Synthesis and spectroscopic properties of linear oligoanilines

Article abstract of DOI:10.1080/15421400490482952

Two types of linear oligomers, namely Ph/NH₂ and NH ₂/NH₂ end-capped oligoanilines have been synthesized by S_NAr coupling/reduction sequence. The ¹H NMR spectra of obtained compounds are strongly infl

Full text of DOI:10.1080/15421400490482952

Mol. Cryst. Liq. Cryst., Vol. 415, pp. 105–114, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490482952
SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF
LINEAR OLIGOANILINES
I. Rozalska, M. Kurylek, M. Franaszek, and I. Kulszewicz-Bajer
Faculty of Chemistry, Warsaw University of Technology,
Noakowskiego 3, 00-664 Warsaw
Two types of linear oligomers, namely Ph=NH2 and NH2=NH2 end-capped oli-
goanilines have been synthesized by SNAr coupling=reduction sequence. The
1H NMR spectra of obtained compounds are strongly influenced by electron
withdrawing or electron donating effects of the terminal groups (NO2 or NH2,
respectively). The UV-Vis spectra of nitroamines exhibit two peaks character-
istic for two types of the phenylene substituents (-NH- or NO2), whereas
UV-Vis spectra of oligoanilines show only one peak attributed to the P-P* tran-
sition in the phenylene rings. The oxidation of oligoanilines was also followed
by UV-Vis spectroscopy.
INTRODUCTION
Polyaniline has attracted considerable attention due to its good stability
and processibility in the conducting state. However, the properties of poly-
aniline depend strongly on its structure. For this reason oligoanilines can
be considered as good model compounds to elucidate the structure, iso-
merization and possible molecular interaction of polyaniline. Aniline oligo-
mers can be divided into three classes depending on their terminal groups,
i.e. oligomers capped with a phenylene ring at one end and an amine group
on the other (Ph=NH2)?, oligomers capped with two amines groups
(NH2-NH2) and oligomers capped with two phenylene rings (Ph=Ph).
Recently, Ph=NH2 and Ph=Ph end-capped oligomers have been synthesized
by the methods involving different types of condensation [1–5].
In this study we report a new method of preparing Ph=NH2 and NH2=NH2
end-capped oligoanilines, namely SNAr coupling of 4-fluoronitrobenzene to
corresponding arylamines followed by the reduction of NO2 groups. The
We wish to acknowledge financial support from the Committee of the Scientific Researche
in Poland (KBN Grant No. 4T09A 082 22).
Address correspondence to I. Kulszewicz-Bajer, Faculty of Chemistry, Warsaw University of
Technology, Noakowskiego 3, Warsaw 00-664. E-mail: ikulsz@chemix.ch.pw.edu.pl
105

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structure of obtained compounds (trimers and tetramers of both types)
1
was characterized by H NMR spectroscopy. The oxidation of oligoanilines
was followed by UV-Vis spectroscopy.
EXPERIMENTAL
N-phenyl-1,4-phenylenediamine,
4,4’-diaminodiphenylamine
sulfate,
triethylamine, dimethyl sulfoxide (DMSO), 4-fluoronitrobenzene, FeCl3,
ammonium persulfate, Sn powder, hydrazine monohydrate were purchased
from Aldrich Chemical Co. Acetone, dichloromethane, ethyl acetate, so-
dium pyrosulfite, HCl, KOH, NaOH were purchased from POCh (Poland).
N-phenyl-1,4-phenylenediamine was crystallized from water and dried over
KOH in vacuum desiccator. Triethyl amine was dried over KOH. DMSO was
˚
stored over thermally activated molecular sieves 4 A. 4-fluoronitrobenzene,
Sn powder, the oxidants and the solvents were used as supplied.
All nitroamines were prepared on the same way. Arylamines (10 mmol),
4-fluoronitrobenzene (10 mmol in the case of NH2=NH2 arylamine or
5 mmol in the case of Ph=NH2 arylamine) and triethylamine (10 mmol)
were dissolved in 15 ml of DMSO under argon atmosphere. The solution
was stirred and heated at 90ꢀC for 3 days. Then it was allowed to cool to
room temperature, purred onto 200 ml of distilled water and filtered. The
brown-orange solid was precipitated and washed several times with dis-
tilled water. The crude product was purified by liquid chromatography
using silica gel. In the case of NO2=NO2 nitroamines the mixture of ace-
tone=CH2Cl2, 1:4 was used as an eluent. In the case of Ph=NO2 nitroamines
the mixture of ethyl acetate=CH2Cl2, 1:4 was used. The powder was recrys-
tallized from methanol.
Nitroamines were reduced to aryloamines using Sn powder and HCl. Nitroa-
mine (2.5 mmol) was stirred with 5 ml of 37% HCl under argon atmosphere for
15 min. Tin powder (40 mmol in the case of NO2=NO2 nitroamines or 20 mmol
in the case of the reduction of Ph=NO2 nitroamines) and 37% HCl (25 ml)
were added in portions and the mixture was refluxed for 3 h until white-yellow
precipitate was formed. After cooling to room temperature NaOH granules
were slowly added with constant stirring to adjust the pH to 12. The white pre-
cipitate was filtered, washed with 20% NaOH solution then with distilled
wt
water. The powder was dried over KOH in vacuum desiccator.
SPECTROSCOPIC MEASUREMENTS
1H NMR spectra were recorded in DMSO-d6 solution on Varian Mercury
400 MHz.
UV-Vis-NIR spectra were registered on Lambda 2 (Parkin-Elmer) spec-
trophotometer in the spectral range of 250–1100 nm.

Synthesis and Spectroscopic Properties of Linear Oligoanilines
107
RESULTS AND DISCUSSION
This synthetic approach is based on the fact that activated fluoroarens
can react with arylamines in dipolar aprotic solvents according to
SNAr mechanism. As activating groups the electron withdrawing groups
like for example NO2 or CN are frequently used. In our method we use
4-fluoronitrobenzene, arylamines and triethylamine as a base to obtain
linear arylnitroamines. SNAr coupling is followed by the reduction of nitro
groups to corresponding amine which can serve as a reagent for next step
of the elongation of the molecule (Scheme 1).
1
The H NMR spectra of synthesized nitroamines in DMSO-d6 are pre-
sented in Figures 1 and 2. The signals in the 9.3–7.8 ppm region are related
to the ÀNHÀ protons. The location of these signals is strongly influenced
by the electron withdrawing effect of the terminal nitro groups. Among our
4 studied oligomers, this effect is the biggest in the case of NO2=NO2 trimer
(the ÀNHÀ proton signal appear at 9.29 ppm) whereas it is the lowest
in the case of Ph=NO2 tetramer. The signals corresponding to aromatic
protons in orto-positions to nitro groups appear ca. 8.05 ppm indepen-
dently of the molecule length. For symmetry reasons the spectra of
NO2=NO2 oligomers are very simple. The spectra of unsymmetric Ph=NO2
oligomers are richer and show. Two triplets located at ca. 7.2 ppm and
6.7 ppm, which confirm the presence of terminal phenylene ring. The
1
observed H NMR spectra agree with the postulated formulas of synthe-
sized oligomers.
SCHEME 1 The synthesis of NO2=NO2 and Ph=NO2 trimers and tetramers. Corre-
sponding oligomers are prepared by the reduction of nitro groups.

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SCHEME 2 Two types of oligomers: Ph=NO2 and NO2=NO2 end-capped oligomers.
The UV-Vis spectra in DMSO of nitroamines (Figs. 3 and 4) exhibit two
peaks located at 264–303 nm and at 430 nm. The high energy peak can be
attributed to the P-P? transition in the benzoid ring with two amine
(ÀNHÀ) substituents, whereas the peak at 430 nm can be ascribed to
the same transition but in the benzoid rings with one amine (ÀNHÀ)
and one nitro groups in p-position. The relative intensities ratio of both
peaks depends on the type of the molecule. The spectrum of Ph=NO2
trimer shows the peaks of comparable intensities. In the case of NO2=NO2
SCHEME 3 Two types of trimers: Ph=NH2 and NH2=NH2 end-capped triamnines.

Synthesis and Spectroscopic Properties of Linear Oligoanilines
109
FIGURE 1 The 1H NMR spectra in DMSO-d6 solution of NO2=NO2 end-capped
oligomers (a) trimer, (b) tetramer.
trimer spectrum the peak related to NH-Ph-NH excitation is small as com-
pared with that corresponding to NH-Ph-NO2 excitation.
The reduction of nitroamines changes the spectroscopic properties of
1
the compounds. The H NMR spectra of oligomers are influenced by the
electro-donating character of amine groups (Fig. 5). The signals of terminal
NH2 group protons appear in the 4.72–4.57 ppm region. The positions of

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I. Rozalska et al.
FIGURE 2 The 1H NMR spectra in DMSO-d6 solution of Ph=NO2 end-capped oligo-
mers (a) trimer, (b) tetramer.
other ÀNHÀ signals are also deplaced to the lower d values as compared to
the signals observed in nitroamines spectra.
Oligoanilines can be easily oxidized and this process can be followed by
UV-Vis spectroscopy measurements. The UV-Vis spectra in DMSO of oligoa-
nilines exhibit only one peak located at ca. 320 nm (Fig. 6). The oxidation
process gives rise to new absorptions. The peak in the vicinity of 1000 nm

Synthesis and Spectroscopic Properties of Linear Oligoanilines
111
FIGURE 3 The UV-Vis spectra in DMSO of NO2=NO2 end-capped oligomers (a) tri-
mer, (b) tetramer.
appears in the initial stages of the oxidation and vanishes when the
oxidation proceeds in the spectra of aniline tetramers whereas it is always
present in the case of trimers spectra. It should be also noted that the
character of the spectra depends on the length of the molecules (trimer
or tetramer) and is not influenced by the nature of the terminal groups
(NH2=NH2 or Ph=NH2).
FIGURE 4 The UV-Vis spectra in DMSO of Ph=NO2 end-capped oligomers (a) tri-
mer, (b) tetramer.

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FIGURE 5 The 1H NMR spectra in DMSO-d6 solution of trimers (a) NH2=NH2,
(b) Ph=NH2 end-capped.
CONCLUSIONS
Two types of aniline oligomers were synthesized by SNAr coupling=
reduction sequence. The spectroscopic properties of oligonitroamines as
well as oligoanilines depend on the length of the molecule and the chemical
nature of the terminal groups. The oxidation of oligoanilines was followed by

Synthesis and Spectroscopic Properties of Linear Oligoanilines
113
FIGURE 6 The UV-Vis spectra in DMSO of Ph=NH2 trimer (a), Ph=NH2 tetramers
(b) oxidized with ammonium persulfate in 0.1 M HCI solutions.
UV-Vis spectroscopy. The spectra of oxidized tetramers exhibit the spectro-
scopic features similar to the observed in polianiline.
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Sci., Part C., 22, 451.

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[2] Rebourt, E., Joule, J. A., & Monkman, A. P. (1997). Polyaniline oligomers; synthesis and
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[3] Zhang, W. J., Feng, J., MacDiarmid, A. G., & Epstein, A. J. (1997). Synthesis of oligomeric
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