New strapped porphyrins as hosts for fullerenes: Synthesis and complexation study

Article abstract of DOI:10.1039/c1ob06643a

New strapped porphyrin-based hosts with different π-conjugated moieties and linkers have been prepared and their ability to bind with fullerenes was studied in dilute solution. We found that the ability of these hosts to bind with fullerenes strongly depe

Full text of DOI:10.1039/c1ob06643a

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PAPER
New strapped porphyrins as hosts for fullerenes: synthesis and complexation
study†
Jean-Benoˆıt Gigue`re and Jean-Franc¸ois Morin*
Received 27th September 2011, Accepted 25th October 2011
DOI: 10.1039/c1ob06643a
New strapped porphyrin-based hosts with different p-conjugated moieties and linkers have been
prepared and their ability to bind with fullerenes was studied in dilute solution. We found that the
ability of these hosts to bind with fullerenes strongly depends on their chemical nature and more
precisely on the substitution pattern of the porphyrin deck. As expected, the more electron-rich hosts
containing either an exTTF or a porphyrin unit as the strap bind fullerenes more efficiently with
association constants of up to 3.9 ¥ 105 M-1. The results clearly demonstrate the potential of such hosts
as a supramolecular scaffold for surface immobilization of pristine fullerenes.
in dilute solution for a guest made from porphyrin and/or
Introduction
exTTF.7d,8g,9
Fullerene C60 is considered as one of the most important building
The next logical step towards the utilization of pristine C60
blocks for the next generation of electronic devices.1 In addition to
in electronic applications is the creation of an organized, stable
being a promising material for non-volatile memory application,2
two-dimensional network of C60 on metallic or semi-conducting
C60 is also very useful as an n-type, electron-conducting material
surfaces by exploiting the recent advances in the supramolecular
for organic electronic devices such as field-effect transistors3 and
solar cells.4 The great interest of the scientific community towards
C60 comes from its triply degenerate LUMO energy level that
can accommodate up to six electrons upon reduction under
optimized conditions.5 However, to be processed in solution, C60
has to be made soluble to allow the formation of thin films or
monolayers. The most common way to enhance the solubility of
C60 is to attach organic moieties directly to the cage through
covalent bonds. Although very useful, this strategy leads to a
significant deterioration of the electronic properties and chemical
stability of C60 since one pair of p-electrons at the 6 : 6 junction is
consumed during the reaction. Therefore, new strategies enabling
the processability of pristine C60 need to be developed.
An increasingly popular method for processing pristine C60 is
to use supramolecular interactions between a p-conjugated host
and the C60 (guest).6 Among several hosts developed recently,
porphyrins7 and extended tetrathiafulvalene (exTTF)8 derivatives
have attracted a lot of attention. Porphyrins have been widely used
to complex with pristine C60 because of its high p-electron density
and low re-organization energy, while exTTF derivatives offer
excellent van der Waals contact with the C60 surface owing to its
curved conformation. After precise conformational optimization,
association constants of up to 108 M-1 have been obtained
chemistry of C60.1d To the best of our knowledge, no efficient
host fulfilling all the requirements for the surface immobilization
of pristine C60 has been developed thus far, although many
hosts for pristine C60 complexation in solution have been de-
veloped. Thus, we decided to address the challenge of surface
immobilization of C60 by preparing a new class of hosts with
strapped porphyrin architecture that could be further organized as
stable self-assembled monolayers on the surface. In recent studies,
Aida et al. showed that very high association constants can be
obtained when a porphyrin “strapped” by another porphyrin
is used as the host for fullerene complexation.7d,9 Although
many research groups reported on the preparation of strapped
porphyrins for different purposes, the Aida report is the only
one showing the potential of such a molecular architecture for
fullerene complexation. However, the use of various p-conjugated
moieties (other than porphyrin) as the strap has not been
accomplished and an exhaustive study on structure–property
relationships for strapped porphyrins as hosts for fullerenes is still
lacking.
As the first step towards the development of highly stable
supramolecular complexes on the surface, we report herein the syn-
thesis and complexation study of new strapped metalloporphyrin-
based hosts for fullerenes (Scheme 1). With this study, we sought
to optimize the structural parameters of the hosts in order to
increase the association constant between strapped porphyrins and
fullerenes. To achieve this goal, different p-conjugated moieties
have been used to strap the porphyrin unit and a complex-
ation study using both C60 and C70 was conducted in dilute
solution.
De´partement de Chimie et Centre de Recherche sur les Mate´riaux Avance´s
(CERMA), Pavillon A.-Vachon, 1045 Ave de la Me´decine, Universite´ Laval,
Que´bec, Canada, G1V 0A6. E-mail: jean-francois.morin@chm.ulaval.ca;
Fax: +1-418-656-7916; Tel: +1-418-656-2812
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1ob06643a
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Scheme 1 Synthesis of the hosts H1–H6.
rearrangement process, called scrambling,11 dramatically decreases
the yield and makes the purification step quite difficult. One way
to suppress the scrambling process is to use a sterically hindered
dipyrromethane.15 In our case, however, the functional group on
the aryl substituent used to induce steric hindrance has to be
carefully chosen since it must not preclude the introduction of
fullerenes inside the host’s pocket. For this reason, we chose
5-(2,6-dichlorophenyl)dipyrromethane (1)12 over the bulkier 5-
mesityldipyrromethane because the van der Waals radii of the
chloride atom is smaller than that of the methyl group and should
cause less steric hindrance towards further complexation with
fullerenes.
As shown in Scheme 2, the synthesis of meso substituted
trans-A2B2 porphyrins 2–4 was achieved via the condensation of
sterically hindered 5-(2,6-dichlorophenyl)dipyrromethane16 1 with
the corresponding aldehydes13 under the carefully controlled con-
ditions developed by Lindsey et al.15 giving minimal scrambling.
Subsequent metallation using zinc acetate afforded the porphyrin
building blocks 2a,b–4 in very good yields (40–51%) for porphyrin
formation. The incorporation of the zinc atom in the porphyrin
center simplified the purification and is necessary for the future
surface immobilization via axial coordination.
Results and discussion
Synthesis of strapped porphyrins
All the hosts, except H7, prepared for this study possess the same
meso-strapped porphyrinic building block, with tethers attached
to the meso-phenyl rings. To attach the straps, five different p-
conjugated moieties have been used, namely 2,6-anthraquinone
(H1 and H2), 2,6-naphthalene (H3), 2,6-triptycene (H4), 2,6-
exTTF (H5 and H7) and porphyrin (H6) (Scheme 1). Porphyrin
with a trans-A2B2-tetraaryl configuration was used as the scaffold
for the synthesis of the strapped porphyrin hosts. This scaffold
has been chosen because it is readily accessible via classical, non-
statistical porphyrin synthesis. In addition, it allows the straight-
forward preparation of a variety of hosts through the attachment
of different linkers and/or p-conjugated bridges directly to the
porphyrin scaffold. This post-functionalization approach greatly
simplifies the purification of the macrocyclic hosts compared to the
route involving the direct formation of strapped porphyrin via the
condensation of a bis-aldehyde-containing strap.10 Moreover, this
approach allows the introduction of chemically sensitive moieties
such as exTTF at the very end of the synthesis.
The acid-catalyzed rearrangement of 5-aryldipyrromethane
during the condensation with an aldehyde to form trans-A2B2
porphyrins can generate a complex mixture of isomers. The
With the porphyrin scaffold on hand, we undertook the
preparation of the p-conjugated bridges. We first chose to focus
our efforts on the preparation of 2,6-dihydroxy substituted
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catalytic amount of NaI and K2CO3 in DMF to afford compound
5 in 74% yield. The diol 5 was di-tosylated with tosyl chloride
and Et3N in CH2Cl2 to afford the bis-tosylated anthraquinone 6
in 94% yield.
The second building block of interest was prepared via a Diels–
Alder reaction between 2,6-dimethoxyanthraquinone,14 prepared
from the two-step reduction of 2,6-dimethoxyanthraflavic acid,
and an excess of benzyne precursor 2-carboxybenzenediazonium
chloride15 with propylene oxide as an acid scavenger. The
racemic C2-symmetric 2,6-dimethoxytriptycene derivative 8 was
thus obtained in 85% yield (Scheme 3). Deprotection of the
methoxy groups was achieved with BBr3 to afford the 2,6-
dihydroxytriptycene 9 in 70% yield.
Scheme 2 Synthesis of the mesoporphyrins.
With the porphyrins and aromatic moieties in hand, the next
step was to form the strapped porphyrin hosts (Scheme 1). The
macrocyclization reactions were performed under classical alkyla-
tion conditions using K2CO3 as a base in DMF under high dilution
conditions (ca. 2 mM) and the desired products were purified using
standard column chromatography on silica gel. First, the bis-(8-
bromooctyloxy)porphyrin 4 was reacted with anthraflavic acid to
afford the host H1 in 50% yield. Likewise, the anthraquinone
appended host H2 bearing the PEO chain was prepared from bis-
hydroxy porphyrin 2b and bis-tosylate anthraquinone 6 in 29%
yield. Similarly, the naphthalene appended host H3 was prepared
from porphyrin 4 and 2,6-dihydroxynaphthalene in 22% yield.
The same procedure was used to form the triptycene appended
host H4 in 41% yield from the corresponding building blocks 4
and 2,6-dihydroxytriptycene 9. The exTTF appended host H5 was
synthesized using 4 and reported 2,6-dihydroxy-exTTF16 10 in 23%
yield. Compound 10 is easily oxidizable and special care has been
taken to minimize its exposure to air.
Fig. 1 Energy-minimized structures of H5 and H7 complexes with C60
(Amber force field using HyperChemTM).
Analogous to Aida’s metalloporphyrin cyclic hosts,6a the
dimeric porphyrin host H6 bearing both free base and zinc
porphyrins was prepared from free base porphyrin 2a and
zinc porphyrin 3 in 12% yield. Following preliminary fullerene
complexation results (vide infra), we also prepared the less
sterically hindered host H7 (Scheme 4). In a similar fashion to the
aromatic moieties with alkyl or ethylene oxide chains as linkers.
The optimal length for the linkers was determined to be an
octyl chain according to geometry optimization performed using
molecular modeling (vide infra).
Commercially available anthraflavic acid is a useful precursor to
prepare a variety of 2,6-substituted aromatic moieties. As shown
in Scheme 3, the anthraquinone derivative bearing the ethylene
oxide strap 6 was synthesized in two steps. Anthraflavic acid was
first alkylated with 2-[2-(2-chloroethoxy)ethoxy]ethanol using a
Scheme 3 Synthesis of ethylene oxide chain-containing anthraquinone
and triptycene units.
Scheme 4 Synthesis of host H7.
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Table 1 Binding constant Ka (M-1) and selectivity factor (Ka,C /Ka,C
)
The next step in our investigation was to study the influence
of the p-conjugated moiety linked to the porphyrin unit. Thus,
we synthesized a host with a small p surface (naphthalene, H3)
and compared it with different p-conjugated moieties that were
expected to show better contact with C60 and C70. H3 shows
detectable association only with C60 (Ka = 5.0 ¥ 103 M-1) in
a mixture of toluene and acetonitrile. Quite surprisingly, no
association was observed with bigger fullerenes such as C70. The
steric hindrance caused by the presence of two chlorine atoms
pointing inside the host “pocket” is likely responsible for this lack
of association between H3 and C70. Moreover, the smaller length of
the naphthalene unit compared to the other p-conjugated moieties
used in this study could be responsible for a significant decrease
in the size of the pocket (shorter strap), thus reducing the affinity
of this particular host for bigger fullerenes like C70.
70
for complexation of C60 and C70 with hosts H1–H7 in different solvents60
b
b
Host Solventa
Ka C60 (M-1) Ka C70 (M-1) Ka,C /Ka,C
70
60
H1
H2
H3
H4
H5
H6
H7
PhMe
—
—
—
—
—
—
—
—
22 000
—
37 000
29 000
390 000
20 000
350 000
—
—
—
—
—
—
—
1.2
—
1.2
1.3
1.2
2.2
2.9
PhMe : ACN (1 : 1) 7000
PhMe
PhMe : ACN (1 : 1)
PhMe
PhMe : ACN (1 : 1) 5000
PhMe 1000
PhMe : ACN (1 : 1) 18 000
PhMe 4000
PhMe : ACN (1 : 1) 31 000
PhMe 23 000
PhMe : ACN (1 : 1) 315 000
PhMe 9000
PhMe : ACN (1 : 1) 120 000
—
—
—
In order to increase the Ka value, a host with a triptycene unit
(H4) was prepared. The triptycene unit was expected to show
improved interactions with fullerene due to its concave p surface
that complements the convex shape of fullerene. As predicted, H4
binds fullerenes more efficiently than H3 in a mixture of toluene
and acetonitrile with Ka values of 1.8 ¥ 104 M-1 for C60 and 2.2 ¥
104 M-1 for C70. Encouraged by these results, we decided to use
exTTF as the p-conjugated moiety (H5). In addition to having
a concave shape, this unit is more electron-rich than triptycene,
thus increasing the strength of the interaction between the host
and fullerenes. As a consequence of this structural improvement,
H5 exhibits higher Ka values in the toluene/acetonitrile mixture
for both C60 and C70 (3.1 ¥ 104 and 3.7 ¥ 104 M-1, respectively)
than H4. Finally, we measured the Ka values for H6, which is a
porphyrin strapped with another porphyrin. Surprisingly, the Ka
values in toluene/acetonitrile were one order of magnitude higher
than those measured for H5 (3.15 ¥ 105 for C60 and 3.9 ¥ 105 M-1 for
C70). This result confirms the great ability of porphyrin derivatives
to bind to C60 in a supramolecular manner.
a PhMe = toluene, ACN = acetonitrile. b UV-visible (298 K). The Ka values
were calculated by the nonlinear least-squares method.19
above-mentioned porphyrins, the alkyl substituted porphyrin 12
was prepared in 60% yield from the TFA catalyzed condensation
of 4,4¢-dimethyl-3,3¢-dibutyl-2,2¢-dipyrrylmethane17 11 with 3-(8-
bromooctyloxy)benzaldehyde. The analog of host H5 was synthe-
sized from porphyrin 12 and 2,6-dihydroxy-exTTF 10 under the
same macrocyclisation conditions to afford host H7 in 13% yield.
Complexation study
The ability of these hosts to bind fullerenes was evaluated by
measuring the association constant (Ka) between the hosts and two
different fullerenes, namely C60 and C70. It is well-known that C70
generally yields a better Ka value than C60 due to its egg shape and
flatter surface, allowing increased p contact with the hosts. The Ka
values have been calculated in dilute solution in pure toluene and in
a 1 : 1 mixture of toluene/acetonitrile. The results are summarized
in Table 1. The complexation between two different fullerenes (C60
and C70) and H1–H7 was studied using UV-visible spectroscopy by
monitoring the spectral changes at the Soret band (around 425 nm)
upon addition of a fullerene solution. In a typical experiment,
aliquots of a toluene solution of fullerene (7.5 ¥ 10-4 M) were
added to a solution of host (1.5 ¥ 10-6 M) in toluene or in an
acetonitrile/toluene mixture.
The first structural parameter studied was the chemical nature of
the flexible linker bridging the porphyrin unit and the p-conjugated
moiety (“strap”). For this study, we chose to compare the straight
alkyl chain and ethylene oxide chain in the anthraquinone-
porphyrin hosts, H1 and H2, respectively. Unfortunately, in both
cases, we were unable to measure the Ka value in toluene due to
the very weak signal lost in UV-visible spectra upon titration with
C60 and C70. However, when a mixture of toluene and acetonitrile
was used, a Ka value of 7.0 ¥ 103 M-1 was measured for H1 with
C60 while H2 exhibited very low binding ability for C60 and C70
(<1.0 ¥ 103 M-1). This difference suggests that straight alkyl chains
are more suitable than ethylene oxide ones as linkers in our host
design. This result is in disagreement with the results published
by Mart´ın et al., suggesting that n–p interactions can take place
between oxygen atoms and fullerenes.18 It should be noted that
Mart´ın et al. used crown ether and not short straight ethylene oxide
for their hosts, meaning that more oxygen atoms are involved in
such an interaction in their case.
A very interesting trend observed in this study is the poor
selectivity of all the hosts towards either C60 or C70. In most of
the reports published thus far, the hosts preferentially bind to
C70 over C60 due to its egg shape and flatter surface, which are
responsible for stronger interactions with the hosts. Even when
very flexible hosts are used, the selectivity factors (Ka,C /Ka,C
)
70
60
are generally much higher than 2.7h However, for the hosts H1–
H6, the selectivity factor is close to unity. Because this peculiarity
of our system is unlikely to be attributed to electronic effects,
we suspected the steric effect to be the cause of this poor
selectivity. As mentioned earlier and shown in Fig. 1, the two
chlorine atoms pointing inside the cavity of the host induce steric
hindrance that reduces the size of the pockets available for the
fullerenes. Obviously, this steric effect is expected to be more
detrimental to the complexation of C70. To confirm this hypothesis,
we synthesized a new strapped porphyrin (H7) by removing the
chlorine atoms on the meso-phenyl units and adding short alkyl
chains on the pyrrole units. exTTF was chosen as the p-conjugated
moiety. Because alkyl chains are more flexible, they should allow
C70 to enter the cavity more easily. Interestingly, H7 binds C60
and C70 more efficiently than H5 with a four-fold increase in the
Ka values when a mixture of toluene and acetonitrile was used.
This significant increase can be attributed to the small increase of
the pocket size, but also to the presence of alkyl chains that can
cause the porphyrin to bend and adopt a concave conformation
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due to steric hindrance (Fig. 1).6a The selectivity factor was also
increased from 1.2 to 2.9 following this simple modification, which
supported our initial hypothesis on steric hindrance induce by
chlorine atoms.
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Another peculiarity of H7 is its ability to bind with C60
only in a 1 : 1 configuration, but with C70 in both 1 : 1 and 2 : 1
configurations. As shown in Fig. 2, two isosbestic points at 423
and 442 nm appeared in the UV/vis spectrum when C70 was added
to a dilute solution of H7 in a mixture of toluene and acetonitrile.
The formation of both types of configuration was assessed using
Job plots, which shows a maximum between 0.5 and 0.6 (see ESI†).
This phenomenon was reported and studied in detail by Aida
et al. for similar macrocyclic hosts.20 The fact that a 2 : 1 complex
has been observed for C70 but not for C60 tends to demonstrate
that the alkyl chains we used to link the p-conjugated bridge and
the porphyrin deck are long enough for C60 complexation, but
a bit too short to accommodate the bigger C70. Hence, the short
strap forces the host to adopt a more open conformation, leaving
one side of the C70 available for further complexation by another
host molecule.
5 (a) R. C. Haddon, L. E. Brus and K. Raghavachari, Chem. Phys. Lett.,
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E. Ort´ı and N. Mart´ın, Angew. Chem., Int. Ed., 2007, 46, 1847; (c) E.
M. Pe´rez, A. L. Capodilupo, G. Ferna´ndez, L. Sa´nchez, P. M. Viruela,
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Fig. 2 (a) UV-visible spectral changes of H7 upon addition of C70 in a
toluene/acetonitrile (1 : 1) mixture at 298 K. Inset: Plot of the UV-visible
changes at 416 nm.
Conclusions
In summary, we prepared seven strapped porphyrin-based hosts
for complexation of fullerenes. As the geometrical complemen-
tarity between the host and the fullerenes (concave/convex
interaction) and the electronic density of the host increase, the
association constants increased to a value of up to 3.9 ¥ 105 M-1.
We also found that the substitution pattern of the porphyrin deck
was an important parameter to take into account in the host
design. The two best hosts, H6 and H7, are now assembled onto
a gold surface and their ability to bind with pristine and reduced
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∑
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20 A. Ouchi, K. Tashiro, K. Yamaguchi, T. Tsuchiya, T. Akasaka and T.
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