Synthesis, spectroscopic, thermal and antimicrobial investigations of charge-transfer complexes formed from the drug procaine hydrochloride with quinol, picric acid and TCNQ

Article abstract of DOI:10.1016/j.molstruc.2012.07.017

Intermolecular charge-transfer or proton-transfer complexes between the drug procaine hydrochloride (PC-HCl) as a donor and quinol (QL), picric acid (PA) or 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) as a π-acceptor have been synthesized and spectroscopically studied in methanol at room temperature. Based on elemental analyses and photometric titrations, the stoichiometry of the complexes (donor:acceptor molar ratios) was determined to be 1:1 for all three complexes. The formation constant (K_CT), molar extinction coefficient (* epsiv;_CT) and other spectroscopic data have been determined using the Benesi-Hildebrand method and its modifications. The newly synthesized CT complexes have been characterized via elemental analysis, IR, Raman, ¹H NMR, and electronic absorption spectroscopy. The morphological features of these complexes were investigated using scanning electron microscopy (SEM), and the sharp, well-defined Bragg reflections at specific 2θ angles have been identified from the powder X-ray diffraction patterns. Thermogravimetric analyses (TGAs) and kinetic thermodynamic parameters were also used to investigate the thermal stability of the synthesized solid CT complexes. Finally, the CT complexes were screened for their antibacterial and antifungal activities against various bacterial and fungal strains, and only the complex obtained using picric acid exhibited moderate antibacterial activity against all of the tested strains.

Full text of DOI:10.1016/j.molstruc.2012.07.017

Journal of Molecular Structure 1030 (2012) 26–39
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectroscopic, thermal and antimicrobial investigations of
charge-transfer complexes formed from the drug procaine hydrochloride
with quinol, picric acid and TCNQ
⇑
Abdel Majid A. Adam
Department of Chemistry, Faculty of Science, Taif University, Al-Hawiah, P.O. Box 888, Zip Code 21974 Taif, Saudi Arabia
h i g h l i g h t s
"
"
"
Three new CT-complexes of procaine hydrochloride drug are characterized.
Various spectroscopic and thermal analyses are used.
XRD confirmed that the obtained complexes located within nanoscale range.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2012
Received in revised form 7 July 2012
Accepted 10 July 2012
Available online 23 July 2012
Intermolecular charge-transfer or proton-transfer complexes between the drug procaine hydrochloride
(PC-HCl) as a donor and quinol (QL), picric acid (PA) or 7,70,8,80-tetracyanoquinodimethane (TCNQ) as a
p
-acceptor have been synthesized and spectroscopically studied in methanol at room temperature. Based
on elemental analyses and photometric titrations, the stoichiometry of the complexes (donor:acceptor
molar ratios) was determined to be 1:1 for all three complexes. The formation constant (KCT), molar extinc-
tion coefficient (eCT) and other spectroscopic data have been determined using the Benesi–Hildebrand
Keywords:
method and its modifications. The newly synthesized CT complexes have been characterized via elemental
analysis, IR, Raman, 1H NMR, and electronic absorption spectroscopy. The morphological features of these
complexes were investigated using scanning electron microscopy (SEM), and the sharp, well-defined Bragg
reflections at specific 2h angles have been identified from the powder X-ray diffraction patterns. Thermo-
gravimetric analyses (TGAs) and kinetic thermodynamic parameters were also used to investigate the ther-
mal stability of the synthesized solid CT complexes. Finally, the CT complexes were screened for their
antibacterial and antifungal activities against various bacterial and fungal strains, and only the complex
obtained using picric acid exhibited moderate antibacterial activity against all of the tested strains.
Ó 2012 Elsevier B.V. All rights reserved.
Procaine hydrochloride
Proton-transfer
Thermal analysis
Antimicrobial activity
1. Introduction
local anesthetic. Furthermore, ever since Koller used cocaine to
anesthetize a cornea in 1844, local anesthetics have been under
Procaine hydrochloride (PC-HCl; 2-(diethylamino)ethyl-4-
aminobenzoate hydrochloride, Formula I) is a synthetic local anes-
thetic drug of the amino ester family that produces a reversible
loss of sensation by diminishing the conduction of sensory nerve
impulses [1,2]. Procaine has long been employed as a pharmaco-
logical agent in the life sciences and in clinical therapeutic studies.
This drug is primarily used to reduce pain from the intramuscular
injection of penicillin, and it is also used in dentistry [3,4]. Procaine
was first synthesized in 1905 under the trade name Novocaine (or
Novocain) as the first injectable synthetic local anesthetic. Before
the discovery of procaine, cocaine was the most commonly used
continuous development. Procaine has low toxicity and low stim-
ulus and is not toxic to the central nervous system [5].
Charge-transfer complexation is of great importance in chemi-
cal reactions, including addition, substitution, condensation [6,7],
biochemical and bioelectrochemical energy-transfer processes
[8], biological systems [9], and drug–receptor binding mechanisms.
For example, drug action, enzyme catalysis, ion transfers through
lipophilic membranes [10], and certain p-acceptors have been suc-
cessfully utilized in the pharmaceutical analysis of some drugs in
pure form or in pharmaceutical preparations [11–17]. Further-
more, charge-transfer complexation is also of great importance in
many applications and fields, such as in non-linear optical
materials, electrically conductive materials [18–21], second-order
non-linear optical activity [22], microemulsions [23], surface
⇑
Tel.: +966 502099808.
E-mail address: majidadam@yahoo.com
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.07.017

Abdel Majid A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
27
was maintained at 5.0 ꢁ 10ꢂ4 M, whereas the concentration of
the p-acceptors (Ca) changed over a wide range of concentrations
(0.25 ꢁ 10ꢂ4–4.00 ꢁ 10ꢂ4 M) to produce solutions with an acceptor
molar reaction that varied from 4:1 to 1:4. The stoichiometry of the
molecular CT complexes was determined from the determination
of the conventional spectrophotometric molar ratio according to
the known methods [34] using a plot of the absorbance of each
CT complex as a function of the Cd:Ca ratio. Modified Benesi–Hilde-
brand plots were constructed [35,36] to allow calculations of the
H2N
O
HN
Cl
O
formation constant, KCT, and the absorptivity, eCT, values for each
CT complex in this study.
Formula I. Molecular structure of procaine hydrochloride.
2.4. Instrumentation
chemistry [23], photocatalysts [24], dendrimers [25], solar energy
storage [26], organic semiconductors [27], and the investigation
of redox processes [28]. Charge-transfer complexes that use organ-
ic species are intensively studied because of their special type of
interaction, which is accompanied by the transfer of an electron
from the donor to the acceptor [29,30]. In addition, the protonation
of the donor from acidic acceptors is a route for the formation of
ion-pair adducts [31–33].
2.4.1. Elemental analyses
The elemental analyses of the carbon and hydrogen contents
were performed by the microanalytical unit at Cairo University,
Egypt, using a Perkin–Elmer CHN 2400 (USA).
2.4.2. Electronic spectra
The electronic absorption spectra of the methanol solutions of
the donor, acceptors and resulting CT complexes were recorded
in wavelength range of 200–800 nm using a Perkin–Elmer Lambda
25 UV/Vis double-beam spectrophotometer at Taif University, Sau-
di Arabia. The instrument was equipped with a quartz cell with a
1.0 cm path length.
This paper describes the formation of CT complexes between
procaine hydrochloride (PC-HCl) as a donor and quinol (benzene-
1,4-diol, QL), picric acid (2,4,6-trinitrophenol, PA), or 7,70,8,80-tetra-
cyanoquinodimethane (TCNQ). The nature and structure of the
final products both in solution and in the solid phase have been
characterized using elemental analysis, infrared (IR), Raman, 1H
NMR and electronic absorption spectroscopy, powder X-ray dif-
fraction and scanning electron microscopy (SEM) to interpret the
behavior of the interactions. The thermal behavior of the obtained
complexes and the kinetic and thermodynamic parameters (Eꢀ, A,
2.4.3. Infrared and Raman spectra
The mid-infrared (IR) spectra (KBr discs) within the range of
4000–400 cmꢂ1 for the solid CT complexes were recorded on a Shi-
madzu FT-IR spectrophotometer with 30 scans at 2 cmꢂ1 resolu-
tion. The Raman laser spectra of the samples were measured on
a Bruker FT-Raman spectrophotometer at Taif University, Saudi
Arabia; the spectrophotometer was equipped with a 50 mW laser.
D
Sꢀ, Hꢀ and Gꢀ) have also been investigated. Finally, the antimi-
D D
crobial activity of the PC-HCl complexes was tested against various
bacterial and fungal strains.
2.4.4. 1H NMR spectra
2. Experimental
1H NMR spectra were collected by the Analytical Center at King
Abdul Aziz University, Saudi Arabia, on a Bruker DRX-250 spec-
trometer operated at 250.13 MHz with a dual 5 mm probe head.
The measurements were performed at ambient temperature using
DMSO-d6 (dimethylsulfoxide, d6) as a solvent and TMS (tetrameth-
ylsilane) as an internal reference. The 1H NMR data are expressed
in parts per million (ppm) and are internally referenced to the
residual proton impurity in the DMSO solvent.
2.1. Materials
Procaine hydrochloride (PC-HCl) (MF = C13H21N2O2Cl) and
p
-acceptors of quinol (QL), picric acid (PA) and 7,70,8,80-tetracyano-
quinodimethane (TCNQ) were of analytical reagent grade (Merck)
and were used without further purification.
2.2. Synthesis of the solid CT complexes
2.4.5. Thermal analysis
Thermogravimetric analysis (TGA) was performed under air
atmosphere between room temperature and 800 °C at a heating
rate of 10 °C/min using a Shimadzu TGA–50H thermal analyzer at
the Central Lab at Ain Shams University, Egypt.
The solid CT complexes of PC-HCl with QL, PA, and TCNQ were
synthesized by mixing 1 mmol of PC-HCl in methanol (10 ml) with
1 mmol of each acceptor in the same solvent. The mixtures were
stirred at room temperature for 15 min, which resulted in the pre-
cipitation of the solid CT complexes. The solid precipitates were fil-
tered, washed several times with methanol, and then dried under
vacuum over anhydrous calcium chloride.
2.4.6. X-ray diffraction patterns
The X-ray diffraction patterns for the obtained CT complexes
were collected on a PANalytical X’Pert PRO X-ray powder diffrac-
tometer at the Central Lab at Ain Shams University, Egypt. The
instrument equipped with a Ge(III) monochromator before the
2.3. Photometric titration measurements
sample and
0.154056 nm was used.
a Cu Ka1 X-ray source with a wavelength of
Photometric titration measurements were performed for the
reactions of the donor with QL, PA, and TCNQ against methanol
as a blank, at wavelengths of 296, 355, and 299 nm, respectively.
A 0.25, 0.50, 0.75, 1.00, 1.50, 2.0, 2.50, 3.00, 3.50 or 4.00 ml aliquot
of a standard solution (5.0 ꢁ 10ꢂ4 M) of the appropriate acceptor in
MeOH was added to 1.00 ml of 5.0 ꢁ 10ꢂ4 M PC-HCl, which was
also dissolved in MeOH. The total volume of the mixture was
5 ml. The concentration of PC-HCl (Cd) in the reaction mixture
2.4.7. SEM and EDX detection
Scanning electron microscopy (SEM) images and energy-
dispersive X-ray spectroscopy (EDX) patterns were collected on a
Jeol JSM-6390 instrument at Taif University, Saudi Arabia. The
instrument was operated at an accelerating voltage of 20 kV.

28
A.M. A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
2.5. Antimicrobial investigation
compounds are active against some of the bacteria and fungi under
investigation. The concentration of each solution was 1.0 ꢁ
10ꢂ3 mol dmꢂ3. Commercial DMSO was used to dissolve the tested
samples.
The antimicrobial activities of the PC-HCl CT complexes and the
pure solvent were determined using a modified Bauer–Kirby disc
diffusion method [37] against different Gram (+) and Gram (ꢂ)
bacteria species: Staphylococcus aureus, Bacillus subtilis, Escherichia
coli and Pseudomonas aeuroginosa. Antifungal screening was stud-
ied against two species: Aspergillus flavus and Candida albicans.
The microanalysis unit at Cairo University, Egypt performed the
3. Results and discussion
3.1. Elemental analysis
investigations. For these investigations, 100 ll of the test bacte-
Elemental analyses (C, H, and N) of the PC-HCl CT complexes
were performed, and the obtained results are as follows:
ria/fungi were grown in 10 ml of fresh media until they reached
a count of approximately 108 cells/ml for bacteria or 105 cells/ml
for fungi [38]. One hundred microliters of the microbial suspension
was spread onto agar plates. The nutrient ager medium for the
antibacterial tests consisted of 0.5% peptone, 0.1% beef extract,
0.2% yeast extract, 0.5% NaCl and 1.5% agar–agar. The medium
for the antifungal tests consisted of 3% sucrose, 0.3% NaNO3, 0.1%
K2HPO4, 0.05% KCl, 0.001% FeSO4 and 2% agar–agar [39]. Isolated
colonies of each organism that might be playing a pathogenic role
were selected from the primary agar plates and tested for suscep-
tibility. The disc diffusion method for the filamentous fungi was
tested using the developed standard method M38-A [40], whereas
the disc diffusion method for yeast was tested using the developed
standard method M44-P [41]. Plates inoculated with filamentous
fungi, bacteria or yeast were incubated at 25 °C, 35–37 °C or
30 °C, respectively. After the plates were incubated for 48 h, the
inhibition (sterile) zone diameters (including disc) were measured
using slipping calipers from the National Committee for Clinical
Laboratory Standards (NCCLS, 1993) [42] and are expressed in
mm. Standard discs of tetracycline (antibacterial agent) and
Amphotericin B (antifungal agent) served as positive controls for
antimicrobial activity, whereas filter discs impregnated with
1 [(PC-HCl)(QL)]:
59.54; %H, 7.05; %N, 7.31, found: %C, 60.01; %H, 7.38; %N, 7.73.
2 [(PC-HCl)(PA)]; 19H24N5O9Cl; mol. wt. = 501.91; calc.: %C,
C19H27N2O4Cl; mol. wt. = 382.92; calc.: %C,
C
45.43; %H, 4.78; %N, 13.95, found: %C, 45.11; %H, 4.55; %N,
14.23.
3 [(PC-HCl)(TCNQ)];
C25H25N6O2Cl; mol. wt. = 477; calc.: %C,
62.89; %H, 5.24; %N, 17.61, found: %C, 63.22; %H, 5.37; %N,
17.25.
The resulting values are in good agreement with the calculated
values, and the suggested values are in agreement with the molar
ratios determined from the photometric titration curves.
3.2. Determination of stoichiometry of the resulting CT complexes
The electronic absorption spectra of PC-HCl,
p-acceptors (QL,
PA, and TCNQ) and the formed CT complexes are shown in Fig. 1.
The spectra demonstrate that the formed CT complexes have
new strong absorption bands attributed to the CT interactions.
These bands are not present in the spectra of the free reactants
and are observed at 296, 395 and 299 nm for the PC-HCl/QL,
PC-HCl/PA and PC-HCl/TCNQ complexes, respectively. The peak
10 ll of DMSO solvent were used as a negative control. An inhibi-
tion zone diameter greater than 8 mm indicates that the tested
CT bands (300 and 395 nm)
CT band (296 nm)
3.0
2.5
2.0
1.5
2.5
2.0
1.5
1.0
1.0
PA
PC-HCl
CT- complex
QL
PC-HCl
CT- complex
0.5
0.5
0.0
250
0.0
200
275
300
325
350
375
400
300
400
500
Wavelength (nm)
Wavelength (nm)
CT band (299 nm)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
TCNQ
PC-HCl
CT- complex
300
400
500
Wavelength (nm)
Fig. 1. Electronic absorption spectra of PC-HCl CT-complexes at the detectable peak.

Abdel Majid A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
29
2.0
PC-HCl/PA
PC-HCl/QL
3.5
3.0
2.5
2.0
1.5
1.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
296 nm
395 nm
0.2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
ml added of PA
ml added of QL
PC-HCl/TCNQ
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
299 nm
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
ml added of TCNQ
Fig. 2. Photometric titration curves for PC-HCl/QL, PC-HCl/PA, and PC-HCl/TCNQ systems at detectable peaks of 296, 395 and 299 nm, respectively.
absorbance values that appeared in the spectra assigned to the
formed CT complexes were measured and plotted as function of
the Cd:Ca ratio according to the known method. Photometric titra-
tion plots based on these measurements are shown in Fig. 2. The
stoichiometry ratio of the complex formation, PC-HCl: acceptor
was found to be 1:1 for all of the complexes. Based on the obtained
data, the formed charge transfer complexes were formulated as
[(PC-HCl)(QL)], [(PC-HCl)(PA)] and [(PC-HCl)(TCNQ)].
3.3. Determination of the formation constant and the molar extinction
coefficient
The spectrophotometric titrations of the intermolecular
charge-transfer complexes formed from the reactions of PC-HCl
with QL, PA and TCNQ acceptors indicated the formation of 1:1
CT complexes; therefore, the formation constant (KCT) and the
molar absorptivity (e) of these complexes were calculated by
PC-HCl/PA
PC-HCl/QL
2.2
1.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
1.0
0.8
0.6
0.4
0.2
100 150 200 250 300 350 400 450 500 550
100 150 200 250 300 350 400 450 500 550
(Cdo+Cao)X10
-6
(Cdo+Cao)X10
-6
PC-HCl/TCNQ
1.2
1.0
0.8
0.6
0.4
0.2
100 150 200 250 300 350 400 450 500 550
-6
(Cdo+Cao)X10
Fig. 3. The modified Benesi–Hildebrand plots of PC-HCl/QL, PC-HCl/PA and PC-HCl/TCNQ systems at detectable peaks of 296, 395 and 299 nm, respectively.

30
A.M. A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
Table 1
Spectrophotometric results of the PC-HCl CT-complexes.
Complex
kmax (nm)
ECT (eV)
K (L molꢂ1
)
emax (L molꢂ1 cmꢂ1
)
f
l
IP
D
RN
D )
G° (25 °C) (kJ molꢂ1
PC-HCl/QL
PC-HCl/PA
PC-HCl/TCNQ
296
395
299
4.20
3.15
4.16
3.66 ꢁ 104
3.44 ꢁ 104
2.81 ꢁ 104
4.27 ꢁ 104
2.40 ꢁ 104
4.43 ꢁ 104
46.14
10.38
27.30
53.87
29.52
41.65
10.93
9.63
10.88
32.70
32.70
32.70
1.19
0.89
1. 18
ꢂ37,446
ꢂ40,522
ꢂ36,795
Briegleb and Czekalla [46] theoretically derived the relationship to
obtain the resonance energy (RN), which given below:
applying the 1:1 modified Benesi–Hildebrand equation in Eq. (1)
[35]:
CT ¼ 7:7 ꢁ 10ꢂ4=½h
m
CT =½RNꢃ ꢂ 3:5ꢃ
ð6Þ
ðCaCdÞ=A ¼ 1=K
e
þ ðCa þ CdÞ=
e
ð1Þ
e
where Ca and Cd are the initial concentrations of the acceptor and
the donor, respectively, and A is the absorbance of the strong de-
tected CT band. When the (CaCd)/A values for the 1:1 charge transfer
complex are plotted against the corresponding (Ca + Cd) values, a
where eCT is the molar absorptivity coefficient of the CT complex at
the maximum of the CT absorption, mCT is the frequency of the CT
peak, and RN is the resonance energy of the complex in the ground
state, which contributes to the stability constant of the complex (a
ground-state property). The energy (ECT) of the n ? pꢀ and
p–
pꢀ
straight line is obtained with a slope of 1/
. The modified Benesi–Hildebrand plots are shown in Fig. 3,
whereas the obtained KCT and values are shown in Table 1. In gen-
eral, these complexes exhibit high values for both the formation
constants (KCT) and the extinction coefficients ( ). The high values
e and an intercept of 1/
Ke
interactions between the donor (PC-HCl) and the acceptors was cal-
culated using the equation derived by Briegleb [47]:
e
ECT ¼ ðhmCT Þ ¼ ð1243:667=kCT
Þ
ð7Þ
e
of KCT reflect the expected high stabilities of the formed CT com-
plexes as a result of the expected strong donation from the PC-
HCl drug that contains two amino groups and one acetate group.
The data also reveal that the [(PC-HCl)(QL)] complex exhibits higher
where kCT is the wavelength of the CT band. The standard free en-
ergy changes of complexation ( G°) were calculated from the for-
D
mation constants using the equation derived by Martin et al. [48]:
D
Gꢄ ¼ ꢂ2:303RT log KCT
ð8Þ
values for both KCT and eCT compared with the other two complexes.
where
D
G° is the free energy change of the CT complexes (kJ molꢂ1),
R is the gas constant (8.314 J molꢂ1K), T is the absolute temperature
in K, and KCT is the formation constant of the complex (L molꢂ1) at
room temperature.
3.4. Determination of the spectroscopic and physical data
The spectroscopic and physical data, such as the standard free
energy (DG°), the oscillator strength (f), the transition dipole mo-
The calculated spectroscopic and physical values (f,
l, IP, RN and
ment ( ), the resonance energy (RN), and the ionization potential
l
D
G°) for the PC-HCl complexes using these Eqs. (2)–(8) are pre-
(IP), were estimated for samples dissolved in methanol at 25 °C.
The calculations can be summarized as follow;
From the CT absorption spectra, the oscillator strength (f) can be
estimated using the approximate formula [43]:
Z
sented in Table 1. The [(PC-HCl)(QL)] complex exhibits consider-
ably higher values of both oscillator strength (f) and transition
dipole moment (l) compared to the other complexes. The ob-
served high values of f indicate a strong interaction between the
donor–acceptor pairs with relatively high probabilities of CT tran-
sitions [49]. Among the numerous applications of CT complexes,
one important application is the calculation of the ionization po-
tential (IP) of the donor. The calculated IP value for the highest filled
molecular orbital that participates in the CT interaction of the PC-
HCl drug is approximately 10.45. The ionization potential of the
electron donor has been reported to be correlated with the
charge-transfer transition energy of the complex [50]. Further evi-
dence for the nature of CT interactions is the calculation of the
f ¼ 4:319 ꢁ 10ꢂ9
eCT dm
ð2Þ
R
where eCT
dm
is the area under the curve of the extinction coeffi-
cient of the absorption band in question plotted as a function of fre-
quency. To a first approximation,
f ¼ 4:319 ꢁ 10ꢂ9eCT m1=2
ð3Þ
where
1/2 is the half-bandwidth in cmꢂ1, i.e., the width of the band at half
the maximum extinction. The transition dipole moment ( ) of the
eCT is the maximum extinction coefficient of the CT band, and
standard free energy change (DG°). The obtained values of DG°
m
for the PC-HCl/QL, PC-HCl/PA and PC-HCl/TCNQ are ꢂ37.5, ꢂ40.5
and ꢂ36.8 kJ molꢂ1, respectively; these values indicate that the
interaction between the PC-HCl and the acceptors is exothermic
l
PC-HCl CT complexes have been calculated from Eq. (4) [44]:
1=2
and spontaneous. The
as the formation constants of the CT complexes increase. The val-
ues of G° become more negative as the bond between the compo-
DG° values, in general, are more negative
l
¼ 0:0958½eCT m1=2
=mmax
ꢃ
ð4Þ
The transition dipole moment is useful for determining if the
orbital
D
transitions are allowed; the transition from a bonding
p
nents becomes stronger and the components are therefore
subjected to more physical strain or loss of freedom.
to an antibonding pꢀ orbital is allowed because the integral that
defines the transition dipole moment is nonzero. The ionization
potentials (IP) of the PC-HCl donors in their charge-transfer com-
plexes were calculated using the empirical equation derived by
Aloisi and Pignataro represented in Eq. (5) [45]:
3.5. IR and Raman spectra
Infrared and Raman spectral studies shed light on the donation
location in the donor species, and the differences occur in the spec-
tra of the obtained charge-transfer complexes. The peak assign-
ments for the important characteristic IR and Raman spectral
bands for the formed PC-HCl charge-transfer complexes are shown
in Table 2, whereas the full assignments for all of the IR bands in
the spectrum are listed in Tables 3–5. The full IR and Raman spec-
tra of the CT complexes are shown in Figs. 4 and 5, respectively. A
IP ðeVÞ ¼ 5:76 þ 1:53 ꢁ 10ꢂ4mCT
ð5Þ
where mCT is the wavenumber in cmꢂ1 that corresponds to the CT
band formed from the interaction between the donor and the
acceptor. The electron-donating power of a donor molecule is mea-
sured by its ionization potential, which is the energy required to re-
move an electron from the highest occupied molecular orbital.

Abdel Majid A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
31
Table 2
Assignments of the characteristic IR and Raman spectral bands (cmꢂ1) for [PC-HCl)(QL)] and [(PC-HCl)(PA)] complexes.
d(NHþ3
)
def
d(NHþ3
)
sym
q )
(NHþ3
Complex
m(NH)
IR
Raman
IR
Raman
IR
Raman
IR
Raman
[(PC-HCl)(QL)]
[(PC-HCl)(PA)]
3201
3174
3057
3008
1607
1609
1606
1554
1311
1338
1267
1335
845
801
867
825
Table 3
Table 4
Characteristic infrared frequencies (cmꢂ1
and their complex.
)
a,b and tentative assignments for PC-HCl, QL
Assignmentsb
Characteristic infrared frequencies (cmꢂ1
)
a,b and tentative assignments for PC-HCl, PA
and their complex.
PC-HCl
QL
Complex
PC-HCl
PA
Complex [(PC-HCl)(PA)] Assignmentsb
[(PC-HCl)(QL)]
3361 s
3315 s
3207 s
3416 br 3404 m, br
3103 ms 3174 m
3079 m
m
m
m
m
m
(NAH); PC-HCl
as(CAH); PC-HCl
(OAH); PA
3361 s
3315 s
3207 s
3262 br
3031 m
3384 s
3359 sh
3302 s
m
m
m
m
m
(NAH); PC-HCl
as(CAH); PC-HCl
(OAH); QL
3003 m
(CAH); aromatic
(NHþ3
)
3201 ms
(CAH); aromatic
(NHþ3
)
2955 vs 2980 sh 2798 m
2854 vs 2872 w 2742 m, br
m
s(CAH) + mas(CAH)
2955 vs
2854 vs
2857 m
2836 m
2716 m
2948 ms
2880 w
2762 w
2665 ms
ms(CAH) +
m
as(CAH)
Hydrogen bonding
Hydrogen bonding
2587 s
2496 s
–
–
m
(+NAH); PC-HCl
1694 vs
–
1737 vs
m
(C@O); PC-HCl
2587 s
2496 s
2590 w
2467 vw
2483 mw
m
(+NAH); PC-HCl
(C@O); PC-HCl
1645 ms 1632 vs 1609 vs
1606 vs 1608 vs 1560 vs
1520 ms 1529 vs
d(NAH); PC-HCl
as(NO2); PA
d(NHþ3
def, complex
(C@C) (in-ring), aromatic
(C@O) + (C@N)
m
1694 vs
1866 m
1855 m
1686 vs
m
)
m
m
1645 ms
1606 vs
1520 ms
1628 w
1609 w
1518 vs
1607 vs
1517 vs
d(NAH); PC-HCl
d(NHþ3
def, complex
(C@C) (in-ring), aromatic
(C@O) + (C@N)
m
)
d(CAH) deformation
Ring breathing bands
m
m
m
1453 vs 1432 s
1428 ms
m
m
(CAH); alkanes; PC-HCl
(C@C) (in-ring), aromatic
d(CAH) deformation
Ring breathing bands
d(CAH) deformation
d(NHþ3
sym, complex
(CAC) + (CAO) +
1453 vs
1363 s
1477 vs
1466 s
m
m
(CAH); alkanes; PC-HCl
(C@C) (in-ring), aromatic
1363 s
1343 ms 1338 vs
1312 w
)
m
m
mas(CAN)
d(CAH) deformation
d(NHþ3
sym, complex
(CAC) + (CAO) +
CAH rock, alkanes; PC-HCl
m
1366 ms
1360 mw
1311 s
)
sNO2
m
m
m
as(CAN)
1316 s
1263 w
1271 vs
m
m
s(CAN); PC-HCl
(CAO); PC-HCl, PA
CAH rock, alkanes; PC-HCl
1272 vs 1150 ms 1165 ms
1316 s
1272 vs
1173 s
1116 s
1074 m
1049 m
1244 vs
1222 vs
1210 vs
1164 ms
1097 m
1281 vs
1243 m
1210 ms
1173 ms
1116 s
m
s(CAN); PC-HCl
1173 s
1116 s
1074 m
1049 m
1086 s
917 vs
1110 s
1080 m
1014 m
915 m
d(CAH) in plane bending
drock; NH
m(CAO); PC-HCl, QL
d(CAH) in plane bending
drock; NH
q
(NHþ3 ), complex
850 s
771 vs
829 w
781 s
732 s
801 m
751 m
714 s
1008 m
CAH out of plane bending
drock, CH2 rock
NAH wag skeletal vibrations
d(ONO); PA
q
(NHþ3 ), complex
850 s
771 vs
827 s
759 vs
845 ms
763 s
CAH out of plane bending
drock, CH2 rock
NAH wag skeletal vibrations
CNC deformation
638 m
703 s
652 sh
522 ms
634 mw
542 mw
CNC deformation
638 m
616 m
525 ms
624 m
515 m
a
s, strong; w, weak; m, medium; sh, shoulder; v, very; vs very strong; br, broad.
m, stretching; ms, symmetrical stretching; mas, asymmetrical stretching; d,
a
b
s, strong; w, weak; m, medium; sh, shoulder; v, very; vs very strong; br, broad.
b
m,
stretching; ms
,
symmetrical stretching; mas
,
asymmetrical stretching; d,
bending.
bending.
acceptors. These bands are attributed to the stretching vibration
of the intermolecular hydrogen bond formed through the transfer
of proton from the acidic center on the QL and PA acceptors to
the basic center of the PC-HCl donor (ANH2) [51]. This assumption
is strongly supported by the appearance of the main characteristic
absorption bands that result from the stretching and bending
comparison of the relevant IR spectral bands of the free donor, PC-
HCl and acceptors (QL, PA, and TCNQ) with the corresponding
bands in the IR spectra of the isolated solid CT complexes clearly
indicated that the characteristic bands of PC-HCl exhibit small
shifts in frequency (Tables 3–5) and changes in their band intensi-
ties. This result could be attributed to the expected changes in
symmetry and electronic configurations upon the formation of
the CT complexes. The IR spectra of the PC-HCl/QL and PC-HCl/
PA complexes are characterized by a broad medium band that ap-
pears between 2400 and 2800 cmꢂ1 (2665 cmꢂ1 for the QL com-
plex; 2742 cmꢂ1 for the PA complex), which does not appear in
the spectra of the free PC-HCl donor or those of the QL and PA
deformation of the NHþ3 group. For example, the
m(NH),
m
def(NHþ),
3
dsym(NHþ) and
q
(NHþ3 ) vibrations in the [(PC-HCl)(QL)] and [(PC-
3
HCl)(PA)] complexes occur at approximately 3200, 1600, 1300
and 800 cmꢂ1, respectively. The presence of these bands confirmed
that the complexation occurs through the protonation of the ANH2
group of the PC-HCl donor via a proton-transfer phenomenon from
the acidic center of each acceptor to the lone pair of electrons on

32
A.M. A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
Table 5
electron-withdrawing process and the conjugated electron system.
This behavior will decrease the CN bond order and therefore lower
its vibrational wavenumber value upon complexation.
a,b
Characteristic infrared frequencies (cmꢂ1
TCNQ and their complex.
)
and tentative assignments for PC-HCl,
Assignmentsb
PC-HCl
TCNQ
Complex
3.6. 1H NMR spectra
[(PC-HCl)(TCNQ)]
3361 s
3315 s
3207 s
3137 ms
3050 s
3740 vw
3618 vw
3135 m, br
3049 m
m
m
m
(NAH); PC-HCl
as(CAH); PC-HCl
(CAH); aromatic
The 1H NMR spectra of PC-HCl complexes were measured in
DMSO-d6, d ppm, 400 MHz at room temperature and are given in
Fig. 6. The chemical shifts (d) of the different types of protons of
the CT complexes are given below. The results obtained from ele-
mental analyses, infrared spectra, and photometric titrations are
in agreement with the 1H NMR spectra, which allows an interpre-
tation of the mode of interaction between the donor and the accep-
tor. The reaction of PC-HCl as the donor with QL as the acceptor
2955 vs
2854 vs
2969 mw
2851 mw
–
ms(CAH) + mas(CAH)
2587 s
2496 s
2220 s
2690 w, br
2179 vs
2135 s
m
(+NAH); PC-HCl
m
(C„N); TCNQ
(C@O); PC-HCl
1694 vs
–
1718 s
m
yielded
a new charge-transfer complex, 4-((2-(diethylammo-
1645 ms
1606 vs
1520 ms
1540 vs
1597 w
1563 vs
d(NAH); PC-HCl
nio)ethoxy)carbonyl)benzenaminium 4-hydroxyphenolate chlo-
ride, which produced signals at (Fig. 6): d = 1.22 (t, 6H, 2CH3),
3.15 (2q, 4H, 2CH2), 3.45 (t, 2H, N+ACH2), 4.51 (t, 2H, OACH2),
6.10 (b, 3H, NHþ3 hydrogen bonded with picrate), 6.58 (2d appear
t, 4H, ArAH phenolate protons), 6.68–7.70 (2d, 4H, ArAH, anilini-
um protons), 8.71 (s, 1H, ArAOH, phenolate), and 10.74 (b, 1H,
NH+Clꢂ). The 1H NMR spectrum of this complex indicated that
the phenolic proton (AOH) signal, which is observed at
d ꢅ 8.59 ppm in the spectrum of the QL acceptor, decreased in
intensity with a downfield shift for the non-hydrogen-bonded
one (d ꢅ 8.71) in the spectrum of the CT complex. This result indi-
cates the involvement of one AOH group in chelating through the
deprotonation from the QL acceptor to the PC-HCl donor and an
overall decrease of the negative charge on the quinol ring due to
the formation of the complex. In addition, the disappearance of
the ANH2 protons (PC-HCl) and the appearance of a signal at
6.10 ppm indicate the involvement of the ANH2 groups in the com-
plexation process. The 1H NMR spectrum also presents a signal at
d = 10.74, which is assigned to the NH+Clꢂ proton. Based on these
data, the structure suggested for the PC-HCl/QL complex is shown
in Formula II.
m
m
(C@C) (in-ring), aromatic
(C@O) + (C@N)
m
d(CAH) deformation
Ring breathing bands
1453 vs
1363 s
–
1433 vw
1404 m
m
m
(CAH); alkanes; PC-HCl
(C@C) (in-ring), aromatic
d(CAH) deformation
(CAC) + (CAO) + as(CAN)
CAH rock, alkanes; PC-HCl
1352 ms
1333 s
m
m
m
1316 s
1272 vs
1173 s
1116 s
1074 m
1049 m
1285 vw
1205 vw
1117 ms
1044 vw
1270 vs
1179 s
1109 ms
1017 m
ms(CAN); PC-HCl
m(CAO); PC-HCl, QL
d(CAH) in plane bending
drock; NH
850 s
771 vs
997 vw
962 vw
860 vs
861 m
770 m
CAH out of plane bending
drock, CH2 rock
NAH wag skeletal vibrations
808 vw
638 m
473 vs
631 vw
573 vw
476 w
CNC deformation
a
s, strong; w, weak; m, medium; sh, shoulder; v, very; vs very strong; br, broad.
stretching; ms, symmetrical stretching; mas, asymmetrical stretching; d, bending.
The reaction between PC-HCl and PA afforded 4-((2-(diethylam-
monio)ethoxy)carbonyl)benzenaminium 2,4,6-trinitrophenolate
chloride. 1H NMR (DMSO-d6, d ppm) (Fig. 6): d = 1.24 (t, 6H,
2CH3), 3.20 (2q, 4H, 2CH2), 3.66 (t, 2H, N+ACH2), 4.49 (t, 2H,
b
the ANH2 group in the PC-HCl donor to form NHþ3 ammonium
based on acid–base theory [52–54].
In the IR spectra of the [(PC-HCl)(TCNQ)] complex, the charac-
teristic bands of PC-HCl (
a significant decrease in intensity, and the band that results from
OACH2), 4.50 (b, 3H, NHþ hydrogen bonded with picrate), 6.69
3
(d, 2H, ArAH, at C(2 and 6) anilinium ring), 7.73 (d, 2H, ArAH, at
C(3 and 5) anilinium ring), 8.79 (s, 2H, ArAH, picrate protons),
9.82 (b, 1H, NH+Clꢂ). The 1H NMR spectrum of this complex re-
vealed that the signal at d = 11.94 ppm, which is assigned to the
AOH proton of the free picric acid acceptor [55], was absent in
the spectrum of this complex, which is attributed to the formation
of the CT complex. The formation of the complex was confirmed by
the appearance of the characteristic broad signal observed at
4.50 ppm in the spectrum of the complex; this signal is attributed
m
(ANH2); 3361 and 3315 cmꢂ1) exhibit
the m(C„N) vibration of the free TCNQ acceptor that appeared at
2220 cmꢂ1 shifted to 2135 cmꢂ1 in the complex. This observation
clearly indicates that the ANH2 group in the donor and the AC„N
group in the acceptor participated in the complexation process. Be-
cause TCNQ lacks acidic centers, the molecular complexes can be
concluded to form through
from the HOMO of the donor to the LUMO of the acceptor. The
pꢀ CT complex is formed via the benzene ring (electron-rich
p
?
pꢀ and/or n ? pꢀ charge migration
to the formation of (NHþ) with the disappearance of the (ANH2)
3
signal. The 1H NMR confirms that the reaction between PC-HCl
and PA occurred in a 1:1 ratio, and the suggested structure of this
complex is given in Formula III.
p
?
group) of the PC-HCl drug and the TCNQ reagent (electron accep-
tor) [55,56]. However, the absence of a few bands at approximately
2600–2400 cmꢂ1 (hydrogen bonding) in the spectrum of this com-
plex strongly indicates that the interaction mode between PC-HCl
and the TCNQ acceptor occurs through the migration of a H+ ion to
one of the cyano groups in the acceptors to form a positive ion
(AC„N+H) that associates with the anion to form ion pairs
[57,58]. The cyano group (AC„N) is an electron-withdrawing
group that exists in TCNQ in a conjugated bonding system. The
4CN groups in TCNQ withdraw electrons from the aromatic ring,
and such a process will make the aromatic ring an electron-accept-
ing region. The pꢀ–CN electron density appears to increase and
more easily accept a proton from the donor because of the
The 1H NMR spectrum for the CT complex between PC-HCl and
TCNQ is shown in Fig. 6 and is summarized as follows: d = 1.28 (t,
6H, 2CH3), 3.16 (2q, 4H, 2CH2), 3.67 (t, 2H, N+ACH2), 4.57 (t, 2H,
OACH2), 6.41 (s, H, NH+ hydrogen bonded with TCNQ), 6.60 (d,
2H, ArAH ArAH, anilinium protons), 7.04 (2d, 4H, ArAH, anilinium
protons), 7.23 (d, 2H, ArAH, TCNQ C2,C6), 7.33 (d, 2H, ArAH, TCNQ
C3, C5), 7.50 (s, H, ArAH, NHꢂ), 10.14 (b, 1H, NH+Clꢂ). In the
charge-transfer reaction between PC-HCl and TCNQ, the proton
of the ANH2 group in PC-HCl is transferred to a nitrogen atom of
TCNQ to form an ion-paired compound, specifically 4-((2-(diethyl-
ammonio)ethoxy)carbonyl)benzenamidocyano [4-(dicyanometh-
ylene)cyclohexa-2,5-dien-1-ylidene]ethanenitrilium chloride. Two

Abdel Majid A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
33
[(PC-HCl)(QL)] complex
[(PC-HCl)(PA)] complex
4000 3500 3000 2500 2000 1500 1000 500
4000 3500 3000 2500 2000 1500 1000
500
-1
-1
Wavenumbers (cm )
Wavenumbers (cm )
[(PC-HCl)(TCNQ)] complex
4000 3500 3000 2500 2000 1500 1000 500
-1
Wavenumbers (cm )
Fig. 4. Infrared spectra of PC-HCl CT-complexes.
new signals were observed in the 1H NMR spectrum of this CT
complex at 6.41 and 7.50 ppm, which are assigned to the protons
of (NH+) and (NHꢂ), respectively. These signals are not detected
in the spectrum of the free donor, which indicates that the ANH2
and AC„N groups are primarily involved in the formation of the
CT complex between PC-HCl and TCNQ. The migration of the H+
ion from the NH2 in the donor to one of the four cyano groups in
the TCNQ acceptor resulted in the formation of a positive ion
(AC„N+H), which is associated with the anion NHꢂ; this result is
also confirmed from the disappearance of the ANH2 signal in the
spectrum of PC-HCl. Because of the formation of the CT complex
between PC-HCl and TCNQ, the four similar protons on the aro-
matic ring of TCNQ became two doublet signals that appeared at
d 7.23 (C2, C6) and 7.33 (C3, C5) ppm. The intensities and chemical
shifts of the aromatic signals were significantly affected by the
complexation process and the accompanying changes in the struc-
tural configuration. According to these observations, the suggested
structure of PC-HCl/TCNQ complex is illustrated in Formula I.
0.04
0.03
0.02
0.01
0.00
[(PC-HCl)(QL)] complex
0
3500 3000 2500 2000 1500 1000 500
-1
Wavenumbers (cm )
0.20
0.15
0.10
0.05
0.00
[(PC-HCl)(PA)] complex
3.7. Thermogravimetric studies
The thermogravimetric analysis provided information about the
thermal stabilities of the prepared charge-transfer complexes and
about the differences in the physical behavior of the starting and
resulting compounds [59]. The PC-HCl CT complexes are stable at
room temperature and can be stored for several months without
any changes. The obtained complexes were studied by thermo-
gravimetric analysis from ambient temperature to 800 °C under
air atmosphere. The TG curves were redrawn as mg mass loss ver-
sus temperature (TG). Typical TG curves of PC-HCl CT complexes
are presented in Fig. 7, and the thermoanalytical results are listed
in Table 6. Based on the TG curves, the overall loss of mass is 100%
for [(PC-HCl)(QL)], 99.99% for [(PC-HCl)(PA)] and 82.65% for [(PC-
HCl)(TCNQ)].
3500 3000 2500 2000 1500 1000 500
0
-1
Wavenumbers (cm )
Fig. 5. Raman spectra of PC-HCl CT-complexes.

34
A.M. A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
Fig. 6. 1H NMR spectrum of (A) PC-HCl/QL, (B) PC-HCl/PA and (C) PC-HCl/TCNQ complexes.
O
O
NH
Cl
Cl
NH
O
O
H3N
H3N
O
O
O2N
NO2
NO2
Formula III. Suggested structure of the [(PC-HCl)(PA)] complex.
OH
Formula II. Suggested structure of the [(PC-HCl)(QL)] complex.
The obtained data indicate that the [(PC-HCl)(QL)] complex is
thermally stable in the 25–200 °C temperature range. Decomposi-
tion of the complex began at ꢅ200 °C and finished at ꢅ450 °C. The
thermal decomposition of the complex is a one-step process within
the range of 200–800 °C, which is attributed to the loss of 7C2H2,
HCl, 5CO2, NH3, NO2 and 4.5H2 molecules, representing a weight
loss of obs. = 100, cal. = 99.89%. The [(PC-HCl)(PA)] complex began
decomposing at ꢅ110 °C in three clear decomposition steps within
the 25–800 °C temperature range. The first decomposition step
(obs. = 10.19%, calc. = 10.66%) within the temperature range

Abdel Majid A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
35
temperature range 25–350 °C corresponding to loss of HCl, 2CO2,
4HCN and NH3 molecules representing a weight loss of (obs. =
38.11%, calc. = 38.89%). The second decomposition step found
within the temperature range 350–800 °C which corresponds to
the liberation of 6C2H2, NO2 and 2.5H2 molecules with some car-
bon atoms remaining in the final residue.
NC
C
CN
CN
HN
O
O
HN
Cl
NH
3.8. Kinetic thermodynamic studies
Thermodynamic parameters: the Coats–Redfern method [60]
and the Horowitz–Metzger [61] method.
Formula IV. Suggested structure for the [(PC-HCl)(TCNQ)] complex.
3.8.1. Coats–Redfern equation
The Coats–Redfern equation (9), which is an atypical integral
method, can be represented as:
25–190 °C is attributed to the liberation of HCl and NH3 molecules.
The second decomposition step within the 190–315 °C tempera-
ture range (obs. = 52.29%, calc. = 51.80%) is reasonably explained
by the loss of 3C2H2, CO2 and 3NO2 molecules. The third decompo-
sition step existed within the temperature range 315–800, which is
reasonably by the loss of 6C2H2, NO2 and H2 molecules. The ther-
mal analysis curve of the [(PC-HCl)(TCNQ)] complex indicates that
the decomposition occurs in two main stages in the temperature
range of 25–800 °C. The first decomposition step within the
Z
Z
ꢀ
n
da=ð1 ꢂ
a
Þ ¼ ðA=
u
Þ
eꢂE =RT dT
ð9Þ
0!1
T1!T2
For convenience of integration, the lower limit T1 is usually ta-
ken as zero. After integration, this equation can be represented as:
ln½ꢂlnð1 ꢂ
a
Þ=T2ꢃ ¼ ꢂEꢀ=RT þ ln½AR=
u
Eꢀꢃ
ð10Þ
8
7
6
5
4
3
2
1
12
10
8
6
4
2
0
[(PC-HCl)(QL)]
[(PC-HCl)(PA)]
0
0
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
Temp [oC]
Temp [oC]
8
7
6
5
4
3
2
[(PC-HCl)(TCNQ)]
1
0
100 200 300 400 500 600 700 800
Temp [oC]
Fig. 7. TG curves of PC-HCl CT-complexes.
Table 6
Thermal decomposition data for the PC-HCl CT-complexes.
Complex
Stage
TG range (°C)
Mass loss (%)
Found
Evolved moiety
Calculated
99.89
PC-HCl/QL
I
25–800
100.00
7C2H2 + HCl + 5CO2 + NH3 + NO2 + 4.5H2
(C19H27N2O4Cl)
PC-HCl/PA
(C19H24N5O9Cl)
I
II
III
25–190
190–315
315–800
10.19
52.29
37.51
10.66
51.80
37.46
HCl + NH3
3C2H2 + CO2 + 3NO2
6C2H2 + NO2 + H2
PC-HCl/TCNQ
(C25H25N6O2Cl)
I
II
25–350
350–800
–
38.11
44.54
17.35
38.89
43.40
17.61
HCl + 2CO2 + 4HCN + NH3
6C2H2 + NO2 + 2.5H2
7C
Residue

36
A.M. A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
where
a is the fraction of the sample decomposed at time t, T is the
The plot of log[log(w /w )] versus h was constructed and was
a
c
observed to be linear, and the slope, Eꢀ, was calculated. The pre-
exponential factor, A, was calculated from:
derivative peak temperature, A is the frequency factor, R is the gas
constant, Eꢀ is the activation energy, and
u is the linear heating rate.
A plot of the left-hand side (LHS) against 1/T was constructed. Eꢀ is
the energy of activation in kJ molꢂ1 and was calculated from
the slope, and A in (sꢂ1) was calculated from the intercept. The
Eꢀh=RT2s ¼ A=½
u
expðꢂEꢀ=RTsÞꢃ
ð13Þ
From the TG curves, the activation energy, Eꢀ, the entropy of
D
Sꢀ, in (J Kꢂ1 molꢂ1) was calculated using
activation,
D D
Sꢀ, the enthalpy of activation, Hꢀ, and the Gibbs free
entropy of activation,
the equation:
energy,
D
Gꢀ, were calculated from:
Hꢀ ¼ Eꢀ ꢂ RT and
D
Gꢀ ¼
D
Hꢀ ꢂ T
D
Sꢀ
D
Sꢀ ¼ R lnðAh=kTsÞ
ð11Þ
D
The linear curves from the Coats–Redfern and Horowitz–Metz-
where k is the Boltzmann constant, h is Planck’s constant, and Ts is
the DTG peak temperature.
ger plots are shown in Fig. 8, and the evaluated kinetic parameters
for the first stages based on the Coats–Redfern and Horowitz–
Metzger equations are listed in Table 7. The results indicate that
the kinetic data obtained from the two methods are comparable
and in agreement with each other. Based on the results in Table
7, the following conclusions are drawn:
3.8.2. Horowitz–Metzger equation
The Horowitz–Metzger equation (Eq. (12)) was written in the
following form:
Log ½log ðwa=wcÞꢃ ¼ Eꢀh=2:303RT2 ꢂ log 2:303
ð12Þ
1. A higher value of activation energy suggests a higher thermal
stability. The [(PC-HCl)(TCNQ)] complex exhibits a higher acti-
vation energy value, which indicates the higher thermal stabil-
ity of this complex.
s
where h = T ꢂ Ts, w = w ꢂ w, w is the mass loss at the completion
c
a
a
of the reaction, and w is the mass loss at time t.
-12.0
-12.5
-13.0
-13.5
-14.0
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-14.5
PC-HCl/QL, (CR), Stage 1
PC-HCl/QL, (HM), Stage 1
-1.4
0.00165 0.00170 0.00175 0.00180 0.00185
-40 -30 -20 -10
0
10
20
30
1000/T(K-1
θ (Κ)
)
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-12.3
-12.6
-12.9
-13.2
-13.5
-13.8
-14.1
-14.4
PC-HCl/PA, (CR), Stage 1
PC-HCl/PA, (HM), Stage 1
0 20 40 60
0.0019 0.0020 0.0021 0.0022 0.0023 0.0024
-40
-20
1000/T(K-1
)
θ (Κ)
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-12.9
-13.2
-13.5
-13.8
-14.1
-14.4
-14.7
-15.0
PC-HCl/TCNQ, (HM), Stage 1
-60 -40 -20
20 40 60 80
PC-HCl/TCNQ, (CR), Stage 1
0.00140 0.00147 0.00154 0.00161 0.00168 0.00175
0
1000/T(K-1
)
θ (Κ)
Fig. 8. The diagrams of kinetic parameters of PC-HCl complexes using Coats–Redfern (CR) and Horowitz–Metzger (HM) equations.

Abdel Majid A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
37
Table 7
Kinetic parameters determined using the Coats–Redfern (CR) and Horowitz–Metzger (HM).
Complexes
Stage
Method
Parametersa
r
Eꢀ
A
D
Sꢀ
D
Hꢀ
D
Gꢀ
[(PC-HCl)(QL)]
[(PC-HCl)(PA)]
[(PC-HCl)(TCNQ)]
1st
1st
1st
CR
HM
CR
HM
CR
HM
3.31 ꢁ 104
4.02 ꢁ 104
3.32 ꢁ 104
7.09 ꢁ 104
4.80 ꢁ 104
5.63 ꢁ 104
7.56
ꢂ2.32 ꢁ 102
ꢂ2.06 ꢁ 102
ꢂ2.34 ꢁ 102
ꢂ1.80 ꢁ 102
ꢂ2.29 ꢁ 102
ꢂ2.06 ꢁ 102
2.92 ꢁ 104
3.63 ꢁ 104
2.79 ꢁ 104
6.58 ꢁ 104
4.29 ꢁ 104
5.11 ꢁ 104
1.38 ꢁ 105
1.33 ꢁ 105
1.74 ꢁ 105
1.78 ꢁ 105
1.85 ꢁ 105
1.79 ꢁ 105
0.99654
0.99457
0.99654
0.99457
0.98426
0.98012
1.65 ꢁ 102
7.57
4.85 ꢁ 103
1.48 ꢁ 101
2.28 ꢁ 102
a
Units of parameters: E in kJ molꢂ1, A in sꢂ1
,
D
S in J molꢂ1 Kꢂ1
,
D
H and
D
G in kJ molꢂ1
.
2. The activation energy values (Eꢀ) of the PC-HCl complexes
arranged in order of thermal stability are [(PC-HCl)(TCNQ)] >
[(PC-HCl)(PA)] > [(PC-HCl)(QL)].
3. Satisfactory values for the correlation coefficients from the
Arrhenius plots of the thermal decomposition steps were
observed to be r ꢅ 1 in all cases, which indicates a good fit with
the linear function and reasonable agreement between the
experimental data and the kinetic parameters.
Table 8
XRD spectral data of [(PC-HCl)(QL)] and [(PC-HCl)(PA)] complexes.
Complex
2h (°) d Value Full width at
Relative Particle
intensity size
(Å)
half maximum
(FWHM)
(%)
(nm)
[(PC-HCl)(QL)]
[(PC-HCl)(PA)]
17.286 5.126
26.273 3.386
0.30
0.45
100
100
4.880
3.306
4. The thermal decomposition process of all of the PC-HCl com-
plexes is non-spontaneous, i.e., the complexes are thermally
stable.
half the scattering angle (the Bragg diffraction angle), and b is the
full-width at half-maximum (FWHM) of the X-ray diffraction line
(additional peak broadening) in radians.
3.9. X-ray powder diffraction investigation
Table 8 presents the XRD spectral data for the [(PC-HCl)(QL)]
and [(PC-HCl)(PA)] complexes, including the values of the Bragg
angle (2h), the full-width at half-maximum (b, FWHM) of the
prominent intensity peak, the interplanar spacing between atoms
(d), the relative intensity and the calculated particle size (D) in
nm. The particle size of the complexes were estimated according
to the highest value of intensity compared with the other peaks
and were found to be ꢅ5 and ꢅ3 nm for the [(PC-HCl)(QL)] and
[(PC-HCl)(PA)] complexes, respectively. These values confirmed
that the particle sizes are located within the nanoscale range.
To investigate the crystal structures of the obtained PC-HCl CT-
complexes, X-ray powder diffraction patterns in the range of
5° < 2h < 65° for the [(PC-HCl)(QL)] and [(PC-HCl)(PA)] complexes
were examined, and the recorded patterns are shown in Fig. 9.
An inspection of these patterns reveals that all of the systems are
semi-crystalline. As evident from Fig. 9, the main characteristic
scattering peak of the [(PC-HCl)(QL)] complex occurs at 17.286°,
whereas this peak occurs at 26.723° in the diffraction pattern of
the [(PC-HCl)(PA)] complex. Based on these investigations, the
sharp and well-defined Bragg peaks at specific 2h angles confirm
the semi-crystalline nature of the investigated CT complexes. The
particle size of these two complexes were estimated from their
XRD patterns based on the highest intensity value compared with
the other peaks using the well-known Debye–Scherrer formula gi-
ven in Eq. (14) [62]:
3.10. SEM and EDX studies
Scanning electron microscopy (SEM) provides general informa-
tion about the microstructure, the surface morphology, the particle
size, the microscopic aspects of the physical behavior and the chem-
ical composition of the respective PC-HCl charge-transfer com-
plexes and demonstrates the porous structures of the surface of
these complexes. In addition, the chemical compositions of the com-
plexes were determined using energy-dispersive X-ray diffraction
(EDX). SEM surface images of the [(PC-HCl)(QL)], [(PC-HCl)(PA)]
and [(PC-HCl)(TCNQ)] complexes along with their EDX spectra are
shown in Fig. 10. Analysis of the SEM images of the PC-HCl com-
plexes shows that the sizes of the particles are quite different with
different acceptors. Furthermore, these images show a small particle
size with nanoscale features that have a tendency to form agglomer-
ates with shapes that differ from those of the starting materials. The
uniformity and similarity between the particles of the synthesized
PC-HCl complexes indicate that the morphological phases of these
complexes have a homogeneous matrix. Based on these observa-
tions, the [(PC-HCl)(QL)] and [(PC-HCl)(PA)] complexes are semi-
crystalline, and some single-phase formations exhibit well-defined
D ¼ Kk=bCosh
ð14Þ
where D is the apparent particle size of the grains, K is a constant
(0.94 for Cu grid), k is the X-ray wavelength used (1.5406 Å), h is
900
800
700
600
500
400
300
200
100
0
PC-HCl/QL
PC-HCl/PA
shapes with a particle size of ꢅ10
lm. The results also indicate for
the particles of the QL complex are flake-shaped and that those of
the PA complex are stone-shaped, whereas the [(PC-HCl)(TCNQ)]
complex particles exhibits different shapes with a particle size of
ꢅ100
lm and a cracked surface. The chemical analysis results from
10
20
30
40
50
the EDX analysis for the formed complexes show a homogenous dis-
tribution of each acceptor. In the EDX profile, the peaks refer to all
elements that constitute the molecules of these complexes; these
2θ
Fig. 9. X-ray diffraction pattern for [(PC-HCl)(QL)] and [(PC-HCl)(PA)] complexes.

38
A.M. A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
Fig. 10. SEM images and EDX spectra of (A) [(PC-HCl)(QL), (B) [(PC-HCl)(PA)] and [(PC-HCl)(TCNQ)] complexes.
elements are clearly identified, and the results confirm the proposed
structures.
negative bacteria strains, Escherichia coli (E. coli) and Pseudomonas
aeuroginosa (P. aeuroginosa), using the disc diffusion method. Fur-
thermore, the CT complexes were also screened for their antifungal
properties against two species, A. flavus and C. albicans, in DMSO
using the standard agar disc diffusion method. The activity was
determined by measuring the inhibition zone diameter values
(mm) of the investigated complexes against the microorganisms.
Tetracycline was used as a standard drug for comparison of the
3.11. Antimicrobial activities
The antibacterial activity of newly synthesized CT complexes
were tested in vitro against two Gram-positive bacteria strains,
Staphylococcus aureus (S. aureus) and B. subtilis, and two Gram-
Table 9
The inhibition diameter zone values (mm) for PC-HCl CT-complexes.
Sample
Inhibition zone diameter (mm/mg sample)
Bacteria
Fungi
Bacillus subtilis
Escherichia coli
Pseudomonas aeuroginosa
Staphylococcus aureus
Aspergillus
flavus
Candida
albicans
(G+)a
(Gꢂ)
(Gꢂ)
(G+)
Control: DMSO
Standard
Tetracycline (Antibacterial
agent)
Amphotericin B (antifungal
agent)
0.0
34.0
–
0.0
32.0
–
0.0
34.0
–
0.0
30.0
–
0.0
–
0.0
–
18.0
19.0
PC-HCl/QL
PC-HCl/PA
PC-HCl/TCNQ
12.0
12.0
0.0
11.0
12.0
11.0
9.0
13.0
0.0
9.0
12.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
a
G: Gram reaction.

Abdel Majid A. Adam / Journal of Molecular Structure 1030 (2012) 26–39
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Fig. 11. Statistical representation for antibacterial activity of PC-HCl complexes.
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Charge-transfer interactions between the drug procaine hydro-
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7,70,8,80-tetracyanoquinodimethane (TCNQ) as a
p-acceptor were
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e
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