Calix[4]arene-linked bisporphyrin hosts for fullerenes: Binding strength, solvation effects, and porphyrin fullerene charge transfer bands

Article abstract of DOI:10.1021/ja066031x

A calix[4]arene scaffolding has been used to construct bisporphyrin ("jaws" porphyrin) hosts for supramolecular binding of fullerene guests. Fullerene affinities were optimized by varying the nature of the covalent linkage of the porphyrins to the calixar

Full text of DOI:10.1021/ja066031x

Published on Web 11/15/2006
Calix[4]arene-Linked Bisporphyrin Hosts for Fullerenes:
Binding Strength, Solvation Effects, and Porphyrin-Fullerene
Charge Transfer Bands
Ali Hosseini,† Steven Taylor,† Gianluca Accorsi,‡ Nicola Armaroli,*,‡
Christopher A. Reed,*,§ and Peter D. W. Boyd*,†
Contribution from the Department of Chemistry, The UniVersity of Auckland,
PriVate Bag 92019, Auckland, New Zealand, Molecular Photoscience Group, Istituto per la
Sintesi Organica e la FotoreattiVita` (ISOF), Consiglio Nazionale delle Ricerche (CNR),
Via Gobetti 101, 40129 Bologna, Italy, and Department of Chemistry, UniVersity of California,
RiVerside, California 92521
Received August 18, 2006; E-mail: chris.reed@ucr.edu; pdw.boyd@auckland.ac.nz; armaroli@isof.cnr.it
Abstract: A calix[4]arene scaffolding has been used to construct bisporphyrin (“jaws” porphyrin) hosts for
supramolecular binding of fullerene guests. Fullerene affinities were optimized by varying the nature of the
covalent linkage of the porphyrins to the calixarenes. Binding constants for C60 and C70 in toluene were
explored as a function of substituents at the periphery of the porphyrin, and 3,5-di-tert-butylphenyl groups
gave rise to the highest fullerene affinities (26 000 M-1 for C60). The origin of this high fullerene affinity has
been traced to differential solvation effects rather than to electronic effects. Studies of binding constants
as a function of solvent (toluene < benzonitrile < dichloromethane , cyclohexane) correlate inversely
with fullerene solubility, indicating that desolvation of the fullerene is a major factor determining the magnitude
of binding constants. The energetics of fullerene binding have been determined in terms of ∆H and ∆S
and are consistent with an enthalpy-driven, solvation-dependent process. A direct relationship between
supramolecular binding of a fullerene guest to a bisporphyrin host and the appearance of a broad NIR
absorption band have been established. The energy of this band moves in a predictable manner as a
function of the electronic structure of the porphyrin, thereby establishing its origin in porphyrin-to-fullerene
charge transfer.
Introduction
tion reaction into the Marcus inverted region, as in the
photosynthetic reaction center (PRC). The PRC achieves long-
Fullerenes and porphyrins possess unique redox and photo-
electronic properties that make them highly suitable chro-
mophores for the assembly of photoactive fullerene-porphyrin
constructs. With irradiation, these assemblies can undergo
photoinduced electron transfer (PET) between the donor por-
phyrin and the acceptor fullerene. Early studies of covalently
linked fullerene-porphyrin dyads showed PET from the first
excited singlet state of the porphyrin to the fullerene, with a
high quantum yield.1,2 Extensive work since then has shown
that the small three-dimensional reorganization energies of
fullerenes, following rapid electron transfer, slow the back
electron transfer rate.3-8 This helps move the charge recombina-
lived charge separation using a sequence of short-range electron
transfer reactions along a redox gradient, leading to a spatially
and electronically separated radical ion pair consisting of a
bacteriochlorophyll dimer radical cation (the “special pair”) and
a quinone radical anion. Recently, using elaborate combinations
of covalently connected porphyrin donors and fullerene accep-
tors, along with auxiliary ferrocene acceptors, the key features
of this process have been duplicated with lifetimes comparable
to the PRC.9-11
Covalent methods of linking fullerenes and porphyrins have
been augmented by coordinate,12-15 supramolecular,16-24 and
(8) Kobori, Y.; Yamauchi, S.; Akiyama, K.; Tero-Kubota, S.; Imahori, H.;
Fukuzumi, S.; Norris, J. R., Jr. Proc. Natl. Acad. Sci. U.S.A. 2005, 102,
10017-10022.
† Auckland University.
‡ CNR-ISOF.
§ University of California, Riverside.
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Y.; Fukuzumi, S. J. Phys. Chem. A 2004, 108, 541-548.
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1994, 60, 537-541.
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J. AM. CHEM. SOC. 2006, 128, 15903-15913
15903

A R T I C L E S
Hosseini et al.
H-bonding methodologies.25-27 We have chosen the supramo-
lecular assembly route because of its versatility and simplicity.
Fullerenes and porphyrins are naturally attracted to each other,
and this new supramolecular recognition element allows the easy
construction of host-guest complexes and novel 3D as-
semblies.28
The attraction of a fullerene to a porphyrin was first
recognized in the molecular packing of a crystal structure of an
early fullerene-porphyrin dyad.29 Since then, the close approach
(ca. 2.7 Å) of the fullerene to the center of the porphyrin has
been found in dozens of the porphyrin-fullerene cocrystal-
lates,28 and the interaction persists in solution.
In order to obtain high binding constants in discrete host-
guest complexes, two appropriately oriented porphyrins are
necessary to complex the fullerene. Cyclic bisporphyrins, first
designed by Aida, Tashiro, and co-workers, showed large
binding constants and selectivities.16,17 Acyclic porphyrin hosts
were first prepared using Pd-linked 4-pyridylporphyrins,18,19 and
a variety of other scaffolds and linkers have been re-
ported.20-22,30,31 In the present work, we use a calixarene scaffold
to hold two porphyrins at the required ca. 12 Å separation.
Calixarenes have a very well developed derivatization chemistry
on both the “upper” and “lower” rims,32 allowing easy synthesis
of bisporphyrin hosts. Two such systems have been reported
and their fullerene binding capacities explored.31,33
Figure 1. Schematic representation of a lower rim calixarene bisporphyrin
with varied X linkers.
been reported for a variety of bisporphyrin hosts, but there is
limited understanding of interplay of solvation energies, struc-
tural constraints, and electronic influences.17,19 Another property
in need of deeper understanding is the porphyrin-to-fullerene
charge transfer absorption. Broad, low-energy absorptions
(above 700 nm) have been reported in solid-state fullerene-
porphyrin assemblies,34-39 covalent dyads which allow the close
approach of the chromophores,40-48 and a supramolecular
complex.49 We will show in this paper that there is a direct
relationship between supramolecular binding of the fullerene
and the appearance of a NIR band. Moreover, the energy of
this band can be systematically varied with porphyrin-type in
an understandable manner. This band is important as a direct
indicator of the energetics of the charge-separated state and, in
combination with emission spectroscopy, leads to information
on the lifetime of charge separation.
Our approach has been to concentrate on lower-rim-appended
bisporphyrins which allow systematic variation in the nature
of the linkage (-X- in Figure 1), the type of porphyrin, its
solubility properties, and, most importantly, lead to an under-
standing of the factors affecting fullerene binding constants.
Large fullerene binding constants are necessary for meaning-
ful photophysical studies because such measurements must be
done at low concentrations (i.e., low optical densities), where
undesired dissociation becomes favored. Binding constants have
Results and Discussion
Synthesis and Structure. The six bisporphyrin calixarenes
used in the present work are illustrated in Figure 2. All exploit
trans-functionalization on the lower rim. Compound 1 has a
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Calix[4]arene-Linked Bisporphyrin Hosts for Fullerenes
A R T I C L E S
Figure 2. Bisporphyrin hosts 1-6.
direct linkage of the porphyrin to a phenolic residue via
esterification. p-Tolyl substituents on the porphyrin were chosen
for solubility reasons and for convenient 1H NMR identification
purposes. Compound 2 extends the linkage by one methylene
group and switches the ester to an amide. Compound 3 is the
three-methylene analogue of 2. Despite extensive effort, we have
been unable to develop a method to produce the compound with
two methylene groups. Compounds 4 and 5 are analogues of
the one-methylene compound 2 having pentafluorophenyl and
3,5-tert-butylphenyl substituents on the porphyrin meso posi-
tions. Similarly, compound 6 replaces the tetraarylporphyrin in
2 with an octaalkyl diaryl porphyrin.
The esterification linkage reaction to produce compound 1
was only achieved in low yield despite several variations in
the methodology. The amide linkages in compounds 2-6 were
all carried out in good yield using dicyclohexylcarbodiimide
(DCC) coupling. The monosubstituted tetrarylporphyrins were
all prepared by standard methods, and the conjugates 1-6 were
routinely prepared in 50-100 mg quantities with 50-60%
yields. Purification was readily achieved via silica gel column
chromatography.
Compounds 1-6 were chosen as synthetic targets based on
molecular modeling, in a manner similar to our earlier work on
Pd-linked bisporphyrins.19 These calculations indicated that in
the trans-linked, lower-rim, bisporphyrins the calixarenes would
adopt the commonly observed “pinched cone” conformation.
This is borne out experimentally in the 1H NMR spectra of these
compounds where the methylene groups in the calixarenes show
AB patterns with JHH ) 13.2-13.4 Hz, typical of this
conformation.50
Figure 3. Variable-temperature 13C NMR spectrum of 13C-enriched C60
in the presence of 0.3 equiv of 2 in toluene-d8.
The 1:1 complexation of C60 with a bisporphyrin host was
easily established for compound 1 because the 1:1 adduct
precipitates immediately upon mixing toluene solutions of host
and guest. Moreover, the complex remains intact as the major
ion in MALDI mass spectrometry, [1 + C60 + H]+ ) 2734.
The precipitate can be dissolved in chloroform, and from the
relative intensities of the porphyrin Soret band and the C60 UV
bands, the 1:1 stoichiometry was confirmed. For 2, 4, and 5,
1:1 complexation was established via 13C NMR spectroscopy
by previously reported methods.19 As shown in Figure 3, a 3:1
mixture of 13C-enriched C60 and 2 in toluene-d8 shows a single
resonance for C60 at 300 K due to time averaging of bound and
unbound C60. Upon cooling, this peak broadens and at 183 K
splits into separate signals for bound (139.7 ppm) and unbound
(142.6 ppm) C60 in a ratio of 2:1. The same behavior was
(50) Ostaszewski, R.; Stevens, T. W.; Verboom, W.; Reinhoudt, D. N.;
Kaspersen, F. M. Recl. TraV. Chim. Pays-Bas 1991, 110, 294-298.
9
J. AM. CHEM. SOC. VOL. 128, NO. 49, 2006 15905

A R T I C L E S
Hosseini et al.
Table 1. Binding Constants for Fullerenes of 1-6 and Metalated
Derivatives of 5 in Toluene
1
1
K, C60 (M-
)
K, C70 (M-
)
1
2
3
4
5
H2
H2
H2
H2
2000 ( 90a
8700 ( 300a
2900 ( 300a
7000 ( 1600b
26000 ( 4000b
19000 ( 800b
11000 ( 1700b
14000 ( 3000b
2570 ( 24a
38600 ( 500b
13000 ( 6000b
234000 ( 20000b
207000 ( 25000b
120000 ( 9000b
53000 ( 27000b
H2
Zn(II)
Cu(II)
Ni(II)
H2
6
a Fluorescence (295 K). b UV-visible (298 K).
Figure 4. Variable-temperature 1H NMR spectrum in the t-butyl region
of 5 in the presence of 3 equiv of 13C-enriched C60 in toluene-d8.
dominant ions arising from toluene-acetonitrile solutions were
2+ ions of the 1:1 complexes.
Fullerene Binding Constants in Toluene. The binding
constants of 1, 2, and 3 for C60 and C70 were measured by
fluorescence quenching titration in toluene and are listed in
Table 1. The reliability of these measurements was confirmed
for 2 by following fullerene binding in the 1H NMR spectrum.
The internal N-H protons of the porphyrin at -2.17 ppm are
shifted upfield by the ring current of the fullerene and, under
fast exchange conditions at room temperature, provide an
alternate measurement of the binding constant. The values
agreed within 300 M-1. Compound 2 has a significantly higher
binding constant for C60 (8700 M-1) than commonly observed
in acyclic systems, and it is also larger than the value observed
in the phenyl analogue (4920 M-1).31 It is intriguing that the
subtle change of phenyl for p-tolyl in three of the peripheral
porphyrin substituents in 2 leads to a near doubling of the
binding constant. This led us to investigate the effect of
substituent variation on the porphyrin in compounds 4-6.
Compounds 4 and 5 show the effect of pentafluorophenyl
and 3,5-di-tert-butylphenyl substituents in place of tolyl in 2.
In addition, an octaalkylporphyrin derivative, 6, was prepared.
The binding constants for C60 and C70 were measured by UV-
vis titration and are listed in Table 1. The 3,5-di-tert-butylphenyl
substituents in 5 give rise to the highest binding constant for
both C60 and C70. From crystal structures of pentafluorophenyl
porphyrin/fullerene cocrystallates,51,52 weak attractive ortho-C-
F‚‚‚C60 interactions have been identified that might be expected
to increase the binding constant of 4 versus that of 2. However,
this is not seen; they have slightly lower binding constants for
C60 and for C70. From crystallography, 3,5-di-tert-butylphenyl
groups also appear to have a weak attractive interaction to
fullerene,19 and 5 does show considerable enhancement of
fullerene binding relative to 2. This lack of predictability
suggests that differential solvation energies play a major role
in dictating affinities, and this idea is supported by the solvent
variation studies discussed below.
Figure 5. Molecular modeling structure of 5‚C60 showing the close
approach of the 10,20-tert-butylphenyl substituents to C60 and the more
distant approach of the 15-tert-butylphenyl substituent.
observed for 4 and 5. The upfield shifts of the fullerene in the
bound states are the result of ring currents effects of the
porphyrin π system on the fullerene. A reciprocal ring cur-
rent effect of C60 on the porphyrin is seen on the internal
1
porphyrin N-H groups.19 The free-base H chemical shift of
the free base at -2.0 ppm moves to -2.3 ppm upon complex-
ation of C60.
The proximity of the ortho-C-F bonds of the porphyrin to
the bound fullerene in 4‚C60 is reflected in the downfield shift
of their 19F resonances relative to free base 4, whereas the meta
and para signals show insignificant shifts. Similarly, there is a
1
downfield shift of the t-butyl methyl groups in the H NMR
spectrum of 5‚C60 relative to free base 5. At low temperatures,
where free rotation of the 3,5-tert-butylphenyl groups at the 10
1
and 20 positions is slow on the NMR time scale, separate H
signals are seen for the t-butyl groups on each face of the
porphyrin (Figure 4). The downfield signal at 1.56 ppm is
assigned to the t-butyl groups in close proximity to C60; the
upfield signal at 1.45 ppm is assigned to t-butyl groups exterior
to the host cavity, and a signal at 1.42 ppm is assigned to the
t-butyl groups on the freely rotating phenyl group at the 15
position. As seen in Figure 5, the “jaws” shape of the
bisporphyrin host places the 10,20-phenyl groups closer to the
fullerene guest than the 15-phenyl group, leading to face
discrimination of the t-butyl groups at the 10,20 positions but
not the 15.
Compound 6, with octaalkyl substituents and only two phenyl
substituents, shows the lowest binding constant for C60. This
may be due to competition from porphyrin-porphyrin face-to-
face association, a commonly observed structural motif in less
sterically face-hindered porphyrins.
A pyrrolidine-derivatized C60, N-methyl-2-phenyl-3,4-fulle-
ropyrrolidine,53 was compared with C60 for binding to 5. As
(51) Hosseini, A.; Hodgson, M. C.; Tham, F. S.; Reed, C. A.; Boyd, P. D. W.
Cryst. Growth Des. 2006, 6, 397-403.
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(53) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519-526.
The 1:1 stoichiometry of binding of both C60 and C70 to 5
was also established by electrospray mass spectrometry. The
9
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Calix[4]arene-Linked Bisporphyrin Hosts for Fullerenes
A R T I C L E S
Table 2. Binding Constants for Fullerenes of 5 as a Function of Solvent
CH3CN/
toluene
1:19
CH3CN/
toluene
1:9
CH3CN/
toluene
1:3
CH3CN/
toluene
1:1
C6H12
C6H5CN
CH2Cl2
toluene
solubility
(mg/mL)
log K
0.036
0.41
0.25
2.60
0.920
4.61
0.710
4.85
0.267
5.26
0.031
6.28
6.61
4.69
4.91
4.42
expected for a fullerene where the addend has only a small steric
and electronic influence, the binding constant, 6000 M-1, was
smaller but comparable.
the order: toluene > chlorobenzene > dichlorobenzene. This
follows the solubility order for fullerenes and is consistent with
dominance of the fullerene desolvation energy in determining
relative magnitudes of the binding constants.
Previous studies of the role of porphyrin metalation in
controlling fullerene binding constants have shown somewhat
variable trends.28 As shown in Table 1 for C60 and C70 binding
to 5, the affinities of metalloporphyrins are all lower than those
of the corresponding free base. This tendency has been observed
with most but not all porphyrin hosts. A subtle interplay of
differential solvation energies and metalloporphyrin-fullerene
interaction energies is believed to control these binding con-
stants.28
The most recent theoretical analysis of porphyrin-fullerene
interaction energies54 shows the need for detailed experimental
studies on the energetics of binding and the role of the solvent.
Only with studies of this kind will a separation of solvation
versus intrinsic binding effects become possible. For this reason,
we have explored the solvent and temperature dependence of
fullerene binding constants for 2 and 5.
Bisporphyrin 5 was chosen for solvent-dependent studies
because it has the highest binding constant for C60 and C70 in
toluene compared with other hosts. In addition, it is soluble in
a range of organic solvents. As shown in Table 2, titration of a
concentrated solution of C60 in toluene into a dilute solution of
5 in benzonitrile showed an increase in the binding constant to
50 000 M-1, compared to 25 000 M-1 in toluene. Similar
titrations in dichloromethane afforded an association constant
of 90 000 M-1. More dramatically, titrations carried out in
cyclohexane showed an increase in binding constant to 4 000 000
M-1. The relative solubility of C60 decreases in the order:
toluene > benzonitrile > dichloromethane . cyclohexane, again
suggesting that desolvation of the fullerene is the dominant
factor determining the energetics of host-guest formation.
Mixtures of acetonitrile/toluene provide an interesting option
for the control of the association constant because the solubility
of C60 decreases in a nonlinear fashion over a large range as
the v/v % of acetonitrile increases.55 A solution of 5 in a mixture
of acetonitrile/toluene was prepared such that the solubility of
C60 at 1:1 v/v solvent ratio (0.031 mg/mL) was comparable to
that in cyclohexane (0.036 mg/mL). The absorption spectrum
of this mixture shows no evidence of cluster formation as has
been found in solutions of porphyrins and fullerenes in higher
ratios of acetonitrile, where broad intense absorptions are
observed in the visible and near-infrared regions relative to the
spectra of the parent porphyrin and fullerene.36 In this case, the
binding constant was of the same order of magnitude (1 000 000
M-1) as the binding constant in cyclohexane. At lower ratios
of acetonitrile (1:3, 1:9, and 1:19), where the solubility of C60
ranges from 0.267 to 0.920 mg/mL, the decrease in binding
constant is very significant (Figure 6). The essential linearity
of the plot of log K versus C60 solubility is intriguing because
the correlation between binding constant and solvent composi-
tion implies a direct correlation of binding constant with the
C60 solvation energy. These data continue to support the idea
that desolvation of the fullerene is the dominant contributor to
the energetics of fullerene binding in solution.
Solvent Dependence of Binding Constants. The influence
of solvent on the supramolecular host-guest interaction is
pronounced. The energetics of binding can conceptually be
broken down into desolvation of the individual host and guest,
intrinsic binding affinities, and resolvation of the host-guest
complex. The formation of a host-guest complex will be
favored in solvents in which either or both of the host-guest
components are weakly solvated or in which the solvation free
energy of the complex exceeds that of the components.
Traditionally, the consideration for the choice of solvents for
titration of a host with a fullerene has been limited by the
solubility of fullerenes in organic solvents. The solubility of
C60 increases in the order: benzene (1.70 mg/mL) < toluene
(2.80 mg/mL) < p-xylene (3.14-5.90 mg/mL) < chlorobenzene
(5.7-7.0 mg/mL) < CS2 (7.7-7.9 mg/mL) , o-dichloroben-
zene (13.8-27.0 mg/mL).55 Haino and co-workers studied the
solvent effect on the association of both C60 and C70 with mono-
and biscalix[5]arenes.56-58 The association constants decrease
in the order: toluene > benzene > CS2 > o-dichlorobenzene.
With the apparent exception of toluene, the association constant
decreases with increasing solubility of the fullerene. Aida and
Tashiro have investigated the competition between complexation
and solvation in C120 complexes of a cyclic bisporphyrin.59
Again, the association constant for the fullerene decreases in
Enthalpy and Entropy of Fullerene Binding. The temper-
ature dependence of the binding constant (K) of 2 with C60 was
measured at six different temperatures using 1H NMR titration
spectroscopy in toluene-d8. The association of C70 with 2 in
toluene is sufficiently large to allow the determination of the
thermodynamics of binding by UV-vis spectroscopy. The
enthalpy (∆H) and entropy (∆S) of complex formation with
C60 and C70 with 2 were evaluated by a van’t Hoff plot of ln-
(K) versus 1/T. As illustrated for C60 in Figure 7, these van’t
Hoff plots are linear with a positive slope. This indicates that
the formation of these host-guest complexes is exothermic and
(54) Jung, Y.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2006, 8, 2831-
2840.
(55) Beck, M. T.; Mandi, G. Fullerene Sci. Technol. 1997, 5, 291-310.
(56) Haino, T.; Yanase, M.; Fukazawa, Y. Angew. Chem., Int. Ed. 1998, 37,
997-998.
(57) Yanase, M.; Matsuoka, M.; Tatsumi, Y.; Suzuki, M.; Iwamoto, H.; Haino,
T.; Fukazawa, Y. Tetrahedron Lett. 2000, 41, 493-497.
(58) Haino, T.; Yanase, M.; Fukunaga, C.; Fukazawa, Y. Tetrahedron 2006,
62, 2025-2035.
(59) Tashiro, K.; Hirabayashi, Y.; Aida, T.; Saigo, K.; Fujiwara, K.; Komatsu,
K.; Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc. 2002, 124, 12086-
12087.
9
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A R T I C L E S
Hosseini et al.
free-base and Zn tetraphenylporphyrins.60,61 This suggests little
or no interaction between porphyrin macrocycles in 5 and Zn25.
The absorption spectra of solutions containing the fullerene
adducts are shown in Figure 9. Adduct formation is clearly
signaled by the substantial intensity decrease (-40%) and red
shift (9 nm) of the porphyrin Soret bands (either free-base or
Zn-type) compared to the receptors alone, and the appearance
of new and weak absorption bands above 700 nm is attributable
to ground state porphyrin-to-fullerene charge transfer (CT)
interactions. Similar low-energy bands have been observed in
previously investigated face-to-face porphyrin-fullerene con-
jugates.44,45,47
The CT absorption bands of the adducts with a free-base
porphyrin are at shorter wavelength (around 720 nm) relative
to those involving the corresponding Zn-porphyrin receptor (ca.
780 nm). This is a consequence of the lower oxidation potential
of Zn-porphyrins.44
Within the same receptor, the CT band tends to shift to lower
energy with C70 relative to C60, reflecting the slightly lower
reduction potential of C70 compared to that of C60.62 Notably,
in the present four adducts where the carbon sphere is
sandwiched between two porphyrin units, CT bands show a
pronounced shoulder with molar extinction coefficient of the
order of ca. 3000 M-1 cm-1, a value substantially higher than
that exhibited in similarly apolar toluene solution by conjugates
having just one porphyrin donor moiety for the fullerene
acceptor (50 M-1 cm-1).45
Figure 6. Variation of the binding constant for C60 (log K) with 5 as a
function of fullerene solubility in toluene/acetonitrile solvent mixtures (v/v
%).
A direct relationship between supramolecular binding of the
fullerenes to a bisporphyrin host and the appearance of a NIR
band can be demonstrated with Zn25·C60, where the component
absorptions are well separated and solubility is high. Titration
of a solution of C60 into a concentrated solution of Zn25 (3
× 10-5 M) correlates with the appearance of a new absorption
band at ∼770 nm (Figure 10). A least-squares fit of the increase
in the intensity of the 770 nm band as a function of C60
concentration gave a formation constant of 21,900 ( 2700 M-1
at 296 K, comparable to the binding constant obtained inde-
pendently by UV-vis titration (19,390 ( 800 M-1 at 298 K).
Evidence that this NIR band is a porphyrin-to-fullerene charge
transfer (CT) in origin is obtained by systematic variation of
the porphyrin. The energy of this band increases for C60 in
order: Zn25 < Cu25 < 5 < Ni25. This correlates with the lower
lying energy of the HOMO of the porphyrin as calculated by
DFT methods. As illustrated in Figure 11, the lower the energy
of the porphyrin HOMO, the greater the gap to the fullerene
LUMO. The CT band is not observed in 4‚C60 presumably
because the electron-withdrawing fluoro substituents increase
the energy of this band into the visible region, where it is masked
by the more intense Q-bands of the porphyrin. Indeed, a
broadening of the porphyrin bands in the visible region is
apparent, but deconvolution cannot be reliably carried out.
Fluorescence spectra of the receptor molecules 5 and Zn25
in cyclohexane are depicted in the inset of Figure 8; spectral
shapes, fluorescence quantum yields, and related singlet lifetimes
(Table 4) are in line with monomeric tetraphenylporphyrins.60,61
Figure 7. Plot of ln(K) versus 1/T for the binding of C60 with 2. The arrow
indicates a value of K determined from a fluorescence titration.
enthalpy driven. The enthalpy of binding for C70 is higher by 4
kJ mol-1, which correlates with the higher binding constant of
C70 relative to that of C60. The entropies of formation are small
and positive (Table 3), indicating a small entropic contribution
to the binding energies. This indicates that the negative entropy
contribution expected from an associative reaction is countered
by an increase in solvent disorder upon complexation compared
to the individual fullerene and porphyrin components.
Similar measurements for C60 with 5 in both toluene and
cyclohexane again gave linear van’t Hoff plots with positive
slopes, indicating enthalpy-driven complex formation. The ∆H
and ∆S values for C60 with 5 in toluene are both somewhat
larger in magnitude than those found for 2 (see Table 3). These
increases rationalize the trebling of the binding constant of 5‚
C60 relative to 2‚C60. On the other hand, for 5‚C60 in cyclo-
hexane, ∆H doubles in magnitude relative to that in toluene
and ∆S is small but negative. The increase in ∆H is sufficiently
large that the unfavorable entropy change is overcome and the
binding constant increases by a factor of ca. 100. This can be
understood in terms of the poor solvation of a fullerene by
cyclohexane relative to toluene.
Absorption and Luminescence Spectra: Ground and
Excited State CT Interactions. The absorption spectra of the
receptor molecules 5 and Zn25 in cyclohexane are reported in
Figure 8. They are characterized by the typical Soret and
Q-bands in the visible spectral region, with molar extinction
coefficients approximately twice as intense as in those of regular
(60) Flamigni, L.; Armaroli, N.; Barigelletti, F.; Balzani, V.; Collin, J. P.;
Dalbavie, J. O.; Heitz, V.; Sauvage, J. P. J. Phys. Chem. B 1997, 101,
5936-5943.
(61) Flamigni, L.; Barigelletti, F.; Armaroli, N.; Collin, J. P.; Sauvage, J. P.;
Williams, J. A. G. Chem.sEur. J. 1998, 4, 1744-1754.
(62) Xie, Q. S.; Perez-Cordero, E.; Echegoyen, L. J. Am. Chem. Soc. 1992,
114, 3978-3980.
9
15908 J. AM. CHEM. SOC. VOL. 128, NO. 49, 2006

Calix[4]arene-Linked Bisporphyrin Hosts for Fullerenes
A R T I C L E S
Table 3. Thermodynamic Parameters for the Binding of Fullerenes to 2 and 5
2
5
C60 Toluene
C70 Toluene
C60 Toluene
C60 Cyclohexane
∆
Hc
∆
Sc
∆
Hc
∆
Sc
∆
Hc
∆
Sc
∆
Hc
∆
Sc
1
1
1
1
1
1
1
1
kJ mol-
J K-1 mol-
kJ mol-
J K-1 mol-
kJ mol-
J K-1 mol-
kJ mol-
J K-1 mol-
-21 ( 1.5
4.5 ( 4.9
-25 ( 2.1
4.1 ( 6.6
-22.7 ( 4.1
8.3 ( 14.9
-40.3 ( 8
-8.4 ( 12
Figure 10. NIR-UV-vis spectrum of titration of Zn25 with C60 in toluene
Figure 8. Electronic absorption spectra of 5 (black), Zn25 (red), C60 (green),
and C70 (blue) in cyclohexane; the spectral window above 420 or 460 nm
has been multiplied by a factor of 20. Inset: Fluorescence spectra of 5
(black) and Zn25 (red) (λexc ) 425 nm, o.d. ) 0.1, cyclohexane).
(Zn25 spectrum subtracted). [C60]:[Zn25] ) 1:2, 1:1, 3:2, 4:2, 5:2, 6:2, 7:2,
8:2.
Figure 9. Electronic absorption spectra of solutions containing 5/C60
(black), 5/C70 (blue), Zn25/C60 (red), and Zn25/C70 (green) in cyclohexane;
the spectral window above 620 nm has been multiplied by a factor of 70.
Details on the preparation of these samples, which contain a concentration
of the supramolecular adduct of about 1.0 × 10-6 M and an excess of C60
or C70 (ca. 5 times higher), are given in the Experimental Section.
Figure 11. Illustrations of the HOMO and LUMO of a tetraphenylpor-
phyrin‚C60 complex and a schematic representation of the energy levels of
the frontier orbitals.
Table 4. Porphyrin Fluorescence and Lifetime Data in
Cyclohexane Solution
Upon predominant excitation of the porphyrin moieties within
the supramolecular adducts 5‚C60, 5‚C70, Zn25‚C60, and Zn25‚
λ
em (%)a,b
(nm)
τc
(ns)
Φ
em (%)a
C
70 (425 nm, Soret band), dramatic quenching of the porphyrin
5
654
592
654
592
654
592
5.2
2.3
0.4
0.3
0.04
0.03
10.2
fluorescence is observed (Table 4). The residual fluorescence
in 5‚C60 and Zn25‚C60 is mainly attributable to some amount
of unbound porphyrin receptor (see Experimental Section); this
is confirmed by the lifetime values, which are identical to those
of the reference molecules. The residual porphyrin emission
turns out to be 10 times weaker in the adducts with C70, and
this is consistent with the higher association constants observed
with the larger carbon cage. For 5‚C70, a short lifetime
component was observed, which is attributed to the quenched
free-base porphyrin within the adduct.
Zn25
5‚C60
2.4
10.1d
2.4d
Zn25‚C60
5‚C70
0.6e-9.8d
2.4d
Zn25‚C70
a λexc ) 425 nm, Soret band. b Highest energy emission feature. λexc
) 407 nm, laser diode. d Residual fluorescence from some unbound receptor
(see text). e Shorter component attributable to quenching of the porphyrin
in the adduct.
c
Emission maxima are at higher energy for 5‚C60 and 5‚C70
(Figure 12) compared to Zn25‚C60 and Zn25‚C70 (Figure 13),
in line with the CT absorption trends. In the case of 5‚C60 and
5‚C70, a tail of residual porphyrin fluorescence has been removed
to obtain the profiles depicted in Figure 12. Emission maxima
By recording the emission spectra of 5‚C60, 5‚C70, Zn25‚C60,
and Zn25‚C70 down to the NIR region, low-energy bands
attributable to charge transfer emissive states are detected with
lifetimes shorter than 1 ns (Table 5).45,48
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A R T I C L E S
Hosseini et al.
Table 5. CT Luminescence and Lifetime Data at 298 K in
Cyclohexane
emission band, tested in the case of the highest signal
(Zn25‚C70), is also detected upon direct excitation in the CT
absorption bands (784 nm), although the signal is very weak
due very low absorbance and excitation power (Figure 13). This
demonstrates that 5‚C60, 5‚C70, Zn25‚C60, and Zn25‚C70 have
to be considered electron donor/acceptor complexes and not
exciplexes since their interaction is not exclusively established
in the excited state but also in the ground state.64
The CT bands are of interest because information relevant
to the rate of photoinduced electron transfer can be obtained
from consideration of their energies, bandwidths, extinction
coefficients, and inter-chromophore distance. The rate of
electron transfer is proportional to the square of the electronic
coupling constant V in the Marcus-Hush relationship for the
rate of electron transfer kET (eq 1).65,66
a
λmax
τc
(nm)
(ns)
5‚C60
848
0.6
0.6
840b
876
5‚C70
866b
982
Zn25‚C60
Zn25‚C70
e0.3c
e0.3c
962b
992
972b
a λexc ) 425 nm. b Corrected values for the photomultiplier response.
c λexc ) 407 nm, laser diode; close to the lower limit of instrumental time
resolution, experimental uncertainty (50%.
(∆G° + λ)2
1/2
4π3
p2λkBT
ET
ket )
V2 exp -
(1)
(
)
[
]
4λkBT
where λ ) reorganization energy, ∆G°ET ) reaction free energy,
kB ) Boltzman constant, p ) Planck constant, V ) electronic
coupling (electronic matrix element), and T ) absolute tem-
perature.
The CT bands in the absorption spectra may be used to
estimate the electronic coupling (V) between the fullerene and
porphyrin using eq 2.67
2.06 × 10-2(ꢀmax
ν
max∆ν1/2)1/2
V )
(2)
Rcc
Figure 12. Emission spectra of 5‚C60 (solid line) and 5‚C70 (dashed line)
in cyclohexane solution (o.d. ) 0.5, λexc) 425 nm). The tail of the porphyrin
residual fluorescence has been mathematically subtracted.
where V ) electronic coupling (in cm-1), ꢀmax ) extinction
coefficient of the CT band at maximum (in mol-1 cm-1), νmax
) frequency of the CT band (in cm-1), ∆νmax ) full width at
half-height (in cm-1), and Rcc ) porphyrin center to fullerene
center distance (in Å); ∆νmax was obtained from a Gaussian fit
to the lower energy portion of the CT band, and Rcc was taken
as 6.25 Å, the common value observed in fullerene-porphyrin
cocrystallates.68 The electronic coupling (V) between the bispor-
phyrin and C60 decreases in the order of Cu(II) (790 cm-1) >
Zn(II) (760 cm-1) > free base (700 cm-1).
Finally, we address the question of how the CT band is
affected by solvent. In a self-associated fullerene-porphyrin
dyad, Guldi et al. observed a red shift of 27 nm in energy of
the CT band upon increasing the solvent polarity from toluene
(ꢀ ) 2.4) to benzonitrile (ꢀ ) 26).46 On the other hand,
Chukharev et al. observed no significant difference in either
the energy of the CT band or the electronic coupling between
the two chromophores upon changing the solvent from toluene
(ꢀ ) 2.4) to cyclohexane (ꢀ ) 2.0).43 We find there is no
significant shift in energy (νmax) of the CT band in Zn25‚C60 in
cyclohexane (778 nm), toluene (772 nm), or benzonitrile (766
nm). This indicates that neither ground state nor the excited
state is highly solvated, consistent with the fullerene guest being
well encapsulated by the porphyrin host. Nevertheless, the band
Figure 13. Emission spectra of Zn25/C60 (A: o.d. ) 0.5, λexc) 425 nm)
and Zn25/C70 (B: o.d. ) 0.5, λexc) 425 nm; C: o.d. ) 0.07, λexc) 548
nm; D: o.d. ) 0.06, λexc) 784 nm) in cyclohexane solution.
of C70 adducts are slightly bathochromically shifted and, at least
in the case of the Zn25 receptor where the spectra are well
separated from the residual porphyrin fluorescence, relatively
more intense than for C60 analogues (Figure 13).
By using corrected luminescence spectra and pristine C60 in
toluene as reference, the emission quantum yield of Zn25‚C70
was estimated to be of the order of 10-4, that is, similar to C60-
porphyrin conjugates investigated earlier.48 The low-emission
quantum yield is related to the very low energy of the emitting
state, according to the energy gap law.63 Notably, the CT
(64) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Har-
wood: Chichester, U.K., 1991; p 248.
(65) Marcus, R. A. J. Phys. Chem. 1989, 93, 3078-3086.
(66) Marcus, R. A. Angew. Chem., Int. Ed. 1993, 105, 1161-1172. See also:
Angew. Chem., Int. Ed. Engl. 1993, 32, 1111-1121.
(67) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100,
13148-13168.
(68) Boyd, P. D. W.; Hodgson, M. C.; Chaker, L.; Rickard, C. E. F.; Oliver, A.
G.; Brothers, P. J.; Bolskar, R.; Tham, F. S.; Reed, C. A. J. Am. Chem.
Soc. 1999, 121, 10487-10495.
(63) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145-164.
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Calix[4]arene-Linked Bisporphyrin Hosts for Fullerenes
A R T I C L E S
recorded after each addition. A plot of absorbance (A) versus C60
concentration was used to obtain the fullerene binding constant using
a nonlinear fitting method.69
widths (ν1/2) vary somewhat, and this leads to some variation
in the electronic coupling values, with benzonitrile (830 cm-1
)
slightly larger than cyclohexane (790 cm-1) or toluene (760
cm-1).
Spectroscopic Studies. The spectroscopic investigations were carried
out in cyclohexane (Carlo Erba spectroscopic grade) in fluorimetric
cuvettes with an optical path of 1 cm. Absorption spectra were recorded
with a Perkin-Elmer λ9 spectrophotometer. Visible and NIR lumines-
cence spectra were obtained with an Edinburgh FLS920 spectrometer
equipped with either a Peltier-cooled Hamamatsu R-928 (vis region)
or a supercooled (193 K) Hamamatsu R5509-72 PMT detector (NIR
region); in the latter configuration, a TM300 emission monochromator
with NIR grating blazed at 1000 nm was used. An Edinburgh Xe900
450 W xenon arc amp was used as light source; the emission calibration
curve was supplied by the manufacturer. Emission lifetimes were
determined with the single photon counting technique by means of the
same Edinburgh FLS920 spectrometer using a laser diode as excitation
source (1 MHz, λexc ) 407 nm, 200 ps time resolution after
deconvolution) and the above-mentioned PMTs as detectors.
In order to make spectroscopic investigations on the supramolecular
adducts, it is necessary to find a balance between two contrasting
factors: the need to get a strong fraction of associated receptor and
the need to have a relatively low optical density of the solutions for
recording spectra under acceptable optical conditions. In light of this
and of the association constant of 5 and Zn25 with C60 in cyclohexane,
that is, 2 × 106 M-1, spectroscopic investigations were carried out as
follows. Solutions containing a mix of 1.0 × 10-6 M ïf 5 or Zn25 and
5.0 × 10-6 M of C60 or C70 were prepared, obtaining a fraction of
associated receptor exceeding 90%. Under these conditions, we get ca.
1.0 × 10-6 M of the supramolecular adducts 5‚C60, 5‚C70, Zn25‚C60,
and Zn25‚C70, an excess 4 times more concentrated of the unbound
fullerene molecule, and very little concentration of the free receptor
(e10-7 M). Under these conditions, it is possible to excite, in the adduct,
the porphyrin moiety on the very intense Soret band with a >95%
selectivity while keeping the optical density at a reasonable value (o.d.
) 0.5-0.6) for the detection of weak luminescence signals in the near-
infrared spectral region. Spectroscopic experiments by preferential
excitation of the fullerene chromophore (e.g., 330 nm for C60) were
not carried out, given the large unbound excess of the carbon allotrope
under the working conditions.
Synthesis. Reactions were carried out in oven-dried glassware under
an atmosphere of dry, oxygen-free nitrogen. Analytical grade organic
solvents were dried by standard methods.71 The progress of reactions
was monitored by silica gel thin layer chromatography (TLC). The
synthesis of the porphyrin and calix[4]arene precursors is detailed in
the Supporting Information. C60, C70, and 13C-enriched C60 were
purchased from SES RESEARCH, Houston, Texas.
5,11,17,23-Tetra-tert-butyl-25,27-di[(4-carboxyphenyl)-10,15,20-
tris-(p-methylphenyl)porphyrin]-26-28-dihydroxycalix[4]arene, 1.
The synthetic procedure is based on the report of Neises and Steylich.72
TTP-COOH (0.0998 g, 0.142 mmol), p-tert-butylcalix[4]arene (0.0458
g, 0.0706 mmol), and DMAP (0.0035 g, 0.0286 mmol) were combined
in dry dichloromethane (5 mL), and the mixture was cooled to e0 °C.
DCC (0.0308 g, 0.149 mmol) in dry dichloromethane was added
dropwise and the reaction mixture allowed to warm to room temperature
followed by stirring overnight under dinitrogen gas. Precipitated urea
was removed by filtration through celite, and precipitated porphyrin
product(s) were washed through the celite with chloroform. The filtrate
solvent was removed in vacuo and the resulting residue dissolved in
chloroform and filtered once more to remove the last traces of urea.
Silica gel was added to the filtrate and solvent removed in vacuo to
adsorb the porphyrin products on to the gel in preparation for column
chromatography. Elution with dichloromethane/n-hexane (3:1) produced
the first band, which was further purified by preparative layer
Conclusions
Calix[4]arenes are convenient scaffolds for the construction
of bisporphyrins. The easy variation of the covalent linkage at
the lower rim allows positional optimization of the two
porphyrins for maximal fullerene binding. The attachment of
porphyrins via an amide linker with a single methylene spacer
(compound 2) proved to be most suited for the acceptance of a
fullerene guest. In addition, fullerene binding is affected by the
choice of peripheral substituents on the porphyrin and those on
the upper rim of the calixarene.
Systematic studies on the binding constants of fullerene guests
into bisporphyrin hosts indicate a strong dependence on dif-
ferential solvation energies. In particular, the inverse correlation
of binding constant with solubility of fullerenes in various
solvents shows that the desolvation of the fullerene is a major
contributor to the energetics of fullerene binding. The thermo-
dynamics of binding are consistent with an enthalpy-driven,
solvation-dependent process, making solvent choice a powerful
way to control binding constants.
By showing a direct relationship between supramolecular
binding of a fullerene guest to a bisporphyrin host and the
appearance of a broad NIR absorption band, whose energy
increases with stability of the porphyrin HOMO, the porphyrin-
to-fullerene charge transfer nature of this band has been placed
on a firm footing.
Experimental Section
General. 1H and 13C NMR spectra were obtained on a Bruker AM-
400 spectrometer, and 19F NMR spectra were recorded on a Bruker
AM-300 spectrometer. FAB mass spectra were recorded on a VG-70
spectrometer, MALDI-TOF on a PerSeptive Biosystems Voyager-DE
STR spectrometer using a dithranol matrix, and electrospray mass
spectra on an Agilent Technologies 1956 MSD mass spectrometer.
Binding constants were measured by fluorescence quenching titrations
on a Hitachi F2000 spectrometer, by UV-vis titration on a Hewlett-
Packard 8452 diode array spectrometer with NESLAB RTE 111 water
bath. A Perkin-Elmer Lambda 35 UV-vis-NIR spectrometer was used
to measure charge transfer bands. NMR spectra were recorded in CDCl3
containing TMS as reference and filtered through K2CO3 to remove
traces of HCl. The temperature of the NMR probe and the sample was
1
calibrated using the H NMR spectra of dry methanol.
Fluorescence titrations were carried out using stock solutions of
bisporphyrins (3 mL) in a quartz cuvette (1 cm path length). The
solution was excited at the wavelength of the first Q-band (e.g.,
excitation λ ) 516 nm for 2), and the fluorescent emission intensity
was monitored at the peak recorded from a wavelength scan. C60 in
toluene was added in 10 µL aliquots and the emission intensity (I)
recorded after each addition. A plot of I versus [C60] was carried out
to yield the binding constant by a nonlinear least-square methods.69,70
UV-vis titrations were carried out using stock solutions of bispor-
phyrins (3 mL) in a quartz cuvette (1 cm path length). The Soret band
in the spectrum was monitored at the wavelength recorded for the free
bisporphyrin. A concentrated C60 solution in toluene was added in 10
µL aliquots to both the blank and sample cells and the absorbance
(69) Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97, 4552-4557.
(70) Encinas, S.; Bushell, K. L.; Couchman, S. M.; Jeffery, J. C.; Ward, M. D.;
Flamigni, L.; Barigelletti, F. J. Chem. Soc., Dalton Trans. 2000, 1783-
1792.
(71) Riddick, J. A.; Bunger, W. B. Organic SolVents: Physical Properties and
Methods of Purification; Wiley: New York, 1986.
(72) Neises, B.; Steglich, W. Angew. Chem., Int. Ed. Engl. 1978, 17, 522-524.
9
J. AM. CHEM. SOC. VOL. 128, NO. 49, 2006 15911

A R T I C L E S
Hosseini et al.
chromatography (plc) eluting with toluene/n-hexane (3:1). The slower
moving of the two major plc bands was extracted from the silica gel
with dichloromethane and solvent removed in vacuo to give the desired
arene, 4. A solution of 11,17,23-tetra-tert-butyl-25,27-dicarboxymethoxy-
26,28-dihydroxycalix[4]arene (46 mg, 0.060 mmol) and dicyclohexyl-
carbodiimide (51 mg, 0.25 mmol) was stirred for 10 min in dichlo-
romethane (8 mL). 5-(para-Aminophenyl)-10,15,20-tris(C6F5)porphyrin
(121 mg, 0.13 mmol) in dichloromethane (3 mL) was added. Purifica-
tion using flash column chromatography with dichloromethane as eluent
gave 4 (96 mg, 63%). FAB-MS: found (M)+ 2528 m/z for C136H88-
F30N10O6. 1H NMR (CDCl3): δ ) -2.90 (s, 4 H, -NH-), 1.19 (s, 18
H, -C(CH3)3), 1.32 (s, 18 H, C(CH3)3), 3.72 (d, J ) 13.3 Hz, 4 H,
Ar-CH2-Ar), 4.49 (d, J ) 13.3 Hz, 4 H, Ar-CH2-Ar), 4.98 (s, 4 H,
O-CH2), 7.17 (s, 4 H, ArH), 7.24 (s, 4 H, ArH), 8.25 (d, J ) 8.6 Hz,
4 H, ArH), 8.32 (d, J ) 8.6 Hz, 4 H, ArH), 8.81-8.85 (m, 12 H, Hâ),
9.30 (d, J ) 4.7 Hz, 4 H, Hâ), 11.20 (s, 2 H, OH) ppm. UV-vis
(toluene) λmax (nm), ꢀ (mol-1 L cm-1): 419 nm (ꢀ 487 145), 511 (ꢀ
38 547), 544 (ꢀ 11 806), 588 (ꢀ 13 010), 642 (ꢀ 2879).
1
product as a purple residue (0.0043 g, 3%). H NMR (C6D6): δ )
-2.89 (s, 4 H, -NH-), 1.15 (s, 18 H, -C(CH3)3), 1.35 (s, 18 H,
-C(CH3)3), 2.18 (s, 12 H, -CH3), 2.50 (s, 6 H, -CH3), 3.71 (d, J )
14 Hz, 4 H, -CH2-), 4.51 (d, J ) 14 Hz, 4 H, -CH2-), 6.45 (d, J )
7.6 Hz, 8 H), 6.56 (d, J ) 7.2 Hz, 8 H), 7.34 (d, J ) 7.6 Hz, 4 H),
8.03 (d, J ) 4.8 Hz, 4 H), 8.08 (d, J ) 8.0 Hz, 4 H), 8.25 (d, J ) 4.8
Hz, 4 H), 8.36 (d, J ) 7.6 Hz, 4 H), 8.55 (d, J ) 4.8 Hz, 4 H), 8.82
(d, J ) 4.4 Hz, 4 H), 9.00 (d, J ) 8.4 Hz, 4 H) ppm. HR FAB-MS:
calcd for [M+] m/z ) 2012.96438; found 2012.96265. UV/vis (tolu-
ene): λmax ) 417 (ꢀmax ) 530 000), 520 (29 000), 555 (19 000), 594
(11 000), 650 nm (8600 L‚mol-1‚cm-1).
General Procedure for Coupling of Monoaminoporphyrins (p-
MAP) with Calix[4]arene. A solution of 25,27-dicarboxy-26-28-
dihydroxycalix[4]arene (0.037 mmol) and dicyclohexylcarbodiimide
(0.16 mmol) in dichloromethane (3 mL) was stirred for 10 min, and
para-MAP (0.075 mmol) in dichloromethane (1 mL) was added and
the mixture was stirred overnight. The solvent was removed, and the
residue was dissolved in chloroform (50 mL) and washed with water
(30 mL). The product was purified by flash column chromatography
on silica gel. Where necessary, the silica gel was washed with 4 column
volumes of hexane to remove dicyclohexylurea prior to elution of the
amide.
5,11,17,23-Tetra-t-butyl-25,27-di[methoxy(4-amidophenyl)-10,15,-
20-tris-(3,5-di-tert-butylphenyl)porphyrin]-26-28-dihydroxycalix-
[4]arene, 5. A solution of 11,17,23-tetra-tert-butyl-25,27-dicar-
boxymethoxy-26,28-dihydroxycalix[4]arene (86 mg, 0.11 mmol) and
dicyclohexylcarbodiimide (96 mg, 0.46 mmol) in dichloromethane (10
mL) was stirred for 10 min. 5-(para-Aminophenyl)-10,15,20-tris-(3,5-
di-tert-butylphenyl)porphyrin (240 mg, 0.25 mmol) in dichloromethane
(5 mL) was added. The mixture was stirred overnight, and 5 was then
purified via flash chromatography using chloroform:hexane (1:2) as
eluent (170 mg, 57%). FAB-MS: found (M + H)+ 2663 m/z for
1
C
184H214N10O6 + H. H NMR (CDCl3): δ ) -2.75 (s, 4 H, -NH-),
5,11,17,23-Tetra-tert-butyl-25,27-di[methoxy(4-amidophenyl)-10,-
15,20-tris-(p-methylphenyl)porphyrin]-26-28-dihydroxycalix[4]-
arene, 2. A solution of 11,17,23-tetra-tert-butyl-25,27-dicarboxymethoxy-
26,28-dihydroxycalix[4]arene (29.8 mg, 0.039 mmol) and dicyclohexyl-
carbodiimide (33 mg, 0.16 mmol) was stirred for 10 min in dichlo-
romethane (3 mL). 5-(para-Aminophenyl)-10,15,20-tris-(para-meth-
ylphenyl)porphyrin (50 mg, 0.075 mmol) in dichloromethane (1 mL)
was added. Flash column chromatography with dichloromethane:hexane
(1:1) gave 2 (43 mg, 54%). FAB-MS: found (M)+ 2072 m/z for
1.17 (s, 18 H, -C(CH3)3), 1.31 (s, 18 H, -C(CH3)3), 1.41 (s, 72 H,
10,20-C(CH3)3), 1.479 (s, 36 H, 15-C(CH3)3), 3.67 (d, J ) 13.5 Hz, 4
H, Ar-CH2-Ar), 4.48 (d, J ) 13.6 Hz, 4 H, Ar-CH2-Ar), 4.92 (s,
4 H, O-CH2), 7.14 (s, 4 H, ArH), 7.21 (s, 4 H, ArH), 7.69 (t, J ) 1.7
Hz, 4 H, ArH), 7.74 (t, J ) 1.8 Hz, 2 H, ArH), 8.00 (d, J ) 1.8 Hz,
8 H, ArH), 8.05 (d, J ) 1.7 Hz, 2 H, ArH), 8.28 (s, 8 H, Hâ), 8.85 (s,
2 H, NHCO), 8.81 (d, J ) 5 Hz, 4 H, ArH), 8.83 (d, J ) 5 Hz, 4 H,
ArH), 8.99 (d, J ) 4.6 Hz, 4 H, Hâ), 9.14 (d, J ) 4.1 Hz, 4 H, Hâ),
11.10 (s, 2 H, OH) ppm. UV-vis (toluene) λmax (nm), ꢀ (mol-1 L cm-1):
419 nm (ꢀ 487 145), 511 (ꢀ 38 547), 544 (ꢀ 11 806), 588 (ꢀ 13 010),
642 (ꢀ 2879).
C
142H130N10O6. 1H NMR (C7D8): δ ) -2.13 (s, 4 H, -NH-), 0.52 (s,
18 H, C(CH3)3), 1.17 (s, 18 H, C(CH3)3), 2.10 (s, 12 H, Ar-CH3),
2.13 (s, 6 H, Ar-CH3), 3.20 (d, J ) 13.3 Hz, 4 H, Ar-CH2-Ar), 4.19
(d, J ) 13.3 Hz, 4 H, Ar-CH2-Ar), 4.35 (s, 4 H, -OCH2), 7.21 (d,
J ) 7.7 Hz, 8 H, Ar-CH2-Ar), 7.25 (d, J ) 7.9 Hz, 4 H, ArH), 7.27
(s, 8 H, ArH), 8.00 (d, J ) 8.0 Hz, 12 H, ArH), 8.25 (d, J ) 8.6 Hz,
4 H, ArH), 8.59 (d, J ) 8.6 Hz, 4 H, ArH), 8.91 (s, 8 H, Hâ), 9.11 (d,
J ) 4.8 Hz, 4 H, Hâ), 9.31 (s, 2 H, CONH), 9.49 (d, J ) 4.7 Hz, 4 H,
Hâ), 11.22 (s, 2 H, Ar-OH) ppm. UV-vis (toluene) λmax (nm): ꢀ
(mol-1 L cm-1), 421 nm (ꢀ 449 657), 517 (ꢀ 25 174), 551 (ꢀ 16 527),
592 (ꢀ 9633), 650 (ꢀ 8090).
Zn25. A saturated solution of Zn(II) acetate in methanol (0.5 mL)
was added to a solution of 5 (50 mg, 0.019 mmol) in chloroform (20
mL), and the mixture was refluxed until no free-base porphyrin was
observed in the UV-vis spectrum. The solvents were removed, and
Zn25 was purified by flash chromatography using dichloromethane/
ether (9:1) as eluent (38 mg, 73%). FAB-MS: found (M)+ 2788 m/z
for C184H210N10O6Zn2. 1H NMR (CDCl3): δ ) 1.18 (s, 18 H,
-C(CH3)3), 1.32 (s, 18 H, -C(CH3)3), 1.42 (s, 72 H, 10,20-C(CH3)3),
1.49 (s, 36 H, 15-C(CH3)3), 3.68 (d, J ) 13.4 Hz, 4 H, Ar-CH2-Ar),
4.49 (d, J ) 13.3 Hz, 4 H, Ar-CH2-Ar), 4.93 (s, 4 H, O-CH2), 7.15
(s, 4 H, ArH), 7.225 (s, 4 H, ArH), 7.70 (t, J ) 1.5 Hz, 4 H, ArH),
7.75 (t, J ) 1.6 Hz, 2 H, ArH), 8.02 (d, J ) 1.6 Hz, 8 H, ArH), 8.05
(d, J ) 1.7 Hz, 4 H, ArH), 8.31 (s, 8 H, ArΗ), 8.87 (s, 2 H, NHCO),
8.92 (d, J ) 4.7 Hz, 4 H, Hâ), 8.95 (d, J ) 4.7 Hz, 4 H, Hâ), 9.02 (d,
J ) 4.7 Hz, 4 H, Hâ), 9.27 (d, J ) 4.7 Hz, 4 H, Hâ), 11.10 (s, 2 H,
OH) ppm. UV-vis (toluene) λmax (nm), ꢀ (mol-1 L cm-1): 423 nm (ꢀ
695 333), 551 (ꢀ 38 450), 590 (ꢀ 11 371).
5,11,17,23-Tetra-tert-butyl-25,27-di[propoxy(4-amidophenyl)-10,-
15,20-tris-(p-methylphenyl)porphyrin]-26-28-dihydroxycalix[4]-
arene, 3. A solution of 5,11,17,23-tetra-tert-butyl-25,27-dicarboxypro-
poxy-26-28-dihydroxycalix[4]arene (31.5 mg, 0.037 mmol) and
dicyclohexylcarbodiimide (33 mg, 0.16 mmol) was stirred for 10 min
in dichloromethane (5 mL). 5-(para-Aminophenyl)-10,15,20-tris-(para-
methylphenyl)porphyrin (50 mg, 0.075 mmol) in dichloromethane (2
mL) was added. Flash column chromatography with dichloromethane:
hexane (1:1) gave 3 (42 mg, 54%). FAB-MS: found (M)+ 2128 m/z
1
for C146H138N10O6. H NMR (CDCl3): δ ) -2.81 (s, 4 H, -NH-),
Cu25. A saturated solution of Cu(II) acetate in methanol (0.5 mL)
was added to a solution of 5 (51 mg, 0.019 mmol) in chloroform (20
mL), and the mixture was refluxed until no free-base porphyrin was
observed in the UV-vis spectrum. The solvents were removed, and
Cu25 was purified by flash chromatography using dichloromethane/
ether (9:1) as eluent (43 mg, 81%). FAB-MS: found (M)+ 2785 m/z
for C184H210N10O6Cu2. 1H NMR (CDCl3): δ ) 1.16 (s, 18 H,
-C(CH3)3), 1.28 (s, 18 H, -C(CH3)3, 1.39 (br s, C(CH3)3), 3.61 (br s,
4 H, Ar-CH2-Ar), 4.36 (br s, 4 H, Ar-CH2-Ar), 4.77 (br s, 4 H,
O-CH2), 7.11 (s, 4 H, ArH), 7.17 (s, 4 H, ArH), 7.60 (br s, ArH),
8.78 (br s, ArH), 10.89 (br s, Hâ). UV-vis (toluene) λmax (nm), ꢀ (mol-1
L cm-1): 417 nm (ꢀ 666 274), 540 (ꢀ 35 379).
1.22 (s, 18 H, -C(CH3)3), 1.25 (s, 18 H, -C(CH3)3), 2.59 (s, 6 H,
-CH3), 2.70 (s, 12 H, -CH3), 3.31 (t, J ) 6.4 Hz, 4 H, CH2), 3.31 (d,
J ) 12.9 Hz, 4 H, Ar-CH2-Ar), 4.27-4.31 (m, 8 H, CH2), 4.75 (d,
J ) 13.0 Hz, 4 H, Ar-CH2-Ar), 6.99 (s, 4 H, ArH), 7.38 (d, J ) 7.8
Hz, 4 H, ArH), 7.53-7.56 (m, 8 H, ArH), 7.94 (d, J ) 7.8 Hz, 4 H,
ArH), 8.07-8.10 (m, 8 H, ArH), 8.48 (s, 2 H, CONH), 8.74 (d, J )
4.7 Hz, 4 H, Hâ), 8.79-8.85 (m, 12 H, Hâ), 9.78 (s, 2 H, ArOH) ppm.
UV-vis (toluene), λmax (nm), ꢀ (mol-1 L cm-1): 421 nm (ꢀ 413 334),
517 (ꢀ 19 382), 552 (ꢀ 11 841), 592 (ꢀ 6276), 650 (ꢀ 5537).
5,11,17,23-Tetra-t-butyl-25,27-di[methoxy(4-amidophenyl)-10,15,-
20-tris(pentafluorophenyl)porphyrin]-26-28-dihydroxycalix[4]-
9
15912 J. AM. CHEM. SOC. VOL. 128, NO. 49, 2006

Calix[4]arene-Linked Bisporphyrin Hosts for Fullerenes
A R T I C L E S
1
Ni25. A saturated solution of Ni(II) acetate in methanol (0.5 mL)
was added to a solution of 5 (48 mg, 0.018 mmol) in chloroform (20
mL), and the mixture was refluxed until no free-base porphyrin was
observed in the UV-vis spectrum. The solvents were removed, and
Ni25 was purified via chromatography using dichloromethane ether (9:
1) as eluent (44 mg, 89%). FAB-MS: found (M - 2H)+ 2773 m/z for
found (M + H)+ 2470 m/z. H NMR (CDCl3): δ ) -2.37 (s, 4 H,
-NH-), 0.95 (t, J ) 7.4 Hz, 12 H, CH3), 1.06 (t, J ) 7.4 Hz, 12 H,
CH3), 1.15 (s, 18 H, -C(CH3)3), 1.35 (s, 18 H, -C(CH3)3), 1.49 (s, 36
H, C(CH3)3), 1.65 (q, J ) 7.4 Hz, 8 H, CH2CH3), 1.71 (q, J ) 7.4 Hz,
8 H, CH2CH3), 2.13 (m, J ) 7.5 Hz, 8 H, CH2CH2CH3), 2.45 (s, 6 H,
CH3), 2.61 (s, 6 H, CH3), 3.69 (d, J ) 13.4 Hz, 4 H, Ar-CH2-Ar),
3.93-3.98 (m, 8 H, -CH2CH2CH2CH3), 4.50 (d, J ) 13.3 Hz, 4 H,
Ar-CH2-Ar), 4.96 (s, 4 H, Ar-O-CH2), 7.12 (s, 4 H, ArH), 7.78 (t,
J ) 1.7 Hz, 2 H, ArH), 7.90 (d, J ) 1.75 Hz, 4 H, ArH), 8.17 (d, J )
8.5 Hz, 4 H, ArH), 8.34 (d, J ) 8.5 Hz, 4 H, ArH), 8.53 (s, 2 H,
NHCO), 10.20 (s, 4 H, meso-CH), 11.03 (s, 2 H, OH) ppm. UV-vis
(toluene) λmax (nm), ꢀ (mol-1 L cm-1): 412 nm (ꢀ 385 051), 507 (ꢀ
34 381), 539 (ꢀ 9948), 576 (ꢀ 13 715), 628 (ꢀ 3386).
1
(C136H84F30N10O6Ni2 - 2H). H NMR (CDCl3): δ ) 1.15 (s, 18 H,
-C(CH3)3), 1.30 (s, 18 H, -C(CH3)3), 1.37 (s, 72 H, C(CH3)3), 1.43
(s, 36 H, C(CH3)3), 3.64 (d, J ) 13.4 Hz, 4 H, Ar-CH2-Ar), 4.43 (d,
J ) 13.2 Hz, 4 H, Ar-CH2-Ar), 4.86 (s, 4 H, O-CH2), 7.11 (s, 4 H,
ArH), 7.19 (s, 4 H, ArH), 7.62 (t, J ) 1.5 Hz, 4 H, ArH), 7.67 (t, J )
1.7 Hz, 2 H, ArH), 7.79 (d, J ) 1.6 Hz, 8 H, ArH), 7.83 (d, J ) 1.7
Hz, 4 H, ArH), 8.01 (d, J ) 8.5 Hz, 4 H, ArH), 8.12 (d, J ) 8.5 Hz,
4 H, ArH), 8.73 (s, 10 H), 8.77 (d, J ) 5 Hz, 4 H, Hâ), 8.97 (d, J )
5 Hz, 4 H, Hâ), 10.94 (s, 2 H, OH) ppm. UV-vis (toluene) λmax (nm),
ꢀ (mol-1 L cm-1): 417 nm (ꢀ 666 274), 540 (ꢀ 35 379).
5,11,17,23-Tetra-tert-butyl-25,27-di[methoxy(4-amidophenyl)-10,-
15,20-tris-(3,4,5-trimethoxyphenyl)porphyrin]-26-28-dihydroxycalix-
[4]arene, 6. A solution of 11,17,23-tetra-tert-butyl-25,27-dicar-
boxymethoxy-26,28-dihydroxycalix[4]arene (20.4 mg, 0.014 mmol) and
dicyclohexylcarbodiimide (21 mg, 0.10 mmol) in dichloromethane (5
mL) was stirred for 10 min. 5-(para-Aminophenyl)-[2,8,12,18-tetrabu-
tyl-3,7,13,17-tetramethyl-15-(3,5-di-tert-butylphenyl)]-21H,23H-por-
phyrin (51 mg, 0.059 mmol) in dichloromethane (2 mL) was added,
and the mixture was stirred overnight. The solvent was then removed,
and the residues were dissolved in chloroform (50 mL) and washed
with water (30 mL). Removal of solvent and flash chromatography
with CHCl3/heptane (1:1) gave porphyrin 6 (21 mg, 27%). FAB-MS:
Acknowledgment. We thank Dr. Dayong Sun for determining
mass spectra. This work was supported by the Marsden Fund
of the Royal Society of New Zealand (UOA0507), The
University of Auckland Research Committee, the National
Institutes of Health (GM 23851), and the Italian National
Research Council (commessa PM-P04-ISTM-C1-ISOF-M5).
Supporting Information Available: Details of the synthesis
and characterization of porphyrin and calix[4]arene precursors
to compounds 1-6. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA066031X
9
J. AM. CHEM. SOC. VOL. 128, NO. 49, 2006 15913