Azo-hydrazo tautomerism and inclusion complexation of 1-phenylazo-2- naphthols with various solvents and β-cyclodextrin

Article abstract of DOI:10.1007/s10895-010-0642-0

Spectral characteristics of sudan I (SDI), sudan II (SDII) and mordant violet-5 (MV5) have been studied in various solvents and β-cyclodextrin (β-CD). The inclusion complex of the above molecules with β-CD was analyzed by UV-visible, fluorometry, and DFT methods. The solvent study shows that azo-hydrazo tautomer is present in these molecules. The increase in the fluorescence intensity and a large bathochromic shift in the S₁ state indicate these molecules forms 1:1 inclusion complex with β-CD.

Full text of DOI:10.1007/s10895-010-0642-0

J Fluoresc (2010) 20:961–972
DOI 10.1007/s10895-010-0642-0
ORIGINAL PAPER
Azo-hydrazo Tautomerism and Inclusion Complexation
of 1-phenylazo-2-naphthols with Various Solvents
and β-cyclodextrin
A. Antony Muthu Prabhu & G. Venkatesh &
N. Rajendiran
Received: 11 December 2009 /Accepted: 16 March 2010 /Published online: 30 March 2010
#
Springer Science+Business Media, LLC 2010
Abstract Spectral characteristics of sudan I (SDI), sudan II
(SDII) and mordant violet-5 (MV5) have been studied in
various solvents and β-cyclodextrin (β-CD). The inclusion
complex of the above molecules with β-CD was analyzed
by UV-visible, fluorometry, and DFT methods. The solvent
study shows that azo-hydrazo tautomer is present in these
molecules. The increase in the fluorescence intensity and a
large bathochromic shift in the S1 state indicate these
molecules forms 1:1 inclusion complex with β-CD.
al. studied intensively the tautomeric equilibrium and the
quantitative information based on the solvents effects [7, 8]
and temperature.
In addition to the solvent effects, studies on the inclusion
complexation of organic molecules with CD can also
provide some useful information about the geometry of
guest molecules. CDs are water soluble oligosaccharides
which form inclusion complexes with a large number of
organic and inorganic molecules. A generally accepted
reason for choosing CDs, a class of cyclic oligosaccharides
with 6–8 D-glucose units linked by α-1,4-glucose bonds, as
the starting materials to construct the supramolecular
architectures is that the truncated cone-shaped hydrophobic
cavities of CDs have a remarkable ability to include various
guest molecules either in solution or in the solid state to
form the functional host-guest inclusion complexes [9–12]
which can be subsequently used as the building blocks of
supramolecular aggregates. Among the various families of
organic compounds used as guest molecules, the chromo-
phoric guests, such as azobenzenes [13–23] are of particular
importance because they can exhibit appreciable spectral
changes upon complexation by inclusion with CDs in
solution, and thus can be applied as the versatile spectral
probes to investigate host-guest interactions. However,
comparative studies on the inclusion complexation and
assembly behavior of CDs with azo derivatives are still
rare, to the best of our knowledge, although azo derivatives
are widely focused upon because of their potential to
construct photo driven molecular machines [13]. The ability
of CDs to accommodate guest molecules of the appropriate
size in their cavities has been utilized by many investigators
to control the photochemical properties and chemical
reactions of organic molecules.
.
.
Keywords 1-phenyazo-2-naphthol β-cyclodextrin
.
Inclusion complex Azo-hydrazo tautomerism
Introduction
Although the tautomerism in phenyl azonaphthol is known
for more than a century [1], the number of studies dealing
with these compounds has risen only in the last few
decades. This is not surprising, because on one hand
tautomerism is an elementary processes in living systems
and advanced technological application [2–4] and on the
other hand it remains of substantial fundamental interest for
method development in molecular spectroscopy [5, 6] and
in most of these cases the individual tautomers cannot be
physically separated. It is well known that the position of
the tautomeric equilibrium in solutions can be shifted by
changing environmental factors such as solvent properties;
temperature and the corresponding spectra can be recorded
but not analyzed by spectrophotometry [5, 6]. Antonov et
:
:
A. Antony Muthu Prabhu G. Venkatesh N. Rajendiran (*)
Department of Chemistry, Annamalai University,
Annamalai Nagar 608 002( Tamilnadu, India
e-mail: drrajendiran@rediffmail.com
The application of aromatic azo dyes for both conven-
tional [5] and sophisticated ‘high tech’ [23] purposes are

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J Fluoresc (2010) 20:961–972
well-known. Their applications as optical sensors and for
molecular memory storage due to the halo-photo and
thermochromic properties attract considerable attention
form both fundamental and applied point of view [24–26]
and there are many reports on the inclusion complexation
of cyclodextrins (CDs) with azo dyes [27, 28].
alizarin violet-5). The important basic structures for the
dyes are given in Fig. 1.
Experimental
The aim of the present study is to report the spectral
changes of three dyes in different solvents and β-
cyclodextrin. In this work, we have examined the
substituent effects on the inclusion complexation of β-
CD with water soluble azo dyes (1-phenyazo-2-naphthol
(SDI, Sudan I), 2, 4-dimethyl-1-phenyazo-2-naphthol
(SDII, Sudan II) and mordant violet-5 (MV5, acid
SDI, SDII, MV5 and β-CD were obtained from Sigma-
Aldrich chemical company and used as such. The purity of
the compound was checked by similar fluorescence spectra
when excited with different wavelengths. Triply distilled
water was used for the preparation of aqueous solutions.
The solutions were prepared just before each measurement.
The concentration of these compounds was of the order of
MV 5
SD II
SD I
MV 5
Azo
Hydrazo
ΔH= -121.4366 kcal/mol
ΔH = -113.6077 kcal/mol
H11-N1= 1.76
μg =6.379
μg= 6.057
O2-H11= 7.29
O3-H11= 7.44
O4-H11= 7.44
H4-O2= 12.48
H4-O4= 13.47
H7-N2= 1.76
H2-H7= 7.34
H2-O1= 7.25
H8-H11= 4.91
H8-O5= 4.76
H4-N= 12.75
H3-O2= 13.13
H3-O3= 13.71
H3-O4= 13.71
H4-O3= 13.47
H3-N= 13.07
O2-N1= 6.72
O3-N1= 7.44
O4-N1= 7.44
H7-H11= 4.14
H5-O3/O4= 12.35
SD II
SD I
Azo
Hydrazo
H1-H6= 7.78
H10-H16= 6.89
H8-H15= 5.40
ΔH =66.502 kcal/mol
ΔH =63.9856 kcal/mol
O-H6= 7.07
μg = 1.488
μg = 1.461
H8-H16= 5.40
H3-H11= 12.35
H12-H14= 8.49
H3-H10= 12.35
H11-H15= 8.49
H8-H14= 5.40
H3-H12= 12.31
Azo
Hydrazo
H1-H6= 7.78
N2-H3= 6.20
H4-H10= 11.71
ΔH =81.15754 kcal/mol, μg = 1.831
ΔH= 84.4394 kcal/mol,
N1-H10= 6.10
μg = 4.804
H2-H11= 8.75
H3-H10= 11.46
O-H10= 8.00
H2-H5= 6.75
N2-H4= 6.40
H5-H9= 9.55
O-H6= 7.07
Fig. 1 CAChe-DFT structure—MV 5, SD II and SD I

J Fluoresc (2010) 20:961–972
963
4×10−4 to 4×10−5 M. The concentration of β-CD solutions
was varied from 1×10−3 to 1×10−2 M.
Absorption spectral measurements were carried out with
a Hitachi Model U-2001 UV-visible spectrophotometer and
fluorescence measurements were made with a Shimadzu RF
5301 spectrofluorometer. The pH values in the range 2.0–
12.0 were measured on an Elico pH meter model LI-120.
The results in Table 1 and very low εmax values indicate
n-π* transition is present in these molecules. The observed
absorption and fluorescence spectral changes are interpreted
in terms of azo-hydrazo equilibrium [29]. It is evident that
increasing the polarity of solvents leads to an increase in
the amount of the ‘hydrazo’ form, whereas the opposite is
not true for the ‘azo’ form [5–8]. Table 1 results also shows,
methyl substituent hardly affect the position of the azo-
hydrazo tautomeric equilibrium compared to hydroxyl
substituent.
Results and discussion
Generally solvent effects on tautomerism equilibrium
can be separated into specific solute-solvent interaction
(e.g. hydrogen bonding) and non-specific bulk interactions.
Based on the previous investigations concerning the
tautomerism of azonaphthol compounds [30–32] the sol-
vents used can be divided into several groups depending on
their type of interactions with the respective azo or hydrazo
tautomer: (i) tautomer H–OH solvent interaction (via,
movable proton and the oxygen of the solvent) in protic
solvents; (ii) tautomer O–H solvent interaction (via the
tautomeric oxygen and the proton/protons from the sol-
vents) in solvents like CHCl3, CH2Cl2 and CH3CN (iii)
tautomer H–O solvent interaction (via, the movable proton
and the double bonded oxygen from the solvent) in solvents
like DMSO, ethyl acetate.
Effect of solvents
The absorption and fluorescence spectra of SDI, SDII
and MV5 have been recorded in solvents of different
polarity and hydrogen bonding tendency and the relevant
data are complied in Table 1. The fluorescence spectra are
shown in Fig. 2. The absorption spectral shape of the
SDII resembles with SDI but the absorption maxima of
SDII are red shifted than that of SDI. The large red shift
and broad absorption and emission spectra in MV5
suggest intramolecular hydrogen bond (IHB) present in
this molecule. The increasing order of red shift is:
SDI <SDII <MV5. The above sequence indicates the
position of the substituent in the aromatic ring is the key
factor for the absorption and emission behavior. Increas-
ing the polarity and proton donor capacity of solvents
has no significant effect, both in the absorption and
emission bands which indicate that the azo-hydrazo
tautomer is present in these molecules. It is well known
the solvent molecules can have dispersive interactions
with the solute and also can act as proton acceptor and
proton donor to the solute molecules. If π-π* transition is
the lowest energy transition, the first two interactions
leads to a red shift and the third leads to a blue shift in the
absorption and fluorescence spectra.
The hydroxyl group can behave in two ways: The
solvent molecules can have dispersive interactions with the
solute and also can act as proton acceptor and proton donor
to the solute molecules. If π-π* transition is the lowest
energy transition, the first two interactions leads to a red
shift and the third leads to a blue shift in the absorption and
fluorescence spectra. Protic solvents interact with the lone
pair electrons on the solute oxygen atom of hydroxyl group
to form a hydrogen bond and hydrogen atom of the
hydroxyl group also makes a hydrogen bond with solvents.
In the former, a blue shift and in the latter, a red shift in the
absorption spectra should be observed. Thus a blue shift in
Solvents which do not participate in specific interactions
The present systems, which are characterized by a strong
intramolecular hydrogen bond (IHB), one expects a some
what different behaviour towards the solvents used. For
instance, the dominating tautomer H–OH solvent interac-
tion with alcohol in SDI, SDII and MV5 would require
breaking of the IHB bond in these compounds. Thus for
these molecules, a tautomer O–H solvent interaction with
alcohol as well as CH2Cl2/CH3CN should be energetically
more favorable. Specific interactions of the tautomer H–O
solvent interaction (DMSO) should hardly be possible
because of unfavorable steric repulsions. Further as
mentioned above, the protic and aprotic solvent values in
Table 1 indicate involvement of hydrogen bonds by H-atom
of the solvent and the oxygen (hydroxyl or azo) of the
solute for the second type of solvent. In agreement with the
results in Table 1, CH–O hydrogen bonds involving alkyl
halogenides have been shown to behave very much like
conventional OH–O hydrogen bonds [33]. Not surprisingly,
these interactions appear to be stronger in the case of the
quinine tautomer thus shifting the tautomer towards this
form. Further, Antonov [5–8] et al. have already reported in
phenyl azonaphthol tautomeric equilibrium shifting towards
the quinone form ‘hydrazo’ for donor substituted deriva-
tives with increasing solvent polarity. As can be seen there
is a trend, for an increasing difference between the dipole
λ
abs in water suggests the formation of the H-bond with the
lone pair thus inhibiting its interaction with π-cloud. The
red shift in aprotic solvents is due to usual dipole-dipole
effect on the π-π* transition or to the hydrogen donating
character of the hydroxyl group.

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250
200
IHB bond strengths, whereas the N–H lengthening due to
strong H-bond formation is small. It is also evident from
these data that the stronger H-bond (shorter N–O distance)
is associated with the less stable tautomer [34, 35].
MV 5
2
150
100
50
4
5
The tautomeric behavior of azo naphthols differs
considerably from that of the corresponding azophenols
[36] which exist mainly in the azo form at room
temperature. Even in polar solvents such a difference could
be caused by the loss of aromaticity in going from azo to
hydrazo form, while in azonaphthols this effect is compen-
sated by the transfer of aromaticity within the naphthalene
fragment. In azophenols, the number of delocalized
electrons in the tautomeric phenyl ring is reduced from
six to four on going to the hydrazo form because of the
engagement of two of those electrons in the strong C=N
and C=O bonds. Thus, the phenyl ring looses much of its
aromaticity. In the naphthalene compounds, this effect is
compensated by the second aromatic ring [37]. The
experimental (UV-Vis) absorption spectra also support this
finding. At room temperature the absorption spectrum at
320–430 nm and 500 nm corresponds to azo and hydrazo
forms respectively [38]. Further, the quantum yield for
these compounds are fairly low, it could be explained by
non-radiative decay paths to the low-lying n-π* states of
the nitrogen atoms [38]. What more interest is that the
excitation spectra are identical irrespective of emission
wavelength, which indicates that the observed emission
originates from one hydrazo tautomer only.
3
1
6
500 550
400
800
600 650
450
700 750
Wavelength (nm)
125
100
SD II
3
5
4
50
25
6
2
1
400
800
600
450
700 750
SD I
500 550
650
Wavelength (nm)
200
150
100
50
4
2
5
6
3
1
500 550
The Stokes shifts of SDI, SDII and MV5 measured in
different solvents were correlated with the BK [39] and
800
400 450
700 750
650
600
Wavelength (nm)
Fig. 2 Fluorescence spectra of MV 5, SD II and SD I in different
solvents at 303 K: (λexcitation: 420 nm): (1) cyclohexane (2) ethyl
acetate (3) acetonitrile (4) 2-propanol (5) methanol and (6) water
moments of the respective tautomeric forms (i.e. hydrazo
being more polar than azo form). But in the case of non
planarity, the substituent cannot really affect the tautomeric
fragment.
The results in Table 1 predict the donor substituent in
MV5 to cause bathochromic shifts of the longest wave-
length absorption bands for both tautomeric forms. A
similar dependence of the UV-visible spectra has been
found for the isomeric 2-hydroxy-1-naphthaldehyde [30].
However, the band positions of the UV-visible spectra are
essentially independent of solvent polarity with an expected
higher polarity of the hydazo tautomer and thus preferential
stabilization by polar solvents. Also, negligible solvent shift
of the various band maxima of the individual tautomers is
observed. Therefore donor substituents apparently stabilize
the hydazo tautomer, at least in polar solvents. Moreover,
N–O bond distance apparently gives a good indication of
Fig. 3 Plot of Stokes shifts (cm−1) of MV5, SD II and SD I vs. ET(30)
and BK solvent parameters: (1) cyclohexane (2) diethyl ether (3)1,4-
dioxane (4) ethylacetate (5) dichloromethane (6) acetonitirle (7)
t-butyl alcohol (8) 2-butanol (9) 2-propanol (10) ethanol (11) methanol
and (12) water

J Fluoresc (2010) 20:961–972
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Reichardt–Dimroth ET(30) [40] parameters (Table 1). It can
be seen that, the plots of Δνss versus BK are non-linear,
charge transfer interactions to the ET(30) value, a non-
linearity is observed for dioxane solvent.
−
suggesting that the solute-solvent interactions are not very
large and that general interactions play a major role in the
spectral characteristics of these compounds. The plot of
Δνss versus ET(30) are given in Fig. 3. As expected, these
plots are solvent effects and hydrogen bonding effects.
However, as there is no contribution from the solute-solvent
Effect of β-CD
−
Table 2, Figs. 4 and 5 shows the absorption and emission
spectra of SDI, SDII and MV5 in aqueous solutions
(pH∼7) containing different concentrations of β-CD. The
Table 2 Absorption and fluorescence maxima (nm) of MV 5, SD II and SD I at different concentrations of β-CD
No.
1
Concentration of β-CD M
Water (without β-CD)
MV 5
SD II
SD I
labs
log ε
lflu
labs
log ε
lflu
labs
log ε
lflu
539-502
410
4.26
590
335
559
517
329
270
221
557
516
325
256
221
556
514
325
256
221
555
514
325
260
220
554
513
326
257
217
555
515
326
257
217
555
514
324
260
218
320
199
−13.33
2.05
1.95
2.25
1.98
585
344
536
496
324
230
1.92
1.96
1.85
2.08
586
337
308
262
3.90
4.25
220
2
3
4
5
6
7
0.001
539-502
410
594
445
335
2.04
1.96
1.88
2.10
2.39
2.03
1.93
2.19
587
424
343
532
499
326
229
1.95
1.96
1.82
2.13
588
447
337
306
265
4.01
4.26
220
0.002
539-502
410
594
445
336
589
423
344
531
499
326
229
1.95
1.96
1.82
2.13
580
447
337
305
264
4.26
4.30
216
0.004
539-502
410
594
446
336
2.03
1.94
2.24
2.38
590
423
344
496
316
233
1.98
1.89
2.24
578
447
338
304
264
4.28
4.30
216
0.006
539-502
410
595
446
336
2.03
1.94
1.91
590
423
344
497
320
231
2.00
1.86
2.14
575
447
338
304
262
4.33
4.32
0.008
539-502
410
597
446
337
2.04
1.96
1.85
2.01
2.31
2.04
1.96
1.88
2.10
2.39
590
423
344
496
315
229
2.03
1.87
2.22
575
447
338
304
262
4.42
4.33
0.010
539-502
410
598
447
337
590
424
345
496
316
230
2.04
1.75
2.16
575
447
338
304
261
4.46
8
Excitation wavelength
320
320
9
Binding constant (M−1
)
191
429
333
115
324
10
ΔG KJ / mole
−13.21
−15.26
−14.63
−11.96
−14.56

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1.5
1.0
0.6
0.5
0.4
0.3
0.2
the same wavelength and the fluorescence intensity ratio of
the hydrazo to the azo tautomer (Ib/Ia) increases as the
concentration of β-CD increases. The similarity in spectral
changes in β-CD solutions suggest that the structural
geometry of the SDI/β-CD and SDII/β-CD inclusion
complexes are similar to MV5/β-CD inclusion complex in
terms of the orientation of guest molecules. Thus, the
polarity and viscosity variations may not play an important
role in these molecules [41–43].
MV 5
0
15
5
10
[β -CD] × 10-3 mol dm-3
7
1
0.5
200
350
650
275
425
500
575
The presence of isosbestic point in the compounds
indicates the formation of 1:1 inclusion complex. In
general, the existence of an isosbestic point in the
absorption spectra is an indication of the formation of well
defined 1:1 complex. The formation constant of SDI is
small compared with SDII/MV5 with β-CD. This is
probably because, SDI molecule is not tightly encapsulated
into the cavity, whereas in MV5, due to presence of
naphthalene ring and ortho hydroxy group, it may tightly
encapsulate in the β-CD cavity.
Wavelength (nm)
0.9
0.6
0.6
0.5
0.4
0.3
0.2
SD II
0
15
5
10
[β -CD] × 10-3 mol dm-3
7
1
0.3
200
340
610
410
480
270
550
Wavelength (nm)
400
200
175
150
If
125
0.9
0.6
0.6
0.5
0.4
0.3
0.2
MV 5
SD I
7
300
100
15
[β -CD] × 10-3 mol dm-3
0
5
10
15
5
10
0
1
[β -CD] × 10-3 mol dm-3
200
100
7
1
7
1
0.3
420 480
660
360
540
600
Wavelength (nm)
200
320
560
380
440
260
500
800
Wavelength (nm)
400
350
If
SD II
300
250
200
Fig. 4 Absorbance spectra of MV 5, SD II and SD I in different
β-CD concentrations (M): (1) 0 (2) 0.001 (3) 0.002 (4) 0.004 (5)
0.006 (6) 0.008 and (7) 0.01
15
5
10
0
600
400
200
[β -CD] × 10-3 mol dm-3
7
1
7
1
absorption spectra of these molecules in β-CD solutions show
little change with those in water as observed for the solvent
polarity dependence, reflecting little azo-hydrazo tautomer
equilibrium in the ground state. Upon increasing the
concentration of β-CD the molar extinction coefficient
slightly increases at the same wavelength. The slight increase
in the absorption with the addition of β-CD has been
attributed to the enhanced dissolution of the guest molecule
through the detergent action of β-CD [41–43].
In contrast to the weak absorbance between the SDI,
SDII and MV5 (4×10−5 M) and β-CD, the fluorescence
intensities of these molecules shows significant change with
change in β-CD concentration (Fig. 5). In the β-CD free
aqueous solution, three emission maxima can be seen even
though the emission quantum yield of both the azo
emission and the hydrazo emission (around 430 nm and
580 nm respectively) is extremely low. Upon addition of
β-CD, all the emission intensities are gradually enhanced at
480
360
540
600 660
420
Wavelength (nm)
600
300
250
SD I
If
200
150
100
450
300
150
15
5
10
0
7
1
[β -CD] × 10-3 mol dm-3
1
7
480
360
540
600 660
420
Wavelength (nm)
Fig. 5 Fluorescence spectra of MV 5, SD II and SD I in different
β-CD concentrations (M): (λexcitation: 320 nm): (1) 0 (2) 0.001 (3)
0.002 (4) 0.004 (5) 0.006 (6) 0.008 and (7) 0.01

J Fluoresc (2010) 20:961–972
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40
According to the above equations, a plot of 1/I-I0 versus
1/[β-CD]2 (both absorption and fluorescence) gives an
upward curves (this figure is not given). However a plot of
1/ΔA versus 1/[β-CD] and 1/I-I0 versus 1/[β-CD] reveals a
linear relationship as shown in Figs. 6 and 7. This analysis
reflects the formation of 1:1 inclusion complex between
these compounds and β-CD.
MV5
SD II
SD I
30
20
10
The free energy change of these complex were calculated
from the following equation
0
200
400
600
800
1000
1200
1/[β-CD] mol dm-3
Fig. 6 Absorption spectra of Benesi–Hildebrand plot for the
complexation of MV 5, SD II and SD I with β-CD (Plot of 1/ΔA
vs. 1/[β-CD])
$G ¼ ꢀRT ln K
ð4Þ
As can be seen from Table 2, ΔG is negative which
suggests that the inclusion process proceeded simultaneously
at 303 K. The negative values in the experimental
temperature indicate the inclusion process is an exothermic
and enthalpy controlled process. The hydrophobic interaction
between the internal wall of β-CD and guest molecules is
an important factor for the stability of inclusion com-
plexes. In MV5, SDI and SDII, it may safely be
considered that the difference in the magnitude of the
hydrophobic interaction is related to that of the contact
area of the guest molecule for the internal wall of β-CD.
Several driving forces have been postulated for the
inclusion complexation of CD with guest compounds [46]:
(1) van der Waals forces, (2) hydrophobic interactions, (3)
hydrogen bonding, (4) release of distortional energy of CD
by binding guest and (5) extrusion of ‘high energy water’
from the cavity of CD upon inclusion complex formation.
Tabushi [47] proposed a thermodynamic model for the
process of CD inclusion complex formation. Based on the
thermodynamic parameter (ΔG) calculated for the inclusion
complexes, we conclude that the hydrogen bonding
interaction, van der Waals interaction, and breaking of the
water cluster around the polar compounds are mainly
dominate the driving force for the inclusion complex
formation.
It is well known that the strength of interaction is also
dependent on the size of the CD cavity and size of the
substituent in the inclusion complex [48]. This means that
the interaction is more sensitive to the size of substituents
and the CD in the complexation. The CDs are truncated,
right-cylindrical, cone-shaped molecules, 7.8 Å heights
with a central cavity. The diameters of the narrower and
wider rim of the cavity for β-CD are 5.8 Å and 6.5 Å,
respectively [49]. It is well known that the van der Waals
force including the dipole-induced dipole interactions are
proportional to the distance between the SDI, SDII, MV5
and the wall of the CD cavity and to the polarizabilities of
these components. It is thus a short range interaction.
Therefore, the neutral SDI, SDII and MV5 may embed
deeper in the β-CD cavity than its anion. The phenyl
moiety may achieve a maximum contact area [50] with the
internal surface of the cavity of the β-CD; hence, the
For 1:1 complex between β-CD and guest molecule the
following equilibrium can be written:
SD þ " ꢀ CD Ð SD:::::" ꢀ CD
ð1Þ
The formation constant ‘K’ and stoichiometric ratios of
the inclusion complex of azonaphthol compounds were
determined according to the Benesi–Hildebrand relation
[44, 45] assuming the formation of a 1:1 host-guest
complex:
1
1
1
¼
þ
ð2Þ
I ꢀ I0 I0 ꢀ I0 KðI ꢀ I0Þ½" ꢀ CDꢁ
1
1
1
¼
þ
ð3Þ
I ꢀ I0 I0 ꢀ I0
2
KðI0 ꢀ I0Þ½" ꢀ CDꢁ
where [β-CD]0 represents the initial concentration of β-
CD, ‘I0’ represents absorbance and fluorescence intensities
in the absence of β-CD (water only), ‘I’ represents
Absorbance and fluorescence intensities in the presence of
different β-CD (0.001–0.009 M β-CD concentration and
I′ represents the inclusion complex formation completed
β-CD concentration (or) absorbance and emission intensity
measured at the highest concentration of β-CD (0.01 M
β-CD concentration). The K values were obtained from the
slope and the intercept of the plots shown in Figs. 6 and 7.
0.040
MV5
SD II
SD I
0.035
0.030
0.025
0
1200
400
600
800
1000
200
1/[β-CD] mol dm-3
Fig. 7 Fluorescence spectra of Benesi–Hildebrand plot for the
complexation of MV 5, SD II and SD I with β-CD (Plot of 1/I-I0
vs. 1/[β-CD])

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J Fluoresc (2010) 20:961–972
interaction of the phenyl ring with β-CD would play an
important role.
with β-CD. The ‘K’ value is a reasonable measure of
hydrogen bonding and the change in hydrogen bonding of
SDI, SDII and MV5 are caused only by the hydrogen ion
concentrations. Since the hydroxyl substituent locates near
the wider rim of the CD cavity and carboxyl group locates
narrower range of the CD cavity, the ‘K’ values are
proportional to the hydrogen bonding interactions. The
difference in the slope in Figs. 6 and 7 for these molecules
and β-CD complexes indicates that the interactions of
hydrogen atoms, especially MV5 with β-CD is much
stronger than SDI and SDII. This is because of MV5
interaction is approximate to the hydrogen bonding contact
but in SDI and SDII they are somewhat weak, since the
hydroxyl group is far from the internal surface of the β-CD
cavity in the inclusion complexes. Thus for MV5, the
association constant with β-CD is greater than with SDI
and SDII (Table 2).
In general, the inclusion of CDs with guest compounds
is also affected by hydrophobic and electronic interactions
[51]. Since CDs have a permanent dipole [52], the primary
hydroxyl end is positive and the secondary hydroxyl end is
negative in the glucose units of CDs. The stability of
binding by hydrophobic interaction is partly the result of
van der Waals force but is mainly due to the effects
of entropy produced on the water molecules [53]. In
aqueous solution, a hydrophobic guest compound is
restricted by the water shell formed by the hydrogen
bonding network [54]. It has a strong tendency to break
down the water cluster and penetrate the non-polar cavity of
CD. This process is an exothermic due to entropic gain
[55]. The association constants for the inclusion of β-CD
with guest compounds were observed to be proportional to
the substituent hydrophobic constant of the guest.
From the above results, we notice some interesting
points: (i) the ‘K’ value for MV5 and SDII is higher than
that of SDI which may be attributed to the more
However, in the molecules the hydrogen bonding
interactions play major roles in the inclusion complexation
Fig. 8 Proposed inclusion complex structure of MV 5, SD II and SD I

J Fluoresc (2010) 20:961–972
971
hydrophobic interaction between the naphthalene ring and
the internal wall of β-CD; (ii) the small binding constant
for SDI implies that the phenyl ring is not more tightly
embedded in the β-CD cavity; (iii) in MV5, the large blue
shifted absorption and emission spectra suggests one of the
hydroxy group is present in the interior part of the β-CD
cavity. The results in Table 2 conforms the molecular
disposition of MV5 in the inclusion complex is similar to
that of SDI and SDII. Further it is a well known fact that
hydrophobicity is the driving force to the formation of
inclusion complexes. Since phenyl group is more hydro-
phobic than naphthol part hence phenyl ring may include in
the β-CD cavity. It is also recognized that the naphthol
group is located outside the β-CD cavity (i.e. in hydrophilic
environments) [41–43].
Considering the above results, three different types of
inclusion complex formation between these molecules and
β-CD are possible: (i) with the phenyl ring captured, (ii)
with naphthol ring captured and (iii) 1:2 inclusion complex.
Since these molecules are neutral and have large size
compared to β-CD they can be included in either of the two
directions in the neutral β-CD cavity and may also be
responsible for the formation of third type of inclusion
complexes in the β-CD solution as demonstrated in Fig. 8.
The fluorescence spectra of these molecules are same in the
presence of high concentration of β-CD. As the intensity of
hydrazo maxima have increased with increasing β-CD
concentration, it seems that the naphthalene ring protrude
into the bulk phase. This is reasonable because naphthol-
quinone ring is more polar and can form H-bonds with
either of the hydroxyl groups of the β-CD cavity rim or
bulk water molecules or both. It should be preferable for the
naphthol quinone group to protrude in to the polar aqueous
phase. Even though both azo group and hydrazo group can
be hydrogen bonded with water, the orientations of these
groups can be different with regard to water molecule by
forming different patterns of β-CD inclusion complexes.
Moreover, because of tautomerization process, protonation
may take place in the azo nitrogen atom; hence interaction
with β-CD hydroxyl group is not possible; thus azo part
can easily get entrapped in the β-CD cavity.
naphthalene ring exposed to the bulk phase as shown in
Fig. 8. The emission intensities of the molecules are
increased with increasing β-CD concentration at the same
wavelength supporting this implication. The absorption and
fluorescence spectral changes are also supporting the
formation of a 1:1 inclusion complex between these
molecules and β-CD.
Conclusions
The following conclusions can be drawn from the above
studies:(i) in all solvents, SDI, SDII and MV5 molecules
exhibit azo-hydrazo tautomers, (ii) large red shift is noticed
for MV5 in all solvents and β-CD solutions indicating IHB
is also present in this molecule and (iii) In β-CD solutions,
the increase in the fluorescence intensity and a large
bathchromic shift in S1 state indicates the above molecules
forms 1:1 inclusion complex with β-CD.
Acknowledgement This work is supported by the Department of
Science and Technology, New Delhi, (Fast Track Proposal Young
Scientist Scheme No. SR/FTP/CS-14/2005) and University Grants
Commission, New Delhi (Project No. F-31-98/2005 (SR)).
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