8. The NMR results indicate the
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan
ROC. E-mail: cchan@mx.nthu.edu.tw; Fax: +886 3 5711082; Tel: +886 3
5724998
† Electronic supplementary information (ESI) available: 1H, 13C and
2D HETCOR spectra of mono-PDA and 2,6-diPDA. See DOI:
10.1039/b609506b
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some disubstituted PDA and PDA were also formed. NMR
studies were performed to understand the electronic nature of
the butylthio groups in both mono-PDA and disubstituted PDA.
Fig. 1 The 1H NMR chemical shift values (d ppm) of aniline and
2-butylthioaniline and the NOE result for 2-butylthioaniline in DMSO-d6.
butylthio group of 2-butylthioaniline prefers to act as an electron
withdrawing substituent rather than electron releasing substituent.
Such electron withdrawing nature of alkylthio groups in the
model compounds was also clearly demonstrated by their 13C
NMR data. The 13C NMR chemical shifts were unambiguously
assigned using 2D NMR (HETCOR) experiments and the results
are summarized in Fig. 2. The results clearly showed that the
chemical shift value of the carbon ortho to the SBu group was
shifted downfield from d 128.8 in aniline (which is its meta carbon)
to d 134.5 in 2-butylthioaniline.
Scheme 1 CRS reaction of PDI with butane-1-thiol.
The structure of the mono-PDA was unambiguously charac-
terized using the NMR studies. For the mono-PDA, the proton
signals for the center ring B (Fig. 3) can be clearly distinguished
from those protons of the terminal rings A and C based on the
splitting patterns of the protons and their integration intensity.
The two correlated sets of ortho-, meta-, and para-proton signals
associated with either ring A or C were clearly identified using
homodecoupling experiments. NOE experiments were further
performed to unambiguously assign the proton sets for rings A
and C. The two non-equivalent NH proton signals are identified
using deuterium exchange experiments. The NOE results and the
final assignments are summarized in Fig. 3. The 1H NMR results
clearly demonstrate that the butylthio substituent shifts ring A
more upfield than ring C, probably due to the resonance electron
donating nature of the butylthio group. To further confirm this
hypothesis, the 13C NMR of mono-PDA was recorded and the
results are presented in Fig. 4; the assignments were based on the
HETCOR experiments. The results indeed confirmed that ring A
is shifted more upfield than ring C. It is also interesting to note that
the ortho carbon (d 117.1) and para carbon (d 115.4) to the SBu
group in the central phenyl ring comes very much more upfield
than the meta carbon (d 124.9).
Fig. 2 The 13C NMR chemical shift values (d ppm) of aniline and
2-butylthioaniline in DMSO-d6.
The electronic behaviour of the alkylthio group in the simple
aniline system seems to contradict the observations in our polymer
(Pan-SBu) systems. We rationalized that this may be due to the
energy mismatching of the atomic orbitals for the 2nd and 3rd
period elements (i.e., carbon and sulfur) which hinders the electron
donating resonance of the alkylthio group to the aromatic ring.
Additionally, the ring electrons may also delocalize into the vacant
3d orbital of sulfur, making the alkylthio group slightly electron
withdrawing. We felt that the difference in the electronic influence
of the alkylthio groups between the monomer and polymer systems
may be caused by the obvious difference in their conjugation
lengths. Because, in general, as the conjugation length increases,
the band gap becomes smaller due to the rising of the energy level
of the HOMO and the lowering of the energy level of the LUMO.2
The polarizability of the electron cloud of the backbone would
also increase with the conjugation degree. We believe that both
the lowering of the LUMO level and the increased polarizability
in the polymer systems should help facilitate the electron-donating
resonance of the alkylthio group. As conjugation interaction plays
an important role in lowering the LUMO and increasing the po-
larizability, it would be more appropriate to examine the electronic
nature of the alkylthio group based on an aniline system with a
moderate conjugation length. Therefore, an aniline trimer with a
butylthio substituent seems to be the ideal candidate, because it is a
minimum redox unit of polyaniline and it is still simple enough for
unambiguous spectroscopic characterisation. The trimer molecule
PDA (i.e., N,Nꢀ-diphenyl-1,4-phenylenediamine) with a butylthio-
substituent was thus prepared by the CRS method as shown in
Scheme 1. Interestingly, along with the mono-substituted PDA,
The fact that 1H, 13C chemical shifts of mono-PDA (a secondary
aniline system) come relatively downfield compared to those of
aniline (a primary aniline), can be attributed to the difference in
their chemical structures. For a more appropriate comparison,
Fig. 3 The NOE results of mono-PDA and its peak assignments.
Fig. 4 The 13C NMR chemical shift values (d ppm) of mono-PDA in
DMSO-d6.
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another secondary aniline, i.e., diphenylamine was used as a
model compound and studied with 1H, 13C and HETCOR NMR
experiments and the results are given in Fig. 5. It is interesting to
note that the chemical shifts (both 1H and 13C) of diphenylamine
are almost identical with the chemical shifts of ring C of mono-
PDA while those of ring A of mono PDA are shifted more upfield.
Furthermore, the 1H and 13C NMR results of mono-PDA and
2,6-diPDA were then compared with those of unsubstituted PDA
(Fig. 8) to better understand the substituent effect of SBu on the
aromatic ring. The 1H and 13C chemical shift values clearly show
the upfield trends for ring A and Aꢀ of mono-PDA and 2,6-diPDA
compared to that of the unsubstituted PDA.
Fig. 8 The 1H and 13C NMR chemical shift values (d ppm) of unsubsti-
tuted PDA in DMSO-d6.
Fig. 5 The 1H and 13C NMR chemical shift values (d ppm) of dipheny-
lamine in DMSO-d6.
All the above NMR results of both mono-PDA and disubsti-
tuted PDA clearly prove that the phenylamino group (ring A or
Aꢀ) at the ortho position (a conjugated position) to the butylthio
group comes more upfield than the phenylamino group at the
meta position (a non-conjugated position) as illustrated in Fig. 9.
As expected, in the aniline systems with moderate conjugation
length, such as mono-PDA and 2,6-diPDA, the butylthio groups
act as an electron releasing substituent, unlike that in the simple
aniline system of 2-butylthioaniline.
Similarly, the disubstituted PDA (formed in ∼10% yield) was
also characterized using the deuterium exchange, homodecou-
pling, HOMOCOSY and NOE experiments. Interestingly, the 1H
NMR spectrum of the disubstituted PDA indicated that it is not
as symmetrically substituted as would be expected for 2,5- or 2,3-
disubstituted PDA’s. Two sets of ortho-, meta-, and para-proton
signals corresponding to the two nonequivalent terminal phenyl
rings Aꢀ and Cꢀ plus one singlet peak associated with the two
equivalent protons of the central phenyl ring Bꢀ were observed.
The 1H NMR results suggested that it is a 2,6-disubstituted
PDA (2,6-diPDA)7 (Fig. 6). Furthermore, the two NH peaks
(d 7.03 and d 8.29) were clearly identified by the deuterium
exchange experiment. Based on the homodecoupling experiments,
the proton sets that are associated with either of the two terminal
phenyl rings Aꢀ or Cꢀ were identified. NOE experiments were
carried out to further confirm the actual physical location of the
SBu group in relation to the Aꢀ and Cꢀ rings (Fig. 6). The NOE
results clearly demonstrated that substitutions of SBu groups make
the ring Aꢀ appear more upfield shifted than ring Cꢀ.
Fig. 9 The upfield and downfield phenylamino groups in the mono-PDA
and 2,6-diPDA.
Further study indicated that the formation of disubstituted
PDA was mainly caused by the reoxidation of the mono-PDA by
unsubstituted PDI (i.e., N,Nꢀ-diphenyl-1,4-phenylenediimine) to
form mono-PDI (i.e., butylthio-substituted PDI) (Fig. 10) which
then underwent the subsequent second CRS reaction and resulted
in the disubstituted PDA. Interestingly, the preferential formation
of the 2,6-diPDA also sheds evidence for the electron releasing
nature of the butylthio group in these trimer systems.
Fig. 6 The NOE results of 2,6-diPDA and its peak assignments.
The same conclusion can also be obtained from the 13C NMR
results of 2,6-diPDA as summarized in Fig. 7. Once again, the
assignments were done using HETCOR experiments. The results
clearly showed that the Bꢀ and Aꢀ rings of 2,6-diPDA are both
further shifted upfield from the corresponding B and A rings of
mono-PDA, due to the contribution of one additional electron-
donating butylthio group.
Fig. 10 Plausible mechanism for the preferential formation of 2,6-diPDA.
For the mono-PDI, the basicity of its Na will be preferentially
increased due to the resonance delocalization of the lone pair
electrons on sulfur, thereby allowing it to be preferentially
protonated over its counterpart Nb which then makes the C6
position more susceptible towards nucleophilic attack by butane-
1-thiol and thus results in the formation of the 2,6-diPDA (Fig. 10).
Fig. 7 The 13C NMR chemical shift values (d ppm) of 2,6-diPDA in
DMSO-d6.
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The hypothetical redox reaction between butylthio-substituted
mono-PDA and PDI to form butylthio-substituted mono-PDI
and PDA (Fig. 11) has been confirmed by our control experiments
1
based on monitoring the H NMR of an equimolar mixture of
mono-PDA and PDI in d8-THF or DMSO-d6. The results showed
that the redox reaction in d8-THF (at room temperature) reached
its equilibrium position after ca. 6 h and yielded 75% of mono-PDI
and PDA. While in DMSO-d6 the reaction rates were much slower
at room temperature (25 ◦C) and 40 ◦C, forming only 32% and 40%
of mono-PDI respectively at 6 h. When the reaction temperature
was raised to 70 ◦C, a similar final equilibrium position as in
d8-THF was easily attained within 2 h.
Fig. 13 The 1H NMR monitoring of the redox reaction between PDI and
mono-PDA in DMSO-d6 at 70 ◦C.
Fig. 11 The redox reaction between mono-PDA and PDI.
increases the electron density of the trimer through its resonance
electron donating effect.
1
For the sake of easier comparison, the H NMR spectra of
Such electron donating nature of butylthio group in the trimer
systems was also confirmed by our CV studies (Fig. 14), showing
that the oxidation potential values E◦ (vs. SCE) of PDA’s decrease
as the number of butylthio groups increases: PDA (0.389 V) >
mono-PDA (0.346 V) > 2,6-diPDA (0.196 V).
Similar electronic behaviour has also been observed for other
alkylthio-substituted trimers, which were prepared via the same
CRS reaction in THF between PDI and other functionalized
alkanethiols like 2-mercaptoethanol, 4-mercaptobutanol, mercap-
topropanesulfonic acid sodium salt, and thiophenol.9 Interest-
ingly, in addition to the corresponding mono-PDA as the major
product, a 10–20% of the 2,6-disubstituted PDA was always
observed as the sole or predominant disubstituted product in
the individual reactants (PDI and mono-PDA) and the products
(PDA and mono-PDI) are given in Fig. 12. Due to the presence
of E/Z isomers associated with the two PDI compounds, their
NMR signals [Fig. 12(a), (d) ] are more complicated than their
corresponding PDA compounds [Fig. 12(c), (b) ]. The complete
assignment for PDI has been already reported in the literature.8
For simplicity, only the chemical shift ranges associated with the
a proton (-SCH2) and the terminal methyl proton (-CH3) of the
butylthio group are presented in Fig. 13. The equilibrium constant
of this reaction was measured to be about equal to or greater than
9, suggesting that the mono-PDA is more readily oxidized than
PDA, possibly due to the presence of the butylthio group which
Fig. 12 The 1H NMR spectra of (a) PDI, (b) mono-PDA, (c) PDA, and (d) mono-PDI in DMSO-d6.
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Synthesis of 2-butylthioaniline
To a stirred solution of 2-aminobenzenethiol (5 mmol, 0.63 g) in
20 mL of THF was added sodium hydride (5 mmol, 0.113 g), and
the contents were stirred at room temperature for 1 h. The reaction
was monitored by TLC and after the disappearance of the starting
material, the solvent was removed under reduced pressure and the
residue was dissolved in chloroform and washed with water (2 ×
20 mL). The chloroform layer was dried over anhydrous MgSO4
and the solvent removed under reduced pressure to yield 0.83 g of
2-butylthioaniline in 92% yield. 1H NMR (500 MHz, DMSO-d6,
TMS as standard): d 7.21 (dd, 1H, J = 7.5 Hz, 1.5 Hz), 7.00 (td,
1H, J = 7.6 Hz, 1.8 Hz), 6.76 (dd, 1H, J = 8 Hz, 1.5 Hz), 6.51
(td, 1H, J = 7.3 Hz, 1.6 Hz), 5.16 (s, 2H, NH2), 2.70 (t, 2H, J =
7.0 Hz), 1.45 (quint, 2H, J = 7.4 Hz), 1.35 (sextet, 2H, J = 7.3 Hz),
0.84 (t, 3H, J = 7.3 Hz); 13C NMR (125 MHz, DMSO-d6 at d 39.5
as standard): d 149.1 (C), 134.5 (CH), 128.9 (CH), 116.4 (CH),
116.1 (C), 114.2 (CH), 33.0 (CH2), 31.1 (CH2), 21.1 (CH2), 13.5
(CH3).
Synthesis of PDI
Fig. 14 The CV results of PDA, mono-PDA and 2,6-dPDA in an aqueous
solution (pH = 1.3) containing 0.05 M TsOH.
N,Nꢀ-Diphenyl-1,4-phenylenediamine (1 g, 3.84 mmol) was dis-
solved in 75 mL of acetone and stirred at room temperature. To
this stirred solution was added silver oxide (1.33 g, 5.76 mmol)
in portions. The color of the solution changed from black to
orange color. After stirring for 1 h, the product precipitated as
an orange solid, which was collected and then recrystallized in
all these cases, suggesting that the electron donating nature of
these alkylthio groups are also effectively functioning in these
moderately conjugated aniline trimer systems.
1
cyclohexane. Yield: 0.94 g, 95% (a mixture of E/Z isomers). H
Conclusions
NMR (500 MHz, DMSO-d6, TMS as standard): d 7.42 (t, E, 4H,
J = 7.75 Hz), 7.39 (t, Z, 4H, J = 8.5 Hz), 7.19 (t, E, 2H, J =
7.0 Hz), 7.17 (t, Z, 2H, J = 7.5 Hz), 7.12 (d, Z, 2H, J = 1.5 Hz),
7.02 (dd, E, 2H, J = 10.5 Hz, 2.0 Hz), 6.89 (d, E, 4H, J = 7.5 Hz),
6.86 (d, Z, 4H, J = 7.75 Hz), 6.83 (dd, E, 2H, J = 10.0 Hz, 2.5 Hz),
6.75 (d, Z, 2H, J = 2.5 Hz); 13C NMR (125 MHz, DMSO-d6 at d
39.5 as standard): d 157.86 (C), 157.81 (C), 149.8 (C), 149.7 (C),
137.7 (CH), 136.4 (CH), 129.12 (CH), 129.08 (CH), 125.3 (CH),
125.07 (CH), 125.03 (CH), 124.3 (CH), 120.4 (CH), 120.2 (CH).
Exact mass: 258.1157; LRMS (EI-MS) 258 [M]+.
In conclusion, although the alkylthio group displayed a weak
electron withdrawing effect in the aniline monomer systems, it
however clearly showed a resonance electron-donating effect in
the higher conjugated aniline trimer as well as in the polymer
systems. Such interesting reversing electron behaviour may provide
a vital clue to the possible polymerization mechanism. During the
copolymerization course between aniline and 2-butylthioaniline, if
the 2-butylthioaniline monomer attaches to the tail of the growing
polymer chain (i.e., the phenyl ring end), the butylthio group then
becomes part of a higher conjugated system and starts behaving
as a resonance electron donating group. When this reversal in
electronic behaviour occurs, the butylthio group may begin to
compete much more significantly with the amino group for
directing the subsequent polymer chain linkage positions, leading
to the formation of significant amounts of the non-conjugated
(i.e., 1,3-ring linkage) defect structures and resulting in poorly
conductive copolymers.
Synthesis of mono-PDA and 2,6-diPDA
Butane-1-thiol (5 mmol, 0.44 g, 0.51 mL) dissolved in 25 mL
of degassed tetrahydrofuran was taken in a flame dried three
neck round bottomed flask equipped with an addition funnel,
condenser, and a rubber septum. The system was air exchanged
with nitrogen three times. To this system was then added dropwise
a solution of PDI (5 mmol, 1.29 g) dissolved in 25 mL of degassed
tetrahydrofuran. The contents were stirred at rt under nitrogen
for 6 h. As the reaction proceeded, the orange color of the
imine disappeared and a pale yellow color was seen. The reaction
was monitored by TLC (eluant: 10% EA in hexane). After the
complete disappearance of the starting materials the solvent was
removed under reduced pressure using rotary evaporator. The
crude product was purified by column chromatography using
hexane followed by 4% EA in hexane as the eluants. Mono-PDA
was obtained as a major product in 74% yield (1.21 g). The 2,6-
diPDA was obtained in 10% yield (0.22 g). PDA, the fully reduced
amine was obtained in 11% yield (0.14 g).
Experimental
The thiols were purchased from Aldrich and used as such.
PDA was obtained from Aldrich. ACS grade tetrahydrofuran
refluxed and distilled over benzophenone–sodium and HPLC
grade methanol were used for the reactions. NMR spectra were
studied using Varian Unity Inova 500 MHz and Bruker Avance
DMX 600 MHz. HRMS were recorded with the Thermo Finnigan
Model MAT 95 XL. LRMS was measured by TRIO 2000
Micromass.
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1H and 13C NMR of mono-PDA
CV measurements for the PDA’s
1H NMR (500 MHz, DMSO-d6, TMS as standard): d 8.11 (s, 1H,
NH), 7.22 (t, 2H, J = 8.0 Hz), 7.15 (s, 1H, NH), 7.11 (t, 2H, J =
7.8 Hz), 7.08 (d, 1H, J = 8.5 Hz), 7.05 (d, 1H, J = 2.5 Hz), 7.04 (d,
2H, J = 7.8 Hz), 6.91 (dd, 1H, J = 8.3 Hz, 2.5 Hz), 6.80 (t, 1H, J =
7.2 Hz), 6.73 (d, 2H, J = 8.0 Hz), 6.66 (t, 1H, J = 7.0 Hz), 2.80 (t,
2H, J = 7.5 Hz), 1.54 (quint, 2H, J = 7.5 Hz), 1.37 (sextet, 2H, J =
7.4 Hz), 0.85 (t, 3H, J = 7.5 Hz); 13C NMR (125 MHz, DMSO-d6
at d 39.5 as standard): d 13.55 (CH3), 21.5 (CH2), 30.6 (CH2), 31.2
(CH2), 114.4 (CH), 115.4 (CH), 116.3 (CH), 117.1 (CH), 117.8
(CH), 119.4 (CH), 124.9 (CH), 128.9 (CH), 129.2 (CH), 132.7 (C),
133.1 (C), 139.8 (C), 143.9 (C), 146.5 (C). Exact mass: 348.1660;
LRMS (EI-MS) 348 [M]+.
Cyclic voltammograms (CV) for the PDA compounds were
measured with a potentiostat (CHI 605A) in a three-electrode
electrochemical cell with an aqueous solution of 0.05M TsOH
as electrolyte, using a platinum plate (0.5 cm × 0.5 cm) as the
working electrode and a gold plate (1 cm × 4 cm) as the counter
electrode and a saturated calomel electrode (SCE) as the reference
electrode. The redox potential (E◦) were calculated as the average
of the anodic and cathodic peak potentials.
Acknowledgements
The authors acknowledge NSC for funding this research program.
1H and 13C NMR of 2,6-diPDA
1H NMR (500 MHz, DMSO-d6, TMS as standard): d 8.29 (s, 1H,
NH), 7.26 (t, 2H, J = 8.0 Hz), 7.11 (d, 2H, J = 7.5 Hz), 7.03 (s,
1H, NH), 7.01 (t, 2H, J = 7.5 Hz), 6.86 (t, 1H, J = 7.25 Hz), 6.75
(s, 2H), 6.54 (t, 1H, J = 7.0 Hz), 6.39 (d, 2H, J = 7.5 Hz), 2.74
(t, 4H, J = 7.25 Hz), 1.52 (quint, 4H, J = 7.6 Hz), 1.34 (sextet,
4H, J = 7.5 Hz), 0.85 (t, 6H, J = 7.5 Hz); 13 C NMR (125 MHz,
DMSO-d6 at d 39.5 as standard): d 13.5 (CH3), 21.5 (CH2), 30.0
(CH2), 30.3 (CH2), 109.4 (CH), 112.4 (CH), 116.4 (CH), 117.2
(CH), 120.1 (CH), 126.3 (C), 128.6 (CH), 129.2 (CH), 139.7 (C),
142.6 (C), 143.0 (C), 147.0 (C). Exact mass: 436.2007; HRMS
(EI-MS) 436.2012 [M]+.
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3 E. M. Genies, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. Met.,
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4 (a) C. C. Han, S. P. Hong, K. F. Yang, M. Y. Bai, C. H. Lu and C. S.
Huang, Macromolecules, 2001, 34, 587; (b) References 2 and 3 in ref. 4a;
(c) C. C. Han, K. F. Yang, S. P. Hong, A. Balasubramanian and Y. T.
Lee, J. Polym. Sci., Polym. Chem. Ed., 2005, 43, 1767; (d) G. D’Aprano,
M. Leclerc, G. Zotti and G. Schiavon, Chem. Mater., 1995, 7, 33.
5 C. C. Han, unpublished results.
6 M. Angelopoulos, G. E. Asturias, S. P. Ermer, A. Ray, E. M.
Scherr and A. G. MacDiarmid, Mol. Cryst. Liq. Cryst., 1988, 160,
151.
7 In separate experiments, we have prepared 2,5- and 2,3-disubstituted
PDA’s and they showed a single set of phenyl and NH protons.
8 M. Sandberg and T. Hjertberg, Synth. Met., 1987, 29, E257.
9 MeOH is used for the reaction with mercaptopropanesulfonic acid
sodium salt as it is insoluble in THF.
1H and 13C NMR of PDA
1H NMR (500 MHz, DMSO-d6, TMS as standard): d 7.9 (s, 2H,
NH), 7.17 (t, 4H, J = 8 Hz), 7.02 (s, 4H), 6.95 (d, 4H, J = 8 Hz),
6.71 (t, 2H, J = 7.25 Hz); 13 C NMR (d 150 MHz, DMSO-d6 at d
39.5 as standard): d 144.9 (C), 136.5 (C), 129.1 (CH), 119.8 (CH),
118.4 (CH), 115.2 (CH).
3516 | Org. Biomol. Chem., 2006, 4, 3511–3516
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