J. Am. Chem. Soc. 2001, 123, 11327-11328
Enhanced Hydrogen Bonding Induced by Optical
11327
Excitation: Unexpected Subnanosecond
Photoinduced Dynamics in a Peptide-Based
[2]Rotaxane
George W. H. Wurpel,† Albert M. Brouwer,*,†
Ivo H. M. van Stokkum,‡ Angeles Farran,§ and
David A. Leigh*,§
Laboratory of Organic Chemistry
UniVersity of Amsterdam, Nieuwe Achtergracht 129
NL-1018 WS Amsterdam, The Netherlands
Faculty of Sciences, Vrije UniVersiteit, De Boelelaan 1081
NL-1081 HV Amsterdam, The Netherlands
Centre for Supramolecular and Macromolecular Chemistry
Department of Chemistry, UniVersity of Warwick
CoVentry CV4 7AL, UK
Figure 1. Rotaxane 1 and thread 2. The hydrogen bond pattern shown
in 1 corresponds to that found in the X-ray structure of a similar exo-
pyridyl macrocycle containing rotaxane (Supporting Information).
ReceiVed March 30, 2001
Control of the shuttling process in rotaxanes using external
sources has been the subject of considerable recent research.1
Photoinduced motions are particularly interesting because of the
convenient use of this stimulus in possible future device applica-
tions. Up to now, however, translational shuttling in photoactive
rotaxanes has required external reagents or catalysts to occur
(sacrificial reductants, sensitizers etc.), and generally takes place
over relatively long time scales.2 Here we report on the excited-
state dynamics of rotaxane 1, and show that in nonpolar solvents
at room temperature an unexpected photoinduced co-conforma-
tional3 change takes place that requires no external assistance and
occurs on a subnanosecond time scale. It involves a rearrangement
in the pattern of hydrogen bonds between thread and macrocycle
leading to a shift of the latter from the peptide site toward the
anthracene-9-carboxamide stopper which has a greatly enhanced
hydrogen bonding affinity in the excited state.
Figure 2. (A) Fluorescence spectra of rotaxane 1 (continuous) and thread
2 (dotted) in DMSO (note the vertical offset). (B) Fluorescence spectra
of 1 (continuous) and 2 (dotted) in 1,4-dioxane and 2 in 2,2,2-trifluoro-
ethanol (dashed). All spectra have been scaled to the same absorbance at
the excitation wavelength (330 nm).
Rotaxane 1 (Figure 1) consists of a thread containing a glycyl-
glycine recognition motif used to promote rotaxane formation4,5
1
and a bulky anthracene stopper. H NMR studies, supported by
simultaneously binding to both amide carbonyl groups in the
normally preferred manner for peptide rotaxanes.4,5
X-ray crystallography of a close analogue of 1 (both included as
Supporting Information), show that in relatively nonpolar solvents
(e.g. CDCl3, 1,4-dioxane, etc.) 1 principally adopts co-conforma-
tions with the macrocycle surrounding the peptide part of the
thread (Figure 1). It appears that the nonplanarity of the
anthracene-9-carboxamide unit prevents the macrocycle from
Rotaxane 1 was originally intended as an “inactive” model
compound for studies aimed at investigating energy transfer or
electron transfer between the mechanically interlocked compo-
nents of rotaxanes. Since energy transfer or electron transfer from
the excited anthracene to this macrocycle is impossible on
thermodynamic grounds6 one would expect the fluorescence of
1 to be virtually identical with that of the thread 2. As shown in
Figure 2A, this is in fact the case in dimethyl sulfoxide (DMSO),
a solvent in which the hydrogen bonding between the peptide
station and the macrocycle is disrupted and the macrocycle resides
on the alkyl chain of the thread, far from the anthracene unit.4 In
non-hydrogen bond disrupting solvents such as 1,4-dioxane
(Figure 2B), however, the fluorescence of the rotaxane is
considerably broader and more intense than that of the thread.
Since some of the vibrational fine structure of the anthracene
emission is retained, it seems likely that the broad emission band
is composed of two contributions, viz. one that resembles the
thread spectrum and an additional broad red-shifted band.
The fluorescence of anthracene-9-carboxamide has been studied
by Werner and Rogers,7 who observed that the spectrum is
broadened and red-shifted in the strongly hydrogen bond donating
solvent 2,2,2-trifluoroethanol. The thread 2 shows the same effect
‡ Vrije Universiteit.
§ University of Warwick. E-mail: David.Leigh@ed.ac.uk. Current address:
Department of Chemistry, University of Edinburgh, The King’s Buildings,
West Mains Road, Edinburgh EH9 3JJ, UK.
(1) For comprehensive reviews see: (a) Benniston, A. C. Chem. Soc. ReV.
1996, 25, 427. (b) Sauvage, J. P. Acc. Chem. Res. 1998, 31, 611-619. (c)
Balzani, V.; Gomez-Lopez, M.; Stoddart, J. F. Acc. Chem. Res. 1998, 31,
405-414. (d) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew.
Chem., Int. Ed. 2000, 39, 3348-3391.
(2) (a) Murakami, H.; Kawabuchi, A.; Kotoo, K.; Kunitake, M.; Nakashima,
N. J. Am. Chem. Soc. 1997, 119, 7605-7606. (b) Armaroli, N.; Balzani, V.;
Collin, J. P.; Gavin˜a, P.; Sauvage, J. P.; Ventura, B. J. Am. Chem. Soc. 1999,
121, 4397-4408. (c) 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. (d) 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.
(3) “Co-conformation” refers to the relative positions of the mechanically
interlocked components with respect to each other, see: Fyfe, M. C. T.; Glink,
P. T.; Menzer, S.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2068-2070.
(4) Lane, A. S.; Leigh, D. A.; Murphy, A. J. Am. Chem. Soc. 1997, 119,
11092-11093.
(6) The lowest excited singlet state is located on the anthracene-
carboxamide moiety and has an E00 ) 3.22 eV. Its oxidation potential (1.6 V
vs SCE) is not sufficient to allow reduction of any group in the ring.
(7) Werner, T. C.; Rodgers, J. J. Photochem. 1986, 32, 59-68.
(5) Leigh, D. A.; Murphy, A.; Smart, J. P.; Slawin, A. M. Z. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 728-732.
10.1021/ja015919c CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/20/2001