Synthesis, spectroscopic and thermal structural investigations of the charge-transfer complexes formed in the reaction of 1-methylpiperidine with σ- and π-acceptors

Article abstract of DOI:10.1016/j.saa.2009.09.055

The reactions of the electron donor 1-methylpiperidine (1MP) with the π-acceptors 7,7,8,8-tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil = CHL) and iodine (I₂) were studied spectrophotometrically in chloroform at room temperature. The electronic and infrared spectra of the formed molecular charge-transfer (CT) complexes were recorded. The obtained results showed that the stoichiometries of the reactions are not fixed and depend on the nature of the acceptor. Based on the obtained data, the formed charge-transfer complexes were formulated as [(1MP)(TCNE)₂], [(1MP)(DDQ)]·H₂O, [(1MP)(CHL)] and [(1MP)I]I₃, while in the case of 1MP-TCNQ reaction, a short-lived CT complex is formed followed by rapid N-substitution by TCNQ forming the final reaction products 7,7,8-tricyano-8-piperidinylquinodimethane (TCPQDM). The five solids products were isolated and have been characterized by electronic spectra, infrared spectra, elemental analysis and thermal analysis.

Full text of DOI:10.1016/j.saa.2009.09.055

Spectrochimica Acta Part A 75 (2010) 134–141
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopic and thermal structural investigations of the
charge-transfer complexes formed in the reaction of 1-methylpiperidine
with ␴- and ␲-acceptors
Amina A. Fakhrooa,∗, Hassan S. Bazzib, Adel Mostafab, Lamis Shahadaa
a Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, P.O. Box 2713, Doha, Qatar
b Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2009
Received in revised form 7 September 2009
Accepted 25 September 2009
The reactions of the electron donor 1-methylpiperidine (1MP) with the ␲-acceptors
7,7,8,8-tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-
p-benzoquinone (DDQ), 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil = CHL) and iodine (I2) were
studied spectrophotometrically in chloroform at room temperature. The electronic and infrared spectra
of the formed molecular charge-transfer (CT) complexes were recorded. The obtained results showed
that the stoichiometries of the reactions are not fixed and depend on the nature of the acceptor. Based
on the obtained data, the formed charge-transfer complexes were formulated as [(1MP)(TCNE)2],
[(1MP)(DDQ)]·H2O, [(1MP)(CHL)] and [(1MP)I]I3, while in the case of 1MP–TCNQ reaction, a short-lived
CT complex is formed followed by rapid N-substitution by TCNQ forming the final reaction products
7,7,8-tricyano-8-piperidinylquinodimethane (TCPQDM). The five solids products were isolated and have
been characterized by electronic spectra, infrared spectra, elemental analysis and thermal analysis.
© 2009 Elsevier B.V. All rights reserved.
Keywords:
Charge-transfer
CHL
DDQ
Iodine (I2)
1-Methylpiperidine (1MP)
TCNQ
TCNE
1. Introduction
␴-acceptor iodine. All reactions were carried out in chloroform as a
solvent. The obtained results enable us to investigate the structure
The study of charge-transfer interactions between various elec-
tron donors contains donor atoms, such as nitrogen, oxygen or
sulfur and ␲- and ␴-acceptors has attracted increased inter-
est over the last years. This is due to the important role of
charge-transfer complexes played in biological systems and for
quantitative estimation of drugs besides that they act as inter-
mediates in a wide variety of reactions involving nucleophiles
and electron deficient molecules [1]. Some of the CT complexes
show interesting physical properties such as electrical conduc-
tivities [2–7]. It has been cleared that both polyazacyclic and
aza-aromatics compounds react with ␴- and ␲-acceptors to form
stable CT complexes. The several applications that CT complexes
can make includes electronics, solar cells, optical devices and others
[8–12]. In this paper, we report the formation of new CT complexes
formed on the reaction of 1-methylpiperidine (1MP) with different
types of ␲-electron acceptors. The ␲-acceptors used are 7,7,8,8-
tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE),
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 2,3,5,6-
tetrachloro-1,4-benzoquinone (chloranil = CHL) in addition to the
of these new complexes.
2. Experimental
2.1. Materials and measurements
All chemicals used in this study were of analytical reagent
grade and used without further purification. 1-Methylpiperidine
(1MP), TCNQ, TCNE, DDQ, chloranil and iodine were pur-
chased from Sigma–Aldrich, USA. The electronic absorption
spectra of the reactants, 1-methylpiperidine (1MP), 7,7,8,8-
tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE),
2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ),
2,3,5,6-
tetrachloro-1,4-benzoquinone (chloranil, CHL) and iodine (I2)
as well as the reaction products were recorded in the region
1100–280 nm using a lambda 950 Perkin Elmer UV-Vis spectrom-
∗
Corresponding author. Tel.: +974 4851633; fax: +974 4851639.
E-mail address: afakhro2@qu.edu.qa (A.A. Fakhroo).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.09.055

A.A. Fakhroo et al. / Spectrochimica Acta Part A 75 (2010) 134–141
135
Table 1
Spectroscopic data for the formed 1-methylpiperidine (1MP) acceptor complexes.
Complex
Color
Absorption (nm)
Stoichiometry
(donor:acceptor)
[TCPQDM]
[(1MP)(TCNE)2]
Dark green
Dark brown
829, 744, 640, 464, 570 1:2
419, 400
1:2
1:1
1:1
1:2
[(1MP)(DDQ)]·H2O Dark brown
716, 569, 463
754, 545
[(1MP)(CHL)]
[(1MP)I]I3
Dark brown
Light yellow 388, 273
eter with a quartz cell of 1.0 cm path length and elemental analysis
was done using model 2400 Perkin Elmer C,H,N,S/O elemental
analyzer.
Photometric titration measurements were performed for the
reactions between the donor 1-methylpiperidine (1MP) and each
of the acceptors TCNQ, TCNE, DDQ, chloranil and iodine in CHCl3
at 25 ◦C in order to get the reaction stoichiometries according to
the literature method [13–15]. The measurements were performed
under the conditions of fixed donor 1MP concentrations while
those of the acceptors TCNQ, TCNE, DDQ, chloranil and iodine were
changed over a wide range, to produce in each case solutions where
the molar ratio of donor:acceptor varies from 1:0.25 to 1:4. The
peak absorbencies of the formed CT complexes were measured for
all solutions in each case and plotted as a function of the acceptor
to donor molar ratio. The infrared spectra of the reactants and the
formed CT complexes (KBr pellets) were recorded on a Spectrum
one Perkin Elmer FTIR spectrophotometer.
Fig. 1. Electronic absorption spectra of 1-methylpiperidine (1MP)–CNQ reaction in
CHCl3. (A) [1MP] = 5 × 10−3 M; (B) [TCNQ] = 3 × 10−3 M; (C) 1:2 1MP–CNQ mixture
[1MP] = 5 × 10−3 M and [TCNQ] = 3 × 10−3 M.
quickly forming a stable green product. Fig. 1 shows the electronic
spectrum recorded at region 440–900 nm of the reaction reactants
1MP and of 7,7,8,8-tetracyanoquinodimethane (TCNQ) along that
of the final dark green product. Interestingly, both reactants do not
show any absorption in the region of study while the reaction prod-
uct has a number of five absorptions at 829 nm, 744 nm, 640 nm,
570 nm, and 464 nm. The formation of the short-lived yellow prod-
uct which gives the stable green reaction product is understood on
the basis that 1MP acts as an electron donor reacts with ␲-acceptor
TCNQ forming 1MP–TCNQ CT complex. This short-lived product
(yellow in color) undergoes elimination reaction forming the green
dark final product which is identified by the thermal measurements
to be 7,7,8-tricyano-8-piperidinylquinodimethane (TCPQDM), for-
mula I. A general mechanism for the reaction is proposed as follows:
2.2. Preparation of the solid CT complexes
The five solid CT complexes formed in the reaction of 1-
methylpiperidine (1MP) with each of TCNQ, TCNE, DDQ, chloranil
and iodine were prepared by the addition of a saturated solution
of the donor 1-methylpiperidine (60 ml) to a saturated solution of
each acceptor TCNQ, TCNE, DDQ, chloranil and iodine (80 ml) in
CHCl3.
The formed precipitate in each case was filtered off, washed
with minimum amounts of CHCl3 and dried in vacuum over
P2O5. The complexes were characterized spectrophotometrically
(FTIR and UV–vis) and elemental analysis (theoretical values are
shown in brackets): 7,7,8-tricyano-8-piperidinylquinodimethane
(C16H14N4) dark green complex (MW: 262 g): C, 72.88% (73.28
%); H, 5.21 % (5.34 %); N, 21.81 % (21.37 %); [(1MP)(TCNE)2] dark
brown complex (MW: 355.37 g): C, 60.80% (60.78 %); H, 3.70 %
(3.66 %); N, 35.41 % (35.46 %); [(1MP)(DDQ)]·H2O dark brown com-
plex (MW: 344.19 g): C, 48.80 % (48.81 %); H, 4.40 % (4.36 %); N,
12.27 % (12.20%); [(1MP)(chloranil)] dark brown complex (MW:
345.07 g): C, 41.74 % (41.73 %); H, 3.82 % (3.77 %); N, 4.10 % (4.06
%) and [(1MP)I]I3 light yellow complex (MW: 606.81 g): C, 11.91%
(11.86%); H, 2.19% (2.14%); N, 2.35% (2.31%).
(i) Formation of CT complex:
TCNQ + 1MP → [(1MP)(TCNQ)] (yellow, short-lived)
(ii) N-substitution by TCNQ:
[(1MP)(TCNQ)] → 7, 7,
8-tricyano-8-piperidinylquinodimethane(TCPQDM)
+ CH3CN (green, stable)
3. Results and discussion
3.1. Electronic spectra
The electronic absorption bands of the CHCl3 solution
of the formed solid complexes [TCPQDM], [(1MP)(TCNE)2],
[(1MP)(DDQ)]·H2O, [(1MP)(CHL)] and [(1MP)I]I3 are given in
Table 1. The elemental analysis of these solid products strongly sup-
ports the proposed complex structures and in good agreement with
stoichometrics reaction based on photometric titration method as
indicated below.
3.1.1. Reaction of 1MP with TCNQ
Upon the addition of the 1-methylpiperdine (1MP) to a solu-
tion of TCNQ in CHCl3, a yellow color is formed which changes very
Our results agree quite well with that reported by Frey et al.
[16] on their reaction between the ␲-acceptors tetracyanoethylene
and electron donor like aniline where the later undergoes rapid

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Fig. 4. Electronic absorption spectra of 1-methylpiperidine (1MP)–DDQ reaction
in CHCl3. (A) [1MP] = 5 × 10−3; (B) [DDQ] = 5 × 10−3 M; (C): 1:1 1MP–DDQ mixture,
[1MP] = 5 × 10−3 M and [DDQ] = 5 × 10−3 M.
Fig. 2. Electronic absorption spectra of 1-methylpiperidine–TCNE reaction in
CHCl3. (A) [1MP] = 5 × 10−3; (B) [TCNE] = 1 × 10−3 M; (C) 1:2 1MP–CNE mixture,
[1MP] = 5 × 10−3 M and [TCNE] = 1 × 10−3 M.
to note that the electronic spectra, in rang between 400 nm and
900 nm for 1MP–TCNQ and 300–500 nm for 1MP–TCNE complexes
(Figs. 1 and 2) present lower energy absorption, which are assigned
as charge-transfer (CT) transitions. The maximum of this transition
were observed at 464 nm for 1MP–TCNQ complex and at 419 nm
and 400 nm for 1MP–TCNE complex in CHCl3 medium. Also, the
electronic spectrum of the 1MP–TCNQ complex exhibits typical
long-wavelength absorptions of the TCNQ radical anion at 829 nm,
744 nm, 640 nm, and 570 nm. The data, all together, suggest that
the TCNE species presents a higher charge uptake capability than
TCNQ.
N-substitution by TCNE:
C6H5NH2 + C2(CN)4 → C6H5NH(C2(CN)3) + HCN
And with the work of Aljaber and Nour [17] on the reaction of TCNE
and O-phenylenediamine,
C6H5NH2 + C2(CN)4 → C6H5NH(C2(CN)3) + HCN
3.1.2. Reaction of 1MP with TCNE
Fig. 2 shows the electronic absorption spectrum of the reactions
of tetracyanoethylene (TCNE) with the donor 1MP, while none of
the reactants’ spectra display any measurable absorption in the
region of measurement, 500–300 nm. The resulting CT complex
shows absorption centered on 400 nm and 419 nm for 1MP–TCNE
reaction. This absorption is associated with the strong change in
color observed upon mixing of reactants (brown from colorless
solutions), and reflect the electronic transitions in the formed CT
complexes.
Photometric titration measurements based on 400 nm and
419 nm absorptions bands were performed in order to determine
the reactions stoichiometries in CHCl3 (Fig. 3). Two straight lines
have been obtained. The elemental analysis of the obtained solid
CT complex has clarified the stoichiometric ratio of 1MP/TCNE to
be 1:2. On the basis of these experimental data, the CT complex
obtained can be formulated as [(1MP)(TCNE)2]. It is of interest
3.1.3. Reaction of 1MP with DDQ
Fig. 4 shows the electronic spectrum recorded in the region
400–1100 nm of the reaction of 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) with 1MP. Similar to the reactions with the
previous two acceptors, strong changes in color are observed upon
mixing. Dark brown color indicates the formation of the 1MP–DDQ
charge-transfer complex and is associated with the electronic
transitions at 716 nm, 569 nm and 463 nm. Photometric titration
measurements were performed for the 1MP–DDQ reaction in CHCl3
as shown in Fig. 5.
Photometric titration based on the absorption at 716 nm,
569 nm and 463 nm shows
a reaction stoichiometry of 1:1
(1MP:DDQ) and indicate that this absorption belong to the CT com-
plex formulated as [(1MP)(DDQ)]. Elemental analysis showed that
Fig. 3. Photometric titration curve for 1-methylpiperidine (1MP)—TCNE reaction in
Fig. 5. Photometric titration curve for 1-methylpiperidine (1MP)–DDQ reaction in
CHCl3 based on the 400 nm and 419 nm absorptions.
CHCl3 based on the 463 nm, 569 nm and 716 nm absorptions.

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137
Fig. 8. Electronic absorption spectra of 1-methylpiperidine (1MP)–iodine in CHCl3.
(A) [1MP] = 5 × 10−3 M; (B) [iodine] = 1 × 10−3 M and (C) 1:2 1MP–iodine mixture,
[1MP] = 5 × 10−3 M and [iodine] = 1 × 10−3 M. The band at 273 nm of diluted solution
of 1MP/iodine CT complex (inset).
Fig. 6. Electronic absorption spectra of 1-methylpiperidine (1MP)–chloranil
reaction in CHCl3. (A) [1MP] = 5 × 10−3
;
(B) [chloranil] = 5 × 10−3 M; (C) 1:1
1MP–chloranil mixture, [1MP] = 5 × 10−3M and [chloranil] = 5 × 10−3 M.
the 1MP–DDQ CT complex can be formulated as [(1MP)(DDQ)]·H2O.
The DDQ CT complex has lowering molar ration than TCNQ and
TCNE. In addition, it has different band structure and wavelength
value than TCNQ and TCNE CT complexes. In case of 1MP–DDQ CT
complex also exhibited two well-defined CT-absorption at 716 nm
and 463 nm and one weak at 569 nm. While TCNE has intense and
multiple absorption at 419 nm and 400 nm and TCNQ has only one
intense band at 464 nm and four weak bands at 829 nm, 744 nm,
640 nm, 570 nm, respecyively. This discrepancy between the three
CT complexes due to difference of the electronic structure and could
be associated with the molecular steric hindrance.
TCNE and DDQ, the electronic spectra of 1MP–chloranil CT complex
have two weak absorptions at 754 nm and 545 nm. The two CT-
absorption are shown to belong to one species, both have the same
photometric titration behavior. The variation of the CT-absorption
of TCNQ, TCNE, DDQ, and CHL should be related to the electron
affinity of each acceptor with the 1MP donor that is controlled by
many factors, such as the molecular geometry, steric hindrance as
well as the type of electron withdrawing groups attached to the
acceptor.
3.1.5. Reactions of 1MP with iodine
Fig. 8 shows the electronic spectrum recorded in the region
250–600 nm of the reaction of iodine with 1MP. Similar to the
reactions with the previous acceptors, strong changes in color
are observed upon mixing of CHCl3 solutions of iodine with the
donor 1MP. Yellow color indicates the formation of the 1MP–iodine
charge-transfer complex and is associated with the electronic tran-
sitions at 388 nm and 273 nm. Photometric titration measurements
were performed for the 1MP–iodine reaction in CHCl3 as shown in
Fig. 9.
Photometric titration based on the absorption at 388 nm shows
a reaction stoichiometry of 1:2 (1MP:iodine). This reaction stoi-
chiometry was supported by the obtained elemental analysis of
the formed solid products. It is of interest to see that the formed
iodine complex 1MP–iodine has strong absorptions at 388 nm and
3.1.4. Reactions of 1MP with chloranil (CHL)
Fig.
6 shows the electronic spectrum recorded in the
region 400–1100 nm of the reaction of 2,3,5,6-tetrachloro-1,4-
benzoquinone (chloranil, CHL) with 1MP. Similar to the reactions
with the previous acceptors; strong changes in color are observed
upon mixing. Dark brown color indicates the formation of the
1MP–CHL charge-transfer complex and is associated with the elec-
tronic transitions at 754 nm and 545 nm. Photometric titration
based on the absorption at 754 nm (Fig. 7) shows a reaction sto-
ichiometry of 1:1 (1MP:CHL) and indicates that these absorptions
belong to the CT complex formulated as [(1MP)(CHL)]. Elemental
analysis showed that the 1MP–CHL complex can be formulated
as [(1MP)(CHL)]. As mentioned above with the acceptors TCNQ,
Fig. 7. Photometric titration curve for 1-methylpiperidine (1MP)–chloranil reaction
Fig. 9. Photometric titration curve for 1-methylpiperidine (1MP)–iodine reaction in
in CHCl3 based on the 754 nm absorption.
CHCl3 based on 388 nm absorption.

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Fig. 10. The plot of (CA0 × CD0 /A) values against (CA0 + CD0
) values for the
[(1MP)(DDQ)]·H2O reaction in CHCl3 at 716 nm, 569 nm and 463 nm.
Fig. 11. The plot of (CA0 × CD0 /A) values against (CA0 + CD0 ) values for the [(1MP)(CHL)]
reaction in CHCl3 at 754 nm.
273 nm. These two absorptions are well known to be character-
−
in conjugation with an aromatic ring which causes high delocaliza-
tion leads to a great increase in the Lewis acidity of the acceptor
DDQ, and hence the higher value of K for [(1MP)(DDQ)] complex
compared with that of chloranil.
istic of the formation of the polyiodide ion I3 [18,19]. Based on
this, the structure of the formed 1MP–iodine CT complex should b−e
[(1MP)I+]I3−. The formation of the triiodide complex [(1MP)I]+I3
could be understood on the basis of the formation of the CT–iodide
intermediate Eqs. (1) and (2) followed by the formation of triio-
dide as the final product Eq. (3). The general mechanism for the
formation of the complex as follows:
An unsuccessful attempt was made to calculate the correspond-
´
ing values of Kc and εc for both the 1:2 complexes [(1MP)(TCNQ)2],
[(1MP)(TCNE)2] and [(1MP)I]+I3−. The 1:2 stoichiometric ratios
have many complicated terms to deal with and we might in the
future overcome this problem and publish these results.
1MP + I2 = (1MP)I2
(1)
(2)
(3)
Followed by the formation of the inner complex
(1MP)I2 = [(1MP)I] + I−
3.3. Infrared spectra
The infrared absorption spectra of 1MP and the complexes
[(1MP)(TCNE)2], [(1MP)(DDQ)]·H2O, [(1MP)(CHL)] and [(1MP)I]I3
as well as 7,7,8-tricyano-8-piperidylquinodimethane (TCPQDM)
are shown in Fig. 12. The infrared band assignments are given in
−
which combines with one iodine molecule to form I3 complex
[(1MP)I]+I− + I2 = [(1MP)I] + I−
3
The formation of the iodide intermediate [(1MP)I]+I− was detected
by the observation of the characteristic absorption of the iodide
ion, I−, at 245 nm in experiments with low concentration of iodine
[21]. This absorption of I− almost disappears upon the use of high
concentration of iodine due to the formation of the triiodide ion.
3.2. Formation constants of CT complexes
The spectrophotometric data were used to calculate the val-
ues of both the formation constants, K (L mol−1), and molar
extinction coefficient, ε (L mol−1 cm−1), of the CT complexes in
CHCl3 [(1MP)(DDQ)], [(1MP)(chloranil)] based on the 1:1 modified
Benesi–Hildebrand equation [20].
CA0CD0l
A
CA0 + CD0
1
Kε
=
+
ε
where CA0 and CD0 are the initial concentrations of the accep-
tors and donor, respectively, while A is the absorbance at the
mentioned CT bands. Plotting the values of (CA0CD0 l/A) against
(CA0+CD0) values of this equation, straight lines were obtained with
slop of 1/ε and intercept of 1/Kε as shown in Figs. 10 and 11.
The obtained values of Kc and εc are 0.565 × 103 L mol−1
and
1.13 × 102 L mol−1 cm−1
for
[(1MP)(DDQ)]·H2O
and
for
0.224 × 103 L mol−1
and
0.127 × 102 Lmol−1 cm−1
[(1MP)(chloranil)].
These results reveal that the KCT values of CT complexes with
DDQ are higher than the corresponding values with chloranil. Such
a variation could be associated with the molecular steric hindrance
of the acceptors, which expected to be higher in the case of DDQ. The
electronic structure of the acceptor chloranil is different from the
DDQ. The DDQ acceptor has two strong withdrawing cyano groups
Fig. 12. Infrared absorption spectra of (A) [1MP]; (B) [(1MP)(TCNQ)2]; (C)
−
[(1MP)(TCNE)2]; (D)[(1MP)(DDQ)]·H2O; (E) [(1MP)(chloranil)]; and (F) [(1MP)I+]I3
.

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139
Table 2. These assignments are based on the comparison of the
spectra of the formed products with the spectra of the free reac-
tants, the donor 1MP and the acceptors TCNQ, TCNE, DDQ, CHL
and iodine. Interestingly, the spectra of the reaction products con-
tain the main infrared bands for both the reactants in each case.
This strongly supports the formation of the CT complexes. How-
ever, the absorptions of 1MP and acceptors in the formed products
show some changes in band intensities and in some cases small
shifts in the frequency wavenumber values. These changes could
be attributed to the expected symmetry and electronic structure
modifications in both donor and acceptor units in the formed prod-
ucts relative to the free molecules. The infrared interpretation for
the CT complexes was dealt with as follows:
The ꢀ(C N) vibrations of the acceptors TCNQ, TCNE, and DDQ
show some changes particularly in terms of band wavenumber val-
ues upon complexation. The CN stretching of TCNQ show a drastic
shift to a lower wavenumber. The distinguish stretching vibra-
tion, ꢀ(C N), of free TCNQ is observed at 2222 cm−1 as single peak,
in the 7,7,8-tricyano-8-piperidylquinodimethane (TCPQDM) com-
plex, two peaks at 2194 cm−1 and 2159 cm−1 value were detected.
The ꢀ(C N) vibrations for free TCNE are observed a doublet at
2196 cm−1 and 2182 cm−1 and for free DDQ at 2203 cm−1. These
vibrations occur at 2200 cm−1 for [(1MP)(TCNE)2] and at 2215 cm−1
for [(1MP)(DDQ)]·H2O. This shift is indicative of higher charge den-
sity on the cyano group of the CT complexes.
The ꢀ(C–N) vibration occurs as a strong and medium absorp-
tion at 1278 cm−1 and 1163 cm−1 in the spectrum of free 1MP,
which are not in the spectra of the free acceptors (TCNQ, TCNE,
DDQ). Nevertheless, upon complexation reaction, the ꢀ(C–N) vibra-
tion appearance as strong at 1298 cm−1and weak at 1185 cm−1
for 7,7,8-tricyano-8-piperidinylquinodimethane (TCPQDM). For
[(1MP)(DDQ)], the ꢀ(C–N) vibration occurs as strong medium
absorption at 1230 cm−1 and 1198 cm−1. For [(1MP)(TCNE)2], the
ꢀ(C–N) vibration band appears as weak absorption at 1273 cm−1
and 1179 cm−1. For [(1MP)(CHL)], the ꢀ(C–N) vibration band
appears as weak and strong at 1276 cm−1 and 1190 cm−1 respec-
tively and as strong medium at 1287 cm−1 and 1272 cm−1 for
[(1MP)I]I3 complex. The presence of ꢀ(C–N) bands in the spectra
of CT complexes strongly supports the charge-transfer interaction
process between the donor 1MP and the five acceptors. Interest-
ingly, ꢀ(C–H) vibration of the 1MP/TCNQ complex occurs as weak
absorption at 2929 cm−1 and very weak absorption at 3049 cm−1
and 2851 cm−1 while on free 1MP occurs as strong absorption at
2935 cm−1 and strong medium absorption at 2853 cm−1
.
3.4. Thermal analysis measurements
Thermogravimetric (TG) and differential thermogravimetric
(DTG) analyses were carried out for the five CT complexes under
a N2 flow in order to confirm their formulae and structures.
The thermograms for 7,7,8-tricyano-8-piperidylquinodimethane
(TCPQDM), [(1MP)(TCNE)2], [(1MP)(DDQ) and [(1MP)(CHL)] com-
plexes are shown in Fig. 13 (A–D) respectively. The full
thermogravimetric data for all complexes are given in Table 3. The
data clearly support the proposed molecular structures of the com-
plexes. However, it is difficult to propose a definite decomposition
reaction in each case.
It was interesting to see that the thermal decomposition
of these complexes exhibits different degradation steps. The
thermograms for the 7,7,8-tricyano-8-piperidylquinodimethane
(TCPQDM), complex (Fig. 13A), shows three main stages of
decomposition occur at 247 ◦C, 335 ◦C and 635 ◦C. At 247 ◦C is
accompanied by a weight loss of 21% of the C4H8 molecule in full
agreement with the calculated value of 21%. At 335 ◦C is accompa-
nied by a weight loss of 10% of the CN molecule in full agreement
with the calculated value of 9.9%. At 635 ◦C is the decomposition

140
A.A. Fakhroo et al. / Spectrochimica Acta Part A 75 (2010) 134–141
Fig. 13. Thermal diagrams for (A) [(1MP)(TCNQ)2, (B) [(1MP)(TCNE)2, (C) [(1MP)(DDQ) and (D) [(1MP)(chloranil).
of the 7,7,8-tricynoquinodimethane (the remaining molecule) and
accompanied by weight loss of 66% in full agreement with the cal-
culated value of 66.1%.
The [(1MP)(TCNE)2] complex decomposes at three tempera-
tures (Fig. 13B). At 225 ◦C and 320 ◦C correspond to 26.3% weight
loss of the 1MP donor and the 741 ◦C correspond to 69.9% weight
loss of the two molecules of TCNE. Both donor and acceptor
have weight loss in agreement with the calculated value see the
table.
The third complex [(1MP)(DDQ)] has four decomposition steps
(Fig. 13C). The first step occurs at 130 ◦C which corresponds to 4.61%
loss of water (confirmed by elemental analysis). The second and
third steps occur at 240 ◦C and 300 ◦C which correspond to 28.8%
weight loss of the 1MP donor, while the fourth step occurs at 673 ◦C
which corresponds to 69.19% weight loss of the acceptor DDQ. It
was found that there is around 3% deviation between the calculated
and the found value of DDQ.
The thermogram of [(1MP)(CHL)] complex has shown five
decomposition steps at 52 ◦C, 199 ◦C, 229 ◦C, 277 ◦C, and 544 ◦C
(Fig. 13D). The first step at 52 ◦C is related to small amount of
volatiles compound (e.g. CHCl3) with weight loss of 1.5%. The sec-
ond and third step is the decomposition of the donor 1MP which
is decomposed at 199 ◦C, 229 ◦C with weight loss 7.5% and 14.5%
respectively. It was to notice that the found value is 22% weight
loss and calculated value is 28.74% of the donor (see the table).
The thermal decomposition of the acceptor chloranil has started
at the fourth decomposition steps at 277 ◦C and has been com-
pletely decomposed at 544 ◦C with weight loss 19.5% and 54.5%
respectively. Interestingly, it was found that the fourth decompo-
sition step is related to the release of two chlorine atoms (2 Cl2)

A.A. Fakhroo et al. / Spectrochimica Acta Part A 75 (2010) 134–141
141
Table 3
Thermogravimetric data for the formed CT complexes.
Complex
DTG max.(◦C)
TG %mass loss found/calc.
Lost species
[TCPQDM]
247
335
635
21.0/21
10/9.9
66/66.1
C4H8
CN
7,7,8-tricynoquinodimethan
[(1MP)(TCNE)2]
225, 320
741
26.3/27.9
69.9/72.1
1MP
TCNE (two molecules)
[(1MP)(DDQ)]·H2O
130
240, 300
673
4.61/5.23
26.2/27.9
69.19/65.95
1.5
21/28.74
H2O (one molecule)
1MP
DDQ
CHCl3
1MP
[(1MP)(chloranil)]
[(1MP)I]I3
52
199, 229
277, 544
240
74/71.26
Chloranil
Decomposition with one
maxima for a complete loss of
the whole compound
which is equivalent to the 20.58% from the whole molecule of CT
complex and the value which has been found was 19.5%. There-
fore, the weight loss mass% of the chloranil is 74% and the found
is 71.26%. The thermogramic analysis of [(1MP)I]I3 complex show
one maxima decomposed at 240 ◦C with complete loss of the whole
compound.
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