Hydrogen-bonding-driven preorganized zinc porphyrin receptors for efficient complexation of C60, C70, and C60 derivatives

Article abstract of DOI:10.1021/ja056509h

This paper describes the self-assembly of a new class of foldamer-based molecular tweezers, whose rigid folded conformations are stabilized by intramolecular hydrogen bonding. Two zinc porphyrin units are introduced to the ends of molecular tweezers Zn₂1 and Zn₂2, while three zinc porphyrin units are incorporated to the S-shaped bi-tweezers Zn₃3, which may be regarded as a combination of two Zn₂1 molecules. Due to the preorganized U-shaped feature, Zn²1 and Zn₂2 are able to strongly complex C₆₀, C₇₀, and C₆₀ derivative 25 in chloroform or toluene in a 1:1 binding stoichiometry, whereas Zn₃3, which possesses two tweezer units, complexes the guests in a 1:2 stoichiometry. More stable complex Zn₃3·24 is formed between Zn₃3 and 24, a linear molecule bearing two C₆₀ moieties at the ends, as a result of the cooperative interaction of two binding sites. Chiral induction is observed for all the three receptors upon complexation with C₆₀-incorporated chiral phenylalanine derivative 29, although the complexation of 29 by the folding receptors is pronouncedly weaker than that of C₆₀ and 25 due to increased steric hindrance. The driving force for the formation of the complexes is the well established π-π stacking between the zinc porphyrin and fullerene units. The ¹H and ¹³C NMR, UV-vis, fluorescent, and circular dichroism spectroscopy have been used to investigate the complexing behavior of the folding receptors and the fullerene guests. The association constants of the corresponding complexes in toluene and chloroform (if possible) have been evaluated with the UV-vis and fluorescent titration experiments.

Full text of DOI:10.1021/ja056509h

Published on Web 11/17/2005
Hydrogen-Bonding-Driven Preorganized Zinc Porphyrin
Receptors for Efficient Complexation of C60, C70, and C60
Derivatives
Zong-Quan Wu, Xue-Bin Shao, Chuang Li, Jun-Li Hou, Kui Wang, Xi-Kui Jiang, and
Zhan-Ting Li*
Contribution from the State Key Laboratory of Bio-Organic and Natural Products Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai 200032, China
Received September 27, 2005; E-mail: ztli@mail.sioc.ac.cn.
Abstract: This paper describes the self-assembly of a new class of foldamer-based molecular tweezers,
whose rigid folded conformations are stabilized by intramolecular hydrogen bonding. Two zinc porphyrin
units are introduced to the ends of molecular tweezers Zn21 and Zn22, while three zinc porphyrin units are
incorporated to the S-shaped bi-tweezers Zn33, which may be regarded as a combination of two Zn21
molecules. Due to the preorganized U-shaped feature, Zn21 and Zn22 are able to strongly complex C60,
C70, and C60 derivative 25 in chloroform or toluene in a 1:1 binding stoichiometry, whereas Zn33, which
possesses two tweezer units, complexes the guests in a 1:2 stoichiometry. More stable complex Zn33‚24
is formed between Zn33 and 24, a linear molecule bearing two C60 moieties at the ends, as a result of the
cooperative interaction of two binding sites. Chiral induction is observed for all the three receptors upon
complexation with C60-incoporated chiral phenylalanine derivative 29, although the complexation of 29 by
the folding receptors is pronouncedly weaker than that of C60 and 25 due to increased steric hindrance.
The driving force for the formation of the complexes is the well established π-π stacking between the zinc
porphyrin and fullerene units. The 1H and 13C NMR, UV-vis, fluorescent, and circular dichroism spectroscopy
have been used to investigate the complexing behavior of the folding receptors and the fullerene guests.
The association constants of the corresponding complexes in toluene and chloroform (if possible) have
been evaluated with the UV-vis and fluorescent titration experiments.
Introduction
Fullerenes and their derivatives are a class of attractive
spherical molecules that have been applied extensively in
discrete research areas. After the first reports of the cocrystal-
lization of C60 or C70 with the porphyrin unit, the search for
highly efficient porphyrin receptors has become intensified.4 A
number of bisporphyrin5 and multiporphyrin receptors6 have
been developed and the resulting porphyryn-fullerene as-
semblies have brought forward many interesting photophysical,
Efficient recognition of the synthetic receptor for a special
molecule or ion requires high structural and binding-site
complementarity between the receptor and the guest. For
spherical guests, macrocyclic receptors are usually of high
efficiency because the recognition sites in these receptors are
pre-organized in the cavity to favorably surround a specific
guest.1 Another class of efficient receptors for spherical guests
are rigid, covalently bonded molecular tweezers, which are able
to employ their two “jaws” to closely hold a binding site-
matching “prey”.2 Nevertheless, the synthesis of both kinds of
receptors are usually of low efficiency or time consuming.3
Moreover, the structural modifications for achievement of a site-
tailored recognition are frequently difficult.
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Balch, A. L.; Dorn, H. C. Nature 1999, 401, 55.
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K.; Saigo, K.; Sakamoto, S.; Yamguchi, K. J. Am. Chem. Soc. 1999, 121,
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17460
J. AM. CHEM. SOC. 2005, 127, 17460-17468
10.1021/ja056509h CCC: $30.25 © 2005 American Chemical Society

Efficient Complexation of C60, C70, and C60 Derivatives
A R T I C L E S
photochemical, and/or electrochemical properties.7 In the past
decade, foldamers, linear molecules that are driven by non-
covalent forces to adopt well-established secondary structures,
have received increasing attention.8 Because of its strength and
directionality, hydrogen bonding is an ideal driving force for
the construction of foldamers. A number of foldamers with rigid
and predictable conformations have been developed based on
rationally designed aromatic amide oligomers.9-16 Some of the
foldamers have been reported as new receptors for recognition
of saccharides14a,14d or alkylammoniums14e or encapsulation of
water.15a Example of foldamers that can promote oxidation of
pyridine has also been described.15b We envisioned that
introduction of additional binding units to rationally designed
folding scaffolds would lead to new generation of tailored
receptors with robust recognition ability. In this paper, we report
the self-assembly of such a class of hydrogen-bonding-driven
porphyrin-appended folding receptors, which exhibit remarkably
high binding affinity for C60, C70, and C60 derivatives in
chloroform and toluene.17-19
(6) For recent examples, see: (a) Kubo, Y.; Sugasaki, A.; Ikeda, M.; Sugiyasu,
K.; Sonoda, K.; Ikeda, A.; Takeuchi, M.; Shinkai, S. Org. Lett. 2002, 4,
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Wolffs, M.; Hoeben, F. J. M.; Beckers, E. H. A.; Schenning, A. P. H. J.;
Meijier, E. W. J. Am. Chem. Soc. 2005, 127, 13484.
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Results and Discussion
Three porphyrin foldamer receptors Zn21, Zn22, and Zn33
have been designed, which were based on the recent observa-
tions that intramolecular three-centered hydrogen bonding20 can
induce linear anthranilamide oligomers to adopt rigid zig-zag
or straight conformation.14c,21 Both Zn21 and Zn22 were
incorporated with two porphyrin units. It was expected that,
due to the existence of the intramolecular hydrogen bonding,
the porphyrin units in both compounds would be arranged
roughly parallel to each other to produce two noncovalently
bonded molecular tweezers. Compound Zn33 might be regarded
as combination of two molecules of Zn21.
(16) (a) Kanamori, D.; Okamura, T.; Yamamoto, H.; Ueyama, N. Angew. Chem.,
Int. Ed. 2005, 44, 969. (b) Masu, H.; Sakai, M.; Kishikawa, K.; Yamamoto,
M.; Yamaguchi, K.; Kohmoto, S. J. Org. Chem. 2005, 70, 1423.
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foldamers, see: (a) Berl, V.; Huc, I.; Khoury, R. G.; Lehn, J.-M. Chem.-
Eur. J. 2001, 7, 2810. (b) Tanatani, A.; Hughes, T. S.; Moore, J. S. Angew.
Chem., Int. Ed. 2002, 41, 325. (c) Inouye, M.; Waki, M.; Abe, H. J. Am.
Chem. Soc. 2004, 126, 2022. (d) Maurizot, V.; Le´ger, J.-M.; Guionneau,
P. Russ. J. Chem. 2004, 53, 1572. (e) Hou, J.-L.; Jia, M.-X.; Jiang, X.-K.;
Li, Z.-T.; Chen, G.-J. J. Org. Chem. 2004, 69, 6228. (f) Arunkumar, E.;
Ajayaghosh, A.; Daub, J. J. Am. Chem. Soc. 2005, 127, 3156. (g) Chang,
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12214.
(18) Metalated “jaws porphyrin” hosts have been reported for complexation of
C
60 and C70, see: (a) Sun, D.; Tham, F. S.; Reed, C. A.; Chaker, L.; Burgess,
M.; Boyd, P. D. W. J. Am. Chem. Soc. 2000, 122, 10704. (b) Sun, D.;
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The synthesis of Zn21 is presented in Scheme 1. Compound
422 was first alkylated to produce aldehyde 5, which then reacted
9
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A R T I C L E S
Scheme 1
Wu et al.
Scheme 2
Scheme 3
with 623 and pyrrole in refluxing propionic acid to afford
porphyrin 8.24 The latter was then hydrolyzed with sodium
hydroxide and further converted to acyl chloride 10 with thionyl
chloride. Treatment of 10 with 1125 afforded H41, which then
reacted with zinc acetate to give Zn21 in quantitative yield. For
the synthesis of Zn22 (Scheme 2), compound 1326 was first
prepared from nitration of 12 in concentrated sulfuric acid and
then reacted with 6 and 7 in refluxing propionic acid to afford
porphyrin 14. This intermediate was reduced to amine 15 and
then reacted with 1627 to yield compound H42, which was then
treated with zinc acetate in methanol and dichloromethane to
afford Zn22 in high yield.
The synthetic route for Zn33 is shown in Scheme 3. Porphyrin
17 was first prepared from the reaction of compounds 10 and
(19) (a) Schuster, D. I.; Cheng, P.; Jarowski, P. D.; Guldi, D. M.; Luo, C.;
Echegoyen, L.; Pyo, S.; Holzwarth, A. R.; Braslavsky, S. E.; Williams, R.
M.; Klihm, G. J. Am. Chem. Soc. 2004, 126, 7257. (b) D’Souza, F.; Smith,
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S.; Hirsch, A.; Guldi, D. M.; Gisselbrecht, J.-P. J. Am. Chem. Soc. 2004,
126, 10370. (d) Hasobe, T.; Kamat, P. V.; Absalom, M. A.; Kashiwagi,
Y.; Sly, J.; Crossley, M. J.; Hosomizu, K.; Imahori, H.; Fukuzumi, S. J.
Phys. Chem. B 2004, 108, 12865. (e) Nakamura, T.; Fujitsuka, M.; Araki,
Y.; Ito, O.; Ikemoto, J.; Takimiya, K.; Aso, Y.; Otsubo, T. J. Phys. Chem.
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Gadde, M. E. Zandler, A. L. McCarty, A. S. D. Sandanayaka, Y. Araki,
O. Ito, Chem. Eur. J. 2005, 11, 4416.
11 in chloroform, and then compound 18 was obtained by
alkylation of 4 in hot DMF. Treatment of 18 with dipyrrole 19
in hot propionic acid produced porphyrin diester 20. This
intermediate was then hydrolyzed with sodium hydroxide and
followed by treatment with thionyl chloride to afford 22. The
latter intermediate was further reacted with 17 in chloroform
to yield porphyrin trimer H63. Finally, treatment of H63 with
zinc acetate produced Zn33 quantitatively. Compounds Zn21,
(20) Gong, B. Chem.-Eur. J. 2001, 7, 4337.
(21) Chen, Y.-Q.; Wang, X.-Z.; Shao, X.-B.; Hou, J.-L.; Chen, X.-Z.; Jiang,
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Eur. J. Org. Chem. 2002, 3276.
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1
(27) Birkinshaw, R. Biochem. J. 1932, 26, 441.
Zn22, and Zn33 have been characterized by the H, 13C NMR,
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Efficient Complexation of C60, C70, and C60 Derivatives
A R T I C L E S
Figure 2. Partial 13C NMR spectrum of (a) C60 (2.0 mM), (b) Zn21 (2.0
mM) + C60 (1:1), (c) Zn22 (2.0 mM) + C60 (1:1), and (d) Zn33 (1.0 mM)
+ C60 (2.0 mM) in CDCl3 containing 5% CS2 at 25 °C.
Moreover, the UV-vis absorbance (Soret band) of all three
compounds in chloroform observes Beer’s law in the concentra-
tion range of less than 50 µM. These results show that the
porphyrin units in these molecules do not significantly interact
with each other both intramolecularly and intermolecularly.
Addition of 1 equiv of C60 to the solution of the three
compounds in chloroform-d caused substantial change of several
Figure 1. Partial 1H NMR spectrum of (a) Zn21 (4.0 mM) + C70 (1:1),
(b) Zn21 (4.0 mM), (c) Zn21 (4.0 mM) + C60 (1:1), (d) Zn43 (2.0 mM), (e)
Zn43 (2.0 mM) + C60 (1:2), (f) Zn22 (4.0 mM), and (g) Zn22 (4.0 mM) +
C60 (1:1) in CDCl3 at 25 °C.
1
signals in their H NMR spectrum.29 For example, the signal
of the amide proton of Zn21 shifted from 10.34 to 10.44 ppm,
while its H-a signal moved from 9.54 to 9.96 ppm (Figure
1c). These observations can be rationalized by considering that
encapsulation of C60 within the porphyrin tweezer reduces the
conformational flexibility of the porphyrins relative to the
bisanthranilamide moiety30 and consequently strengthens the
intramolecular hydrogen bonding. This strengthened hydrogen
bonding again increases the defielding effect of the CdO group
to H-a.10,31 Similar downfield shifting was also observed for
both Zn43 (∆δ: 0.07 ppm for NH and 0.26 ppm for H-a) and
Zn22 (∆δ: 0.13 ppm for both NH and H-a) (parts e and g of
Figure 1). All the results suggest a strong interaction between
the porphyrin tweezers and C60. Similar downfield shifting was
also observed when C60 was replaced with C70. For example,
when 1 equiv of C70 was added to the solution of Zn21 in
chloroform-d, the NH and H-a signals (∆δ 0.12 and 0.46 ppm,
respectively) of Zn21 moved downfield substantially (parts a
and c of Figure 1). The values are even larger than those
observed for Zn21 induced by C60, implying an even stronger
interaction between the porphyrin receptor and C70 (vide
infra).5a,b
and (HR) mass spectroscopy or microanalysis and are of good
solubility in organic solvents such as chloroform and toluene.
Previous X-ray and spectroscopic investigations have revealed
a rigid planar conformation for the diamide skeletons in Zn21,
Zn22, and Zn33 due to the intramolecular three-centered
hydrogen bonding.11a,14c,21 Molecular modeling (see Supporting
Information) showed that the two porphyrin units in the folded
Zn21 and Zn22 form a rigid tweezer, with a spatial separation
of approximately 12 and 13 Å from the center of the porphyrin
units. Such distances are very suitable to sandwich a C60 or C70
molecule.28 Trimer Zn33 may be considered a combination of
two molecules of Zn21 and the spatial separation between its
neighboring porphyrin units should be comparable to that of
Zn21. In principle, Zn33 may exist in a S- or C-shaped
conformation, depending on the orientation of the two peripheral
porphyrin units relative to the central porphyrin unit. The
S-shaped conformation should be of lower energy considering
that it avoids any possible steric hindrance, which is expected
from the two peripheral porphyrin units in the C-shaped
conformation.
The 1H NMR spectra of Zn21, Zn43, and Zn22 in chloroform-d
are shown in Figure 1. All three spectra are of high resolution,
and the signals in the downfield area have been assigned based
on the 2D-NOESY experiments. Although in principle Zn43 has
two sets of different amide protons, only one single signal is
Strong encapsulation of C60 by the porphyrin receptors was
also supported by the 13C NMR spectroscopy (Figure 2). The
spectrum of the 1:1 solution of C60 and Zn21, Zn22, and Zn33
in chloroform and carbon disulfide revealed significant upfield
shifting (∆δ: -3.01, -2.94, and -3.10 ppm, respectively) of
the C60 signal relative to that of the free C60 (143.03 ppm).32
This upfield shifting is obviously resulted from the shielding
or ring current effect of the zinc porphyrin units of the receptors
1
exhibited for the NH protons in its H NMR spectrum. It can
be found that the signal of the amide protons of all the three
compounds appears in the downfield area (10.34, 10.33, and
10.23 ppm, respectively). This observation supports the forma-
tion of the intramolecular three-centered O‚‚‚H-N hydrogen
bonding in these compounds. Dilution of their solutions in
chloroform-d did not produce important shifting of signals
(29) Both C60 and C70 are scarcely soluble in chloroform. Addition of the zinc
porphyrin receptors substantially increases their solubility and the homo-
geneous phase, obtained after sonication, is stable at room temperature.
(30) The X-ray analysis reveals a torsion angle of 16° between the peripheral
benzene and the central benzene in the solid state in the bisanthranilamide
moiety, see ref 14c.
(31) The defielding of the encapsulated C60 might also be partially responsible
for the large downfield shifting of the H-a signal.
(32) All the spectra were recorded in chloroform-d containing 10% (v/v) of
carbon disulfide, in which the 13C NMR spectrum of C60 could be recorded.
1
(<0.008 ppm from 10 to 0.2 mM) in the H NMR spectrum.
(28) (a) Boyd, P. D. W.; Reed, C. A. Acc. Chem. Res. 2005, 38, 235. (b) Satake,
A.; Kobuke, Y. Tetrahedron 2005, 61, 13.
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A R T I C L E S
Wu et al.
Similar spectroscopic changes were also observed when incre-
mental amount of C70 was added to the solution of Zn21 and
Zn22 in toluene and the Ka of complexes Zn21‚C70 and Zn22‚
C70 was determined to be 1.1 ((0.1) × 106 and 9.8 ((1.2) ×
105 M-1, respectively, by the same method. The association
constant of Zn21‚C70 and Zn22‚C70 is pronouncedly greater than
that of the corresponding C60 complexes. This is in accordance
with the above 1H NMR observation and implies that the
complexation of C70 by the porphyrin receptors occurs mainly
at its equatorial region rather than its poles.5b Such a binding
mode can lead to greater π-π contact area from the less curved
region of C70 to the porphyrin units of the receptors.28a
The Job’s plot study described above has established that
receptor Zn33 can complex two fullerene molecules. Also from
the UV-vis titration experiments, we determined the apparent
association constant Ka of the single porphyrin tweezer of Zn33
with C60 and C70 in toluene to be 1.5 ((0.1) × 104 and 6.7
((1.0) × 104 M-1, respectively.36 The association constant of
the present complexes are pronouncedly higher than that of the
complexes of C60 with the palladium-linked bisporphyrin “jaws”,
reported by Boyd and Reed et al.,18 demonstrating that the
intramolecular hydrogen bonding in the present receptors plays
an important role in increasing the binding affinity of the new
preorganized foldamer receptors toward fullerene guest.
In principle, incorporation of two C60 units into one guest
molecule would double the binding site and should, in the
absence of large steric hindrance, increase its binding affinity
to receptor Zn33. On the basis of this consideration, C60 dimer
24 was designed and prepared from amine 23, which could be
produced conveniently from the reaction of C60, glycine and
dodecyl aldehyde in refluxing chlorobenzene.37 For the sake of
comparison, compound 25 was also prepared from the acylation
of 23. Introduction of the long aliphatic chains provides both
24 and 25 with good solubility in chloroform and toluene.
Mixing the same equivalents of Zn33 with 24 in chloroform-d
also led to significant change of the signals of both molecules
Figure 3. TLC (developed in iodine vapor) of (a) C60 (Rf ) ca. 1.0, the
spot is too pale to scan), (b) Zn21 + C60 (1:1), (c) Zn21 + C60 (2:1), and
(d) Zn21. TLC condition: silica gel plate/CS2-CHCl3 (1:2).
upon strong encapsulation of the guest.5a-c Straight-phase thin-
layer chromatography (TLC) analysis, which exhibited a new
spot for the complexes, also indicated the formation of stable
complexes between the receptors and C60 or C70. As an example,
Figure 3 provides the TLC result of the complexation of C60
by Zn21.
Upon addition of C60 to the solution of the receptors in
toluene, the Soret band of the receptors in the UV-vis spectrum
was shifted notably (from 423.6 nm to 427.2, 427.0, and 427.1
nm, respectively, for Zn21, Zn21, and Zn33). Replacement of
C60 with C70 of the identical concentration caused even larger
1
red shift. The results are consistent with the above H and 13C
NMR observations, also suggesting strong electronic interaction
between the porphyrin receptors and the fullerene guest.5a As
examples, parts a and b of Figure 4 present the UV-vis spectra
of Zn21 in toluene in the presence of incremental amount of
C60 and C70. Job’s plot analysis based on the UV-vis experi-
ments, as shown in Figure 5, supported a 1:1 stoichiometry for
complexes Zn21:C60 and Zn22:C60 and a 1:2 stoichiometry for
complex Zn33:C60, which exhibited a largest change of absorb-
ance at the 1:1 and 1:2 ratio of the receptor and C60 when the
total concentration of the two samples was kept unchanged.33,34
1
in the H NMR spectrum. Especially, the NH signal of the
receptor moved downfield remarkably, as observed above for
the complex of Zn21 with C60 or C70. Similar shifting was also
observed in the 1H NMR spectrum of the solution of Zn21 with
25. These results clearly showed that important interaction also
occurs between Zn21 and the new C60 derivatives. Quantitative
binding behavior between Zn33 and 24 and 25 was investigated
in chloroform and toluene with the UV-vis titration method.
The UV-vis spectra of Zn33 in chloroform, obtained with
addition of incremental amount of 24 and 25, are provided in
parts c and d of Figure 4. The data obtained upon addition of
24 to the solution of Zn33 in chloroform were fit to a 1:1 binding
mode, giving Ka ) 1.8 ((0.2) × 107 M-1 for complex Zn33‚
24. The Ka of Zn33‚24 in toluene is obviously beyond the limit
of the UV-vis titration method,28b because the absorbance
change was linearly dependent on the concentration of 24 when
0-1 equiv of 24 was added and became unchanged when more
amount of 24 was added. By virtue of the analysis method
described above for the complexes of Zn33 and C60 or C70, the
1
The H NMR spectra for [Zn21]:[C60] ) 1:0-1:3 in chloro-
form-d at 25 °C revealed a saturation of complexation-induced
chemical shift change at [Zn21]:[C60] ) 1:0 (see Figure 1S in
Supporting Information), also supporting the 1:1 stoichiometry
of their complex. Figure 6 shows the proposed binding mode
for the 1:1 and 1:2 complexes. Because C70 exhibits a stronger
binding affinity than C60,28a it is reasonable to consider that its
complexes with the porphyrin receptors should have similar
binding mode.
The association constants (Ka) of complexes Zn21‚C60 and
Zn22‚C60 in toluene were then evaluated by the UV-vis titration
method. The absorption spectral change of the receptors induced
by addition of C60 displayed a clear isosbestic point both at
429 nm (Figure 3a). On the basis of the change values of
absorbance with [C60], we estimated the Ka of the two complexes
to be 1.0 ((0.1) × 105 and 2.7 ((0.2) × 104 M-1, respectively.35
(33) Job, P. Ann. Chim. Ser. 10 1928, 9, 113.
1
(34) The H NMR titration experiments in chloroform-d also revealed that the
(36) The apparent association constant may be regarded as the averaged value
of the Ka of the discrete zinc porphyrin tweezers when the receptor contains
more than one zinc porphyrin tweezers. For a recent example of the method,
see: Li, W.-S.; Jiang, D.-L.; Suna, Y.; Aida, T. J. Am. Chem. Soc. 2005,
127, 7700.
(37) Herranz, M. A.; Illescas, B.; Martin, N.; Luo, C.; Guldi, D. M. J. Org.
Chem. 2000, 65, 5728.
NH and H-a signals of Zn21 shifted downfield with the addition of C60
and achieved the maximum values when 1 equiv of C60 was added. Further
addition of C60 did not cause shifting of the signals, and the C60 added was
not dissolved neither.
(35) Conners, K. A. Binding Constants: The Measurement of Molecular
Complex Stability Wiley: New York, 1987.
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17464 J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005

Efficient Complexation of C60, C70, and C60 Derivatives
A R T I C L E S
Figure 4. Absorption spectral changes of Zn21 (1.5 µM) upon addition of (a) C60 and (b) C70 in toluene at 25 °C. Absorption spectral changes of Zn33 (0.67
µM) upon on addition of (c) 24 and (d) 25. All the spectra were recorded in chloroform at 25 °C (the absorbance of the fullerene unit had been subtracted
from the spectra; inset, the plot of the absorbance change vs [C60] or [C70]).
Figure 5. Job’s plots: absorbance change of the Soret band at 428 nm of (a) Zn21 vs [Zn21]/([Zn21] + [C60]) and (b) Zn33 vs [Zn33]/([Zn33] + [C60]) in
toluene at 25 °C (the overall concentration ) 5.0 × 10-6 M).
apparent association constant Ka for the complex of the single
tweezer of Zn33 with 25 in chloroform and toluene was
obtained in chloroform, indicating that the addition of polar
methanol would weaken the intramolecular hydrogen bonding
of the receptor and consequently reduced its preorganization.
The complexing behavior of 24 by Zn21 and Zn22 was also
investigated. Job’s plot studies based on the UV-vis experi-
ments revealed a 1:1 stoichiometry for their complexes in both
chloroform and toluene (see Figure 2S in Supporting Informa-
tion). The corresponding association constant in chloroform and
toluene was evaluated to be 4.7 ((0.1) × 104 and 7.6 ((0.6)
× 105 for Zn21‚24 and 4.1 ((0.4) × 104 and 1.8 ((0.2) × 105
for Zn22‚24, respectively. It can be found that the stability of
Zn21‚24 and Zn22‚24 in toluene is significantly higher than that
of the corresponding complexes Zn21‚C60 and Zn22‚C60 but
substantially lower than that of Zn33‚24 in chloroform of high
polarity. This result reflects that the existence of the second
C60 unit in 24 may promote the binding stability of Zn21‚24
determined to be 1.3 ((0.1) × 104 and 8.6 ((0.8) × 104 M-1
,
respectively. The stability of Zn33‚24 is substantially higher than
that of the complex of Zn33 and 25 due to the doubling of its
binding site (Figure 7). On the basis of the UV-vis titration,
the Ka of complexes Zn21‚25 and Zn22‚25 in chloroform and
toluene was also evaluated to be 1.6 ((0.2) × 104 and 6.0 ((1.0)
× 104 M-1 and 7.9 ((1.2) × 103 and 1.2 ((0.1) × 104 M-1
,
respectively. Compared to that of the complexes of C60, the
stability of the complexes of 25 is notably decreased as a result
of the additional aliphatic moiety in 25. To explore the influence
of the solvent polarity on the binding affinity, the Ka of complex
Zn21‚25 in mixture of chloroform and methanol (9:1 v/v) was
also evaluated, which gave rise to a value of 2.3 ((0.3) × 103
M-1. The value is pronouncedly smaller than that of the complex
9
J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005 17465

A R T I C L E S
Wu et al.
Figure 6. The binding mode for complexes (a) Zn21‚C60, (b) Zn22‚C60,
and (c) Zn33‚2C60.
Figure 8. Dynamic exchanging process for complex Zn21‚24.
chloroform-d and did not split even under reduced temperature.
This observation indicates that the exchanging process for
complexes Zn21‚24, as shown in Figure 8, and Zn22‚24 is quick
1
on the H NMR time scale.
Fluorescent studies revealed that the emission of the zinc
porphyrin units of the receptors could be efficiently quenched
by the fullerene guests. All the receptors exhibited typical
emission bands of the zinc porphyrin units at ca. 606 and 643
nm upon excitation at the isosbestic point of their Soret band.38
The quenching results of the first emission with fullerene
receptors in toluene are provided in Table 1. It can be found
that the quenching efficiency of Zn33 by 24 is substantially
higher than any other receptor-guest system. This result is
consistent with the above UV-vis investigation, reflecting the
remarkably increased complexing affinity between Zn33 and 24
due to the doubling of their binding site. Figure 9 presents the
fluorescent spectra of Zn33 in toluene in the presence of
increasing amount of 24. The emission of the porphyrin units
of Zn33 could not be quenched completely even in the presence
of excessive 24,19l which may be rationalized by considering
that the complexation is a dynamic process and there is always
a small amount of free Zn33 in the solution. On the basis of the
change of the emission strength at 606 nm with [24], the Ka of
Figure 7. The proposed binding mode for complex Zn33‚24.
and Zn22‚24 by additional aromatic interaction with the por-
phyrin unit of the receptors, as shown in Figure 8 (with Zn21
as example). Such additional interaction is obviously of low
efficiency compared to that between C60 and the porphyrin units
of a tweezer. The fact that three-component complexes (Zn21)2‚
24 and (Zn22)2‚24 were not formed (in measurable amount)
implies a large spatial repulsion might exist between the two
large bisporphyrin molecules in such possible tricomponent
complexes. Only one set of signals was displayed in the
downfield area of the 1H NMR spectrum of both complexes in
complex Zn33‚24 was evaluated to be 3.4 ((0.4) × 108 M-1 39
.
For the sake of comparison, the Ka of complex Zn21‚25 in
(38) Solladie´, N.; Walther, M. E.; Gross, M.; Duarte, T. M. F.; Bourgogne, C.;
Nierengarten, J.-F. Chem. Commun. 2003, 2412.
(39) (a) Hauke, F.; Swartz, A.; Guldi, D. M.; Hirsch, A. J. Mater. Chem. 2002,
12, 2088. (b) D′Souza, F.; Chitta, R.; Gadde, S.; Zandler, M. E.; McCarty,
A. L.; Sandanayaka, A. S. D.; Araki, Y.; Ito, O. Chem.-Eur. J. 2005, 11,
4416. (c) Sessler, J. L.; Jayawickramarajah, J.; Gouloumis, A.; Torres, T.;
Guldi, D. M.; Maldonado, S.; Stevenson, K. J. Chem. Commun. 2005, 1892.
9
17466 J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005

Efficient Complexation of C60, C70, and C60 Derivatives
A R T I C L E S
Table 1. Fluorescent Quenching Data of Receptors Zn21, Zn22, and Zn33 by the Fullerene Guests in Toluene at 25 °C
Ib ([I0
−
I]/I0)c
(M)d
a
I0
C60
C70
24
25
[C60
]
[C70]
[24]
[25]
Zn21
Zn22
Zn33
262
248
221
252 (0.04)
235 (0.03)
212 (0.04)
165 (0.24)
199 (0.24)
165 (0.25)
179 (0.32)
225 (0.12)
10 (0.95)
230 (0.12)
238 (0.04)
195 (0.04)
3.7 × 10-5
3.1 × 10-5
3.8 × 10-5
3.1 × 10-6
2.8 × 10-6
2.8 × 10-6
1.5 × 10-6
9.1 × 10-6
2.8 × 10-7
8.0 × 10-6
3.4 × 10-5
8.6 × 10-6
a Values of pure receptors at [porphyrin] ) 2.0 × 10-6 M. b Values when 1 equiv of fullerene guest ([fullerene] ) 2.0 × 10-6 M) was added. c Quenching
efficiency. d Guest concentration at [I0 - I]/I0 ) 0.5.
Scheme 4
Figure 9. Fluorescence spectral changes of Zn33 (7.0 × 10-7 M) upon
addition of 24 (0-5.0 × 10-6 M) in toluene at 25 °C. Excitation wavelength
) 429 nm at the isosbestic point of the UV-vis spectra. Inset: the plot of
change emission intensity at 606 nm vs [24].
chloroform and the apparent Ka of complexes of Zn33 with C60
and C70 in toluene were also evaluated by the fluorescent
titration method, which afforded a value of 1.8 ((0.1) × 104,
1.4 ((0.2) × 10-4, and 7.4 ((1.1) × 10-4 M-1, respectively.
These results are in good accordance with those estimated from
the UV-vis titration method.
Given the strong complexing feature between the foldamer-
based porphyrin tweezers and the fullerene guests, the possibility
of supramolecular chiral induction through complexation of the
foldamer receptors toward chiral C60 derivative 29 was ex-
plored.40,41 The synthesis of 29 is shown in Scheme 5. Briefly,
treatment of 26 with 27 in chloroform in the presence of DCC
afforded 28, which reacted with C60 in hot toluene to give 29.
The Ka of complexes Zn21‚29 and Zn22‚29 in chloroform was
Scheme 5
evaluated to be 3.2 ((0.3) × 103 and 1.5 ((0.2) × 103 M-1
,
while the apparent Ka of the complex of the single tweezer of
Zn33 with 29 was determined to be 2.6 ((0.3) × 103 M-1. The
values are notably lower than that of the corresponding
complexes of 25, which can be attributed to the increased size
of the aliphatic moiety of 29 relative to that of 25. Adding R-29
or S-29 to the solution of the porphyrin receptors in chloroform
led to the generation of induced circular dichroism (CD) of
mirror image, as shown in Figure 10 (the CD spectrum of the
C60 guest had been subtracted). The spectral modes of the
complexes of Zn21 and Zn22 are comparable, reflecting a similar
pattern of chiral induction. In contrast, Zn33 displays a strong
Cotton effect in region of the porphyrin Soret band. The results
may be ascribed to the different orientation of the two chiral
guest molecules, relative to each other, in the three-component
complexes, which lead to a different orientation of the three
poprhyrin chromophores in Zn33. Although the relative strength
is significantly varied, the Cotton effects between 500 and 650
nm of all the porphyrin receptors do not shift significantly.
Conclusion
In summary, we have described the self-assembly of a new
class of molecular tweezers whose rigid skeletons are con-
structed based on the intramolecular hydrogen-bonding-driven
aromatic amide foldamers. The new zinc porphyrin-based, well-
ordered tweezers represent new efficient nonring receptors for
complexation of fullerene and fullerene-derived molecules.
Because of their preorganized rigid conformation, the new
folding receptors exhibit a fullerene-binding ability that can be
(40) A chiral porphyrin-C60 triad had been reported, see: Kessinger, R.; Thilgen,
C.; Mordasini, T.; Diederich, F. HelV. Chim. Acta 2000, 83, 3069.
(41) (a) Huang, S.; Nakanishi, S.; Berova, N. Chirality 2000, 12, 237. (b)
Pescitelli, G.; Gabriel, S.; Wang, Y.; Fleischhauer, J.; Woody, R. W.;
Berova, N. J. Am. Chem. Soc. 2003, 125, 7613.
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J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005 17467

A R T I C L E S
Wu et al.
Figure 10. The induced CD spectra in chloroform at 25 °C. (1) Zn21 (3.3 × 10-4 M) in the presence of S-29 (3.3 × 10-3 M) (a), R-29 (3.3 × 10-3 M) (b),
and R-29 (6.6 × 10-3 M) (c); (2) Zn22 (3.3 × 10-4 M) in the presence of S-29 (3.3 × 10-3 M) (a) and R-29 (3.3 × 10-3 M) (d) and Zn33 (2.3 × 10-4 M)
in the presence of S-29 (3.3 × 10-3 M) (b) and R-29 (3.3 × 10-3 M) (c).
Table 2. Summary of the Association Constants of the New
Porphyrin-Fullerene Complexes Determined by the UV-Vis
bonding-induced foldamers or related well-ordered rigid archi-
Titration Method
2. The results well demonstrate the great potential of hydrogen
tectures as building blocks or scaffolds for developing new
synthetic receptors, which opens new possibility in molecular
recognition and self-assembly.
1
1
complex
Ka (M-
)
solvent
complex
Ka (M-
)
solvent
Zn21‚C60
Zn22‚C60
Zn33‚C60
Zn33‚C60
Zn21‚24
Zn22‚24
Zn33‚24
Zn21‚25
Zn21‚25
Zn22‚25
Zn33‚25b
Zn21‚29
Zn33‚29b
1.0 × 105 toluene
Zn21‚C70
Zn22‚C70
Zn33‚C70
Zn33‚C70
Zn21‚24
Zn22‚24
1.1 × 106 toluene
2.7 × 104 toluene
9.8 × 105 toluene
b
b
1.5 104
toluene
6.7 × 104 toluene
Acknowledgment. This work is supported by the Ministry
of Science and Technology (No. 200007801), the National
Natural Science Foundation of China (No. 20321202, 90206005,
20372080, 20332040, and 20425208), and the Chinese Academy
of Sciences for financial support. We also thank Prof. Li-Zhu
Wu for beneficial discussion.
1.4 × 104 toluene
7.6 × 105 toluene
1.8 × 105 toluene
7.4 × 104 toluene
b,c
b,c
4.7 × 104 chloroform
4.1 × 104 chloroform
3.4 × 108 toluene
1.8 × 107 chloroform Zn33‚24c
6.0 × 104 toluene
Zn21‚25
1.6 × 104 chloroform
1.8 × 104 chloroform
7.9 × 103 chloroform
1.3 × 104 chloroform
1.5 × 103 chloroform
2.3 × 103 chloroformd Zn21‚25c
1.2 × 104 toluene
Zn22‚25
8.6 × 104 toluene
Zn33‚25b
3.2 × 103 chloroform Zn22‚29
Supporting Information Available: Detailed experimental
procedures and characterization of the intermediates and target
molecules, examples of the 1H NMR, UV-vis and fluorescent
titration spectra, methods for evaluating the association constants
and binding stoichiometry, energy-minimized conformation of
the receptors, and complete ref 11a. This material is available
free of charge via the Internet at http://pubs.acs.org.
2.6 × 103 chloroform
a The association constants are typically averages of two experiments at
25 °C. b Apparent association constant, representing the average binding
ability of the single tweezer of the receptor to the fullerene guest.
c Determined by the fluorescent titration method. d With 10% (v) of
methanol.
comparable to the cyclic bisporphyrin receptors.5a The associa-
tion constants of the new complexes are summarized in Table
JA056509H
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17468 J. AM. CHEM. SOC. VOL. 127, NO. 49, 2005