Synthesis, spectroscopic, and computational studies of charge-transfer complexation between benzidine with 2,3-dichloro-5,6-dicyano-p-benzoquinone and chloronil

Article abstract of DOI:10.1002/poc.3700

The solid charge transfer (CT) complexes that have been formed from the reactions of donor benzidine (BZ) and the π-acceptors such as 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and chloronil (CHL) have been studied and characterized experimentally and

Full text of DOI:10.1002/poc.3700

Received: 6 July 2016
DOI 10.1002/poc.3700
Revised: 16 February 2017
Accepted: 24 February 2017
R E S E A R C H A R T I C L E
Synthesis, spectroscopic, and computational studies of charge‐
transfer complexation between benzidine with 2,3‐dichloro‐5,6‐
dicyano‐p‐benzoquinone and chloronil
Arunapriya Lakkadi
| Naveen Baindla | Srimai Vuppala | Parthasarathy Tigulla
Department of Chemistry, Osmania
University, Hyderabad, TS, India
The solid charge transfer (CT) complexes that have been formed from the reactions
of donor benzidine (BZ) and the π‐acceptors such as 2,3‐dichloro‐5,6‐dicyano‐p‐
benzoquinone (DDQ) and chloronil (CHL) have been studied and characterized
Correspondence
Parthasarathy Tigulla, Department of
Chemistry, Osmania University, Hyderabad,
TS 500007, India.
experimentally and theoretically. The experimental work which includes the use of
UV‐visible spectroscopy to identify the CT band of the CT‐complex. The composi-
tion of the complexes has been investigated successfully by using spectrophotomet-
ric titration and Job method of continuous variation to be 1:1. Furthermore, to
calculate the formation constant and molar extinction coefficient, we have used
the Benesi‐Hildebrand equation. Infrared and proton nuclear magnetic resonance
spectral studies were used to characterize and confirm the formation of CT‐complex.
The experimental studies were well supported by quantum chemical simulations by
using density functional theory. The computational analysis of molecular geometry,
Mulliken charges, and molecular electrostatic potential surfaces of reactants and
complexes is very much helpful in assigning the CT route. The C═O bond length
of DDQ and CHL increased upon complexation with BZ. We have also observed
that the substantial amount of charge has been transferred from BZ to DDQ and
CHL in the process of complexation. An excellent consistency has been achieved
between experimental and theoretical results.
Email: sarathychem@gmail.com
KEYWORDS
benzidine, charge transfer complexes, DFT studies, electronic absorption spectra, molecular electrostatic
potential
1 | INTRODUCTION
Molecular CT materials have been paying great attention
because of their attractive and realistic targets in materials
In principle, the charge transfer (CT) interactions are formed
between the electron donors and the electron acceptors,
which involves a resonance with a transfer of charge from
the donor molecule to the acceptor molecule.[1–3] The forma-
tion of CT‐complex is based on the interaction of the highest
occupied molecular orbital (HOMO) of the donor with the
lowest unoccupied molecular orbital (LUMO) of the accep-
tor. In general, the CT complexation occurs as an ionic band
in a simple ion‐radical pair interaction.[4]
science. The formation of molecular CT systems accompa-
nied by the changes in magnetic, dielectric properties, and
structural changes. These CT complexes not only exhibit fas-
cinating optical, electrical, and photoelectrical properties but
also play a vital role in many electrophysical and optical
processes.[5] The CT complexation has acquired much impor-
tance in biochemical and bioelectrochemical energy transfer
processes[6], biological systems[7], drug receptor, binding
mechanism, drug action, enzyme catalysis, ion transfers
J Phys Org Chem. 2017;e3700.
wileyonlinelibrary.com/journal/poc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.3700

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LAKKADI ET AL.
through lipophilic membranes,[8] etc. Certain electron donors
and π‐acceptors have been successfully used in the prepara-
tions of drug molecules in pharmaceutical industry.[9,10]
Although several electron donors have been used widely
in pharmaceutical as well as in material science research
field, the benzidine (BZ; 4,4'‐diaminobiphenyl) and its deriv-
atives are being used extensively in the production of dyes.
Moreover, by using the BZ test, the presence of blood/
haemoglobin in urine (haematuria) can be detected.[11] The
haeme moiety of the haemoglobin has peroxidase‐like activ-
ity that brings about the decomposition of hydrogen peroxide
to liberate the nascent oxygen. The peroxidase activity of
hemoglobin catalyzes the oxidation of BZ to form an intense
blue color charge‐transfer complex. Thus, the BZ is an impor-
tant compound since it forms CT complex in the biological
system. The electron‐donating nature of the BZ can be
explored in combination with the electron acceptors. From
these perspectives, it is necessary to further investigate the
properties of the CT complexes that are formed from the
BZ. On the other hand, the electron acceptors such as 2,3‐
dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) and chloronil
(CHL) are most widely used in chemical synthesis for various
applications.
1H NMR data are expressed in parts per million (ppm), refer-
enced internally to the residual proton impurity in DMSO
solvent.
2.3 | Synthesis and elemental analysis
Two solid CT complexes were synthesized from BZ with
DDQ and BZ with CHL by mixing 1 mmol BZ with 1 mmol
of each acceptor in methanol (10 ml). The reaction mixture
was continuously stirred at room temperature for 20 minutes,
which resulted in the precipitation of the solid CTC1 (BZ‐
DDQ) and CTC2 (BZ‐CHL) complexes. The solid precipi-
tates were filtered, washed several times with methanol.
Finally, the solvent was removed and dried under vacuum
over anhydrous calcium chloride. The data of the elemental
analyses are as follows: CTC1‐mol formula = C20H12Cl2N4O2.
Elemental analysis = C, 58.41; H, 2.94; Cl, 17.24; N, 13.62;
O, 7.78. m/z = 410.03 (100.0%), 412.03 (63.9%), 411.04
(21.6%), 413.03 (13.8%), 414.03 (10.2%), 412.04 (2.2%),
415.03 (2.2%), 411.03 (1.5%), 414.04 (1.4%). Lipinski
rule
=
410.034; 6; 2; 3; 0. CTC2‐ mol for-
mula = C18H12Cl4N2O2. Elemental analysis = C, 50.27; H,
2.81; Cl, 32.97; N, 6.51; O, 7.44. m/z = 427.97 (100.0%),
429.96 (63.9%), 429.96 (63.9%), 431.96 (51.1%), 430.97
(24.9%), 428.97 (19.5%), 433.96 (13.1%), 432.96 (11.9%),
431.96 (10.2%), 431.97 (2.3%), 429.97 (1.8%), 434.96
(1.3%), 434.96 (1.3%), 435.95 (1.0%). Lipinski
rule = 427.965; 4; 2; 1; 0.
The main objective of our work is to investigate the elec-
tron‐donating strength of BZ by studying its CT complexes
that are formed with DDQ and CHL, which are structurally
similar but differs in their ionization potentials (IPs). In
particular, isolation and spectral characterization of CT
complexes were conducted in detail.
2 | EXPERIMENTAL
2.4 | Molecular composition of the CT
complexes
2.1 | Materials and stock solutions
The method of continuous variation, a technique first devel-
oped by Job, was applied to determine the molecular compo-
sition (stoichiometry) of the formed CT complexes. The plot
of absorbance versus mole fraction revealed that the maxi-
mum absorbance was recorded at 0.5 mole fraction, which
indicates the formation of a 1:1 CTC1 and CTC2 complexes
in acetonitrile and methanol solvents, respectively. These
photometric titrations were measured for the reactions of
the BZ donor with DDQ and CHL acceptors at 457 and
443 nm, respectively. A 0.25, 0.50, 0.75, 1.00, 1.50, 2.00,
2.50, 3.00, 3.50, and 4.00 ml aliquots of a standard solution
(5.0 × 10‐4M) of the appropriate acceptor in solvent were
added to 1.00 ml of BZ (5.0 × 10‐4M). The total volume
of the mixture was set to 5 ml. The concentration of BZ
(C1) in the reaction mixture was kept fixed at 5.0 × 10‐4M
while the concentration of the π‐acceptors (C2) changed over
a wide range of concentrations from 0.25 × 10‐4M to
4.0 × 10‐4M, to produce solution varying ratio of donor to
acceptor from 4:1 to 1:4. The stoichiometry of the molecular
CT complexes was determined by the application of
All chemicals used in this study were of analytical grade.
Benzidine, DDQ, and CHL were purchased from Avra
chemicals whereas the acetonitrile and methanol were pur-
chased from Sigma‐Aldrich.
2.2 | Instrumentation and physical
measurements
The electronic absorption spectra of the donor, acceptors, and
the resulted CT complexes were recorded in the region of 350
to 700 nm by using a Shimadzu double beam spectrophotom-
eter with a 1 cm quartz cell. The infrared (IR) spectral mea-
surements (KBr discs) of the solid CT complexes were
conducted on Fourier transform IR spectrophotometer in
the 4000 to 250 cm‐1 range. Proton nuclear magnetic reso-
nance (1H NMR) spectra were obtained on a Bruker DRX‐
250 spectrometer. The measurements were conducted at
ambient temperature using dimethyl sulfoxide (DMSO) as
the solvent and tetramethylsilane as an internal reference.

LAKKADI ET AL.
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conventional spectrophotometric molar ratio according to the
known methods.
2.5 | Computational details
The Gaussian 09W[12] program was used for the density
functional theory (DFT) calculations of the reactants and
complexes in the gas phase. The geometries of the reactants
(BZ, DDQ, and CHL) and CT complexes (CTC1 and
CTC2) were fully optimized at Becke 3‐parameter Lee‐
Yang‐Parr
hybrid
exchange‐correlation
functional
(B3LYP)[13] and 6‐31 + G(d,p) basis set. We have used the
GaussView 5.0.8 to draw the input molecular structures[14]
and also to analyze the electron density distribution in fron-
tier molecular orbitals.
3 | RESULTS AND DISCUSSION
3.1 | Electronic spectra
The absorption spectra of donor‐acceptor mixtures were
recorded, and the CT complexes were characterized by the
appearance of new absorption band, which is attributed to
the n‐π* transition. Mixing of violet colored BZ with pale
yellow–colored DDQ yielded a brownish mixture whose
electronic absorption spectrum showed multiple CT bands
at 423, 442, 457, 547, and 587 nm, which were not observed
in the absorption spectrum of either donor or acceptor
(Figure 1A). These transitions are attributed to the electronic
transitions from more than 1 closely located HOMO to 1 or
more LUMOs. The interaction of BZ with DDQ was allowed
to continue at room temperature, and the absorption spectrum
of the reaction mixture (CTC1) was recorded against blank
solution at 5‐minute intervals. These bands are the character-
istic absorption band of DDQ radical anion.[15] Further, the
gradual decrease in intensity of these CT bands observed in
the 400 to 600 nm spectral regions could be due to the con-
sumption of the ion pair. Three peaks at 423, 442, and
457 nm disappeared and gave a stable peak at 441 nm, and
there was not much difference between the peaks at 547
and 587 nm (Figure 1B).
The BZ solution was mixed with CHL solution, and the
interaction of BZ with CHL was allowed to continue at room
temperature, and the absorption spectrum of the reaction
mixture was recorded against blank solution (Figure 2A). A
violet‐colored product was obtained that absorbs maximum
at 443 nm, and absorption intensity of this new absorption
maximum increased with time (Figure 2B). This gradual con-
tinuous increase in the new absorption band with the reaction
time indicates the formation of the CT‐complex CTC2. The
new absorption band was attributed to the complete electron
transfer from the donor to acceptor accompanied by the for-
mation of colored CHL radical anion,[16] which was formed
FIGURE 1 A, Electronic absorption spectra of the charge transfer
complex (green), 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (black),
and benzidine (red) in acetonitrile. B, Absorption spectra of the reaction
mixture (CTC1) measured at varying time intervals (5‐60 minutes)
against the reagent blank in acetonitrile
by the dissociation of an original donor‐acceptor complex.
The observed absorbance of the CT complex was stable for
more than 60 minutes at room temperature.
3.2 | Reaction with π‐acceptors (DDQ and
CHL)
The chemistry involved in the proposed method is based on
the reaction of the donor (BZ) with the π‐acceptors (DDQ
and CHL) to form CT‐complex, which subsequently dissoci-
ates to form radical anion depending on the polarity of the
solvent used. The complete electron transfer from the donor
to the acceptor moiety takes place with the formation of
intense‐color radical anion,[17] according to the following
equation.
D : þA →
½D : →Aꢀ
→
D·þþA·‐
Charge transfer complex
Radical anion
(1)
The dissociation of the (D−A) complex is generally pro-
moted by the high ionizing power of the polar solvent and the
resulting peaks in the absorption spectra of the reaction mix-
ture are similar to the maxima of the radical anion of the
acceptor obtained from the reduction method.

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LAKKADI ET AL.
FIGURE 3 Job's plot of the charge transfer complex CTC1
FIGURE 2 A, Electronic absorption spectra of the charge transfer
complex (blue), chloronil (black), and benzidine (red) in methanol. B,
Absorption spectra of the reaction mixture (CTC2) measured at varying
time intervals (5‐60 minutes) against the reagent blank in methanol
FIGURE 4 Job's plot of the charge transfer complex CTC2. DDQ,
2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone
3.3 | Composition of the CT complex
We have used the Job continuous variation method[18] to
determine the composition (stoichiometry) of the formed
CT complexes CTC1 (DDQ and BZ) and CTC2 (CHL and
BZ). In this method, we have prepared several solutions in
which the concentrations of the donor and the acceptor are
varied but the sum of the concentrations is kept constant.
Using these solutions, the absorption of the solutions is mea-
sured and plotted versus the mole fraction. The plots of CTC1
and CTC2 obtained from this method have been shown in
Figures 3 and 4, respectively. From the figures, it can be
observed that the maximum absorbance occurs at 0.50 mole
fraction, which indicates 1:1 stoichiometry of donor to accep-
tor for CTC1 and CTC2. On the other hand, the 2 straight
lines produced in the spectrophotometric titration[19] method
are intercepting at 1:1 donor to acceptor ratio (Figures 5 and
6) that also confirms the 1:1 stoichiometry. The same
FIGURE 5 Photometric titration plot for CTC1 system
stoichiometry of CTC1 and CTC2 systems was further con-
firmed by the modified Benesi‐Hildebrand plots as shown
in Figures 7 and 8.

LAKKADI ET AL.
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C1 x C2
A
1
KCT
C1 þ C2
¼
þ
;
(2)
ε
ε
whereas C1 and C2 are the initial concentrations of π‐acceptor
(DDQ or CHL) and donor (BZ), respectively, A is the absor-
bance of the CT band. The data obtained for C1 and C2 then
the values of (C1 + C2) and (C1× C2)/A were calculated. The
plot of (C1× C2)/A Vs (C1 + C2) has been found to be a
straight line with a slope of 1/ε and with an intercept of 1/
KCTε (Figures 7 and 8). The K and ε values for CTC1 and
CTC2 complexes were given in Table 1. The formation con-
stant depends on the nature of the acceptor and donor.
FIGURE 6 Photometric titration plot for CTC2 system
3.5 | Determination of the spectroscopic
physical data
The standard free energy of complexation (ΔG0) for each
complex has been calculated using the formation constant.[21]
The free energy changes (ΔG0) of CTC1 and CTC2 were cal-
culated from the following equation and were given in
Table 1.
ΔG0 ¼ −RT lnKCT
(3)
The results of ΔG0 reveal that the CT complex formation
process is spontaneous. The ΔG0 values are found to be more
negative as the formation constant of the CT complex
increases. As the bond between the components becomes
stronger and thus the components are subjected to more phys-
ical strain or loss of freedom, the ΔG0 value becomes more
negative. The more negative value of ΔG0, the farther to
the reaction will proceed to achieve equilibrium.
FIGURE 7 The modified Benesi‐Hildebrand plot of CTC1
The energy of charge transfer (ECT) for CTC1 and CTC2
have been calculated by using the following equation:[21]
1243:667
ECT ¼ ðhυCTÞ ¼
;
(4)
λCT
whereas λCT is the wavelength of the band (CT complexes).
Generally, these complexes have a high formation constant
and extinction coefficient. These high values of K confirm
the expected high stabilities of the CT complexes because
of the extensive donation from the BZ due to the amino
groups. The formation constants also depend on the nature
of the acceptors.
The IPs of the BZ donor in the complexes were calculated
FIGURE 8 The modified Benesi‐Hildebrand plot of CTC2
using the following empirical equation:[21]
IP ðeVÞ ¼ 5:76 þ 1:53 x 10−4 υCT
;
(5)
3.4 | Formation constant and CT energy of CT
complexes
whereas υCT is the wavenumber in cm‐1 that corresponds to
the CT band formed from the interaction between the donor
and the acceptor. The electron‐donating power of the donor
is measured in terms of its IP, the energy required to the
The modified Benesi‐Hildebrand[20] equation has been used
to calculate the formation constant, K (L mol‐1) and molar
extinction coefficient, ε (L mol‐1 cm‐1) of the CT complexes.

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LAKKADI ET AL.
TABLE 1 Spectrophotometric and free energy change results of CT complexes
KCT
‐ΔG°
ECT
(eV)
ε
Complex
(L mol‐1)
λmax (nm)
(kJ mol‐1)
(L mol‐1 cm‐1)
IP
W
RN
CTC1
CTC2
5.872
4.424
457
443
−3671.43
−4355.99
2.72
2.81
0.446
0.602
9.12
9.21
4.5
0.777
0.802
5.03
removal of an electron from its HOMO. The dissociation
energy (W) of formed CT complex was calculated from the
corresponding CT energy (ECT), the ionization potential of
the donor (IP), and electron affinity (EA) of the acceptor.
3.6 | Infrared spectra
The IR absorption spectra of the BZ and the CT complexes
CTC1 and CTC2 were recorded. The complete IR data are
compiled in Table 2, and the spectra are shown in Figure 9.
The donation process from the BZ to the π‐acceptors can
occur either from the lone pair of electrons on the nitrogen
atom of amino groups or from the aromatic rings.[23,24] The
nitrogen atoms have been identified as the donation source
in most of the cases studied. The IR spectroscopy has been
used to distinguish between the 2 possibilities. In this case,
the υ(N–H), υ(C ≡ N), and δ(N–H) bands of BZ donor were
shifted to lower frequencies on complex formation.
The IR spectrum of the molecular complex of DDQ with
BZ indicates that the single υ(C ≡ N) of the free acceptor
molecule that exhibited at 2250 cm‐1 is shifted to a lower
wavenumber of 2249 cm‐1, while the υ(C = O) absorption
band of the free DDQ at 1674 cm‐1 is shifted to lower value
of 1653 cm‐1 on complex formation.
W ¼ IP−EA−ECT
(6)
Briegleb and Czekalla[22] theoretically derived the fol-
lowing relationship to obtain the resonance energy (RN).
7:7 x 10−4
hυCT=½RNꢀ−3:5
εCT
¼
;
(7)
whereas εCT is the molar absorption coefficient of the CT
complex at the maximum of the CT absorption, υCT is the fre-
quency 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).
Comparison of the IR spectral bands of the free BZ, fre-
quencies at 3327 and 3194 cm−1 correspond to NH2
TABLE 2 Characteristic infrared frequencies (cm‐1) and tentative assignments
BZ
DDQ
CTC1
CHL
CTC2
Assignments
υ(H2O) KBr
3402 s
3196
3354
3424
3219
3401br
m(O‐H)
3327 ms
3194
υ (NH) BZ
3028
3019
2847
υs(C_H)
2824
2851
2249
1653
1598
1572
1273
υas(C‐H)
2251
1674
υ(C ≡ N)DDQ
1687
1588
1686
1590
1570
v(C = O), DDQ, chloronil
δNH2
1607
1583
υ(C = C), DDQ, chloronil
υ(C‐N)
1265
1173
820
υ(C‐C) BZ
υ(CH) out of plane waging
NH deformation
υ(C‐Cl) chloronil
υ(CH) out of plane bending BZ
750
743
752
700
509
711
Abbreviations: BZ, benzidine; CHL, chloronil; DDQ, 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ); s, strong; m, medium; υ, stretching; υs, symmetrical stretching;
υas, asymmetrical stretching; br, broad

LAKKADI ET AL.
7 of 11
FIGURE 9 Fourier transform infrared
spectra of the benzidine (top), CTC1
(middle), and CTC2 (bottom) in the range of
4000 to 400 cm‐1
asymmetric‐ and symmetric‐stretching vibrations and these
were found to be shifted and reduced in intensity after com-
plexation. It shows that the ‐NH2 group in the BZ participates
in the complex formation process. It is known that the ‐CN
group is an electron withdrawing group in DDQ in the conju-
gated system that pulls the electrons from the aromatic ring.
It results in a decrease in bond strength of C N and subse-
quently lowers its vibrational wavenumber during the com-
plex formation. Therefore, it can be concluded that the NH2
group of BZ participates in complex formation with DDQ.
Further, υ(C–Cl) of the free acceptor are also shifted to lower
wavenumber values on complexation. From the IR spectrum
of the molecular complex of CHL with BZ, it has been
observed that the υ(C = O) of the free acceptor molecule that
exhibited at 1686 cm‐1 is shifted to a lower wavenumber
value of 1685 cm‐1, while the υ(C = C) absorption band at
donor to the LUMO of the acceptor. The hydrogen bonding
between the BZ proton (–NH2) and the oxygen of the accep-
tor is strongly supported by the appearance of the character-
istic peaks m(O–H) at the wavelengths of 3401 and
3424 cm‐1 for CTC1 and CTC2 complexes, respectively.
3.7 | 1H NMR spectra
The 1H NMR spectra of the CT complex CTC2 were recorded
in DMSO and shown as Figure 10. Using the 1H NMR stud-
ies, we have identified the nature of interactions between the
donor and the acceptor of the resulted product shown in
Scheme 1. The formation of the complex was confirmed by
the appearance of new signals in the spectrum. The broad
peak observed between 3.0 and 3.5 ppm in the spectrum of
the CTC2 is due to the formation of intermolecular hydrogen
bond between the hydrogen of amine (‐NH2) of BZ and oxy-
gen of the CHL. The peaks observed between 7 to 8 ppm are
due to the aromatic nature of BZ.
1588 cm‐1 is shifted to a lower value at 1570 cm‐1 Since
.
DDQ is derived from any acidic centers, thus we may con-
clude that the molecular complexes are formed through
π–π* and/or n–π* charge migration from HOMO of the

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LAKKADI ET AL.
FIGURE 10 H1 NMR spectra of the charge transfer complex CTC2 obtained in dimethyl sulfoxide
SCHEME 1 Schematic representation of
CTC2 formation from benzidine and
chloronil
found to be 0.393 au. The charges on Cl atoms of both
CHL and DDQ were found to be decreased, and the charges
on both the O atoms were found to be increased upon com-
plexation. The charges on the C11, C12, N13, and N14 atoms
in DDQ increased upon complexation. But on complexation
with BZ, the charges were found to vary from atom to atom
as depicted in Tables S3 and S4. The electronic charges of
N21 and N24 in BZ were found to be −0.197 au, which were
decreased upon complexation with DDQ to −0.079 and
−0.101 au and also decreased upon complexation with
CHL to −0.165 and −0.177 au. This indicates that apprecia-
ble amount of electronic charge has been transferred from BZ
(donor) to DDQ and CHL (acceptors). Therefore, it can be
concluded that the Mulliken charges increased for most of
the atoms of the DDQ and CHL and decreased for most of
the atoms of BZ upon complexation.
4 | COMPUTATIONAL STUDIES
4.1 | Geometry optimizations and charge
distributions
The molecular modeling studies have been conducted to con-
firm the formation of CT complexes (CTC1 and CTC2). The
optimized bond lengths of BZ, DDQ, CHL, CTC1, and
CTC2 complexes in the gas phase, calculated by DFT
(B3LYP) method, are given in Tables S1 and S2, and opti-
mized geometries are shown in Figure 11. It has been
observed that the C═O bond length in DDQ and CHL
increased by 0.017 Å (from 1.240 to 1.257 Å) and 0.005 Å
(from 1.238 to 1.243 Å), respectively. On the other hand,
the C─Cl bond length in DDQ (1.780 Å) and in CHL
(1.783 Å) has been increased upon complexation.[25] Interest-
ingly, the C═C bond length in DDQ and in CHL is either
increased or decreased upon complexation. The C─N bond
length in DDQ increases upon complexation (from 1.173 to
1.174 Å). The most of the bond length parameters of BZ moi-
ety are found to be decreased upon complexation.
4.2 | HOMO‐LUMO calculation for CTC1
and CTC2 complexes in the ground state
The Mulliken atomic charges[26] play a vital role in the
application of quantum mechanical calculations to the molec-
ular systems, especially in CT phenomenon. In DDQ, the
Mulliken charges on the C1/C2 and C4/C5 atoms were found
to be 0.151 and −0.264 au while that of on carbon atoms
attached to oxygen, ie, C3 and C6, was found to be
0.332 au. The Mulliken charges on the C1, C3, C4, and C6
atoms of CHL were found to be −0.260 au and while that
of on carbon atoms attached to oxygen, ie C2 and C5, was
The HOMOs and LUMOs of the complexes CTC1 and CTC2
were obtained using the B3LYP method and are shown in
Figures S1 and S2. The electron density distribution pattern
at BZ and DDQ/CHL can be observed from HOMO and
LUMO. In CTC1, HOMO, HOMO‐2, and HOMO‐3 were
localized only on BZ. While for CTC2, the electron density
is localized only on BZ for HOMO, HOMO‐2, HOMO‐3,
and HOMO‐5 whereas it is localized on the CHL for
HOMO‐4.

LAKKADI ET AL.
9 of 11
FIGURE 11 The optimized ground state geometric structures of (A) benzidine, (B) DDQ, (C) CHL, (D) CTC1, and (E) CTC2 obtained at B3LYP/
6‐31 + G(d,p) level
From Tables S5, it is interesting to note that the LUMO
energy level of the CTC1 complex (−0.17 eV) is in compara-
ble with that of DDQ (−0.18 eV), whereas the HOMO
energy level of the CTC1 complex (−0.19 eV) is close to that
of BZ (−0.18 eV). Similarly, from Tables S6, it has been
observed that the LUMO energy level of the CTC2 complex
(−0.16 eV) is in comparable with that of CHL (−0.18 eV),
whereas the HOMO energy level (−0.18 eV) of the CTC2
complex is close to that of BZ (−0.18 eV). This tendency
for localization of frontier molecular orbitals of CTC1 and
CTC2 systems is very much similar to the previously reported
electron donor‐acceptor composite systems.[27,28] From the
above discussion, it is very clear that the orbital interaction
energy arises mainly from the CT between occupied and
unoccupied orbitals. The molecular electrostatic potentials
(MEPs) are useful for correlation of the relative orientation
of the acceptor and donor molecules in the formation of CT
complex.
molecule.[30] The MEP maps are used to visualize variably
charged regions of a molecule[31] and is a good guide to
assess the reactivity of the molecule towards positively or
negatively charged reactants. The different values of the elec-
trostatic potential are represented with different colors. The
red, blue, and green colors represent regions of most electro-
negative, most positive, and 0 electrostatic potentials, respec-
tively. The electrostatic potential increases in the order of,
red < orange < yellow < green < blue.
Significant changes have been observed for the complex
moieties (CTC1 and CTC2) compared to either BZ or DDQ
or CHL, confirming the CT route. The MEPs were computed
at the same level of theory as used for the geometry optimiza-
tion. As shown in Figure 12, the MEP plot of DDQ was char-
acterized by a positive region (blue) that is localized on the
center of the 6‐membered ring with a surface map value of
0.06305 au while the regions of negative potential were
observed around the nitrogen atoms (−0.03671 au) of the
cyano groups and oxygen atom of the carbonyl group
(−0.02628 au). For the BZ, the negative potential
(−0.03691 au) is mainly associated with the center of the
benzene rings due to the presence of π‐electrons and donation
of lone pair electrons of the amine group, a positive potential
(0.04376 au) was located on the N atom of the amine groups
(Figure 12A). In CTC1, where the BZ interacts with DDQ,
4.3 | Molecular electrostatic potential
The MEP[29] is the most convenient electrostatic property to
study the relation between structure and activity of a mole-
cule that describes the interaction energy between the electri-
cal charges generated from electron and nuclei of the

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LAKKADI ET AL.
FIGURE 12 Molecular electrostatic
potential maps for (A) benzidine, (B) 2,3‐
dichloro‐5,6‐dicyano‐p‐benzoquinone, (C)
chloronil, (D) CTC1, and (E) CTC2 in ground
state obtained from density functional theory
calculations
Back View
Side View
Front View
FIGURE 13 The charge density difference plot of the charge transfer complex CTC1
the center positive value of DDQ decreased to 0.02130 au, N
atom of cyano group potential and O atom of the carbonyl
group potential increased to −0.05063 and −0.04245 au,
respectively. The center of the benzene ring potential
decreased to −0.00800 au and N atom of amine group in
BZ potential increased to 0.04909 au upon the CT interaction
(Figure 12D). In CHL, a positive region (blue) was localized
on the center of the 6‐membered ring with a maximum sur-
face value of 0.04754 au while the region of negative poten-
tial (red) was observed around the oxygen atom of the
carbonyl group (−0.03466 au). In CTC2, where the BZ inter-
acts with CHL, the center surface value of CHL decreased to
0.03310 au. The O atom of carbonyl group potential
(−0.04106 au) and N atom of amine group in BZ potential
(0.04958 au) increased upon the CT interaction (Figure 12
E). These results further reinforce the concept that DDQ
and CHL are generally considered as good electron acceptors
in forming CT complexes with various donors.[32]
clearly observed that the charge is deposited on DDQ and is
depleted from BZ molecule upon the formation of CT
complex.
5 | CONCLUSIONS
Charge transfer reactions of BZ with the DDQ and CHL were
investigated experimentally and theoretically. The spectro-
scopic and physical parameters of these complexes were eval-
uated. Ultraviolet–visible, IR, and 1H NMR spectra were
used to characterize the complexes in solution and solid
states. The computational investigation has been conducted
using DFT calculations. The computed geometrical parame-
ters (bond lengths) have shown variation for the complexes
as compared with those of individual reactants. The Mulliken
atomic‐charge analysis revealed that the increased values for
the acceptor and the decreased values for the donor in the
complex as compared with the corresponding isolated spe-
cies, asserting the CT‐complex formation. In addition, the
suggested positioning of the acceptor molecule with respect
to the donor molecule was confirmed theoretically from the
The charge density difference plot of the CT complex
CTC1 has been given in Figure 13. The yellow region repre-
sents charge accumulation whereas the light blue region
represents the charge depletion. From the figure, it has been

LAKKADI ET AL.
11 of 11
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the donor (BZ) to the acceptors (DDQ and CHL) in CT com-
plexes is mirrored in MEPs. The obtained experimental
results were in good accordance with the computational
study. From the results, it is concluded that the CT is taking
place from BZ to DDQ and BZ to CHL.
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One of the authors, AL, is thankful to the UGC, New Delhi,
for granting fellowship, and to The Head, Department of
Chemistry, Osmania University, for providing facilities for
the work.
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Vuppala S, Tigulla P. Synthesis, spectroscopic, and
computational studies of charge‐transfer complexation
between benzidine with 2,3‐dichloro‐5,6‐dicyano‐p‐
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