Synthesis and spectroscopic properties of aniline tetramers. Comparative studies

Article abstract of DOI:10.1039/b311096f

A new synthetic method involving S_NAr coupling of 4-fluoronitrobenzene to arylamines, followed by the reduction of the nitro groups, has been developed. Two types of aniline oligomers, namely Ph/NH ₂ and NH₂/NH₂

Full text of DOI:10.1039/b311096f

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P A P E R
Synthesis and spectroscopic properties of aniline tetramers.
Comparative studies
´
Irena Kulszewicz-Bajer,* Izabela Roz˙alska and Małgorzata Kuryłek
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664
Warsaw, Poland. E-mail: ikulsz@chemix.ch.pw.edu.pl
Received (in Montpellier, France) 10th September 2003, Accepted 23rd January 2004
First published as an Advance Article on the web 27th April 2004
A new synthetic method involving SNAr coupling of 4-fluoronitrobenzene to arylamines, followed by the
reduction of the nitro groups, has been developed. Two types of aniline oligomers, namely Ph/NH2 and
NH2/NH2 tetramers have been prepared by this method. Tetramers in the leucoemeraldine and in the
emeraldine oxidation states were characterized by NMR, UV-Vis-NIR, IR and mass spectroscopies. It shown
that the NH2/NH2 tetramer oxidized to the emeraldine state forms two isomers: syn and anti. The oxidation
of the Ph/NH2 tetramer leads to the creation of positional isomers, depending on the synthetic method.
coupling/reduction sequence allows to prepare tetramers of
Introduction
both the above-mentioned types. Moreover, the SNAr reaction
is carried out in mild conditions and therefore protection of the
amine groups is unnecessary. The obtained tetramers in both
the reduced and oxidized forms are characterized by elemental
analyses, mass spectroscopy, NMR, IR and UV-Vis-NIR spec-
troscopies. The spectroscopic properties of such oligomers are
compared with the properties of the Ph/NH2 tetramer
obtained by oxidative coupling.
Ph/NH2 and Ph/Ph end-capped tetramers contain 4 nitro-
gen atoms but different numbers of phenyl rings (4 and 5,
respectively). The conjugation length (or the length of the per-
ipheric bonds) is different in both cases, therefore the proper-
ties of these compounds cannot be easily compared. In our
study the name ‘‘tetramer’’ is used to denote compounds that
contain 4 phenyl rings and 4 (in the case of Ph/NH2) or 5 (in
the case of NH2/NH2) nitrogen atoms.
Oligoanilines can be considered as good model compounds of
polyaniline, the most technologically important conducting
polymer. Studies of the topological and geometrical isomers
formed after oxidation of amines allow one to understand
the complexity of the molecular interactions in polyaniline.
The most interesting form of polyaniline is emeraldine, which
contains ca. 50% amine and ca. 50% imine groups, that is one
of 4 rings is quinoid, the remaining three being benzoid. For
this reason the shortest model compound for emeraldine must
comprise four aniline repeat units. Many synthetic methods
involving different types of condensation reactions have been
reported as suitable for the synthesis of phenyl/NH2 and phe-
nyl/phenyl end-capped tetramers. Honzl and Tlustakova1
obtained the tetramer by the condensation of N-phenyl-1,4-
phenylenediamine with diethyl succinoylsuccinate, followed
by hydrolysis, decarboxylation and aromatization. The con-
densation of p-phenylenediamine with p-anilinophenol was
reported by Ochi et al.2 An Ullmann-type coupling reaction
between acetanilides and 4-iodonitrobenzene was used by
Rebourt et al.3 The tetramer was obtained by reduction and
deacetylation reactions but no detailed experimental procedure
was given. The phenyl/NH2 end-capped tetramer was pre-
pared by oxidative coupling of N-phenyl-1,4-phenylenedia-
mine in the presence of FeCl3ꢀ6H2O as an oxidant by Zhang
et al.4 The phenyl/phenyl end-capped tetramer was synthe-
sized by Wei et al.5 using p-phenylenediamine, diphenylamine
and (NH4)2S2O8 as a suitable oxidizing agent. Recently, Wang
and MacDiarmid6 have reported the synthesis of a phenyl/
phenyl end-capped tetramer by the condensation of N-phe-
nyl-1,4-phenylenediamine with hydroquinone in the presence
of titanium n-butoxide. A general and elegant way to prepare
different types of oligoanilines was proposed by Sadighi et al.7
who synthesized oligomers by palladium catalyzed amination
of aryl halides, followed by deprotection of the amine groups.
However, it should be noted that the most popular method for
the preparation of aniline tetramers is still oxidative coupling,
due to its simplicity.
Experimental
Materials and methods
All compounds studied were subjected to elemental analyses.
C, H and N were determined by combustion analysis.
1H NMR spectra were recorded on a Varian Mercury 400
MHz and referenced with respect to TMS. IR spectra were
recorded on a Bio-RAD FTS-165 using KBr pellets. UV-
Vis-NIR spectra were registered using a Lambda 2 (Perkin–
Elmer) spectrometer in the spectral range of 200–1100 nm.
Mass spectra were measured using the electron impact
ionization method on an AMD 604 instrument.
N-Phenyl-1,4-phenylenediamine, 4,40-diaminodiphenylamine
sulfate, triethylamine, dimethyl sulfoxide (DMSO), 4-fluoro-
nitrobenzene, FeCl3 , ammonium persulfate, Sn powder and
hydrazine monohydrate were purchased from Aldrich Chemical
Co. Acetone, dichloromethane, ethyl acetate, sodium pyro-
sulfite, HCl, KOH, NaOH were purchased from POCh
(Poland). N-Phenyl-1,4-phenylenediamine was crystallized from
water and dried over KOH in a vacuum desiccator. Triethyl-
amine was dried over KOH. DMSO was stored over thermally
The present study reports a simple method for the synthesis
of phenyl/NH2 (Ph/NH2) and NH2/NH2 end-capped aniline
tetramers by the SNAr reaction of 4-fluoronitrobenzene with
the corresponding aromatic amines. The coupling is followed
by reduction of the nitro groups to arylamines. The iterative
˚
activated 4 A molecular sieves. 4-Fluoronitrobenzene, Sn
powder, the oxidants and the solvents were used as supplied.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 6 9 – 6 7 5
669

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Syntheses
74.57; H, 5.64; N, 18.12; found: C, 73.63; H, 5.44; N, 17.50.
EI-MS: m/z ¼ 380.3 [M þ H]þ.
4,40-Diaminodiphenylamine 1. 4,40-Diaminodiphenylamine
sulfate (ca. 1.2 g) was dissolved in 50 ml of 1 wt % KOH solu-
tion. Sodium persulfite (Na2S2O5 ; 0.6 g) was added to the solu-
tion and the mixture was boiled for 15 min, then filtered and
crystallized. The white crystals were dried over KOH in a
vacuum desiccator. M.p.158–160 ꢁC. 1H NMR (400 MHz,
DMSO-d6) d 6.76 (s, 1H), 6.63 (s, 4H), 6.45 (m, 4H), 4.46 (s,
4H). IR (cmꢂ1) 3409, 3390, 3034, 1612, 1515, 1312, 1275,
823. Anal. calcd for C12H13N3 C, 72.36; H, 6.53; N, 21.11;
found: C, 69.68; H, 6.50; N, 20.39.
Trimer Ph/NO2 5. N-Phenyl-1,4-phenylenediamine (1.106
g, 6 mmol) and 4-fluoronitrobenzene (0.846 g, 6 mmol) were
dissolved in 15 ml of DMSO. Triethylamine (0.85 ml, 6 mmol)
was added to the solution and then the mixture was stirred and
heated at 90 ꢁC for 5 days under argon atmosphere. The pro-
cedure was the same as for the preparation of tetramer
NO2/NO2 2, except that the purification by chromatography
was done with 1:4 ethyl acetate–CH2Cl2 . The product was
recrystallized from methanol as a red powder (0.85 g, 46.5%
yield). M.p. 132–134 ꢁC. 1H NMR (400 MHz, DMSO-d6) d
9.12 (s, 1H), 8.17 (s, 1H), 8.05–8.03 (m, 2H), 7.21 (t, 2H),
7.12–7.11 (m, 4H), 7.06–7.04 (m, 2H), 6.91–6.89 (m, 2H),
6.79 (t, 1H). IR (cmꢂ1) 3380, 3030, 1595, 1520, 1494, 1463,
1319, 1286, 1259, 1185, 1110, 833, 795, 695. Anal. calcd for
C18H15N3O2 : C, 70.82; H, 4.92; N, 13.77; found: C, 70.42;
H, 5.04; N, 13.91.
Tetramer NO2/NO2 2. 4,40-Diaminodiphenylamine (1;
0.995 g, 5 mmol) and 4-fluoronitrobenzene (1.41 g, 10 mmol)
were dissolved in 15 ml of DMSO. Triethylamine (1.7 ml, 12
mmol) was added to the solution and the mixture was stirred
and heated at 90 ꢁC for 3 days under argon atmosphere to
avoid oxidation of the amine. Then the solution was allowed
to cool to room temperature and poured into 200 ml of dis-
tilled water. The brown-orange solid was precipitated, washed
several times with distilled water and dried in air. The crude
product was purified by liquid chromatography (silica gel,
acetone–CH2Cl2 1:4). The powder was recrystallized from
methanol as orange-brown crystals (0.992 g, 45% yield after
recrystallization). M.p.184–186 ꢁC. 1H NMR (400 MHz,
DMSO-d6) d 9.12 (s, 2H), 8.22 (s, 1H), 8.05–8.03 (m, 4H),
7.15–7.09 (m, 8H), 6.91–6.89 (m, 4H). IR (cmꢂ1) 3355, 1602,
1523, 1498, 1299, 1114, 830. Anal. calcd for C24H19N5O4 : C,
65.31; H, 4.31; N, 15.87; found: C, 64.02; H, 4.19; N, 15.45.
Trimer Ph/NH2 6. Trimer Ph/NO2 5 was reduced to tri-
mer Ph/NH2 6 in exactly the same manner as described for
the reduction of tetramer NO2/NO2 2. Trimer Ph/NO2 (5;
0.505 g, 1.65 mmol) was reduced with Sn powder (1.57 g,
13.2 mmol) and 20 ml of 37% HCl. The amine formed was
completely soluble in HCl. After neutralization the white pow-
der was precipitated (0.327 g, 72% yield). M.p. 152–155 ꢁC. 1H
NMR (400 MHz, DMSO-d6) d 7.65 (s, 1H), 7.27 (s, 1H), 7.10
(t, 2H), 6.91–6.89 (m, 2H), 6.84–6.82 (m, 2H), 6.79–6.76 (m,
4H), 6.62 (t, 1H), 6.51–6.49 (m, 2H), 4.72 (s, 2H). IR (cmꢂ1
)
3435, 3370, 3025, 1601, 1526, 1513, 1496, 1311, 1259, 1224,
1172, 821, 746, 697. Anal. calcd for (C18H17N3)ꢀ(H2O)0.5 : C,
76.05; H, 6.34; N, 14.79; found: C, 74.26; H, 5.95; N, 14.45.
EI-MS: m/z ¼ 275.5 [M]þ.
Tetramer NH2/NH2 3. Tetramer NO2/NO2 (2; 0.8 g, 1.81
mmol) was stirred with 4 ml of 37% HCl under argon atmo-
sphere for 15 min at room temperature. Tin powder (3.44 g,
29.0 mmol) and 37% HCl (20 ml) were added in portions
and the mixture was refluxed for ca. 3 h until a whitish yellow
precipitate was formed. After cooling to room temperature 10
ml of 20 wt % NaOH solution and then NaOH granules were
slowly added with constant stirring. The pH of the mixture was
adjusted to 12. The white precipitate was filtered under argon
atmosphere and the product was washed several times with 20
wt % NaOH solution, then with distilled water to pH ¼ 7. The
whitish gray powder was dried over KOH in a vacuum desic-
Tetramer Ph/NO2 7. Tetramer Ph/NO2 (7) was prepared in
the same manner as trimer Ph/NO2 5. Trimer Ph/NH2 (6;
0.65 g, 2.4 mmol), 4-fluoronitrobenzene (0.338 g, 2.4 mmol)
and triethylamine (0.35 ml, 2.4 mmol) were dissolved in 10
ml of DMSO. The mixture was heated at 90 ꢁC for 3 days
under argon atmosphere. The crude product was purified by
chromatography (silica gel, ethyl acetate–CH2Cl2 1:4), then
recrystallized from methanol (0.44 g, 46% yield). M.p. 175–
178 ꢁC. 1H NMR (DMSO-d6) d 9.07 (s, 1H), 8.04–8.01
(m, 2H), 7.95 (s, 1H), 7.88 (s, 1H), 7.16 (t, 2H), 7.09–7.07 (m,
2H), 7.03–7.00 (m, 6H), 6.95–6.94 (m, 2H), 6.87–6.85 (m, 2H),
6.70 (t, 1H). IR (cmꢂ1) 3391, 3311, 3034, 1595, 1524, 1509,
1287, 1223, 1188, 1110, 824, 748, 695. Anal. calcd for
C24H20N4O2 : C, 72.73; H, 5.05; N, 14.14; found: C, 72.05;
H, 4.61; N, 14.01.
1
cator (0.18 g, 75% yield). H NMR (400 MHz, DMSO-d6) d
7.21 (s, 1H), 7.08 (s, 2H), 6.79–6.77 (m, 4H), 6.73–6.71 (m,
8H), 6.48–6.46 (m, 4H), 4.60 (s, 4H). IR (cmꢂ1) 3387, 3345,
3022, 1609, 1511, 1303, 1263, 1223, 817. Anal. calcd for
(C24H23N5)ꢀ(H2O)0.7 : C, 73.77; H, 6.25; N, 17.93; found: C,
74.21; H, 6.24; N, 17.99. EI-MS: m/z ¼ 381.7 [M]+.
Tetramer NH2/NH2-EB 4. Tetramer NH2/NH2 was oxi-
dized to the emeraldine oxidation state by the following proce-
dure. 3 (0.378 g, 0.99 mmol) was suspended in 10 ml of 0.1 M
HCl aqueous solution with constant stirring for 30 min at 0 ꢁC.
Ammonium persulfate (0.226 g, 0.99 mmol) was dissolved in
10 ml of 0.1 M HCl and cooled to 0 ꢁC. The oxidant solution
was added dropwise to the tetramer suspension with strong
stirring. The color of the suspension changed to dark green.
The reaction was carried out for 3 h at 0 ꢁC. The very fine sus-
pension was coagulated in distilled water and then filtered. The
green precipitate was washed several times with distilled water
and was deprotonated in 30 ml of 0.1 M NH3 aqueous solution
for 16 h at room temperature. The color turned to blue. The
precipitate was filtered, washed with distilled water and dried
over KOH. 1H NMR (400 MHz, DMSO-d6) d 7.86/7.85
(s, 1H), 7.06/7.04 (m, 1H), 6.96/6.95 (s, 2H), 6.94–6.91
(m, 1H), 6.89–6.83 (m, 6H), 6.79–6.76 (m, 2H), 6.61–6.59
(m, 2H), 6.56–6.54 (m, 2H), 5.44/5.43 (s, 2H), 4.83 (s, 2H).
IR (cmꢂ1) 3390, 3270, 3032, 1599, 1501, 1380, 1317, 1286,
1267, 1167, 832. Anal. calcd for (C24H21N5)ꢀ(H2O)0.4 : C,
Tetramer Ph/NH2 8. Tetramer Ph/NO2 (7; 0.95 g, 2.4
mmol) was reduced with Sn powder (2.3 g, 19.2 mmol) and
20 ml of 37% HCl as described above. The obtained white
powder was dried over KOH in a vacuum desiccator (0.64 g,
73% yield). 1H NMR (400 MHz, DMSO-d6) d 7.70 (s, 1H),
7.45 (s, 1H), 7.17 (s, 1H), 7.11 (t, 2H), 6.95–6.93 (m, 2H),
6.87–6.84 (m, 6H), 6.77–6.75 (m, 4H), 6.64 (t, 1H), 6.51–6.48
(m, 2H), 4.63 (s, 2H). IR (cmꢂ1) 3389, 3021, 1598, 1527,
1513, 1495, 1304, 1264, 1220, 817, 747, 693. Anal. calcd for
C24H22N4 : C, 78.69; H, 6.01; N, 15.30; found: C, 75.09; H,
5.68; N, 14.27. EI-MS: m/z ¼ 366.7 [M]þ.
Tetramer Ph/NH2-EB 9. Tetramer Ph/NH2 8 was oxi-
dized to the emeraldine oxidation state exactly in the same
manner as tetramer NH2/NH2 3 (see above). 1H NMR
(DMSO-d6) d 8.36, 7.68, 7.24, 7.12, 6.93–6.86, 6.69–6.64,
6.50, 5.49, 4.73. Anal. calcd for (C24H20N4).(H2O)1 :C, 75.39;
H, 5.76; N, 14.66; found: C, 70.62; H, 5.32; N, 13.64.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
670
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 6 9 – 6 7 5

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Tetramer Ph/NH2-EB(MD) 10. Tetramer Ph/NH2 10 in
the oxidation state of emeraldine was synthesized according
to the method of Zhang et al.4 1H NMR (DMSO-d6) d 8.38
(s, 1H), 7.24 (t, 2H), 7.09 (s, 4H), 7.03–6.96 (m, 5H), 6.91–
6.80 (m, 2H), 6.82–6.79 (m, 2H), 6.61–6.60 (m, 2H), 5.53 (s,
2H). IR (cmꢂ1) 3377, 3204, 3028, 1596, 1513, 1496, 1324,
1168, 842, 744, 693. Anal. calcd for (C24H20N4)ꢀ(H2O)0.5 : C,
77.21; H, 5.63; N, 15.01; found: C, 77.58; H, 5.58; N, 14.88.
EI-MS: m/z ¼ 365.3 [M þ H]þ.
(triarylamines) are preferentially formed even with an excess
of amines and at low temperature (50 ꢁC). However, the use
of triethylamine as a base ensures the formation of linear aryl-
nitroamines. In this case the reaction fulfils the normal criter-
ion for an SNAr reaction. As compared to the previously
reported Ullmann-type coupling,3 the present method does
not require protection of the amine groups and proceeds at a
relatively low temperature (90 ꢁC). Compound 2 (tetramer
NO2/NO2) was reduced directly to the tetramer NH2/NH2
3. Tetramer Ph/NH2 8 was obtained by an iterative
coupling/reduction sequence.
Tetramer Ph/NH2(MD) 11. Tetramer Ph/NH2-EB(MD) 10
was reduced to the leucoemeraldine form according to the
procedure of Wei et al.8 Tetramer 10 (0.1 g, 2.7 mmol) was
suspended and refluxed in hydrazine monohydrate (30 ml,
0.62 mol) under argon atmosphere for 24 h. The whitish
gray powder was dried in vacuum. 1H NMR (DMSO-d6) d
7.69 (s, 1H), 7.45 (s, 1H), 7.17 (s, 1H), 7.11 (t, 2H), 6.94–
6.92 (m, 2H), 6.87–6.83 (m, 6H), 6.76–6.74 (m, 4H), 6.64 (t,
1H), 6.50–6.48 (m, 2H), 4.63 (s, 2H). IR (cmꢂ1) 3390, 3921,
1600, 1529, 1510, 1498, 1308, 1220, 817, 745, 695. Anal. calcd
for C24H22N4 : C, 78.69; H, 6.01; N, 15.30; found: C, 77.19; H,
5.81; N, 14.99. EI-MS: m/z ¼ 366.3 [M]þ.
Tetramer Ph/NH2-EB(MD) 10 was also prepared by the
oxidative coupling method of Zhang et al.4 The spectroscopic
properties of these three tetramers in the leucoemeraldine and
the emeraldine oxidation states have been studied. The syn-
thetic routes used to obtain the different types of tetramers
are presented in Scheme 1.
1
In Fig. 1 the H NMR solution spectra of tetramers NH2/
NH2 [3, Fig. 1(a)] and Ph/NH2 [8, Fig. 1(b)] in the leuco-
emeraldine state are presented. The spectra of the Ph/NH2
tetramers, obtained either by the SNAr coupling/reduction
sequence 8 or via the oxidative coupling/reduction method
11, were exactly the same. Magnetically inequivalent protons
are indicated in Scheme 2.
The signals of terminal NH2 group protons appear at 4.60
and 4.63 ppm in the spectra of NH2/NH2 3 and Ph/NH2
(8 or 11) tetramers, respectively. The positions of the other
–NH– signals are influenced by the number of electron-donat-
ing amine groups and by the character of the neighboring aro-
matic rings. Thus, the signal at 7.08 ppm can be attributed to –
NHb– and that at 7.21 ppm to –NHc– protons in the spec-
trum of NH2/NH2 tetramer 3. The positions of the –NHb–
and –NHc– signals are displaced to 7.17 and 7.45 ppm in the
spectrum of Ph/NH2 tetramer 8. The peak related to the
–NHd– proton is located at 7.70 ppm. The greater nucleophilic
character of the NH2/NH2 tetramer as compared to the Ph/
NH2 one is also evidenced in the position of the aromatic
Results and discussion
It is widely known that diarylamines are formed by the reac-
tion of activated fluoroarens with arylamines in dipolar aprotic
solvents. We have chosen this type of reaction for the prepara-
tion of phenyl/NH2 and NH2/NH2 end-capped tetramers of
aniline. In the presence of a base the reaction of arylamines
with 4-fluoronitrobenzene results in the formation of arylni-
troamines (see Scheme 1).
Monodirectional or bidirectional growth of the chain length
is possible by using unsymmetrical or symmetrical arylamines.
We found that in the presence of K2CO3 or KF, agents fre-
quently used for this type of reaction,8,9 branched amines
Scheme 1 The syntheses of Ph/NH2 and NH2/NH2 tetramers in the leucoemeraldine and emeraldine oxidation states.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 6 9 – 6 7 5
671
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4

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Fig. 1 The 1H NMR spectra in DMSO-d6 solution of (a) NH2/NH2
tetramer 3, (b) Ph/NH2 tetramer 8 in the leucoemeraldine oxidation
state.
proton signals. These appear at lower d values in the spectrum
of the NH2/NH2 tetramer 3 as compared to those characteris-
tic of Ph/NH2 tetraamine 8. It should be noted that both types
of molecules correspond to AA0BB0 systems. Therefore, for
aromatic proton signals we do not observe simple doublets
but multiplets. In the spectrum of NH2/NH2 tetramer 3 the
multiplets at 6.48–6.46, 6.73–6.71, 6.79–6.77 ppm can be
attributed to the H1 , H2 þ H3 , H4 protons, respectively. In
the spectrum of the Ph/NH2 tetramer 8 the signals appear at
6.50–6.48 (H1), 6.76–6.74 (H2 and H3), 6.87–6.83 (H4 , H5 ,
H6), 6.94–6.92 (H7) ppm. The appearance of two triplets at
7.11 (H8) and 6.64 (H9) ppm confirms the presence of the term-
Fig. 2 IR spectra of (a) NH2/NH2 tetramer 3, (b) Ph/NH2 tetramer
8, (c) Ph/NH2(MD) tetramer 11.
stretching vibrations appear at 1528, 1513 and 1495 cmꢂ1 in
the spectra of Ph/NH2 tetramers and at 1511 cmꢂ1 with a
shoulder at 1495 cmꢂ1 in the spectrum of NH2/NH2 tetra-
amine 3. Similar modes were observed by Boyer et al.10 for
Ph/NH2 oligomers. The comparison of the bands located in
this region suggests that the peak at 1528 cmꢂ1 can be assigned
to the vibration of the terminal phenyl ring of the Ph/NH2 tet-
ramer (8 or 11), because it is absent in the spectrum of 4,40-dia-
minodiphenylamine (not presented in Fig. 2) as well as in the
spectrum of the NH2/NH2 tetramer 3. There are some differ-
1
inal phenyl ring in the Ph/NH2 tetramer 8. The observed H
NMR spectra agree with the postulated formula of the synthe-
sized tetramers. Moreover, the last method results in the
formation of a tetramer coupled only in the para position.
In Fig. 2 the IR spectra of the three tetramers 3, 8 and 11 are
shown. They exhibit features typical of the reduced form, that
is the leucoemeraldine oxidation state. The bands originating
from N–H or NH2 stretching vibrations can be distinguished
at 3390 cmꢂ1 for Ph/NH2 tetramers 8 and 11 and at 3387
cmꢂ1 with a shoulder at 3345 cmꢂ1 for NH2/NH2 tetramer
3. The modes due to a combination of C–H bending and CC
ences in the interpretation of the nature of the peaks located
11
at 1303 and 1264 cmꢂ1. Choi and Kertesz
proposed that
the mode at 1303 cmꢂ1 is due to C–N stretching in the Ci point
group whereas Quillard et al.12 suggested that the C–N stretch-
ing can be related to the peak at 1264 cmꢂ1. Comparing the
intensities of the peaks shown in the spectra of the tetramers
we are inclined to follow the interpretation given by Quillard
et al. Moreover, the peaks attributed to the C–H out-of-plane
deformation of 1,4-disubstituted rings at 816 cmꢂ1 and mono-
substituted rings at 746 and 695 cmꢂ1 appear in the spectra of
Ph/NH2 tetramers 8 and 11 whereas only the peak at 817 cmꢂ1
is present in the NH2/NH2 tetramer 3 spectrum.
Tetramers Ph/NH2 8 and NH2/NH2 3 synthesized by the
SNAr coupling/reduction sequence were oxidized to the
emeraldine form using ammonium persulfate in a 0.1 M HCl
solution. This oxidation can be conveniently followed by
UV-Vis-NIR spectroscopy. In the reduced state for both
NH2/NH2 3 and Ph/NH2 8 tetramers only one peak is
observed, at 325 and 322 nm, respectively, ascribed to the p–p*
transition in the aromatic ring. The oxidation process gives
rise to new peaks in the less energetic region of the spectrum
Scheme 2 Two types of tetramers: NH2/NH2 3 and Ph/NH2 (8 or
11) end-capped tetraamines.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
672
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 6 9 – 6 7 5

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(Fig. 3). In particular, for NH2/NH2 tetramer 3, in the first
step of the oxidation two new peaks appear at 426 and ca.
1000 nm. Further oxidation causes a hypsochromic shift in
the position of all peaks present in the spectrum and the
appearance of a new peak at ca. 750 nm, which dominates
the spectrum at the end of the oxidation process. Thus, the
oxidized tetramer can be characterized by the following
absorption bands: 306, 410 and 730 nm. The behavior of the
Ph/NH2 tetramer 8 is similar [(Fig. 3(b)]. The most important
feature observed in the oxidation induced spectra of NH2/
NH2 3 and Ph/NH2 8 consists in the fact that the band in
the vicinity of 1000 nm appears in the initial stages of the
oxidation and vanishes as the oxidation proceeds. It should
be noted that the spectra of the oxidized tetramers are charac-
teristic of their protonated forms, which is not unexpected
because the oxidation reaction is carried out in an acidic
medium.
After deprotonation the absorption spectrum (not shown) of
oxidized NH2/NH2 tetramer 4 shows peaks at 320 nm, which
can be attributed to a p-p* transition, and at 608 nm ascribed
to an exciton transition in the quinoid ring. In the spectrum of
deprotonated, oxidized Ph/NH2 tetramer 9 the maxima are
located at 306 and 588 nm in DMSO solution and at 290
and 550 nm in CHCl3 solution. The positions of these peaks
are in excellent agreement with the location of the peaks
observed in the spectrum of Ph/NH2-EB(MD) tetramer 10
and agrees with the data published by MacDiarmid et al.13
The oxidation of both tetramers can be confirmed by the IR
spectra. The main difference between the spectra of tetramers
in the leucoemeraldine state and that in the emeraldine state
is the appearance of a peak at 1596 cmꢂ1 due to CC stretching
in the quinoid ring and a peak at 1167 cmꢂ1 ascribed to C–H
bending deformations in the quinoid ring.14 The mode related
to C–H out-of-plane deformation is also displaced to 832 cmꢂ1
in the spectrum of oxidized Ph/NH2 tetramer 9 and to 842
cmꢂ1 in the spectrum of the oxidized NH2/NH2 molecule 4
as compared to the case of the reduced tetramers.
However, oxidation of tetramers can lead to the formation
of different positional isomers, shown in Scheme 3, as was pos-
tulated by MacDiarmid et al.13. Tetramer Ph/NH2 can form
three positional isomers. Tetramer NH2/NH2 4 is symmetric
and can form only two isomers. The1H NMR spectrum of oxi-
dized NH2/NH2-EB tetramer 4 is shown in Fig. 4. The oxida-
tion of this tetraamine to the emeraldine state causes the loss of
symmetry. In the spectrum three signals at 7.86, 5.44 and 4.83
ppm can be attributed to amine protons. This suggests that
after oxidation the molecule exists in the form of isomer A.
The formation of isomer C can be excluded since its formation
would lead to four signals. The appearance of quinoid ring
and imine nitrogens in the structure of the molecule causes a
displacement of the amine proton signals in comparison to
the signals observed in the spectrum of the reduced tetraamine.
The signals of terminal amine group protons are shifted from
4.60 ppm in the spectrum of reduced NH2/NH2 tetraamine 3
to 4.83 and 5.44 ppm and can be assigned to NH2 protons
of the groups located further and closer to the quinoid ring,
respectively. The quinoid ring influences also the position of
the signal of –NH– proton, which shifts from 7.08 ppm to
7.86 ppm upon the oxidation [compare Fig. 1(a) and Fig. 4].
Moreover, one can observe that except for the signal at 4.83
ppm all other signals are split. The simplest interpretation of
these spectral features involves the presence of syn and anti
isomers (see Scheme 4). We tried to interpret the spectrum
according to this hypothesis and estimate the relative amounts
of both isomers. We assumed that the signals, for example at
7.86 (7.85), 6.56 (6.54), 6.79 (6.76) are related to the same type
of protons but in the anti and syn isomers. The ratio of the
areas of the mentioned peaks is estimated as 1:0.75, thus the
ratio of the amount of both isomers should be the same. Addi-
tionally, we assumed that the anti isomer, which has the lower
steric hindrance, would be present in higher concentration.
According to these assumptions the multiplets located at
6.56–6.54 ppm can be attributed to H7 , at 6.61–6.58 ppm to
H1 and at 6.79–6.77 ppm to H6 protons. The intense and broad
Fig. 3 The UV-Vis-NIR spectra in DMSO of (a) NH2/NH2 tetramer
3, (b) Ph/NH2 tetramer 8 oxidized with ammonium persulfate in 0.1 M
HCl solution.
Scheme 3 Positional isomers of oxidized Ph/NH2 and NH2/NH2
tetramers.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 6 9 – 6 7 5
673

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consideration our earlier assumptions (anti-to-syn ratio of
1:0.75). The area ratios of the corresponding peaks measured
from the spectrum is 1.6:3.4:2, which is in an agreement with
our predictions.
We have also studied two Ph/NH2 tetramers in the emer-
aldine oxidation state, the Ph/NH2-EB(MD) tetramer 10
obtained by oxidative coupling and the Ph/NH2-EB tetramer
9 prepared by the SNAr coupling/reduction sequence and then
oxidized. The 1H NMR spectra of both tetramers are shown in
Fig. 5. It has been reported by MacDiarmid et al.13 that in
DMSO-d6 solution only isomer B was detected (see Scheme
3). Cao et al.15 stated that in DMF-d6 solution the isomeric
forms B and C co-exist. However, comparison of our spectrum
of the Ph/NH2-EB(MD) tetramer 10 with the spectrum of the
NH2/NH2-EB tetramer 4 shows that only isomer A is formed.
The signal of the terminal amino group protons are located at
5.53 ppm, similar to the case of the signal of NH2(a1) in the
spectrum of NH2/NH2-EB 4, indicating a rather close vici-
nity of the quinoid ring. Additionally, the signal of the
–NH– proton is placed at 8.38 ppm and is shifted ca. 0.68
ppm in comparison to the –NHd– signal observed at 7.70
ppm in the Ph/NH2 tetraamine 11 spectrum [see Fig. 1(b)].
A similar –NH– signal displacement is detected in the
spectrum of the NH2/NH2-EB tetramer 4. The signal at
7.09 ppm can be assigned to quinoid protons. The triplet at
7.24 ppm is ascribed to H7 protons. However, the second tri-
plet of H8 was not clearly visible and we suppose that it could
be located at ca. 6.83 ppm, overlapping with the signals of
other aromatic protons. The assignment of other signals is
marked in Fig. 5(a).
The spectrum [Fig. 5(b)] of the oxidized Ph/NH2 tetraa-
mine in the EB state 9 is richer compared to that of Ph/
NH2-EB(MD) 10. This spectrum can correspond to a mixture
of the positional isomers A and B because two signals appear
at 4.73 and 5.49 ppm, which can be related to the NH2(a2)
and NH2(a1) protons. Moreover, two signals that can be
attributed to –NH– protons are located at 7.68 and 8.36
ppm. The integration of the signals at 4.73/5.49 ppm
gives the ratio 3.3:1 and the integration of the signals at
Fig. 4 The 1H NMR spectrum in DMSO-d6 of the NH2/NH2-EB
tetramer 4.
multiplet at 6.89–6.83 can be assigned to other protons of the
phenylene rings. However, the assignment of the quinoid pro-
tons is not so clear. The peaks at 6.96 and 6.95 ppm can be
ascribed to this type of proton but the integration of these
peaks gave a too low value to correspond to four protons.
Nevertheless, the quinoid protons are influenced by H2 and
H4 protons and are not equivalent, as it is shown in Scheme
4. Thus, we can distinguish three types of protons marked as
H3a , H3b and H3c . If the signals at 7.06–7.04 ppm and at
6.94–6.91 ppm are also due to the quinoid protons the area
ratios of the signals 7.06–7.04 (H3c): 6.96–6.95 (H3a): 6.94–
6.91 (H3b) ppm should correspond to 1.5:3.5:2, taking into
Fig. 5 The 1H NMR spectra in DMSO-d6 of (a) Ph/NH2-EB(MD)
tetramer 10 and (b) Ph/NH2-EB tetramer 9.
Scheme 4 Anti and syn isomers of the oxidized NH2/NH2 tetramer 4.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
674
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 6 9 – 6 7 5

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7.68/8.36 ppm results in a 3.8:1 ratio. Thus, the isomer B is
formed in preference to isomer A in this case. The signal at
7.12 ppm can be assigned to quinoid protons. The peak at
7.24 ppm can correspond to H7 protons whereas the triplet
0
of H7 should be more shifted and can overlap with the signal
at 7.68 ppm (–NHb2).
Acknowledgements
We wish to acknowledge financial support from the Commit-
tee of Scientific Research in Poland (KBN Grant No. 4T09A
082 22).
It should be stressed that in the spectra of 9 and 10 pre-
sented in Fig. 5 the signals are not split as it is observed in
the spectrum of 4 in Fig. 4. The question why we do not
detect the syn and anti isomers in these cases remains open.
The most trivial explanation is that only one type of isomer
(syn or anti) is formed, however, other hypotheses cannot
be excluded.
References
1
2
3
4
5
6
7
J. Honzl and M. Tlustakova, J. Polym. Sci., Part C: Polym.
Symp., 1968, 22, 451–462.
M. Ochi, H. Furusho and J. Tanaka, Bull. Chem. Sci. Jpn., 1994,
67, 1749–1752.
E. Rebourt, J. A. Joule and A. P. Monkman, Synth. Met., 1997,
84, 65–66.
W. J. Zhang, J. Feng, A. G. MacDiarmid and A. J. Epstein,
Synth. Met., 1997, 84, 119–120.
Y. Wei, C. Yang, G. Wei and G. Feng, Synth. Met., 1997, 84,
289–290.
W. Wang and A. G. MacDiarmid, Synth. Met., 2002, 129,
199–205.
J. P. Sadighi, R. A. Singer and S. L. Buchwald, J. Am. Chem. Soc.,
1998, 120, 4960–4976.
Conclusions
Two types of aniline tetramers, namely NH2/NH2 and Ph/
NH2 end-capped oligomers, were synthesized by an SNAr cou-
pling/reduction sequence. Additionally, the Ph/NH2-EB(MD)
tetramer was obtained by oxidative coupling. The spectro-
scopic properties of these tetramers in the leucoemeraldine
and emeraldine oxidation states were compared. In the
reduced form both types of Ph/NH2 tetraamines are spectro-
scopically indistinguishable. They differ, however, in the oxi-
dized state. The oxidative coupling led to the formation of
positional isomer A whereas the oxidation of the Ph/NH2 tet-
raamine gave a mixture of isomers A and B. In the case of
NH2/NH2-EB tetramer two geometrical isomers, syn and anti,
were present.
8
9
J. H. Gorvin, J. Chem. Soc., Perkin Trans. 1, 1988, 1331–1335.
J. J. Kulagowski and C. W. Rees, Synthesis, 1980, 215.
10 M. I. Boyer, S. Quillard, G. Louarn and S. Lefrant, Synth. Met.,
1999, 101, 782–783.
11 C. H. Choi and M. Kertesz, Macromolecules, 1997, 30, 620–630.
12 S. Quillard, G. Louarn, S. Lefrant and A. G. MacDiarmid, Phys.
Rev. B, 1994, 50, 12 496–12 508.
13 A. G. MacDiarmid, Y. Zhou, J. Feng, G. T. Furst and A. M.
Shedlow, Polym. Prepr (Am. Chem. Soc., Div. Polym. Chem.).,
1999, 40, 246–247.
14 G. Louarn, M. Lapkowski, S. Quillard, A. Pron, J. P. Buisson and
S. Lefrant, J. Phys. Chem., 1996, 100, 6998–7006.
15 Y. Cao, S. Li, Z. Xue and D. Guo, Synth. Met., 1986, 16, 305–309.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 6 9 – 6 7 5
675