Complexation of 4-Dimethylaminoazobenzene with Various Kinds of Cyclodextrins: Effects of Cyclodextrins on the Thermal cis-to-trans Isomerization

Article abstract of DOI:10.1002/kin.10074

On the basis of the change in electronic and induced circular dichroism spectra for complex formation, the complexation of 4-dimethylaminoazobenzene (DAAB) with four kinds of cyclodextrins (α- and β-cyclodextrin (CD), heptakis(2,6-di-O-methyl)-β-cyclodextrin, and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin) was studied in methanol-water and dimethyl sulfoxide-water mixtures. It was found that the trans and cis isomers of DAAB form two different types of complex (inclusion and lid type) with CDs, depending on the kinds of CDs and solvents. Further, we have examined the effect of CDs on the thermal cis-to-trans isomerization of DAAB. The accelerated or decelerated effect on the thermal isomerization was observed upon adding CDs. The effects of CDs on the thermal isomerization are discussed in connection with the complexation of the cis-isomer of DAAB with CDs.

Full text of DOI:10.1002/kin.10074

Complexation of 4-
Dimethylaminoazobenzene
with Various Kinds
of Cyclodextrins: Effects
of Cyclodextrins on the
Thermal cis-to-trans
Isomerization
YOSHIMI SUEISHI, HIROAKI HISHIKAWA
Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushima Naka, Okayama 700-8530, Japan
Received 22 October 2001; accepted 24 April 2002
DOI 10.1002/kin.10074
ABSTRACT: On the basis of the change in electronic and induced circular dichroism spec-
tra for complex formation, the complexation of 4-dimethylaminoazobenzene (DAAB) with four
kinds of cyclodextrins (�- and �-cyclodextrin (CD), heptakis(2,6-di-O-methyl)-�-cyclodextrin,
and heptakis(2,3,6-tri-O-methyl)-�-cyclodextrin) was studied in methanol–water and dimethyl
sulfoxide–water mixtures. It was found that the trans and cis isomers of DAAB form two dif-
ferent types of complex (inclusion and lid type) with CDs, depending on the kinds of CDs and
solvents. Further, we have examined the effect of CDs on the thermal cis-to-trans isomeriza-
tion of DAAB. The accelerated or decelerated effect on the thermal isomerization was observed
upon adding CDs. The effects of CDs on the thermal isomerization are discussed in connection
with the complexation of the cis-isomer of DAAB with CDs. �C 2002 Wiley Periodicals, Inc. Int
J Chem Kinet 34: 481–487, 2002
INTRODUCTION
ous workers [1–3]. One of the important applications
of inclusion complexes is in the control of chemical
reactions. The isomerization of azobenzenes has at-
tracted much attention because of their photochromic
and thermochromic behaviors. Several studies have
been reported on the effects of CDs on the cis-trans
photoisomerization of azobenzenes [4,5]. On one hand,
the effect of CDs on the thermal cis-to-trans isomer-
ization of azobenzenes has not been well established
yet, though Sanchez and Rossi have reported about
Cyclodextrins (CDs) are the cyclic oligosaccharides,
which can form inclusion complexes with various
molecules. The conformation and dynamic properties
of inclusion complexes have been studied by numer-
Correspondence to: Y. Sueishi; e-mail: ysueishi@cc.okayama-u.
ac.jp.
�
c 2002 Wiley Periodicals, Inc.

4
82
SUEISHI AND HISHIKAWA
the effects of ␤-CD on the thermal isomerizations of
some azobenzenes [6]. We have examined the effects
of various kinds of CDs on the thermal isomerization
of 4-dimethylaminoazobenzene (DAAB) in methanol–
water and dimethyl sulfoxide (DMSO)–water mix-
tures, and an unexpected significant effect of CDs
was observed. In this paper, we have used four kinds
of CDs (␣- and ␤-CDs, heptakis(2,6-di-O-methyl)-
with water, as a solvent. A sample solution (DAAB ca.
�
5
� 3
4 � 10 mol dm ) involving a small quantity of
piperidine was irradiated with projection lamp to pro-
duce the cis-isomer of DAAB [7]. The thermal isomer-
ization from cis-to-trans isomer at 298 K was moni-
toredbythechangeinthemaximumabsorption(�max =
450 nm) of the DAAB trans-isomer in mixture solvents
by using a Hitachi U-3200 spectrophotometer. The
thermal cis-to-trans isomerization obeys first-order ki-
netics and the observed rate was reproducible to within
� 5%. Circular dichroism spectra were obtained with
a JASCO J-720 spectropolarimeter.
␤
-cyclodextrin (DM-␤-CD), and heptakis(2,3,6-tri-
O-methyl)-␤-cyclodextrin (TM-␤-CD)). We have fo-
cused attention on the complexation of DAAB with
four kinds of CDs in mixture solvents, and the effects
of CDs on the thermal isomerization are discussed in
detail.
RESULTS AND DISCUSSION
Complexation of the trans-Isomer
of DAAB with CDs
EXPERIMENTAL
Materials
Figure 1 shows the absorption spectra of the trans-
isomer of DAAB in methanol–water and DMSO–water
mixtures containing various concentrations of ␤-CD.
When the ␤-CD concentration was increased, absorp-
tion peaks shifted to shorter wavelengths. Judging from
the existence of isosbestic points, the trans-isomer was
found to form 1:1 complexes with ␤-CD. The associa-
tion constant (KT) between the trans-isomer and ␤-CD
can be expressed as follows:
DAAB was prepared according to the reported method
and recrystallized from benzene, mp 391–392 K (lit.
392–393K)[7,8]. VariouskindsofCDs(␣-and␤-CDs,
DM-␤-CD, andTM-␤-CD)werepurchasedfromWako
Pure Chemicals and used without further purification.
Methanol, DMSO, and water were distilled before
use.
Measurements
T + CD � T-CD
Since DAAB is insoluble in water, we used methanol–
water (30:70 v/v) and DMSO–water (30:70 v/v) mix-
tures, which means 30 ml of organic distilled to 100 ml
(1)
[T � CD]
KT =
[T][CD]
Figure 1 Absorption spectra of the trans-isomer of DAAB in DMSO–water and methanol–water mixtures containing various
�
5
� 3
� 4
concentration of ␤-CD. (a) DMSO–water mixture: [DAAB] = 4.15 � 10 mol dm and [␤-CD] = (1) 0, (2) 4.44 � 10
,
�
4
� 3
� 3
� 3
� 5
� 3
(
3) 8.88 � 10 , (4) 1.78 � 10 , and (5) 2.66 � 10 mol dm . (b) Methanol–water mixture: [DAAB] = 2.07 � 10
�
3
� 4
� 4
� 4
� 4
mol dm and [␤-CD] = (1) 0, (2) 2.22 � 10 , (3) 4.44 � 10 , (4) 6.66 � 10 , (5) 8.88 � 10 , and (6) 1.33 � 10
�
3
mol dm . Inset: Circular dichroism spectra of complexes between the trans-isomer and ␤-CD.

COMPLEXATION OF 4-DIMETHYLAMINOAZOBENZENE WITH VARIOUS KINDS OF CYCLODEXTRINS
483
where T and T-CD denote the trans-isomer of DAAB
and its complex, respectively. In the presence of excess
CD, the concentration of CD is approximated with the
initial concentration of CD ([CD]0). The spectral data
in Fig. 1 can be analyzed according to the Benesi–
Hildebrand (B-H) equation (Eq. 2) [9]:
Table I Equilibrium Constants Between the trans-
Isomer of DAAB and CDs at 298 K
KT (mol dm3)
� 1
DMSO–water
(30:70 v/v)
Methanol–water
(30:70 v/v)
Cyclodextrin
␣
␤
-CD
-CD
3940 (lid)
87.5 (lid)
3360 (inclusion)
630 (inclusion)
6140 (lid)
[
T]0[CD]0
1
[CD]0
972 (inclusion)
1810 (inclusion)
223 (inclusion)
=
+
(2)
1
E
KT1ε
1ε
DM-␤-CD
TM-␤-CD
Here [T]0 is the total concentration of DAAB, 1ε is
the difference in the molar extinction coefficients for
bound and unbound DAAB, and 1E is the change in
the absorption intensity of DAAB solution on adding
CD. As shown in Fig. 2, a linear relationship between
benzoate anions with CDs, Namor et al. [11] have sug-
gested that two different types of complexation oc-
cur as a result of a change in the reaction medium:
One is the inclusion complex and the other reason-
able possibility is the formation of the lid type com-
plex where azobenzene is located in one of the rims
of CD and not in the CD cavity. In order to depict
the lid type complexes of ␣- and ␤-CDs with DAAB,
the Corey–Pauling–Koltun (CPK) space-filling mod-
els of these systems were examined. The CPK model
shows that the distance between two nitrogen atoms
of the dimethylamino and azo groups is smaller than
the inner diameter of ␤-CD [12] (Scheme 1). There-
fore, though complexation of azobenzene derivatives
with CDs may be specific, the trans-isomer of DAAB
is considered to form stronger hydrogen bonding with
OH groups on the rim of ␣-CD through the nitrogen
atoms of DAAB (Scheme 1), which is responsible for
the large KT value for the lid type complex formation
of ␣-CD in DMSO–water, compared to that for ␤-CD.
Similar KT values for the lid type complexation were
[T]0[CD]0/1E and [CD]0 was obtained. The KT values
were obtained by dividing the slope by the intercept,
and are given in Table I, together with those with var-
ious kinds of CDs. Hirai et al. [10] indicated that the
magnitude of the induced circular dichroism is propor-
tional to the amount of the inclusion complexes pro-
duced. In Fig. 1, the induced circular dichroism spectra
for the complexes of the DAAB trans-isomer with ␤-
CD in methanol–water and DMSO–water are shown.
In methanol–water mixture, induced circular dichroism
appears in the vicinity of 420 nm, which indicates the
inclusion complex formation. However, the DAAB-␤-
CD system in DMSO–water mixture does not show any
induced circular dichroism by themselves. It is noted
that the complex between DAAB and ␤-CD in DMSO–
water mixture is not an inclusion complex, though the
change in the electronic spectra is observed by adding
␤
-CD. For the complexation with DM-␤-CD and TM-
3
␤
-CD, theinducedcirculardichroismspectraappeared,
reported by Namor et al. [11]: KT = 1.58–5.89 � 10
�
1
3
and those for the complexation with ␣-CD did not
appear.
Based on the thermodynamic study for the com-
plex formation of substituted (p-hydroxyphenylazo)-
mol dm between substituted azobenzenes and ␣-CD
in N,N-dimethylformamide. For the inclusion complex
formation, there are two possible isomeric 1:1 com-
plexes: one of these has the phenyl ring inserted and
Figure 2 Determination of the association constant of ␤-CD with the DAAB trans-isomer according to the Benesi–Hildebrand
relationship in (a) DMSO–water and (b) in methanol–water.

4
84
SUEISHI AND HISHIKAWA
substitution by O-methyl groups at the inner side (the
O(3)H group) of ␤-CD narrows the inner diameter of
the cavity [13–15]. This is responsible for the small KT
value for inclusion by TM-␤-CD. In the cases of DM-
␤
-CD and TM-␤-CD, the lid type complex may not be
formed because of methylation of the hydroxyl groups
on the CD rim.
There have been many reports on the solvent ef-
fects of the complexation with CD. In binary aqueous-
organic solvents, Connors et al. [16] have interpreted
the effects of organic solvents on CD-complex forma-
tions in terms of competitive complexing. It is recog-
nized that the addition of organic solvents decreases re-
markably the equilibrium constant for association with
CD [16,17]. Matsui et al. [18] examined the solvent ef-
fects on the CT absorption band of pyridinio derivatives
�
withI inDMSO–waterandmethanol–watermixtures.
With increasing methanol and DMSO, the absorption
maximum of the CT complex shifted to the direction of
longer wavelengths and the effect was larger in DMSO
than in methanol. This suggests that in our study the ad-
dition of DMSO makes the environment around DAAB
less polar, compared with that of methanol. The fact
that DAAB does not form inclusion complex with ␤-
CD in DMSO–water can be attributed to the similar-
ity between the environment in mixture solvents and
microenvironment in the CD cavity. Moreover, since
the association constant of DMSO with CD is larger
than that of methanol [16], our observations may in-
volve the influence of competitive complexing by or-
ganic solvents. For the solvent effect on the lid type
complex formation, although the kind of interaction
that is operative has not been clarified, microscopic
solvent-polarity effect is operative for the lid type
Scheme 1
the other has the dimethylaminophenyl ring inserted.
However, the existence of isosbestic points and linear-
ity of B–H plots for absorption and circular dichro-
ism spectral changes suggest one kind of inclusion
complex formation. Judging from hydrophobic na-
ture of the CD cavity and mesomeric effect of the
dimethylamino group of the DAAB guest, we consider
that the dimethylaminophenyl ring is included by CD
(Scheme 1). An X-ray diffraction study has suggested
that the substitution by a methyl group at the O(2) and
O(6) positions in ␤-CD lengthens the CD cavity from
ca. 0.8 to 1.1 nm without any significant distortion of
the ring and enhances the inclusion ability [13–15]. In
fact, DM-␤-CD forms the stable inclusion complex and
theassociationconstantKT forDM-␤-CDinmethanol–
water is about twice larger than that for ␤-CD. The en-
hanced inclusion stability of DM-␤-CD is ascribed to
the enlarged hydrophobic cavity of the CD host. On the
other hand, KT for TM-␤-CD in methanol–water is re-
markably small compared with that for DM-␤-CD, as
shown in Table I. The methylations at the O(2), O(3),
and O(6) positions in ␤-CD considerably reduce the
stability of inclusion complexes. The TM-␤-CD ring
is distorted in contrast with the case of DM-␤-CD and
Figure 3 Induced circular dichroism spectra of DAAB
�
5
� 3
(2.07 � 10 mol dm ) in methanol–water: (1) 0, (2) 2.22
�
4
� 4
� 4
� 3
� 10 , (3) 4.44 � 10 , and (4) 8.88 � 10 mol dm
of ␤-CD. Inset: the plot of [T]0[␤-CD]0/11ECD against
␤-CD]0.
[

COMPLEXATION OF 4-DIMETHYLAMINOAZOBENZENE WITH VARIOUS KINDS OF CYCLODEXTRINS
485
in methanol–water mixture forms the inclusion com-
plex with ␤-CD. The types of the complexations with
various kinds of CDs in mixture solvents are clarified
in Table I.
Effect of CD on the Thermal
Isomerization
Figures 4 and 5 show the significant effects of CDs
on the thermal cis-to-trans isomerization. As shown in
Fig. 4, the accelerated effect on the thermal isomeriza-
tion was observed with increasing the ␣-CD concentra-
tion. Upon addition of ␤-CD, the accelerated effect on
the isomerization rate was observed in DMSO–water,
while the decelerated effect was obtained in methanol–
water (Fig. 5). The addition of DM-␤-CD caused de-
crease in the isomerization rates, independent of sol-
vents. Further, the thermal isomerization rate is inde-
pendent of the addition of TM-␤-CD. The mechanism
of the thermal cis-to-transisomerization in the presence
of CDs can be expressed as follows:
Figure 4 Dependence of the observed rate constants with
the [␣-CD] for the thermal isomerization of DAAB at 298 K:
(
N) in DMSO–water and (�) in methanol–water.
complexation, resulting in the large KT value for ␣-CD
in methanol–water.
In complexation of ␤-CD in methanol–water, the
changeinelectronicspectrainvolvesbothinclusionand
lid type complex formations. To investigate the com-
plexing ability of DAAB toward ␤-CD in methanol–
water, we have compared the KT values estimated from
analyses ofinduced circulardichroism spectra with that
of electronic ones. Figure 3 shows the induced circu-
lar dichroism spectra in the vicinity of 420 nm for the
h␯
T �ꢀ C
k 1
C �ꢀ T
k2
C-CD �ꢀ T-CD
KC
C + CD )* C-CD
␤
-CD complex with DAAB in methanol–water mix-
ture. The magnitude of the induced circular dichroism
increases with an increase in the concentration of ␤-
CD. From analyses of the circular dichroism spectra,
the KT value for the inclusion complex formation in
methanol–water mixture was calculated to be 971 �
where C and C-CD denote the cis-isomer of DAAB and
its complex with CD, respectively. KC is the associa-
tion constant between the cis-isomer and CDs. The rate
constants (k1) for the thermal isomerization in DMSO–
�
3
water and methanol–water (at [CD] = 0 mol dm
)
�
1
3
� 1
� 1
1
10 mol dm , which is in good agreement with
were determined as 0.0162 s and 0.0398 s , re-
spectively. Under the condition [CD] � [DAAB], the
observed rate constant (kobs) for the thermal cis-to-trans
the value estimated from analyses of the electronic
spectra. This suggests that the trans-isomer of DAAB
Figure 5 Dependence of the observed rate constants with the CD concentration for the thermal isomerization of DAAB at
98 K in (a) DMSO–water and (b) methanol–water: ( ) ␤-CD, (
) DM-␤-CD, and (�) TM-␤-CD.
2
N
�

4
86
SUEISHI AND HISHIKAWA
Table II Rate Constants for the Thermal Isomerization and the Equilibrium Constants Between cis-Isomer of DAAB
and CDs at 298 K
KC (mol 1 dm3)
�
k1 (10
� 2 � 1
s
)
k2 (10
� 2 � 1
s )
Solvent
Cyclodextrin
DMSO–water (30:70 v/v)
␣-CD
44.5
340
1400
600
700
1740
1.62
4.37
2.10
0.127
5.80
2.85
␤
-CD
1.62
1.62
3.98
3.98
3.98
DM-␤-CD
␣-CD
Methanol–water (30:70 v/v)
␤
-CD
DM-␤-CD
0.237
isomerization can be given by
k1 + k2 KC[CD]0
kobs =
(3)
1
+ KC[CD]0
From fitting the experimental data in Figs. 4 and 5 to
Eq. (3), the k2 and KC values can be determined and
are given in Table II.
We consider that the above kinetic results are deeply
correlated to the nature of the complexation between
the cis-isomer and CDs. It is noted that the accelerated
effects on the thermal isomerization by CD is observed
in the system formed the lid type complex between CD
and the trans-isomer. As suggested earlier, the trans-
isomer of DAAB forms the two different types of com-
plexes with CDs. Since the distance between the ni-
trogen atoms of dimethylamino and azo groups in the
DAAB cis-isomer is same as that in the trans-isomer,
the cis-isomer may form the lid type complex with CD
in analogy with that of the trans-isomer. In the ther-
mal isomerization of azobenzenes, it is recognized that
the isomerization reaction is catalyzed by acid, and the
isomerization rate accelerates by the protonation to the
azo group [6,19]. As depicted below, a similar situa-
tion is realized by the formation of the lid type complex.
Scheme 2
inclusion from the two phenyl groups side (direction B)
and the other is the inclusion from the azo group side
direction A). Since the distance between two phenyl
groups is larger than the inner diameter of ␤-CD, en-
trance from direction B is impossible. Therefore, ␤-CD
may include the cis-isomer from the azo group side
(
(
direction A), and some part of the cis-isomer is in-
cluded in the cavity (Scheme 2). Despite our attempts
to observe the induced circular dichroism spectra for
the inclusion complex of the unstable cis-isomer with
CDs, we have been unsuccessful. When DM-␤-CD
was added, the decelerated effect (k2/k1 = 0.060 in
methanol–water) is remarkably large compared with
that by ␤-CD (k2/k1 = 0.72 in methanol–water). This
suggests that the cis-isomer of DAAB is included more
deeply into the DM-␤-CD cavity, which is responsible
for the enhanced inclusion stability of DM-␤-CD. In
the presence of TM-␤-CD, the fact that the thermal
isomerization of DAAB is not affected by TM-␤-CD
indicates that TM-␤-CD cannot form an inclusion com-
plex with the cis-isomer. Judging from the inclusion
abilities of DM-␤-CD and TM-␤-CD, above results are
reasonable.
This leads to the decrease in the double bond character
of the N N bond, which favors the rotational mecha-
nism, resulting in the accelerated effect. On the other
hand, as reported by Nishimura et al. [7], when the ther-
mal isomerization of azobenzenes is not catalyzed by
acid, the thermal isomerization of DAAB proceeds via
inversion of the nitrogen atom of the azo group through
a linear (sp-hybridized) transition state (in-plane lat-
eral shift). In the cis-DAAB inclusion complexation,
the following two type of inclusions by CD give rise
to the decelerated effect on the thermal isomerization
of the inversion mechanism (Scheme 2): One is the
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