Study of inclusion complex of β-cyclodextrin and diphenylamine: Photophysical and electrochemical behaviors

Article abstract of DOI:10.1016/j.saa.2011.02.030

The photophysical, electrochemical and photoprototropic behaviors of diphenylamine (DPA) in aqueous β-cyclodextrin (β-CD) solution have been investigated using absorption spectroscopy and cyclic voltammetric techniques. Absorption of the neutral and catio

Full text of DOI:10.1016/j.saa.2011.02.030

Spectrochimica Acta Part A 79 (2011) 169–178
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Study of inclusion complex of ␤-cyclodextrin and diphenylamine:
Photophysical and electrochemical behaviors
K. Srinivasana, K. Kayalvizhia, K. Sivakumarb, T. Stalina,∗
a Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003, Tamilnadu, India
b Department of Chemistry, SCSVMV (Deemed University), Kanchipuram 631 561, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The photophysical, electrochemical and photoprototropic behaviors of diphenylamine (DPA) in aqueous
␤-cyclodextrin (␤-CD) solution have been investigated using absorption spectroscopy and cyclic voltam-
metric techniques. Absorption of the neutral and cationic form of DPA is enhanced due to the formation
of a 1:1 complex with ␤-CD. The formation of this complex has been confirmed by Benesi–Hildebrand
plot and docking studies by RasMol tool methods. The solid complex of ␤-CD with DPA is investigated by
FT-IR, XRD and AFM methods. The thermodynamic parameters (ꢀG, ꢀH and ꢀS) of inclusion process are
also determined. The pKa values of neutral-monocation equilibria have been determined with absorption
(conjugate acid–base) titrations. A mechanism is proposed to explain the inclusion process.
© 2011 Elsevier B.V. All rights reserved.
Received 25 September 2010
Received in revised form 6 February 2011
Accepted 16 February 2011
Keywords:
␤-Cyclodextrin
Diphenylamine
Inclusion complex
pH effects
Cyclic voltammetry
RasMol tool
1. Introduction
Ramamurthy, Eaton, and Co-workers [13,14] have exploited the
use of CDs as host to examine photochemical and photophysical
Cyclodextrins (CDs) are cyclic oligosaccharides with internal
cavities capable of forming complexes with hydrophobic organic
molecules in aqueous solutions [1]. The inner diameters of the cav-
processes that occur in molecules complexed within them and to
compare their behavior in aqueous solutions and in the solid state.
Their studies dealt mostly with intramolecular events, with only
a single molecule present in the CD cavity. Furthermore, these
studies seem to indicate differences in the extent of inter- and
intramolecular hydrogen bond/intramolecular charge transfer for
the molecules in aqueous solutions of cyclodextrins. Applications
of cyclodextrins and their derivatives cover various areas of the
chemistry, including the sensing of organic molecules and organic
pollutants, analytical chemistry, pharmaceuticals, food and other
industrial areas [15–17].
The inclusion of a guest in a ␤-CD cavity consists basically of a
substitution of the included water molecules by the less polar guest
[18]. The process is energetically favoured 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 spite of the fact that the “driving force” of complexation
is not yet completely understood, it seems that it is the result of
various effects [18]; (a) substitution of the energetically unfavoured
polar–apolar interactions (between the included water and the ␤-
CD cavity on the one hand, and between water and the guest on the
other) by the more favoured apolar–apolar interaction (between
the guest and the cavity), and the polar–polar interaction (between
bulk water and the released cavity-water molecules). (b) ␤-CD-ring
strain release on complexation, (c) Van der Waals interactions and
hydrogen bonds between host and guest molecules.
˚
˚
˚
ities are approximately 4.5 A in ␣-CD, 7.0 A in ␤-CD and 8.5 A in
␥-CD [2]. The CDs are capable of incorporating a high range of
guest molecules based on hydrophobic and geometrical cavities
(Scheme 1). They have a toroidal shape with an internal hydropho-
bic surface and an external hydrophilic surface (Scheme 1) and they
are acting as a host molecule. These cyclodextrins are well known
as they form stable host–guest inclusion complexes which have the
interesting property of including organic, inorganic and biological
molecules in their cavities [3,4]. Efforts have been made to mod-
ify CDs so as to enhance their catalytic powers [5,6]. The physical
chemistry of complexation by CDs has been extensively studied
[1–6].
The chemical properties of cyclodextrins combined with their
non-toxic character to humans have led to their use in phar-
maceuticals, as food additives as well as in the environmental
de-contamination procedures of wastewater, aquifer, air, and soil
[7,8]. Particularly cyclodextrins and their derivatives have been
used to remove contaminations by the formation of inclusion com-
plexes or to enhance the solubility of several compounds [9–12].
∗
Corresponding author. Tel.: +91 9944266475; fax: +91 4565 225202.
E-mail address: tstalinphd@rediffmail.com (T. Stalin).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.02.030

170
K. Srinivasan et al. / Spectrochimica Acta Part A 79 (2011) 169–178
Scheme 1. (A) Chemical structures of ␣-CD, ␤-CD and ␥-CD. (B) The internal and external widths for ␤-cyclodextrin are noted in the figure and height of the cavity is
˚
˚
presumed to be ∼8 A and width size is 15.3 A.
In this study, we report the absorption characteristics of
diphenylamine (DPA) in different ␤-cyclodextrin concentrations.
For the past one decade, the corresponding author has largely been
involved in studying the photophysical properties [19–25] and
electrochemical properties [26] of various organic fluorophores.
This molecule shows 1:1 inclusion with ␤-CD molecule, and is
clearly confirmed by spectral analysis as well as docking method
by RasMol tool [27]. This stimulated us to carry out a study on
diphenylamine.
Chattopadhyay et al. [30] have studied photochemical conver-
sion of diphenylamine (DPA) to carbazole (CAZL) fluorometrically
in aerated aqueous and aqueous ␤-CD environments. KothaiNayaki
et al. [31] have studied the solvatochromism and prototropism
of DPA in aqueous solution. The techniques of UV–Vis, FT-IR,
XRD, AFM, CV and thermodynamic parameters have been used
to examine the effects of ␤-cyclodextrin upon complexation of
diphenylamine. And the formation constants of the complexes
were calculated.
Diphenylamine (DPA) and its derivatives are most commonly
used as stabilizers in nitrocellulose-containing explosives and pro-
pellants, in the perfumery, and as antioxidants in the rubber and
elastomer industry. DPA is also widely used to prevent post-harvest
deterioration of apple and pear crops. It is used for the production of
dyes, pharmaceuticals, photography chemicals and further small-
scale application [28]. First reports showed that DPA was found in
soil and groundwater. Some ecotoxicological studies demonstrated
the potential hazard of various diphenylamines to the aquatic
environment and to bacteria and animals. Studies on the biodegrad-
ability of DPA and its derivatives are very sparse. Therefore, further
investigation is required to determine the complete dimension of
the potential environmental hazard [29] and to introduce possible
(bio) remediation techniques for sites that are contaminated with
this class of compounds.
2. Experimental
2.1. Instruments
The absorption spectral (UV–vis spectrum) measurements were
carried out with Shimadzu UV-2401PC double-beam spectropho-
tometer. The pH values in the range 2.0–12.0 were measured using
Elico pH meter LI-120. Electrochemical studies were carried out
using Auto lab electrochemical analyzer, it used to apply potential
on the working electrode equipped with a three-electrode glassy
carbon electrode (diameter: 1 mm) is served as a working elec-
trode system. Reference electrode was saturated calomel electrode
(SCE) and counter electrode was platinum wire. All experiments
were carried out at 30 1 ◦C. The working electrode was polished

K. Srinivasan et al. / Spectrochimica Acta Part A 79 (2011) 169–178
171
Table 1
to a mirror with 0.05 ␮m alumina aqueous slurry, and rinsed with
triply distilled water before each experiment. The supporting elec-
trolyte was pH ∼1 (0.1 M H2SO4 + 0.1 M Na2SO4) and pH ∼7 (0.1 M
KH2PO4 + 0.1 M NaOH) buffer. Powder X-ray diffraction spectra
were taken by XPert PRO PANalytical diffractometer. FT-IR was
recorded using Nicolet 380, Thermo Electron Corporation Spec-
trophotometer and surface morphology was recorded using AFM
diCP-II, veeco USA.
Absorption maxima (nm) and log ε of DPA at different concentrations of ␤-CD in pH
∼1 and pH ∼7 solutions.
S.No.
Concentration
of ␤-CD (M)
pH ∼1
pH ∼7
ꢁmax
log ε ꢁmax
log ε
(nm)
(nm)
1
2
3
Without ␤-CD
0.002
0.012
280.0
282.0
283.0
682
−16.4
−6436
−21.2
3.89 279.0
4.17 281.5
4.18 283.0
343
4.03
4.20
4.22
Binding constant (M−1
)
ꢀG (kJ mol−1
ꢀH (kJ mol−1
)
)
−14.7
2.2. Reagents
−6436
ꢀS (kJ mol−1 k−1
)
−21.2
␤-Cyclodextrin {(␤-CD), were obtained from Sd fine chemical
Company} was used without further purification. Diphenylamine
(DPA, Loba Chemical Reagents Company) and other chemical
reagents were analytical reagent grade. Triply distilled water was
used to prepare all solutions. All solvents used were of the high-
est grade (Spectrograde). 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) [32] for the solutions below
pH ∼2 (using a H2SO4–H2O mixture) and Yagil basicity scale (H–)
[33] for solutions above pH ∼12 (using a NaOH–H2O mixture) were
employed. The solutions were prepared just before taking measure-
ments. The concentrations of the solutions were of the order (10−4
3. Results and discussion
3.1. Effect of ˇ-cyclodextrin
The UV–visible absorption spectral data of diphenylamine (DPA)
in different concentrations of ␤-CD are complied in Table 1. The
absorption peaks of DPA (1 × 10−4 mol dm−3) in pH ∼1 (Fig. 1) and
pH ∼7 (Fig. 2) appear at 280–283 nm (pH ∼1) and 279–283 nm (pH
∼7). Upon increasing the concentration of ␤-CD in both cases, no
clear isosbestic point is observed in the absorption spectra. In the
presence of ␤-CD, a slight red shift is observed in the absorption
maxima at pH ∼1 (280–283 nm) and a small red shift (279–283 nm)
at pH ∼7. The absorption spectra show a slight change in absorption
maxima even in the presence of the highest concentration of ␤-CD
used (12 × 10−3 mol dm−3) and there is no change in the absorbance
by further addition of ␤-CD. At pH ∼7, DPA exists as a neutral form;
hence we also recorded at pH ∼1 because monocation might be
present in acidic condition. The absorption maxima and the spectral
shape of DPA molecule in both pHs are almost same, so far there
has been no formation of monocation in pH ∼1. This behavior has
been attributed to the enhanced dissolution of the DPA molecule
through the hydrophobic interaction between DPA and ␤-CD. These
results indicated that DPA molecule is entrapped in the ␤-CD cavity
to form 1:1 inclusion complex. In both cases, the binding constant
for the formation of DPA-␤-CD complex has been determined by
analyzing the changes in the intensity of absorption maxima with
the ␤-CD concentration. In the case of inclusion complex formed
to 10−5 mol dm−3). The stock solution of DPA is 1 × 10−2 mol dm−3
.
2.3. ˇ-Cyclodextrin solution preparation
1 × 10−2 mol dm−3 concentration of stock solution of DPA was
prepared using ethanol and 0.2 ml of this stock solution was added
into 10 ml volumetric flasks and made up to the mark using the
following concentration of ␤-CD solutions 0, 2, 4, 6, 8, 10 and
12 × 10−3 mol dm−3 respectively and shaken thoroughly, so that
the final concentration of DPA in each flask is 2 × 10−4 mol dm−3
.
All the absorption spectra were recorded at 30 1 ◦C.
2.4. Preparation of ˇ-cyclodextrin solution for electrochemical
studies
The stock solution of ␤-CD (12 × 10−3 mol dm−3) was prepared
using pH ∼1 (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, 10 and 12 × 10−3 mol dm−3 of ␤-CD were prepared using
pH ∼1 and pH ∼7 buffers, so that the final concentration of DPA in
each flask is 2 × 10−4 mol dm−3. The cyclic voltammograms were
recorded after the solution was kept static for at least 24 h.
2.5. Preparation of solid inclusion complex of DPA with ˇ-CD
Accurately weighed 1 g of ␤-CD was placed into a 50 ml coni-
cal flask and 30 ml triply distilled water was added and then this
solution was oscillated enough. After that, 0.15 g DPA was put in to
a 50 ml beaker and 20 ml was ethanol added and put over electro-
magnetic stirrer to be stirred until it was dissolved [34]. Then DPA
solutions were slowly poured into ␤-CD solution. The above-mixed
solution was continuously stirred for 48 h at room temperature. The
reaction mixture was kept in refrigerator for 48 h. At this time, we
observed that white precipitate is formed. The precipitate was fil-
tered by G4 crucible and washed with triply distilled water. After
being dried in oven at 50 ◦C for 12 h, off white powder was obtained.
This is solid inclusion complex of DPA with ␤-CD and it was further
analysed by FT-IR, AFM and XRD methods.
Fig. 1. The absorption spectra of DPA (pH ∼1) in different ␤-CD concentrations (a)
0.0 M, (b) 0.002 M, and (c) 0.012 M.

172
K. Srinivasan et al. / Spectrochimica Acta Part A 79 (2011) 169–178
Scheme 2. The proposed structure of inclusion complex of DPA with ␤-CD for 1:1
inclusion complex.
Fig. 2. The absorption spectra of DPA (pH ∼7) in different ␤-CD concentrations (a)
0.0 M, (b) 0.002 M, and (c) 0.012 M.
At constant temperature
ꢀH − ꢀG
between DPA and ␤-CD, the equilibrium can be written as
ꢀS =
T
K
DPA + ␤-CDꢀDPA . . . ␤-CD
The thermodynamic parameters ꢀG, ꢀH and ꢀS for the binding
of the guest molecule to ␤-CD cavity are given in Table 1. As can
be seen from Table 1, ꢀG is negative which suggests that the inclu-
sion process proceeded spontaneously at 303 K. ꢀH and ꢀS are also
negative in the experimental temperature range, which indicates
that the inclusion process is an exergonic and enthalpy controlled
process. The negative enthalpy change (ꢀH) arose from the van der
Waals interaction, while the negative entropy change (ꢀS) is the
steric barrier caused by molecular geometrical shape and the limit
of ␤-CD cavity to the freedom of shift and rotation of DPA molecule.
The experimental results indicated that the inclusion reaction of
␤-CD with DPA was an exothermic reaction accompanied with neg-
ative ꢀS. In this case, the actions that enthalpy and entropy change
played were on the contrary. The conclusion of these thermody-
namic parameters is that changes in ꢀH are largely compensated
for by changes in ꢀS.
The binding constant ‘K’ and stoichiometric ratios of the
inclusion complex of DPA can be determined according to the
Benesi–Hildebrand [35] relation assuming the formation of a 1:1
host–guest complex.
1
1
ꢀε
1
=
+
A − A0
K[DPA]0ꢀε[␤-CD]0
where A and A0 are the difference between the absorbance of DPA
in the presence and absence of ␤-CD, ꢀε is the difference between
the molar absorption coefficient of DPA and the inclusion complex,
[DPA]0 and [␤-CD]0 are the initial concentration of DPA and ␤-CD,
respectively. Fig. 3 depicts a plot of 1/A − A0 vs. 1/[␤-CD] for DPA (in
both pHs) and a good linear correlation was obtained, confirming
the formation of a 1:1 inclusion complex. From the intercept and
slope values of this plot, K is evaluated and the binding constants
for neutral DPA (pH ∼7, 343 M−1) are considerably less than those
of its acidic condition form (pH ∼1, 682 M−1) at 303 K. The binding
constants are sensitive to change in the pH values, which reveal
that selective inclusion associated with the species form (neutral
and acidic condition) of DPA.
Considering the above discussions, the possible inclusion mech-
anism is proposed. Naturally, the inclusion complex formation
between DPA and ␤-CD is possible with the imine group captured
inside the ␤-CD cavity and it is shown in Scheme 2.
3.2. Electrochemical method
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
The formation of inclusion complex of DPA and ␤-CD was
also confirmed by electrochemical analytical method. The elec-
trochemical behavior of DPA has been widely researched in same
observation [36]. If ␤-CD interacted with DPA, different electro-
chemical properties would be observed. First, some DPA was put
into ␤-CD aqueous solution (pH ∼1 and pH ∼7) with sufficient
stirring. Then the cyclic voltammograms were recorded after the
solution was kept static for at least 24 h.
ꢀG = −RT In K
From Figs. 4 and 5 it is observed that the anodic peak current,
ipa, decreased drastically with increasing concentration of ␤-CD
in both pHs (pH ∼1 and pH ∼7). The anodic peak potential, Epa
,
shifted in positive direction when ␤-CD was increased in both
cases. The result showed that the inclusion complex between ␤-
CD and DPA was formed when ␤-CD was added into DPA aqueous
solution. The diffusion coefficient of the inclusion complex from
bulk layer to electrode surface was very slow than that of the DPA
monomer itself, which leaded the current decrease. On the other
hand, because DPA monomer entered into the hydrophobic cavity
Fig. 3. Benesi–Hildebrand plot of 1/A − A0 vs. 1/[␤-CD] for DPA in pH ∼1 and pH ∼7
solutions.

K. Srinivasan et al. / Spectrochimica Acta Part A 79 (2011) 169–178
173
Table 2
CV for DPA–␤-CD in pH ∼1 buffer solution and scan rate 100 mV s−1, DPA (Conc.
2 × 10−4 M) solution and 0–12 × 10−3 M ␤-CD concentrations.
␤-CD con-
centration
(M)
Epa (V)
Ipa (␮A)
Ipc (␮A)
−4.28
Epc (V)
(Epa − Epc)/
2(mV)
Without
␤-CD
0.710
15.09
0.374
168
0.002
0.004
0.006
0.008
0.010
0.012
0.724
0.737
0.754
0.777
0.790
0.790
4.24
2.92
1.65
1.36
0.95
0.93
−3.00
−3.03
−2.08
−2.93
−2.17
−2.16
0.364
0.367
0.296
0.375
0.307
0.307
180
185
229
201
242
243
964
Binding
constant
(M−1
)
Table 3
CV for DPA–␤-CD in pH ∼7 buffer solution and scan rate 100 mV s−1, DPA (Conc.
2 × 10−4 M) solution and 0–12 × 10−3 M ␤-CD concentrations.
␤-CD con-
centration
(M)
Epa (V)
Ipa (␮A)
Ipc (␮A)
−1.531
Epc (V)
(Epa − Epc)/
Fig. 4. CV for DPA in pH ∼1 buffer solution, scan rate 100 mV s−1, DPA (Conc.
2 × 10−4 M) solution in different concentrations of ␤-CD (a) 0.0 M, (b) 0.002 M, (c)
0.004 M, (d) 0.006 M, (e) 0.008 M, (f) 0.010 M and (g) 0.012 M.
2(mV)
Without
␤-CD
0.473
10.690
0.159
157
0.002
0.004
0.006
0.008
0.010
0.012
0.484
0.512
0.514
0.518
0.519
0.521
7.726
4.165
3.129
1.975
1.881
1.781
−2.162
−1.709
−1.136
−0.922
−0.901
−0.789
−0.051
−0.026
−0.044
−0.052
−0.041
−0.038
268
269
274
285
280
280
463
of ␤-CD, it was reasonable that the electrochemical oxidation of the
inclusion complex was more difficult than that of DPA monomer
itself, which would lead the anodic peak potential shift in positive
direction, the anodic peak current, iap, decreased and also higher
activation energy would be observed in both pHs.
From Tables 2 and 3 the oxidation potential of DPA is 0.710 V,
0.473 V and the reduction potential is 0.374 V, 0.159 V in pH ∼1
and pH ∼7 respectively. The difference between the oxidation and
reduction potentials is 168 mV and 157 mV for pH ∼1 and pH ∼7
respectively, so this reaction in both pH media is quasi reversible. By
increasing the ␤-CD concentration from 2 × 10−3 M to 12 × 10−3 M,
the difference between the oxidation and reduction potentials is
increased to 242 mV and 280 mV for pH ∼1 and pH ∼7 respec-
tively. Because of this reaction moves towards irreversibility and
because DPA monomer entered into the hydrophobic cavity of ␤-
CD, it was reasonable that the electrochemical oxidation of the
inclusion complex was more difficult than that of DPA itself.
Binding
constant
(M−1
)
The oxidation potential of DPA in pH ∼1 is higher (0.710 V) than
that in pH ∼7 (0.473 V), this is due to the formation of monocation
(acidic condition) in pH ∼1, because of this monocation oxidation
observed at higher potential (0.710 V) and the peak current also
increases (15.09 mV) due to loss of protonated H+. In pH ∼7 the
lone pair of electron gets oxidized at lower potential (0.473 V) and
peak current also decreases (10.69 mV). In pH ∼1 system 2e− and
1H+ are involved in electrochemical reaction [37] and in pH ∼7
system only 2e− is involved [37,38].
Fig. 6 shows the relationship between the anodic peak potential
of DPA and the solution pH. It is found that the potentials (Epa
)
shifted negatively with increasing pH, indicating that protons take
part in the oxidation process of DPA at the GCE surface. The anodic
peak potential (Epa) is proportional to the solution pH in the range of
Fig. 5. CV for DPA in pH ∼7 buffer solution, scan rate 100 mV s−1, DPA (Conc.
2 × 10−4 M) solution in different concentrations of ␤-CD (a) 0.0 M, (b) 0.002 M, (c)
0.004 M, (d) 0.006 M, (e) 0.008 M, (f) 0.010 M and (g) 0.012 M.
Fig. 6. The anodic peak potential (Epa) is proportional to the solution pH ∼1–9.

174
K. Srinivasan et al. / Spectrochimica Acta Part A 79 (2011) 169–178
Scheme 3. Electrochemical reaction mechanism of DPA, (a) pH ∼1 system and (b) pH ∼7 system.
Fig. 7. Benesi–Hildebrand plot of 1/[1/IHG − IG] vs. 1/[␤-CD] for DPA in pH ∼1 and
pH ∼7.
Fig. 8. The peak current of DPA changes at 0.710 V and 0.473 V with different ␤-CD
concentrations in pH ∼1 and pH ∼7 respectively.
1–9. The linear-regression equation of DPA is described as following
with a correlation coefficient of 0.9923.
Benesi–Hildebrand [35] relation assuming the formation of a 1:1
host–guest complex.
Epa (V) = 0.7472 − 0.0368pH
1
1
ꢀI
1
=
+
IHG − IG
K[DPA]0ꢀI[␤-CD]
Thus the probable electrochemical reaction mechanism of DPA
(denoted as DPAH) is given in Scheme 3.
where IG is the oxidation peak current of guest molecule of DPA, and
IHG is the oxidation peak current of inclusion complex of DPA–␤-
CD. IHG − IG is the difference between the oxidation peak current
The binding constant
K and stoichiometric ratios of the
inclusion complex of DPA can be determined according to the
Scheme 4. Various prototropic equilibria of DPA in aqueous and ␤-CD medium.

K. Srinivasan et al. / Spectrochimica Acta Part A 79 (2011) 169–178
175
Table 4
Prototropic maxima (absorption) of DPA in with and without ␤-CD medium.
With ␤-cyclodextrin
Without
␤-cyclodextrin [31]
Species
ꢁmax
pKa
ꢁmax
pKa
(nm)
(nm)
Monocation
Neutral
Monoanion
282.0
283.5
–
−0.85
2.0–15.75
–
255.0
279.0
298.4
0.78
2–14
15.58
of inclusion complex and DPA. ꢀI is the difference between the
molar peak current coefficient of the inclusion complex and DPA,
[DPA]0 and [␤-CD]0 are the initial concentrations of DPA and ␤-CD,
respectively.
A plot of [1/IHG − IG] vs. 1/[␤-CD] gives a straight line for both
pH solutions as shown in Fig. 7. A good linear correlations were
obtained, confirming the formation of a 1:1 inclusion complex for
both pH solutions. From the intercept and slope values of this plot
K was evaluated, the binding constant for DPA: ␤-CD is 964 M−1
and 463 M−1 in pH ∼1 and pH ∼7 respectively. When the ␤-CD
concentrations are higher than 8 × 10−3 mol dm−3, the oxidation
peak current remains unchanged and there is no change in the
peak potential differences by further addition of ␤-CD (Fig. 8). This
behavior has been attributed to the enhanced dissolution of the
DPA molecule through the hydrophobic interaction between DPA
and ␤-CD. These results indicate that DPA molecule is entrapped in
Fig. 9. Absorption spectra of DPA in different prototropic species at 303 K with ␤-CD
(a) monocation and (b) neutral.
the ␤-CD cavity to form inclusion complex and it proved electro-
chemically.
Fig. 10. FT-IR spectra of (A) ␤-CD, (B) DPA and (C) DPA–␤-CD solid complex in KBr.

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K. Srinivasan et al. / Spectrochimica Acta Part A 79 (2011) 169–178
Scheme 5. (A) Ball and stick representation of ␤-cyclodextrin. The oxygen atoms of –OH groups are shown as red balls, hydrogen atoms are not shown. (B) Ball and stick
representation of DPA. The hydrogen atoms are shown as magenta balls and nitrogen atom as blue ball. (C) Ball and stick representation of proposed inclusion complex
structure of DPA with ␤-cyclodextrin. The hydrogen atoms of DPA are shown as magenta balls and nitrogen atom as blue ball. The –OH group oxygen atoms of ␤-cyclodextrin
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 the
article.)
3.3. Effect of pH with ˇ-CD medium
When pH is increased above 14 no noticeable change is observed in
the absorption spectra, whereas blue shift is observed in neutral to
monocation of DPA in ␤-CD when compared to aqueous (without ␤-
CD) medium [31]. But the monoanion is not formed above pH ∼14
in ␤-CD medium. Further the pKa value for the monocation–neutral
equilibrium of DPA in ␤-CD is higher than in aqueous medium; this
finding confirms DPA molecule encapsulated in the ␤-CD cavity.
Further it is confirmed by RasMol tool (Scheme 5) [27].
The absorption spectra of DPA have been studied in the pH
range H0 −0.84 to H 15.75 in ␤-CD medium. KothaiNayaki et al.
[31] studied the absorption spectra of DPA in the pH range with-
out ␤-CD, H0 −3 to H 16 in aqueous solution, there are three
prototropic species (monocation–neutral–monoanion) present in
aqueous medium. But in the case of ␤-CD medium it presents two
species (Table 4 and Fig. 9) (neutral and monocation). When com-
pared to aqueous medium there is some appreciable change in the
absorption maxima of neutral to monocation (Fig. 9). The equilib-
rium of DPA could be due to the presence of resonating structure
as shown in Scheme 4. The absorption maxima of DPA have been
taken in 12 × 10−3 M ␤-CD aqueous solutions in the pH range (H0
−0.85 to H 15.75). Noticeable change is observed in the absorp-
tion spectral intensity, when pH was decreased below pH ∼1.0 and
the absorption maximum also decreased in 2 M H2SO4 solution.
Though a new blue-shifted absorption spectrum was observed at
282 nm due to the formation of the monocation by the protonation
of imino group and there is no clear isosbestic point observed for
this ground state equilibrium between the cation and neutral forms.
Scheme 6. DPA–␤-CD 1:1 host–guest mechanism.
Fig. 11. XRD pattern of (A) ␤-CD, (B) DPA and (C) DPA–␤-CD solid complex.

K. Srinivasan et al. / Spectrochimica Acta Part A 79 (2011) 169–178
177
Fig. 12. Atomic force microscope photographs of (a) ␤-CD, (b) DPA and (c) DPA–␤-CD solid complex in 3D and 2D view.
3.4. FT-IR spectral studies
a new crystal structure of solid complex.
The host molecule (␤-CD) reacts with guest molecule (DPA) to
form a host–guest solid complex. The reaction sketch is as follows
(Scheme 6): according to the above reaction solid complex for-
mation may be confirmed by FT-IR spectroscopy (Fig. 10) because
the bands resulting from the included part of the guest molecule
are generally shifted or their intensities altered [39]. If ␤-CD and
DPA form a solid inclusion complex, the non-covalent interactions
between them such as hydrophobic interactions, Van der Waals
interactions and hydrogen bonds lower the energy of the included
part of DPA, it thus reducing the absorption intensities of the cor-
responding bands. We can see that there are apparent differences
between the spectra of ␤-CD, DPA and solid inclusion complex. For
example, the 1595.2 cm−1 absorption peak (Fig. 10B) which could
be assigned to the stretching vibration of N–H, its absorption inten-
sity was reduced (Fig. 10C) obviously as the solid complex was
formed. In addition, 1319.3 cm−1 absorption peak which may be the
C–N stretching vibration of benzene ring skeleton, its absorption
intensity was reduced and shifted to 1313.0 cm−1 (Fig. 10C) in the
solid complex. Based on these facts we can conclude that hydropho-
bic group (>NH) of DPA is included into the ␤-CD cavity. Moreover,
the absorption peak at 746.0 cm−1 and 698.3 cm−1 (Fig. 10B) are
the characteristic peaks of monosubstituted benzene; decrease of
its absorption intensity (Fig. 10C) confirms the solid complex for-
mation. It is interested that decrease of absorption intensity N–H
at 1595.2 cm−1 proves that depth of guest molecule DPA insert into
␤-CD cavity. In the past it was thought that benzene ring was only
included when the molecule containing benzene ring reacted with
␤-CD to form a supramolecular inclusion complex [40]. From the
above discussions we can conclude that DPA molecule was included
into ␤-CD cavity.
3.6. Atomic force microscopic morphological observations
Atomic force microscopes (AFMs) are well suited for visualiz-
ing the surface texture of the deposited films, especially when the
surface feature sizes are far below three micron. The AFM 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 ␤-CD and DPA by AFM (Fig. 12a and b respectively) method,
and then we also observed the surface morphological structure of
solid inclusion complex (Fig. 12c). Pictures clearly elucidated the
difference of each other. Modification of crystals can be assumed
as a proof of the formation of a solid inclusion complex. Measur-
ing the surface texture of ␤-CD, DPA and solid complex films with
horizontal length scale of less than 3 ␮m and a vertical length scale
of 159.7 nm, 35 nm and 136 nm respectively. This solid inclusion
complex formation may lead to the preparation of nanomaterials
using ␤-CD.
4. Conclusions
The following conclusion can be arrived at from the above stud-
ies; (i) DPA forms 1:1 inclusion complex with ␤-CD (ii) prototropic
reactions in ␤-CD medium indicate, NH group present in the upper
rim (polar) of the ␤-CD cavity (iii) FT-IR, XRD, AFM and RasMol tool
results suggest that DPA formed a solid inclusion complex with ␤-
CD (iv) the above studies demonstrate that in DPA, ICT interactions
play a significant role in ␤-CD aqueous/polar medium.
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