Synthesis, spectroscopic and thermal studies on solid charge-transfer molecular complexes formed in the reaction of 1-(2-aminoethyl)piperidine with π- And σ-acceptors

Article abstract of DOI:10.1016/j.molstruc.2012.06.049

The solid charge-transfer molecular complexes formed in the reaction of the electron donor 1-(2-aminoethyl) piperidine (AEP) with the σ-acceptor iodine and π-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), 7,7,8,8-tetracyanoquinodimethane (TCNQ

Full text of DOI:10.1016/j.molstruc.2012.06.049

Journal of Molecular Structure 1029 (2012) 45–52
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectroscopic and thermal studies on solid charge-transfer molecular
complexes formed in the reaction of 1-(2-aminoethyl)piperidine
with p- and r-acceptors
⇑
Adel Mostafa, G. Benjamin Cieslinski, Hassan S. Bazzi
Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar
h i g h l i g h t s
"
"
"
We studied the C-T reactions of the donor 1-(2-aminoethyl)piperidine with
We analyzed the obtained CT-complexes via FT-IR, UV-Vis, elemental and thermal analysis.
We calculated KCT, CT, G, ECT and Ip calculated
p- and r-electron acceptors.
e
D
a r t i c l e i n f o
a b s t r a c t
Article history:
The solid charge-transfer molecular complexes formed in the reaction of the electron donor 1-(2-amino-
ethyl) piperidine (AEP) with the -acceptor iodine and -acceptors 2,3-dichloro-5,6-dicyano-1,4-benzo-
Received 10 April 2012
Received in revised form 31 May 2012
Accepted 25 June 2012
r
p
quinone (DDQ), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,4,4,6-tetrabromo-2,5-cyclohexadienone
(TBCHD)were studied in chloroform at 25 0C. These were investigated through electronic spectra, infrared
spectra, thermal and elemental analysis. The obtained results showed that the formed solid CT-complexes
Available online 2 July 2012
have the formulas ½ðAEPÞIꢀþIꢁ, [(AEP)(DDQ)2], [(AEP)(TCNQ)2] and [(AEP)(TBCHD)] in full agreement with
3
Keywords:
1-(2-Aminoethyl)piperidine
Iodine
DDQ
TCNQ
TBCHD
CT-complexes
the known reaction stoichiometries in solution as well as the elemental measurements. The formation
constant KCT (L molꢁ1), the molar extinction coefficient eCT (L molꢁ1 cmꢁ1), the free energy change
DG°
(cal molꢁ1), the charge transfer energy ECT, and the ionization potential Ip were calculated for the CT-com-
plexes ½ðAEPÞIꢀþIꢁ, [(AEP)(DDQ)2], [(AEP)(TCNQ)2] and [(AEP)(TBCHD)].
3
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
The formation of a particular polyiodide species depends
strongly on the nature of the donor base and in some cases on
The study of charge-transfer complexes formed in the reactions
- and -electron acceptors with organic electron donors has
considerable interest and a growing importance owing to their sig-
nificant physical and chemical properties such as electrical con-
ductivity. The chemical and physical properties of charge-transfer
the method of preparation [11–13]. The electron acceptors
7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3-dichloro-5,
6-dicyano-1,4-benzoquinone (DDQ) are known to form stable, col-
ored CT-complexes with many donor bases. However, the reaction
stoichiometries depend on many factors such as the nature of
donor and acceptor, and in some cases, on the solvent used. The in-
creased interest in the study of charge-transfer interactions stems
from the various applications of CT-complexes, including electron-
ics, solar cells, and optical devices [14]. These interactions play
very important roles in biological systems [15]. Also, CT-complexes
act as intermediates in a wide variety of reactions involving nucle-
ophiles and electron deficient molecules.
of
r
p
(CT) complexes formed in the reactions of p- and r-electron accep-
tors with different donors like crown ethers amines, polysulfur
bases and oxygen-nitrogen mixed bases have been the subjects
of many studies both in solution and in solid state chemistry [1–
10]. The electron donor used in this study, 1-(2-aminoethyl)piper-
idine (AEP), is a derivative of piperidine and contains both primary
and tertiary nitrogen atoms (electron donating atom). It is used as
a reactant for synthesis of analogs of anticancer agents.
This paper reports the formation of four new CT-complexes
obtained from the reaction of 1-(2-aminoethyl)piperidine (AEP)
with the
(DDQ), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,4,4,6-
tetrabromo-2,5-cyclohexadienone (TBCHD), and -acceptor iodine
p-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
⇑
Corresponding author. Tel.: +974 4423 0018; fax: +974 4423 0060.
E-mail address: bazzi@tamu.edu (H.S. Bazzi).
r
0022-2860/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.06.049

46
A. Mostafa et al. / Journal of Molecular Structure 1029 (2012) 45–52
(I2) in CHCl3. The aim of this study is to characterize the reaction
products and make assessments of the correct nature and stoichi-
ometry of the formed CT-complexes.
while those of the acceptors iodine, DDQ, TCNQ, and TBCHD were
changed over a wide range to produce reaction solutions where the
molar ratio of donor to acceptor varied from 1:0.25 to 1:4.
The peak absorbencies of the formed CT complexes were mea-
sured for all solutions and plotted as a function of the acceptor
to donor molar ratio.
The infrared spectra of the reactants and the formed CT com-
plexes (KBr pellets) were recorded on a Spectrum One Perkin-
Elmer FT-IR Spectrophotometer.
2.2. Preparation of the solid CT-complexes
The four solid CT-complexes formed in the reaction of AEP with
iodine, DDQ, TCNQ and TBCHD were prepared in CHCl3 by the
drop-wise addition of a saturated solution (65 ml) of each of the
donors to a saturated solution (80 ml) of each of the acceptors. In
each case the mixing of reactants was associated with a strong
change in color. The resulting precipitate in each case was filtered
off, washed with minimum amounts of CHCl3 and dried in vacuum
over P2O5. The complexes were characterized using spectroscopic
techniques (FTIR and UV-vis) and by elemental analysis shown in
Table 1.
3. Results and discussion
3.1. Electronic absorption spectra
Upon mixing of CHCl3 solutions of iodine with the donor 1-(2-
aminoethyl)piperidine (AEP), a strong change in color was ob-
served as the solution turned into an intense dark brown color.
This color, not associated with either of the reactants, indicates
the existence of charge-transfer interactions between iodine and
the donor. The electronic absorption spectra of the reactants along
with the CT-complex formed with iodine/AEP is shown in Fig. 1.
Strong absorption bands, due to the formed product, appeared
at 364 and 294 nm for AEP-I2. Photometric titrations for the reac-
tion in CHCl3 were performed.
2. Experimental
The molar ratio was found to be 1:2, as shown in Fig. 2. This
reaction stoichiometry was supported by the obtained elemental
analysis of the formed solid products. However, the two absorption
bands of the formed iodine complex around 364 and 294 nm in
Fig. 1, are known to be characteristics of the formation of the poly-
iodide ion, Iꢁ3 [17]. Based on this, þthe structure of the formed CT-
complex was projected as ½ðAEPÞIꢀ Iꢁ3 .
2.1. Materials and measurements
The chemicals used in this study were purchased from Sigma-
Aldrich, USA, and used without further purification. The electronic
absorption spectra of the CHCl3 solutions of the solid CT-complexes
formed in the reactions of the donor1-(2-aminoethyl)piperidine
(AEP) and the acceptors iodine, 2,3-dichloro-5,6-dicyano-1,
4-benzoquinone (DDQ),7,7,8,8-tetracyanoquinodimethane (TCNQ)
and 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) as well as
the reaction products were checked in the region 1000–200 nm
using a Lambda 950 Perkin Elmer UV–Vis Spectrometer with
quartz cell of 1.0 cm path length.
Elemental analysis was done using a Perkin Elmer CHNSO ele-
mental analyzer model 2400 Series II. The infrared spectra of the
reactants, AEP, DDQ, TCNQ and TBCHD (iodine has no infrared
activity) and the obtained CT-complexes (KBr pellets) were re-
corded on a Perkin Elmer FT-IR Spectrometer Model Spectrum One.
Thermogravimetric (TG) and differential thermogravimetric
(DTG) analysis were carried out for all reactants and CT-complexes
using a Perkin Elmer Model Pyris 6 TGA computerized thermal
analysis system. The rate of heating of the sample was kept at
10 °C minꢁ1 under nitrogen flow at 20 ml minꢁ1. Copper sulfate
pentahydrate was used as the calibration standard.
The formation of the [(AEP)I]+ ions is analogous to those previ-
ously reported in studies on related systems [18,19]. For example,
hexamethylenetetramine (HMTA) and 1,4,7,10,13,16-hexathiacyc-
looctadecane (HTCOD), when reacted to iodine in CHCl3 form
[(HMTA)2I]+I3ꢁ and [(HTCOD)2]+Iꢁ5 . The formation of the triiodide
complex ½ðAEPÞIꢀþI3ꢁ is explained by the formation of the CT-iodide
intermediate [Eq. (a)] followed by the formation of triiodide as the
final product (Eq. (b)).
The formation of iodide intermediate [(AEP)I]+Iꢁ is character-
ized by its absorption at 245 nm [20], particularly in experiments
with a low iodine to donor ratio.
ðAEPÞ þ I2 ! ½ðAEPÞꢀI2
ðaÞ
followed by the formation of the inner complex;
½ðAEPÞꢀI2 ! ½ðAEPÞIꢀþIꢁ
which combines with one iodine molecule to form Iꢁ3 complex
Photometric titration measurements were performed for the
reactions between the donor AEP and each of the acceptors in
CHCl3 at 25 °C in order to determine the reaction stoichiometries
according to a literature method [4,16]. The measurements were
conducted under the conditions of fixed donor AEP concentrations
½ðAEPÞIꢀþIꢁ þ I2 ! ½ðAEPÞIꢀþIꢁ
ðbÞ
3
Fig. 3 shows the electronic spectrum recorded in the region 400–
1000 nm of the reaction of DDQ with AEP. Similar to the reaction

A. Mostafa et al. / Journal of Molecular Structure 1029 (2012) 45–52
47
Table 1
Elemental analysis values for the solid CT-complexes of the donor AEP with the acceptors iodine, DDQ, TCNQ and TBCHD with theoretical values in parentheses.
Name
MW (g/mol)
635.84
Color
C% (theo%)
H% (theo%)
2.47 (2.52)
N% (theo%)
4.42 (4.40)
I% (theo%)
½ðAEPÞIꢀþIꢁ3
Dark brown
13.18 (13.21)
79.79 (79.84)
[(AEP)(DDQ)2]
[(AEP)(TCNQ)2]
[(AEP)(TBCHD)]
582.24
536.6
537.92
Dark red
Brown
Dark brown
47.46 (47.40)
69.37 (69.33)
29.04 (29.0)
2.78 (2.75)
4.49 (4.47)
3.38 (3.35)
14.39 (14.43)
26.12 (26.09)
5.23 (5.21)
na
na
na
Fig. 1. Electronic absorption spectra of 1-(2-aminoethyl) piperidine (AEP)-I2
Fig. 3. Electronic absorption spectra of 1-(2-aminoethyl)piperidine (AEP)-DDQ
reaction in CHCl3. (A): [AEP] = 5 ꢂ 10ꢁ3; (B): [DDQ] = 5 ꢂ 10ꢁ3M; (C): 1:2 AEP-DDQ
mixture, [AEP] = [DDQ] = 5 ꢂ 10ꢁ3 M.
reaction in CHCl3. (A): [AEP] = 5 ꢂ 10ꢁ3
;
(B): [I2] = 1 ꢂ 10ꢁ3 M; (C): 1:2 AEP-I2
mixture, [AEP] = 5 ꢂ 10ꢁ3 M and [I2] = 1 ꢂ 10ꢁ3 M.
Fig. 4. Photometric titration curve for 1-(2-aminoethyl)piperidine (AEP)-DDQ
reaction in CHCl3 measured at 480 nm and 745 nm.
Fig. 2. Photometric titration curve for 1-(2-aminoethyl)piperidine (AEP)-iodine
reaction in CHCl3 measured at 364 nm.
with the iodine acceptor, a strong change in color was observed
upon mixing.
A dark red1 color indicates the formation of the AEP-DDQ charge-
transfer complex and is associated with the electronic transitions at
745 and 480 nm.
Photometric titration measurements were performed for AEP-
DDQ reaction in CHCl3 as shown in Fig. 4. The results showed that
the donor-DDQ molar ratio was found to be 1:2. This is in agree-
ment with the obtained elemental analysis of the solid CT-complex
which accordingly can be formulated as [(AEP)(DDQ)2].
Fig. 5 shows the electronic absorption spectra of the reaction of
TCNQ with the donor AEP. While none of the reactants’ spectra dis-
play any measurable absorption in the region 450–1000 nm, the
resulting CT-complex shows strong absorptions centered on 847
and 748 nm reaction. These absorptions are associated with the
Fig. 5. Electronic absorption spectra of 1-(2-aminoethyl)piperidine (AEP)-TCNQ
reaction in CHCl3. (A) [AEP] = 5 ꢂ 10ꢁ3; (B) [TCNQ] = 1 ꢂ 10ꢁ3M; (C) 1: 2 AEP-TCNQ
mixture, [AEP] = 5 ꢂ 10ꢁ3 M and [TCNQ] = 1 ꢂ 10ꢁ3 M.
1
For interpretation of color in Figs. 1–12 and 14 the reader is referred to the web
version of this article.

48
A. Mostafa et al. / Journal of Molecular Structure 1029 (2012) 45–52
Fig. 6. Photometric titration curve for 1-(2-aminoethyl) piperidine (AEP)-TCNQ
reaction in CHCl3 measured at 748 nm and 847 nm.
Fig. 8. Photometric titration curve for 1-(2-aminoethyl) piperidine (AEP)-TBCHD
reaction in CHCl3 measured at 520 nm.
Table 2
Spectroscopic data for the CHCl3 solutions of solid CT-complexes of AEP with the
acceptors iodine, DDQ, TCNQ and TBCHD.
Complex
Color
Absorptiona
(nm)
Stoichiometry
(donor:acceptor)
½ðAEPÞIꢀþIꢁ3
Dark brown
364s, 294s
1:2
[(AEP) (DDQ)2]
[(AEP)(TCNQ)2]
[(AEP)(TBCHD)]
Dark red
Brown
Dark brown
745s, 480s
847s, 748 m
520 h
1:2
1:2
1:1
a
The reactants AEP, iodine, DDQ, TCNQ and TBCHD have no measurable
absorptions in the region of study with used concentrations; m, medium; s, strong;
h, hump.
indicating variation in shifts of electron density as a result of the
transition. This distinguishes it from the
the ligand.
Intensity: CT absorptions bands are intense and often lie in the
ultraviolet and visible portion of the spectrum.
The formation constant (KCT) and molar extinction coefficient
(eCT) values for the formed CT-complexes of the donor 1-(2-amino-
ethyl) piperidine (AEP) with iodine, DDQ, TCNQ and TBCHD in
p ?
pꢃ transitions on
Fig. 7. Electronic absorption spectra of 1-(2-aminoethyl)piperidine (AEP)-TBCHD
reaction in CHCl3. (A) [AEP] = 5 ꢂ 10ꢁ3; (B) [TBCHD] = 5 ꢂ 10ꢁ3 M; (C): 1:1 AEP-
TBCHD mixture, [AEP] = [TBCHD] = 5 ꢂ 10ꢁ3 M.
strong color change observed upon mixing of the reactants (from a
brown to a yellow solution), and reflects the electronic transitions
in the formed CT-complexes.
Photometric titration measurements based on these absorp-
tions were performed in order to determine the reaction stoichi-
ometry in CHCl3 as shown in Fig. 6. The results showed that the
donor to TCNQ molar ratio was found to be 1:2. This is in agree-
ment with the elemental analysis of the solid CT-complex. On
the basis of these experimental data, the complex obtained can
be formulated as [(AEP)(TCNQ)2].
Fig. 7 shows the electronic spectrum recorded in the region
400–700 nm of the reaction of TBCHD with AEP. A strong color
change to dark brown was observed upon mixing and indicates
the formation of the AEP-TBCHD charge-transfer complex. This
complex was associated with the electronic transitions at 520 nm.
Photometric titration measurements were performed for the
AEP-TBCHD reaction in CHCl3 as shown in Fig. 8. The results
showed that the donor-TBCHD molar ratio was found to be 1:1
and the obtained elemental analysis of the solid CT-complex accu-
rately matched the molar ratio of 1:1 and was formulated as
[(AEP)(TBCHD)]. Table 2 shows the spectroscopic data of the result-
ing CT-complexes.
CHCl3 at 25 °C were calculated.
A0D0l
A
1
A0 þ D0
¼
þ
ð1Þ
ke
e
The 1:1 modified Benesi–Hildebrand equation (Eq. (1)) [21] was
used to calculate the values of the formation constant, KCT (L molꢁ1),
and the molar extinction coefficient eCT (L molꢁ1 cmꢁ1), for the
complex [(AEP)(TBCHD)].
2
ðA0Þ D0l
1
A0ðA0 þ 4D0Þ
¼
þ
ð2Þ
A
k
e
e
The corresponding spectral parameters for the complexes
½ðAEPÞIꢀþI3ꢁ, [(AEP)(DDQ)2] and [(AEP)(TCNQ)2] were calculated
using the known equation (Eq. (2)) of 1:2 complexes [22]:
Where A0 and D0 are the initial concentrations of the acceptors
and donors, respectively, A is the absorbance of the mentioned CT
bands, and ‘ is the cell path length (1 cm). The data obtained
throughout this calculation is given in Table 3. Plotting the
A0D0‘/A against (A0 + D0) from equation (1) and plotting (A0)2D0‘/
A versus A0(A0 + 4D0) from equation (2), linear lines were obtained
with a slope of 1/eCT and an intercept of 1/KCTeCT as shown in
Figs. 9–12.
Identification of charge transfer bands can be illustrated as
follows:
The color: of CT-complexes is reflective of the relative energy
balance resulting from the transfer of electronic charge from donor
to acceptor.
These complexes give high values of both the formation con-
Solvatochromism: in solution, the transition energy and there-
fore the complex color varies with variation in solvent permittivity,
stant (KC) and the molar extinction coefficient (
values of KCT confirm the expected high stabilities of the formed
eCT). These high

A. Mostafa et al. / Journal of Molecular Structure 1029 (2012) 45–52
49
Table 3
Spectrophotometric and free energy results of CT-complexes of ½ðAEPÞIꢀþI3ꢁ, [(AEP)(DDQ)2], [(AEP)(TCNQ)2] and [(AEP)(TBCHD)] in CHCl3.
Complex
KCT (L molꢁ1
)
kmax (nm)
ꢁ
D
G° (cal molꢁ1
)
ECT (eV)
eCT (L molꢁ1 cmꢁ1
5.90 ꢂ 103
)
65.0 ꢂ 103
364
6.56 ꢂ 103
3.42
½ðAEPÞIꢀþIꢁ3
[(AEP)(DDQ)2]
[(AEP)(TCNQ)2]
[(AEP)(TBCHD)]
42.6 ꢂ 103
125.3 ꢂ 103
0.10 ꢂ 103
480
847
520
6.31 ꢂ 103
6.95 ꢂ 103
2.73 ꢂ 103
2.59
1.47
2.39
0.454 ꢂ 103
0.154 ꢂ 103
0.062 ꢂ 103
Fig. 11. Spectral determination of formation constant and molar extinction
coefficient of CT-complex [(AEP)(TCNQ)2] at 847 nm.
Fig. 9. Spectral determination of formation constant and molar extinction coeffi-
cient of CT-complex ½ðAEPÞIꢀþI3ꢁ at 364 nm.
Fig. 12. Spectral determination of formation constant and molar extinction
coefficient of CT-complex [(AEP)(TBCHD)] at 520 nm.
Fig. 10. Spectral determination of formation constant and molar extinction
coefficient of CT-complex [(AEP)(DDQ)2] at 480 nm.
iodine complex. As a result of the expected strong donation of
AEP, the high value of
eCT agrees with the existence of the triiodide
ion, Iꢁ3 , which is known to have a high absorptivity value [23,24].
D
G° (cal molꢁ1) values of the complexes
CT-complexes as a result of the expected high donation of 1-(2-
aminoethyl) piperidine. The formation constants are strongly
dependent on the nature of the used acceptors including the type
of electron withdrawing substituent.
The free energy change
½ðAEPÞIꢀþIꢁ, [(AEP)(TCNQ)2], [(AEP) (DDQ)2] and [(AEP)(TBCHD)]
3
were calculated from Gibbs free energy of formation according to
Eq. (3) [25,26]:
The obtained data shows that the CT-complex [(AEP)(TBCHD)]
D
Gꢄ ¼ ꢁRT ln KCT
ð3Þ
has
a much lower value of KCT compared with that of
[(AEP)(DDQ)2], [(AEP)(TCNQ)2] and ½ðAEPÞIꢀþIꢁ. This demonstrates
3
a much higher stability of these three complexes compared with
TBCHD. This could be related to several factors. First, the steric hin-
drance is relatively high in the case of TBCHD due to the larger
atomic size of the bromine substituent compared with the chlorine
in DDQ. Also, TCNQ has four cyano groups which are strong elec-
tron withdrawing groups. Second, the aromatic ring in TBCHD
has a lower electron accepting ability compared to DDQ or TCNQ,
related to the lower electron withdrawing process of the substitu-
ent bromine in TBCHD compared to chlorine in DDQ. This allows
for stronger electron donation from the AEP base to the three
acceptors, DDQ, TCNQ and þiodine, compared with TBCHD.
where DG° is the free energy of the charge transfer complexes; R the
gas constant (1.987 cal molꢁ1 °C); T the temperature in Kelvin, and
KCT the formation constant of donor–acceptor complexes (L molꢁ1).
The
tained results of
cess is exothermic in nature and spontaneous.
The results of G° are generally more negative as the formation
constants of the CT-complexes increase. As the bond between the
components becomes stronger and thus the components are sub-
jected to more physical strain or loss of freedom, the values of
D
G° values of the complexes are given in Table 3. The ob-
D
G° reveal that the CT-complexes formation pro-
D
D
G° become more negative. The more negative the value for DG°,
The CT-complex ½ðAEPÞIꢀ I3ꢁ shows the highest value of both KCT
the farther to the right the reaction will proceed in order to achieve
equilibrium.
and eCT. The high value of KCT reflects the high stability of the

50
A. Mostafa et al. / Journal of Molecular Structure 1029 (2012) 45–52
The charge transfer energy ECT: there is a corresponding relation
between the amount of transferred charge and the change of the
energy originating from charge transfer. As the amount of trans-
ferred charge increases, the total energy decreases and the lumi-
nescence intensity increases. Therefore the energy of charge
transfer from guest to host may be derived from the charge trans-
fer. ECT of the formed solid CT-complexes was calculated using the
following equation (4) [27,28]:
1243:667
ECT
¼
ð4Þ
kCT
where kCT is the wavelength (nm) of the studied CT-complexes
½ðAEPÞIꢀþI3ꢁ, [(AEP)(DDQ)2], [(AEP)(TCNQ)2] and [(AEP)(TBCHD)].
The ECT values calculated from equation (4) are listed in Table 3.
%T
These results (KCT, eCT, DG° and ECT) explain that the obtained solid
CT-complexes formed in the reaction of the donor AEP and the
acceptors iodine, TCNQ, DDQ and TBCHD have high CT energy and
formation constants (KCT).
The ionization potential is the difference in potential between
the initial state, in which the electron is bound, and the final state,
in which it is at rest of infinity, can be determined from the CT
energies of the CT band of the complexes. For the acceptors iodine,
DDQ, TCNQ and TBCHD, the relationship becomes the following
[27–29]:
1:5
Ip ꢁ 5:2
ECT ¼ Ip ꢁ 5:2 þ
ð5Þ
where Ip is the ionization potential (eV) and ECT is the charge trans-
fer energy of the formed solid CT-complexes. The obtained values of
ionization potential are 6.9, 6.5, 5.6 and 6.4 eV for the obtained CT-
complexes ½ðAEPÞIꢀþI3ꢁ, [(AEP)(DDQ)2], [(AEP)(TCNQ)2] and
[(AEP)(TBCHD)], respectively. It has been reported that the ioniza-
tion potential of the electron donor may be correlated with the
charge transfer transition energy of the complex [30].
Wavenumber (cm-1)
Fig. 13. Infrared absorption spectra of: (A) 1-(2-aminoethyl) piperidine (AEP), (B)
[(AEP) (DDQ)2], (C) [(AEP)(TCNQ)2], (D) [(AEP)(TBCHD)], and (E) ½ðAEPÞIꢀþI3ꢁ
.
Table 4. These assignments are based on the comparison of the
spectra of the formed products with the spectra of the free reac-
tants, the donor AEP and the acceptors iodine, DDQ, TCNQ and
TBCHD.
The spectra of the reaction products contain the main infrared
bands for both AEP and each of the accepting reactants. This sup-
ports the formation of the donor-acceptor CT-complexes. However,
3.2. Infrared absorption spectra
The infrared absorption spectra of the donor 1-(2-aminoethyl)
piperidine (AEP) along with those of the formed complexes
½ðAEPÞIꢀþIꢁ, [(AEP)(DDQ)2], [(AEP)(TCNQ)2] and [(AEP)(TBCHD)]
3
are shown in Fig. 13. The infrared band assignments are given in
Table 4
Infrared wavenumbers (cmꢁ1) and tentative band assignments for AEP, [(AEP)(DDQ)2], [(AEP)(TCNQ)2], ½ðAEPÞIꢀþIꢁ3 and [(AEP)(TBCHD)].
½ðAEPÞIꢀþIꢁ3
[AEP]
[(AEP)(DDQ)2]
[(AEP)(TCNQ)2]
[(AEP)(TBCHD)]
Assignments
3363s
3285m
3192w
2928s
2285s
3437s
3263w
3422s
3280w
3417s
3082w
3429s
3242w
m
(H2O); KBr
m(NAH); AEP
3021w
2956w
2215m
2928s
2945m
2868w
2928s
2854m
m(CAH); AEP
2854m
2746ms
2134ms
m(C„N); DDQ,TCNQ
1699ms
1574s
1635m
1563m
1383w
1349m
1256w
1242w
1157m
1123m
1041m
994m
m
(C@O); DDQ, TBCHD
1598s
1383m
1347w
1243w
1156m
1171w
1126m
1041m
994m
m(C@C); DDQ, TBCHD, TCNQ
1382w
1346ms
1277ms
1396m
1349w
1239ms
1220w
1160w
1121m
1083w
990m
1338w
1319w
1297ms
1256w
1174w
1147w
1028w
998w
Free and complexed AEP
m
(CAN); AEP
(CAC); AEP
1154ms
1124ms
1041ms
992ms
963m
m
d(CH)deformation, AEP
968w
963w
965w
963w
864ms
784m
888m
798w
861w
870w
784w
859m
d(CH); out of plan
Wag; AEP
754ms
756ms
755w
757w
757w
NH deform-ation
m, medium; s, strong ; w, weak; br, broad;
m
, stretching; d, bending.

A. Mostafa et al. / Journal of Molecular Structure 1029 (2012) 45–52
51
105
100
105
100
90
90
DTG
DTG
80
70
60
50
80
70
60
50
TG
40
30
40
30
20
20
10
0
TG
10
(A)
(B)
100
0
30
200
400
600
800
950
30
200
300
400
500
Temperature (oC)
Temperature (oC)
105
100
105
100
DTG
DTG
90
90
80
70
80
70
60
50
40
30
20
10
0
60
50
40
30
20
10
0
TG
TG
TG
(C)
(D)
30
200
400
600
950
30
200
400
600
800
900
800
Temperature (oC)
Temperature (oC)
Fig. 14. (A–D) Thermograms of the CT- complexes; (A) [(AEP)(DDQ)2], (B) ½ðAEPÞIꢀþIꢁ, (C) [(AEP)(TBCHD)] and (D) [(AEP)(TCNQ)2].
3
Table 5
Thermal data for the CT-complexes ½ðAEPÞIꢀþI3ꢁ, [(AEP)(DDQ)2], [(AEP)(TCNQ)2], and [(AEP)(TBCHD)].
Complex
Reaction stoichiometry
donor:acceptor
DTG max (°C)
TG% mass loss found/
calculated
Lost species
½ðAEPÞIꢀþIꢁ3
1:2
1:2
174
56.0/60.1
39.3/39.9
37.9/38.5
20.8/22.3
[(AEP)I2]
(I2)
[AEP, CN, Cl2]
[C7NO2]
308, 392, 407
145, 187, 272
681
[(AEP)(DDQ)2]
950
Uncertain with 39.2% carbon decomposition
residue
[(AEP)(TCNQ)2] 1:2
[(AEP)(TBCHD)] 1:1
122
34.3/33.6
65.7/66.4
[(AEP)(CN)2]
[TCNQ], [C10H4N2]
205, 270, 335, 360, 455,
708
126, 213
34.18/36.85
62.8/63.1
[(AEP)(Br)]
[C6H2Br3O]
349, 387, 421, 654, 855
the absorptions of AEP and acceptors in the formed products show
changes in band intensities and some small shifts in the frequency
wavenumber values. These changes may be explained by the ex-
pected symmetry and electronic structure modifications in both

52
A. Mostafa et al. / Journal of Molecular Structure 1029 (2012) 45–52
donor and acceptor units in the formed products compared with
weight loss of 34.3% very close to the calculated value of 33.6%. The
temperatures of 205, 270, 335, 360, 455 and 708 °C correspond to
the loss of the acceptor [TCNQ + C10H4N2]. The total weight loss of
those steps is 65.7%, close to the calculated value of 66.4% as shown
in Table 4. The deviation between the observed and calculated val-
ues was small (0.7%).
those of the free molecules. For example, the
m(NAH) vibrations
of the free AEP in [(AEP)(DDQ)2]are at 3263 and 3054 cmꢁ1 while
in the [(AEP)(TCNQ)2] and ½ðAEPÞIꢀþIꢁ, one absorption is only ob-
3
served in each case at 3280 and 3082 cmꢁ1, respectively.
The outlined changes in
m(NAH) upon complexation support
the involvement of the nitrogen atom of the amino group in the do-
nor AEP through the CT-interaction process. It may indicate that
4. Conclusion
the
m(C„N) vibrations of the acceptors TCNQ and DDQ change in
terms of band wavenumber values upon complexation. The
Charge-transfer interactions between the donor AEP, the
-acceptor iodine, and the p-acceptors DDQ, TCNQ and TBCHD
m
(C„N) vibration for free TCNQ is observed at 2223 cmꢁ1and for
r
free DDQ at 2230 cmꢁ1. These vibrations occur at 2134 and
2176 cmꢁ1 in the spectrum of [(AEP)(TCNQ)2] and at 2215 cmꢁ1
for[(AEP)(DDQ)2].
were studied in CHCl3 at 25 °C. We were able to show that the
reaction stoichiometry is the same for the acceptors DDQ, TCNQ
and iodine (1:2) but differ with TBCHD which is 1:1; the resulting
CT-complexes were shown to have the formulas ½ðAEPÞIꢀþI3ꢁ,
[(AEP)(DDQ)2], [(AEP)(TCNQ)2], and [(AEP)(TBCHD)]. Our results
indicate that the nitrogen atom of the amino group in the donor
AEP is involved in the complexation with the acceptors. The mea-
surements show the donor-acceptor molar ratio differs only with
the acceptor (TBCHD) reaction and were found to be 1:1. This could
be due to multiple factors. First, the steric hindrance is relatively
high in the TBCHD due to the larger substituent bromine atomic
size compared that of chlorine in DDQ. This will weaken the
interaction between AEP and TBCHD compared with that with
the other three acceptors. Second, the aromatic ring in TBCHD
has lower electron accepting ability and a lower electron with-
drawing process of the substituent bromine in TBCHD compared
with the other acceptors. This allows stronger electron donation
from AEP base to iodine, DDQ, and TCNQ compared to TBCHD.
3.3. Thermal analysis measurements
Thermogravimetric (TG) and differential thermogravimetric
(DTG) analysis were carried out under a N2 flow (20 ml minꢁ1) in
order to confirm the formulas and structures of the four solid CT-
complexes. Fig. 14A–D show the thermograms of [(AEP)(DDQ)2],
½ðAEPÞIꢀþIꢁ, [(AEP)(TBCHD)] and [(AEP)(TCNQ)2] respectively.
3
The thermogravimetric data for these complexes is shown in
Table 5. The obtained data support the calculated formulas and
structures of the formed CT-complexes. The degradations steps
and their associated temperatures vary from one complex to an-
other depending on the type of constituents as well as their stoi-
chiometry. These two factors have pronounced effects on the
type of bonding, relative complex stabilities and geometries.
The first CT-complex [(AEP)(DDQ)2] shown in Fig. 14A decom-
poses in four temperature steps at 145, 187, and 272 °C corre-
sponding to the loss of [(AEP)(CN)(Cl2)] with total weight loss of
37.9% (38.5% calculated) and at 681 °C corresponds to loss of
[C7NO2] with weight loss of 20.8 % (22.3% calculated). The lost spe-
cies are not certain at 950 °C because the complex loss is not com-
plete with a carbon residue of 39.2% by weight at that temperature.
The combustion of the complex was performed under an inert
atmosphere of nitrogen gas.
References
[1] R. Foster, Organic Charge Transfer Complexes, Academic Press, London, 1969.
pp. 51 and 387.
[2] A. Mostafa, H.S. Bazzi, Spectrochim. Acta Part A 74 (2009) 180.
[3] A. Mostafa, H.S. Bazzi, Spectrochim. Acta Part A 79 (2011) 1613.
[4] H.S. Bazzi, A. Mostafa, S.Y. AlQaradawi, E.M. Nour, J. Mol. Struct. 842 (2007) 1.
[5] H.S. Bazzi, S.Y. AlQaradawi, A. Mostafa, E.M. Nour, J. Mol. Struct. 879 (2008) 60.
[6] S.Y. AlQaradawi, H.S. Bazzi, A. Mostafa, E.M. Nour, Spectrochim. Acta Part A 71
(2008) 1594.
It is interesting to see that the triiodide complex ½ðAEPÞIꢀþI3ꢁ
shown in Fig. 14(B) decomposes in four degradation steps. The loss
at 174 °C corresponds to the loss of [(AEP)I]+Iꢁ with a weight loss of
56% very close to the calculated value of 60.1%. This step is fol-
lowed by three degradations at 309, 392 and 407 °C corresponding
to the loss of [I2] species with a total weight loss of 39.3% with a
0.6% deviation from the calculated value (39.9%). This is evident
that the AEP represents 20.16% of the complex, Table 5.
[7] J. Casaszar, Acta Phys. Chim. 29 (1998) 34.
[8] E.M. Nour, Spectrochim. Acta Part A 167 (2000) 56.
[9] A.S.N. Murthy, A.P. Bhardwaj, Spetrochim. Acta 39A (1983) 415.
[10] S.Y. AlQaradawi, E.M. Nour, Spectrosc. Lett. 37 (2004) 337.
[11] E.M. Nour, L.H. Chen, J. Laane, J. Raman Spectrosc. 467 (1986) 17.
[12] P.J. Trotter, P.A. White, Appl. Spectrosc. 32 (1978) 323.
[13] E.M. Nour, L.A. Shahada, Spectrochim. Acta 44A (1988) 1277.
[14] S. Licht, Sol. Energy Mater. Sol. Cells 35 (1995) 305.
[15] H. Salem, J. Pharm. Biomed. Anal. 29 (2002) 527.
[16] D.A. SKoog, Principle of Instrumental Analysis, Sunder College Publishing, New
York, 1985 (Chapter7).
The third complex [(AEP)(TBCHD)] is shown in Fig. 14C. This
thermogram is relatively complicated. It decomposes at seven tem-
peratures at 126, 213, 349, 387, 421, 654 and 855 °C. The temper-
atures of 126 °C and 213 °C show total weight loss of 37.18%
corresponding to the loss of the donor with one bromine atom
[(AEP)(Br)] species very close to the calculated value of 36.85%.
The other decomposition temperatures at 349, 387, 421, 654 and
855 °C are associated with a total weight loss of 62.82% corre-
sponding to the loss of the acceptor losing one bromine atom in
the first two decomposition steps at temperatures 126 °C and
213 °C[C6H2Br3O]in agreement with the calculated value of 63.15%.
The fourth complex [(AEP)(TCNQ)2] shown in Fig. 14(D) has six
main degradation steps at 122, 205, 270, 360, 455 and 708 °C. The
temperature 122 °C corresponds to the loss of [(AEP)(CN)2] with a
[17] W. Kiefer, H.J. Bernstein, Chem. Phys. Lett. 16 (1972) 5.
[18] G.A. Bowmaker, R.J. Knappstein, J. Chem. Soc. Dalton (1977) 1928.
[19] E.M. Nour, Spectrochim. Acta 47A (1991) 743.
[20] M. Mizuno, J. Tanaka, I. Harda, J. Phys. Chem. 85 (1981) 1789.
[21] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703.
[22] A. El-Kourashy, Spectrochim. Acta A 37 (1981) 399.
[23] L. Andrews, E.S. Prochaska, A. Loewenschuss, Inorg. Chem. 19 (1980) 463.
[24] K. Kaya, N. Mikami, Y. Udagawa, M. Ito, Chem. Phys. Lett. 16 (1972) 151.
[25] M. Arslan, H. Duymus, Spectrochim. Acta A 67 (2007) 573–577.
[26] A.A.A. Boraei, Spectrochim. Acta A 58 (9) (2002) 1895.
[27] G. Briegleb, Z. Angew. Chem. 72 (1960) 401.
[28] G. Briegleb, Z. Angew. Chem. 76 (1964) 326.
[29] Moamen S. Refat, Akram M. El-Didamony, Ivo Grabchev, Spectrochim. Acta A
67 (2007) 58.
[30] M. Pandeeswaran, K.P. Elango, Spectrochim. Acta A 65 (2006) 1148.