Inclusion complexation of phenoxyaliphatic acid derivatives of 3,3′-bis(indolyl)methanes with β-cyclodextrin

Article abstract of DOI:10.1007/s10895-014-1373-4

The inclusion complexation behavior of phenoxyaliphatic acid derivatives of 3,3′-bis(indolyl)methane (BIMs 1-5) with β-cyclodextrin (β-CD) were investigated in both solution and solid state by means of UV-Visible, fluorescence spectroscopy, FT-IR and

Full text of DOI:10.1007/s10895-014-1373-4

J Fluoresc (2014) 24:925–931
DOI 10.1007/s10895-014-1373-4
ORIGINAL PAPER
Inclusion Complexation of Phenoxyaliphatic Acid Derivatives
of 3,3′-bis(indolyl)methanes with β-Cyclodextrin
A. Antony Muthu Prabhu & G. S. Suresh Kumar
Received: 10 December 2013 /Accepted: 24 February 2014 /Published online: 13 March 2014
#
Springer Science+Business Media New York 2014
Abstract The inclusion complexation behavior of
phenoxyaliphatic acid derivatives of 3,3′-bis(indolyl)methane
(BIMs 1–5) with β-cyclodextrin (β-CD) were investigated in
both solution and solid state by means of UV-Visible, fluores-
chemists [1]. BIMs exhibit the various applications in phar-
maceutical industry [2–4]. Shao et al. reported that bis(3-
indolyl)methane skeleton is used as a chemosensor with the
anion F− and AcO− in organic aprotic solution [5]. Kim and
coworkers studied that bis(3-indolyl)methane skeleton has
been demonstrated to be an efficient chromogenic and fluo-
rescent moiety for metal ion sensing [6]. Due to the versatile
application possibilities of BIMs, the syntheses of these com-
pounds have become an interesting target for synthetic
chemists.
1
cence spectroscopy, FT-IR and H NMR techniques. The
nature of the host–guest inclusion complex between BIMs
and β-CD has been elucidated. The experimental results con-
firmed the existence of 1:1 inclusion complex of BIMs with
β-CD. The binding constants describing the extent of forma-
tion of the complexes have been determined using Benesi-
Hildebrand plots using UV-Vis and fluorescence spectrosco-
py. BIMs exhibited an affinity for β-CD. The spectral studies
suggested the phenyl ring along with alkyl substitutions of
BIMs is present inside of β-CD cavity.
Cyclodextrins (CDs) and their derivatives are as models of
biological receptor–substrate interactions is a significant topic
in chemistry and biochemistry [7]. CDs can form inclusion
complexes with a variety of size suitable organic and inorgan-
ic molecules [8]. Therefore, many properties of guest mole-
cules are affected remarkably, such as chemical reactivity [9],
volatility [10], absorption spectrum [11], and so on [12].
These changes in chemical and physical properties are of both
theoretical and practical interest. CDs and their derivatives
have attracted attention as enzyme models [13], chromato-
graphic phases [14], additives for foods [15], pharmaceuticals
[16] etc. In particular, CD derivatives were applied in drugs to
increase the solubility, bio-utility [17], stability [18] etc.
In this regard, the present investigation has been carried out
the study of the photophysical properties of phenoxyaliphatic
acid derivatives of 3,3′-bis(indolyl)methane in its ground and
excited states in the presence of β-CD. The solid inclusion
complexes of these molecules also were prepared and charac-
terized using FT-IR and 1H NMR techniques. In the last few
years, our emphasis has been to study the effect of solvents of
different polarity, acid–base concentrations, natural-, and
modified α- and β-cyclodextrins on the spectral characteris-
tics of different fluorophores [19–23]. And also we have been
synthesized the biologically active phenoxyaliphatic acid and
ester derivatives of 3,3′-bis(indolyl)methanes from indole and
formylphenoxyaliphatic acids in the presence of potash alum
Keywords Phenoxyaliphatic acid derivatives of
.
.
3,3′-bis(indolyl)methanes β-cyclodextrin Inclusion
complexes
Introduction
The importance of bis(indolyl)methanes (BIMs) and their
derivatives is well recognized by natural as well as synthetic
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-014-1373-4) contains supplementary material,
which is available to authorized users.
A. Antony Muthu Prabhu (*)
Department of Chemistry, Manonmaniam Sundaranar University,
Tirunelveli 627 012, Tamilnadu, India
e-mail: antonyphdchem@yahoo.com
G. S. Suresh Kumar
Faculty in Chemistry, Department of Science and Humanities,
Dr. Mahalingam College of Engineering and Technology
(Dr. MCET), Udumalai Road, Pollachi 642 003, Tamilnadu, India

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J Fluoresc (2014) 24:925–931
[24] and potassium titanyl oxalate [25] as a catalyst. This
prompted us to study the effect of β-cyclodextrin on
phenoxyaliphatic acid derivatives of 3,3′-bis(indolyl)methane
using photophysical studies.
smart and fluorescence measurements were made by using a
Elico sl 170 spectrofluorimeter. Infrared spectra were recorded
on a JASCO FT-IR Model 410 spectrophotometer (in KBr
pellet). Band positions are reported in reciprocal centimetres
(cm−1). 1H NMR spectra were recorded on a Bruker (Avance)
300 MHz NMR instrument using TMS as internal standard
and CDCl3 as solvent.
Experimental
Materials
Results and Discussion
Using our previous reported method [24], BIMs (1–5) were
synthesized from indole and formylphenoxyaliphatic acid(s)
in the presence of potash alum. BIMs (1–5) were isolated
using column chromatography. The chemical name of the
molecules are given one by one: (i) 2-{2-[bis(1H-indol-3-
yl)methyl]}benzene (BIM-1), (ii) 2-{2-[bis(1H-indol-3-
yl)methyl]phenoxy}acetic acid (BIM-2), (iii) 2-{2-[bis(1H-
indol-3-yl)methyl]-6-methoxyphenoxy}acetic acid (BIM-3),
(iv) 4-{2-[bis(1H-indol-3-yl)methyl]phenoxy}acetic acid
(BIM-4), and (v) 4-{2-[bis(1H-indol-3-yl)methyl]-6-
methoxyphenoxy}acetic acid (BIM-5). The optimized struc-
tures of BIMs were shown in Fig. 1 using DFT method. β-
Cyclodextrin was purchased from Sigma-Aldrich chemical
company and used without further purification. Triply distilled
water was used for the preparation of aqueous solutions.
Effect of β-Cyclodextrin
Table 1 summarizes the absorption maxima, molar extinction
coefficient and fluorescence maxima of BIMs and β-CD
inclusion complexes in aqueous solution. Figure 2 depicts
the absorption and fluorescence spectra of BIM1 in aqueous
solution containing various concentrations of β-CD (vide
Absorption and fluorescence spectra of BIM 2–5 are given
in the supporting information—Fig. S1 and S2). The insert
Fig. 2 show the changes of both absorbance and fluorescence
intensity was observed as a function of the concentration of β-
CD. The absorption spectrum of BIMs in aqueous solution
(2×10−5 M) exhibited λmax around 290 nm for π–π* transi-
tion with less or more molar absorptivity of 35.1570×
103 M−1 cm−1 for all molecules. Upon increasing the β-CD
concentrations, the absorbance of BIMs are increased at the
same wavelength without the shift of the absorption maxi-
mum. The increase in absorbance is due to the detergent action
of β-CD and is attributed to the additional dissolution of the
guest adsorbed on the surface of the walls of the container
[27–29]. The above interesting results are due to all molecules
are transferred from more protic environment (bulk aqueous
phase) to less protic β-CD cavity environment.
No clear isosbestic point was observed in the absorption
spectra and the absorbance changes are very small. In general,
the existence of an isosbestic point in the absorption spectra is
an indication to the formation of well defined 1:1 complexes
[27–29]. The possibilities proposed for this deviation are (i)
more than one guest molecule could have got accommodated
within a single β-CD cavity, (ii) due to the space restriction of
β-CD cavity, more than one type of complex each having 1:1
stoichiometry might have been formed, (iii) the solution con-
taining methanol (1 %) could have made some interaction
between these components, and (iv) the strong detergent ac-
tion of β-CD could have prevented the formation of isosbestic
point [30–33]. In β-CD solutions, increase in the absorbance
indicates that the aromatic ring is encapsulated in the non-
polar β-CD cavity.
Preparation of Inclusion Complexes in Solution
A stock solution of BIMs was prepared in methanol and the
concentration of stock solution was 2×10−3 M. Exactly 0.2 ml
of this stock solution was transferred into each 10 ml volu-
metric flask. To this, varying concentrations of β-CD solution
(range from 1.0×10−3 to 1.0×10−2 M) was added. The mixed
solution was diluted to 10 ml with triply distilled water and
shaken thoroughly. The final concentration of BIMs in all the
flasks was 4×10−5 M.
Preparation of Inclusion Complexes in Solid State
The inclusion complexes of the BIMs with β-cyclodextrin
were prepared as per co-precipitation method [26]. The solu-
tion of these molecules in required concentrations were added
drop by drop to β-cyclodextrin solution of the required con-
centration. The mixtures were stirred for a period of 48 h and
filtered. The filtrate was cooled for 24 h in refrigerators. The
precipitate obtained was filtered through G-4 crucible, washed
with water and dried in air for 24 h. After drying in an oven at
60 °C for 24 h, a pale pink colour solid was obtained.
Instruments
Fluorescence spectra of BIMs with β-CD were measured
in aqueous solution at pH 7.2. The maximum absorption
wavelength of BIMs was chosen for excitation. The excitation
and emission maxima of BIMs are presented in Table 1. With
Absorption spectral measurements were carried out with a
Systronics-double beam UV-visible spectrophotometer 2203

J Fluoresc (2014) 24:925–931
927
Fig. 1 The optimized structures of a BIM 1, b BIM 2, c BIM 3, d BIM 4 and e BIM 5 using DFT method
the addition of β-CD to the solution of BIMs, the fluorescence
intensity was markedly enhanced (Fig. 2). The increase of
fluorescence intensity is due to the formation of complexes of
β-CD with BIMs could be explained as the guest molecules
were entrapped in the cavity of β-CD. The microenvironment
around β-CD with smaller polarity and stronger rigidity
would restrict the freedom of guest molecules inside the cavity
and increase the fluorescence intensity. Furthermore, the steric
hindrance of β-CD torus can protect the excited states from
non-radiative and quenching process that normally occur in
bulk buffer solution and enhance the fluorescence efficiencies
of guest molecules [34]. The fluorescence intensity of BIMs
gradually increased with increasing the concentration of β-
CD and finally reached a saturation point (Fig. 2).
Table 1 Absorption and emission spectral studies of BIM 1–5 in different concentrations of β-cyclodextrin
Concentration of β-CD M BIM 1
BIM 2
BIM 3
BIM 4
BIM 5
λabs
log ε λflu
λabs
log ε λflu
λabs
log ε λflu
λabs
log ε λflu
λabs
log ε λflu
Water
0.002
0.004
0.006
0.008
0.01
290
230
290
230
290
230
290
230
290
230
290
230
290
3.11 350
4.01
3.14 350
4.03
3.16 350
4.07
3.19 350
4.11
3.23 350
4.15
3.26 350
4.18
290
230
290
230
290
230
290
230
290
230
290
230
290
3.38 350
4.21
3.40 350
4.23
3.43 350
4.25
3.47 350
4.28
3.51 350
4.30
3.54 350
4.33
290
230
290
230
290
230
290
230
290
230
290
230
290
3.40 350
4.25
3.42 350
4.28
3.45 350
4.30
3.48 350
4.33
3.51 350
4.36
3.54 350
4.39
290
230
290
230
290
230
290
230
290
230
290
230
290
3.65 350
4.38
3.68 350
4.40
3.70 350
4.43
3.72 350
4.46
3.75 350
4.49
3.78 350
4.52
290
230
290
230
290
230
290
230
290
230
290
230
290
3.68 350
4.40
3.71 350
4.43
3.73 350
4.46
3.76 350
4.48
3.79 350
4.51
3.82 350
4.54
Excitation wavelength
Binding constant (M−1
ΔG KJ / mole (−ve)
)
322
410
386
457
408
502
448
557
531
635
14.54
15.15 15.00
15.42 15.14
15.66 15.37
15.92 15.80
16.25

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J Fluoresc (2014) 24:925–931
0.80
0.60
bonding, hydrophobic and van der Waals forces), size and
shape of the guest molecules relative to the host cavity.
0.4
0.3
0.2
BIM1
4
12
M
0
8
[β-CD] x 10-3
0.40
0.20
Solvatochromic Studies
7
To deduce the polarity effect on the microenvironment of
BIMs within the β-CD cavity, we have studied a
solvatochromic study by comparing the absorption and the
fluorescence of BIMs in solvents with increasing polarity
[24]. The spectral properties of BIMs in different solvents
are compared with that of the inclusion complexes
(BIMs–β-CD). With comparison of these results of BIMs,
the absorption maximum is slightly red shifted from cyclo-
hexane to water and in inclusion complexes. No significant
change was observed in the absorption maxima in β-CD
compared to water except the increase in absorbance. The
emission spectra of inclusion complexes closely resemble to
BIMs spectra in alcohol, suggesting that the inclusion com-
plexes formed is in an alcohol-like environment provided by
the rings of hydroxyl groups at each end of β-CD cavity. The
solvatochromic shifts reveal that the hydrogen bonding is
present along with dipole interactions.
1
360
200
320
240
280
Wavelength (nm)
600
450
700
BIM1
If 500
300
0
4
8
12
M
300
150
[β-CD] x 10-3
7
1
380
Wavelength (nm)
460
340
420
300
Fig. 2 Absorption and fluorescence spectra of BIM1 in different β-
cyclodextrin concentrations (M): (1) 0, (2) 0.002, (3) 0.004, (4) 0.006,
(5) 0.008 and (6) 0.01. Insert fig.: absorbance and fluorescence intensity
vs. β-CD concentrations
The free energy change was calculated from the binding
constant (K):
Interestingly, at higher β-CD concentrations, the absorption
and emission spectral shape of all molecules are same indi-
cating the formation of similar type of inclusion complexes.
ΔG ¼ −RTlnK
ð2Þ
Determination of Binding Constant
The value of thermodynamic parameter ΔG for the forma-
tion of the guest molecules to β-CD is given in Table 1.
Change in Gibbs free energy (ΔG) associated with various
BIMs–β-CD complexations was evaluated from their respec-
tive K values. Table 1 reveals that there is a strong host–guest
binding ability between β-CD and BIMs quantitatively. It can
be concluded that these complexation process is highly feasi-
ble, which is evident from the negative value of ΔG for
BIMs–β-CD inclusion complexes.
The binding constant for all the inclusion complexes has been
determined by analyzing the changes in the intensity of ab-
sorption and fluorescence maxima with β-CD concentration.
In order to determine the stoichiometry of the inclusion com-
plexes, the dependence on β-CD of the BIMs absorbance and
fluorescence were analyzed by using the Benesi-Hildebrand
equation [35]. 1:1 (Eq. 1) complexes from BIMs and β-CD
are as shown below:
Further, the calculated parameter show negative values of
ΔG, suggesting that the formation of inclusion complexes
between BIMs and β-CD were an entropy driven process in
which the key role is played by van der Waals forces [37]. In
this mechanism, when water molecules bind inside the β-CD
cavity they are replaced by the guest molecules during com-
plexation; due to the above process entropy is released with
consequent reduction of the energy of the system. The nega-
tive free energy change can be attributed to the reduced
configurational freedom of the entrapped guest molecules.
The fluorescence enhancement in the inclusion complexes
indicated that BIMs were interactive within the less protic
interior of the β-CD cavity than in the aqueous phase.
Considering the above discussions, the possible inclusion
mechanism is proposed as follows. Generally different types
of inclusion complexes may be formed with BIMs and β-CD
1
1
1
¼
þ
ð1Þ
I−I0
I0−I0 KðI−I0Þ½β−CDŠ
Where K is the binding constant, I0 is the initial absorption/
fluorescence intensity of free guest molecules, I′ is the
absorption/fluorescence intensity of β-CD inclusion complex
and I is the observed absorption/fluorescence intensity. Ac-
cording to Eq. 1, a plot of 1/I−I0 versus 1/[β-CD] (both
absorption and fluorescence) gives a straight line (Fig. 3). This
analysis reflects the formation of 1:1 inclusion complex be-
tween BIMs and β-CD [36]. The binding constant (K) for the
inclusion complexes were determined from the slope and
intercept of the B-H plots (Table 1). The binding constant
value depends on several parameters like weak forces (H-

J Fluoresc (2014) 24:925–931
929
120
is captured or the benzene ring is captured whereas in 1:2
inclusion complex both indole and aromatic rings may be
captured by the β-CD cavity. In 1:1 inclusion complexes,
the indole ring is embedded in the β-CD environment and
the –NH group could form a hydrogen bond with the second-
ary hydroxyl group of the bigger cyclodextrin side, whereas
the phenyl ring is included in the β-CD environment and not
possible to form a hydrogen bond with the secondary or
primary hydroxyl group of β-CD cavity. We may expect the
dual emission in the β-CD environment for the 1:2 inclusion
complexes between BIMs and β-CD. N. Rajendiran et al. [38,
39], reported that the tricylic compounds was formed different
types of inclusion complexes with β-CD. In these molecules,
the two emission wavelengths were observed in the presence
of β-CD due to the formation of different types of inclusion
complexes. Further, the Benesi–Hildebrand analysis reflects
that one type of inclusion complex is formed between BIMs
and β-CD.
BIM1
BIM2
BIM3
BIM4
BIM5
90
60
30
0
0
200
400
800
1/[β-CD] M-1
1200
1000
600
0.08
0.06
0.04
BIM1
BIM2
BIM3
BIM4
BIM5
0.02
0
This is further supported by density functional theory cal-
culation (DFT). The internal diameter of the β-CD is approx-
imately 6.5 Å and its height is 7.8 Å. To determine the
dimensions of BIMs geometry of the ground state was opti-
mized by DFT. Based on single crystal XRD data [24] and
DFT for BIM-2, the vertical distance between H1–H15 is
8.58 Å whereas the horizontal distance between H5–H11 is
0
200
400
800
1200
1000
600
1/[β-CD] M-1
Fig. 3 The Benesi-Hildebrand plot of absorption and fluorescence spectra
of BIM 1–5. Plot of 1/A−A0 vs. 1/[β-CD] and Plot of 1/I−I0vs. 1/[β-CD]
(Fig. 4) (vide supporting information, BIMs 3–5–β-CD
Fig. S3). In 1:1 inclusion complexes, one of the indole rings
Fig. 4 The proposed models of
inclusion complex of a BIM1, b
BIM2 with β-CD for 1:1 and 1:2
inclusion complex
a
b
1:1 complex
1:2 complex

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J Fluoresc (2014) 24:925–931
11.45 Å. The vertical and horizontal distance of the particular
bond lengths for the other four molecules was calculated using
DFT method only. This calculation revealed that the length
between the aromatic and indole rings in BIMs are greater
than that of β-CD cavity, they cannot be encapsulated
completely within the β-CD cavity. Therefore we concluded
that BIMs formed 1:1 inclusion complex with β-CD. The
phenyl ring with alkyl substitutions of BIMs is embedded in
the β-CD cavity and two indole rings are present in the
aqueous phase.
From the above findings, it is clear that in β-CD solutions,
hydrophobicity is the driving force for encapsulation of the
molecules inside the β-CD cavity. Generally the hydrophobic
part likes to go inside the deep core of the non-polar β-CD
cavity [40–42].
of BIMs–β-CD complexes. In general, the resonances of the
protons of β-CD located within or near the cavity (H-3, 5, 6)
shows remarkably large shifts in the mixture. A minor shift is
observed for the resonance of H-1, 2, 4 located on the exterior
of β-CD [46]. In particular, the resonance of the protons of β-
CD, located within or near the cavity showed remarkably
large up-field shift (−0.20, −0.19 ppm) in the complex, which
suggested that the resonance of H-3 and H-5 are shielded
largely in the complex, the phenyl ring with alkyl substitution
must penetrate deeply into the cavity. A minor shift (−0.04,
−0.07 ppm) was observed for the resonance of H-atoms
located on the exteriorof β-CD.
Since BIMs contain three parts, two indole rings and one
phenyl ring with substitution, this may lead to two isomeric
1:1 complex and 1:2 complex. To ascertain the structure of the
inclusion complexes, 1H NMR spectroscopy studies of BIMs
were therefore under taken. The proton signals of BIMs
phenyl ring with substitutions showed up or down-field shifts
between the free and complex form, indicating that they are
affected as a result of complexation. It can be deduced from
this information that phenyl ring with substitution probably
entered the inner cavity of β-CD. These observations proved
the reality of the inclusion complexation and showed that the
driving forces for the formation of the inclusion complexes are
hydrophobic interactions [47].
Solid Inclusion Complexes Studies
FT-IR Spectral Studies
The formation of inclusion complexes was confirmed by FT-
IR spectroscopy because bands resulting from the included
part of the guest molecules are generally shifted or their
intensities altered [43]. If β-CD and BIMs form inclusion
complexes, the non-covalent interactions between them such
as hydrophobic interactions, van der Waals interactions, and
hydrogen bonds are lowered the energy of the inclusion part of
BIMs which reduce the absorption intensities of the corre-
sponding bands. We can see that there are apparent differences
between the spectra of BIMs–β-CD and that some character-
istic IR peaks of BIMs change obviously by comparison of the
FI-IR spectrograms of BIMs and the inclusion complex of β-
CD (vide supporting information, Fig. S4).
The −NH stretching frequency for BIMs appears around
3,419–3,368 cm−1 and is slightly moved in the inclusion
complexes spectra to a shorter wave number. The –C=O
stretching frequency is present at 1,725 cm−1 and is largely
red shifted in the inclusion complexes to 1,742 cm−1. The
observed changes in the FT-IR spectra of BIMs–β-CD inclu-
sion complexes are due to the restriction of the vibration of
free BIMs upon encapsulation into β-CD cavity, indicating
the formation of inclusion complexes from BIMs and β-CD.
The phenyl ring with alkyl substitutions in BIMs was inserted
into the cavity of β-CD.
Conclusion
The present work demonstrated that the aqueous solubility of
BIMs was enhanced considerably by formation of an inclu-
sion complex with β-CD. Thus, the β-CD may be useful in
improving the dissolution and the bioavailability of BIMs in
pharmaceutical formulation. The spectral data suggested that
the formation of a stable 1:1 stoichiometric complex of β-
CD–BIMs. The spectral studies suggested that the phenyl ring
along with alkyl substitutions of BIMs is present inside of β-
1
CD cavity. FT-IR and H NMR spectra strongly confirmed
that the inclusion complexes are formed between BIM 1–5
and β-CD.
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