Fluorimetric and prototropic studies on the inclusion complexation of 3,3′-diaminodiphenylsulphone with β-cyclodextrin and its unusual behavior

Article abstract of DOI:10.1016/j.saa.2010.06.021

The photophysical and photoprototropic properties of 3,3′- diaminodiphenylsulphone (3DADPS) in aqueous β-cyclodextrin (β-CDx) solution have been investigated using absorption and fluorescence spectral techniques. β-CDx forms 1:1 inclusion complex with 3DA

Full text of DOI:10.1016/j.saa.2010.06.021

Spectrochimica Acta Part A 77 (2010) 473–477
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Fluorimetric and prototropic studies on the inclusion complexation of
3,3ꢀ-diaminodiphenylsulphone with ␤-cyclodextrin and its unusual behavior
Muthu Vijayan Enoch1, Rajaram Rajamohan, Meenakshisundaram Swaminathan∗
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India
a r t i c l e i n f o
a b s t r a c t
The photophysical and photoprototropic properties of 3,3ꢀ-diaminodiphenylsulphone (3DADPS) in aque-
ous ␤-cyclodextrin (␤-CDx) solution have been investigated using absorption and fluorescence spectral
techniques. ␤-CDx forms 1:1 inclusion complex with 3DADPS as revealed by steady state and time-
resolved fluorescence spectroscopy. This inclusion complex formation was also confirmed by the FT-IR
and SEM image analysis of solid complex prepared by co-precipitation method. The existence of twisted
intramolecular charge transfer state and the unusual red shift observed during inclusion complexation
were analyzed and discussed. The ground and excited state acidity constants in ␤-CDx are reported. Based
on the photophysical and photoprototropic characteristics of 3DADPS in ␤-CDx, the structure of the 1:1
inclusion complex is proposed.
Article history:
Received 12 January 2010
Received in revised form 29 May 2010
Accepted 15 June 2010
Keywords:
3,3ꢀ-Diaminodiphenylsulphone
Prototropism
␤-Cyclodextrin
TICT
Inclusion complex
© 2010 Elsevier B.V. All rights reserved.
Molecular recognition is a phenomenon in which a ‘receptor’
or a ‘host’ molecule forms a stable complex with one or more
‘guest’ molecules through weak forces such as hydrogen bonds,
van der Waal’s and electrostatic interactions. Of many important
hosts, cyclodextrins (CDxs) have been widely used over the last
decade [1]. Cyclodextrins are cyclic oligosaccharides of six to eight
␣,d-glucopyranose units (namely ␣, ␤ and ␥-CDx respectively). Sig-
nificant interest centered on complexation by cyclodextrins is due
to their binding ability on several organic molecules of different
size and shapes [2].
serve as tools for studying the effect of acidity on pharmaceuti-
cally important compounds. The fluorescence and proton transfer
characteristics of some amino substituted diphenyl, diphenylsul-
phones, diphenylethers, fluorene and thiazole in ␤-cyclodextrin
have been reported from our laboratory [15–20].. Generally organic
fluorophores show a blue shift in fluorescence during inclusion
complexation with ␤-CDx. We observed an unusual red shift in both
florescence maxima of 3DADPS during inclusion complexation. In
this paper, we report the fluorescent and prototropic behaviors of
3,3ꢀ-diaminodiphenylsulphone in ␤-cyclodextrin.
Numerous studies on the photophysical and photochemical
properties of organic guest species complexed with cyclodextrins
have been reported [3–7]. Intrinsic, site-specific, fluorescent probes
for understanding drug–protein interactions are of current inter-
est [8–12] and the analysis of photophysical behavior of such
molecules in cyclodextrin can give information about the correct
choice among such probes.
Diphenylsulphones have been reported to be pharmaceuti-
cally important as antimalarial and antileprotic drugs [13] and
their photophysical and photoprototropic properties have been
reported [14]. Absorption and fluorescence spectral techniques
1. Experimental
1.1. Materials and methods
3,3ꢀ-Diaminodiphenylsulphone (Aldrich) and ␤-CDx (S.D. Fine
Chemicals) were used as received. The purity of 3DADPS was
checked by obtaining identical fluorescence spectra at different
excitation wavelengths. The absorption spectra were recorded on a
JASCO model 7800 spectrophotometer. Fluorescence spectra were
obtained from JASCO FP-770 spectrofluorimeter. Fluorescence life-
times were determined using a time-correlated picosecond single
photon counting spectrofluorimeter (Tsunami, Spectraphysics).
pH values in the range of 2–6 were measured on an ELICO
pH meter model LI-1OT. A modified Hammett’s acidity scale
(using a H2SO4–H2O mixture) was used for making solutions
∗
Corresponding author. Tel.: +91 9842381967.
E-mail address: chemres50@gmail.com (M. Swaminathan).
Present address: Department of Chemistry, Karunya University, Coimbatore.
1
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.06.021

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M.V. Enoch et al. / Spectrochimica Acta Part A 77 (2010) 473–477
below pH 1.5. FT-IR spectra were obtained with Avatar-330 FT-IR
spectrophotometer using KBr pellet. Microscopic morphological
structure measurements were performed with JEOL-JSM 5610LV
scanning electron microscope (SEM). The isosbestic wavelengths
were used for measuring the fluorescence intensities for fluorimet-
ric titration at any analytical wavelength.
1.2. Preparation of solid inclusion complex
The solid 3DADPS–␤-CDx complex was prepared using the fol-
lowing method. 3DADPS solution in methanol was added slowly
to the saturated solution of ␤-CDx in water and a suspension was
formed. The suspension was stirred at 40 ◦C for 30 min and stirring
continued at room temperature for 24 h. The obtained mass was
filtered through 0.45 ␮m membrane filter and dried at 40 ◦C in an
oven for 24 h. The dried complex was ground to fine powder.
Fig. 1. Fluorescen0ce spectra of 3DADPS with various concentrations of ␤-CDx: (1)
2. Results and discussion
0 M, (2) 4 × 10−4 M, (3) 8 × 10−4 M, (4) 1.2 × 10−3 M, (5) 1.6 × 10−3 M, (6) 2.0 × 10−3
M
and (7) 2.4 × 10−3 M.
2.1. Inclusion complexation
the neutral form. A dual emission with two maxima at 333.0 and
411.0 nm was observed for 3DADPS in aqueous solution. It was
reported that the shorter (SW) and longer (LW) wavelength bands
in water were from the locally excited (LE) and twisted intramolec-
ular charge transfer (TICT) states respectively [14]. These two bands
are red shifted with the increase in intensity by the addition of ␤-
CDx and the red shift is more with SW band than with LW band.
In general organic fluorophores exhibit a blue shifted fluorescence
with increase in intensity. The red shifts reveal that both LE and
TICT are stabilized by the addition of ␤-CDx. The larger red shift in
SW band must be due to the hydrogen bonding interaction between
the sulphonyl group of 3DADPS and hydroxyl groups present in the
outer rim of ␤-CDx. Formation of TICT state is difficult in ␤-CDx
inclusion complex as the rotation is restricted in the complex. But
TICT emission was observed in some of the inclusion complexes
formed with ␤-CDx [21]. In those cases it was found that the half
of the molecule was inside the cavity leaving the other half out-
side the cavity. This structure will not restrict the rotation of the
molecule in the excited singlet state. In this case also the inclu-
sion complex may have similar structure. Hence the structure of
the complex must be more favorable for both hydrogen bonding
interactions and TICT state formation. The complexation seems to
be complete at 2.4 × 10−3 M ␤-CDx concentration as there was no
change in fluorescence intensity by further addition of ␤-CDx.
The fluorescence excitation spectra of 3DADPS recorded at dif-
ferent emission wavelengths viz., 333 and 412 nm without and with
␤-CDx are the same and they resemble their absorption spectra.
This indicates that the two maxima in the emission spectra are from
the same precursor in the ground state.
The absorption spectral data of 3,3ꢀ-diaminodiphenylsulphone
(3DADPS) with different concentrations of ␤-CDx at pH 6 is given
in Table 1. There is a very small change in the molar extinction
coefficient with a slight red shift in longer wavelength absorption
at 307 nm. The small red shift may be due to the complexation of
3DADPS by ␤-CDx.
The fluorescence spectra of 3DADPS at pH 6 with various con-
centrations of ␤-CDx are shown in Fig. 1. The fluorescence spectral
data is given in Table 1. At pH 6, the 3DADPS molecule exists in
Table 1
Absorption and fluorescence spectral data of 3DADPS at different concentrations of
␤-CDx.
Concentration of
Absorption maximum
Fluorescence
␤-CDx (M)
ꢀabs (nm) (log ε)
maximum ꢀemi (nm)
(excitation 290 nm)
0
228.0 (4.82)
307.0 (3.81)
333
411
4.0 × 10−4
8.0 × 10−4
1.2 × 10−3
1.6 × 10−3
2.0 × 10−3
2.4 × 10−3
227.4 (4.83)
307.2 (3.81)
340
420
227.6 (4.83)
307.2 (3.82)
355
422
227.2 (4.83)
307.2 (3.82)
359
422
227.4 (4.84)
307.8 (3.83)
361
424
227.4 (4.84)
308.0 (3.83)
362
424
The formation of inclusion complex was also revealed by the
fluorescence decay curves obtained at different concentrations
of ␤-CDx. The fluorescence lifetimes of the 3DADPS molecule in
aqueous and in ␤-CDx were determined from the decay curves
227.4 (4.84)
308.0 (3.83)
364
424
Table 2
Time-resolved fluorescence spectral data of 3DADPS (excitation wavelength = 278 nm; emission wavelength = 432 nm).
2
Concentration of ␤-CDx (M)
Lifetime (s)
Relative amplitudes
100
␹
Standard deviation
0
1.08 × 10−9
1.06
1.05
1.88 × 10−10
8.0 × 10−4
1.04 × 10−9
44.32
55.68
4.2 × 10−11
2.38 × 10−9
1.38 × 10−10
1.6 × 10−3
2.4 × 10−3
1.01 × 10−9
2.69 × 10−9
21.45
78.55
1.05
1.02
4.38 × 10−11
1.62 × 10−10
1.03 × 10−9
11.87
88.13
5.01 × 10−11
1.73 × 10−10
3.21 × 10−9

M.V. Enoch et al. / Spectrochimica Acta Part A 77 (2010) 473–477
475
Fig. 2. Benesi–Hildebrand plot for the 1:1 complexation of 3DADPS in ␤-CDx.
and are given in Table 2. Single exponential decay was observed
with 3DADPS in aqueous solution of ␤-CDx. By the addition of ␤-
CDx, single exponential decay curve becomes biexponential. This
shows the presence of two species i.e., 3DADPS and its complexed
form. Two lifetimes were obtained in the presence of ␤-CDx. With
increase in the concentration of ␤-CDx, the lifetime of the ␤-CDx
complexed form of 3DADPS increases owing to the vibrational
restriction in the S1 state. The amplitude of the complexed form
also increases due to increase in complex formation and that of the
free species decreases.
With aqueous d(+)glucose solution, the absorption and fluores-
cence spectra of 3DADPS are not significantly changed and this
confirms that all the spectral changes observed in ␤-CDx are due to
inclusion complexation and not due to any non-inclusional com-
plex formation.
Fig. 3. FT-IR spectra of: (a) pure 3DADPS and (b) 3DADPS–␤-CDx complex.
equation (1) for the 1:1 complex
1
I − I0
1
1
=
+
(1)
Iꢀ − I0
K [Iꢀ − I0] [ˇ-CDx]
where K is the binding constant, I0 is the intensity of fluorescence
of 3DADPS without ␤-CDx, I is the fluorescence intensity with a
certain concentration of ␤-CDx and I is the intensity with the high-
est concentration of ␤-CDx. Fig. 2 shows the Benesi–Hildebrand
plot of 1/(I − I0) vs 1/[␤-CDx]. The linearity of the plot indicates the
The stoichiometry and binding constant of the 3DADPS–␤-CDx
inclusion complex are determined using the Benesi–Hildebrand
Fig. 4. SEM images of: (a) pure 3DADPS (×500), (b) 3DADPS–␤-CDx complex (×500), (c) pure 3DADPS (×1000) and (d) 3DADPS–␤-CDx complex (×1000).

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M.V. Enoch et al. / Spectrochimica Acta Part A 77 (2010) 473–477
Fig. 5. Absorption spectra of 3DADPS with ␤-CDx at various pH: (1) pH 1.60, (2) pH
2.02, (3) pH 2.44, (4) pH 2.81 and (5) pH 3.15.
Fig. 7. Fluorimetric titration curves for the monocation–neutral equilibrium of
3DADPS with ␤-CDx.
formation of 1:1 complex between 3DADPS and ␤-CDx. From the
slope of the straight line, the binding constant K was calculated to
be 398.25 M−1
.
2.2. Effect of acidity
The Gibbs free energy change (ꢁG) of the complex formation
was determined from the binding constant at 303 K using Eq. (2).
The absorption and the fluorescence spectra of 3DADPS with ␤-
CDx have been investigated in the H0/pH range of −1 to 4. With the
decrease of pH from 4, the absorption maximum at the longer wave-
length is blue shifted from 304 to 274 nm. Protonation of NH2 group
leads to a blue shift and hence this blue shifted spectrum is due to
the formation of monocation. The pH dependence of the absorption
spectrum of 3DADPS in ␤-CDx is shown in Fig. 5. There are two isos-
bestic points at 260 and 286 nm, indicating the monocation–neutral
equilibrium of 3DADPS in ␤-CDx.
The fluorescence spectra of 3DADPS at different H0/pH in ␤-CDx
are given in Fig. 6. At pH 4 and above, there are two fluorescence
bands at 315 (shoulder) and 424 nm. This is due to the neutral form
of 3DADPS. At pH below 0.33 a blue shifted fluorescence with a
maximum at 310 nm was observed and it is due to the monocation
of 3DADPS.
ꢁG = − RT ln K
(2)
The negative ꢁG value of −15.09 kJ/mol shows that the complex
formation is spontaneous.
Supporting evidence for the inclusion complexation of 3DADPS
with ␤-CDx in solid state was also obtained from FT-IR spectral
study and SEM images of pure and solid complex of 3DADPS with
␤-CDx. A comparison of the FT-IR spectra of pure and the solid
complex of 3DADPS with ␤-CDx (Fig. 3) reveals that the stretch-
ing frequencies of the N–H, C–H and C–S–C bands of pure 3DADPS,
which appeared at 3381.47, 3093.17 and 1308.61 cm−1 are shifted
to 3268.23, 2926.31 and 1305.59 cm−1 in the solid inclusion com-
plex. This effect can be attributed to the formation of inclusion
complex between 3DADPS and ␤-CDx.
The fluorimetric titration curves for the neutral–monocation
equilibrium are shown in Fig. 7. Decrease of pH from 4 causes a
decrease in the intensity of fluorescence at 424 nm. The fluores-
cence of the monocation at 310 nm starts appearing only from pH
2. The monocation is weakly fluorescent and there is a overlap of
fluorescence of neutral form with the monocation. Hence these two
curves do not meet exactly at the centre of inflection. But the meet-
ing point may be taken as close to the excited state pKa value and
it is found to be 1.35. The ground and excited state pKa values are
compiled in Table 3.
SEM images of 3DADPS and its inclusion complex with two dif-
ferent magnifications are shown in Fig. 4. The morphologies clearly
elucidated the difference of the solid states. This difference may be
taken as a proof for the formation of inclusion complex between
3DADPS and ␤-CDx.
*
The difference in pKa values in aqueous and ␤-CDx solution is
not large. This indicates that the protonated amino group lies out-
side the ␤-CDx cavity. Various bond lengths of 3DADPS calculated
using MOPAC/AM1are given in Fig. 8a. Based on the above facts,
the structure of the 1:1 inclusion complex of 3DADPS in ␤-CDx is
proposed as in Fig. 8b. This structure aligns with the experimen-
tal observations from complexation of 3DADPS by ␤-CDx and the
effect of acidity on the complex.
Table 3
Ground and excited state pKa values of 3DADPS.
EquilibriumMonocation ꢀ neutral
Ground state pKa
Excited state pKa
Without ␤-CDx
With ␤-CDx
3.18
2.62
1.40a
1.35
Fig. 6. Fluorescence spectra of 3DADPS with ␤-CDx at various pH: (1) pH 4.5, (2) pH
3.8, (3) pH 3.0, (4) pH 2.4, (5) pH 1.8, (6) pH 1.3, (7) pH 0.8 and (8) pH 0.3.
a
From Ref. [14].

M.V. Enoch et al. / Spectrochimica Acta Part A 77 (2010) 473–477
477
Fig. 8. Schematic diagram of the inclusion complex (1:1) of 3DADPS with ␤-CDx.
3. Conclusions
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Twisted intramolecular charge transfer (TICT) state formation
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The unusual red shift observed in fluorescence of the 3DADPS by
the addition of ␤-CDx is due to the hydrogen bonding interaction
between the hydroxyl group of the ␤-CDx and the sulphonyl group
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FT-IR spectral study and SEM images in the solid state. The pro-
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We are grateful to the University Grants Commission, New
Delhi, for financial support (Project No. 200 F.49). We also thank
the National Centre for Ultrafast Processes (NCUFP), University of
Madras, Chennai, for fluorescence lifetime measurements.
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