Study of inclusion complex of β-cyclodextrin and Ortho-Anisidine; Photophysical and electrochemical behaviors

Article abstract of DOI:10.1016/j.molstruc.2010.12.026

Effect of solvents, buffer solutions of different pH and β-cyclodextrin on the absorption spectra of Ortho-Anisidine (O-AN) have been investigated. The host-guest inclusion complexation between β-CD and O-AN were investigated by spectrophotometric (UV-Vis

Full text of DOI:10.1016/j.molstruc.2010.12.026

Journal of Molecular Structure 987 (2011) 214–224
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Study of inclusion complex of b-cyclodextrin and Ortho-Anisidine; photophysical
and electrochemical behaviors
⇑
K. Srinivasan, J. Vaheethabanu, P. Manisankar, T. Stalin
Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Effect of solvents, buffer solutions of different pH and b-cyclodextrin on the absorption spectra of Ortho-
Anisidine (O-AN) have been investigated. The host–guest inclusion complexation between b-CD and O-AN
were investigated by spectrophotometric (UV–Visible) and electrochemical (CV) methods. The UV–Visi-
ble and CV studies indicated the b-CD forms 1:1 inclusion complex with O-AN. In electrochemical studies
the anodic peak current (ipa) decreased drastically with increasing b-CD and the anodic peak potential
(Epa) shifted in positive direction. These result also evidence for the formation of inclusion complex
between b-CD and O-AN. A mechanism is proposed to explain the inclusion complex. The inclusion inter-
Received 29 September 2010
Received in revised form 10 December 2010
Accepted 10 December 2010
Available online 21 December 2010
Keywords:
b-Cyclodextrin
Ortho-Anisidine
Inclusion complex
pH Effects
Cyclicvoltammetry
RasMol tool
action was examined and the thermodynamic parameters (DG, DH and DS) of inclusion process are also
determined. The experimental results indicated that the inclusion process is an exergonic and spontane-
ous process. Stable solid inclusion complex were prepared and characterized by FT-IR, Scanning Electron
Microscopy (SEM) and X-ray diffraction and docking studies by RasMol tool methods.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
The inclusion of a guest in a b-CD cavity consists basically of a
substitution of the included water molecules by the less polar
The absorption spectral characteristics and their intensities of
an aromatic molecule are dependent on the nature as well as on
the position of the substituent in the aromatic ring [1]. The proton
transfer processes of intramolecular hydrogen bonded molecules
are topics of current research activity [2]. Solvent effects on proton
transfer to solvent raise interesting problems concerning static and
dynamic processes, The proton transfer of aromatic molecules
which are hydrogen bonded in the ground state undergoes changes
markedly depending on the kind of substitution on the aromatic
ring.
The inclusion process of organic molecules with cyclodextrins
(CDs) usually results in a modulation of the physicochemical prop-
erties of guest molecules, such as increased solubility, improved
chemical stability and reduced toxicity and so on [3]. Therefore,
it would be of great importance to comprehensively understand
the chemistry of cyclodextrins and inclusion behavior of organic
molecules with CDs, a family of three well known commercially
available cyclic oligosaccharides containing 6,7,8-glucopyranose
guest [4]. The process is energetically favored by the interactions
of the guest molecule with the solvated hydrophobic cavity of
the host. In this process entropy and enthalpy changes have an
important role. In this study, we report for the absorption charac-
teristics of O-AN in different solvents, pH and b-cyclodextrin effects
on the spectral characteristics. For the past one decade, the corre-
sponding author has largely been involved in studying photophys-
ical properties [5–12] of various organic fluorophores. This
molecule shows 1:1 inclusion with b-CD molecule, it clearly con-
firmed by spectral analysis as well as docking studies have been
obtained using RasMol tool [13,14]. This stimulated us to carry
out a study on O-AN.
Applications of O-AN in various field like as catalysis and chem-
icals processing, chemical synthesis, colorants for paper, dyestuffs,
pigments and optical brighteners. In electrochemical synthesis of
polymer material using the O-AN [15] to form thin films and com-
posite material [16]. O-AN used as reducing agent in gold nano par-
ticle preparation in aqueous solution [17].
units and are referred to as
a
-, b- and
c
-CDs, (Scheme 1) respec-
So that, the corresponding author doing the electrochemical
analysis related to sensor work using modification of glassy carbon
electrode with the help of multiwall carbon nanotube and b-cyclo-
dextrin polymer material is used in electrochemical sensor. This
stimulated us to preliminarily carry out to the study the effect of
solvents, pH and b-CD on spectral characteristics of O-AN. In this
paper we have reported the inclusion complex of O-AN with
b-CD in solution phase as well as solid phase, the formation of
tively. b-CD is mainly used for the inclusion process because low
cost, perfect cavity size and adopt a bucket-like arrangement with
a hydrophilic exterior and hydrophobic interior (Scheme 1).
⇑
Corresponding author. Tel.: +91 4565 225206; fax: +91 4565 225202.
E-mail address: tstalinphd@rediffmail.com (T. Stalin).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.12.026

K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
215
Scheme 1. (a) Chemical structures of
to be ꢀ8 A° and width size is 15.3 A.
a-CD, b-CD and c-CD. (b) The internal and external widths for b-cyclodextrin are noted in the figure the height of the cavity is presumed
inclusion complex were characterized by absorption spectroscopy
(UV–Vis) and electrochemical (Cyclicvoltammetry, CV) methods.
The solid complex were prepared and characterized by XRD, SEM
and FT-IR techniques and docking studies by RasMol (backbone)
representation of the 3D structure of inclusion complex.
(O-AN) purchased from Loba Chemical Reagents Company. Triply
distilled water was used to prepare all solutions. All solvents used
were of the highest grade (AR). Solutions in the pH range 2.0–12.0
were prepared by adding the appropriate amount of NaOH and
H3PO4. A modified Hammett’s acidity scale (H0) [18] for the solu-
tions below pH ꢀ 2 (using a H2SO4–H2O mixture) and Yagil basicity
scale (H ) [19] for solutions above pH ꢀ 12 (using a NaOH–H2O
ꢁ
2. Experimental
mixture) were employed. The solutions were prepared just before
taking measurements. The concentrations of the solutions were of
the order (10ꢁ4–10ꢁ5 mol dmꢁ3). The stock solution of O-AN is
2.1. Instruments
5.1 ꢂ 10ꢁ2 mol dmꢁ3
.
The UV–Vis spectra (absorption spectral measurements) were
carried out with Shimadzu UV-2401PC double-beam spectropho-
tometer, The pH values in the range 2.0–12.0 were measured on
Elico pH meter Li-120, Electrochemical studies were carried out
using autolab electrochemical analyzer it used to apply potential
on the working equipped with a three-electrode glassy carbon
electrode (GCE) (diameter: 1 mm) is served as a working electrode.
Reference electrode was saturated calomel electrode (SCE) and
platinum wire as counter electrode. All experiments were carried
out at 30 1 °C. The working electrode was polished to a mirror
2.3. b-Cyclodextrin solution preparation
5.1 ꢂ 10ꢁ2 mol dmꢁ3 concentration of stock solution of O-AN
were prepared using ethanol and 0.1 ml of this stock solution were
added into 10 ml volumetric flasks and made up to the mark using
following concentration of b-CD solutions 0, 2, 4, 6, 8, 10 and
12 ꢂ 10ꢁ3 mol dmꢁ3 respectively and shaken thoroughly. So that
the final concentration of O-AN in each flask is 5.1 ꢂ
10ꢁ4 mol dmꢁ3
. All the absorption spectra were recorded at
with 0.05 lm alumina aqueous slurry, and rinsed with triply dis-
30 1 °C.
tilled water before each experiment. The supporting electrolyte
was pH ꢀ 2 and pH ꢀ 7 buffers (0.1 M H2SO4 + 0.1 M Na2SO4 and
0.1 MKH2PO4 + 0.1 M NaOH). Powder X-ray diffraction spectra
were taken by XPert PRO PANalytical diffractometer. FT-IR was re-
corded using Nicolet 380, the surface morphology were taken by
Hitachi S 3000 H SEM.
2.4. Preparation of b-cyclodextrin solution for electrochemical studies
The stock solution of b-CD (12 ꢂ 10ꢁ3 mol dmꢁ3) was prepared
using pH ꢀ 2 (0.1 M H2SO4 + 0.1 M Na2SO4) and pH ꢀ 7 (0.1 M
KH2PO4 + 0.1 M NaOH) buffer solutions. From the stock solution
2, 4, 6, 8 and 10 ꢂ 10ꢁ3 mol dmꢁ3of b-CD were prepared using
pH ꢀ 2 and pH ꢀ 7 buffers. So that the final concentration of
O-AN in each flask is 5.1 ꢂ 10ꢁ4 mol dmꢁ3. The cyclic voltammo-
grams were recorded after the solution kept static for at least 24 h.
2.2. Reagents
b-Cyclodextrin {(b-CD), were obtained from Sd fine chemical
Company} and used without further purification. Ortho-Anisidine

216
K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
2.5. Preparation of solid inclusion complex of O-AN with b-CD
Accurately weighed 1 g of b-CD was placed into 50 ml conical
flask and 30 ml triply distilled water added and then oscillated this
solution enough. After that, 0.11 g O-AN was put into a 50 ml bea-
ker and 20 ml ethanol added and put over electromagnetic stirrer
to stir until it was dissolved [20]. Then slowly poured O-AN solu-
tion into b-CD solution. The above mixed solution was continu-
ously stirred for 48hrs at room temperature. The reaction
mixture was put and kept into refrigerator for 48hrs. At this time,
we observed that white precipitate is formed. The precipitate was
filtered by G4 crucible and washed with triply distilled water. After
dried in oven at 50 °C for 12 h, off white powder was obtained. This
is solid inclusion complex of O-AN with b-CD and it further ana-
lysed by FT-IR, XRD and SEM methods.
3. Results and discussion
3.1. Effect of solvents
The absorption spectra of ortho-anisidine (O-AN) have been
studied in solvents of various polarities and hydrogen bonding
abilities. The relevant data are complied in Table 1 along with
the spectral data of aniline and O-toludine [21]. When compared
to aniline and O-toludine the absorption maxima of O-AN is red
shifted in all solvents. This is because the intramolecular hydrogen
bonding (IHB) present between O and H of the amino and methoxy
groups respectively in O-AN. The red shift increases according to
following sequence toludine > aniline > ortho-anisidine. The above
sequence indicates the position of the substituent in the phenyl
ring is the key factor for the absorption behavior, because of the
different electronic densities of the HOMO on each atom. Thus in
O-AN, substitution at ortho position would form IHB, whereas meta
and para position would stabilize the charge transfer (CT) charac-
ter. Examine of this results that the position and/or the molar
extinction co-efficient of this band are highly influenced by the
nature of the polar substituents in the phenyl ring.
Fig. 1. Absorption Spectra of O-AN in selected solvents at 303 K: The concentration
of O-AN is 5.1 ꢂ 10ꢁ4 M (a) cyclohexane, (b) 1, 4-dioxane, (c) 2-propanol, (d) ethanol
and (e) water.
attributed to (p–
pꢃ) transition with the heterocyclic moiety (i.e.
intramolecular hydrogen bonding ring) of the O-AN molecule.
3.2. Effect of hydrogen ion concentration
The absorption spectra of different prototropic species of O-AN
were recorded at different acid–base concentration and the rele-
vant data are compiled in Table 2 and Fig. 2. In the pH range
3.5–15.62 absorption maxima resemble the spectra observed in
non-aqueous solvents and thus can be assigned to the neutral spe-
cies [22]. The larger red shift (285 nm) was observed in the absorp-
tion maxima, could be due to the exists of neutral species in O-AN.
The absorption spectra (Fig. 1) of O-AN in selected solvents are
characterized only
a
longer wavelength band (LW) with
a
With a decrease on pH from 3.5, the absorption maxima
maximum around 287.5 nm. An analysis of solvatochromic shifts
of O-AN reveals that the absorption maxima are blue shifted from
cyclohexane to water. This results imply that this band can be
(275.5 nm) are blue shifted in pH ꢀ 3.0.The blue shifts in O-AN
with increase of hydrogen ion concentration (pH ꢀ 3.0–2.5) sug-
gest the formation of the monocation, formed by protonating to
Table 1
Absorption maxima (nm) and log
e of O-Anisidine, Aniline and O-Toluidine in selected solvents.
S. no
Solvents
O-AN
Aniline
O-Toluidine
kmax (nm)
log
e
kmax (nm)
log
e
kmax (nm)
log e
1.
2.
3.
4.
5.
6.
7.
8.
Cyclohexane
1,4-Dioxane
Acetonitrile
Ethylacetate
2-Propanol
Methanol
Ethanol
287.5
250.5
3.60
2.80
283.0
235.0
3.20
3.90
283.0
234.0
3.37
3.94
287.5
256.0
3.60
2.70
286.0
238.0
3.30
3.96
286.4
236.2
3.36
3.94
286.0
–
3.56
–
286.0
238.0
3.29
3.98
286.8
236.0
3.37
3.95
286.0
–
3.56
–
–
–
286.0
236.2
3.30
3.80
284.5
265.0
3.70
2.95
284.0
232.0
3.22
3.86
282.6
233.4
3.30
3.88
283.0
–
3.37
–
284.0
232.0
3.22
3.88
281.5
232.0
3.18
3.92
285.5
254.5
3.50
2.80
284.0
232.0
3.22
3.90
282.0
233.0
3.20
3.88
Water
285.0
255.5
3.50
2.90
280.0
–
3.20
–
280.0
–
3.20

K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
217
Table 2
Various prototropic maxima (absorption spectra) and pKa values of O-AN in without
and with b-CD medium.
Species
Without b-cyclodextrin
With b-cyclodextrin
kmax (nm)
pKa
kmax (nm)
pKa
Monocation
Neutral
275.5
285.0
3.0
3.5–15.62
275.5
285.5
3.0
3.5–15.62
Fig. 3. Absorption spectra of O-AN (Conc. 5.1 ꢂ 10ꢁ4 M) at (pH ꢀ 2) different b-CD
concentrations (1) 0.0 M, (2) 0.002 M, (3) 0.004 M, (4) 0.006 M, (5) 0.008 M, (6)
0.010 M and (7) 0.012 M.
effect of pH studies. Half wave potential for organic compounds are
therefore markedly pH-dependant. Furthermore alteration of the
pH may result in a change in the reaction product. It should be
emphasized that an electrode process that consumes or produces
hydrogen ion will alter the pH of the solution at the electrode sur-
face. Electrical potential develops if the solution has a different pH
from the solution in the probe.
The observed in the equilibria using absorption data could be
due to (i) the use of band maxima rather than using 0–0ꢃ transi-
tions and the different solvent relaxation for the conjugate species
involved in the equilibria in different electronic states and (ii) in-
crease in the acidity and basicity of the amino group in S1 state
is such that the order of the prototropic reactions in the S0 state
are changed. Similar observations are noted in case of molecules
containing both the electron donating and electron accepting func-
tional groups [24].
Fig. 2. Absorption spectra of different prototropic species of O-AN at 303 K;
concentration 5.1 ꢂ 10ꢁ4 M (a) neutral (b) monocation.
the amino group. The absorption maxima present in the monoca-
tion of O-AN formed by protonation of the amino group consistent
with the result observed by others for the similar protonation [22].
Moreover, when the protonation occurred at the amino group,
there would have been a blue shift in the absorption spectra as no-
ticed in this compound [23]. This blue shift may be caused by the
removal of conjugation and also by changing the polarity of the
solvents (below pH ꢀ 3.0). In the case of O-AN, absorption maxima
occur at 285 nm because the pair of electrons on nitrogen atom is
in conjugation with the p-bond system of the benzene ring. In its
acidic solutions, (pH ꢀ 3), a blue shift was caused and absorption
occurs at lower wavelength (275 nm). The monocation formed in
pH ꢀ 3 acidic solutions, the electron pair is no longer present and
hence conjugation is removed. The pKa value for the equilibria is
determined spectrophotometrically and is given in Table 2. The
pKa ꢀ 3.0 value of O-AN are less than aniline (pKa ꢀ 4.0) which
indicates that O-AN molecules more acidic than aniline.
3.3. Effect of b-cyclodextrin
Table 3 and Figs. 3 and 4 show the absorption maxima of O-AN
(5.1 ꢂ 10ꢁ4 mol dmꢁ3) in pH ꢀ 2 (monocation) and pH ꢀ 7 (neu-
tral) solutions containing different concentrations of b-CD. At
In electro analytical technique the electrode processes ordinar-
ily involve hydrogen ions, the typical reaction being represented as
Table 3
Absorption maxima (nm) and log
e of O-AN at different concentrations of b-CD in pH ꢀ 7 and pH ꢀ 2 solutions.
S. no
Concentration of b-CD (M)
pH ꢀ 7
kmax (nm)
pH ꢀ 2
kmax (nm)
log
e
log e
1
2
3
4
5
6
7
Without b-CD
0.002
0.004
0.006
0.008
284.5
285.0
285.0
285.0
285.0
285.5
286.0
3.47
3.50
3.51
3.52
3.53
3.54
3.54
223
ꢁ13.67
ꢁ3994.8
ꢁ13.09
275.5
275.5
275.5
275.5
275.5
275.5
275.5
3.22
3.09
3.07
3.05
3.04
3.02
3.02
962
ꢁ17.36
ꢁ3994.8
ꢁ13.08
0.010
0.012
Binding constant (Mꢁ1
)
D
D
D
G (kJ molꢁ1
H (kJ molꢁ1
S (kJ molꢁ1 kꢁ1
)
)
)

218
K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
Scheme 2. The proposed structure of inclusion complex of O-AN with b-CD for 1:1
inclusion complex.
complex has been determined by analyzing the changes in the
intensity of absorption maxima with the b-CD concentration. In
the case of inclusion complex formed between O-AN and b-CD
the equilibrium can be written as
Fig. 4. Absorption spectra of O-AN (Conc. 5.1 ꢂ 10ꢁ4 M) at (pH ꢀ 7) different b-CD
concentrations (1) 0.0 M, (2) 0.002 M, (3) 0.004 M, (4) 0.006 M, (5) 0.008 M, (6)
0.010 M and (7) 0.012 M.
O-AN þ b-CD ꢀ O-AN . . . b-CD
pH ꢀ 7 O-AN exists as a neutral form only hence we also recorded
spectrum at pH ꢀ 2 as a cationic form. The absorption spectra of
these two forms (monocation, neutral) are drastically different. In
both cases, no clear isosbestic point is observed in the absorption
spectra. In the presence of b-CD, there is no significant change is
observed in the absorption maxima at pH ꢀ 2. The absorption
spectra show the change in absorption maxima even in the pres-
ence of the highest concentration of b-CD used (12 ꢂ
10ꢁ3 mol dmꢁ3) and there is no change in the absorbance by
further addition of b-CD (Fig. 5) in pH ꢀ 2 and pH ꢀ 7 solutions.
Whereas the absorption spectra of O-AN in pH ꢀ 7 is seen to
The binding constant K and stoichiometric ratio of the inclusion
complex of O-AN can be determined according to the Benesi–
Hildebrand [25] relation assuming the formation of
host–guest complex.
a 1:1
K½O-ANꢄ0 ½b-CDꢄ0
where A-Ao is the difference between the absorbance of O-AN in the
presence and absence of b-CD, is the difference between the
D
e
molar absorption co-efficient of O-AN and the inclusion complex
[O-AN]0 and [b-CD]0 are the initial concentration of O-AN and
b-CD, respectively. Fig. 6 depicts a plot of 1/A-Ao as a function of
1/[b-CD] for O-AN. Good linear correlations were obtained, confirm-
ing the formation of a 1:1 inclusion complex. From the intercept
and slope values of this plot K was calculated [pH ꢀ 2 = 962 Mꢁ1
and pH ꢀ 7 = 223 Mꢁ1] at 303 K (Table 3).
undergo
a slight red shift is observed (284.5–286.0 nm). At
pH ꢀ 2, O-AN exist in their cationic form and absorption spectra
do not show any change in absorption maxima even in the pres-
ence of the highest concentration of b-CD (12 ꢂ 10ꢁ3 mol dmꢁ3
)
with the indication of cationic form (NH3 group), Even though
the O-AN located inside the b-CD cavity the NH2 group located at
the rim which will unable acid–base equilibrium in pH ꢀ 7. This
behavior has been attributed to the enhanced dissolution of the
O-AN molecule through the hydrophobic interaction between
O-AN and b-CD. These results indicate that O-AN molecule is
entrapped in the b-CD cavity to form inclusion complex. In both
cases, the binding constant for the formation of O-AN: b-CD
The determination of the thermodynamic parameters in this
inclusion process in the free energy change can be calculated from
the binding constant ‘K’ by the following equation
D
G ¼ ꢁRT ln K
At constant temperature
D
H ꢁ
DG
D
S ¼
T
Fig. 5. Absorption intensity of O-AN changes (a) at 275.5 nm and (b) at 284.5 nm with different b-CD concentrations in pH ꢀ 2 and pH ꢀ 7 respectively.

K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
219
Fig. 6. Benesi–Hildebrand plot of 1/A ꢁ A0 vs. 1/[b-CD](a) pH ꢀ 2 and (b) pH ꢀ 7 solutions.
The thermodynamic parameters
of the guest molecule to b-CD cavity is given in Table 3. As can be
seen from Table 3, G is negative which suggests that the inclusion
process proceeded spontaneously at 303 K. H and S are also
D
G,
D
H and
D
S for the binding
steric barrier caused by molecular geometrical shape and the limit
of b-CD cavity to the freedom of shift and rotation of O-AN mole-
cule. The experimental results indicated that the inclusion reaction
of b-CD with O-AN was an exothermic reaction accompanied with
D
D
D
negative in the experimental temperature range, which indicates
that the inclusion process is an exergonic and enthalpy controlled
negative
change played were on the contrary. The conclusion of these ther-
modynamic parameters is that changes in H are largely compen-
sated for by changes in S.
DS. In this case, the actions that enthalpy and entropy
process. The negative enthalpy change (
D
H) arose from the vander
S) is the
D
Waal’s interaction, while the negative entropy change (
D
D
Scheme 3. (a) Ball and stick representation of b-cyclodextrin. The oxygen atoms of –OH groups are shown as red balls, hydrogen atoms are not shown. (b) Ball and stick
representation of O-AN. The hydrogen atoms are shown as magenta balls, nitrogen atom as blue, oxygen as red ball and carbon as grey balls. (c) Ball and stick representation of
proposed inclusion complex structure of O-AN with b-CD. The hydrogen atoms of O-AN are shown as magenta balls nitrogen atom as blue ball, oxygen as red ball and carbons
as grey balls. The –OH group oxygen atoms of b-CD are shown as red balls, hydrogen atoms are not shown. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)

220
K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
Table 4
CV for O-AN:b-CD in pH ꢀ 2 buffer, scan rate 100 mV sꢁ1, O-AN (Conc.5.1 ꢂ 10ꢁ4 M) solution in different concentration of b-CD (0–12 ꢂ 10ꢁ3M).
b-CD Concentration (M)
Epa (V)
ipa
(lA)
Ipc
(lA)
Epc (V)
Epa ꢁ Epc/2 mV
Without b-CD
0.002
0.004
0.006
0.008
0.712
0.712
0.715
0.718
0.724
0.725
0.734
50.34
46.08
44.56
43.46
43.03
42.48
42.36
ꢁ25.29
ꢁ25.06
ꢁ24.90
ꢁ24.86
ꢁ24.08
ꢁ23.43
ꢁ23.28
0.228
0.226
0.226
0.224
0.220
0.219
0.218
242
244
244.5
247
252
253
258
417.7
0.010
0.012
Binding constant (Mꢁ1
)
Table 5
CV for O-AN: b-CD in pH ꢀ 7 buffer, scan rate 100 mV sꢁ1, O-AN (Conc. 5.1 ꢂ 10ꢁ4 M) solution in different concentration of b-CD (0–12 ꢂ 10ꢁ3 M).
b-CD Concentration(M)
Epa (V)
ipa
(l
A)
Ipc
(lA)
Epc (V)
Epa ꢁ Epc/2 mV
Without b-CD
0.002
0.004
0.006
0.008
0.516
0.520
0.524
0.533
0.554
0.556
0.583
38.95
36.72
35.22
33.72
31.93
31.76
31.73
ꢁ15.99
ꢁ15.77
ꢁ15.52
ꢁ15.50
ꢁ15.04
ꢁ14.71
ꢁ14.07
ꢁ0.101
ꢁ0.197
ꢁ0.198
ꢁ0.246
ꢁ0.308
ꢁ0.318
ꢁ0.327
308
358
361
389
277
437
455
173
0.010
0.012
Binding constant (Mꢁ1
)
Considering the above discussions, the possible inclusion com-
plex mechanism is proposed. Naturally, the inclusion complex for-
mation between O-AN and b-CD are possible with the amino group
captured inside of the b-CD cavity in pH ꢀ 2 solution, it shows in
Scheme 2.
3.4. Possible inclusion complex of O-AN in b-CD cavity
In 1:1 inclusion complex, the aromatic moiety is embedded in
b-CD cavity and the amine group forms hydrogen bonding with
other b-CD-OH groups. The formation of the 1:1 host:guest com-
plex is clearly demonstrated in Scheme 2. Such a 1:1 inclusion
complex structure gains further stabilization energy by hydrogen
bonding between hydroxyl groups of the primary and secondary
rims of the b-CD molecule, water should be excluded from such a
structure, although the small solvent molecule could penetrate
the central cavity to interact with the amino group. However, most
likely this group should bind to the oxygens of the glucosidic
bridges as such as interaction (Scheme 2) was observed.
Fig. 7. CV for O-AN:b-CD in pH ꢀ 2 buffer, Scan rate 100 mV sꢁ1, O-AN (Conc.
5.1 ꢂ 10ꢁ4 M) solution in different concentration of b-CD (1) 0.00 M, (2) 0.002 M,
(3) 0.004 M, (4) 0.006 M, (5) 0.008 M, (6) 0.010 M and (7) 0.012 M.
In 1:1 complex, the binding constants for protonated O-AN
(pH ꢀ 2) are considerably greater than those of their neutral forms
Fig. 8. CV for O-AN:b-CD in pH ꢀ 7 buffer, Scan rate 100 mV sꢁ1, O-AN (Conc.5.1x10ꢁ4M) solution in different concentration of b-CD (1) 0.00 M, (2) 0.002 M, (3) 0.004 M, (4)
0.006 M, (5) 0.008 M, (6) 0.010 M and (7) 0.012 M.

K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
221
(pH ꢀ 7). The binding constant are very sensitive to change of pH
values, which further supported the selective inclusion associated
with the species form of O-AN. Of the two species, we should note
that the b-CD can readily include the protonated species than the
neutral species, because the former is more hydrophobic than
the latter. The higher binding constants in pH ꢀ 2 implied that
+
the NH3 group present in the interior within the inner part of
the b-CD cavity. This pattern in the binding constant has been
detected in the complexes between benzene derivatives [26] and
b-CD.
Fig. 9. Influence of the solution pH at the GCE on the anodic peak potential of O-AN (containing 5.1 ꢂ 10ꢁ4 mol Lꢁ1).
Fig. 10. Benesi–Hildebrand plot of 1/IG ꢁ IHG vs. 1/[b-CD] in (a) pH ꢀ 2 and (b) pH ꢀ 7 solution.
Fig. 11. The peak current of O-AN changes (a) at 0.712 V and (b) at 0.516 V with different b-CD concentrations in pH ꢀ 2 and pH ꢀ 7 respectively.

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K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
To substantiate the above discussion, the effect of b-CD on the
prototropic equilibrium between neutral and monocation, the pH
dependent changes in the absorption of the O-AN molecule in b-
CD solution have been recorded (Table 2). The absorption spectra
of O-AN have been studied in the pH range from ꢁ1.0 to 12 (figure
not given). In the ground state neutral to monocation are blue
Fig. 12. FT-IR Spectra of solid complex (a) b-CD, (b) O-AN and (c) O-AN:b-CD complex.

K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
223
inclusion complex, [O-AN]0 and [b-CD]0 are the initial concentration
of O-AN and b-CD, respectively.
shifted in b-CD medium similar aqueous medium, it indicating
O-AN molecule entrapped in b-CD cavity. Further it confirmed by
RosMol tool (Scheme 3) [13,14].
A plot of 1/IGꢁ IHG verses 1/[b-CD] gives a straight line for
pH ꢀ 2 and pH ꢀ 7 medium as shown in Fig. 10. Good linear corre-
lations were obtained, confirming that the formation of a 1:1 inclu-
sion complex for pH ꢀ 2 and pH ꢀ 7 solutions. From the intercept
and slope values of this plot binding constant ‘K’ values was eval-
uated, the binding constant for O-AN:b-CD was 417.7 Mꢁ1 and
173 Mꢁ1 in pH ꢀ 2 and pH ꢀ 7 solution. When the b-CD concentra-
tions higher than 10 ꢂ 10ꢁ3 mol dmꢁ3, the oxidation peak current
remain unchanged by further addition of b-CD in pH ꢀ 2 and
pH ꢀ 7 (Fig. 11) solutions. This behavior has been attributed to
the enhanced dissolution of the O-AN molecule through the hydro-
phobic interaction between O-AN and b-CD. These results indicate
that O-AN molecule was entrapped in the b-CD to form inclusion
complex and it proved by electrochemically.
3.5. Electrochemical studies
The formation of the inclusion complex of O-AN and b-CD was
also confirmed by electrochemical method. The electrochemical
behavior of O-AN has been researched in the following same obser-
vation [27]. If b-CD interacted with O-AN, different electrochemical
properties would be observed in Tables 4 and 5.
From Fig. 7 (pH ꢀ 2) and Fig. 8 (pH ꢀ 7) the anodic peak current,
ipa, decreased drastically with increasing the concentration of b-CD
in pH ꢀ 2 and pH ꢀ 7 solution. The anodic peak potential (Epa
)
shifted in positive direction when b-CD is increased in both cases.
The result showed that the inclusion complex between b-CD and
O-AN. The diffusion co-efficient of the inclusion complex from bulk
layer to electrode surface was very slow than that of the O-AN
monomer itself, which leaded the current decrease. On the other
hand, because O-AN monomer entered into the hydrophobic cavity
of b-CD, it was reasonable for that the electrochemical oxidation of
the inclusion complex was more difficult than that of O-AN mono-
mer itself, which would lead the anodic peak potential shift in po-
sitive direction, the anodic peak current, ipa decreased and also
higher activation energy would be observed in both pH ꢀ 2 and
pH ꢀ 7 solutions.
3.6. FT-IR spectral studies
The solid complex formation may be confirmed by FT-IR spec-
troscopy (Fig. 12) because the bands resulting from the included
part of the guest molecule are generally shifted or their intensities
altered [28]. In O-AN the two peaks are observed at 3456.30 cmꢁ1
and 3372.03 cmꢁ1 due to the asymmetric and symmetric stretch-
ing of primary amine. The characteristics peak of –C–N stretching
vibration appeared at 1272 cmꢁ1 for primary aromatic amine.
The characteristics peaks of –C@C– stretching vibration appeared
at 1613.72 cmꢁ1 and 1502.17 cmꢁ1 for aromatic nuclei. For disub-
stituted benzene the peak appeared at 735.1 cmꢁ1. From the
Fig. 12c asymmetric and symmetric peaks of 3456.3 cmꢁ1 and
3372 cmꢁ1 are not observed in this spectrum is due to the primary
amine entrapped in b-CD cavity. This is confirmed by the charac-
teristic –C–N stretching vibration at 1272 cmꢁ1 also not observed,
due to the aniline moiety into the b-CD cavity. The characteristic
stretching vibration of –C@C– at 1613.7 cmꢁ1 and 1502.2 cmꢁ1
intensities are reduced and shifted to 1615.32 cmꢁ1 and
1506.36 cmꢁ1 this is due to the benzene ring entrapped into the
b-CD cavity. The characteristic –C–H stretching vibration of methyl
group (or) –CH3 group appear at 2950.7 cmꢁ1 and 2836.0 cmꢁ1 for
asymmetric and symmetric vibration (Fig. 12b) but these two peak
are not observed in this spectra (Fig. 12c), and the characteristic of
From Tables 4 and 5 the oxidation potential of O-AN is 0.712 V,
0.516 V and the reduction potential is 0.228 V, ꢁ0.101 V in pH ꢀ 2
and pH ꢀ 7 respectively. The difference between the oxidation and
reduction potential is 242 mV and 308 mV for pH ꢀ 2 and pH ꢀ 7
respectively, so this reaction in both pH medium is quasi revers-
ible. Increasing the b-CD concentration from 2 ꢂ 10ꢁ3
M to
12 ꢂ 10ꢁ3M the difference between the oxidation and reduction
potential is increased to 258 mV and 455 mV for pH ꢀ 2 and
pH ꢀ 7 respectively, due to this reaction moves towards irrevers-
ible because O-AN monomer entered into the hydrophobic cavity
of b-CD, it was reasonable for that the electrochemical oxidation
of the inclusion complex was more difficult than that of O-AN itself.
The oxidation potential of O-AN in pH ꢀ 2 is higher (0.712 V)
than pH ꢀ 7 (0.516 V), this is due to the formation of monocation
in pH ꢀ 2, because of this monocation oxidation occurs at higher
potential (0.712 V) and the peak current also increases
(50.34 mV) due to the loss of protonated H+. Fig. 9 shows the rela-
tionship between the anodic peak potential of O-AN and the solu-
tion of various pH. It is found that the potentials (Epa) shifted
negatively with increasing pH, it indicating that protons takes part
in the oxidation process of O-AN at the GCE surface. The anodic
peak potential (Epa) is proportional to the solution pH in the range
of 1–9. The linear-regression equation of O-AN is described as fol-
lowing with a correlation co-efficient of 0.992.
–C–O
stretching
band
appear
at
1220.1 cmꢁ1
and
1340.6 cmꢁ1(Fig. 12b) but in this spectrum the intensity reduced
and shifted to 1222.9 cmꢁ1 and 1330.7 cmꢁ1 (Fig. 12c) because
the O–CH3 group also entrapped into b-CD cavity. This is confirmed
EpaðVÞ ¼ 0:779 ꢁ 0:036pH;
In pH ꢀ 7 the lone pair of electron get oxidized at lower poten-
tial (0.516 V) and peak current also decreases (38.95 mV).The bind-
ing constant K and stoichiometric ratios of the inclusion complex of
O-AN can be determined according to the Benesi–Hildebrand [25]
relation assuming the formation of a 1:1 host–guest inclusion com-
plex between b-CD and O-AN.
K½O-ANꢄ0 I½b-CDꢄ0
where IG is the oxidation peak current of guest molecule of O-AN,
and IHG is the oxidation peak current of inclusion complex of
O-AN:b-CD and IG ꢁ IHG difference between the oxidation peak cur-
rent of O-AN and O-AN:b-CD inclusion complex,
DI is the difference
between the molar peak current co-efficient of O-AN and the
Fig. 13. Powder X-ray diffraction spectra of b-CD and O-AN:b-CD inclusion complex.

224
K. Srinivasan et al. / Journal of Molecular Structure 987 (2011) 214–224
Fig. 14. Scanning Electron Microscope photographs (Pt. coated) of (a) b-CD and (b) solid complex of O-AN:b-CD.
by the –C–H asymmetric bending vibration of –CH3 group ap-
peared at 1458.64 cmꢁ1 in Fig. 12b but not appeared in Fig. 12c
so that the methoxy group entrapped into the b-CD cavity. The
characteristics peak of disubstituted benzene in Fig. 12b appeared
at 735.09 cmꢁ1 but it is not observed in Fig. 12c. So that bifunc-
tional group of amine and methoxy groups is entrapped into the
b-CD cavity. In the past it was thought that benzene ring was only
included when the molecule containing benzene ring reacted with
b-CD to form a supramolecular inclusion complex [29]. From the
above discussions we can concluded that of O-AN molecule was in-
cluded into b-CD cavity.
gest O-AN formed a solid inclusion complex with b-CD (iv) the
above studies demonstrate that in O-AN, ICT interactions play a sig-
nificant role in b-CD aqueous/polar medium.
References
[1] H. Suzuki, Electronic Absorption Spectra and Geometry of Organic Molecules,
Academic Press, New York, 1967, p. 568.
[2] S.L. Wang, J.M. Lin, S.E. Shu, C.Y. Chern, J. Photochem. Photobiol. A: Chem. 210
(2010) 54–60.
[3] L. Jinxia, M. Zhanga, J. Chaoa, S. Shuang, Spectrochim. Acta Part A 73 (2009)
752–756.
[4] G. Astray, C.G. Barreiro, J.C. Mejuto, R.R. Otero, J.S. Gandara, Food Hydrocoll. 23
(2009) 1631–1640.
[5] T. Stalin, N. Rajendiran, Spectrochim. Acta 61A (2005) 3087–3096.
[6] T. Stalin, R. Anitha Devi, N. Rajendiran, Spectrochim. Acta 61A (2005) 2495–
2504.
[7] T. Stalin, N. Rajendiran, J. Mol. Struct. 794 (2006) 35–45.
[8] T. Stalin, N. Rajendiran, J. Photochem. Photobiol. A; Chem. 177 (2006)
144–155.
[9] T. Stalin, K. Sivakumar, N. Rajendiran, Spectrochim. Acta 62 (2005) 991–999.
[10] T. Stalin, N. Rajendiran, Chem. Phys. 322 (2006) 311–322.
[11] T. Stalin, N. Rajendiran, J. Incl. Phenom. 55 (2006) 21–29.
[12] [a] T. Stalin, N. Rajendiran, J. Photochem. Photobiol. A; Chem. 182 (2006) 137–
150;
[b] T. Stalin, N. Rajendiran, Ind. J. Chem. 45 (2006) 1113–1120.
[13] S.S. Maity, S. Samanta, P.S. Sardar, A. Pal, S. Dasgupta, S. Ghosh, Chem. Phys.
354 (2008) 162–173.
3.7. Powder X-ray diffraction spectrum
The formation of the inclusion complex can be confirmed by
X-ray diffractometer [30,31]. Fig. 13 is the powder X-ray diffraction
spectra of b-CD and inclusion complex. The X-ray spectrum of the
inclusion complex shown in Fig. 13b was evidently different from
that of b-CD monomer itself shown in Fig. 13a. The difference
between both spectra indicated that the inclusion complex is due
to the interaction of b-CD with O-AN.
3.8. Scanning Electron Microscope morphological observations
[14] K. Sivakumar, S. Balaji, J. Chem. Sci. 119 (2007) 571–579.
[15] S. Patil, Poly. Inter. 46 (1998) 99–105.
Scanning Electron Microscope (SEM) are well suited for visual-
ize the surface texture of the deposited films. The SEM analysis is
ideal for quantitatively measuring the nanometric dimensional
surface roughness and for visualizing the surface nano-texture of
the deposited film. First we observed surface morphological struc-
ture of b-CD (Fig. 14a) by SEM, and then we also observed the sur-
face morphological structure of inclusion complex Fig. 14b. These
pictures clearly elucidated the difference of each other. Modifica-
tion of crystals can be assumed as a proof of the formation of a so-
lid inclusion complex.
[16] G. Wang, Z. Yang, X. Li, C. Li, Carbon 43 (2005) 2564–2570.
[17] Xinhua Dai, Yiwei Tan, Jian Xu, Langmuir 18 (2002) 9010–9016.
[18] M.J. Jorgenson, D.R. Harter, J. Am. Chem. Soc. 85 (1963) 878–883.
[19] G. Yagil, J. Phys. Chem. 71 (1967) 1034–1044.
[20] T. Stalin, M. Shanmugam, D. Ramesh, V. Nagalakshmi, R. Kavitha, R.
Rajamohan, Spectrochim. Acta Part A 71 (2008) 125–132.
[21] N. Rajendiran, Afinidad LVI 481 (1999) 170–174.
[22] N. Rajendiran, M. Swaminathan, J. Photochem. Photobiol. Sect. A 90 (1995)
109–116.
[23] N. Rajendiran, M. Swaminathan, Bull. Chem. Soc. Jpn. 68 (1995)
2797–2808.
[24] N. Rajendiran, M. Swaminathan, Bull. Chem. Soc. Jpn. 69 (1996) 2447–2452.
[25] H.A. Bensi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707.
[26] G. Gaitano, P.R.S. Rozas, J.R. Isasi, A.G. Martínez, G. Tardajos, J. Phys. Chem. B
108 (2004) 14154–14162.
4. Conclusions
[27] M. Chen, G. Diao, E. Zhang, Chemosphere 63 (2006) 522–529.
[28] L. Szente, vol. 3, Pergamon Press, Oxford, 1996, pp. 253–278.
[29] H.Y. Wang, J.H. Xia Guang Feng, Y.L. Pang, Spectrochim. Acta Part A 65 (2006)
100–105.
[30] S. Scalia, A. Molinari, A. Casolari, A. Maldotti, Eur. J. Pharm. Sci. 22 (2004) 241–
249.
The following conclusion can be arrived at from the above stud-
ies; (i) O-AN forms 1:1 inclusion complex with b-CD (ii) prototropic
reactions in b-CD medium indicates, NH2 group present in the b-CD
cavity in pH ꢀ 2 (iii) FT-IR, XRD, SEM and RosMol tool results sug-
[31] T. Pralhad, K. Rajendrakumar, J. Pharm. Biomed. Anal. 34 (2004) 333–339.