2
50
O.K. Abou-Zied / Spectrochimica Acta Part A 62 (2005) 245–251
4. Conclusions
The complexes of AZ with -CD and ␥-CD were studied
using absorption, fluorescence, and induced-circular dichro-
ism spectroscopy. The changes in the absorption and flu-
orescence spectra of AZ as a function of varying the ini-
tial concentrations of CDs suggest the inclusion of AZ in-
side the CD cavities. The data were analyzed in the light
of Benesi–Hildebrand and non-linear square fits. The results
show that AZ forms 1:1 and 1:2 inclusion complexes with

-CD and ␥-CD, respectively. Binding constants were calcu-
−1
lated for the two complexes to be 780 ± 150 M for AZ:-
5
−2
CD and (4.5 ± 0.86) × 10 M for AZ:(␥-CD)2. The bind-
ing constant in the latter is due to a stepwise equilibrium
mechanism involving the formation of a 1:1 complex in the
first step with a subsequent formation of a 1:2 complex with
−
1
−1
binding constants of K1 = 775 M and K2 = 580 M , re-
spectively. Both ␥-CD molecules contribute almost equally to
thebindingconstantinAZ:(␥-CD)2. Induced-circulardichro-
ism spectra confirm the formation of inclusion complexes in
which the AZ molecule exists inside the -CD cavity in an ax-
ial mode, whereas the inclusion of AZ inside ␥-CD is nearly
axial. The latter observation is explained in terms of the effect
of the wide ␥-CD cavity size in which AZ can acquire some
flexibility.
Fig. 7. (a) Absorption of AZ aqueous solution; (b) induced-circular dichro-
−3
ism spectra of AZ containing 5 × 10 M of -CD (solid line) and ␥-CD
(dashed line).
The smaller ellipticity value in the case of the AZ:(␥-CD)2
complex can be interpreted in terms of the change in the angle
between the electronic transition dipole moment of the guest
(AZ) and the symmetry axis of the host (CD) [37]. The large
cavity size of ␥-CD (i.d. 0.95 nm) should allow AZ to acquire
a position which creates some angle between its long axis
and the CD-symmetry axis (see Scheme 2). This is restricted
to some extent in the case of the AZ:-CD complex due to
the smaller inner diameter of the -CD cavity (0.78 nm). In
addition, the inclusion of AZ by two ␥-CD molecules results
in a large space available for the AZ molecule to acquire some
unrestricted geometries. The orientation of the AZ molecule
inside the ␥-CD cavities leads to more interaction with the
inner walls and explains the large K1 and K2 values obtained
above.
Acknowledgments
This work was supported by the Sultan Qaboos Univer-
sity. The author would like to thank Prof. Gary Scott and
the University of California, Riverside for the use of their
spectrophotometer to measure the induced-circular dichro-
ism spectra.
No evidence for any AZ dimer formation inside one or two
-CD cavities (2:1 or 2:2) was found in this work. Although
References
␥
the results obtained in this paper show only the formation
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The stoichiometry of the inclusion of 2-acetylnaphthalene in
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[
[
[
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