Hydrogen bond-assembled fullerene molecular shuttle

Article abstract of DOI:10.1021/ol0274110

(Figure presented) A novel [2]rotaxane has been prepared in which fullerene C₆₀ behaves as both a stopper and a photoactive unit. The amphiphilic nature of the rotaxane thread can be used to shuttle the macrocycle from close to the fullerene sp

Full text of DOI:10.1021/ol0274110

ORGANIC
LETTERS
2003
Vol. 5, No. 5
689-691
Hydrogen Bond-Assembled Fullerene
Molecular Shuttle
,‡
,§
Tatiana Da Ros,† Dirk M. Guldi,* Angeles Farran Morales,§ David A. Leigh,*
,†
Maurizio Prato,* and Riccardo Turco†
Dipartimento di Scienze Farmaceutiche, UniVersita` di Trieste, Piazzale Europa 1,
34127 Trieste, Italy, Radiation Laboratory, UniVersity of Notre Dame, Indiana 46556,
and School of Chemistry, UniVersity of Edinburgh, The King’s Buildings,
West Mains Road, Edinburgh EH9 3JJ, United Kingdom
prato@uniV.trieste.it; guldi.1@nd.edu; daVid.leigh@ed.ac.uk
Received December 5, 2002
ABSTRACT
A novel [2]rotaxane has been prepared in which fullerene C behaves as both a stopper and a photoactive unit. The amphiphilic nature of the
60
rotaxane thread can be used to shuttle the macrocycle from close to the fullerene spheroid (in nonpolar solvents) to far away (in polar
solvents). The differing location of the macrocycle in dichloromethane and dimethyl sulfoxide gives rise to effects detectable by 1H NMR and
time-resolved spectroscopy.
Photoactive rotaxanes have attracted increasing interest in
recent years due to their potential as components of molecular
devices.1 Fullerene C60 is particularly well suited as a
rotaxane building block for several reasons. First of all, it is
a bulky group, ideal for acting as a stopper. Second, C60 is
highly electro- and photoactive.2 Its incorporation into these
potentially machine-like structures could give rise to a series
of electrochemically induced or photoinduced events such
as energy transfer or electron transfer for the construction
of photoswitches, modulation of luminescence, etc.
Here we describe the synthesis of a novel hydrogen bond-
assembled [2]rotaxane, 1, which utilizes C60 as an active
component.3 The photophysical behavior of the rotaxane is
also reported, probing the influence of the position of the
macrocycle on the fullerene triplet excited state. The system
is based on the hydrogen bond-directed assembly of a
† Universita` di Trieste.
‡ University of Notre Dame.
§ University of Edinburgh.
(1) (a) Benniston, A. C. Chem. Soc. ReV. 1996, 427-435. (b) Murakami,
H.; Kawabuchi, A.; Kotoo, K.; Kunitake, M.; Nakashima, N. J. Am. Chem.
Soc. 1997, 119, 7605-7606. (c) Armaroli, N.; Balzani, V.; Collin, J. P.;
Gavin˜a, P.; Sauvage, J. P.; Ventura, B. J. Am. Chem. Soc. 1999, 121, 4397-
4408. (d) Ashton, P. R.; Ballardini, R.; Balzani, V.; Credi, A.; Dress, K.
R.; Ishow, E.; Kleverlaan, C. J.; Kocian, O.; Preece, J. A.; Spencer, N.;
Stoddart, J. F.; Venturi, M.; Wenger, S. Chem. Eur. J. 2000, 6, 3558-
3574. (e) Brouwer, A. M.; Frochot, C.; Gatti, F. G.; Leigh, D. A.; Mottier,
L.; Paolucci, F.; Roffia, S.; Wurpel, G. W. H. Science 2001, 291, 2124-
2128. (f) Wurpel, G. W. H.; Brouwer, A. M.; van Stokkum, I. H. M.;
Morales, A. F.; Leigh, D. A. J. Am. Chem. Soc. 2001, 123, 11327-11328.
(g) Stanier, C. A.; Alderman, S. J.; Claridge, T. D. W.; Anderson, H. L.
Angew. Chem., Int. Ed. 2002, 41, 1769-1772. (h) See also Special issue
of Acc. Chem. Res. 2001, 34 ( 6).
(2) (a) Fullerenes and Related Structures; Hirsch, A., Ed.; Springer:
Berlin, 1999; Vol. 199. (b) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000,
33, 695-703. (c) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998,
31, 593-601. (d) Mart´ın, N.; Sa`nchez, L.; Illescas, B.; Pe´rez, I. Chem.
ReV. 1998, 98, 2527-2547.
(3) Copper-complexed rotaxane with two fullerene stoppers has previ-
ously been reported (Diederich, F.; Dietrich-Buchecker, C.; Nierengarten,
J.-F.; Sauvage, J.-P. J. Chem. Soc., Chem. Commun. 1995, 781-782), and
its properties have been studied (Armaroli, N.; Diederich, F.; Dietrich-
Buchecker, C.; Flamigni, L.; Marconi, G.; Nierengarten, J.-F.; Sauvage,
J.-P. Chem. Eur. J. 1998, 4, 406-416).
10.1021/ol0274110 CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/06/2003

Scheme 1. Synthesis of Fullerene Molecular Shuttle 1
benzylic amide macrocycle around a dipeptide thread, a
ppm compared to the analogous protons of the thread, 5,
indicating that the macrocycle resides on the peptide residue
in these solvents (1-peptide co-conformer). In DMSO-d6,
however, the peptide methylene groups appear at similar
chemical shifts in both thread 5 and rotaxane 1, and it is the
alkyl chain protons that are shielded by nearly 1 ppm by the
aromatic rings on the macrocycle, indicating that the mac-
rocycle has shuttled down the thread to produce the 1-alkyl
chain co-conformer (Scheme 1).
method that has previously been shown to provide a
straightforward route to functionalized, solvent-switchable
molecular shuttles.4
Fulleropyrrolidine 2, prepared according to a previous
work,5 was condensed with Boc-gly-gly6 to afford fullero-
dipeptide 3 (Scheme 1).7 After deprotection in acidic
conditions, the resulting free amino group was coupled with
acid 4 to give the thread 5. Macrocyclization around the
glycylglycine residue of 5 gave the fullerene [2]rotaxane 1
in 25% yield using standard rotaxane-forming conditions (p-
xylylenediamine, isophthaloyl dichloride, Et3N, CHCl3).
It has previously been shown that amphiphilic peptide
[2]rotaxanes exist as different co-conformers depending on
the polarity of the environment.4 In nonpolar solvents such
as dichloromethane or chloroform, the macrocycle hydrogen
bonds to the peptide residue; in highly polar solvents such
as DMSO, the hydrogen bonding between the macrocycle
and the peptide is disrupted by the competing solvent and
polarophobic interactions locate the macrocycle preferentially
over the alkyl chain. Indeed in CDCl3 or CD2Cl2, the glycyl
methylene protons of rotaxane 1 are shielded by nearly 1
Computer modeling of the co-conformations in the dif-
ferent types of solvents was performed using the Spartan
program on a Silicon Graphics workstation (Figure 1).
In dichloromethane, the hydrogen bonding between the
dipeptide and the macrocycle brings the latter very close to
the fullerene spheroid. In DMSO, on the other hand, the
macrocycle is located on the aliphatic portion of the thread
and there should be no significant interactions between the
fullerene moiety and the aryl groups of the macrocycle. The
differences in co-conformation encouraged us to investigate
the proximity effects of the macrocycle on the fullerene
moiety using time-resolved spectroscopy.
The fullerene fluorescence and the fullerene triplet excited-
state features in rotaxane 1, thread 5, and a fullerene ref-
erence that bears a solubilizing triethylenglycol chain
[N-(CH2CH2O)3CH3 fulleropyrrolidine 6]8 were investigated
in both CH2Cl2 and DMSO. In CH2Cl2, the fluorescence of
(4) Lane, A. S.; Leigh, D. A.; Murphy, A. J. Am. Chem. Soc. 1997, 119,
11092-11093.
(5) (a) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993,
115, 9798-9799. (b) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31,
519-526.
(6) Chen, F. M. F. Synthesis 1978, 928-929.
(7) For a review on fullerene peptides, see: Bianco, A.; Da Ros, T.;
Prato, M.; Toniolo, C. J. Pept. Sci. 2001, 7, 208-219.
(8) Wang, P.; Chen, B.; Metzger, R. M.; Da Ros, T.; Prato, M. J. Mater.
Chem. 1997, 7, 2397-2400.
690
Org. Lett., Vol. 5, No. 5, 2003

Figure 1. Space-filling model of rotaxane 1 in CH2Cl2 (top, peptide
co-conformer) and DMSO (bottom, alkyl chain co-conformer).
the C60-rotaxane 1 and the C60-thread 5 is noticeably
quenched (25% fluorescence quenching) relative to the
fulleropyrrolidine reference.2b This is indicative of a weak
electronic interaction between the fullerene in its singlet
excited state and the adjacent amide linkages. Overall, this
leads to a decrease in the fullerene fluorescence quantum
yield. The proximity of the macrocycle to the fullerene
actually contributes negligibly to the fluorescence quenching.
In contrast, in DMSO the strong interaction of the weakly
redox active amide linkages with the solvents prevails over
the possible engagement of the amide groups with the
photoexcited fullerene. As a consequence, the fluorescence
quantum yields of the C60-rotaxane complex and the C60-
thread become comparable with that of the fulleropyrrolidine
reference.
Figure 2. Triplet-triplet spectra of rotaxane 1 (black spectrum),
N-TEG-fulleropyrrolidine 6 (red spectrum), and thread 5 (blue
spectrum) in DMSO (a) and CH2Cl2 (b) following 337 nm laser
excitation with a Nitrogen Laser of a solution that exhibited an
absorption of 0.5 at the 337 nm excitation wavelength.
Finally, the triplet lifetimes follow exactly the trend seen
in the fluorescence quantum yields. While in DMSO, the
triplet lifetime is ∼17 µs and nearly identical for all three
compounds; only those of the C60-rotaxane 1 and the C60-
thread 5 in dichloromethane are shorter by a factor of 1.7
(∼10 µs).
In conclusion, we have synthesized a novel fullerene-
containing [2]rotaxane in which the macrocycle can be
shuttled between co-conformations that position it either close
to or far away from the fullerene moiety. The proximity of
the macrocycle does not affect fluorescence but has a
significant effect on the triplet-triplet spectra of the fullerene
fragment.
The triplet-triplet spectra of thread 5, rotaxane 1, and
fullerene reference 6 all show identical spectral features in
DMSO. In particular, the two maxima at 700 and 380 nm
have nearly superimposeable extinction coefficients (ꢀ700 nm
≈ 16 100 M-1 cm-1 and ꢀ380
≈ 10 300 M-1 cm-1, re-
nm
spectively) (Figure 2a).2b Again, we rationalize this observa-
tion with insignificant perturbations of the fullerene core by
both the macrocycle (distance effect) and the amide groups
of the thread (neutralized by the hydrogen bonding solvent).
However, the situation is notably different in dichloro-
methane (Figure 2b). The most obvious difference is seen
between the triplet spectra of the C60-rotaxane 1 and the C60-
thread 5. These deviate substantially from each other,
suggesting that the triplet spectra are sensitive to effects
stemming from the close proximity of the macrocycle and/
or the inductive effects of the intercomponent hydrogen
bonding. To underpin this argument, it is interesting to note
that the triplet spectra of C60-thread 5 and fulleropyrrolidine
reference 6 resemble both each other and those recorded in
DMSO quite well.
Acknowledgment. This work was carried out with partial
support from the EU (RTN “EMMMA”), MIUR (PRIN
2002, prot. 2002032171), and the Office of Basic Energy
Sciences of the U.S. Department of Energy. D.A.L. is an
EPSRC Advanced Research Fellow (AF/982324). This is
document NDRL-4437 from the Notre Dame Radiation
Laboratory.
Supporting Information Available: Full experimental
details pertaining to the preparation of 1 and 3-5. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL0274110
Org. Lett., Vol. 5, No. 5, 2003
691