Efficient one-pot synthesis of rotaxanes bearing electron donors and [60]fullerene

Article abstract of DOI:10.1021/ol9016645

An easy one-pot procedure to synthesize rotaxanes bearing electron donors and C₆₀ is described. The straightforward strategy, based on copper(I)-templated synthesis and "click" chemistry, proved to be very efficient and versatile, allowing the

Full text of DOI:10.1021/ol9016645

ORGANIC
LETTERS
2
009
Vol. 11, No. 18
152-4155
Efficient One-Pot Synthesis of
Rotaxanes Bearing Electron Donors and
4
[60]Fullerene
Jackson D. Megiatto, Jr.,* Robert Spencer, and David I. Schuster*
Chemistry Department, New York UniVersity, New York City, New York 10003
jackson.megiatto@nyu.edu; daVid.schuster@nyu.edu
Received July 21, 2009
ABSTRACT
An easy one-pot procedure to synthesize rotaxanes bearing electron donors and C60 is described. The straightforward strategy, based on
copper(I)-templated synthesis and “click” chemistry, proved to be very efficient and versatile, allowing the preparation of porphyrin- and
ferrocene-stoppered fullerene-rotaxanes in high yields. As revealed by NMR analysis and computational studies, the highly flexible
porphyrin-fullerene rotaxane can assume different conformations, which are most likely driven by attractive interactions between porphyrin
and fullerene moieties.
In natural photosynthesis, photoinduced energy and electron-
transfer reactions are the key steps in the process that
converts sunlight into chemically useful products. In these
lecular concepts have been introduced into the synthesis of
D-A systems to permit the construction of more realistic
and more complex models. Controlling the position and
interactions of chromophores by means of hydrogen bonds,
metal coordination, and π-π interactions has led to new
possibilities in managing the electron-transfer dynamics in
1
systems, the noncovalent interactions between electron donor
(
D) and acceptor (A) moieties create a well-defined energy
gradient which drives electron-transfer reactions, generating
2
4
long-lived charge-separated states. In artificial photosyn-
artificial photosynthetic model systems.
thesis, much effort has been directed toward understanding
these interactions and elucidating the electron-transfer mech-
anisms. A better understanding of these phenomena is
crucial for developing more efficient and inexpensive pho-
tovoltaic cells that convert light into electricity.
Among potentially useful supramolecular systems, inter-
locked structures, such as rotaxanes and catenanes, are
promising architectures for the preparation of noncovalently
linked D-A systems. Catenanes are comprised of two or
more interlocked rings, while rotaxanes possess a ring
3
The design and synthesis of artificial photoactive electron
donor-acceptor systems that mimic structures involved in
natural photosynthesis require the organization of several
components in an elaborate molecular topology. Supramo-
(
3) For representative examples, see: (a) Sgobba, V.; Giancane, G.;
Conoci, S.; Casilli, S.; Ricciardi, G.; Guldi, D. M.; Prato, M.; Valli, L.
J. Am. Chem. Soc. 2007, 129, 3148. (b) Figueira-Duarte, T.; Lloveras, V.;
Vidal-Gancedo, J.; Gegout, A.; Delavaux, N. B.; Welter, R.; Veciana, J.;
Rovira, C.; Nierengarten, J. F. Chem. Commum. 2007, 42, 4345. (c) Regehly,
M.; Ermilov, E. A.; Helmreich, M.; Hirsch, A.; Jux, N.; Roeder, B. J. Phys.
Chem. B 2007, 111, 998. (d) Santos, J.; Grim, B.; Islescas, B. M.; Martin,
N. Chem. Commun. 2008, 45, 5993. (e) Kira, A.; Umeyama, T.; Matano,
Y.; Yoshida, K.; Isoda, S.; Park, J.; Kang, K.; Imahori, H. J. Am. Chem.
Soc. 2009, 131, 3198.
(
(
1) Gust, D.; Moore, T. A. Science 1989, 244, 35.
2) Balzani, V. Electron Transfer in Chemistry; Wiley-VCH: Weinheim,
Germany, 2003; Vols. 1-5. Gust, D.; Moore, T. A.; Moore, A. L. Acc.
Chem. Res. 2001, 34, 40.
10.1021/ol9016645 CCC: $40.75
Published on Web 08/17/2009
 2009 American Chemical Society

5
threaded on a rod bearing bulky substituents at its termini.
The interlocked components can undergo submolecular motions
by application of external stimuli. For example, in the case of
rotaxanes, the ring can be reversibly switched between two
different stations on the rod by application of an input such as
light, applied voltage, temperature changes, or changes in
solvent polarity. Such submolecular motions of the rings in
catenanes and rotaxanes have been explored as elegant prin-
ciples for the construction of molecular machines.
In the field of artificial photosynthesis, the use of inter-
locked structures as linkers between electron donor and
acceptor moieties has recently been explored. A number of
such systems incorporating electron donors and C60 have
8
,9
been prepared and studied by our group and others. In
9
our laboratory, we have used the Cu(I) template synthesis,
developed by Sauvage and co-workers, to assemble rotax-
10
9
Figure 1. Examples of structures studied in our previous work:
anes containing zinc(II)-porphyrins (ZnP) as stoppers and
(a) porphyrin-stoppered fullerene-rotaxanes and (b) fullerene-
stoppered porphyrin-rotaxanes.
9a
C60 on the macrocyclic ring (for an example, see Figure 1a).
The alternative configuration, in which ZnP is on the ring
and C60 acts as the stopper, has also been investigated (Figure
9
b
1
b). Upon irradiation, these ZnP-C60-based Cu(I)-rotaxanes
materials. Multiple chromatographic separations were re-
quired to afford the pure target rotaxanes, which were isolated
in low yields.
undergo a series of energy- and electron-transfer processes
that lead to a charge-separated radical pair (CSRP) state,
•
+
+
•-
ZnP -Cu -C60 , with lifetimes in the microsecond time
domain. In the case of the fullerene-stoppered porphyrin
rotaxane, Figure 1b, the CSRP lifetime in THF is 32 µs, the
longest lifetime reported to date for an interlocked D-A
system in solution at ambient temperatures.
Herein, we report an easy and expeditious one-pot
procedure to synthesize rotaxanes linked to electron donors
and C60. In recent contributions to the field of interlocked
11
molecules, we have described efficient protocols that allow
the preparation of functionalized macrocycles as well as [2]-
1
0
The preparation of these ZnP-C60 linked Cu(I)-rotaxanes
requires a long multistep synthesis. In our previous approach
to such materials, classical ester coupling was used for the
final “stoppering” reaction. As a result, the final product
was a complex mixture of interlocked and noninterlocked
and [3]catenanes based on Cu(I)-template synthesis and
1
2
“
click” chemistry. Since Cu(I) can act as both template
and catalyst, we have developed a one-pot procedure for the
final “stoppering” reactions, affording rotaxanes containing
electron donor and C60 moieties in one step with very high
yields. The preparations of ferrocene- and porphyrin-stop-
pered fullerene-rotaxanes are described here to demonstrate
the versatility of this protocol.
The building blocks and the synthetic strategy are shown
in Scheme 1 (see the Supporting Information for details).
First, C60 was attached to previously prepared phenanthroline
macrocycle 1 using the Bingel-Hirsch reaction to afford 2.
Macrocycle 2 (1 equiv) was then dissolved in 2 mL of 7:3
9
(
4) (a) Santos, J.; Grimm, B.; Illescas, B. M.; Guldi, D. M.; Martin, N.
Chem. Commum. 2008, 45, 5993. (b) Gayathri, S. S.; Wielopolski, M.; Perez,
E. M.; Fernandez, G.; Sanchez, L.; Viruela, R.; Orti, E.; Guldi, D. M.;
Martin, N. Angew. Chem.Int. Ed. 2009, 48, 815. (c) Kira, A.; Umeyama,
T.; Matano, Y.; Yoshida, K.; Isoda, S.; Park, J.; Kang, K.; Dongho, K.;
Imahori, H. J. Am. Chem. Soc. 2009, 131, 3198. (d) Sarova, G. H.;
Hartnagel, U.; Balbinot, D.; Sali, S.; Jux, N.; Hirsch, A.; Guldi, D. M
Chem.sEur. J. 2008, 14, 3137. (e) Hagemann, O.; Jørgensen, M.; Krebs,
F. C. J. Org. Chem. 2006, 71, 5546. (f) Collin, J.-P.; Harriman, A.; Heitz,
V.; Odobel, F.; Sauvage, J.-P. J. Am. Chem. Soc. 1994, 116, 5679.
(
5) Sauvage, J. P.; Dietrich-Buchecker, C. O. Molecular Catenanes,
Rotaxanes and Knots; Wiley-VCH: Weinheim, Germany, 1999.
6) (a) Serrel, V.; Lee, C. F.; Kay, E. R.; Leigh, D. A. Nature 2005,
45, 523. (b) Green, J. E.; Choi, J. W.; Boukai, A.; Bunimovich, Y.;
2 2 3 3 4 6
CH Cl /CH CN (v/v), to which [Cu(CH CN) ][PF ] (1 equiv)
was added as a solid. The solution was stirred for 30 min,
(
11
and previously reported phenanthroline diazide 3 was added
4
Johnston-Halperin, E.; Delonno, E.; Luo, Y.; Scheriff, B. A.; Xu, K.; Shin,
as a solid. The resulting dark brown solution was stirred for
Y. S.; Tseng, H. R.; Stoddart, J. F.; Heath, J. R. Nature 2007, 445, 414.
3
h under an inert atmosphere with magnetic stirring to
(
7) (a) Badijic, J. D.; Credi, V.; Silvi, S.; Stoddart, J. F. Science 2004,
generate pseudo-rotaxane 4. Alkynylferrocene 5 or alkynyl-
3
03, 1845. (b) Mateo-Alonso, A.; Guldi, D. M.; Paolucci, F.; Prato, M.
Angew. Chem., Int. Ed. 2007, 46, 8120.
8) (a) Watanabe, N.; Kihara, N.; Furusho, Y.; Takata, T.; Araki, Y.;
13
2 2
zinc-porphyrin 6 (3 equiv), dissolved in 1 mL of CH Cl ,
(
was then added to the brown solution, followed by the
addition of sulfonated bathophenanthroline (SBP) (4 equiv),
CuI (2 equiv), sodium ascorbate (SA) (4 equiv), and 1,8-
Ito, O. Angew. Chem., Int. Ed. 2003, 42, 681. (b) Clifford, J. N.; Accorsi,
G.; Cardinali, F.; Nierengarten, J. F.; Armaroli, N. C. R. Chimie 2006, 9,
005. (c) Illescas, B. M; Santos, J; D ´ı az, M. C.; Mart ´ı n, N.; Atienza, C. M.;
1
Guldi, D. M. Chem.sEur. J. 2007, 30, 5027. (d) Saha, S.; Flood, A. H.;
Stoddart, J. F.; Impellizzeri, S.; Silvi, S.; Venturi, M.; Credi, A. J. Am.
Chem. Soc. 2007, 129, 12159. (f) Mateo-Alonso, A.; Ehli, C.; Rahman,
A. G. M.; Guldi, D. M.; Fioravanti, G.; Marcaccio, M.; Paolucci, F.; Prato,
M. Angew. Chem. 2007, 46, 3521. (g) Mateo-Alonso, A.; Fioravanti, G.;
Marcaccio, M.; Paolucci, F.; Rahman, A. G. M.; Ehli, C.; Marois, J. S.;
Cantin, K.; Desmarais, A.; Morin, J. F. Org. Lett. 2008, 10, 33.
(10) (a) Dietrich-Buchecker, C. O.; Sauvage, J.-P. Tetrahedron Lett.
1983, 24, 5095. (b) Dietrich-Buchecker, C. O.; Sauvage, J.-P. Chem. ReV.
1987, 87, 795. (c) Dietrich-Buchecker, C. O.; Sauvage, J.-P. Tetrahedron
1990, 46, 503.
(11) (a) Megiatto, J. D., Jr.; Schuster, D. I. J. Am. Chem. Soc. 2008,
130, 12872. (b) Megiatto, J. D, Jr.; Schuster, D. I. Chem.sEur. J. 2009,
15, 5444.
(
9) (a) Li, K.; Schuster, D. I.; Guldi, D. M.; Herranz, M. A.; Echegoyen,
L. J. Am. Chem. Soc. 2004, 126, 3388. (b) Li, K.; Bracher, P. J.; Guldi,
D. M.; Herranz, M. A.; Echegoyen, L.; Schuster, D. I. J. Am. Chem. Soc.
(12) (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem. 2001,
113, 2056. (b) Meldal, M.; Tornøe, C. W. Chem. ReV. 2008, 108, 2952.
(13) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941.
2
004, 126, 9156. (c) Schuster, D. I.; Li, K.; Guldi, D. M.; Ramey, J. Org.
Lett. 2004, 6, 1919. (d) Schuster, D. I.; Li, K.; Guldi, D. M. C.R. Chimie.
006, 9, 892. (e) Li, K. Ph D. Dissertation, New York University, 2004.
2
Org. Lett., Vol. 11, No. 18, 2009
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Scheme 1
.
Building Blocks, Precursors, and Synthetic Strategy Used for the Preparation of Ferrocene- and Porphyrin-Fullerene-Based
a
Rotaxanes
a
DBU ) 1,8-diaza[5.4.0]bicycloundec-7-ene. SA ) sodium ascorbate. SBP ) sulfonated bathophenanthroline.
diaza[5.4.0]bicycloundec-7-ene (DBU) (2 equiv), all dis-
solved in 3 mL of an oxygen-free mixture of 1:1 H
2
O/EtOH
1
1b
(
v/v), as in our previous study. The reaction mixture was
stirred at room temperature for 12 h under an inert
atmosphere. The organic phase was separated, washed with
water, and stirred for 2 h with a saturated methanol solution
1
4
of KPF
evaporated under reduced pressure, the remaining light brown
solid was extracted with CH Cl , and the solution was filtered
through paper. For rotaxane 7, final purification was achieved
by flash chromatography on silica using CH Cl as the eluent.
The first product eluted was unreacted alkynylferrocene.
Elution with CH Cl /MeOH (97:3, v/v) afforded target
6
to effect anion exchange. The solvents were
2
2
2
2
2
2
rotaxane 7 as a brown solid in 92% yield.
For rotaxane 8, final purification was achieved by column
chromatography on silica, using CH Cl /MeOH (gradient
2 2
from 0 to 3%, v/v) as the eluent. The first material eluted
was unreacted alkynylporphyrin 6, while the second (purple
solid, 20% yield) fraction was the noninterlocked bis-
porphyrin dumbbell compound (MALDI-TOF m/z 2752, [M
+
+
H] ). The third eluted material was target rotaxane 8,
isolated as a purple solid in 75% yield. The lower yield
obtained for 8 compared to that of 7 is rationalized in terms
of the size difference between alkynylferrocene 5 and
alkynylporphyrin 6. Steric effects in the final 1,3-dipolar
cycloaddition reaction should be greater for formation of 8
vs 7, in which case the reaction to give 8 is expected to be
slower. As a consequence, it is not surprising that the
pseudorotaxane precursor 4 may be more susceptible to
10
unthreading during the final rotaxane assembly, accounting
for the lower yield observed for 8 vs 7.
Both rotaxanes were fully characterized by standard
spectroscopic techniques. The H NMR spectrum of 7 (Figure
) shows the expected upfield shift for the phenyl protons
1
3
Figure 2. Partial H NMR spectra (400 MHz, CDCl , 300 K) of
1
macrocycle 2, Cu(I) rotaxane 7, and thread 3.
2
of the phenanthroline fragments entwined around the Cu(I)
1
0
center. The methylene groups adjacent to the triazole rings
appear at 4.52 ppm, while the protons on the triazole rings
appear at 7.60 ppm. The ferrocene protons are observed at
MALDI-TOF results (m/z 2576 [M - PF
6
]+) are in
agreement with the structure of rotaxane 7 shown in Scheme
1
6
1
1
5
1. The H NMR spectrum of rotaxane 8 is more complex
4
.95, 4.45, and 4.25 ppm.
(
see the Supporting Information), suggesting that it is
-
(
(
14) Both Cu(I)-rotaxanes have been isolated as PF
6
salts.
+
15) Ornelas, C.; Aranzaes, J. R; Cloutet, E.; Alvez, S.; Astruc, D.
(16) Since both rotaxanes bear a positive charge (Cu ), the ionization
-
Angew. Chem., Int. Ed. Engl. 2007, 46, 872.
process in MALDI experiments occurs with loss of a counterion PF
6
.
4154
Org. Lett., Vol. 11, No. 18, 2009

conformationally flexible due to attractive interactions be-
tween ZnP and C60 moieties as well as the high flexibility
of the polyoxoethylene linkers (see below).
from the second one, in accord with the biexponential
fluorescence decay. The edge-to-edge distances between the
two porphyrins and the C60 moiety are approximately 4.0
and 24.0 Å. Interestingly, for an analogous rotaxane lacking
the C60 moiety, computations show that the extended
structure with porphyrins close to each other is the most
stable (see the Supporting Information), presumably due to
π-π interactions between the ZnP moieties.
Fluorescence experiments support this hypothesis. In our
9
previous studies of less flexible ZnP-C60 rotaxanes, in
which both macrocycle and thread were prepared using
diethylene glycol “arms” (Figure 1), steady-state fluorescence
analysis showed modest quenching of the ZnP singlet excited
state. The fluorescence lifetime for the porphyrin-stoppered
rotaxane in Figure 1a was 1.0 ns, compared to 3.2 ns for a
model ZnP system. This relatively inefficient quenching is
not at all typical of covalently linked porphyrin-fullerene
1
7
hybrid molecules, for which lifetimes are typically 0.1 ns
or less. This suggests that fluorescence quenching in the first-
generation rotaxanes (Figure 1) does not involve direct
1
9
interaction between the ZnP* and C60 moieties. Further
investigation revealed that the (phen) Cu(I) complex quenches
2
the ZnP fluorescence (by energy transfer) and that the
intramolecular electronic interactions between ZnP-C60 take
9
d
2
place via the (phen) Cu(I) complex.
The steady-state fluorescence spectrum of rotaxane 8 in
dichloromethane (DCM) (Figure 3) reveals unexpectedly
strong quenching of the porphyrin fluorescence compared
to that of reference porphyrin 6. Time resolved fluorescence
experiments with 8 in DCM indicate two different decay
components, with lifetimes of 0.42 and 1.94 ns.
Figure 4. Molecular model of free-base rotaxane 8. Energy was
minimized with 3,5-di-tert-butylphenyl groups in the meso positions
of each porphyrin, which were later removed for clarity, along with
most hydrogen atoms.
In conclusion, we report a straightforward and versatile
method to prepare electron donor-fullerene rotaxanes in high
yields. This approach is very promising for the synthesis of
more elaborate C60-containing interlocked structures, such
as [2]- and [3]catenanes. The quenching of the ZnP fluo-
rescence in C60-rotaxane 8 was much stronger than expected,
2
indicating that the ZnP, C60, and (phen) Cu(I) complex are
in closer proximity than expected on the basis of the extended
structure. Time-resolved fluorescence experiments revealed
two decay components, consistent with a structure in which
one of the ZnP groups is close to both the C60 moiety and
2
the (phen) Cu(I) complex, while the second one is at a much
greater distance. Support for such a low energy folded
conformation was obtained computationally. A complete and
detailed investigation of the photophysical properties of the
rotaxanes reported in this paper is in progress.
Figure 3
ZnP-C60 rotaxane 8 in CH
.20) at the 424 nm excitation wavelength.
.
Fluorescence spectra of alkyne zinc porphyrin 6 and
2
Cl with matching absorption (OD )
2
0
Acknowledgment. We are deeply grateful to the National
Science Foundation (Grant CHE-0647334) for financial
support. R.S. was a recipient of grants from the Dean’s
Undergraduate Research Fund at NYU. This investigation
was conducted in part in a facility constructed with support
from Research Facilities Improvement Grant No. C06 RR-
The fluorescence lifetime should be directly related to the
distance between the chromophores. Indeed, the 3D structure
of the lowest energy conformation of free-base rotaxane 8,
computed using PM3 minimization, is folded as shown in
Figure 4. C60 and the (phen)
2
Cu(I) complex are much closer
16572-01 from the National Center for Research Resources,
to one of the two porphyrins and at a considerable distance
National Institutes of Health.
(
17) Vail, S. A.; Krawczuk, P. J.; Guldi, D. M.; Palkar, A.; Echegoyen,
L.; Tom e´ , J. P. C.; Fazio, M. A.; Schuster, D. I. Chem.sEur. J. 2005, 11,
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Supporting Information Available: Experimental details
for the preparation and spectral characterization of all
products and their precursors. This material is available free
of charge via the Internet at http://pubs.acs.org.
3
Braslavsky, S. E. Tetrahedron 2006, 62, 1928. Schuster, D. I.; Cheng, P.;
Jarowski, P. D.; Guldi, D. M.; Luo, C.; Echegoyen, L.; Pyo, S.; Holzwarth,
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004, 126, 7257.
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