9
toward a wide range of hydrophobic species. We believe
that these host-guest interactions can disrupt the substantial
π-π stacking of phthalocyanines, rendering nonaggregated
species in aqueous media. This simple approach has been
reported for porphyrins,10 which in general have a weaker
aggregation tendency compared with phthalocyanines. To our
knowledge, the complexation of phthalocyanines with cy-
clodextrins has only been little studied. Ruebner and Breslow
et al. reported a series of cyclodextrin dimers which can
complex and solubilize several mono-sulfonyl or carboxy
phthalocyanines in water.11 Nyokong et al. studied briefly
the effects of cyclodextrins on the photophysical properties
of several hydrophobic phthalocyanines in DMSO. In this
work, we used various spectroscopic and molecular modeling
methods to show unambiguously the complexation of a series
of tetrakis- and octakis(4-carboxyphenoxy) phthalocyanines
with â-cyclodextrin (â-CD) and heptakis(2,3,6-tri-O-methyl)-
â-cyclodextrin (TMe-â-CD) (Figure 1), and demonstrate that
1
2
Figure 2. Structures of phthalocyanines 1-4.
The UV-vis spectra of 1-4 in THF showed (a) very sharp
and intense Q-band(s), which followed the Lambert-Beer
law very well (see Figures S1 and S2 in the Supporting
Information). The results suggested that these compounds
are essentially nonaggregated in this solvent. In the presence
of NaOH, these compounds were soluble in water, and the
Q-band(s) became significant broadened and shifted to the
blue, particularly for the tetrasubstituted analogues. This is
a strong indication of the formation of face-to-face or
1
5
H-aggregates.
The complexation of 3 and â-CD in an alkaline aqueous
medium (pH 12) was first studied by fluorescence spectros-
copy and the stoichiometry determined by a continuous
1
6
variation method. The spectra of mixtures of these two
compounds in different ratios (total concentration ) 10 µM)
showed an emission at ca. 710 nm with different intensity.
By plotting the area of the fluorescence peak versus the mole
fraction of 3, the curve clearly showed a maximum at 0.34
Figure 1. Schematic structures of â-CD and TMe-â-CD.
such host-guest interactions could be an effective way to
reduce the aggregation of phthalocyanines in water and
enhance their singlet oxygen formation.
The 4-carboxyphenoxy phthalocyanines 1-4 (Figure 2)
were selected for the study because they could be readily
soluble in alkaline aqueous media and the aryl substituents
were expected to preferentially reside in the hydrophobic
cavity of cyclodextrins. The metal-free phthalocyanines 1
(see Figure S3 in the Supporting Information). This suggested
that phthalocyanine 3 complexes with â-CD in a 1:2 manner.
Figure 3 shows the changes in the UV-vis spectrum of 3
in water (pH 12) upon addition of â-CD (from 8- to 183-
fold). It can be seen that at higher â-CD concentrations, the
Soret band at 333 nm shifts to 355 nm, while the broad
Q-band at 643 nm becomes a sharp band at 684 nm.17 This
shows that â-CD can disrupt the aggregation of this phtha-
locyanine probably through host-guest interactions. Ac-
cording to a 1:2 binding model, eq 1 can be derived and
(as a mixture of constitutional isomers) and 3 were prepared
in ca. 30% yield by treating 4-(4-methoxycarbonylphenoxy)-
phthalonitrile or 4,5-bis(4-methoxycarbonylphenoxy)phtha-
lonitrile with lithium in 1-pentanol followed by in situ
alkaline hydrolysis with LiOH and subsequent protonation
with HCl. These compounds were then metalated with Zn-
1 2
used to determine the stepwise binding constants K and K
1
8
by a nonlinear regression method:
2
(OAc)
2
2
‚2H O to give the zinc analogues 2 and 4 in 56%
I
0
+ I
+ K [â-CD] + K K [â- CD]
0
1 1 0 2 1 2 0
K [â-CD] + I K K [â-CD]
I )
(1)
and 63% yield, respectively (characterization data and spectra
2
1
1
0
1
2
1
3,14
for 1-4 are given in the Supporting Information).
where I is the intensity (either the absorbance at a particular
(
9) See the whole issue of: Chem. ReV. 1998, 98, 1741-2076, D’Souza,
V. T., Lipkowitz, K. B., Eds.
10) (a) Konishi, T.; Ikeda, A.; Asai, M.; Hatano, T.; Shinkai, S.;
Fujitsuka, M.; Ito, O.; Tsuchiya, Y.; Kikuchi, J.-i. J. Phys. Chem. B 2003,
07, 11261. (b) Wu, J.-J.; Ma, H.-L.; Mao, H.-S.; Wang, Y.; Jin, W.-J. J.
Photochem. Photobiol. A: Chem. 2005, 173, 296.
11) (a) Ruebner, A.; Kirsch, D.; Andrees, S.; Decker, W.; R o¨ der, B.;
(
(13) The hexadecacarboxy analogues were prepared similarly. See: Choi,
C.-F.; Tsang, P.-T.; Huang, J.-D.; Chan, E. Y. M.; Ko, W.-H.; Fong, W.-
P.; Ng, D. K. P. Chem. Commun. 2004, 2236.
(14) Compounds 2-4 were prepared previously directly from the
corresponding carboxy-substituted phthalonitriles, but very few characteriza-
tion data were given. See: (a) W o¨ hrle, D.; Eskes, M.; Shigehara, K.;
Yamada, A. Synthesis 1993, 194. (b) Nazeeruddin, M. K.; Humphry-Baker,
R.; Gr a¨ tzel, M.; W o¨ hrle, D.; Schnurpfeil, G.; Schneider, G.; Hirth, A.;
Trombach, N. J. Porphyrins Phthalocyanines 1999, 3, 230. (c) Maree, S.
E.; Nyokong, T. J. Porphyrins Phthalocyanines 2001, 5, 782.
1
(
Spengler, B.; Kaufmann, R.; Moser, J. G. J. Inclusion Phenom. Mol.
Recognit. Chem. 1997, 27, 69. (b) Baugh, S. D. P.; Yang, Z.; Leung, D.
K.; Wilson, D. M.; Breslow, R. J. Am. Chem. Soc. 2001, 123, 12488.
(12) Tau, P.; Ogunsipe, A. O.; Maree, S.; Maree, M. D.; Nyokong, T. J.
Porphyrins Phthalocyanines 2003, 7, 439.
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Org. Lett., Vol. 9, No. 13, 2007