Cyclic metalloporphyrin dimers and tetramers: Tunable shape-selective hosts for fullerenes

Article abstract of DOI:10.1039/c2dt31126g

Covalently linked cyclic metalloporphyrin dimers and tetramers have been demonstrated to be good shape-selective hosts for fullerene guests. The fullerene affinities of these hosts can readily be tuned by modulating the covalent linkage and the metal ions in the porphyrin subunits. A rigid Zn(porphyrin) dimer with conjugated bis(alkynyl) linkers exhibits a high selectivity towards C₇₀ over C₆₀ in toluene (K _a,C70/K_a,C60 = ～28). For the host structures examined, a synergistic combination of rigidity in the linker and electropositive Al ions gives rise to the strongest binding of C₇₀. In the case of a bisected Zn(porphyrin) tetramer, two well-defined cavities exist; however, due to their comparatively small size, only one C₆₀ can be accommodated. Studies of fullerene binding as a function of metal ion in a porphyrin divider suggest that the right combination of shape and steric match is essential to exploit both van der Waals and local-charge/induced-dipole interactions.

Full text of DOI:10.1039/c2dt31126g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2012, 41, 12156
www.rsc.org/dalton
PAPER
Cyclic metalloporphyrin dimers and tetramers: tunable shape-selective hosts
for fullerenes†
Byungman Kang,‡ Ryan K. Totten,‡ Mitchell H. Weston, Joseph T. Hupp and SonBinh T. Nguyen*
Received 25th May 2012, Accepted 4th August 2012
DOI: 10.1039/c2dt31126g
Covalently linked cyclic metalloporphyrin dimers and tetramers have been demonstrated to be good
shape-selective hosts for fullerene guests. The fullerene affinities of these hosts can readily be tuned by
modulating the covalent linkage and the metal ions in the porphyrin subunits. A rigid Zn(porphyrin)
dimer with conjugated bis(alkynyl) linkers exhibits a high selectivity towards C70 over C60 in toluene
(Ka,C70/Ka,C60 = ∼28). For the host structures examined, a synergistic combination of rigidity in the
linker and electropositive Al ions gives rise to the strongest binding of C70. In the case of a bisected
Zn(porphyrin) tetramer, two well-defined cavities exist; however, due to their comparatively small size,
only one C60 can be accommodated. Studies of fullerene binding as a function of metal ion in a porphyrin
divider suggest that the right combination of shape and steric match is essential to exploit both
van der Waals and local-charge/induced-dipole interactions.
rigidity of cyclic di- and tetrameric porphyrin hosts as well as
their metal sites.
Introduction
The tetrapyrrole unit in porphyrins is well-known to interact
favorably with the curved surfaces of fullerenes, both in solution
and in the solid state, via a combination of π–π and van der
Waals interactions.1–5 These interactions have been exploited
in the design of a variety of porphyrin-based hosts for
fullerenes,3,6–9 among which covalently linked cyclic porphyrin
dimers and trimers have been reported to give association con-
stants as high as ∼108 M−1 for C60 and C70.10–13 In these con-
structs, the electron-rich walls of multiporphyrin hosts and their
close contacts with fullerenes were described as the key factors
that drive strong intermolecular interactions. Furthermore, it was
found that the metal center of a metalloporphyrin can also inter-
act with the fullerene in an attractive fashion. For instance,
varying the metal center in a series of cyclic metalloporphyrin
dimers can modulate their association constants with fullerenes
by more than two orders of magnitude.10 While several studies
have been carried out to elucidate the effect of the metal species
in the porphyrin core toward fullerene binding,10,14 the shape
selectivity for fullerene isomers by multiporphyrin hosts has not
been examined in a systematic fashion.14–17 We hypothesized
that the selective binding of fullerenes with different shapes and
sizes can be achieved by modulating the cavity shape and
To evaluate the aforementioned hypothesis, we chose cofacial
porphyrin dimers A, covalently connected via alkynyl, alkenyl,
or alkyl groups, whose porphyrin cores can be easily modified
with electropositive metal ions (Fig. 1a).18 Such dimers have
been shown by us to be capable of complexing a large p-nitro-
phenyl diphenyl phosphate (PNPDPP) (C60 (∼10 Å) < PNPDPP
(∼12 Å) < C70 (∼13 Å)). We were further intrigued by the possi-
bility of achieving a 2 : 1 guest : host binding stoichiometry
inside a bisected Zn(porphyrin) tetramer (B, Fig. 1b), especially
because the large cavity of this host has recently been shown to
Department of Chemistry and International Institute for
Nanotechnology, Northwestern University, 2145 Sheridan Road,
Evanston, IL 60208, USA. E-mail: stn@northwestern.edu;
Fax: +1 847 491-7713; Tel: +1 847-467-3347
†Electronic supplementary information (ESI) available: Complete
experimental details of syntheses; compound characterization data and
spectra; UV-vis/fluorescence titration experimental methods, spectra, and
data analysis. See DOI: 10.1039/c2dt31126g
Fig. 1 Schematic representation of selective fullerene binding by co-
facial porphyrin dimers possessing linkers with tunable rigidity and
bisected Zn(porphyrin) tetramers.
‡Authors contributed equally.
12156 | Dalton Trans., 2012, 41, 12156–12162
This journal is © The Royal Society of Chemistry 2012

View Article Online
complex up to two PNPDPP guests.19 By incorporating a trans-
10,20-bis(4-pyridyl)porphyrin “divider”, this large cavity can be
partitioned into two “conjoined” cofacial porphyrin hosts poten-
tially capable of encapsulating two C60 guests. We reasoned that
such a bisected porphyrin tetramer, with smaller cavity sizes and
more rigid “walls” than the aforementioned dimers, would have
better interactions with C60. Along with shape optimization for
C60 binding, the metal site in the porphyrin divider can be
readily tuned to allow us to determine the effect that different
metalloporphyrins have on the affinity for C60 by the bisected
tetramers.
or trihexylsilyl substituents in the meso positions of the
porphyrin building block allows these dimers and tetramers to
maintain solubility in common solvents such as methylene
chloride, tetrahydrofuran, and toluene. An additional potential
advantage of using these substituents is that they could
further engage in CH–π interactions with bound fullerene to
increase the van der Waals contacts in the inclusion complex.22
Finally, the delocalization of the porphyrin electron density
into a silyl-protected acetylene would increase the positive
character of the metal center of the porphyrin, further strengthen-
ing any local-charge/induced-dipole interaction with fullerene
species.18
Results and discussion
The binding of C60 and C70 within the cavity of cyclic
diporphyrin hosts
In designing a flexible, cofacial porphyrin dimer host (A) that
can encapsulate fullerenes in a shape-selective fashion, we were
interested in exploring secondary interactions between the
linkers of A and the fullerene guest. In particular, tuning the
structural rigidity of these linkers from purely alkyl to alkenyl,
and then conjugated diynyl, may result in pre-organization of the
host towards a specific fullerene shape, leading to optimal selec-
tivity. Moreover, the inclusion of π electrons into the hydro-
carbon linkers could potentially enhance the binding through
additional π–π interactions between the host and the fullerene.
In addition to rigidifying the porphyrin cavity towards differ-
ent fullerene guests, we are also interested in investigating the
effect that electropositive metal ions in the core of the porphyrin
may have towards fullerene binding. For instance, a cyclic IrIII-
(porphyrin) dimer has been shown to give the highest reported
binding constant (>109 M−1 in C6H6) to C60 to date.11 Such
strong binding has been attributed to the formation of an η2
complex between the 6 : 6 ring-juncture double bond of the
fullerenes and the IrIII metal center, which is strong enough to
override the side-on mode normally associated with the van der
Waals interactions between the host and C70 and “fix” the latter
into an end-on mode via strong covalent bonds to the IrIII-
(porphyrin) panels.11 As we are interested in obtaining porphyrin
hosts that can strongly bind fullerenes through purely non-
covalent interactions, we reasoned that local-charge/induced-
dipole interactions between the highly polarizable fullerenes and
electropositive metalloporphyrins would lead to large association
constants without the need for covalent bond formation. In such
cases, the strength of this interaction increases with the charge of
the metal ion and polarizability of the guest molecule. Given the
formal +3 charge of AlIII, and its inability to form η2 complexes
with olefins, porphyrin-embedded AlIII ions can be expected to
induce significant dipoles with proximal C60 and C70, which
have average polarizabilities (α) of ∼80 and ∼100 Å3, respect-
ively (cf. α(C6H6) = ∼10 Å3).20,21 For comparison, the analo-
gous Zn(porphyrin) host is not expected to bind fullerene as
strongly given its +2 charge (which will be partially screened by
the anionic porphyrin) and its softer ionic character.
The binding stoichiometry of C60 within the cavity of a saturated
Zn(porphyrin) dimer A1 was evaluated using UV-vis spectro-
scopy and the continuous variation method.23 As expected, a
Job plot, obtained by monitoring the Soret absorption of A1
(0.5 × 10−6 M) upon addition of C60 in toluene, indicated the
formation of a 1 : 1 complex (Fig. 3). To corroborate the com-
plexation between fullerene and A1, we also employed variable-
temperature (VT) 13C NMR spectroscopy.8,10 A 3 : 1 mixture of
13C-enriched C60 (2.4 × 10−3 M) and A1 (0.8 × 10−3 M) in
toluene-d8 shows a single resonance (142.0 ppm) for C60 at
296 K due to the time averaging of the signals for bound
and unbound C60 (Fig. 4b). Upon cooling, this peak broadens
and splits into separate resonances for bound (139.8 ppm) and
unbound (142.7 ppm) C60 in a ratio of 2 : 1 at 183 K (Fig. 4e).
The observed upfield shift of C60 in the bound state is attributed
to the shielding effect of the electron-rich π system of the por-
phyrin ring on the fullerene.8
Notably, the 1 : 1 adduct of fullerene (C60 or C70) with A1
was found to remain intact as the major molecular ion (Fig. 3)
when the adduct was subjected to analysis by matrix-assisted
laser desorption ionization time-of-flight (MALDI-ToF) mass
spectrometry. Upon ionization, the MALDI-ToF spectrum of a
1 : 2 mixture of A1 (0.5 × 10−3 M) and C60 (1.0 × 10−3 M) in
toluene exhibited a single peak at m/z 2808.77 ([A1·C60]+),
suggesting that the 1 : 1 inclusion complex is the only one
present (Fig. 5a). Similarly, MALDI-ToF mass analysis of a
mixture of A1 and C70 revealed a strong molecular-ion peak at
m/z 2927.99 for the 1 : 1 complexed material (Fig. 5b). Assum-
ing that the ionization cross-sections of A1·C60 and A1·C70 are
the same, A1 appears to have a higher preference for C70: the
MALDI-ToF mass spectrum of a mixture of A1 : C60 : C70
(1 : 2 : 2) only exhibits a signal for the inclusion complex A1·C70
(Fig. 5c). Along with UV-vis and VT 13C NMR spectroscopies,
this demonstrates the absence of concentration-dependent effects
on the function of the multiporphyrin host.
Cofacial porphyrin dimers A and bisected Zn(porphyrin)
tetramers B (unsaturated or saturated (Zn-PP)4·(M-DPyDPP))
used in the present work are shown in Fig. 2 (M-DPyDPP =
The shape-selective binding of C60 and C70 with cofacial
porphyrin dimers
5,15-bis(4-pyridyl)-10,20-diphenylmetallo-porphyrin).
Synth-
The ability of cofacial porphyrin dimers A1–A6 to bind fuller-
enes was evaluated by measuring the respective association con-
stants (Ka) using UV-vis absorption spectroscopy. In a typical
eses of these cyclic porphyrin assemblies have recently been
reported by us.18,19 The incorporation of 3,5-di-tert-butylphenyl
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 12156–12162 | 12157

View Article Online
Fig. 2 Molecular structures of the cofacial porphyrin dimers A1–A6 and the bisected porphyrin tetramers B1–B7.
experiment, a solution of A1 (0.5 × 10−6 M in toluene) was
titrated with small aliquots of a solution of C60 or C70 in toluene
(to a total of 80 or 7 molar equiv, respectively; see section V in
the ESI†). An increase in the concentration of C60 produces an
appreciable decrease in absorption in the region of the Soret
band, along with the appearance of two isosbestic points at 414
and 429 nm (Fig. 6a). A similar trend in the UV-vis titration
curve was also observed when a solution of A1 in toluene was
titrated with C70 (Fig. 6b). Incremental addition of C70 caused a
modest bathochromic shift with a concomitant decrease of the
Soret band intensity. On the basis of the 1 : 1 binding mode and
the nonlinear fit of the adsorption titration data,23 Ka values of
C60 and C70 with A1 were estimated to be ∼3.3 × 104 M−1 and
∼7.0 × 105 M−1, respectively.
be attributed to a combination of larger size, elongated shape,
and “flat” surface, that allow for better overall van der Waals
contact with the porphyrin units.1,10 This stronger binding of C70
was further evidenced by the aforementioned MALDI-ToF MS
experiment where a 1 : 2 : 2 mixture of A1 : C60 : C70 only shows
a signal for A1·C70 (Fig. 5c). Notably, the preferential binding of
C70 is greater in A3, which has rigid 3,5-octadiyn-1,8-diyl
linkers, than in A1, which has flexible 1,8-octadiyl ones (Ka,C70
/
Ka,C60 = ∼28 for A3 and Ka,C70/Ka,C60 = ∼21 for A1, respect-
ively). We note that such selectivity for C70 over C60 are among
the highest reported for Zn(porphyrin) dimer hosts (Ka,C70/Ka,C60
= 2–25).7,15,22,24 The conformational rigidity in A3, together
with the good shape-complementarity between the porphyrin
host and the oval-shaped C70 guest, leads to a maximization of
van der Waals contacts as indicated by a ∼1.7-fold increase in
As shown in Table 1, Zn(porphyrin) dimers A1, A2, and A3
all have greater affinities for C70 than C60. The differences can
K
a,C70 for A3 over A1 (Table 1, cf. entries 1 and 3).
12158 | Dalton Trans., 2012, 41, 12156–12162
This journal is © The Royal Society of Chemistry 2012

View Article Online
Fig. 3 UV-vis absorption spectra of A1 at different mole fraction of
60. Inset: Job plot for the determination of the binding stoichiometry
C
between A1 and C60 in toluene.
Fig. 5 MALDI-ToF mass spectra for the mixtures containing: (a) A1
and C60 (2 equiv) in toluene, (b) A1 and C70 (2 equiv) in toluene, (c) A1
in the presence of C60 and C70 (2 equiv each) in toluene.
Fig. 4 Variable-temperature 13C NMR spectra of 13C-enriched C60 in
the (a) absence or (b–e) presence of A1 (0.33 equiv) in toluene-d8.
Given the known attractive π–π interactions between fullerenes
and porphyrins,3,25 we reasoned that dimer A5, with its electro-
positive AlIII centers, would have additional and stronger
localized-charge/induced-dipole interactions with fullerenes
compared to the analogous Zn(porphyrin) dimer A1 and free-
base dimer A4. Indeed, the binding affinity of A5 for C60
(Fig. 7a) was roughly two orders of magnitude stronger than
those observed for A1 and A4 (Ka,C60 for A5/Ka,C60 for A1 =
112 and Ka,C60 for A5/Ka,C60 for A4 = 86, Table 1). For C70
(Fig. 7b), A5 again exhibited a high affinity in toluene (∼8.1 ×
106 M−1), albeit only a ∼12-fold increase from the Ka,C70
observed for A1. Further supporting the existence of local-
charge/induced-dipole interactions between the electropositive
AlIII ions and fullerenes is the similarity in the binding affinities
of A5 for C70 over C60: that the Ka,C70/Ka,C60 ratio for A5 is only
∼2 compared to ∼20 for that of A1 suggests that these inter-
actions are strong enough to override the shape preference of this
dimer host for C70.
Fig. 6 Absorption spectra of A1 (0.5 × 10−6 M) in toluene at 296 K
upon titration with (a) C60 and (b) C70. Insets: the absorption changes at
422 nm and the result of fitting the experimental data.
hypothesized that this rigidity may influence and/or enhance
both the selectivity and binding affinity of A5 towards C70.
Indeed, Al(porphyrin) dimer A6, with 3,5-octadiyn-1,8-diyl
Since the conformational rigidity imparted by the 3,5-octa-
diyn-1,8-diyl linkers renders A3 to bind C70 more favorably, we
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 12156–12162 | 12159

View Article Online
Table 1 Association constants of cofacial porphyrin dimers with C60
and C70 in toluenea
Association constant (Ka, M−1
)
Cyclic porphyrin dimer
C60
C70
A1
A2
A3
A4
A5
A6
(3.3 0.4) × 104
(5.0 0.9) × 104
(4.3 0.3) × 104
(4.5 0.3) × 104
(3.7 0.6) × 106
(2.8 0.1) × 106
(7.0 0.2) × 105
(6.9 0.1) × 105
(1.2 0.5) × 106
(6.3 0.1) × 105
(8.1 1.9) × 106
(1.4 0.7) × 107
a All association constants were measured by UV-vis titration at 296 K.
Fig. 8 1H NMR spectra of (a) unsaturated (Zn-PP)4 (1.3 mM), (b)
H2-DPyDPP divider (1.3 mM), and (c) a 1 : 1 mixture of unsaturated
(Zn-PP)4 (1.3 mM) and H2-DPyDPP (1.3 mM). Spectra a–c were
obtained in toluene-d8 at 296 K.
Fig. 9 UV-vis absorption spectra of B1 at different mole fraction of
C
60. Inset: Job plot for the determination of the binding stoichiometry
between B1 and C60 in toluene.
tetramer assembly and could be used to partition the latter into
two conjoined cavities.19 The H NMR spectrum of assembly
1
Fig. 7 Absorption spectra of A5 (0.5 × 10−6 M) in toluene at 296 K
upon titration with (a) C60 and (b) C70. Insets: the absorption changes at
406 nm and the result of fitting the experimental data.
B1 ((Zn-PP)4·(H2-DPyDPP)) differs significantly from those for
unsaturated (Zn-PP)4 and H2-DPyDPP divider (Fig. 8): the
pyridyl α- and β-protons of the encapsulated H2-DPyDPP dis-
played substantial upfield shifts (3.16 and 5.36 ppm) from those
observed for free H2-DPyDPP (8.88 and 8.09 ppm) (cf. Fig. 5b
and c), indicative of the strongly shielded environment due to
the coordination of the pyridyl group to Zn(porphyrin)
centers.26,27
To determine the stoichiometry for encapsulation of C60 by
the bisected cavity of B, a Job plot was constructed from the
UV-vis absorption data of a mixture of B1 and C60 at constant
total concentration (0.5 × 10−6 M in toluene).23 Unexpectedly, as
can be seen in the inset of Fig. 9, a maximum is observed at a
mole fraction value of only 0.5, indicating a 1 : 1 stoichiometry
for the tetramer : fullerene complex. This 1 : 1 binding mode
linkers, afforded the highest Ka for C70 (1.4 × 107 M−1) as com-
pared to the aforementioned dimers (Table 1). The C70/C60 selec-
tivity factor was also increased from ∼2 to ∼5 following this
modification, supporting our claim that a synergistic effect
results from the combination of AlIII ion and rigid hydrocarbon
linkers in A6.
The binding of C60 with bisected Zn(porphyrin) tetramers
As reported previously, 5,15-bis(4-pyridyl)-10,20-diphenylpor-
phine (H2-DPyDPP) can be completely complexed by (Zn-PP)4
12160 | Dalton Trans., 2012, 41, 12156–12162
This journal is © The Royal Society of Chemistry 2012

View Article Online
Table 2 Association constants of bisected Zn(porphyrin) tetramers
B1–B6 with C60 in toluenea
Bisected Zn(porphyrin)
tetramer
Association constant
(Ka, M−1
Divider
)
B1
B2
B3
B4
B5
B6
H2-DPyDPP
(4.6 1.7) × 104
(7.5 2.0) × 104
(7.0 1.9) × 104
(6.7 1.9) × 104
(6.3 2.2) × 104
(4.4 1.5) × 104
ZnII-DPyDPP
CuII-DPyDPP
CoII-DPyDPP
Scheme 1 Schematic description of the 1 : 1 binding mode between
the bisected Zn(porphyrin) tetramer and C60
MeO-AlIII-DPyDPP
Me-RhIII-DPyDPP
.
a All association constants were measured by UV-vis titration at 296 K.
enhanced relative to that of B1, which features a free-base H2-
DPyDPP divider. Interestingly, B5 and B6, which contain AlIII
and RhIII ions, respectively, do not show any increase in Ka,C60
in comparison to that of B1. This insensivity to the metallopor-
phyrin divider confirms our early suspicion about the highly
restrictive steric environment of the cavity, which eliminates any
potential metal-dependent effects. That is, when the cavity is so
restricted such that van der Waals interactions become the
primary factor dictating fullerene binding, the tight fit of the
guest inside the host would render impotent any possible attrac-
tive local-charge/induced-dipole interactions between the fuller-
ene and the metalloporphyrin walls.
To confirm the aforementioned suspicion that (Zn-PP)4·
(M-DPyDPP) is too restrictive, we synthesized a saturated (Zn-
PP)4 tetramer in which the unsaturated 4-octen-1,8-diyl linkers
in the unsaturated (Zn-PP)4 were reduced to flexible 1,8-octadiyl
linkers of approximately equal length (see section IV in the
ESI†). As expected, based on the UV-vis titration of saturated
(Zn-PP)4·(MeO-AlIII-DPyDPP) (B7) with C60 in toluene (see
Fig. S20 in the ESI†), the affinity of B7 for C60 is ∼2.4-fold
greater than that of the unsaturated B5 (Ka,C60 = ∼1.5 × 105 M−1
for B7 and ∼6.3 × 104 M−1 for B5). This encouraging result
indicates the important contribution of a flexible cavity such as
B7 in stabilizing C60: C60 is better bound when the cavity
between the MeO-AlIII-DPyDPP divider and the Zn(porphyrin)
wall is not as conformationally restricted. Unfortunately, no
association of B7 with the larger fullerene was detected,
suggesting that the partitioned cavity in B7 is still too small.
Fig. 10 Absorption spectra of B1 (0.5 × 10−6 M) in toluene at 296 K
upon titration with C60. Inset: the absorption changes at 442 nm and the
result of fitting the experimental data.
suggests that the binding of a second C60 to the remaining
vacant conjoined cavity is severely inhibited by the tight fitting
of one C60 into the first cavity. Presumably, complexation leads
to a slight expansion of the C60-encapsulated site, making the
second cavity smaller and unable to accommodate another C60
(Scheme 1).
The titration experiment of B1 (0.5 × 10−6 M in toluene)
against C60 revealed that the Soret band of the metalloporphyrin
became hypochromic with two isosbestic points appearing at
425 and 460 nm (Fig. 10). These spectral characteristics coincide
with our aforementioned findings for cofacial porphyrin dimer–
fullerene complexation, indicating a close contact between the
porphyrin moieties in B1 and C60. Assuming a 1 : 1 stoichio-
metry, the nonlinear least-square fitting of the titration data
afforded a Ka value of 4.6 × 104 M−1 for C60 in toluene. In con-
trast, the UV-vis titration of C70 into B1 showed no absorption
changes in its Soret and Q bands, suggesting that the partitioned
cavity in B1 is too small and rigid to associate with C70. Com-
pared to the larger cavity of A1 (up to 15.6 Å, see section VII in
the ESI†), which is able to accommodate both C60 and C70, the
smaller dimensions of the bisected cavity B1 (∼11.1 to 13.2 Å)
provide a tight fit only for C60.
To assess the effect of a metalloporphyrin divider on the C60
binding affinity of bisected tetramer B, we synthesized several
M-DPyDPP dividers and inserted them into hollow (Zn-PP)4 to
generate new bisected cavities B2–B6. From the observed UV-
vis absorption spectral profiles of these assemblies upon titration
with C60, Ka,C60 values were obtained. As shown in Table 2, the
fullerene affinities of B2–B6, possessing M-DPyDPP complexes
of first-row transition metals (ZnII, CuII, and CoII), are slightly
Conclusions
In summary, we have demonstrated that matching the cavity of a
porphyrin host to the shape of a fullerene guest, such as the case
of our “rectangular” dimer to the ovoid C70, can notably enhance
the selective binding of the fullerene. Increasing the rigidity of
the linkages in the host by replacing flexible octyl chains with
more rigid 4-ethenyl and 3,5-octadiynyl ones can further
enhance the interaction between the cavity and the guest. When
the match is most optimal, further metal interactions can be
beneficial: the high association constant of Al(porphyrin) dimers
A5 and A6 with fullerenes indicates a strong interaction between
the electropositive AlIII centers in the porphyrin walls and the
highly polarizable fullerenes.
In the case of the bisected Zn(porphyrin) tetramer B1–B6
with 4-octen-1,8-diyl linkages, only one of the two well-defined
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 12156–12162 | 12161

View Article Online
9 K. Tashiro, T. Aida, J. Y. Zheng, K. Kinbara, K. Saigo, S. Sakamoto and
K. Yamaguchi, J. Am. Chem. Soc., 1999, 121, 9477–9478.
10 J.-Y. Zheng, K. Tashiro, Y. Hirabayashi, K. Kinbara, K. Saigo, T. Aida,
S. Sakamoto and K. Yamaguchi, Angew. Chem., Int. Ed., 2001, 40,
1858–1861.
11 M. Yanagisawa, K. Tashiro, M. Yamasaki and T. Aida, J. Am. Chem.
Soc., 2007, 129, 11912–11913.
12 G. Gil-Ramírez, S. D. Karlen, A. Shundo, K. Porfyrakis, Y. Ito,
G. A. D. Briggs, J. J. L. Morton and H. L. Anderson, Org. Lett., 2010,
12, 3544–3547.
conjoined cavities is able to accommodate C60 due to the overall
small size of the parent cavity. Together with the lack of binding
by C70, the negligible effects that different metalloporphyrin
dividers have on the binding affinity for C60 clearly point to the
importance of having the right combination of shape and steric
match if one is to take advantage of both van der Waals and
local-charge/induced-dipole interactions. In such cases, making
the cavity flexible enough so that both effects can be utilized
should result in the best binding constant possible.
13 A. R. Mulholland, C. P. Woodward and S. J. Langford, Chem. Commun.,
2011, 47, 1494–1496.
14 D. Sun, F. S. Tham, C. A. Reed, L. Chaker and P. D. W. Boyd, J. Am.
Chem. Soc., 2002, 124, 6604–6612.
15 L. P. Hernández-Eguía, E. C. Escudero-Adán, I. C. Pintre, B. Ventura,
L. Flamigni and P. Ballester, Chem.–Eur. J., 2011, 17, 14564–14577.
16 Y. Shoji, K. Tashiro and T. Aida, J. Am. Chem. Soc., 2010, 132, 5928–
5929.
17 Y. Shoji, K. Tashiro and T. Aida, J. Am. Chem. Soc., 2006, 128, 10690–
10691.
18 R. K. Totten, P. Ryan, B. Kang, S. J. Lee, L. J. Broadbelt, R. Q. Snurr,
J. T. Hupp and S. T. Nguyen, Chem. Commun., 2012, 48, 4178–4180.
19 B. Kang, J. W. Kurutz, K.-T. Youm, R. K. Totten, J. T. Hupp and
S. T. Nguyen, Chem. Sci., 2012, 3, 1938–1944.
Acknowledgements
This material is based upon research sponsored by the AFOSR
under agreement FA-9550-07-1-0534. We acknowledge DTRA
(grant HDTRA1-10-1-0023) and the DoE (grant DE-FG02-
03ER15457) for additional financial support. We thank Professor
George C. Schatz for his insightful discussions.
20 I. Compagnon, R. Antoine, M. Broyer, P. Dugourd, J. Lermé and
D. Rayane, Phys. Rev. A: At., Mol., Opt. Phys., 2001, 64, No. 025201.
21 K. J. Miller and J. A. Savchik, J. Am. Chem. Soc., 1979, 101,
7206–7213.
22 A. Hosseini, S. Taylor, G. Accorsi, N. Armaroli, C. A. Reed and
P. D. W. Boyd, J. Am. Chem. Soc., 2006, 128, 15903–15913.
23 K. Hirose, J. Inclusion Phenom. Macrocyclic Chem., 2001, 39,
193–209.
24 Y. Shoji, K. Tashiro and T. Aida, J. Am. Chem. Soc., 2004, 126,
6570–6571.
25 Y. Sun, T. Drovetskaya, R. D. Bolskar, R. Bau, P. D. W. Boyd and
C. A. Reed, J. Org. Chem., 1997, 62, 3642–3649.
Notes and references
1 P. D. W. Boyd, M. C. Hodgson, C. E. F. Rickard, A. G. Oliver,
L. Chaker, P. J. Brothers, R. D. Bolskar, F. S. Tham and C. A. Reed,
J. Am. Chem. Soc., 1999, 121, 10487–10495.
2 E. M. Pérez and N. Martín, Chem. Soc. Rev., 2008, 37, 1512–1519.
3 P. D. W. Boyd and C. A. Reed, Acc. Chem. Res., 2005, 38, 235–242.
4 C. M. Drain, A. Varotto and I. Radivojevic, Chem. Rev., 2009, 109,
1630–1658.
5 L. P. Hernández-Eguía, E. C. Escudero-Adán, J. R. Pinzón, L. Echegoyen
and P. Ballester, J. Org. Chem., 2011, 76, 3258–3265.
6 K. Tashiro and T. Aida, Chem. Soc. Rev., 2007, 36, 189–197.
7 M. Dudič, P. Lhoták, I. Stibor, H. Petříčkova and K. Lang, New J. Chem.,
2004, 28, 85–90.
26 S. Anderson, H. L. Anderson, A. Bashall, M. McPartlin and
J. K. M. Sanders, Angew. Chem., Int. Ed., 1995, 34, 1096–1099.
27 E. B. Fleischer and A. M. Shachter, Inorg. Chem., 1991, 30,
3763–3769.
8 D. Sun, F. S. Tham, C. A. Reed, L. Chaker, M. Burgess and
P. D. W. Boyd, J. Am. Chem. Soc., 2000, 122, 10704–10705.
12162 | Dalton Trans., 2012, 41, 12156–12162
This journal is © The Royal Society of Chemistry 2012