Spectrophotometric studies on the inclusion formation of β-cyclodextrin with 3,4-(methylenedioxyphenyl) benzylideneaniline

Article abstract of DOI:10.1023/A:1015868825524

Complexation of β-cyclodextrin with 3,4-(methylenedioxyphenyl) benzylideneaniline has been studied in aqueous solution by means of spectrophotometry. It was observed that the complexation attains equilibrium very slowly. The equilibrium constant of the 1:

Full text of DOI:10.1023/A:1015868825524

Journal of Solution Chemistry, Vol. 31, No. 3, March 2002 (�C 2002)
Spectrophotometric Studies on the Inclusion
Formation of �-Cyclodextrin with
3,4-(Methylenedioxyphenyl) Benzylideneaniline
Jian Chen,1 Dezhi Sun, and Changgen Feng1
�
2
Received June 22, 2001; revised January 9, 2002
Complexation of �-cyclodextrin with 3,4-(methylenedioxyphenyl) benzylideneaniline
has been studied in aqueous solution by means of spectrophotometry. It was observed
that the complexation attains equilibrium very slowly. The equilibrium constant of the
1
:1 complexation between �-CD and MDBA was determined, and its dependence on
temperature examined. Findings are discussed in light of host–guest intermolecular
interactions. Two possible mechanisms of complexation are proposed.
KEY WORDS: �-Cyclodextrin; benzylideneaniline; complexation; reorganization.
1. INTRODUCTION
Cyclodextrins (CD) are starch hydrolysis products. In these molecules the
glucose units in chair conformation are linked by �-1,4 bonds. This geometry
gives the CDs the overall shape of a truncated cone with the wider end formed
by the secondary 2- and 3-hydroxyl groups and the narrower end by the primary
6-hydroxyls. The number of glucose units determines the shape and size of the
cavity. The cavity is relatively hydrophobic, compared to water, while the external
faces are hydrophilic. Because the hydrophobic central cavity is suitable for the
inclusion of various organic molecules, �-cyclodextrin (�-CD) was one of the
first receptor molecules whose ability to bind organic molecules was recognized
and extensively studied by various experimental techniques. �-CD is stable in
alkaline and neutral solutions, which are nearly transparent in the UV-visible range.
Thus �-CD–organic guest systems are suitable for spectrophotometric studies. The
formation of complexes is driven by weak interactions between host CDs and guest
1Department of Mechanics and Engineering, Beijing Institute of Technology, Beijing, 100081, P. R.
China; email: uschenzhz@263.net.
2
Department of Chemistry, Zhejiang University, Hanzhou, P. R. China.
2
11
0
095-9782/02/0300-0211/0 �C 2002 Plenum Publishing Corporation

2
12
Chen, Sun, and Feng
Scheme 1. Molecular structure of MBDA.
molecules and is a consequence of molecular recognition between host and guest
molecules. The processes involved in inclusion complexation by CDs have been
(1)
asserted to be very rapid.
3
,4-(Methylenedioxyphenyl) benzylideneaniline (MDBA) is an important
(2,3)
intermediate in synthesis of antibacterial drugs.
Used as a donor–acceptor
�
-conjugated bridge (D�A), its derivatives are regarded as promising materials
(4–6)
for nonlinear optical devices.
They are generally nonplanar molecular struc-
tures with the twist angle � (see Scheme 1) of the anilino benzene ring dependent on
the properties of the substitutents and their positions on the benzene ring.( The
nonplanar structures of benzylideneanilines differ from those of �-isoelectronic
trans-stilbeneandtrans-azobenzene, andhavesignificanteffectsontheirelectronic
7,8)
(9–12)
behavior and their nonlinear optical properties.
In order to gain insight into the effect of the nonplanar structure on molec-
ular recognition, we have studied the complexation between �-CD and MDBA.
Spectrophotometric data show that the inclusion complexation between �-CD and
MDBA is actually a very slow process. The electronic absorption spectra take
about 5 days to reach equilibrium. The equilibrium constant for 1:1 complexation
between �-CD and MDBA is found to be surprisingly large. We discuss these
results to interpret this phenomenon.
2. EXPERIMENTAL
�
-CD was recrystallized twice in redistilled water. MDBA was obtained by
condensation of substituted benzaldehyde and aniline. Its purity was established
1
by thin layer chromatography and [ H]NMR spectroscopy. Aqueous MDBA so-
�
lutions were prepared in the temperature range 65 to 68 C at a concentration of
�
3
about 47 �mol-dm . Weighed amounts of about 12 mg of �-CD were put into a
0-ml volumetric flask of the MDBA solution to obtain a mixed solution ready for
1
use. Pipetting a given volume of the prepared mixed solution and adjusting vol-
umes with the prepared MDBA solution, a series of sample solutions containing a
fixed concentration of MDBA but varying concentrations of �-CD were prepared,
transferred to a thermostat, and stirred intermittently during complexation between
�
-CD and MDBA. The water used in the experiments was deionized by passage

Spectrophotometric Studies of Inclusion Formation
213
through a mixed-bed ion exchanger and then twice distilled in a quartz sub-boiling
purifier.
Absorption spectra were recorded on a UV-2401PC spectrophotometer
�
(
Shimadzu, Japan) including a temperature controller (� 0.1 C). The redistilled
water was taken as the sample reference. The temperature of complexation was
controlled by a thermostat within � 0.2 C for more than 5 days.
�
3
3
. RESULTS AND DISCUSSIONS
.1. Inclusion Formation
A series of sample solutions (Series I), containing a fixed concentration of
�
3
MBDA (47 �mol-dm ) and differing amounts of �-CD, were studied with the
�
spectrophotometer after they had been kept in the thermostat at 32.8 C for 2 days.
The recorded spectra given in Fig. 1 indicate that with �-CD concentration up
�
3
to the maximum of 71 �mol-dm there has been no appreciable change in the
shape of absorbance peaks, although some changes in peak intensities are seen.
�
Subsequently, after the solutions had been stirred intermittently and held at 32.8 C
for 5 days, the UV-visible absorption spectra appeared stable, and the complex-
ation had presumably reached equilibrium. These fully equilibrated UV-visible
absorption spectra are shown in Fig. 2. The absorbance peaks of the complexed
MDBA molecule are shifted toward the shorter wavelength. Except for the first
set fo measurements on Series I, all measurements on aqueous �-CD + MDBA
solutions were performed on equilibrated samples.
Fig. 1. UV-visible absorption spectra of MDBA (spectrum A) and �-CD + MDBA
solutions (Series I) at 306.0 K after complexation for 2 days. Spectra are shown for
�
3
three �-CD concentrations, the highest being 71 �mol-dm . The MDBA concen-
�
3
tration is 47 �mol-dm
.

2
14
Chen, Sun, and Feng
Fig. 2. UV-visible absorption spectra of MDBA and �-CD + MDBA solutions
Series I) at 306.0 K after complexation for 5 days. Absorbance peaks are shifted
toward shorter wavelength with increasing �-CD concentration.
(
Aseriesofsampleswereacidifiedtoabout0.18mol-dm 3 H byaddingaque-
�
+
ous HCl. The UV-visible absorption spectra spectra of these samples are given in
Fig. 3. The spectra are very similar to those determined in neutral solutions both in
peak shapes and intensities, inasmuch as the lower intensities can be attributed to
the dilution accompanying the acidification. As the �-CD concentration in the so-
�
3
lution is increased from 0 to 71 �mol-dm , i.e., the mole ratio of �-CD to MDBA
is varied from 0 to 1.5, the absorption spectra of the acidified samples exhibit two
different features: (1) spectra for samples with the mole ratio less than 1:1 appear
shifted toward violet; (2) spectra for samples with ratio close to or greater than 1:1
+
show little change at shorter wavelengths. The first effect can be attributed to H
stabilizing the MDBA ground state of the n–� transition in solution. The second
effect can be explained in terms of the formation of inclusion compounds. When
the mole ratio is near 1:1 or greater, most of the MDBA molecules in solution exist
in the inclusion form: the concentration of the free MDBA molecules approaches
zero. The cage skeleton of �-CD isolates the complexed MDBA molecules from
+
H ions in the aqueous solution.
When the complexed host molecule �-CD was hydrolyzed in acid solution, it
was observed that the equilibrated electronic absorption spectrum of the complex
reverted to that of acidified MDBA aqueous solution because of the breakup of the
cage structure of the host �-CD molecule. This observation is in accordance with
the formation of an inclusion compound.
No differences between basic and neutral MDBA solutions are apparent at
longer wavelengths (Fig. 4). If the imino nitrogen of MDBA exists as an imide
ion in neutral aqueous solution, adding base should promote the dissociation of
the salt, and cause spectroscopic changes associated with the chromophore C == N.

Fig. 3. UV-visible absorption spectra for neutral (B) and acidified (A) aqueous
MDBA and �-CD + MDBA solutions: (a) No �-CD; (b) mole ratio of �-CD to
�
3
MDBA 0.6; (c) mole ratio 1.5. The MDBA concentration is 47 �mol-dm
.

2
16
Chen, Sun, and Feng
Fig. 4. UV-visible absorption spectra for neutral (A) and basic (B) MDBA solutions
�
3
(
47 �mol-dm ).
However, it has been reported that the imide salts of N-benzylideneanilines are too
prone to hydrolysis to be stable in aqueous solution.( Therefore, MDBA does
not exist in the salt form in neutral aqueous solution.
13)
In summary: (1) the MDBA molecules were stable both during the preparation
of the solution and during complexation with �-CD; (2) the MDBA molecule and,
particularly, its chromophore C == N, entered and was indeed accommodated within
the cavity of �-CD to form an inclusion compound.
3.2. Mathematical Treatment
The formation of complexes is considered for the following three cases:
(
a) H + G = H � G
K1
(b) H + G = H � G
K1
K2
H � G + G = H � G2
(
c) H + G = H � G
K1
K2
H � G + H = H2 � G
We take case (b) as an example to illustrate the mathematical analysis of the
observed absorbance data sets.
[
H � G]
[H]0 � [H � G] � [H � G2])([G]0 � [H � G] � 2[H � G2])
H � G2]
[G]0 � [H � G] � 2[H � G2])[H � G]
K1 =
K2 =
(
(
[

Spectrophotometric Studies of Inclusion Formation
217
where [H]0 and [G]0 represent the initial concentrations of host and guest, re-
spectively. According to the Beer–Lambert law, the absorbance of solution is
represented at a given wavelength as
Abs = E1[G] + E2[H � G] + E3[H � G2] + E4[H]
Where Ei represents the molar absorptivity of species i. The objective function
may then be represented as
X
2
Objmin
=
{Abs(i) � E1[G] � E2[H � G] � E3[H � G2] � E4[H]}
i
Constraints on the above equation are
X
(
{Abs(i) � E [G] � E [H � G] � E [H � G ] � E [H]} [H � G]) = 0
1
2
3
2
4
i
X
(
{Abs(i) � E [G] � E [H � G] � E [H � G ] � E [H]} [H � G ]) = 0
1
2
3
2
4
2
i
Solutions to this problem can be found by nonlinear optimization. When K2 is
zero, this case becomes equivalent to case (a). A similar method can be used for
case (c). The results of calculations by the above procedure showed that �-CD
and MDBA form a 1:1 inclusion complex in dilute aqueous solution and that
complexes with other stoichiometry are not present. The stability constants of the
1:1 complex obtained at different temperatures and using different wavelengths for
measurements are listed in Table I. As expected, the stability constants calculated
from the absorbance data sets obtained at the same temperature but at different
wavelengths are in agreement. The molar absorptivity of MDBA in chloroform
3
3
� 1
� 1
(14)
solution was reported previously as 7.8 � 10 dm -mol cm at 311.5 nm.
This value is close to that at 314.6 nm listed in Table I.
3
.3. Complexation Thermodynamics
It is noteworthy that the stability constants (Table I) are invariant with tem-
perature. Therefore, from van’t Hoff’s relation
d ln K
1H
=
�
RT 2
dT
it must be concluded that the overall process for the complexation between �-CD
and MDBA in aqueous solution is completely controlled by the entropic effect.
The entropic effect is generally contributed by: (1) dehydration of guest and host
molecules, such as �-CD and MDBA, (2) a change of host configuration, and (3)
formation of a complex between host and guest molecules, which would involve
an entropic cost. Guest molecules in complexation will lose degrees of freedom in
translation and rotation, while vibrational degrees of freedom for will be gained by
complexation.( An entropy-controlled process can thus produce a net entropy
15)

2
18
Chen, Sun, and Feng
Table I. Molar Absorptivity of MBDA in Aqueous Solutions
and Stability Constant of the 1:1 Complex between MDBA and
�
-CD at Three Temperatures from Spectrophotometric Data at
Three Wavelengthsa
�
3
� 3
10 Kb
Wavelength
10 � Molar absorptivity
T = 299.0 K
2
2
3
51.0
86.0
14.5
4.0
7.2
7.6
420
420
428
T = 306.2 K
2
2
3
51.2
86.2
14.6
3.9
7.6
8.4
420
420
420
T = 311.6 K
2
2
51.0
86.0
4.0
6.2
420
420
a
b
3
� 1
� 1
Units: wavelength, nm; absorptivity, dm -mol -cm ; K,
dm -mol
Allowed error, 2.5%.
3
� 1
.
gain to drive the host–guest complexation. Since dehydration and a change in host
configuration entail entropy gains, it is expected that these factors will play an
important role in the formation of the complex between �-CD and MDBA.
On the other hand, the host–guest inclusion can be discussed as a general
phenomenon in terms of the enthalpy–entropy compensation effect. For a com-
pensation to exist, we have
T 1S = �1H + T 1So
The Inoue group proposed that the intercept T 1So of this linear relation can
be used as a quantitative measure of the extent of desolvation occurring upon
(16)
formation of the inclusion compound.
In this context, from their viewpoint,
the thermodynamic parameters of the complexation between �-CD and MDBA
can be regarded as a point in a compensation relationship. We then have, using
3
3
� 1
K = 420 � 10 dm -mol (Table I)
1
H = 0
T 1S = T 1So = 32 kJ-mol�
1
�
at 25 C. The large net entropy gain for this complexation can be attributed to
the extensive dehydration of MDBA when it enters into the lipophilic cavity of the
host �-CD. However, the net entropic gain is much greater than indicated by the

Spectrophotometric Studies of Inclusion Formation
219
intercept value (T 1So = 13.0 kJ/mol) for the inclusion complexation of a wide
�
(16)
variety of guest molecules with �-, �-, and � -CD in aqueous solution at 25 C.
Thus, it is doubtful that the large entropy change accompanying the complexation
can be explained completely by dehydration.
3.4. Inclusion Formation Mechanism
In the �-CD molecule, a ring of hydrogen bonds is formed intramolecu-
larly between the 2-hydroxyl and the 3-hydroxyl groups of adjacent glucose units.
This hydrogen-bonding ring gives �-CD a remarkably rigid structure. CDs used
as host molecules can show molecular recognition toward guest molecules in di-
(1)
verse conditions; this recognition process generally occurs very rapidly. To our
knowledge, there have been no reports on such a slow and entropy-controlled
complexation as in the present study. �-CD possesses a smaller cavity than �-CD.
Yoshida et al.( studied molecular recognition by �-CD of �-isoelectronic trans-
azobenzenes. Their rate constants determined for complexation of �-CD with
hydroxyphenylazo derivatives of sulfanilic acid testify that these inclusions pro-
ceeded quickly.
1)
The Wenz group reported studies on the inclusion complexation between
1
(17)
�
-CD and stilbene by [ H]NMR and calorimetric titration.
The formation of
the 1:1 inclusion compound �-CD with (E)-stilbene was accompanied by a low
�
1
� 1
enthalpy gain (� 11.3 kJ-mol ), but also some entropic gain (5.0 kJ-mol at
room temperature), which is less than that of the complexation between �-CD and
MDBA. The results were attributed to the formation of a loose complex because
the guest (E)-stilbene molecule is too slender to completely fill the host �-CD
cavity. Although the twist angle � is about 20 degrees, as estimated by AM1, the
difference in molecular shape examined by means of the CPK model between (E)-
stilbene and MDBA is too small to cause steric hindrance to the entry of MDBA
into �-CD. This suggests that dimensions of MDBA should not hinder it from
rotating or librating inside the cavity.
�
-CD offers two different interfaces to a guest to form an inclusion com-
pound. If we can ignore the longitudinal anisotropies of �-CD, two possibilities
for molecular recognition by �-CD can primarily be considered (see Scheme 2).
The two possibilities involve an equilibrating state (also known as intermediate
species)inreachingequilibrium. Atequilibrium, theC == NchromophoreofMDBA
is accommodated in the cavity of �-CD, and the absorption spectrum of the equili-
brated complex is observed. Since the nonbonding electron pairs of the glucosidic
oxygen bridges are directed toward the inside of the cavity, producing a high elec-
tron density and lending it some Lewis base character, the polar end of MDBA,
the 3,4-methylenedioxyphenyl moiety, is more likely to undergo intermolecular
interaction with water and/or the hydrogen-bonding rim of �-CD than to enter
the hydrophobic cavity because of interactions between the polar end of MDBA

2
20
Chen, Sun, and Feng
Scheme 2. Two possible complexation routes.
and the glucosidic oxygen bridges of the cavity. In the meantime, because of the
interactions, the inclusion compound can dissociate energy-favorably only in a
direction away from the anilino moiety of MDBA, thus decreasing the probability
of dissociation of the inclusion compound. Therefore, it is probably reasonable to
propose that the MDBA nonpolar end, the anilino moiety, first penetrates into the
�
-CD cavity. Subsequently, an intramolecular rearrangement takes place, allow-
ing the complexed MDBA molecule to fit favorably into the �-CD cavity to attain
equilibrium (Scheme 3a). The first step releases energy 1H, due to hydropho-
bic interactions between the anilino moiety of MDBA and �-CD, which may be
assumed have a value close to that of inclusion complexation between benzene
and �-CD in aqueous solution. A subsequent intramolecular rearrangement then
cancels the enthalpy gain obtained in the first step to maintain a zero net enthalpy
gain for the overall inclusion complexation process (Scheme 3).
As mentioned above, the first step in the complexation process, the equi-
librating complexation, is a binding process accompanied by an enthalpy gain
(Scheme 3). According to the shift principle of equilibrated reactions, it is ex-
pected that an increase in temperature will be unfavorable to the formation of
equilibrating species. Experimentally it was observed that absorption spectra for
a series of �-CD + MDBA solutions gave the same spectrum as that of aqueous
Scheme 3. Energy vs. complexation coordinate.

Spectrophotometric Studies of Inclusion Formation
221
MDBA solutions and showed no appreciable changes after 9 days in the thermostat
�
at 43.9 C.
Clearly, obtaining an additional entropy gain is decisive for the complexation
to overcome the enthalpy gain obtained in the first step toward equilibrium. We
suggest two possible mechanisms in attempting to explain the strange behavior
of this inclusion complexation. Assuming that the inclusion complexation has no
discernible effect on the original structure of �-CD in the equilibrated complex,
formation of the equilibrating inclusion compound overcomes the energy barrier
due to the repulsive interaction between the glucosidic oxygen bridges and the
C == N chromophore of MDBA, so that the anilino moiety penetrates farther into
the cavity to attain equilibrium. Thus the glucosidic oxygen bridges are stuck
between the C == N and the oxygen-bridge moiety of MDBA in equilibrium, just
like a rotaxane. However, the net entropy gain obtained seems to be a bit too
large for the overall inclusion complexation. As to the aspect of configuration
entropy, orientations of complexed guest molecule inside the cavity cannot be
distinguished from each other because of the toroidal symmetry of the cavity.
There have been no reports in the literature about configurational entropy for guest
molecules complexed with �-CD.
Another proposed mechanism is based on a guess that there occurs some
geometric distortion of the cage skeleton of �-CD: i.e., the reorganization of the
original hydrogen-bond network of �-CD initiated by the polar interaction be-
tween the ether oxygen atoms of MDBA and the �-CD rim. This reorganization
would occur as inclusion complexation, which causes the guest molecule to be
anchored by disruption of original �-CD hydrogen-bond network and subsequent
formation of a hydrogen bond between �-CD and MDBA to block the dissocia-
tion of this �-CD + MDBA inclusion compound. There would be no appreciable
enthalpy effects of the overall complexation, but a large net entropy gain. Hence,
the complexation between �-CD and MDBA would exhibit a large stability con-
stant. In view of the process associated with the breaking of the original hydro-
gen bonding networks of �-CD, it can be assumed that this reorganization is a
slow process compared to the formation of complexes without the distortion of
�
-CD.
4. CONCLUSION
Molecular recognition by cyclodextrins constitutes a fundamental model pro-
cess in host–guest chemistry. Despite their apparent simplicity, the factors deter-
mining the recognition mechanism are not yet fully elucidated. Considering spec-
trophotometric data, we discussed the two possible mechanisms for the molecular
recognition by �-CD of MDBA in dilute aqueous solution on the basis of a hy-
pothesis that the synthesized MDBA contains no appreciable amount of isomers,
such as (Z)-MDBA, other than (E)-MDBA.

2
22
Chen, Sun, and Feng
One mechanism supposes that the 1:1 inclusion complexation between �-CD
and MDBA is accompanied by distortion of the �-CD skeleton initiated by in-
tramolecular polar interactions between the host and the complexed MDBA, and
that this reorganization is a rate-determining step (RDS) that produces a large en-
tropy gain. This reorganization would cause the complexed guest molecule to be
anchored and, consequently, would inhibit the dissociation of the inclusion com-
pound. On the other hand, another mechanism attributes the net entropy gain to
the inclusion complexation itself between MDBA and �-CD, without the distor-
tion of �-CD. Its RDS is for the anilino moiety to penetrate farther into the �-CD
cavity to form an equilibrated inclusion compound of a pseudorotaxane structure.
A theoretical simulation of the inclusion behavior is underway.
ACKNOWLEDGMENT
Dr. L. Zhou provided 3,4-(methylenedioxyphenyl) benzylideneaniline.
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