Spectroscopic investigation of the donor-acceptor interaction between o-phenylenediamine and tetracyanoethylene in dichloromethane

Article abstract of DOI:10.1016/j.saa.2007.08.026

The interaction between the electron donor o-phenylenediamine (o-phendn) and the π-acceptor tetracyanoethylene (TCNE) has been investigated spectrophotometrically in dichloromethane at 25 °C. The results indicate that the reaction proceeds via two major steps. The formation of the 1:2 charge-transfer complex [(o-phendn)₂, (TCNE)₄] in which two o-phenylenediamine molecules undergoes slow N-substitution by one TCNE forming the final reaction product 2,2-di-o-aminoaniline-1,1-dicyanoethylene and 2HCN molecules. The reaction was studied using electronic, infrared, photometric titration and cyclic voltametric measurements. A general mechanism for the reactions is proposed.

Full text of DOI:10.1016/j.saa.2007.08.026

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Spectrochimica Acta Part A 70 (2008) 997–1000
Spectroscopic investigation of the donor–acceptor interaction between
o-phenylenediamine and tetracyanoethylene in dichloromethane
Amina S. Aljaber∗, El-Metwally Nour
Department of Chemistry and Earth Science, College of Art and Sciences, Qatar University,
P.O. Box 2713, Doha, Qatar
Received 5 March 2007; accepted 1 August 2007
Abstract
The interaction between the electron donor o-phenylenediamine (o-phendn) and the ␲-acceptor tetracyanoethylene (TCNE) has been investigated
spectrophotometrically in dichloromethane at 25 ◦C. The results indicate that the reaction proceeds via two major steps. The formation of the 1:2
charge-transfer complex [(o-phendn)2, (TCNE)4] in which two o-phenylenediamine molecules undergoes slow N-substitution by one TCNE forming
the final reaction product 2,2-di-o-aminoaniline-1,1-dicyanoethylene and 2HCN molecules. The reaction was studied using electronic, infrared,
photometric titration and cyclic voltametric measurements. A general mechanism for the reactions is proposed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Charge-transfer; o-Phenylenediamine; Tetracyanoethylene; Spectra
1. Introduction
The study of the charge-transfer interactions in solution
between various electron donors with donor atoms like nitro-
gen, oxygen or sulfur and ␴- and ␲-electron acceptors have been
the subject of many investigations [1–6]. More recently, atten-
tion has been given to the isolation and investigation of physical
2. Experimental
properties of some CT-complexes in the solid state. Some of
these complexes show interesting electrical conductivity prop-
Reagent grade chemicals were used throughout. Tetracya-
erties and have found applications in many forms of electronics
noethylene (TCNE) was obtained from Aldrich Chem. Co.,
and solar cells [7,8].
while o-phenylenediamine (o-phendn) was obtained from BDH.
The electronic absorption spectra of the reactants TCNE and
o-phendn as well as of the reaction products in the solvents
dichibromethane were recorded in the region 300–900 nm using
a Perkin-Elmer double beam spectrometer model EZ-210 with
quartz cell of 1 cm path length. The mid-infrared spectra of the
To continue our studies of charge-transfer interaction
[2,4,9–11], the present paper represents the results obtained
from electronic and vibrational absorptions and cyclic voltame-
try measurements on the chemical products formed in the
reaction of o-phenylenediamine as an electron donor and the ␲-
acceptor tetracyanoethylene. The aim of the work is to determine
same reactants and the final reaction product were recorded
the reaction stoichiometry as well as to investigate the nature
in KBr discs in the region 4000–400 cm−1 using a Nicolet
and structure of the reaction products in both solution and solid
FT-IR spectrometer model 670. Photometric titrations were
state.
preformed for the reaction between TCNE and o-phendn in
dichloromethane at 25 ◦C. In these titrations, the concentra-
tion of o-phendn in the reaction mixture were kept fixed at
5 × 10−3 M while that of the acceptor TCNE was changed
∗
over a wide range of concentrations to produce solution of o-
Corresponding author. Fax: +974 4851049.
E-mail address: a.s.aljaber@qu.edu.qa (A.S. Aljaber).
phendn:TCNE molar ratio varying from 1:0.25 to 1:6. The peak
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.08.026

998
A.S. Aljaber, E.-M. Nour / Spectrochimica Acta Part A 70 (2008) 997–1000
Fig. 3. Structure for 2,2-di-o-aminoaniline-1,1-dicyanoethylene (DOAA-
PDCE).
Fig. 1. Electronic absorption spectra of o-phenylenediamine–TCNE
in CH2Cl2. (A) [o-phendn]=5 × 10−3 M; (B) [TCNE]=1 × 10−4 M;
(C) 1:2 [o-phenedn–TCNE] blue mixture, [o-phendn]=5 × 10−3 M and
[TCNE]=1 × 10−4 M; (D) final pale brown product, [DOAAPDCE]
=∼1 × 10−6 M.
absorbance at 735 nm of the formed CT-complex was measured
for each reaction mixture and plotted as a function of the TCNE
to o-phendn molar ratio in order to determine the stoichiome-
try of the reaction according to the known method [12]. Cyclic
voltammetry measurements were preformed with Oxford Elec-
trode Potentiostat/wave generator by using a three-electrode
configuration, including platinum working electrode, platinum
coil auxiliary electrode, and a silver wire as reference electrode.
Dichloromethane was used as the solvent, and tetrabutylammo-
niumhexafluorophosphate [0.1 M (n-Bu4N) PF6] as a supporting
electrolyte.
The o-phendn–TCNE solid product was isolated by the addi-
tion of saturated solution of o-phendn (30 ml) to a saturated
solution of TCNE (50 ml) in CH2Cl2. Upon the addition of
these two colorless reactants, a dark blue color is formed which
fades away very slowly forming stable light brown precipi-
tate. The precipitate was filtered off and washed several times
with small amounts of dichloromethane, and dried under vac-
uum. The formed light brown product was characterized by
Fig. 4. Structure for 1,1-dicyano-2-cyano-2-anminobenzene.
its electronic and infrared spectral and elemental analysis data.
It was identified as 2,2-di-o-aminoaniline-1,1-dicyanoethylene
(C16H14N6); analysis: C, 65.87 (66.20%); H, 4.91 (4.83%) and
N, 28.58 (28.97%) (calculated values are shown in parenthesis).
3. Result and discussion
A blue color is formed upon the addition of o-
phenylenediamine to a solution of TCNE in dichloromethane.
This blue color is stable for about 1–2 h. It fades away very
slowly forming a pale brown precipitate. The solubility of this
brown solid is very low of the order of 10−6 M. Fig. 1 shows
Fig. 2. Photometric titration curve for o-phenylenediamine–TCNE reaction
(blue species) in CH2C2. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of the article.)
Fig. 5. Cyclic voltammogram for the blue intermediate formed in the reaction
of o-phendn–TCNE in CH2Cl2.

A.S. Aljaber, E.-M. Nour / Spectrochimica Acta Part A 70 (2008) 997–1000
999
the electronic absorption spectra in the region 900–300 nm
of the reactants o-phenylenediamine (5 × 10−3 M) and TCNE
(1 × 10−4 M) and their blue short-lived product as well as
the spectra of the final pale brown product (1 × 10−6 M) in
dichloromethane. The short lived blue product has a strong
absorption around 735 nm while the final pale brown product
has much stronger absorption around 480 nm. Both the reactants
do not show any absorption in the region of study. Photomet-
ric titration measurements through the first hour of the reaction
were used to determine the charge-transfer reaction stoichiom-
etry for the short-lived blue reaction intermediate based on its
strong absorption at 735 nm. The o-phenylenediamine–TCNE
molar ratio was found to be 1:2 (Fig. 2) and the formed blue CT-
intermediate complex is formulated as [(o-phenylendiamine)2
(TCNE)4]. This blue intermediate is slowly converted to the
pale brown precipitate as the final product which is identi-
fied on the basis of its elemental analysis and infrared spectral
measurements to be 2,2-di-o-aminoaniline-1,1-dicyanoethylene
[DOAADCE] (Fig. 3), through that two o-phenylenediamine
molecules undergo N-substitution by TCNE forming the reac-
tion product [DOAADCE] and 2HCN molecules:
[(o-Phenylenediamine)2 (TCNE)]
→ (C6H4(NH2)2(NH))2(C2(CN)2) + 2HCN
Our conclusion of the N-substitution reaction of o-
phenylenediamine with TCNE is similar to that known [13] in
Table 1
Infrared wavenumbersa (cm−1
)
and tentative assignments for o-
phenylenediamine (o-phendn); TCNE and the reaction product 2,2-di-o
-aminoaniline-1,1-dicyanoethylene (DOAADCE)
(o-Phendn)
TCNE
(DOAADCE)
Assignmentsb
3396m
3364m
3280wm
(N H); free o-phendn
3167m
3122m
3084m
2964wm
2216vs
ν(N H); (DOAADCE)
ν(C H); aromatic
2261s
ν(C N), TCNE and
(DOAADCE) ring stretching
vibrations, o-phedn and
(DOAADCE)
2228m
2178s
1629m
1591m
1502s
1637vs
1599vs
1506w
1488w
δ(NH2), δ(NH) and δ(C H),
o-phendn and (DOAADCE)
1458m
1476m
1432s
1380m
1315wm
1268m
1231wm
1148vw
1273m
1241wm
o-Phendn and (DOAADCE)
1155vs
1115m
TCNE
o-Phendn
o-Phendn, TCNE and
(DOAADCE)
1151m
1138w
1109mw
1056m
1030m
1086
1020wm
958ms
934m
1021m
998m
933m
TCNE
o-Phendn, TCNE and
(DOAADCE)
928m
916m
804m
814wm
749m
824m
759m
736s
NH and NH2 deformations
696wm
578m
670m
604m
580wm
δ(CH), out of plane ring
bending, aromatic
555ms
556w
532wm
TCNE and (DOAADCE)
δ(CH), out of plane ring
bending aromatic
543wm
462w
438m
447wm
442m
a
m: medium; s: strong; v: very; w: weak.
ν: streching; δ: bending.
Fig. 6. Infrared absorption spectra of (A) o-phenylenediamine, (B) TCNE and
(C) the final product [DOAADCE].
b

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A.S. Aljaber, E.-M. Nour / Spectrochimica Acta Part A 70 (2008) 997–1000
the reaction of TCNE and the electron donor aniline where the
compoundC6H5NH(C2(CN)3) (Fig. 4), isformedasthe reaction
product according to the equation:
of the NH2 groups from each of the reacted two o-phendn in the
addition elimination reaction forming the 2HCN molecules and
2,2-di-o-aminoaniline-1,1-dicyanoethylene (DOAADCE).
C6H5NH2 + C2(CN)4 → C6H5NH(C2(CN)3) + HCN
4. Conclusion
Similar behavior occurs in the reaction of the donor
pyrrolidine and ␲-acceptor 7,7-8,8-tetracyanoquinodimethane
(TCNQ) to form the product 7,7-dipyrrolidino-8,8-dicyanoqui-
nodimethane and two molecules of HCN [14].
The electron donor o-phenylenediamine reacts with the
␲-acceptor tetracyanoethylene (TCNE) in CH2Cl2 at room
temperature to form the relatively short-lined blue charge-
transfer which undergoes elimination reaction forming the final
pale brown product 2,2-di-o-aminoaniline-1,1-dicyanoethylene
(DOAADCE).
Cyclic voltammetry method was used to prove the
charge-transfer interaction forming the blue CT-intermediate
[(o-phendn)2 (TCNE)4]. Fig. 5 shows the cyclic voltammogram
for the CT-complex recorded in potential ranged from −500 to
+500 mV versus silver-metal electrode. The E1/2 of the free o-
phenylendiamine equals to 165 mV shifted to higher value at
290 mV on complexation with TCNE. This shift indicating that
the donor o-phendn becomes more difficult to be oxidized and
the acceptor TCNE has acquired some electron charge from the
donor confirming the formation of the charge-transfer complex
between o-phendn and TCNE.
Infrared spectra for the reactants o-phendn, TCNC and the
reaction find product (DOAADCE) are shown in Fig. 6 and
their band assignments are given in Table 1. The spectrum of
the reaction product has most of the same features for both
reactants o-phendn and TCNE, but with some changes in band
wavenumber values and intensities. This is normal due to the
expected changes in molecular symmetry and electronic struc-
ture of the reactants upon complexation. Interesting, the ν(N H)
and ν(C N) vibrations of both the free o-phendn and TCNE
are observed at higher wavenumber values than those of the
product (DOAADCE). These changes support our conclusion
of the involvement of two of the CN groups of TCNE and one
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