Synthesis, spectral investigations, antimicrobial activity and DNA-binding studies of novel charge transfer complex of 1,10-phenanthroline as an electron donor with π-acceptor p-Nitrophenol

Article abstract of DOI:10.1016/j.molstruc.2010.05.031

Proton or charge transfer (CT) complex of donor, 1,10-phenanthroline (Phen) with π-acceptor, p-Nitrophenol (PNP) has been studied spectrophotometrically in methanol at room temperature. The binding of the CT complex with calf thymus (ct) DNA has been inve

Full text of DOI:10.1016/j.molstruc.2010.05.031

Journal of Molecular Structure 977 (2010) 189–196
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectral investigations, antimicrobial activity and DNA-binding studies
of novel charge transfer complex of 1,10-phenanthroline as an electron donor with
p-acceptor p-Nitrophenol
*
Ishaat M. Khan , Afaq Ahmad
Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh 202002 UP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Proton or charge transfer (CT) complex of donor, 1,10-phenanthroline (Phen) with p-acceptor, p-Nitrophe-
Received 30 March 2010
Received in revised form 22 May 2010
Accepted 24 May 2010
nol (PNP) has been studied spectrophotometrically in methanol at room temperature. The binding of the CT
complex with calf thymus (ct) DNA has been investigated by fluorescence spectrum, to establish the ability
of the CT complex of its interaction with DNA. Stern–Volmer quenching constant (Ksv) has also been calcu-
Available online 1 June 2010
lated. The formation constant (KCT), molar extinction coefficient (eCT), free energy (D
Go) and stoichiometric
ratio of the CT complex have been determined by Benesi–Hildebrand equation. The stoichiometry was
found to be 1:1. The CT complex was screened for its pharmacology as antibacterial and antifungal activity
against various bacterial and fungal strains, showing excellent antibacterial and antifungal activity. The
newly synthesized CT complex has been characterized by FTIR spectra, elemental analysis, 1H NMR, elec-
tronic absorption spectra. TGA–DTA studies were also carried out to check the stability of CT complex.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Charge transfer
DNA-binding
Pharmacology
UV-vis
Fluorescence spectroscopy
1H NMR
1. Introduction
gands Phen and its derivatives have been the subject of research
interests in recent years [15–18] due to the potential application
Charge transfer interaction within a molecular complex formed
from an electron donor, D and electron acceptor, A, involves reso-
nance with a transfer of charge from D to A [1,2]. Therefore, the ex-
cited state of the complex is easily achieved by direct excitation in
the charge transfer band of the complex [3]. In general, the charge
transfer complexation occurs as an ionic bond in simple low-radi-
cal pair interaction. The charge transfer complexes play the impor-
tant roles in many biological systems such as DNA-binding,
antibacterial, antifungal, insecticides and ion transfer through lipo-
philic membranes [4–10]. Besides, CT complexes act as intermedi-
ates in a wide variety of reactions involving nucleophiles and
electron deficient molecules [11]. They can also be used in the
study of drug acceptor binding mechanism [10]. The formation of
the charge transfer complexes was utilized in the development of
simple, rapid and accurate spectrophotometric methods for the
analysis of ganciclovir in pure form as well as in its pharmaceutical
formulation [12,13]. The fluorescence spectroscopy method has
been used to investigate the interaction of selected antibiotic com-
pounds with DNA [14]. The redox properties of CT complexes have
been found to promote DNA-binding. The bidentate nitrogen li-
as DNA cleaving agents and nonradioactive nuclei acid probes.
Many priority phenols are known for their toxicity and carcinoge-
netic character [19,20].
p
?
pꢀ and n ? pꢀ molecular complexes of Phen derivatives
with chloranil, picric acid and chloranilic acid were investigated
spectrophotometrically [21]. The spectral data of the complexes
obtained on the reaction of iodine with twin site donors such as
1,10-phenanthroline and its methyl and chloro derivatives, 1,7-
and 4,7-phenanthroline and 2,20-bipyridine were also obtained
[22]. Charge transfer interactions of
p-acceptor p-Nitrophenol
(PNP) and other phenols with N,N0-bis [2-hydroxyethyl]-1,4,6,8-
napthalenediimide (BHENDI) were investigated spectrophotomet-
rically and spectroscopically to obtain their important properties
[23]. The interest to study these particular organic compounds
was to have an idea of them pharmacology. These compounds
are used as parent material in pharmaceutical insecticides and
the excess of thiocarbamides can be determined as charge transfer
complexes. The Phen and its derivatives show high catalytic activ-
ity [24–26]. The importance and applications of Phen and PNP,
prompted us to synthesize and carry out spectrophotometric stud-
ies of CT complex forward from Phen and PNP. The studies were
also detected towards DNA-binding and antimicrobial activities.
This paper is in continuation of our studies on charge transfer
interactions [27–29]. Herein, the synthesis and behavior of CT com-
* Corresponding author. Tel.: +91 9412174753, +91 5712703515.
E-mail addresses: drishaatamu@gmail.com (I.M. Khan), drafaqahmad7@gmail.
com (A. Ahmad).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.05.031

190
I.M. Khan, A. Ahmad / Journal of Molecular Structure 977 (2010) 189–196
plexation between Phen with PNP as
p
-acceptor are reported. The
bance ratio A260/A280 (ꢃ1.8). Fluorescence measurements were
made with a 150 W xenon lamp and a slit width of 5 nm. A fixed
concentration of the compound (30 M) was taken in a quartz cell
and increasing amounts of ct-DNA solution was titrated using
increments of 5 M. The fluorescence spectra were recorded in
stoichiometry and other physical parameters of the charge transfer
complex under investigation were determined using Benesi–Hilde-
brand methods [30]. Studying the CT complex interaction with DNA
is one of the most important aspects in biological investigations
aimed at discovering and developing new type of antiproliferative
agents [31]. DNA-binding studies of synthesized CT complex were
done by fluorescence spectroscopy. The CT complex was also
screened for its antifungal and antibacterial activity against various
strains. The CT complex was characterized by instrumental tech-
niques such as FTIR, 1H NMR, TGA–DTA, and electronic absorption.
l
l
the range of 300–450 nm upon excitation at 280 nm at 310 K.
2.3.4. Antibacterial activity
The antibacterial activity of newly synthesized CT complex was
tested in vitro against two Gram-positive bacteria Staphylococcus
aureus (MSSA 22) and Bacillus subtilis (ATCC 6051) and two
Gram-negative bacteria Escherichia Coli (K 12) and Pseudomonas
aeruginosa (MTCC 2488) strains using disc diffusion method
[32,33]. The discs measuring 5 mm in diameter were prepared
from Whatmann No. 1 filter paper sterilized by dry heat at
140 °C for 1 h. The sterile discs previously soaked in a concentra-
tion of the test complex were placed in a nutrient agar medium.
The plates were invested and kept in an incubator for 24 h at
37 °C. The inhibition zone thus formed was measured (in mm) after
2. Experimental
2.1. Materials and reagents
1,10-Phenanthroline (Merck), p-Nitrophenol (Thomas Baker),
Tetracycline, Nystatin and agar (Hi-Media, Mumbai) were used as
supplied. Analytical grade (AR) methanol (Merck), DMSO (Merck)
and HCl (Merck) were used without any further purification. Calf
thymus DNA (ct-DNA) was purchased from Sigma (USA).
24 h. The screening was performed for 100 lg/ml concentration of
test CT complex and antibiotic disc, Tetracycline (30
lg/disc, Hi-
Media) was used as control.
The nutrient broth which, logarithmic serially twofold diluted
amount of test CT complex and controls, was inoculated within
the range 10ꢂ4–10ꢂ5 cfu/ml. The cultures were incubated for 24 h
at 37 °C and growth was monitored visually and spectrophotomet-
rically like [34]. The lowest concentration (highest dilution) re-
quired to arrest the growth of bacteria is regarded as minimum
inhibitory concentration (MBC). To obtain the diameter of zone,
0.1 ml volume was taken each and spread on agar plates. The num-
ber of colony forming units (cfu) was counted after 24 h of incuba-
tion at 35 °C.
2.2. Analyses
The electronic absorption spectra of the donor (Phen), acceptor
(PNP) and the resulting CT complex in methanol were recorded at
room temperature in the region of 700–200 nm using a Shimadzu
UV–vis Spectrophotometer model UV 1700 Pharma Spec with a
1 cm quartz cell paths length and DNA-binding studies of synthe-
sized CT complex were carried out by RF-5301 fluorescence Spec-
trometer. FTIR spectra of reactants and the CT complex were
recorded using KBr discs on the FTIR Spectrometer model Spectro-
spec 2020, 1H NMR spectra of the CT complex, donor and acceptor
were recorded in DMSO using Bruker advance II 400 NMR Spec-
trometer and elemental analysis were done by Elementar Vario
EL III Carlo Erba 1108. The thermal analyses (TGA and DTA) was
carried out under nitrogen atmosphere with a heating rate of
20 °C/min for thermogravimetric analysis (TGA) and DTA using
Shimadzu model DTG-60H Thermal Analyzers.
2.3.5. Antifungal activity
The newly synthesized CT complex was also screened for its
antifungal property against Aspergillus niger (Laboratory isolate),
Candida albicans (IQA-109) and Penicillium sp (Laboratory isolate)
in DMSO using standard agar disc diffusion method [35]. The syn-
thesized CT complex was dissolved in DMSO. All cultures were rou-
tinely maintained on Sabouraud’s dextrose agar (SDA, Hi-media,
Mumbai) and incubated at 28 °C. Spore of fungal stain lawning
was formed from 7 days old culture on sterile normal solution
and diluted to obtain approximately 105 cfu/ml. The culture was
centrifuged at 1000 rpm, pellets was resuspended and diluted in
sterile NSS to obtain a viable count 105 cfu/ml. The inoculum of
non-sporing fungi C. albicans was performed by growing the cul-
ture in SD broth at 37 °C overnight. With the help of spreader,
0.1 ml volume of the approximately diluted fungal culture suspen-
sion was taken and spread on agar plates. The fungal activity of CT
2.3. Procedures
2.3.1. Preparation of standard stock solutions
A standard solution of 1,10-phenanthroline (donor) at a concen-
tration of 1 ꢁ 10ꢂ2 M was prepared in different volumetric flask by
dissolving 0.099 g of pure Phen in 50 ml of methanol and diluting
with the same solvents and a standard solution of PNP 10ꢂ2
M
(acceptor) was prepared by dissolving 0.069 g of pure PNP in a
50 ml volumetric flask using methanol.
complex was compared with Nystatin (30 lg/disc Hi-Media) as
standard drug. The cultures were incubated for 48 h at 37 °C and
the growth was monitored. Antifungal activity was determined
by measuring the diameters of the zone (mm) in triplicates sets.
2.3.2. Synthesis of the solid CT complex
The solid CT [(Phen)(PNP)] product was synthesized by mixing
the saturated solution of PNP (0.139 g, 1 mmol) in CH3OH (10 ml)
with a saturated solution of (0.198 g, 1 mmol) in CH3OH (10 ml).
The resulting solution was stirred for 15 min with heating at
50 °C and kept for 4 days at room temperature. The solid CT com-
plex was separated as needle of off white CT complex which was
filtered off and dried under vacuum over CaCl2. The result of ele-
mental analysis are: (theoretical values are shown in brackets):
[(Phen)(PNP)], (C18H15N3O4) CT complex (M/w: 337.29 g): C,
63.99% (64.09%); H, 4.47% (4.48%); N, 12.40% (12.46%).
2.3.6. Determination of stoichiometry and formation constant
Stoichiometry is determined using the Benesi–Hildebrand meth-
od (straight linemethod) from stock solutions of acceptor and donor.
Ontheotherhand, theformation constant(KCT)and molarextinction
coefficient of the CT complex determined by Benesi–Hildebrand
equation [30] valid under the condition [A]0 ꢄ [D]0 or [D]0 ꢄ [A]0,
for 1:1 donor–acceptor complexes [36–38]. The concentration of do-
nor was kept fixed at 4.5 ꢁ 10ꢂ5 M and that of the acceptor varied
from 0.8 ꢁ 10ꢂ4 to 7.56 ꢁ 10ꢂ4 M. The electronic absorption spectra
of the complex were measured in methanol at room temperature.
The preparation of the molecular complex and the working proce-
dure has been described elsewhere [39,40].
2.3.3. DNA-binding
Calf thymus DNA was dissolved in tris–HCl buffer (0.1 M, pH
7.4). The purity of the DNA solution was checked from the absor-

I.M. Khan, A. Ahmad / Journal of Molecular Structure 977 (2010) 189–196
191
tively electron poor molecules. When a solution contains both an
electron rich and electron poor compound, they tend to associate
with one another in loose interaction known as electron-donor–
acceptor (EDA) complexes [42]. The new, low energy absorptions
are observed in solutions containing both a donor and an acceptor
3. Results and discussion
3.1. Observation of CT bands and determination of formation constant
Taking into consideration the presence of an electron donating
group in the electron donor system under investigation, on mixing
the methanol solution of the donor and acceptor according to the
condition [A]0 ꢄ [D]0 or [D]0 ꢄ [A]0 for 1:1 donor–acceptor com-
plexes [36–38], different wavelength, kCT, of CT transition is ob-
served relative to the wavelengths of donor and acceptor. This is
further supported by the measurement of the absorption band of
CT complex of Phen with p-acceptor PNP in methanol which pro-
duced CT characteristic broad bands in the UV region. In this re-
gion, both donor and acceptor are not absorbing.
The electronic absorption spectra of charge transfer complexes
formed from different concentration and excess concentration of
PNP with fixed concentration of Phen were recorded in methanol
at room temperature and are shown in Fig. 1. The data for forma-
tion constant and molar extinction coefficient are reported in Table
1. The increase in absorption intensities for CT complex an addition
of the acceptor part in the reaction mixture are reported in Table 1.
The new absorption bands are observed at 316 nm and 233 nm
which are neither of donor nor of acceptor and band for maximum
absorbance have been consider as CT band [41] which is formed
due to transfer of charge. It has shown to the transfer of phenolic
proton from acceptor to donor by way of hydrogen bonding, as-
signed as N+AH–O–. The absorption intensities of the new bands
increase with increases in the concentration of the acceptor in
the mixture. The Phen is relatively electron rich and PNP is rela-
0.000016
0.000014
0.000012
0.000010
0.000008
0.000006
0.000004
0.000002
0
2000 4000 6000 8000 10000 12000 14000
1/[A]0
Fig. 2. Benesi–Hildebrand plot of the charge transfer complex of 1,10-phenanthro-
line with p-Nitrophenol, [D]o/A vs. 1/[A]o in methanol at room temperature.
Fig. 3. Fluorescence emission spectra of CT complex of 1,10-phenanthroline and p-
Nitrophenol in the absence (—) and presence of increasing amount of DNA from (a)
to (i) (—).The arrow shows the intensity changes on increasing the CT complex
concentration.
Fig. 1. Electronic absorption spectra of mixture of charge transfer complexes of; (1)
Phen (4.5 ꢁ 10ꢂ5 M)
+
PNP (0.8 ꢁ 10ꢂ4 M); (2) Phen (4.5 ꢁ 10ꢂ5 M)
+ PNP
(1.06 ꢁ 10ꢂ4 M); (3) Phen (4.5 ꢁ 10ꢂ5 M) + PNP (1.2 ꢁ 10ꢂ4 M); (4) Phen
(4.5 ꢁ 10ꢂ5 M) + PNP (1.8 ꢁ 10ꢂ4 M); (5) Phen (4.5 ꢁ 10ꢂ5 M) + PNP (2 ꢁ 10ꢂ4 M);
(6) Phen (4.5 ꢁ 10ꢂ5 M) + PNP (2.64 ꢁ 10ꢂ4 M) and (7) Phen. (4.5 ꢁ 10ꢂ5 M) + PNP
(7.56 ꢁ 10ꢂ4 M) in methanol at room temperature, are shown with increasing
concentration of acceptor from bottom to top.
Table 1
Absorption data for spectrophotometric determination of stoichiometry and forma-
tion constant (KCT) and molar extinction coefficient (eCT) of the CT complex of Phen
and PNP in methanol at room temperature.
Concentration Concentration Absorbance Formation
Molar extinction
coefficient (eCT
of acceptor
of donor (M) at kCT
316 nm
constant
)
(ꢁ10ꢂ4 M)
(KCT) l molꢂ1 l cmꢂ1 molꢂ1
2.203 ꢁ 103 4.401 ꢁ 105
0.80
1.06
1.20
1.80
2.00
2.64
7.56
4.5 ꢁ 10ꢂ5
0.406
0.513
0.588
0.769
0.935
1.090
4.118
Fig. 4. Stern–Volmer plot for the binding of of CT complex of 1,10-phenanthroline
and p-Nitrophenol with DNA.

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I.M. Khan, A. Ahmad / Journal of Molecular Structure 977 (2010) 189–196
Table 2
Antibacterial and antifungal activity of synthesized 100
lg/ml concentration of novel CT complex of Phen and PNP.
Diameter of zone of inhibition in mm at 100 g/ml and DMSO as control
l
Bacteria
CT complex
Tetracycline
Fungi
CT complex
Nystatin
Staphylococcus aureus
Bacillus subtilis
Escherichia Coli
35.3 ( 1.89)
30.7 ( 1.20)
26.0 ( 0.82)
21.5 ( 1.33)
20 ( 0.23)
21 ( 0.55)
25 ( 0.88)
24 ( 0.63)
Aspergillus niger
Candida albicans
Penicillium sp
46 ( 1.99)
48 ( 1.88)
45 ( 1.33)
24 ( 0.24)
23 ( 0.58)
25 ( 0.88)
Pseudomonas aeruginosa
Table 3
Infrared frequenciesa (cmꢂ1) and band assignments for (A) Phen (B) PNP and (C)
[(Phen) (PNP)] compound.
A
B
C
Assignmentsb
3321
m
s, br
3384 m, br
3064 w
2955 w
m
m
m
m
(OAH) of PNP; CTC
(CAH); aromatic
(C@C); CH aromatic
s(CAH); CH2 A CH3
m
(+NH)
3059 w
2960 w
3084 w
2980 w
2948 w
2950 w
2930 w
2878 w
Hydrogen bonding
2811 w
2744 mw
2682 w
2625 w, br
1650 sh
1690 w
1613 mw
m
m
(C@C); aromatic
as(NO2); PNP
1589
1492
m
m
s
s
1592
1494
m
m
s
s
1587 m
1509
d(CH); CH def
1500
1463 sh
d(CH); aromatic
1422
1339 w
m
s
1325
1278
1218
1169 s
1106
1007 w
m
m
m
s
s
s
s
1422 s
1334 s
1303 s
1270 w
1214 w
1187 w
m
m
m
as(CN)
Fig. 5. FTIR spectra of (A) 1,10-phenanthroline; (B) p-Nitrophenol; (C) Charge
transfer complex of p-Nitrophenol and 1,10-phenanthroline.
(CAO); CAOH
s(NO2); PNP
by Mulliken [43] as charge transfer interactions involving the exci-
tation of an electron on the donor to empty orbital on the acceptor.
The formation constant (KCT) for the CT complex of donor–acceptor,
with molar extinction coefficient (eCT) were determined by the Benesi–
Hildebrand [30] equation using the absorbance of the CT complex.
m
1138 mw
1091
1037 mw
988 w
1102
m
s
m
s(CN)
1094 sh
1042 w
1009 w
889 w
963 w
850 ms
819 w
d(CH); in-plane bend
854
m
s
837 s
840 ms
½Dꢅ0=A ¼ 1=KCTeCT ꢆ 1=½Aꢅ0 þ 1=eCT
ð1Þ
778 w
762 w
740 s
694 w
622 m
637 mw
757 s
766 w
752 m
730 s
d(CH); CH-rock
d(NH); NH def
where [A]0 and [D]0 are the initial concentrations of the acceptor
and donor, respectively, A is the absorbance of the donor–acceptor
mixture at kCT measured against the solvent as reference and eCT is
the molar extinction coefficient. KCT is the formation constant of the
CT complex. In this case a typical linear plot is obtained and is
shown in Fig. 2. The correlation coefficient for this plot was about
0.9. The values of eCT and KCT are determined from the slop of such
627
694 s
628
m
s
s
711 w
688 w
634 w
622 w
535 w
493 w
465 w
405 w
m
525
447
411
536 mw
496 s
417 w
d(CH); out-of-plan
plot and are 4.401 ꢁ 105 l cmꢂ1 molꢂ1 and 2.303 ꢁ 103 l molꢂ1
,
a
respectively. The transition energy of CT complex is calculated to
be 3.935 eV. The values of the formation constant (KCT) are depen-
dent on the nature and geometry of acceptors and donors. The stoi-
chiometry of the CT complex was obtained as the straight line of
Benesi–Hildebrand plots as shown in Fig. 2 and was found to be
1:1, similar to the stoichiometry of CT complexes of derivatives of
1,10-phenanthroline with aromatic nitrophenols [10,44].
br, broad; m, medium; s, strong; sh, shoulder; w, weak;
bending.
m, stretching, d,
b
Stretching and bending.
3.3. Pharmacology
The synthesized CT complex to be tested was dissolved in
DMSO to obtain 100 g/ml stock solutions. The diameter zones
l
were measured which exhibited the growth of tested microorgan-
ism. The CT complex showed excellent microbial activities against
various bacterial and fungal strains.
3.2. Determination of free energy (D
Go)
The values of KCT were used to calculate ꢂ
D
Go values according
ð2Þ
to the equation [45,46].
3.3.1. DNA-binding studies of CT complex using fluorescence
spectroscopy
The fluorescence quenching technique is often used to monitor
the molecular interactions because of its high sensitivity [47–49].
The interactions of CT complex of Phen and PNP, with calf thymus
D
Go ¼ ꢂRT ln K
The free energy, ꢂ
D
Go, iscalculated to be21.054 kJ molꢂ1, indicat-
ing that the formation ofthe complex is exothermic and spontaneous.

I.M. Khan, A. Ahmad / Journal of Molecular Structure 977 (2010) 189–196
193
Fig. 6. 1H NMR spectrum of charge transfer complex of 1,10-phenanthroline with p-Nitrophenol.
DNA was studied by fluorescence spectroscopy. Fluorescence
quenching is usually classified as dynamic quenching and static
quenching [50]. Dynamic quenching results from collision between
fluorophore and quencher whereas static quenching is due to

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I.M. Khan, A. Ahmad / Journal of Molecular Structure 977 (2010) 189–196
OH
N+
N
N
O−
H
+
N
NO2
NO2
N+
H
O−
N
NO2
Scheme 1. Proton transfer mechanism between 1,10-phenanthroline and p-Nitrophenol.
ground-state complex formation between fluorophore and
quencher [51]. However the characteristic Stern–Volmer plot of
combined quenching (both static and dynamic) is an upward cur-
vature. The binding of CT complex with calf thymus DNA, was
studied by monitoring the changes in the intrinsic fluorescence
of CT complex at varying DNA concentrations. Fig. 3 gives the rep-
resentative fluorescence emission spectra of the CT compound
upon excitation at 280 nm. The addition of DNA caused a gradual
decrease in the fluorescence emission intensity of the compound
suggesting changes in the microenvironment of the fluorophore
upon interaction with DNA. CT complex has effect on the fluores-
cence intensity of ct-DNA being quenched. To determine the
DNA-binding ability of the compound, fluorescence intensity data
were analyzed by the Stern–Volmer equation [52].
The CT complex exhibited good inhibitory results against E. coli
and P. aeruginosa as reported in Table 2.
3.3.3. Antifungal activity studies
The synthesized CT complex was also examined for its anti-
fungal activity and Nystatin was used as standard drug for compar-
ison of antifungal results. The test CT complex exhibited excellent
inhibitory results for A. niger (Laboratory isolate), C. albicans (IQA-
109) and Penicillium sp (Laboratory isolate) as given in Table 2. The
data revealed that the CT complex has produced the marked
enhancement in the potency as antifungal agent.
3.4. Comparative study of FTIR of CT complex and reactants
FoF ¼ 1 þ Ks
v
½Qꢅ
ð3Þ
A careful investigation of the important characteristic peaks of
the FTIR spectra of Phen, PNP and their 1:1 CT complex are re-
corded and shown in Fig. 5. Assignments of the characteristic infra-
red spectral bands of the free acceptor and donor as well as the
formed CT complex are reported in Table 3. It is observed that
the formation of CT complex is strongly supported by observing
the main infrared bands of the reactant Phen and PNP in the prod-
uct spectrum. It is observed that the positions of the most bands of
donor and acceptor in the complex spectrum show shift in the fre-
quency and changes in their band intensities compared with those
of the free Phen and PNP. These changes strongly support the for-
mation of CT complex and could be attributed to the expected
symmetry and electronic structure modifications in both donor
and acceptor units in the formed product relative to the free mol-
ecules. In general, spectra the CT complex show the characteristic
bands due to various modes of vibration of the acceptor part, shift
to lower wavenumber while those of the donor part acquire a
counter shift. The shift of the nCH bands of the donor to higher
wavenumber is considered to be a criterion for CT complex inter-
action [53] involving the transfer of an electron from HOMO of
the donor to the LUMO of the acceptor or transfer of proton from
acceptor to donor. These types of bands are due to the stretching
where F and F0 are the fluorescence intensity with and without the
quencher (ct-DNA), Ksv the Stern–Volmer quenching constant, and
[Q] the concentration of the quencher. The Ksv for the CT complex
was obtained from the slope of the Stern–Volmer graph as shown
in Fig. 4 and was found to be 1.4 ꢁ 104 l Mꢂ1 which depicts that
the quenching may be static or dynamic.
3.3.2. Antibacterial activity studies
The biological activities of the synthesized CT complex have
been studied for its antibacterial and antifungal activities using
disc diffusion method [32,33] in vitro against two Gram-positive
bacteria Staphylococcus auras (MSSA 22) and B. subtilis (ATCC
6051) and two Gram-negative bacteria E. Coli (K 12) and P. aerugin-
osa at concentration of 100 lg/ml. Tetracycline was used as stan-
dard drug for the comparison of bacterial results and screening
data are given in table. The newly synthesized CT complex have ex-
erted significant inhibitory activity against the growth of the tested
bacterial strains and data reveal that CT complex have significant
influence on the antibacterial profile of S. aureus and B. subtilis.

I.M. Khan, A. Ahmad / Journal of Molecular Structure 977 (2010) 189–196
195
peaks appear at 3321 cmꢂ1 (very strong, broad), 1589 cmꢂ1 (very
strong) and 1613 cmꢂ1 (medium, weak), respectively for PNP
which is more acidic than alcohol. The infrared spectrum of CT
complex shows a medium broad band for the N+AHꢆꢆꢆOꢂ within
the range 3224–3500 cmꢂ1 and stretching vibrations of the C@N,
C@C and NO2 peaks appear at 1422 (very strong), 1650 cmꢂ1 (sh)
and 1592 cmꢂ1 (very strong), respectively. It is also observed that
some peaks for Phen and PNP do not appear in the IR spectra of
CT complex of them. This behavior is in accordance with the charge
migration from the donor to the acceptor [55,56], which give an
additional evidence for interaction between Phen and PNP. There-
fore, the FTIR data provided evidence for the existence of a new
bands of medium weak intensity in the spectra of CTC prepared
and indicated that the formation of quaternary amine species i.e.,
+NH, from which a labile proton is expected. Therefore, these re-
sults reveal that the CT complex has just some changes in their
band intensities and shift of some band frequency values.
3.5. 1H NMR spectra
The nuclear magnetic resonance (1H NMR) spectra of the CT
complex, donor and acceptor were recorded in DMSO. The spec-
trum CT complex is shown in Fig. 6. The 1H NMR spectrum of the
CT complex was compared with the reactants and structure of do-
nor, acceptor and proposed CT complexation are given in scheme 1.
The changes in the chemical shift values of donor and acceptor
moieties in the 1H NMR spectrum of CT complex have been observed
relative to the free donor and acceptor same as the 1H NMR spectra of
CT complexes of 2,9-dimethyl-1,10-phenanthroline and 8-hydroxy-
quinoline and with picric acid and p-Nitrophenol, respectively
[21,28]. The phenolic proton in the acceptor (PNP) usually appears
at d = 10.77 ppm [28] which is disappeared after complexation to
make hydrogen bond with one nitrogen atom of Phen because H of
AOHhasmoreacidic characterthanother Hatomsof PNP. Therefore,
H of AOH became more easy to liberate. It is clearly obvious that new
peak in spectrum of the CT complex at d = 3.32 ppm (br s, 1H, +NAH
of CTC) is assigned to +NH proton of CT complex due to proton trans-
fer from acceptor donor which indicates the involvement of the phe-
nolic AOH and one nitrogen atom of Phen. The doublets at
d = 6.91 ppm and d = 8.10 ppm have been assigned to the one of each
two protons of the same kind in PNP moiety in the CT complex
whereas in free PNP they were observed at d = 6.95 ppm and
d = 8.25 ppm respectively [28]. These upfield shifts in frequencies
have been attributed to an increased electron density on PNP part
of the CT complex due to the existence of the (N+AH) charge transfer
interaction between the donor and acceptor molecules. The triplet
and singlet peaks at d = 7.75 ppm and d = 7.99 ppm are observed
with small change in chemical shift and assigned to the one of each
two protons in CTC. The two doublet at d = 8.477 ppm and
d = 9.09 ppm are observed in the spectrum of the CT complex and
each peak are assigned to the two proton.
Fig. 7. TGA–DTA curves for (A) 1,10-phenanthroline; (B) p-Nitrophenol; (C) charge
transfer complex of 1,10-phenanthroline and p-Nitrophenol.
3.6. Comparative study of thermograms for 1,10-phenanthroline, p-
Nitrophenol and their CT complex
mode of proton attached to a quaternary nitrogen atom [54].The
stretching vibrations of the (C@N) and (C@C) peaks appear at
1422 cmꢂ1 (very strong) and 1690 cmꢂ1 (weak), in phenanthroline,
respectively, and stretching vibration of AOH, ANO2 and C@C
Thermal analysis was carried out for Phen, PNP and their CT
complex. A heating rate of 20 °C/min within the temperature range
Table 4
Weight loss, enthalpy (DH), and degradation temperature (T), for CT complex of 1,10-phenanthroline and p-Nitrophenol.
Step
CT complex of Phen and PNP
1,10-Phenanthroline
Weight loss (%)
p-Nitrophenol
Weight loss (%)
Weight loss (%)
D
H (J/g)
T (°C)
D
H (J/g)
T (°C)
D
H (J/g)
T (°C)
I
II
III
3.56
81.56
5.65
+21.55
+729.80
ꢂ127.36
142.37
298.99
499.77
9.37
87.22
+121.69
ꢂ117.10
111.93
280.67
87.26
6.04
+64.34
+600.64
254.83
226.09

196
I.M. Khan, A. Ahmad / Journal of Molecular Structure 977 (2010) 189–196
of 25–800 °C was used. The combined TGA and DTA thermograms
for charge transfer complex along with acceptor and donor are pre-
sented in Fig. 7. It is clearly observed that [(Phen) (PNP)] exhibit
three step degradation (Fig. 7C) which is typical thermal behavior
of derivatives of PNP as reported elsewhere [23]. In first step,
3.558% of the compound is lost at around 142.2 °C which is thought
to be a consequence of crystallization. This can also be reflected by
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4. Conclusions
The electron donor 1,10-phenanthroline reacts with the
p-
acceptor p-Nitrophenol in methanol at room temperature to form
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Acknowledgments
Authors thank to the Chairmen Department of Chemistry for
providing research facilities. One of the authors (I.M. Khan) grate-
fully acknowledges University Grant Commission, India for provid-
ing financial assistance.
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