A cyclic porphyrin trimer as a receptor for fullerenes

Article abstract of DOI:10.1021/ol101393h

(Equation Presented). A cyclic porphyrin trimer has been synthesized which has a high affinity for fullerenes. It forms 1:1 complexes with C₆₀ and C₇₀ with association constants of 2×10⁶ and 2×10⁸ M^-1, respectively, in toluene. Its affinities for C₈₆ and La@C₈₂ are too strong to measure by fluorescence titration. The solvent dependence of the association constants shows that solvation of both the guest and the host influence the binding strength.

Full text of DOI:10.1021/ol101393h

ORGANIC
LETTERS
2010
Vol. 12, No. 15
3544-3547
A Cyclic Porphyrin Trimer as a Receptor
for Fullerenes
Guzma´n Gil-Ram´ırez,†,‡ Steven D. Karlen,†,‡ Atsuomi Shundo,†
Kyriakos Porfyrakis,‡ Yasuhiro Ito,‡ G. Andrew D. Briggs,‡ John J. L. Morton,‡
and Harry L. Anderson*,†
Chemistry Research Laboratory, Department of Chemistry, UniVersity of Oxford,
12 Mansfield Road, Oxford OX1 3TA, U.K., and Department of Materials, UniVersity
of Oxford, Parks Road, Oxford OX1 3PH, U.K.
harry.anderson@chem.ox.ac.uk
Received June 16, 2010
ABSTRACT
A cyclic porphyrin trimer has been synthesized which has a high affinity for fullerenes. It forms 1:1 complexes with C60 and C70 with association
constants of 2 × 106 and 2 × 108 M-1, respectively, in toluene. Its affinities for C86 and La@C82 are too strong to measure by fluorescence
titration. The solvent dependence of the association constants shows that solvation of both the guest and the host influence the binding
strength.
Paramagnetic endohedral fullerenes are well-known for their
long coherence times (T2); for example, N@C60 has a T2 of
300 µs, which is an order of magnitude longer than those of
other molecular radicals.1 This property makes them promis-
ing qubits for quantum information processing.2 In order to
develop materials for this application, we need a way of
holding several endohedral fullerene units in precise spatial
arrangements, so that the entanglement between their spins
can be manipulated. A molecular framework functionalized
with noncovalent fullerene receptors would be the ideal
scaffold because it could bind the fullerene quantitatively at
the final stage of the synthesis, while scarcely perturbing
the environment around the endohedral paramagnetic atom.
Such receptors will need to have high affinities for fullerenes
to achieve quantitative binding in dilute solution. Here we
report the synthesis of a receptor designed for this applica-
tion. A single phthalimide connection point is rigidly attached
to the receptor, so that in the future several such receptors
can be linked together to create multispin arrays with precise
geometries. This receptor has a high binding affinity for C60
in toluene, and the affinity increases by at least 2 orders of
magnitude on moving to larger fullerenes (C70, C86, and
La@C82). These results imply that receptor 1 will bind
strongly to other endohedral fullerenes such N@C60.
† Department of Chemistry.
‡ Department of Materials.
(1) (a) Morton, J. J. L.; Tyryshkin, A. M.; Ardavan, A.; Porfyrakis, K.;
Lyon, S. A.; Briggs, G. A. D. J. Chem. Phys. 2006, 124, 014508. (b) Morton,
J. J. L.; Tyryshkin, A. M.; Ardavan, A.; Porfyrakis, K.; Lyon, S. A.; Briggs,
G. A. D. Phys. ReV. B 2007, 76, 085418.
Among the wide range of receptors for fullerenes reported
in the literature,3 those based on porphyrins display the
highest binding affinities.4,5 Many structures with a single
(2) Benjamin, S. C.; Ardavan, A.; Briggs, G. A. D.; Britz, D. A.;
Gunlycke, D.; Jefferson, J.; Jones, M. A. G.; Leigh, D. F.; Lovett, B. W.;
Khlobystov, A. N.; Lyon, S. A.; Morton, J. J. L.; Porfyrakis, K.; Sambrook,
M. R.; Tyryshkin, A. M. J. Phys.: Condens. Mater. 2006, 18, S867–S883.
(3) Pe´rez, E. M.; Mart´ın, N. Chem. Soc. ReV. 2008, 37, 1512–1519.
(4) Boyd, P. D. W.; Reed, C. A. Acc. Chem. Res. 2005, 38, 235–242
.
10.1021/ol101393h  2010 American Chemical Society
Published on Web 07/02/2010

porphyrin-fullerene interaction have been characterized in
the solid state,4,6,7 but these complexes readily dissociate in
solution, and most solution-phase receptors utilize two or
more porphyrin units.5,8-12 We reasoned that a cyclic
porphyrin trimer in which three porphyrins are preorganized
to chelate to the same fullerene guest might provide very
strong binding. No covalent macrocyclic receptors have
previously been reported that exhibit this type of tridentate
porphyrin-fullerene interaction, although 3:1 coordination
geometries have been reported in solid-state complexes,7
metal-coordinate self-assemblies,11 and complexes of tripodal
porphyrin trimers.12 Four-coordinate complexes, in which
two porphyrin dimers chelate to the same fullerene molecule
have also been reported.13
A search of the Cambridge Structural Database shows that
the mean value of this C60 centroid to porphyrin centroid
parameter is 6.28 ( 0.08 Å (mean for 21 zinc porphyrin
C60 interactions).14 We calculate that the D3h symmetric
analogue of receptor 1 with 3,3′-biphenyl links, at all three
corners, would have a cavity centroid to porphyrin centroid
distance of 5.91 Å, making it too small to accommodate C60.
Thus the roles of the phthalimide unit in the design of 1 are
to expand the cavity and to provide a rigid attachment point.
Cyclic porphyrin trimer 1 was synthesized by Sonogashira
coupling of 3,4-diiodophthalimide 215 and the alkyne-
terminated linear porphyrin trimer 3 (Scheme 1). The linear
trimer 4 was prepared by Suzuki coupling of porphyrins 5
and 7 and deprotected to give 3. Porphyrin monomers 6 and
7 were obtained by statistical reaction of with dipyrromethane
8 with aldehydes 9 and 10.16 The most difficult part of this
synthesis turned out to be preparation of the porphyrin
bisboronic acid 5. After testing several routes to this
compound, it was obtained in 25% yield from 6 by
palladium-catalyzed boronylation.
Molecular mechanics calculations on a variety of potential
receptors identified porphyrin trimer 1 as a promising target.
In the calculated structure of the 1·C60 complex (Figure 1),
1
The H NMR spectrum of the cyclic porphyrin trimer 1
confirms its C2V symmetry. Its purity and identity were
1
1
established by H NMR, 13C NMR, H-1H NOESY, GPC,
MALDI-TOF MS, and UV-vis spectroscopy (see Support-
ing Information).
Boyd and co-workers have shown that fullerene-porphyrin
interactions are highly solvent-dependent, and that the
strengths of these interactions can be maximized by selecting
solvents in which C60 has a low solubility.9c Thus we chose
to investigate the interaction of trimer 1 with C60 in a variety
of solvents covering a wide rage of C60 solubilities. UV-vis
titrations were carried out in THF, cyclohexane, carbon
tetrachloride, toluene, and o-dichlorobenzene. In all cases,
the formation of a 1:1 complex with a red-shifted Soret band
was detected.17,18 A typical set of titration spectra is shown
in Figure 2a, and the binding constants measured in these
five solvents are summarized in Table 1.
Figure 1. Calculated structure of the C60·1 complex (MM+,
Hyperchem 8.0). Distances shown from the centroid of the
porphyrins to the centroid of the fullerene.
the distances from the centroid of the fullerene to the
centroids of the three porphyrins are 6.42, 6.44, and 6.48 Å.
The solubility data19 provide a good measure of the ability
of each solvent to solvate C60. As expected, the worst solvent
(cyclohexane) gives the highest binding constant [(2.1 ( 0.3)
× 107 M-1], while the best solvent (o-dichlorobenzene) gives
the lowest binding constant [(2.2 ( 0.2) × 104 M-1].
However, THF and carbon tetrachloride do not fit the trend:
they give weaker binding constants than expected based on
(5) Tashiro, K.; Aida, T. Chem. Soc. ReV. 2007, 36, 189–197
.
(6) Boyd, P. D. W.; Hodgson, M. C.; Rickard, C. E. F.; Oliver, A. G.;
Chaker, L.; Brothers, P. J.; Bolskar, R. D.; Tham, F. S.; Reed, C. A. J. Am.
Chem. Soc. 1999, 121, 10487–10495
.
(7) (a) Hosseini, A.; Hodgson, M. C.; Tham, F. S.; Reed, C. A.; Boyd,
P. W. Cryst. Growth Des. 2006, 6, 397–403. (b) Olmstead, M. M.; Nurco,
D. J. Cryst. Growth Des. 2006, 6, 109–113
.
(8) (a) Tashiro, K.; Aida, T.; Zheng, J.-Y.; Kinbara, K.; Saigo, K.;
Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc. 1999, 121, 9477–9478.
(b) Zheng, J.-Y.; Tashiro, K.; Hirabayashi, Y.; Kinbara, K.; Saigo, K.; Aida,
T.; Sakamoto, S.; Yamaguchi, K. Angew. Chem., Int. Ed. 2001, 40, 1857–
1861. (c) Shoji, Y.; Tashiro, K.; Aida, T. J. Am. Chem. Soc. 2004, 126,
(14) See Supporting Information for an analysis of zinc porphyrin C60
interactions from the Cambridge Crystallographic Database.
(15) Terekhov, D. S.; Nolan, K. J. M.; McArthur, C. R.; Leznoff, C. C.
J. Org. Chem. 1996, 61, 3034–3040.
6570–6571
.
(9) (a) Sun, D.; Tham, F. S.; Reed, C. A.; Chaker, L.; Boyd, P. D. W.
J. Am. Chem. Soc. 2002, 124, 6604–6612. (b) Kundra´t, O.; Ka´s, M.;
Tkadlecova´, M.; Lang, K.; Cvacka, J.; Stibor, I.; Lhota´k, P. Tetrahedron
Lett. 2007, 48, 6620–6623. (c) Hosseini, A.; Taylor, S.; Accorsi, G.;
Armaroli, N.; Reed, C. A.; Boyd, P. D. W. J. Am. Chem. Soc. 2006, 128,
(16) (a) Sessler, J. L.; Johnson, M. R.; Creager, S. E.; Fettinger, J. C.;
Ibers, J. A. J. Am. Chem. Soc. 1990, 112, 9310–9329. (b) McCallien,
D. W. J.; Burn, P. L.; Anderson, H. L. J. Chem. Soc., Perkin Trans. 1
1997, 2581–2586.
(17) Analysis of titration data was carried out using Specfit/32 software
v3.0.38; see Supporting Information.
15903–15913
(10) Marois, J.-S.; Cantin, K.; Desmarais, A.; Morin, J.-F. Org. Lett.
2008, 10, 33–36
(11) Schmittel, M.; He, B.; Mal, P. Org. Lett. 2008, 10, 2513–2516
.
(18) Formation of the 1·C60 complex is accompanied by the appearance
of a weak charge-transfer band at 650-800 nm (ε ≈ 1000 M-1 cm-1, in
toluene) as reported for other C60-porphyrin complexes; see: Imahori, H.;
Tkachenko, N. V.; Vehmanen, V.; Tamaki, K.; Lemmetyinen, H.; Sakata,
.
.
(12) (a) Takai, A.; Chkounda, M.; Eggenspiller, A.; Gros, C. P.; Lachkar,
M.; Barbe, J.-M.; Fukuzumi, S. J. Am. Chem. Soc. 2010, 132, 4477–4489.
(b) Tong, L. H.; Wietor, J.-L.; Clegg, W.; Raithby, P. R.; Pascu, S. I.;
Y.; Fukuzumi, S. J. Phys. Chem. A 2001, 105, 1750–1756
.
(19) (a) Ruoff, R. S.; Tse, D. S.; Malhotra, R.; Lorents, D. C. J. Phys.
Chem. 1993, 97, 3379–3383. (b) Beck, M. T.; Ma´ndi, G. Fullerene Sci.
Technol. 1997, 5, 291–310. (c) Tomiyama, T.; Uchiyama, S.; Shinohara,
H. Chem. Phys. Lett. 1997, 264, 143–148.
Sanders, J. K. M. Chem.sEur. J. 2008, 14, 3035–3044
.
(13) Ouchi, A.; Tashiro, K.; Yamaguchi, K.; Tsuchiya, T.; Akasaka,
T.; Aida, T. Angew. Chem., Int. Ed. 2006, 45, 3542–3546.
Org. Lett., Vol. 12, No. 15, 2010
3545

Scheme 1. Synthesis of the Cyclic Porphyrin Trimer 1
( 0.7) × 106 M-1], satisfactory solubility of both components
and ease of comparison with published work on other
receptor-fullerene complexes. Preliminary experiments with
the higher fullerenes (C70, C86, and La@C82) showed that
their complexes with 1 in toluene are all too strong to
quantify by UV-vis titration, so we switched technique to
fluorescence titration to enable stronger association constants
to be measured by working at lower concentrations. Binding
to fullerenes quenches the fluorescence of the receptor
(Figure 2b), presumably via photoinduced electron transfer
from the receptor to the fullerene. The fluorescence titration
of 1 with C60 gave an association constant of (8.3 ( 0.7) ×
105 M-1, which is acceptably close to the value from UV-vis
titration. The results for the higher fullerenes are summarized
in Table 1. The C70·1 complex is 2 orders of magnitude more
stable than C60·1 (i.e., KC /KC ≈ 100), while C86·1 and
Table 1. Binding Constants of 1 with Fullerenes in Several
Solvents, and the Solubility of Fullerenes in These Solvents
solubility
(mg/mL)19
a
fullerene
solvent
K (M-1
)
C60
C60
C60
C60
C60
C70
C86
La@C82
cyclohexane
THF
CCl4
(2.1 ( 0.3) × 107
(3.3 ( 1.5) × 105
(9.1 ( 2.4) × 105
(1.6 ( 0.7) × 106
0.036
0.06
0.32
2.8
27
1.1
-
-
toluene
o-dichlorobenzene (2.2 ( 0.2) × 104
toluene
toluene
toluene
(1.6 ( 0.7) × 108
>109
>109
a All association constants were measured at 298 K; values for C60 were
measured by UV-vis titration, whereas those for the higher fullerenes were
measured by fluorescence titration.
70
60
La@C82·1 are too stable to measure by fluorescence titration
(K > 109 M-1). Strong binding to C70 and C86 was confirmed
by mass spectrometry: when a MALDI-TOF mass spectrum
was recorded for an equimolar mixture of C60, C70, C86, and
receptor 1, peaks were observed for C70·1 and C86·1, but not
for C60·1 (see Supporting Information).
their C60 solubility data, probably because they are good
solvents for zinc porphyrin units.20 The variation in binding
constant with solvent reflects the strength of solvation of both
the host and the guest.
This solvent study enabled us to identify toluene as the
best compromise in terms of high association constant [(1.6
Comparison with published work on other fullerene
receptors8 reveals that cyclic timer 1 binds C60 in toluene
slightly more strongly than cyclic zinc porphyrin dimers,
indicating that the presence of the third porphyrin unit has a
small but positive influence on the affinity for C60. However,
the cooperative effect of the third porphyrin becomes
(20) The data in Table 1 imply that 1 equiv of receptor 1 increases the
solubility of C60 in cyclohexane by a factor of 1050 ( 150. We have
observed that the presence of 1 increases the solubility of C60, but we
have not measured the solubility of the 1:1 complex. Other receptors have
previously been reported to enhance the solubility of fullerenes; see: Pantos,
G. D.; Wietor, J.-L.; Sanders, J. K. M. Angew. Chem., Int. Ed. 2007, 46,
2238–2240.
3546
Org. Lett., Vol. 12, No. 15, 2010

C70, C86, and La@C82 enables them to interact well with all
three walls of the receptor. The electronic polarization of
La@C82 may also contribute toward its strong binding.21 The
surprisingly large value of KC /KC for receptor 1 indicates
70
60
that its cavity is slightly too large for binding C60, as deduced
above from molecular modeling and crystallographic data.
In conclusion, a cyclic trimer of porphyrins 1 has been
synthesized, and its binding to C60 has been studied in a range
of solvents. There is no clear trend relating the solubility of
C60 in each solvent to the affinity of C60 for receptor 1 in
that solvent, but desolvation of the host and of the guest is
evidently important. The highest value for the binding
constant to C60 is obtained in cyclohexane. Other fullerenes
(C70, C86, and La@C82) show very high binding constants
to receptor 1 in toluene. The value of K for C70·1 is 1 × 108
M-1, which is substantially higher than those of previous
receptors based on zinc porphyrins. The affinities for C86
and La@C82 are beyond the limit of the fluorescence titration
techniques (K > 109 M-1). To the best of our knowledge,
this is the first report of binding of endohedral fullerenes to
a molecular receptor.21 It is well-known that metallopor-
phyrins such as those of rhodium(III) bind fullerenes much
more strongly than zinc porphyrins,3,5,8b,13 thus changing the
metal in 1 should lead to even higher fullerene affinities.
The high affinity for La@C82 displayed by receptor 1 in
toluene indicates that covalently linked assemblies, decorated
with multiple copies of this receptor, should be capable of
complexing multiple spin-active fullerenes, providing a
promising system for studying spin entanglement.
Acknowledgment. We thank the EPSRC (Grant EP/
F028806/01) for financial support and the EPSRC Mass
Spectrometry Service (Swansea) for mass spectra. We
acknowledge use of the Cambridge Structural Database.
Figure 2. (a) Absortion spectral change of 1 (0.77 µM) in toluene
at 298 K upon titration with C60 (0-27 µM), 10 mm path length.
Inset: plot of (A424-A412) against number of equivalents of C60:
circles ) experimental data. (b) Fluorescence spectra during the
titration of 1 (0.093 µM) with C60 (0-7.2 µM) in toluene at 298 K
(λex ) 413 nm). Inset: plot of ∆I615 against number of equivalents
of C60: diamonds ) experimental data. All titrations were performed
at constant receptor concentration.
Supporting Information Available: NMR spectra, de-
tailed experimental procedures, UV-vis and fluorescence
titration data. This material is available free of charge via
the Internet at http://pubs.acs.org.
OL101393H
dramatic with the higher fullerenes. The stability of the C70·1
complex is much higher than those of previously reported
(21) Elution profiles on porphyrin-functionalized silica indicate that the
interaction of La@C82 with porphyrins is stronger than those of C60 or C70:
Xiao, J.; Savina, M. R.; Martin, G. B.; Francis, A. H.; Meyerhoff, M. E.
J. Am. Chem. Soc. 1994, 116, 9341–9342.
zinc porphyrin receptors (which typically have KC /KC
≈
70
60
10).8,9 We are not aware of previous binding results with
C86 or endohedral fullerenes. Evidently, the greater size of
Org. Lett., Vol. 12, No. 15, 2010
3547